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CFD simulation of effects of operating parameters and product on heat transfer and mass loss in the stack of bagged potatoes
Abstract
The heat and mass transfer within the stacked bags of potato depend on many parameters of both product and operating conditions. Most important of these are: rate of metabolic heat generation, porosity of the bulk medium, diameter of the product, resistance of the product skin preventing moisture loss, and temperature as well as RH of the storage air. Therefore, the effect of the parameters of the product and the operating conditions on heat and mass transfer in the stack of bagged potatoes during the transient cooling and at steady state were studied using the CFD modeling approach. It was found that increasing the porosity of the bulk medium as well as product diameter reduced the product temperature and moisture loss during the cooling. The metabolic heat of respiration and storage air temperature increased the temperature of the product and moisture loss during the transient cooling and steady state. Moisture loss and RH in the bulk medium increased with increasing the skin mass transfer coefficient. Increasing the storage air temperature linearly increased the average product temperature at steady state. It was also observed that in comparison to storage air temperature, the RH of the storage air had more influence on the moisture loss from the product. The cool-down time was decreased with the increase in bulk medium porosity, product diameter and metabolic heat of respiration. It was found that large variations in moisture loss could be expected if the characteristics of the product and storage conditions varied from the prescribed ones.
... However, based on this principle, forced-air cooling and pressure cooling have appeared, which are precooling technologies that facilitate rapid air cooling to replace room cooling [1,16]. Among them, the speed and temperature of the cold airflow are considered to be the pivotal elements affecting the effect of forced-air cooling [17][18][19]. Furthermore, larger airflow is necessary to demand a higher static pressure, which may not only augment the gross energy consumption but also cause moisture loss, wilting, and cold damage to FVS [20,21]. ...
... However, there are still some problems in numerical modeling, such as the accuracy and simplification of a model. The errors of these predictions and experimental studies were all greater than 10% in the numerical simulation of HMT during the precooling period of FVS [19,31]. Therefore, a simple, accurate, and efficient numerical model has always been expected by researchers. ...
The weight loss and spoilage of post-harvest fruits and vegetables (FVS) occur as a result of physiological and biological processes, the rates of which are influenced primarily by product temperature. In order to maintain the freshness of FVS and reduce losses, it is necessary to cool the product as soon as possible after harvest. Precooling is considered such an effective technique because it quickly removes field heat from FVS, thereby preventing deterioration and senescence. With the increasing demand for fresh FVS, the optimization of precooling technology has received extensive attention, especially the research on its basic principle, that is, the heat and mass transfer (HMT) between FVS and the precooling environment. Therefore, this paper reviews the advantages and disadvantages of several main precooling techniques, their HMT processes, the research methods and detection techniques of HMT, and the simulation and application based on numerical technology. Precooling techniques include room cooling, forced-air cooling, hydrocooling, and vacuum cooling. These advanced detection techniques for HMT include magnetic resonance imaging, particle image velocimetry, infrared thermography, nuclear magnetic resonance, bioelectric impedance analysis, dilatometry, thermogravimetric analysis, and X-ray CT. HMT research mainly adopts porous media method, direct numerical simulation, cell growth simulation. Their applications focus on computational fluid dynamics and the lattice Boltzmann method. Furthermore, this paper highlights the application of the computer field in FVS precooling and provides perspectives on the directions for future research.
... Despite an error of approximately 20% for the velocity prediction, this model provided acceptable results for the cooling rate and weight loss, which were helpful for the practical design of cold rooms. Using a similar CFD model, Chourasia and Goswami (2007) studied the effects of operation and produce parameters, such as heat generation by respiration, bulk porosity, produce size and relative humidity, on heat and mass transfer within stacked bags of potato. All parameters affected the produce temperature and moisture loss, and, hence, the cooling time. ...
... The average difference in temperature between the experimental and simulated data was 1.2°C. Large variations in moisture loss were observed if the characteristics of the produce and storage conditions varied from the prescribed conditions (Chourasia & Goswami, 2007). Tanaka, Konishi, Kuroki, Hamanaka, and Uchino (2012) numerically studied the cooling performance of a partially loaded cold room with different loading patterns of produce containers. ...
Background
Precooling is a critical step in the postharvest cold chain. Studies of the precooling of fruit and vegetables are based on the strong interactions between modelling, engineering, physiology and commercial outcomes. In recent years, new progress in precooling has been achieved. These achievements include different cooling strategies, research into precooling mechanisms, and numerical simulations. This review aims to provide the most recent information about precooling and promote its application in the fruit and vegetable industry.
Scope and approach
Different precooling strategies are evaluated with respect to the cooling rate, cooling uniformity, and multiscale simulation. An overview of mathematical modeling approaches used to quantitatively describe precooling processes for computer-aided designs is provided. The effect of precooling on fruit quality at the physiological and molecular levels is outlined.
Key findings and conclusions
Numerical simulations have become widely used to improve the precooling performance. Cooling homogeneity, in particular, has attracted increasing attention in recent studies because of the substantial effects of cooling homogeneity on the precooling efficiency and produce quality. The spatial scale of numerical simulations of the precooling process has started to become more precise and specific. Recent numerical simulations have focused on the bin and package scale. Models of transport processes at multiple spatial scales are investigated using multiscale modeling. Moreover, the effect of precooling on produce quality has recently received increasing attention. In addition, the investigation of the effect of precooling on fruit at the metabolomic and genomic levels has become an emerging trend and has provided deeper insights into the molecular mechanisms underlying the effect of precooling treatments on fruit.
... This phenomenon causes a high-temperature region at the rear portion of the cold store [10]. S. Duret et al. 2014, in his experiment, show that return air velocity is higher as compare to air supply velocity in the rear section of cold storage, it causes relatively low heat transfer at the rear section in comparison with cooling coil side [11,12,13]. CFD modeling of cold store units required some specific features as there are large airflow obstacles by means of stockpiling of agricultural goods, air-conditioning systems affect room boundary condition, the evaporator unit can be treated as a source of momentum and low air velocity ± 3m/s with turbulence [14,15,20]. ...
... Error is considerable as per the literature review suggested the same, it because of standard assumptions made during CFD analysis. M. L. Hoang et al., demonstrate a relative error of 26% for airflow velocity calculation thru SST k-model [12,13,16,18], also Zhang et al indicated that the prediction accuracy of 3D CFD model for air temperature was with the average percentage errors of 7.5%, when compared to measured values [23] and Moonne et al. found error 10% for the same [24]. As discussed above indicate the auxiliary draught system can improve the airflow distribution in the cold chamber. ...
... Computational Fluid Dynamics (CFD) is such a powerful tool that uses a computer to solve numerically the basic conservation equations of fluid flow, heat and mass transfer for realistic problems in chemical, mechanical, pharmaceutical, biological and food engineering (Chourasia & Goswami, 2007;Hoang et al. 2003;Hoang et al. 2000;Tassou & Xiang 1998;Xu & Burfoot 1999). With the enhancement of computing power and efficiency, and the availability of affordable CFD packages, CFD has been accepted as a successful method in engineering. ...
... The air circulation in a cool room must ensure the distribution of 1-MCP gas in the whole cool room and on the fruit. The uniform distribution of 1-MCP is strongly influenced by the design of storage room, arrangement & stacking of fruit bins inside the room, bin design & materials of construction as well as storage conditions (Chourasia & Goswami 2007;Hoang et al. 2000Hoang et al. , 2003Delele et al. 2009;Ambaw et al. 2010). ...
In most cases, postharvest and food processing technologies are commodity and location specific. It enhances and augments per capita food availability from a unit arable land and other resources by preventing avoidable postharvest losses and adding more value to the food produce. At present situation, most of the food produce is harvested during a specific season and area. The major challenge in this area is to reduce postharvest losses and provide consumer with best quality produce using improved storage methods. Quality is a result of appropriate production practices, careful harvesting, storage and transport. Several factors can be mentioned, but loss of quality due to the continued natural ripening or aging of produce is a main concern in postharvest storage. Fumigation using 1-Methylcyclopropene (1-MCP) gas to control ripening is a recent method that has been proposed to better control for extending the shelf life and reducing the postharvest quality loss of food produce.
At commercial scale, the cost of real experiments is very high. Therefore computer based simulation and evaluation tools are an attractive and alternative. These tools allow engineers in the design cycle to test different concepts and to finally optimize a complete process or system. Computational Fluid Dynamics (CFD) is such a powerful tool that uses a computer to solve numerically the basic conservation equations of fluid flow, heat and mass transfer for realistic problems in chemical, mechanical, pharmaceutical, biological and food engineering. It has been noticed from recent studies that the effectiveness of the postharvest treatments is partly determined by the uniformity of the distribution of the active substance over the whole storage area. The air circulation in the cool room must ensure the distribution of the active substance in the cool room and on the fruit. The uniformity of the distribution of the 1-MCP gas is thus strongly influenced by the design of the cool room, arrangement and stacking of fruit bins inside cool room, and bin design and materials of construction.
The main objective of the present study was to develop guidelines for the 3-D distribution of 1-MCP gas that is as uniform as possible with the lowest possible dose in a loaded cool room with apple fruits.
... Furthermore, the cooling unit and humidifying equipment occupied space in the container, but their effects were ignored as well. The following set of equations describes the mass, momentum, and heat transfer in the models (Anonymous, 2003;Chourasia and Goswami, 2007). ...
... CFD simulation has been widely used in the description and design of post-harvest facilities, including products and packaging (Defraeye et al., 2014;Delele et al., 2013a, b;Ferrua and Singh, 2009), storage facilities (Chourasia et al., 2007;Delele et al., 2009;Foster et al., 2006), and refrigerated transport (Moureh et al., 2009a, b;Tapsoba et al., 2007). This study addresses the optimization of the gas exchange system for transportation with CA. ...
