Applied Thermal Engineering

Published by Elsevier
Online ISSN: 1359-4311
Publications
Conference Paper
Heat recovery from moving beds of solid particulate that have to be cooled for process requirements is usually carried out by one or more hoods collecting the cooling air fed underneath the bed. However, in many process plants the bed operation is characterized by continuous variations of solid inlet temperature and flow rate. As a consequence, the design of the heat recovery plant is usually performed by assuming a hypothetical steady-state operation and resorting to mean values of the process parameters. Sizing optimization of the air capturing hoods becomes difficult to perform under such circumstances, resulting in poor performances of the whole heat recovery plant from the economical standpoint. Furthermore, control system design appears particularly critical especially for more complex heat recovery schemes. With the aim to make a contribution towards the modeling of the real-time behaviour of the cooling bed, in this paper a dynamic simulation approach has been developed based on two-dimensional schematization and on time-dependent convective-conductive heat transfer. The model enables the transient analysis of the cooling bed operation and could be used as a useful tool in control and optimal design studies. In this work reference has been made to the cooling section of an iron-ore sintering bed of the Ilva steelworks in Taranto for operating conditions definition and model verification
 
Article
This paper presents an efficiency analysis, accounting for both energy and exergy considerations, of a design for a cogeneration-based district energy system. A case study is considered for the city of Edmonton, Canada, by the utility Edmonton Power. The original concept using central electric chillers, as well as two variations (one considering single-effect and the other double-effect absorption chillers) are examined. The energy- and exergy-based results differ markedly (e.g., overall energy efficiencies are shown to vary for the three configurations considered from 83% to 94%, and exergy efficiencies from 28% to 29%, respectively). For the overall processes, as well as individual subprocesses and selected combinations of subprocesses, the exergy efficiencies are generally found to be more meaningful and indicative of system behaviour than the energy efficiencies.
 
Article
In construction, the use of Phase Change Materials (PCM) allows the storage/release of energy from solar radiation and internal loads. The application of such materials for lightweight construction (e.g., a wood house) makes it possible to improve thermal comfort and reduce energy consumption. The heat transfer process between the wall and the indoor air is convection. In this paper, we have developed a numerical model to evaluate several convective heat transfer correlations from the literature for natural, mixed and forced convection flows. The results show that the convective heat transfer highly influences the storage/release process in case of PCM walls. For the natural convection, the numerical results are highly dependent on the correlation used and the results may vary up to 200%. In the case of mixed and forced convection flows, the higher is the velocity, the more important is the storage capacity.Highlights► We study effect of inside convection correlation on energy stored in PCM wall. ► We developed a 1D conduction model for multilayer walls, with phase change material. ► Correlations have been constructed for mixed convection in all flow regimes. ► Up to 200% variation of energy stored in PCM layer, depending on convection correlation. ► Ventilation can increase the energy stored in the PCM layer.
 
Article
A laboratory model of a thermally driven adsorption refrigeration system with activated carbon as the adsorbent and 1,1,1,2-tetrafluoroethane (HFC 134a) as the refrigerant was developed. The single stage compression system has an ensemble of four adsorbers packed with Maxsorb II specimen of activated carbon that provide a near continuous flow which caters to a cooling load of up to 5 W in the 5–18 °C region. The objective was to utilise the low grade thermal energy to drive a refrigeration system that can be used to cool some critical electronic components. The laboratory model was tested for its performance at various cooling loads with the heat source temperature from 73 to 93 °C. The pressure transients during heating and cooling phases were traced. The cyclic steady state and transient performance data are presented.
 
Article
Two-phase flow analysis for the evaporation and condensation of refrigerants within the minichannel plate heat exchangers is an area of ongoing research, as reported in the literatures reviewed in this article. The previous studies mostly correlated the two-phase heat transfer and pressure drop in these minichannel heat exchangers using theories and empirical correlations that had previously been established for two-phase flows in conventional macrochannels. However, the two-phase flow characteristics within micro/minichannels may be more sophisticated than conventional macrochannels, and the empirical correlations for one scale may not work for the other one. The objective of this study is to investigate the parameters that affect the two-phase heat transfer within the minichannel plate heat exchangers, and to utilize the dimensional analysis technique to develop appropriate correlations. For this purpose, thermo-hydrodynamic performance of three minichannel brazed-type plate heat exchangers was analyzed experimentally in this study. These heat exchangers were used as the evaporator and condenser of an automotive refrigeration system where the refrigerant R-134a flowed on one side and a 50% glycol–water mixture on the other side in a counter-flow configuration. The heat transfer coefficient for the single-phase flow of the glycol–water mixture was first obtained using a modified Wilson plot technique. The results from the single-phase flow analysis were then used in the two-phase flow analysis, and correlations for the refrigerant evaporation and condensation heat transfer were developed. Correlations for the single-phase and two-phase Fanning friction factors were also obtained based on a homogenous model. The results of this study showed that the two-phase theories and correlations that were established for conventional macrochannel heat exchangers may not hold for the minichannel heat exchangers used in this study.
 
Article
This paper is a continuation of the authors previous work. In the present paper, the performance of the refrigeration cycle using a two-phase ejector as an expansion device is experimentally investigated. Refrigerant R-134a is used as working fluid. Motive nozzles having three different outlet diameters are tested. New experimental data that have never been seen before are presented on the effects of the external parameters i.e. heat sink and heat source temperatures on the coefficient of performance and various relevant parameters i.e. primary mass flow rate of the refrigerant, secondary mass flow rate of the refrigerant, recirculation ratio, average evaporator pressure, compressor ratio, discharge temperature and cooling capacity. The effects of size of the motive nozzle outlet on the system performance are also discussed.
 
