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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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).
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.
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.
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%.
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.
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
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.
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.
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.
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.