Maintaining a suitable gas composition in a transportation container with controlled atmosphere is important. When O2 concentration is below the suitable value, gas exchange is activated to improve O2 concentration. Computational fluid dynamics was used to compute the effects of exhaust valve location, driving speed, and valve diameter on the gas exchange process. Gas exchange time can be reduced by relocating the exhaust valve to a lower area and by improving driving speed and valve diameter. Moreover, running the fan can shorten gas exchange time and enhance the distribution uniformity of the O2 volume fraction in a container. The influence of gas exchange on temperature in the container can be reduced by shortening processing time, but this reduction significantly affects the distribution uniformity of O2 volume fraction. A test was conducted to verify the accuracy of the numerical models. A good agreement was found between the simulation and test values of O2 volume fraction during gas exchange. Results reveal the rules and characteristics of gas exchange in fresh-keeping transportation containers with a controlled atmosphere.
... The spatial variations of temperature were also determined by Tassou & Xiang (1998) for wet air-cooled store after 5, 10 and 15 hours, Akdemir & Arin (2006) for evaporative cold store by using a mapping software with measured data, storage temperature varied between 0-5 o C for Tassou & Xiang (1998), 2.7-5.3 o C for Akdemir & Arin (2006). Average temperature differences during cooling period was found as 1.2 o C by Chourasia & Goswami (2007b) used CFD in their research. ...
... There was a good agreement between CFD model estimations and data logger measurements for distribution of relative humidity (%) in the cold store (Chourasia & Goswami 2007a). The relative error (E CFD ) of relative humidity was better than 15.9% as overall Chourasia & Goswami (2007b). ...
The objective of this study was to analyse air temperature and relative humidity distribution in an experimental cold store fully loaded with apples by using both experimental and numerical (CFD) methods. An unsteady three-dimensional computational fluid dynamics model was developed to assess the distribution of temperature and relative humidity in a cold store fully loaded with Granny Smith apples. The storage temperature and relative humidity were maintained at 2 °C and 90%, respectively. The relative humidity and temperature were measured at 36 different points inside the cold store in three different planes. A three-dimensional mathematical model was built for the numerical needs. The numerical model was validated against experimental data from the same facility. Relative error of the model was calculated 13% for temperature and 1.43% for relative humidity. Numerical results obtained from the simulations agreed quite well with experimental data for temperature and relative humidity. Maximum differences were observed near the borders of the cold store which can be attributed to the stronger thermal gradients taking place on these surfaces. A more homogeneous distribution was achieved in the middle of the cold store both for air temperature and relative humidity leading to even smaller errors between measurements and simulations.
... Stamou et al.verified the results from the CFD model for indoor airflow and heat transfer with obtained experimental results [11]. Chourasia et al. done extensive work on potato cold storage experimentally and also applied CFD models to predict the results [12], [13], [14].Chourasia et al. investigated moisture loss in potato and heat transfer in storage under steady-state using the CFD technique [13]. They also developed a 3D model on mass transfer, heat transfer and airflow in a partially impermeable enclosure containing agricultural produce during natural convective cooling [14]. ...
Temperature heterogeneity inside the space of cold room results in fruit quality degradation and weight loss. It is governed by the flow of air inside the room. It is quite challenging to achieve the temperature homogeneity at every location of the room. As this depends upon various operating and design variables.Analysis of flow field and temperature profile inside the cold room using experimental process is a costly and time consuming process. Thus nowadays researchers are using CFD tools to predict the same to save time and resources. Air flow field in the cold room depends on various factors which affects the results produced through CFD analysis. Thus selection of turbulence model for achieving the desired flow conditions becomes crucial.In the present study theairflow pattern and temperature profile of the cold room loaded with bins of apples is analyzed by CFD technique.Turbulence model utilized is SST k-for air turbulences in the room. A new forced duct arrangement system is proposed for improving temperature homegeniety inside the cold room.
... The study showed that products under mixed convection regime are better cooled than those under a forced regime. Chourasia and Goswami (2007) obtained similar results and showed that recirculation due to natural convection helps remove heat from the products, leading to a lower product temperature. Thus, natural convection exerts a positive influence on the heat transfer within a product stack. ...
... The model underpredicted the product temperature. This was thought to be due to errors in the positions of thermocouples, model input values and/or model assumptions.For potato,Chourasia and Goswami (2007a) examined the effect of several parameters, such as rate of generated respiratory heat, void diameter in the medium, RH, tuber size and temperature, on heat and mass transfer patterns in potato bulk storage. The study also included steady-state and transient models and validated the models using experimental measurements of temperature and moisture loss. ...
Harvested sugar beet (Beta vulgaris L.) are stored in cold regions in large piles exposed to ambient weather conditions and fluctuate temperatures during the winter storage period, which lasts for four months. To better understand the impact of air temperature on the pile temperature. A two-dimensional (2D) heat transfer steady-state model was designed to predict the temperature profile of the pile. To validate the model, temperatures obtained from the model were compared with the temperatures measured from onsite commercial piles during the storage seasons from the second season in Reese, MI. The model tended to underestimate the pile temperature (°C). The mean difference between measured and modeled temperature values was significant (P ≤ 0.05). Daily rate of sugar loss (kg/metric ton/day) based on measured and modeled temperatures were calculated and compared for model accuracy. The mean of the daily sugar loss based on the modeled pile temperature was significantly (P≤0.05). Additionally, three zones (upper, middle and lower) of the pile were studied for the model accuracy. There was a significant difference between the modeled and measured pile temperature between the three zones in the second season, whereas the first season didn't show difference between the temperatures of the upper and the middle zones (P≤ 0.05). Moreover, a comparison of predicted sugar loss as a function of pile geometry was conducted under 2012 air temperature and a 3°C increase in air temperature relative to 2012 data.
... It could be used in different geographical areas, seasons, and aeration systems, and could automatically adjust its set point. Chourasia and Goswami (2007), Elgamal et al. (2013), Liu et al. (2016 used the CFD modeling method to study the variation of transient cooling and heat and mass transfer in grain bulk, and predicted the temperature variations, and the simulated values coincided with the measured values. Arias Barreto et al. (2013) used a mathematical model to analyze the effects of storage conditions in silo-bag holding wheat from summer to winter, and predicted the average dry matter loss under different storage conditions and compared them with the critical limits for safe storage of grain and seeds, which did not exceed the limit of grain quality degradation. ...
Grain temperature is one of the most important factors affecting grain security. The temperature field of the underground ecological granary was investigated by field experiment and numerical simulation. The experiment was carried out in the underground grain silo (UGS), a full‐scale engineering model. The UGS temperature data for one consecutive year were obtained through daily on‐site periodic monitoring. Based on the boundary conditions of UGS, including ambient air temperature, heat transfer coefficient, and underground constant temperature zone, a two‐dimensional rotational axisymmetric model is constructed. Then the temperature field of UGS was analyzed by numerical method. The simulation results coincide with the test results, and the conclusions are as follows: (1) The average temperature of UGS is maintained at around 20°C, the upper zone is easy to be affected by the outside, but the central and bottom zone are relatively stable. (2) The average temperature of grain is always below 17°C, and the temperature of the surface layer has inevitable fluctuations due to the influence of the ambient air temperature. (3) The average temperature of grain changes significantly in the early stage, but as time goes on, the grain temperature tends to the temperature in the constant temperature zone.
Practical Applications
Compared with the traditional above‐ground granaries underground granary has the advantages of land saving, grain storage capacity per unit area and long‐term storage of grain at low temperature. Now we have built two large underground granaries to study the various properties of underground granaries, which will be put into normal use later. By studying the temperature field of underground granary, exploring the change of temperature field of long‐term grain storage is beneficial to the popularization and application of underground granary, which is of great significance to ensure food security.
... Stamou and Katsiris (2006) verified the results from the CFD model for indoor airflow and heat transfer with experimental results. Chourasia & Goswami did an extensive work on potato cold storage experimentally and applied CFD models to predict the results (Chourasia and Goswami 2007a, 2007b, 2007c. They investigated heat transfer and moisture loss in potato storage under steady-state using the CFD technique (Chourasia and Goswami 2007). ...
Cold rooms are primarily used to preserve food produce in bulk volume. The presence of bulk food produce creates a non-uniformity in cold air flow distribution. This non-uniformity in cold air results in the uneven cooling of the food produce inside the cold room. Furthermore, the ceiling-type cooling units are used in a cold room where the air flow distribution is influenced by the air entrainment effect (generating secondary air flow), resulting in temperature inhomogeneity inside the cold room. An induced duct arrangement is proposed in the present study to suppress the effect of secondary airflow and enhance the cooling uniformity. A numerical investigation of the 3D CFD model of the cold room with an induced duct arrangement is presented in this paper. A cold room of about 81.86 m³ volume is considered. A numerical study is performed for 20 h in transient cooling conditions. Air velocity and temperature are determined at various locations for two different cold room arrangements. It is found that the introduction of a duct arrangement at the rear wall of the cold room reduces the half cooling time by 10% compared to the normal cold room condition due to the elimination of the entrainment of air. The convective heat transfer coefficient is seen to be increased by 12.7% compared to the normal cold room, while the temperature inhomogeneity inside the room decreases by 14.3% with the introduction of duct arrangement.
... During FAC, the airflow through the package structure is a complex phenomenon, and obtaining detailed information on the heat and mass transfer at various locations without understanding the airflow dynamics is challenging. Numerical studies have proven effective in understanding and solving the above problems by creating three-dimensional airflow distributions and heat transfer during the pre-cooling process [53,54,55,56,57,58,59,60]. ...