Article
A water chiller with an open reciprocating compressor has been used to comparatively assess the performance of HFC-134a with reference to CFC-12 under as close to identical conditions as possible. Performance characteristics of the chiller under retrofit conditions show that HFC-134a offers better cooling load and coefficients of performance vis-a-vis CFC-12 for identical operating conditions. Further heat transfer analysis of data for the condenser shows that the condensing heat transfer coefficients for HFC-134a are superior to CFC-12. The better performance of HFC-134a may be ascribed to the better heat transfer coefficient of HFC-134a over CFC-12.
 
Experimental setup for the adsorption cooling module: 1. computer, 2. data acquisition device, 3. adsorption cooling module, 4. adsorber section, 5. adiabatic section, 6. condenser/evaporator (receiver) section, 7. thermocouples, 8. electrical heating oven, 9. temperature controller.
Comparison of predicted temperatures and experimental data of the adsorber of the cooling module.
Comparison of predicted temperatures and experimental data of the condenser/evaporator of the cooling module.
Article
In order to investigate the performance of the adsorption cooling module (16 mm in diameter and 1020 mm in length) with zeolite 13X and water as the adsorption working pair, a dynamic heat and mass transfer model was established based on the linear driving force (LDF) model. For the working process of the main parts of the module, including adsorber and condenser/evaporator, the coupled dynamic equations were set up for each stage of heating/desorbing and cooling/adsorbing, respectively. The model was then solved using the finite difference method, and the performance of the adsorber and condenser/evaporator of the module were analyzed. The calculated results were validated with experimental data and good agreement was observed. By means of the model, simulation and optimization of the adsorption cooling module can be further studied.
 
Article
An analysis of the coefficient of performance (COP), specific cooling power (Qscp) and exergy losses for a four-bed adsorption heat pump is presented. A composite adsorbent (SWS-1L) and water are the adsorption pair. An optimum cycle time, corresponding to a maximum specific cooling power, was found. This maximum specific cooling power increases almost linearly with the regeneration temperature. For the operation corresponding to the maximum specific cooling power at the regeneration temperature of 120 °C, using the SWS-1L composite adsorbent to substitute a regular-density silica gel in the adsorbers, the COP and Qscp values can be increased by 51% and 38.4%, respectively. At the regeneration temperature of 100 °C and the mode operating time of 360 s, the second-law efficiency of the adsorption heat pump is 20.4%. The cycle exergy loss mainly occurs in the adsorbers. The exergy losses in the condenser and evaporator are small. Among the four processes in the adsorbers, the precooling and preheating processes result in 41.55% and 28.96% of the cycle exergy loss, respectively, while the adsorption and regeneration processes cause 8.44% and 18.97%, respectively. The exergy losses in the precooling and preheating processes mainly result from heat transfer through a significant temperature difference.
 
Article
With reference to the feasibility analysis for CHCP applications in airports performed in Part I of this work, the optimal strategy for repowering and operation of the Malpensa 2000 CHCP plant is here discussed. The analysis starts from experimental energy demand data, available on hourly basis for an entire operational year. After a description of the CHCP lay-out, the optimal management strategy was determined keeping into account the articulated energy tariff system and the technical characteristics of components. A profit-oriented optimization of the repowering actions to take up to 2010 is presented, based on the expected growth of energy consumption due to the scheduled increases in transportation capacity; energy and emission saving were also calculated by simulating plant operation on hourly basis, for several repowering strategies.
 
Article
The developments in process heat transfer from the 1960s until today are reviewed and an attempt made to predict the major new developments by 2010. The review considers a range of topics including the availability of expertise, changes in the structure of the process software industry, innovations in construction, etc. and shows how these are driving mostly-modest changes. Several significant changes are identified but no revolutionary changes are foreseen. These changes cover the design process, improvements in the use of software and the evolution of existing heat exchanger types. No major new heat exchanger types are expected.
 
Article
In the gas-turbine field ‘simple-cycle’ engines (compressor + burner + expander) have been dominant across almost the full spectrum of power-generation and mechanical-drive applications. Paced by aerodynamic and materials-technology advancements, efficiency values have progressed significantly over the last five decades. However, to reduce specific fuel consumption further (by say a step change of 30–40%) and to reduce emissions significantly, more-complex thermodynamic cycles that include the use of exhaust-heat-recovery exchangers are necessary. Clearly, there are discrete applications where the use of recuperators or regenerators will find acceptance on a large scale, an example being for gas turbines rated at less than about 100 kW for hybrid automobiles and small generator sets. The role that recuperators and regenerators can play in future gas turbines is put into perspective in this paper. Innovative engineering concepts will be required to meet the demanding high-temperature operating environment and low-cost requirements, and these will essentially necessitate the utilization of ceramic-composite heat-exchanger configurations that are amenable to large-volume manufacturing methods.
 
Article
A steady-state mathematical modeling and experimental study were carried out to investigate effects of oil circulation in an inverter air conditioner using R-22 and R-407C. To highlight the prediction of oil circulating mass fraction, the simplified sub-model of oil discharge was modified. The predicted results were validated with experimental data. Two tested lubricants were mineral oil (MO) and polyol-ester lubricant (POE). The experiments varied the compressor frequency in the range of 30–50 Hz for each compressor oil level (0.8 l and 1 l). The results showed that the circulating oil flow rates decreased with the reducing compressor frequency and with a lower oil charge. The lubricant concentration affected the system performance at high frequencies. The charged oil quantity of 0.8 l provided more efficient performance than 1 l. The vapor velocity of R-407C is inadequate high enough to entrain the liquid MO at its lowest frequency. The immiscible mixture of R-407C/MO is not suitable used in the inverter air conditioning system. The proposed model obtains better results for the miscible mixture. The model prediction agreed with the measured values in the compressor frequency of 40–50 Hz.
 