Preservation of horticultural produce is vital for quality products to be delivered to customers long after harvest. Product qualities are maintained when field heat is removed. Prompt forced-air cooling (FAC) is preferred for various products to remove this field heat and delay ripening. Packaging modifications are vital in enhancing the FAC process and minimizing energy consumption. The detailed characteristics of fluid flow and heat transfer inside packaging can be best analyzed by computational fluid dynamics (CFD) modeling, which helps develop better packaging designs. This comprehensive review highlights various numerical models used to study the pre-cooling process and discusses different evaluation methods for analyzing the performance of FAC processes. The procedure to develop a CFD model for pre-cooling studies is also discussed in detail. The effect of various parameters like vent hole size, shape, airflow velocity, and cooling temperature on cooling efficiency and uniformity is accentuated through literature. Different boundary conditions, new performance parameters, and guidelines for future studies are also deduced. Further, the study emphasizes the importance of multi-parameter analysis in designing better packaging for optimum cooling performance. This study thus demonstrates the use of CFD methods to study the effect of various parameters on pre-cooling performance and energy consumption while simultaneously comparing various packaging designs to find sustainable design solutions for different horticulture products.
... Similarly, Mahajan et al. [9] discovered that increasing the RH by 20% from 76 to 96% at 4 ºC reduced mushroom moisture loss by 87% while lowering the temperature of food product from 16 to 4 ºC reduced moisture loss by 61% at constant RH (96%). Chourasia and Goswami [10] performed a numerical study in a steady state cold storage and proposed that, when the RH of air decreased by 15% from 95 to 80%, the product (potato) loses 41.8% more moisture. The weight loss or moisture loss changes by 0.0095% per week when the RH of air changes by 1%. ...
Refrigerated storage is widely used to extend the post-harvest life of perishable fruits and vegetables. Inside the refrigerated space, food product encounters difference in encompassing conditions from the field environment to the storage environment, and in this way refrigerated storage keep up with the optimal environment to regulate the physiological changes in food commodities. The present study examines the storage operating condition including temperature, relative humidity (RH), and velocity of the air and their effect on the moisture loss of orange in a loaded cold storage facility. In view of investigation of moisture loss, temperature and RH significantly control the moisture loss in orange, which ranges from 0.9 mg/(cm2.h) (at 27.71 ºC and 70 ± 2% RH) to 0.035 mg/(cm2.h) (at 6 ºC and 92 ± 2% RH). Further, the cold storage facility observed inconsistency in operating conditions between the different locations, and accordingly, a comparative study of operating conditions and moisture loss is performed. The mass transfer coefficient inside the package (crates) is evaluated and found to be useful in calculating the moisture loss for oranges with an error of up to 7.7%.
... M.K. Choursiya, T. K. Goswami. [13] Finding the velocity of air, product temperature, and moisture loss with experiment and simulated the given model with CFD 2D Model at steady state condition. It is found that the result obtained by experiment and CFD model have good agreement. ...
In cold storage maintaining preservation
temperature uniformly for preserving food is difficult to
attain. This paper represents the temperature distribution and
velocity distribution for axial flow evaporator and mix flow
evaporator arrangement in a refrigerated cold storage. The
cooling chamber under analysis has an axial flow cooling
coil arrangement located at top of the wall and mix flow
cooling coil (discharge through centre and circumferential)
located at centre of roof in front of column of bins. A CFD
analysis is done under steady state for air flow distribution
and temperature distribution using a three dimensional
model. In this paper a three dimensional model for a cold
storage [10 m(l)× 8 m(w)×9m(h)] prepared. The
performance analysis done for both the coils arrangement
having the same cooling capacity, air velocity and discharge
temperature. It is found that during mix flow arrangement
the average temperature of cold chamber and cooling
effectiveness was lower than the axial flow cooling coil
arrangement.
... Delele et al. [16] studied a chicory root cold storage humidification system using computational fluid dynamics. Chourasia et al. [17] simulated the specific effects of potato respiration heat production, pores of cargo box and heat transfer coefficient of cargo surface, humidity and temperature of airflow on the refrigeration of goods. In order to understand the influence of the stacking mode of goods on the temperature field of cold storage, based on the conclusions of previous studies, this paper uses CFD software to simulate and analyze the 2 temperature field distribution of the cold storage under different cargo stacking modes when the vent air outlet mode is adopted. ...
The distribution of temperature field in cold storage is affected by different stacking patterns of goods. To explore the influence of goods stacking way on cold storage temperature field, computational fluid dynamics (CFD) was used to simulate the influence on temperature flied of cold storage by different types of stack arrangement methods under a new air outlet design way in this article. The three types of stack arrangement included stacking along the X axis, along Z axis, and along Z axis with 10cm thick dunnage board. The simulation result shows that the temperature distribution inside the cold storage is the most uniform when goods are stacked along the Z-axis and filled with 10cm thick dunnage board. The simulation on this type of arrangement well agreed with measured results. This work would give out research reveal the features of the temperature distribution in cold storage with vent series. This would be helpful on design of the cold storage and the stack arrangement.
... Most of the prior mentioned research work has concentrated on investigating (experimental or numerical) the effect of design and operating parameters on cold room performance. The problem associated with the cooling performance and the energy-saving potential in room cooling depends on many parameters such that location of cooling coil [11], type of product to be stored, shape and size of produce [12,13], packaging arrangement [9,14], wall insulation [15], door opening time [16] and the influence of operating variables include temperature, relative humidity and airflow field [17][18][19]. However, regarding the full-scale experiment work on cold room analysis, only a few experimental works have been carried out. ...
Temperature heterogeneity in the refrigerated cold room promotes food degradation and losses and governs by the airflow field. Achieving the temperature homogeneity of fruits at various locations in the cold room is challenging due to the effect of operating and design variables. This study presents the experimental investigation of airflow field, temperature characteristics (fruits and air), convective heat transfer coefficient, half cooling time and weight loss measurement of fruits loaded in crates inside the cold room. Results showed significant cooling heterogeneity inside the cold room during the cooling period. The maximum cooling heterogeneity identified along the length of the cold room. The crates placed in-front of the air cooling unit observed a higher cooling rate and minimum cooling time in comparison with that placed at the rear-side. During the storage condition, the temperature heterogeneity is varied from 0.87 to 0.97 °C, and the coefficient of temperature heterogeneity ranged from 0.088 to 0.103. The maximum value (79%) of the velocity heterogeneity index obtained at the top plane. This experimental study is beneficial to understand the mechanism of cooling pattern and air distribution in the cold room.
... In recent years, the numerical modelling approach was also used to evaluate and optimize the room cooling process. Chourasia and Goswami (2007) numerically analysed the effects of the product and operating parameters on the cooling time and moisture loss during the cooling of stacks of bagged potatoes in the cold store. They concluded that the product temperature and moisture loss decreased with higher bulk medium porosity and product diameter while increased with the respiration heat and higher air temperature. ...
Precooling is of significant importance for postharvest fruits and vegetables to control the quality degradation and prolong the shelf-life. Current precooling methods include room cooling, forced-air cooling, hydrocooling, vacuum cooling, contact or package icing, and cryogenic cooling, all of which have their advantages and disadvantages. The first two methods with the cooling medium of air are extensively used because of the wide applicable range of fruits and vegetables. Numerous studies have been devoted to cope with the drawbacks of these two air-based precooling methods with various evaluation criteria and optimization methods. A systematic literature review on these studies is firstly conducted with respect to experimental and numerical investigations respectively for the two methods. The main contributions from the previous studies are also summarized respectively with the research objectives and performance metrics. The literature review indicates that the current performance evaluation is limited to apparent parameters and the optimal design is only proposed based on the performance evaluation and comparison. Furthermore, with inspiration from the research in other domains, a scheme of advanced evaluation and optimization for air-based precooling methods is proposed with thermodynamic evaluation metrics and constructal optimization methods from the interdisciplinary perspective.
... A CFD model of air movement through a doorway was developed in other study and verified against conventional and Laser Doppler Anemometry measurements 11 . Distributions of temperature and relative humidity for two different cold stores were determined and air conditioning influenced the moisture loss from the products [12][13][14] . The most important factors limiting the storage life of peach fruit are the density loss and the fungal contamination [15][16][17] . ...
Spatial variation of temperature and relative humidity were estimated with Computational Fluid Dynamics (CFD) at top, middle and base levels for peach storage and verified with measured data in an experimental cold store having a chiller unit. Cooling system set up at +1 o C and 90% relative humidity. Ansys Fluent Software was used for CFD modelling. CFD models were validated by using differences of model estimations and measurements and evaluated by using descriptive statistics and variance analysis. Mean differences between the model and measured temperature and relative humidity were calculated as 1.02ºC and 11.66%, respectively. Fruit firmness, total soluble solid, titratable acidity, pH, respiration rate, external appearance and flavour analyses were determined. AIMS AND BACKGROUND Growing agricultural products is important for human life and the economics of a country, but also their conservation until consumption stage. Healthy storage depends on the cooling system used for assuring a suitable atmosphere for product features, and the correct case and package 1-3. CFD has been used for different purposes 4. Such technique is successful when a good correlation between the numerical simulation and the experiment results can be established, or when the simulations can predict highly complex phenomena 5,6. A model capable of predicting both the air and product temperature was studied in the litterature 7. Particularly, a two-dimensional mathematical model for a mini type cold store was proposed 8. In such studies, air flow distribution was investigated by using computational fluid dynamics inside a cold store 9,10. A CFD model of air movement through a doorway was developed in other study and verified against conventional and Laser Doppler Anemometry measurements 11. Distributions of temperature and relative humidity for two different cold stores were determined and air conditioning influenced the moisture loss from the products 12-14. The most important factors limiting the storage life of peach fruit are the density loss and the fungal contamination 15-17 .
... In recent years, the numerical modelling approach was also used to evaluate and optimize the room cooling process. Chourasia and Goswami (2007) numerically analysed the effects of the product and operating parameters on the cooling time and moisture loss during the cooling of stacks of bagged potatoes in the cold store. They concluded that the product temperature and moisture loss decreased with higher bulk medium porosity and product diameter while increased with the respiration heat and higher air temperature. ...
Precooling is of significant importance for postharvest fruits and vegetables to control the quality degradation and prolong the shelf-life. Current precooling methods include room cooling, forced-air cooling, hydrocooling, vacuum cooling, contact or package icing, and cryogenic cooling, all of which have their advantages and disadvantages. The first two methods with the cooling medium of air are extensively used because of the wide applicable range of fruits and vegetables. Numerous studies have been devoted to cope with the drawbacks of these two air-based precooling methods with various evaluation criteria and optimization methods. A systematic literature review on these studies is firstly conducted with respect to experimental and numerical investigations respectively for the two methods. The main contributions from the previous studies are also summarized respectively with the research objectives and performance metrics. The literature review indicates that the current performance evaluation is limited to apparent parameters and the optimal design is only proposed based on the performance evaluation and comparison. Furthermore, with inspiration from the research in other domains, a scheme of advanced evaluation and optimization for air-based precooling methods is proposed with thermodynamic evaluation metrics and constructal optimization methods from the interdisciplinary perspective.
... Chourasia et al. applied the RNG k-ε turbulence model with the finite volume solution technique. Average overall errors of 19.5%, 8%, 0.5 o C, and 0.61% were found for air velocity, air flow temperature, product temperature and moisture loss from the potatoes respectively [19,20,21,22]. found that the four different models with their individual prediction accuracy regarding the air velocity were the Standard k-ε model with 24.3 % error, Realizable k-ε model with 23.5 % error, RNG k-ε model with 22.4 % error and SST k-ω models with 18.2% error. A comparison between predicted air pressure drop and produce temperature and measured values showed a good agreement with an average relative error of 13.80% and 16.27%, respectively [23, 24 25, 26]. ...
Air distribution enable convective heat transfer in cold storage operation. Thermal behaviors of the cold storage system are based on air transport arrangements. Transport characteristics can handle with auxiliary arrangements such as induce draught system. Experimental investigation for the impact of auxiliary draught system (ADS) on air transportation is carried out. Air transport velocity was measured in the cold chamber with a hot wire anemometer. Experimental results show significant enhancement by three times mid-section air flow velocity and overall one- and half-time greater flow velocity observed, while return air velocity measured almost tow time of general condition during the experiment. COP of plant improve by 21% with 25% less time required to achieve desired temperature. 26% saving in power consumption observed during experiments. Auxiliary draught ensures homogeneous environment inside the plant through proper mixing of air and support convective heat transfer. Designing and analysis of airflow patterns with temperature distribution in large entity like cold storage is a difficult task thus Computational Fluid Dynamics (CFD) can address the issue with high degree of precision. It has been observed that SST K-ℇ model has average 26% error with experimental values
... Note: ρ is fluid density in kg/m 3 ; x, y, and z are the coordinate positions in the fluid; u, v, and ω are the velocity vector of X, Y, and Z directions, respectively, in m/s; k is the turbulent kinetic energy in m 2 /s 2 ; ε is the dissipation rate in m 2 /s 3 ; P r is the Prandtl number; T is the reserve temperature in°C; η is the dynamic viscosity in kg/(m·s); η t is the viscosity coefficient of turbulence; and p is pressure ; and c µ , c 1 , c 2 , σ T are the empirical constants, σ k is the Prandtl number for turbulent kinetic energy, and σ ε is the Prandtl number for the dissipation rate of turbulent kinetic energy; they were set as the values in Table 2 in the k-ε model. Due to the presence of plants in the greenhouse, a porous medium model was used [35]. The model equation applied was: ...
Decreasing the temperature of a greenhouse in summer is very important for the growth of plants. To investigate the effects of a roof sprinkler on the heat environment of a greenhouse, a three-dimensional symmetrical model was built, in which a k-ε (k-epsilon) turbulent model, a DO (Discrete Ordinates) irrational model, a Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) algorithm, and a multiphase model were used to simulate the effects of the roof sprinkler, at different flow rates. Based on the simulation results, it was found that the temperature could be further reduced under a proper sprinkle rate, and the temperature distribution in the film on the roof was more uniform. A test was conducted to verify the accuracy of the model, which proved the validity of the numerical results. The simulation results of this study will be helpful for controlling and optimizing the heat environment of a greenhouse.
... M. L. Hoang et al (2000) use CFD for air flow analysis in Cold store [9]. M. K. Chourasia (2001Chourasia ( , 2007a analyzed potato storage facility with various storing arrangements with the help of CFD tools and demonstrate air flow distribution , heat transfer and energy conservation for cold storage for batter productivity of system; CFD simulation and model selection for same ware validate with the experimental investigation [10], [11], [12], [13]. (2007) were among those researchers who worked on cold storage cool air flow distribution and opting CFD as their supporting tool [14], [15], [16], [17]. ...
Experimental analysis of flow Distribution inside a cold storage is a costly affair, thus many researchers are intensively using computational techniques. In designing of various air distributions arrangements in cold storage Computational Fluid Dynamics (CFD) can play a vital role. There are various factors which affects the results produced through CFD analysis of air circulation in close environment like cold storage. Selection of the specific turbulence model for particular flow condition is a big bottle-neck. This paper discussed CFD approaches and various turbulence models used in cold storage air flow evaluation. Selection of turbulence model affects the analysis as each model use different set of boundary conditions. It has been observed that most of the researchers adopt RANS K-ℇ model because of its simplicity and ease of understanding. While on an average there is 26% error in results produced through RANS turbulence models and LES model provide good results but lots of skills and higher computational capacity required.
... With the actual physical model, constructing the reasonable mathematical model, and the analogue simulation, we can obtain the result which is similar to the experimental research, and can change the condition flexibly, save a lot of time and cost. At present, CFD has been used in all aspects of cold storage research, involving the temperature field, gas flow field, humidity field, pressure field [11,12,13,14], etc. There are some precedents that CFD was used to simulate the airflow in the air space in an air supply duct system [15]. ...
... The storage environment's determinant parameters, primarily the temperature and RH, are critical for extending the shelf-life of fresh produce (Thompson et al., 2002). Several studies have shown that temperature, RH or moisture and air flow heterogeneity inside cold rooms and can affect the quality of the stored produce (Chourasia and Goswami, 2007). Variations in heat transfer coefficients inside cold stores at different positions were determined. ...
The main purpose of this study was to compare the effectiveness of low-cost evaporative cooler (EC), CoolBot-Air-Conditioner (CBAC) and combinations of EC and CBAC in terms of changes in temperature and relative humidity. The transient temperature and relative humidity (RH) distribution and the cooling efficiency of the cooling pads were evaluated. A Comparison of the changes in temperature and RH was also performed. The result indicates that the CBAC cooler can maintain the required temperature of the air inside the empty store room by reducing the temperature to 6°C compared to the ambient air temperature of 34.1°C. The CBAC cooling system achieved a total of 28.1°C temperature reduction. The RH decreased to about 75%,
which is not within the optimum RH range for fruit and vegetables storage requirement. On the other hand, the temperature and RH achieved by EC+CBAC, when operated together (8-15°C and 80-99%) was within the optimum recommended range for fresh produces. The temperature achieved by EC (17-22°C) was not within the optimum range for many fruit and vegetables, while the RH (95-98.6%) achieved was in the optimum range required for the storage of fresh produces. In conclusion, the EC maintained only the RH but didn’t reach the required temperature for short term storage, while EC+CBAC maintained both optimum temperature and RH required for fresh produces. Although, these technologies demonstrated a promising alternative for fresh produces shelf-life extension, further investigation in terms of a detailed fluid dynamics study of the micro-environment inside the cooler with load, taking in to account that the heat of generation from respiring plant materials is required for the optimization of the cooling technologies.
... The storage environment's determinant parameters, primarily the temperature and RH, are critical for extending the shelf-life of fresh produce (Thompson et al., 2002). Several studies have shown that temperature, RH or moisture and air flow heterogeneity inside cold rooms and can affect the quality of the stored produce ( Chourasia and Goswami, 2007). Variations in heat transfer coefficients inside cold stores at different positions were determined. ...
... A literature survey reveals that many research works took an interest in this field for improving the understanding of physical phenomena and proposing some optimization ways through experimental and numerical approaches [5]. The studied products are very highly diversified, including fruit [6][7][8][9], vegetables [10,11] or meat [12,13]. ...
The fresh vegetable farm produce needs to be cooled after the harvest before the shipment and the sale. The cooling duration is very sensitive to several parameters: foodstuffs nature (size, weight, properties), harvest period, packaging shapes, climatic conditions … In this paper, experimental and numerical investigations focused on forced air-cooling and room cooling of fresh cauliflowers are presented. For this vegetable, the optimal storage conditions are 4–7 °C and 90–98% relative humidity. As cauliflower is a voluminous (15–20 cm diameter) and heavy vegetable (600 g to more than 2000 g), the cooling process lasts generally several hours (>10 h) in the case of conventional room cooling. In order to apprehend the cooling kinetics of these products, some experiments carried out on a laboratory experimental set-up and on industrial site are presented. A 1D numerical model is firstly implemented to predict the temperature field and the mass loss of a single product. Several heat transfer mechanisms occur at the surface: convection, evaporation and long-wavelength infrared radiation. Based on in-situ experiments during which product temperature, air temperature and hygrometry were monitored, a second model is developed to simultaneously determine the temperature and the mass loss of several products placed in conventional room cooling. Comparisons between simulations and experiments show relevant results.
... Transpiration rate per unit area of product surface ( _ m) was calculated using a lumped model, neglecting the moisture diffusion inside the product and assuming a local equilibrium of water vapor at the product's surface. To derive the expression for the rate of evaporation, a pseudo steady state was assumed at the surface of the orange and the steady state rate of mass transfer under equilibrium conditions was considered which could be expressed as in Chourasia and Goswami [6] and Delele et al. [7]: ...