Article
In this study, the dynamic performance of an air-source heat pump using alternatives to HCFC-22 is presented under different conditions. The HFC alternatives covered in this study are: R-407a, R-507, as well as NARM-502. Predicted numerical results of the heat-pump performance have been compared with experimental data under various conditions. The comparison showed that our dynamic model accurately predicted the experimental data. Furthermore, our proposed model was employed to predict the dynamic performance key parameters, such as cooling, heating capacities, COPs, pressures and temperatures at the system components, using new alternatives to HCFC-22.
 
Article
This paper presents the experimental performance analysis of a 1.5 TR window air-conditioner, retrofitted with R-407C, as a substitute to HCFC-22. Experimental results showed that R-407C, for the operating conditions covered in this study, had lower cooling capacity in the range 2.1–7.9% with respect to HCFC-22. The coefficient of performance for R-407C was lower in the range 7.9–13.5%. The power consumption of the unit with R-407C was higher in the range 6–7% than HCFC-22. The discharge pressures for R-407C were higher in the range 11–13% than HCFC-22.This paper also presents simulation results of heat exchangers of an HCFC-22 window air conditioner retrofitted with R-407C. The simulation has been carried out using EVAP-COND, a heat exchanger model developed by National Institute of Standards and Technology, U.S.A. The simulated evaporator capacities are within ±3% of the experimentally measured cooling capacities for both refrigerants. Simulation results for R-407C and HCFC-22 are compared. The exit temperatures of R-407C are lower by 1.9 °C to 5.2 °C in the condenser and are higher by 3.2 °C to 3.8 °C in the evaporator than HCFC-22. Evaporating pressures of R-407C are higher by 4.5–5.3% as compared to HCFC-22. The pressure drops of R-407C are lower in both the evaporator and the condenser as compared to HCFC-22. The outlet temperatures of air for HCFC-22 and R-407C in both heat exchangers are nearly the same.
 
Article
Overall energy rating of buildings including installed HVAC systems, as made in the frame of building energy certificates to be introduced in the EU, gets more important on the background of climate protection. Innovative multifunctional system layouts with heat pumps for combined space heating, domestic hot water and further integration of the ventilation system, however, are often not covered by current product test standards and calculation methods. Therefore, Annex 28 in the heat pump program (HPP) of the international energy agency (IEA) has delivered recommendations for comprehensive testing and subsequent performance calculation of residential heat pump systems with combined space- and domestic hot water heating for standardisation organisations. Both testing and performance calculation for combined operating systems are based on existing standards. A first comparison with systems in field monitoring of different types of combined operating heat pumps, including a ventilation compact unit with air-source heat pump, shows deviations between the calculated and monitored seasonal performance in the range of ±6%, approving the feasibility of the method. The recommendations are currently implemented in the frame of the revision of the European heat pump test standards for the domestic hot water testing and calculation standards in the frame of the EU directive on the energy performance of buildings (EPBD).
 
Article
Flow condensing experiments for refrigerant R-290, and R-600a mixed with the lubricating oil (EMKARATE RL 32H) in serpentine small-diameter (2.46 mm) U-tubes are reported. The tests were run at the saturation temperature of 40 °C, vapor qualities of 0.41–0.82, mass flux of 300–600 (kg/m2s) and inlet oil concentrations from 0 to 5 mass% oil. It was found that the condensation heat-transfer coefficients increased as mass flux values, vapor quality and the number of tube bends increased, but it decreased as the oil concentration increased. In addition, the two-phase pressure drops increased with increases in mass flux values, the number of tube bends and the oil concentration.
 
Article
The performance of a refrigerating system with an environment-friendly refrigerant, propane (R-290) as the refrigerant, was experimentally studied. There were two evaporators connected in series within the system under study.The results show that with both lengths of the two capillary tubes fixed, both the mass flow rate of the refrigerant and the suction pressure of the system increase with the condensing pressure. In addition, the cooling capacity of the high-temperature evaporator decreases, but that of the low-temperature evaporator increases. As the condensing pressure is fixed and the length of the capillary tube for the high-temperature evaporator is increased while that for the low-temperature evaporator is fixed, the cooling capacity of the high-temperature evaporator increases while that of the low-temperature evaporator decreases. On the other hand, as the capillary tube for the low-temperature evaporator is lengthened while that for the high-temperature evaporator is fixed, the variations in the cooling capacity of these two evaporators reverse. The enthalpy changes of the refrigerant within the evaporators are strongly affected by the length of the high-temperature capillary tube, while the evaporating pressures are influenced mainly by the length of the low-temperature capillary tube.
 
Article
This paper reports an experimental investigation of heat transfer and pressure drop behavior during condensation of R-600, R-600/R-290 (50 wt.%/50 wt.%) and R-290 in the three-line serpentine small-diameter (2.46 mm) tube bank. Heat transfer coefficients and pressure drop characteristics are measured at a constant heat flux of 5.2 kW/m2, mass flux (205–510 kg/m2 s) and refrigerant quality (0.15–0.84). The heat transfer coefficients for R-600, R-290/R-600 and R-290 are about 155%, 124% and 89% larger as compared with that for R-134a at the same conditions. The condensation flow frictional pressure drop for R-600, R-600/R-290 and R-290 in the present refrigerant mass fluxes are about 69%, 58% and 36% larger as compared with that for R-134a. In addition, the Dobson and Chato correlation provided the best prediction of the average heat transfer coefficients for the present refrigerants with experimental data with an average standard deviation of 12.8%. The pressure drop result shows a satisfactory agreement between the Friedel correlation and the experimental data, with a mean deviation of 15.3%.
 