The cooling performance of fruits and vegetables including transient heat removal, temperature homogeneity and moisture loss inside the cool storages is affected by pallet boxes arrangement. In this paper, the effect of two different types of vented pallet boxes arrangement including in-line and staggered on cooling performance were compared while longitudinal gaps between the pallet boxes and cooling airflow varied at three commonly applied levels. Validated computational fluid dynamics (CFD) models of airflow, heat and mass transfer were applied as an alternative for the experiments. Results showed that staggered array increased the surface heat transfer coefficient at the pallet boxes wall from 18 to 53%, which caused 28 to 38% reduction in ¾ cooling time compared to in-line. The most effectiveness of staggered array on heat transfer coefficient occurred for 0.25m longitudinal gap between the pallet boxes. Staggered array did not improve temperature homogeneity and moisture loss. Results also showed that, an increment of longitudinal gaps from 0.1 to 0.25m caused 10% reduction in ¾ cooling time. Increasing more than 0.25m, did not showed reasonable improvement in cooling performance.
... Such models are not capable of resolving the liquid to vapor exchanges or the solid phase to fluid phase energy flux. This category of modelling is referred to as the Equilibrium approach for heat and mass transfer [15][16][17][18]. By this approach, a specified boundary flux for the fluid to porous regions is empirically assigned utilizing correlations for Nusselt and Sherwood numbers available in the literature. ...
... The last two decades, however, has seen enormous advances in computing power and commercial CFD codes to meet the sophisticated modelling requirements of the food-processing industry (e.g., drying, cooking, sterilization, chilling, cold storage, etc.). Thus, the above-mentioned difficulties can be reduced or avoided by using numerical CFD simulations to create three-dimensional spatio-temporal distributions of airflow and temperature during precooling [33][34][35][36][37][38][39][40]. ...
Optimizing the design of fresh produce packaging is vital for ensuring that future food cold chains are more energy efficient and for improving produce quality by avoiding chilling injuries due to nonuniform cooling. Computational fluid dynamics models are thus increasingly used to study the airflow patterns and heat transfer inside ventilated packaging during precooling. This review discusses detailed and comprehensive mathematical modeling procedures for simulating the airflow, heat transfer, and mass transfer that occurs during forced-air precooling of fresh produce. These models serve to optimize packaging design and cooling efficiency. We summarize the most commonly used parameters for performance, which allows us to directly compare the cooling performance of various packaging designs.
... Examples of its implementation include modeling heat transfer and moisture loss in bagged potatoes during cold storage (Chourasia & Goswami, 2007), modeling heat transfer of infrared heating on strawberries (Tanaka et al., 2007), predicting 3-D dose distribution of electron beam radiation treatment of complex foods (Kim et al., 2007), and simulating radiation distribution and inactivation in wastewater treatment (Crapulli et al., 2010;Elyasi et al., 2006). CFD modeling has proved to be a powerful engineering tool, providing an efficient and effective design solution. ...
The practical usage of UV-C to decontaminate fruits has been limited by the difficulty applying a uniform dose to complicated shapes. Simulation of UV-C radiation treatment was investigated in this study to find the optimal treatment conditions. A setup of strawberries being placed on a UV-transmittable film with radiation sources from the top and bottom was proposed. A 4-lamp model with a horizontal distance of 300 mm between the lamps was found to yield the most uniform dose distribution, with mean radiation intensity of 2.00 W m−2. For this configuration, the treatment time required to achieve one log inactivation of Cladosporium cladosporioides and Penicillium digitatum spores were 3 min 46 s and 1 min 36 s, respectively. Radiation simulation in combination with the inactivation model has been shown to be a useful tool for the optimization of the UV-C inactivation process.
... The velocity of air outlet was 3m/s [8] . As the Semi-Implicit Method for Pressure Linking Equation (SIMPLE) algorithm had the higher precision than other algorithms for transient state simulation [9] , this work chose this algorithm for numerical calculation. The viscosity of air was little which caused the Reynold number (Re) larger than 10 5 (order of magnitude). ...
The sales volume of vertical air conditioner in China mainland has developed rapidly. The researches in the field to the operation of vertical air conditioner are lacked since it is hardly to analyze different kinds of operating condition. Computational Fluid Dynamics (CFD) has been applied in engineering with the advance of the computer science and technology. In this article, both temperature and air flow distributions were analyzed to a meeting room which putted a vertical air conditioner. The model was used and the equations for numerical simulation (e.g. energy, residual, continuity, etc) were chosen as second order to get the higher accuracy. The results to simulation were showed reasonable and could offer a reference to the practical.
... (N.R. Markarian, 2006) developed a mathematical model and software for controlled air flow and temperature storage room. (Chourasia and Goswami, 2007) proposed a CFD model for controlled natural convection cold storage room. ...
Harvested sugarbeet (Beta vulgaris L.) roots are stored in Michigan in large piles and exposed to
ambient weather conditions during winter storage period, which generally lasts three to four months. During
this time, the air temperature may range from as low as -10 °F to as high as 60 °F. Winter air temperatures are
rising in association with long-term trends of increasing global temperatures. Higher air temperatures contribute
to increasing respiration and storage decay due to increasing metabolic activities, and give decay organisms
optimal conditions to increase their growth and aggressiveness. Decay losses can result in losses of 206 g
sugar/ ton beets/ day, that can amount to as much as $1 million dollars per week for the Michigan industry.
Thus, it is important to understand the composition of the pile temperatures by developing a model to predict
pile temperature based on the history of air temperature, pile age, and pile architecture., We designed a 2D
model of the temperature profile of a non-ventilated pile as a function of air speed, direction, relative humidity,
and temperature.
... Proper methods of cooling may ensure the temperature uniformity of products stacked in a cold store and improve the heat transfer efficiency between the products and air (Tanaka et al., 2012). To improve the cooling performance of cold stores, number of studies modeling the airflow pattern and temperature distribution have demonstrated the effectiveness of various methods (Nahor et al., 2005;Chourasia and Goswami, 2007;Konishi et al., 2009). Tanaka et al. (2012) developed a transient threedimensional computational fluid dynamics model to investigate the cooling performance of a partially loaded cold store in the cooling process. ...
Heat transfer during the freezing of guava pulp conditioned in large containers such as in stacked boxes (34 L) and buckets (20 L) and unstacked drums (200 L) is discussed. The air velocities across the cross-section of the tunnel were measured, and the values in the outlet of the evaporator were used as the initial conditions in computational fluid dynamics (CFD) simulations. The model tested was turbulent standard k-ε. The CFD-generated convective heat transfer coefficients were mapped on the surfaces for each configuration and used in procedures for the calculation of freezing-time estimates. These estimates were compared with the experimental results for validation. The results showed that CFD determined representative coefficients and produced good correlations between the predicted and experimental values when applied to the freezing-time estimates for the box and drum configurations. The errors depended on the configuration and the adopted mesh (3-D grid) construction.
... Furthermore, particular solutions have been presented to these models considering numerical methods. These numerical solutions allow solving for different parameters such as mass diffusivity (Bon et al., 1998); heat and mass transfer inside the materials to be dried (Wu et al., 2003;Chourasia and Goswami, 2007). Additionally, the simulated drying process has also been used for materials such as sugar (Rastikian and Capart, 1998), bagasse (Vijayaraj and Saravanan, 2008), fruits (Lagunez-Rivera et al., 2007), and cassava (Durango et al., 2001a). ...
Transient mass transfer process is analyzed for cassava drying (Manihot Esculenta Crantz) in a pack bed. Experiments were performed in a thermally insulated radial dryer, considering cylindrical pieces of non peeled cassava with three different thicknesses: 4, 6, and 8 mm. The void fractions considered were 0.22, 0.49, 0.64 and 0.66, while the temperature values were 50°C and 70°C. The humidity removed from the cassava was measured from 10 pieces randomly selected at the beginning of the process. These pieces were weighed every 15 minutes during a three hours period. From the data gathered a non linear regression model was attained as a function of non dimensional numbers, which is valid for the following ranges: 700≤Re≤1900, 10000≤Sc≤31000, 0<Fo≤4. The fitted regression coefficient is 2 R2adj = 0.87, and the average error when comparing with experimental data is 24%.
Cheese temperature control in the cold chain is essential for quality preservation and waste reduction, especially for soft cheeses, which generate heat due to their microbiological activity. This study first analyses, at steady state, the natural convection effect on the temperature distribution along three pallet rows (from upstream to downstream). Second, it investigates, under unsteady state, the effect of upwind air velocity (0.25 m/s and 0.64 m/s), product heat generation (0 W, 0.05 W, and 0.3 W per product item), and initial product temperature heterogeneity on the cooling rate within a ventilated pallet in a cold room. The cheeses were replaced with plaster cylinders equipped with controllable resistance heaters to simulate heat generation by cheeses. At steady state, the temperature measurements confirmed the presence of a thermal plume on the pallet downstream row when natural convection was predominant (Richardson number = 6.53). Under unsteady state conditions, increasing the air velocity from 0.25 to 0.64 m/s reduced the half cooling time (HCT) and seven-eighths cooling time (SECT) by at least 26% and 37%, respectively. Greater heat generation increased the product temperature but, interestingly, reduced the product cooling time.
Homogeneity and temperature levels within a refrigerated facility are vital in preserving the quality of horticultural products throughout the cold chain to the consumer. These temperatures are affected by different factors at different scales, including the shape and thermal properties of the horticultural products, package design, pallet arrangement, or characteristics of the cold chain unit of operation. Therefore, airflow and heat transfer studies are valuable in evaluating these factors to optimize the cold chain and achieve and maintain an optimal product temperature. This paper provides an overview of the different scales of the numerical model and experimental setup used to assess the influence of the different factors on the cooling performance, as well as any challenges and limitations of each scale. The importance of considering other aspects in the cold chain studies, such as product quality, energy consumption, and package mechanical strength, will be discussed through an integrative approach. Finally, we propose new perspectives on how multi-scale modelling approaches can be helpful in tackling different challenges and how to investigate the effects of different factors on the cooling efficiency.
Throughout this chapter we will explore the computational applications that can help us in the evaluation, calculation and simulation of the thermal treatment of canned foods. Although some basic principles of microbial death kinetics will be recalled, the course is basically focused on the exploration and use of computational applications to evaluate and simulate the heat treatment of low-acid foods, considering C. botulinum as the reference microorganism. I hope that this book chapter will be useful for you and that you will be able to explore all the contents that are planned to be developed: General and technical aspects of the heat treatment of canned foods, heat penetration studies of canned foods, heat treatment evaluation General method, calculation and prediction of heat treatment by Ball’s Method, heat treatment modeling and simulation, and optimization of heat treatment.
Objective of this study was to determine the impact of differences observed in ambient temperature and relative humidity for peach boxes in cold storage. The study was implemented in an experimental cold store with a chiller unit. Cooling system was adjusted at +1 oC with 90% of relative humidity. Changes of ambient factors inside of the boxes and in the cold store for different levels were measured. Differences between ambient factors of the cold store and plastic storage boxes were compared. ‘Glohaven’ peaches (Prunus persica L. Batch) were cold stored for 4 weeks. Quality parameters of the peaches were determined for different levels per week during storage period. According to the results, there was no important difference between ambient factors for the cold store and inside of the storage boxes. The cold store with chiller cooling system worked successfully and distribution of the storage temperature and the relative humidity in the cold store and inside the storage boxes was almost homogeny. Fruit firmness, soluble solid content, titratable acidity, pH values, respiration rate, changes on the starch and its impact on the flavour were found to be significantly meaningful. However, its effects on the quality criteria on top, medium and base levels inside the store were considered statistically insignificant. The measurement results showed that there were some differences among various levels with regards to storage conditions though they had no significant negative effect on quality criteria.
Le fromage, comme tout produit frais, doit être conservé à basse température tout au long de la chaîne du froid. Une particularité des fromages à pâte molle (camembert, par exemple) est qu’ils dégagent un flux de chaleur important à cause de la respiration des microorganismes qui y sont présent. D’un point de vue thermique, suffisamment d’air froid doit entrer dans une palette de fromage pour évacuer la chaleur dégagée afin de maintenir une température stable dans le système. D’un point de vu aéraulique, l’échauffement du produit induit de la convection naturelle à l’intérieur de la palette. Plusieurs modes de convection sont donc présents : la convection forcée imposée par les ventilateurs de la chambre froide autour de la palette et la convection mixte, combinaison de convections forcée et naturelle, à l’intérieur de la palette. Ceci augmente par conséquent la complexité de l’écoulement d’air. Le régime de convection mixte apparait surtout à faible vitesse de ventilation ce qui est souvent le cas lors du stockage en chambre froide. De plus, le système est influencé par un grand nombre de paramètres : l’orientation de la palette, l’ajourage des cartons, la vitesse de soufflage, la puissance de chauffage…L’objectif de la thèse est d'apporter de nouvelles connaissances sur les écoulements d'air et les transferts thermiques au sein d'empilements de produits dégageant de la chaleur et faiblement ventilés. Ces connaissances visent également à répondre à des problématiques industrielles. Les acquis du projet doivent contribuer à améliorer la conception aéraulique des emballages et des modes de palettisation du fromage, qui à ce jour est réalisée de manière empirique. Il convient également de noter que la problématique scientifique abordée dans ce projet (interactions aérodynamiques et thermiques entre convection forcée et naturelle autour et au sein d'empilements d’objets) présente des applications dans d’autres domaines tels que le refroidissement des circuits électroniques.La thèse se compose de deux parties principales : Une étude expérimentale et une étape de modélisation. Dans l’approche expérimentale, une maquette de palette à l’échelle 1 a été conçue dans laquelle les produits ont été remplacés par des blocs de plâtre munis de résistances chauffantes afin de simuler le flux de chaleur dégagé par le produit. Les expérimentations ont été réalisées dans une cellule à température et vitesse d’air maitrisées pour deux vitesses amont : 0.31 et 0.73 m.s-1, trois puissances de chauffage : 0.05 W, 0.15 W et 0.30 W par produit (0.25kg) et deux orientations de la palette. L’utilisation d’un vélocimètre laser par effet Doppler (LDV) et d’un anémomètre à fil chaud ont permis de caractériser les écoulements (profils de vitesses, débit d’air dans les orifices) selon la vitesse amont et la puissance de chauffage. L’implantation de 200 thermocouples répartis dans l’ensemble du montage expérimental a par ailleurs permis d’obtenir les profils de température de l’air et des produits au sein de la palette selon ces mêmes paramètres.La partie modélisation a été réalisée selon deux approches: une simulation détaillée par mécanique des fluides numériques (CFD) avec un maillage de plusieurs millions de cellules et un modèle zonal (une vingtaine de zones) assimilant le système à un réseau hydraulique. Les deux modèles ont été validés grâce aux données expérimentales. L’approche CFD a permis d’obtenir des informations non accessibles à partir des mesures expérimentales ce qui a facilité la compréhension des phénomènes. L’approche zonale, de par sa simplicité d’utilisation et son faible temps de calcul est quant à elle plus adaptée pour une utilisation industrielle.
The effects of recirculation velocity on the volume fraction of oxygen and the temperature distribution in a container with a controlled atmosphere (CA) by liquid nitrogen injection were investigated. A three-dimensional (3-D) model was developed, and the airflow, heat, and mass transfer in the process of liquid nitrogen injection were simulated. A validation test was conducted to verify the accuracy of the model. The results indicate that improving the recirculation velocity will extend the time of liquid nitrogen injection but will reduce the temperature difference in the container and on the surfaces of fruits and vegetables. Different recirculation velocities have little effect on the distribution uniformity of the oxygen volume fraction. The temperature at the holes on the division plate were all above 0 °C when the recirculation velocity was 8 m/s. The results of this study provide a detailed understanding of air transport in a container, which will be useful for the design and optimisation of transportation containers for fruits and vegetables in a CA.
A 3D numerical model using COMSOL software was developed to simulate the profile of ClO2 gas concentration and bacterial inactivation in treatment chambers with different gas inlets and outlets. The multiphysics simulation based on a finite element method (FEM) included compressible flow, convection and diffusion, and especially, pathogen inactivation codes using Java language were embedded into the program. Finally, each component of the model was coupled and simulated in a transient state. The main fluid stream reached a steady state in a short time because the input speed of ClO2 gas was fast compared to the size of the chamber. Though the chamber was a closed system, compressible air was not observed inside the chamber. Various scenarios with different gas inlet and outlet locations can influence the uniformity profile of increasing ClO2 gas concentration and bacterial inactivation. The optimal configuration was finally found by means of the simulation, and there was a good correspondence in pathogen inactivation profiles between CFD simulation and experiment. This study revealed the computational fluid dynamics (CFD) codes which can help the food processing industry in constructing a scaled-up system and establishing optimum sanitizing treatment conditions.
In this paper transient heat transfer in a solar food drier is modeled by using both computational fluid dynamics (CFD) and lumped capacitance modeling techniques. The CFD model is used to simulate the air flow and transient heat transfer within the dryer. A lumped capacitance model, developed from energy exchanges between various components of the drying chamber, is extensively compared with the results of the CFD model. The two models predict both a 40 °C rise in steady-state temperature within the drying chamber. Before reaching this temperature, there is about 8 °C RMS deviation between the transient temperature prediction of the two models. This deviation is mainly caused by the heat transfer coefficient around the food rack shelfs. Fitting these coefficients give rise to a minimal RMS deviation between the two models of 1.8 °C and 0.9 °C on the lower and upper rack shelfs respectively. The transient heat transfer phenomenon from CFD modeling also showed that the spatial distribution of temperature after 30 min is uniform thus validating the very assumption of lumped capacitance modeling. This demonstrates that the lumped capacitance model can supplant the computationally demanding CFD modeling technique in predicting transient heat transfer phenomenon in solar food dryers.
The implementation of early-stage simulation tools, specifically computational fluid dynamics (CFD), is an international and interdisciplinary trend that allows engineers to computer-test concepts all the way through the development of a process or system. With the enhancement of computing power and efficiency, and the availability of affordable CFD packages, the applications of CFD have extended into the food industry for modeling industrial processes, performing comprehensive analyses, and optimizing the efficiency and cost effectiveness of the new processes and systems. Beginning a new series dedicated to contemporary, up-to-date food engineering practices, Computational Fluid Dynamics in Food Processing is the first book of its kind to illustrate the use of CFD for solving heat and mass transfer problems in the food industry. Using a computational grid, CFD solves governing equations that describe fluid flow across each grid cell by means of an iterative procedure in order to predict and visualize the profiles of velocity, temperature, pressure, and other parameters. Starting with an overview of CFD technology and applications, the book illustrates the use of CFD for gaining a qualitative and quantitative assessment of the performance of processes involving heat and mass transfer. Specific chapters cover airflow in refrigerated trucks, retail display cabinets, microwaves, and doorways; velocity in meat dryers and spray drying; thermal sterilization; plate heat exchangers; membrane separation systems; jet impingement ovens; food extrusion and high-pressure processing; prediction of hygiene; design of biosensors; and the fermentation of tea and ripening of cheese. Drawing from an esteemed panel of international professionals and academics, this groundbreaking bookprovides engineers and technologists in research, development, and operations with critical, comprehensive, and readily accessible information on the art and science of CFD technology.
Currently most food products are cooled and frozen in air-blast cold storage to prolong storage time. The airflow field distribution in storage has a great impact on the process of food freezing and energy cost by that. In this paper, a transient model of food freezing considering airflow field was developed to simulation the temperature profile of air and food products during freezing process. A lumped parameter model was used to predict the temperature and moisture profile of air, which connected all other components together, such as air coolers, food products, envelop enclosure and refrigeration system. A finite difference method was employed to model the heat transfer inside food products during freezing, where the mass transfer was neglected as the food products were wrapped with polystyrene films. Unit load factor method was applied to calculate the sensible heat refrigeration capacity and thus the total capacity of air coolers. The simulation was conducted on a large cold storage filled with large quantities of packaged food products. Results show that there are great differences in airflow field distribution at different locations in cold storage, which lead to spacial differences in freezing time required. Inappropriate set point of freezing time prolongs freezing process unnecessarily and leads to extra energy consumption. Operational mode of air coolers has a great impact on the total energy consumption, as they consume energy themselves and release equivalent heat into storage simultaneously.