Article
Flow boiling experiments for refrigerant R-600a, and R-290 mixed with the lubricating oil (EMKARATE) in the serpentine small-diameter (2.46 mm) U-tubes are reported. The tests were conducted at the nominal inlet pressure of 186.2 kPa, vapor qualities (0–0.76), mass flux of 100–320 (kg/m2s) and inlet oil concentrations from 0 to 5 mass% oil. It was noted that a significant degradation of heat transfer coefficients presented at high qualities and high oil concentrations. The present study investigated that whether the average heat transfer coefficient increased as mass fluxes and the numbers of the U-bend increased. In addition, the ratios (the enhanced heat transfer factor, EF) for both R-600a and R-290 refrigerants increased as the oil concentration increased up to 1% then decreased with each increase in oil concentration. Moreover, pressure dropped during evaporation increased with the addition of a lubricant, mass fluxes and the numbers of the U-turn. The values of the pressure drop penalty factor PF were generally larger than l and increased rapidly as the oil concentration and the numbers of the U-turn increased.
 
Article
In order to check the theoretical performance of new working fluids LiBr+H2N(CH2)2OH+H2O, LiBr+HO(CH2)3OH+H2O, and LiBr+(HOCH2CH2)2NH+H2O [LiBr/H2N(CH2)2OH, LiBr/HO(CH2)3OH, and LiBr/(HOCH2CH2)2NH mass ratios were 3.5] which were developed particularly for the air-cooled cycle operation, the theoretical coefficients of performance (COPs) were calculated at various operating conditions. The cooling capacity and crystallization problem were also checked at a specific condition for air-cooled cycle operation. All the solutions were found to be of possibe use as working fluids for the air-cooled absorption chiller as alternatives to the conventional LiBr+H2O solution
 
Article
An experimental investigation of the performance of thermosyphons charged with water as well as the dielectric heat transfer liquids FC-84, FC-77 and FC-3283 has been carried out. The copper thermosyphon was 200 mm long with an inner diameter of 6 mm, which can be considered quite small compared with the vast majority of thermosyphons reported in the open literature. The evaporator length was 40 mm and the condenser length was 60 mm which corresponds with what might be expected in compact heat exchangers. With water as the working fluid two fluid loadings were investigated, that being 0.6 ml and 1.8 ml, corresponding to approximately half filled and overfilled evaporator section in order to ensure combined pool boiling and thin film evaporation/boiling and pool boiling only conditions, respectively. For the Fluorinert™ liquids, only the higher fill volume was tested as the aim was to investigate pool boiling opposed to thin film evaporation. Generally, the water-charged thermosyphon evaporator and condenser heat transfer characteristics compared well with available predictive correlations and theories. The thermal performance of the water-charged thermosyphon also outperformed the other three working fluids in both the effective thermal resistance as well as maximum heat transport capabilities. Even so, FC-84, the lowest saturation temperature fluid tested, shows marginal improvement in the heat transfer at low operating temperatures. All of the tested Fluorinert™ liquids offer the advantage of being dielectric fluids, which may be better suited for sensitive electronics cooling applications and were all found to provide adequate thermal performance up to approximately 30–50 W after which liquid entrainment compromised their performance.
 
Article
Numerical and experimental analyses were carried out to study thermal–hydraulic characteristics of air flow inside a circular tube with different tube inserts. Three kinds of tube inserts, including longitudinal strip inserts (both with and without holes) and twisted-tape inserts with three different twisted angles (α = 15.3°, 24.4° and 34.3°) have been investigated for different inlet frontal velocity ranging from 3 to 18 m/s. Numerical simulation was performed by a 3D turbulence analysis of the heat transfer and fluid flow. Conjugate convective heat transfer in the flow field and heat conduction in the tube inserts are considered also. The experiments were conducted in a shell and tube exchanger with overall counterflow arrangement. The working fluid in the tube side was cold air, while the hot Dowtherm fluid was on the shell side. To obtain the heat transfer characteristics of the test section from the experimental data, the ε-NTU (effectiveness-number of transfer unit) method is applied to determine the overall conductance (UA product) in the analysis.It was found that the heat transfer coefficient and the pressure drop in the tubes with the longitudinal strip inserts (without hole) were 7–16% and 100–170% greater than those of plain tubes without inserts. When the longitudinal strip inserts with holes were used, the heat transfer coefficient and the pressure drop were 13–28% and 140–220%, respectively, higher than those of plain tubes. The heat transfer coefficient and the pressure drop of the tubes with twisted-tape inserts were 13–61% and 150–370%, respectively, higher than those of plain tubes. Furthermore, it was found that the reduction ratio in the heat transfer area of the tube of approximately 18–28% may be obtained if the twisted-tape tube inserts are used.
 
Article
Solid particle erosion in a steam turbine main stop valve bypass valve has been investigated by means of computational fluid dynamics. Previews attempts to couple fluid mechanics and erosion modeling and improvements in the hydrodynamics models together with improvements in the erosion models are reviewed. The solid particle bearing steam flow through the valve was investigated using a 3D numerical model and the finite volume code Fluent V6.0.12, looking for a reduction of the erosion process. The flow simulation was carried out for the valve original and modified designs with changes of the angle of particle impact on the valve surface. Numerical predictions have been carried out using the Renormalization Group (RNG) k–ε turbulence model. To account for the influence of turbulent fluid fluctuations on particle motion, the stochastic tracking Discrete Random Walk model is used, which includes the effect of instantaneous turbulent velocity fluctuations on the particle trajectories. The removal of wall material due to erosion is calculated using the Finnie model developed for ductile materials. The numerical predictions showed a 51% reduction of the erosion rate for the valve modified design due to changes of the particles trajectories and impingement angle (angle of particle impact). The results obtained show that numerical simulation can be used in a predictive manner to solve a real practical design problem.
 