The water loss of fruits and vegetables in precooling is a mass transferring process affected by the heat transfer, it is important for reducing the weight loss of fruits and vegetables to analyze the affecting factors in forced air precooling. Combining with the integrate heat transferring equation of spherical fruits and vegetables, the coupling equation of heat and mass transfer of water loss was derived by analyzing the process of water loss of fruits and vegetables in precooling, including the factors of the parameters of fruits and vegetables and precooling conditions. The effects of the internal water diffusion coefficient, precooling air velocity and precooling air relative humidity on water loss were analyzed. The results show that the structure parameter is the key factor and the effect of the air velocity on the precooling time is significant. The water loss is mainly affected by the structure parameters of fruits and vegetables and decreases with the increase of air velocity. The result was verified by the experiment with apple, which had a similar result with the theoretical analysis.
With the development of computational fluid dynamics, the simulation accuracy and reliability are improved constantly. This technology is widely used in agricultural cold-chain logistics for the past few years which has significances to raise temperature homogeneity of the low-temperature environment, determine the goods stack, reasonably control refrigerating time, and improve the economic benefits of overall cold-chain logistics. This paper mainly described the pre-processing, computation and post-processing of computational fluid dynamics, reviewed the research advance of it in store, transportation and market process of agricultural cold-chain logistics, expounded the research methods and application characteristics of the technology, generalized and analyzed the advantages and deficiencies. Moreover, the paper also looked into the future trends of computational fluid dynamics in agricultural cold-chain logistics applications. ©, 2015, Chinese Society of Agricultural Machinery. All right reserved.
A numerical formulation capable of simulating fluid flow and non-equilibrium heat and mass transfer in three-dimensional conjugate fluid/solid/porous domains is presented. The governing transport equations are presented for the fluid, solid, and porous regions, with special consideration given towards the manner in which moisture is accounted for in the air–water vapour mixture. Mathematical conditions are also presented to ensure that heat and mass transfer occurs smoothly across fluid–porous, fluid–solid and porous–solid interfaces. The developed formulation is validated by simulating direct and indirect evaporative cooling problems. The results demonstrate that the formulation is capable of simulating evaporative cooling in conjugate three-dimensional domains, with and without the addition of sensible heat. Moreover, different simulated cases show that the results are accurate compared to available experimental results, and are physically realistic throughout the domain and at interfaces between conjugate regions. The unsteady problem of drying of an initially saturated porous material is also simulated to demonstrate the non-equilibrium mass transfer feature of the developed formulation. The results show the correct trends in drying time with respect to the flow Reynolds number and the relative humidity of the inlet air–water vapour mixture.
A computer algorithm was developed that estimates the latent and sensible heat loads due to the bulk refrigeration of fruits and vegetables. The algorithm also predicts the commodity moisture loss and temperature distribution which occurs during refrigeration. Part I focused upon the thermophysical properties of commodities and the flowfield parameters which govern the heat and mass transfer from fresh fruits and vegetables. This paper, Part II, discusses the modeling methodology utilized in the current computer algorithm and describes the development of the heat and mass transfer models. Part II also compares the results of the computer algorithm to experimental data taken from the literature, and, describes a parametric study which was performed with the algorithm. In addition, this paper also reviews existing numerical models for determining the heat and mass transfer in bulk loads of fruits and vegetables.
Measurements were made in a commercial potato cold, storage to evaluate some of the operating and geometric parameters (like loading density, percentage free space, potato temperature during cooling, cool-down time, storage air temperature and humidity, loading pattern, power consumption pattern, heat transfer coefficients in evaporator and condenser) related to the losses of stored produce. The loading density of potato in stack and cold storage (534.63 kg/m 3 and 366.01 kg/m 3, respectively) was optiimum. The percentage free space was adequate and found to be 31.54%. Therefore, center-most potatoes achieved safe temperature (12-13°C) in about 10 days and the maximum cool-down time was about 30 days. Hence, the potatoes were safe from the rotting. Also, the temperature of potatoes at the surface was always above the critical temperature for cold injury. However, large variations in storage air temperature were observed during the cool-down period, which increased the weight loss from the potatoes. The average weight loss was 5.3% after 8 months of storage. Steep fluctuations in power consumption were observed due to irregular loading pattern. The power consumption of compressor during transient cooling was found to vary between 34.84 and 84.6 kW.
A general mathematical model for forced air precooling of spherical food products in bulk is developed. The food products are arranged inline to form a rectangular parallelepiped. Chilled air is blown along the height of the package. The governing equations for the transient two-dimensional conduction with internal heat generation in the product, simultaneous heat and mass transfer at the product-air interface and one-dimensional transient energy and species conservation equations for the moist air are solved numerically using finite difference methods. Results are presented in the form of time-temperature histories. Experiments are conducted with model foods in a laboratory scale air precooling tunnel. The agreement between the theoretical and experimental results is found to be good. In general, a single product analysis fails to predict the precooling characteristics of bulk loads of food products. In the range of values investigated, the respiration heat is found to have a negligible effect.
Many product thermal/physical properties used as model inputs are highly variable and some have not been independently determined. The chosen values of these properties can affect the accuracy of weight loss predictions. Peaches were selected as the product to study and three parameters, skin mass transfer coefficient (k(s)), vapor-pressure lowering effect of dissolved solutes (VPL), and radius were identified as having considerable influence on predicted weight loss at 5 to 25°C, 50 to 100% RH, and 0.005 to 5.0 m/s air velocity conditions. The effects of k(s) on predicted weight loss decreased with higher relative humidity and air velocity and lower temperature. VPL affected predicted weight loss most at higher temperature, and lower velocity and relative humidity. Varying input values of k(s) and VPL resulted in predicted weight loss ranges that encompassed 71% of experimental weight loss values for peaches and those values not falling within the predicted values were mostly (88%) underpredicted.
Seasonal variations in ambient temperatures can cause safe moisture contents of stored grain to become unsafe due to migration or redistribution of moisture within the storage. Such migration of moisture generally occurs due to thermal gradients in the stored grain and depends on various factors related to the grain quality, bin size, and local climatic conditions. In this article, a mathematical model for the simultaneous heat and moisture transfer is developed by incorporating a sorption isotherm relationship to predict moisture migration in stored grain due to diffusion. The coupled, nonlinear transport equations are solved numerically by using a control-volume scheme
The effectivity of vent hole designs of a bulk container for seed potatoes concerning water vapour transfer is investigated using a numerical model based on the lattice Boltzmann technique. The model describes the heat and water vapour transport driven by natural convection in the potato container. By numerical study we have found an effective vent hold design which maintains the keeping quality of the potatoes by preventing water condensation.
Losses of potatoes in cold storage occur in all the potato-producing countries and are caused, in part, by excessive high temperatures during storage, which increase rotting, decay and loss of edible quality and nutritive values. In some cases, freezing or chilling injury, also called cold injury, caused by low temperatures may be involved. Other serious losses are caused by mechanical injury from careless or rough handling and by shrinkage or wilting because of moisture loss. Diseases and pests, which develop during favourable conditions in cold storage, may also cause losses in the form of rottage, decay, poor germination and deformation. The extent of losses of potatoes in cold storage depends on a number of factors and the most important ones are temperature, humidity, ventilation and the condition of potatoes stored. Therefore, the matter of reducing the storage losses directly leads to the problem of providing more efficient and appropriate storage arrangements for potatoes. An attempt has been mad e in this review, to critically appraise the types of losses of potatoes in cold storage, their causes, preventive measures and important factors affecting the extent of these losses.
Effective cold storage of bagged potatoes depends on many interrelated geometric and operating parameters. A clear knowledge of this relationship helps to design and operate the cold storage under optimum conditions in order to maintain potato quality during long-term storage. A mathematical model has been developed in this study to predict the effects of cold storage air velocity and temperature and potato heat of respiration, aspect ratio, and bag volume on the temperature profile and cool-down time of the bagged potatoes. It has been found that the steady-state temperature of the product increases with an increase in storage air temperature and volume of the bag. An increase in storage air velocity up to 3.0 m/s significantly lowers the cool-down time and steady-state produce temperature. It has also been found that, for a constant bag volume, a long rectangular shape is preferable to a cubic one. For validation of the model, results from the literature under similar condition have been compared with those predicted, and were found to be in close agreement.
A new method is presented to compute the sensitivity of the temperature course inside conduction heated foods with respect to the surface heat transfer coefficient. The method is based on the finite element formulation of the heat conduction equation. As an illustrative example, a sensitivity analysis of the reheating process of ready-made lasagna is carried out. Sensitivity charts are constructed which relate the dimensionless centre temperature with the Biot and Fourier number for slab, cylinder and sphere shapes. It is shown that in the case of processes with low surface heat transfer coefficient, such as food chilling using air at moderate speeds or convection heating of prepared meals, small deviations in the surface heat transfer coefficient may result in large deviations in the core temperature of the food.
Uniformity of air flow is a key factor affecting the quality of potatoes during storage. Stores with non-uniform air flows often suffer from condensation, consequent rotting and poor quality of the potatoes. A model, solved using computational fluid dynamics software (cfds-cfx4), was produced to identify modifications to air distribution systems that would improve the uniformity of the air flows. The model treats the potatoes as a porous resistance around the ducts and predicts the flows inside the air supply ducts. A commercial store which experienced condensation and rotting each year was modified on the basis of predictions from the model and both problems in the store were eliminated. The model is simple and cost effective enough to be used in commercial applications.