Article
Tungsten inert Gas (TIG) welding takes place in an atmosphere of inert gas and uses a tungsten electrode. In this process heat input identification is a complex task and represents an important role in the optimization of the welding process. The technique used to estimate the heat flux is based on solution of an inverse three-dimensional transient heat conduction model with moving heat sources. The thermal fields at any region of the plate or at any instant are determined from the estimation of the heat rate delivered to the workpiece. The direct problem is solved by an implicit finite difference method. The system of linear algebraic equations is solved by Successive Over Relaxation method (SOR) and the inverse problem is solved using the Golden Section technique. The golden section technique minimizes an error square function based on the difference of theoretical and experimental temperature. The temperature measurements are obtained using thermocouples at accessible regions of the workpiece surface while the theoretical temperatures are calculated from the 3D transient thermal model.
 
Article
This article presents the results of simulations using a zonal airflow model, a radiation model and a humidity model to predict the heat fluxes, the transient airflow patterns as well as the temperature and absolute humidity distributions in a 3D section of an indoor ice rink in Montreal. The results of the simulations are in good agreement with published CFD(Computational Fluid Dynamics) results and experimental measurements. The principal advantage of using the present model to simulate the thermal and airflow fields in an ice rink is the computational time which is very short compared to that used by a CFD model. The effects of the ceiling emissivity and the temperature set point in the stands on heat transfer toward the ice were studied by conducting a parametric study.
 
Article
Two-phase frictional pressure drop characteristics of R410A/oil mixture flow boiling in horizontal small smooth tubes with outside diameters of 5.0 mm and 3.0 mm were investigated experimentally. Experimental conditions cover nominal oil concentrations from 0% to 5%. The test results show that the frictional pressure drop of R410A initially increases with the increase of vapor quality and then decreases, presenting a local maximum in the vapor quality range between 0.6 and 0.8; the presence of oil increases two-phase frictional pressure drop about 0–120% and 0–90% in present test conditions for 5.0 mm O.D. smooth tube and 3.0 mm O.D. smooth tube, respectively, and the increase is evident at high vapor qualities. The vapor-phase multiplier of R410A/oil mixture based on the mixture properties decreases with the decrease of tube diameter. A new vapor-phase multiplier correlation to predict the local frictional pressure drop of R410A/oil mixture flow boiling inside smooth tubes is developed based on local properties of refrigerant–oil mixture, and the deviations of the new correlation are within ±25% from the experimental data.
 
Article
A design program of municipal waste incineration, which includes selective non-catalytic reduction (SNCR) process, was developed, by applying a numerical study. A comprehensive numerical model related with the process of waste-off gaseous fuel was made. The turbulence combustion chemistry involved with NO reaction was modeled by the harmonic mean strategy. Based on the numerical study a computer program was developed which successfully evaluated against the operational data, temperature distribution, NO concentration and flue gas velocity, respectively.
 
Article
Internal channel cooling is employed in advanced gas turbines blade to allow high inlet temperatures so as to achieve high thrust/weight ratios and low specific fuel consumption. The objective of the present work is to study the effect of rib height to the hydraulic diameter ratio on the local heat transfer distributions in a double wall ribbed square channel with 90° continuous attached and 60° V-broken ribs. The effect of detachment of the rib in case of broken ribs on the heat transfer characteristics is also presented. Reynolds number based on duct hydraulic diameter is ranging from 10,000 to 30,000. A thin stainless steel foil of 0.05 mm thickness is used as heater and infrared thermography technique is used to obtain the local temperature distribution on the surface. The images are captured in the periodically fully developed region of the channel. It is observed that the heat transfer augmentations in the channel with 90° continuous attached ribs increase with increase in the rib height to hydraulic diameter ratio (e/D) but only at the cost of the pressure drop across the test section. The enhancements caused by 60° V-broken ribs are higher than those of 90° continuous attached ribs and also result in lower pressure drops. But, with an increase in the rib height, the enhancements are found to decrease in channel with broken ribs. The effect of detachment incase of broken ribs is not distinctly observed. The heat transfer characteristics degraded with increase in the rib height in both attached and detached broken ribbed cases.
 
Article
A visualization study was conducted with the aim of deepening the understanding of the boiling mechanism in a dendritic and micro-porous copper structure for enhanced boiling heat transfer. The unique structure has earlier been shown to enhance heat transfer in pool boiling applications as well as in convective boiling in both small and large channels. Pool boiling tests were conducted in R134a and in the dielectric fluid FC-72 and were visualized with a high speed imaging system. Data on bubble size, bubble frequency density, heat transfer coefficient and the latent and sensible heat flux contributions were collected and calculated at heat flux varying between 2 and 15 W/cm2. The enhanced surface produces smaller bubbles and sustains a high bubble frequency density in both fluids, even at low heat flux. An enhanced latent heat transfer mechanism of up to 10 times, compared to that of a plain reference surface, is the main reason for the improved boiling heat transfer performance on the enhanced surface. The data also suggests that the high nucleation bubble frequency density leads to increased bubble pumping action and thus enhancing single-phase convection of up to 6 times. The results in this study highlight the importance of both two and single-phase heat transfer within the porous structure.Highlights► Boiling tests on an enhanced porous surface were filmed with a high speed camera. ► R134a and FC-72 were used as fluids and a plain copper surface as a reference. ► The porous surface produced small bubbles at high frequency density in both fluids. ► Latent and sensible heat transfer mechanisms were enhanced 10 and 6 times respectively.
 
Article
The two-phase heat transfer coefficient and pressure drop characteristics of refrigerant R22 and R407C (a mixture of R32/R125/R134a, 23%/25%/52%) in a micro-fin tube with nominal diameter 9.52 mm were presented in this study. Experimental data were taken at an evaporation pressure of 600 kPa. The mass flux was between 100 and 300 kg/m2·s and the heat fluxes were between 6 and 14 kW/m2. Experimental data were presented in the form of length-averaged heat transfer coefficients and frictional pressure gradients. The effect of heat flux and mass flux on the heat transfer coefficients were also reported in the present investigation. It is found that the heat transfer coefficients for R407C are considerably lower than R22 (50–80% less), and the pressure drop of R407C is 30–50% lower as compared to R22.
 