Effects of deviations in airflow properties and product properties on cooling of chicory roots were studied based on numerical simulations with a validated transient model of coupled heat and mass transfer in bulks of chicory roots. The model was solved for a one-dimension 1-m deep bulk of chicory roots cooled at various levels of air velocity, temperature and relative humidity. Variations in bulk porosity, heat of respiration and product heat capacity were studied as well. The results led to an improved understanding of the cooling process with respect to control strategies and cold store practice. The smallest sensitivity of product temperature with respect to process conditions was found at low velocity (0.1 ms−1), low temperature (1 °C) and high RH (95%). Small deviations of these conditions could, however, still lead to increased weight losses by as much as 4% after 3 months of storage. Variations of bulk porosity and of respiration rates significantly affected weight loss variations.
The present paper is concerned with a noniterative method for handling
the pressure-velocity coupling of the implicitly discretized fluid flow
equations. The method, called PISO (pressure-implicit with splitting of
operators), utilizes the splitting of operations in the solution of the
discretized momentum and pressure equations. The fields obtained at each
time step are close approximations of the exact solution of the
difference equations with a formal order of accuracy of the order of
powers of delta t depending on the number of operation-splittings used.
The method is cast in a time-dependent form. It is, however, also useful
for steady-state calculations due to its stability for fairly large time
steps. An outline of PISO is presented, and its accuracy is assessed.
Attention is given to the transport equations, the finite-difference
formulation, the methodology for incompressible flow, boundary
conditions, a generalization to compressible flows, and a generalization
of the method to other equations.
The surface heat transfer coefficient is an important parameter to characterise forced convection heating and cooling processes. It expresses the influence of flow phenomena on the heat transfer rate to foods. For thermal process calculations, one often relies on dimensionless correlations that contain the flow properties, namely the free stream air velocity and the turbulence kinetic energy. This paper discusses the sensitivity of the food temperature with respect to these variables for different food geometries and conditions. The study is based on the finite element analysis of the product temperature deviations as a result from deviations of the surface heat transfer coefficient, combined with existing dimensionless correlations for the surface heat transfer coefficient. Sensitivity charts are constructed, which relate the temperature sensitivity with respect to the free stream air velocity and the turbulence kinetic energy to different air flow conditions as a function of time. By constructing sensitivity charts, it is shown that small velocity deviations can lead to large food temperature deviations. The effect of small deviations of the turbulence intensity is not as significant.
1. Under normal English storage conditions, the heat production of mature potatoes drops rapidly from a value of probably about 150 b.th.u./ton/hr. immediately after harvest to about 30–50 b.th.u./ton/hr. Sprouting is accompanied by an increase in the rate of heat production. The initial heat production of immature potatoes may be of the order of 250 b.th.u./ton/hr.
2. As a result of the production of heat, the temperatures in stacks of potatoes will tend to rise to levels above that of the outside air which are just sufficient to cause the convection and conduction necessary to remove the metabolic heat as fast as it is produced.
3. The difference in temperature between the potatoes and the ambient air is a function of the heat production of the potatoes and of the height of the stack, and is practically independent of its other dimensions if these exceed twice the height. Under average conditions during the middle of the storage season, and for heights of storage of from about 6 to 12 ft. it may be taken as a rough practical guide that the average and maximum temperatures of the potatoes will tend to exceed the average temperature of the store air by about 2/3 and 1° F. respectively for every foot of height.
4. Overheating is possible at both the beginning and end of the storage season, when heat production is high and the outside temperatures also possibly high. In general it is safe to store unventilated potatoes to a height of about 6 ft. if they are mature, though if they are harvested with a great deal of earth late storage should not be attempted. If there is no intention of storing late, and the potatoes are fairly clean, storage to aheight of 12 ft. may be permissible. Immature potatoes should not be stored to a height of more than 3 ft.
ABSTRACTA procedure was developed to predict moisture loss from cooling data of potato packed in gunny bags and stacked on wooden platforms in commercial cold stores. To predict the moisture loss, mass transfer coefficients kcand kmwere estimated during the storage period, which were found to decrease with time. The calculated time average kcand kmvalues were 1.83 × 10−4 m/s and 2.31 × 10−10 kg/s·m2·Pa during the transient cooling period and 1.59 × 10−4 m/s and 2.27 × 10−10 kg/s·m2·Pa for the rest of the storage period, respectively. The estimated moisture losses were 4.8, 4.74 and 4.78%, at the center of three different stacks, for a storage period of 8 months. The corresponding experimentally measured weight losses at the center of the same stacks were 5.2, 5.1 and 5.26% with a variation of 11, 7.5 and 10.2%, respectively. Therefore, the procedure adopted in this study may be used to assess the moisture loss from potatoes under the different storage conditions. The effect of relative humidity (RH) and potato temperature on moisture loss was also predicted using the developed procedure. Decrease in RH of the storage air increased the moisture loss. The potatoes stored below 85% RH incurred more than 7% water loss. Therefore, 88–90% RH in the cold store may be used to limit the maximum moisture loss within the permissible limit of 5% even after 8 months of storage. It was also found that increasing the potato temperature exponentially increased the rate of moisture loss.
A computer model is described for calculating energy consumption and moisture loss. The model uses data from an actual potato store in the Cambridge region of the UK. The differences between actual and calculated power consumption values for 1989-1990 and 1990-1991 seasons are +1·0% and -2·3% respectively. For the whole storage season, the weight losses are 5·09% and 5·02% for the 1989-1990 and 1990-1991 seasons, respectively.
Agricultural produce, such as potato, onion, etc. are packed in permeable gunny bags for cooling in a natural convective environment and subsequent long-term storage. Due to permeable boundaries, the heat and mass transfer within the bag is strongly affected by the flow characteristics, especially during the transient cooling process. Therefore, an attempt was made in this study to simulate the three-dimensional airflow and heat transfer in this system using the computational fluid dynamics technique. The mass transfer phenomenon is being modelled in Part II of this paper. It was found that air mainly penetrated through bottom and side surfaces of the bag and escaped from top surface. The velocity contours maintained symmetry about both the horizontal axes in horizontal plane and only about the vertical axis in the vertical plane. The same patterns were observed for isotherms as well. The average natural convective velocity in the bag at steady state was found to be 0·0016 m s−1. The temperature of the product in the bag decreased at a very fast rate during the initial cooling period and then dropped to comparatively slow rates. In the vertical plane, the zone of maximum temperature was observed below the top surface of the bag at the early stages of cooling and it proceeded towards the centre point as the cooling progressed. Steady-state average temperature in the bag was found to be 0·13 K higher than the surrounding air temperature at 274 K. The experimental product temperature at the centre of the bag was found to be higher than that of the simulated one. The maximum and minimum temperature deviation from the experimental values were 3·4 K at 287·1 K and 0·5 K at 279·1 K on 2nd and 16th day of cooling respectively when the measured initial and final temperatures of potato at the centre were 297·1 and 278·1 K, respectively for a total cooling period of 25 days.
The basic features of the buoyancy-driven convection in an open-ended cavity are analyzed and an in depth presentation of the related results are given in this work. It is shown that, at higher Rayleigh numbers, a one to one relationship exists between the frequency of the periodic oscillations in the Nusselt number and the central vortex's oscillations and location inside the cavity. In fact, for any given type of oscillatory pattern (e.g. asymmetric sinusoidal or distorted W), the frequency of the oscillations of the Nusselt number (or the central vortex inside the cavity) increases linearly as the Rayleigh number increases. For air, this linear relationship was explicitly determined. Furthermore, it is determined that, for cases which Ra $ ̌9 × 105, the central vortex's oscillation frequency and/or amplitude becomes much higher and it starts to deviate from any type of repeatable pattern. It is proposed that it is this irregular high frequency and/or amplitude of the central vortex coupled with the flow separation around the mid-section of the lower block that triggers the transition to turbulent flow.
A numerical simulation by using a computational fluid dynamics (CFD) code is carried out to predict heat and mass transfer during vacuum cooling of porous foods on the basis of mathematical models of unsteady heat and mass transfer. The simulations allow the simultaneous prediction of temperature distribution, weight loss and moisture content of the meats at low saturation pressure throughout the chilling process. The simulations are also capable of accounting for the effects of the dependent variables such as pressure, temperature, density and water content, thermal shrinkage, and anisotropy of the food. The model is verified by vacuum cooling of cooked meats with cylindrical shape within an experimental vacuum cooler. A data file for pressure history was created from the experimental pressure values, which were applied in the simulations as the boundary condition of the surface temperature.
A model of three-dimensional natural convection in a confined porous medium with internal heat generation was developed. The governing equations were reduced to one parabolic and one elliptic differential equation. Experiments were performed on cooling model material, representing agricultural products, at different rates of heat generation (0-235 Wm-3) in a closed container (0.76 m × 0.76 m × 0.50 m) with isothermal walls. Compared with conduction only, natural convection accelerated cooling, gave a lower average temperature and moved the location of the maximum temperature from the centre of the container upwards. Measured and calculated cooling curves could be brought into good agreement by introduction of a wall heat transfer coefficient in the model equations.
This paper presents a model for a wet air-cooled store based on computational fluid dynamics. Unlike previous works on cold room modelling published in the literature, the model considers the effects of buoyancy by coupling the fundamental energy and diffusion equations with the momentum equation. It has been found that the consideration of buoyancy is important in regions of low air velocity. The model which can be applied to other types of cold store has been validated using experimental results from a wet air-cooled store. A good agreement was found between the experimental and simulation results. The modelling results identify a number of design problem areas and illustrate the usefulness of the model in investigating the effects of design parameters on the performance of the cold store.
A two-dimensional two-phase model of temperature and moisture distribution during cooling and storage of agricultural products in a cylindrical container is developed. In the model equations the influence of air flow by natural convection is incorporated. The agreement of model calculations with the measured temperature course during self-heating of potatoes or model material is shown. Model calculations indicate the advantage of high air velocities during precooling.
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