Article
In many industrial processes there is a simultaneous need for electric power and refrigeration at low temperatures. Examples are in the food and chemical industries. Nowadays the increase in fuel prices and the ecological implications are giving an impulse to energy technologies that better exploit the primary energy source and integrated production of utilities should be considered when designing a new production plant. The number of so-called trigeneration systems installations (electric generator and absorption refrigeration plant) is increasing. If low temperature refrigeration is needed (from 0 to −40 °C), ammonia–water absorption refrigeration plants can be coupled to internal combustion engines or turbogenerators. A thermodynamic system study of trigeneration configurations using a commercial software integrated with specifically designed modules is presented. The study analyzes and compares heat recovery from the primary mover at different temperature levels. In the last section a simplified economic assessment that takes into account disparate prices in European countries compares conventional electric energy supply from the grid and optimized trigeneration plants in one test case (10 MW electric power, 7000 h/year).
 
Article
While synthetic jets have found more applications in controlling fluid flow especially in aerospace applications, more recently they captured a lot of attention for the thermal management of electronics. While the jet sizes vary and may be large for microfluidic applications, it is preferred that they should be in the meso scale range for removing heat from electronics components. Current study focuses on the heat transfer and acoustic aspects of the small-scale synthetic jets. Synthetic jets designed and developed at the General Electric Global Research Center can provide peak air velocities in excess of 90 m/s from a 1 mm hydraulic diameter rectangular orifice. The jets are driven by a sine wave with an operating frequency of between 3 and 4.5 kHz, providing the highest thermal performance for the current jets. An infrared thermal imaging technique was used to acquire fine scale temperature measurements. Two heater sizes have been studied in the current study to understand the effect of the characteristic length. Several parameters are varied to find the change in the heat transfer rates with the jet location, driving voltage, driving frequency, and heater power. Heat transfer enhancements over the specific heater sizes are presented for the same jet. It is found that the enhancement can be between 4 and 10 times depends on the heater size showing that smaller sizes provide the best jet effectiveness. It is also noted that jet noise can be as large at 73 dB, but possible abatement techniques can decrease this noise level as low as to 30 dB.
 
Article
Hong Kong is a typical subtropical region with frequently high humidity in late spring and summer seasons. Plume from evaporative cooling towers, which service air-conditioning systems of civil buildings, has aroused public concerns since 2000 when the fresh water evaporative cooling towers were allowed to be used for high energy efficiency and environmental issues. This paper presents the evaluation of the plume potential and its effect on the sizing of the plume abatement system in a large commercial office building in Hong Kong for practical application. This evaluation was conducted based on a dynamic simulation platform using the typical meteorological year of Hong Kong since the occurrence of the plume heavily depends on the state conditions of the exhaust air from cooling towers and the ambient air, while the state condition of the exhaust air is determined by the total building cooling load and the control strategies of cooling towers employed mainly for improving energy efficiency. The results show that the control strategies have a significant effect on the plume potential and further affect the system design and sizing of the plume abatement system.
 
Article
Computational simulations are reported of experiments on convective heat transfer to carbon dioxide at a pressure of 75.8 bar, which is just above the thermodynamic critical value of 73.8 bar. These have been carried out using a variable property, elliptic computational formulation incorporating low Reynolds number turbulence models of k − ε and V2F types. Firstly, the simulations were compared with the heat transfer measurements and then they were used in developing an understanding of interesting phenomena observed in the experiments. It has been found that the effect of buoyancy on turbulence production and heat transfer in fluids at supercritical pressure can be very significant even under conditions of relatively ‘low’ buoyancy parameter based on bulk properties. The effect of buoyancy, although complex, can be explained by relating it to the large-property-variation (LPV) region, i.e., the region within the flow field near to the locations where the fluid temperature has the pseudo-critical value. Under certain conditions, a very non-uniform radial distribution of the buoyancy force may be present and cause some reduction of turbulence in the core but a big increase near the wall, resulting in much improved heat transfer. It is clear that new heat transfer correlations are needed to account for such effects on heat transfer to supercritical pressure fluids as they come to be used more and more in new energy systems applications such as, advanced water-cooled nuclear reactors, environmentally friendly air-conditioning and refrigeration systems and high pressure water oxidation plant for waste processing.
 
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The flow experiencing the abrupt contraction suffers from the viscous dissipation due to vast change in flow field across the plane of contraction. In the present study, simulation of flow field and entropy generation rate due to viscous dissipation are carried out for different upstream Reynolds numbers. The flow situation is considered to be isothermal to avoid the influence of heat transfer on the entropy generation rate. A control volume approach is introduced when discretizing the governing equations of flow. In order to secure the grid independent solution the grid independent tests are conducted. It is found that overshooting of axial velocity occurs along the symmetry axis across the plane of contraction. Entropy generation rate enhances with increasing Reynolds numbers. The location of maximum entropy generation rate becomes almost identical for all Reynolds numbers considered in the present study.
 
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This paper presents a new approach based on artificial neural networks (ANNs) to determine the properties of liquid and two phase boiling and condensing of two alternative refrigerant/absorbent couples (methanol/LiBr and methanol/LiCl). These couples do not cause ozone depletion and use in the absorption thermal systems (ATSs). ANNs are able to learn the key information patterns within multidimensional information domain. ANNs operate such as a ‘black box’ model, requiring no detailed information about the system. On the other hand, they learn the relationship between the input and the output. In order to train the neural network, limited experimental measurements were used as training data and test data. In this study, in input layer, there are temperatures in the range of 298–498 K, pressures (0.1–40 MPa) and concentrations of 2%, 7%, 12% of the couples; specific volume is in output layer. The back-propagation learning algorithm with three different variants, namely scaled conjugate gradient (SCG), Pola–Ribiere conjugate gradient (CGP), and Levenberg–Marquardt (LM), and logistic sigmoid transfer function were used in the network so that the best approach can find. The most suitable algorithm and neuron number in the hidden layer are found as SCG with 8 neurons. For this number level, after the training, it is found that maximum error is less than 3%, average error is about 1% and R2 value are 99.999%. As seen from the results obtained the thermodynamic equations for each pair by using the weights of network have been obviously predicted within acceptable errors. This paper shows that values predicted with ANN can be used to define the thermodynamic properties instead of approximate and complex analytic equations.
 
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The development of absorption chillers activated by renewable heat sources has increased due mainly to the increase in primary energy consumption that causes problems such as greenhouse gases and air pollution among others. These machines, which could be a good substitute for compression systems, could be used in the residential and food sectors which require a great variety of refrigeration conditions. Nevertheless, the low efficiency of these machines makes it necessary to enhance heat and mass transfer processes in the critical components, mainly the absorber, in order to reduce their large size.This study used ammonia–water as the working fluid to look at how absorption takes place in a plate heat exchanger operating under typical conditions of absorption chillers, driven by low temperature heat sources. Experiments were carried out using a corrugated plate heat exchanger model NB51, with three channels, where ammonia vapor was injected in bubble mode into the solution in the central channel. The results achieved for the absorption flux were in the range of 0.0025–0.0063 kg m−2 s−1, the solution heat transfer coefficient varied between 2.7 and 5.4 kW m−2 K−1, the absorber thermal load from 0.5 to 1.3 kW. In addition, the effect of the absorber operating conditions on the most significant efficiency parameters was analyzed. The increase in pressure, solution and cooling flow rates positively affect the absorber performance, on the other hand an increase in the concentration, cooling, and solution temperature negatively affects the absorber performance.
 
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An investigation is reported of the thermal performance of an integrated solar water heater with a corrugated absorber surface. The thermal performance of the rectangular collector/storage solar water heater depends significantly on the heat transfer rate between the absorber surface and the water, and on the amount of solar radiation incident on the absorber surface. In this investigation, the surface of the absorber is considered to be corrugated, with small indentation depths, instead of plane. The modified surface has a higher characteristic length for convective heat transfer from the absorber to the water, in addition to having more surface area exposed to solar radiation. The corrugated surface based solar water heater is determined to have a higher operating temperature for longer time than the plane surface. It means during the operation of water heater, more solar energy is converted into useful heat. However, this modification has reduces the efficiency of the system marginally.
 
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This paper describes an experimental and theoretical analysis of the combined heat and mass transfer processes that take place in the absorber of vapor absorption cooling systems. The effect of the main operating variables such as solution inlet temperature, solution flow rate, coolant inlet temperature, coolant flow rate and absorber vapor pressure on the performance of absorber is experimentally investigated. A mathematical model that takes account of the geometrical details of the solution and coolant flow is developed following the traditional heat exchanger analysis to obtain additional information on the performance of the absorber. The governing equations are solved analytically using Laplace Transformation technique. The variations of the concentration of solution and the temperature of the coolant and solution after each horizontal tube are analyzed. There is satisfactory agreement between the predictions of the analytical model and the experimental results. The serpentine arrangement of the coolant flow in the absorber tubes results in a temperature and concentration variation of the solution mainly along the height of the absorber.
 
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Polymers have been proven to be high potential low-cost materials for the design and mass production not only for ordinary solar water heaters but also for very simple large size, modular solar collectors, suitable for easy erection of large solar heating plants. Their major drawback for solar–thermal conversion applications is their low thermal conductivity, which prohibits their use unless an appropriate absorber design is employed. The low thermal conductivity of polymers has imposed the need of a particular absorber design, which is basically composed of a pair of dark, closely spaced parallel plates at the top of which solar radiation is absorbed, forming a thin channel for the flow of the heat transfer fluid. The aim of the present work is to investigate the particular limitations of this polymer plate absorber design, for a wide range of collector loss and convective heat transfer coefficients between heat transfer fluid and absorber plate. The aim is also to calculate the particular collector efficiency factors and conditions under which the associated collector performance parameters should be modified to account for the finite absorber plate conductance. This conductance was proven to be another decisive absorber design parameter, improper selection of which may probably lead to strong deterioration of the collector efficiency.
 
Article
A method of calculating the detrimental effects of the presence of non-condensable gases on the performance of the absorber of the vapour absorption refrigeration systems is presented using data for the lithium bromide/water system with air as a contaminant gas in a film absorber. It is shown that the reduction in the heat and mass transfer as a result of the non-condensable gas can be offset by introducing turbulence into the vapour/gas mixture in the absorber. A calculation of the reduction in the COP, due to a non-condensable gas in the absorber, of a LiBr/ H2O system is also presented.
 
Article
A mathematical model of a single effect, LiBr–H2O absorption heat pump operated at steady conditions is presented. This model took into consideration of crosscurrent flow of fluids for heat and mass exchangers, two-dimensional distribution of temperature and concentration fields, local values of heat and mass transfer coefficients, thermal parameter dependent physical properties of working fluids and operation limits due to the danger of the LiBr aqueous solution hydrates and crystallization. Improvements of the calculation method make this simulation much more convenient and efficient. An improved absorber experiment set-up and a complete absorption heat pump were built and tested for further study. It was found that the mass flux of vapor increased with the increase of absorber pressure, coolant flow rate, spray density of LiBr solution and decrease of coolant and input temperature of solution. And the vapor mass flux increased almost linearly with the increase of absorber pressure. Results derived from this model show agreement within 7% with experimental values.
 
Article
A detailed model is developed for treating the 2-D water vapour absorption into an aqueous solution of LiBr, which is flowing over a vertical tube. Numerical predictions are presented for the spatial variation of solution temperature and concentration in the axial direction and within the LiBr–H2O falling film taking into account the flow and temperature rise of coolant inside the tube. Special care was given to the adequate description of the thermodynamic properties of superheated water vapour, which is treated as a real gas. Emphasis was also given in demonstrating the electrolytic nature of the aqueous solution, introducing thus the effect of variable heat of absorption on the temperature field. A parametric study for examining the effect of absorption pressure, solution mass flow rate and inlet coolant temperature on the absorption process was carried out implementing the model under laminar flow conditions. A good coincidence with experimental observations obtained for solution flows with Re<150 was witnessed, acknowledging thus the predictive capability of the numerical model.
 
Article
The escalating cost of energy during this decade has made co-generation very popular. When cooling is required for either space conditioning or industrial processes, absorption chillers which use waste heat can be utilised. Much of the work is concentrated on the water-cooled absorption chiller. Since countries with hot and dry climates are often short of water, it will also be necessary to develop air-cooled equipment. The purpose of this paper is to investigate the effect of ambient temperature on the surface area of the generator, condenser, evaporator, absorber and solution heat exchanger of an air-cooled absorption machine.
 
Article
An experimental investigation was undertaken to study the performance of the bubble pump for diffusion absorption refrigeration units. The bubble pump is the motive force of the diffusion absorption cycle and is a critical component of the absorption diffusion refrigeration unit. The purpose of the bubble pump (besides the circulation of the working fluid) is to desorb the solute refrigerant from the solution. Therefore the efficiency of the bubble pump will be set by the amount of the refrigerant desorbed from the solution. The performance of the diffusion absorption cycle depends primarily on the efficiency of the bubble pump. A continuous experimental system was designed, built and successfully operated. The experiments were performed in which some of the parameters affecting the bubble pump performance were changed. During the experimental investigation, photographs were taken showing that the bubble pump operates at slug flow regime with a churn flow regime at the entrance of the bubble pump tube. It was obtained that the performance of the bubble pump depends mainly on the motive head and on the heat input to the bubble pump.
 
Article
In this study, a prototype of an aqua–ammonia absorption heat pump system (AHP) using solar energy was investigated. The performance tests of the system were performed for the climate condition of Ankara in Turkey. The system has been designed operating with a parabolic slote type collector to obtain the required temperatures. In the experiments, high temperature water obtained from the collector was used as heat source needed for the generator. The system design configuration was analysed by using the experimental data. The effect of irreversibilities in thermal process on the system performance in AHP were determined. Thermodynamic analysis shows that both losses and irreversibility have an impact on absorption system performance. The study indicates which components in the system need to be improved thermally. This study will contribute the development of the system for the future use of solar-powered food preservation and commercial air conditioning.
 
Article
This paper is concerned with the design, modelling and parametric analysis of a gas-to-thermal fluid heat recovery system from engine exhausts in a trawler chiller fishing vessel to power an NH3-H2O absorption refrigeration plant for onboard cooling production. Synthetic oil was used as heat transfer fluid and recirculated. The major components of the system are fluid-to-solution and gas-to-fluid heat exchangers. Both heat exchangers and the complete system have been modelled. Models are implemented in several computer programs. These models have been used to study the influence of geometric design parameters and thermal operating conditions on heat exchangers and system thermal performance. The analysis of the results allowed us to find the optimum thermal operating conditions that minimise total heat transfer area. Optimal design based on real data was performed and the operating function of exhaust gases by-pass control was obtained and is presented.
 
Article
The enhancement effects of additive on vertical falling film of water into aqueous lithium bromide (LiBr) were studied by an experimental method. Based on the Navier–Stokes equations of falling film absorption, a new dimensionless parameter, surface renewal number Rn, was introduced, and a semi-empirical equation of enhancement factor of additive was obtained. It was shown that the absorption Marangoni number Ma, the surface Marangoni number MaA, and the surface renewal number Rn enhance the heat transfer of absorption, however the adsorption number Π and the Reynolds number Re weaken the heat transfer of absorption. It was proved that the semi-empirical equation agreed well with the experimental results by introduction of the parameters related to surface tension characters presented by the authors into the equation.
 
A schematic representation of a triple-pressure-level absorption refrigeration cycle (EARS).
A schematic representation an ejector-absorber model.
Variation of the normalized exergy loss of the generator with generator temperature at different evaporator and condenser temperatures.
Article
In the absorption refrigeration system (ARS) working with aqua–ammonia, the ejector is commonly located at the condenser inlet. In this study, the ejector was located at the absorber inlet. Therefore, the absorber pressure becomes higher than the evaporator pressure and the system works with triple-pressure-level. The ejector has two main functions: (i) aiding pressure recovery from the evaporator, (ii) upgrading the mixing process and the pre-absorption by the weak solution of the ammonia coming from the evaporator. In addition to these functions, it can also act to lower the refrigeration and heat-source temperatures. Energy analyses show that the system’s coefficient of performance (COP) and exergetic coefficient of performance (ECOP) were improved by 49% and 56%, respectively and the circulation ratio (f) was reduced by 57% when ARS is initiated at lower generator temperatures. Due to the reduced circulation ratio, the system dimensions can be reduced; consequently, this decreases overall cost. The heat source and refrigeration temperatures decreased in the range of 5–15 °C and 1–3 °C, respectively. Exergy analyses show that the exergy loss of the absorber of ARS with ejector had a higher exergy loss than those of the other components. Therefore, a multiple compartment absorber can be proposed to reduce the exergy loss of the absorber of ARS with ejector.
 
Top-cited authors
Luisa F. Cabeza
  • Universitat de Lleida
Harald Mehling
Belen Zalba
  • University of Zaragoza
Ibrahim Dincer
  • Ontario Tech University
Jose Maria Marin
  • University of Zaragoza