Showing papers in "International Journal of Refrigeration-revue Internationale Du Froid in 2001"
TL;DR: In this article, the effects of vapour quality, mass velocity, saturation temperature and temperature difference between saturation and tube wall on the heat transfer coefficient are investigated by analysing the experimental data.
Abstract: This paper reports experimental heat transfer coefficients and pressure drops measured during condensation inside a smooth tube when operating with pure HFC refrigerants (R134a, R125, R236ea, R32) and the nearly azeotropic HFC refrigerant blend R410A. Data taken when condensing HCFC-22 are also reported for reference. The experimental runs are carried out at a saturation temperature ranging between 30 and 50°C, and mass velocities varying from 100 to 750 kg/(m2 s), over the vapour quality range 0.15–0.85. The effects of vapour quality, mass velocity, saturation temperature and temperature difference between saturation and tube wall on the heat transfer coefficient are investigated by analysing the experimental data. A predictive study of the condensation flow patterns occurring during the tests is also presented. Finally comparisons with predictions from the model by Kosky and Staub (Kosky PG, Staub FW. Local condensing heat transfer coefficients in the annular flow regime. AIChE J 1971;17:1037) are reported for all the data sets.
272 citations
TL;DR: In this paper, an ad-sorption cooling system was developed and tested and various operation procedures have been tried and experimental results showed that the heat recovery operation between two adsorption beds will increase the COP by about 25% if compared with one adsorber basic cycle system.
Abstract: An adsorption cooling system was developed and tested and various operation procedures have been tried. The experimental results show that the heat recovery operation between two adsorption beds will increase the COP by about 25% if compared with one adsorber basic cycle system. It was also proved that mass recovery is very effective for heat recovery adsorption cooling operation, which may help to obtain a COP increase of more than 10%. Theoretical analyses on the COP have been completed for various heat and mass recovery cycles, such as basic intermittent adsorption cycle, continuous two-adsorber heat recovery cycle, mass recovery cycle, mass recovery with sensible heat recovery, and mass recovery with both sensible heat and heat of adsorption recovery. The theoretical results are in good agreement with experimental values. Based upon the developed theoretical model, it is possible to predict the COP for various operation procedures of a real adsorption cooling system.
253 citations
TL;DR: In this paper, a critical review to portray precooling as an intricate part of temperature management and highlight the importance of its utilisation for extending the shelf life and maintaining the quality of horticultural products with emphasis on cut flowers is presented.
Abstract: One of the most important factors affecting the postharvest life and quality of horticultural crops is temperature. Quality loss after harvest occurs as a result of physiological and biological processes, the rates of which are influenced primarily by product temperature. As the maintenance of market quality is of vital importance to the success of the horticultural industry, it is necessary not only to cool the product but to cool it as quickly as possible after harvest. The process of precooling is the removal of field heat which arrest the deteriorative and senescence processes so as to maintain a high level of quality that ensures customer satisfaction. This paper provides a critical review to portray precooling as an intricate part of temperature management and to highlight the importance of its utilisation for extending the shelf life and maintaining the quality of horticultural products with emphasis on cut flowers. Various different precooling methods are depicted with the benefits and ambiguities of each shown. The applicability and cooling efficiency of each are also illustrated. Through this review, it is aimed to promote interest in precooling and encourage its use on a more widespread basis.
251 citations
TL;DR: In this paper, a comprehensive review of the significant efforts that have made to mathematically model the coupled heat and mass transfer phenomena that occur during falling-film absorption is presented, including boundary conditions, assumptions, solution methods, results, and validation of these investigations.
Abstract: In absorption space-conditioning systems, the performance of the absorber is critical to the overall system performance, size, and first-cost. The objective of this paper is to provide a comprehensive review of the significant efforts that researchers have made to mathematically model the coupled heat and mass transfer phenomena that occur during falling-film absorption. A detailed review of the governing equations, boundary conditions, assumptions, solution methods, results, and validation of these investigations is presented. This review excludes experimental work in this area, the effect of additives, and the effect of non-absorbable gases. It is shown that most work found in the literature has focused on the particularly simplified case of absorption in laminar vertical films of water-lithium bromide. Fewer researchers have considered the important situations of wavy films, turbulent films, and films on horizontal tubes. Investigations of the ammonia-water fluid pair have been generally more empirical in nature and/or restricted to vertical laminar films. This review is used to highlight key areas which need attention such as film and vapor hydrodynamics, especially the non-periodicity, instability, and recirculatory motion of waves in the vertical wall case and droplets and waves in the horizontal tube case. Also the potential interaction of the heat and mass transfer process on the film hydrodynamics, surface wetting, heat transfer in the vapor phase, and common simplifications to the governing equations should all be considered carefully. Finally, emphasis must be placed on experimental validation of the local conditions and transfer processes within the absorber, not just overall transport values.
239 citations
TL;DR: In this article, the authors developed the equations which can calculate the vapor pressure, enthalpy and entropy of LiBr solutions at such high temperatures, valid from concentrations of 40-65 wt% and also from temperatures of 40 −210°C.
Abstract: Emerging triple-effect LiBr–water absorption chillers operate at higher temperatures and pressures than traditional double-effect chillers. However, there is not enough data about thermodynamic properties of LiBr–water solutions at such high temperatures. Using recently measured data of vapor pressure and heat capacity, we have developed the equations which can calculate the vapor pressure, enthalpy and entropy of LiBr solutions at such high temperatures. The developed equations are valid from concentrations of 40–65 wt.% and also from temperatures of 40–210°C. These equations will be very helpful for the modeling and design of triple-effect LiBr–water chillers.
234 citations
TL;DR: In this article, the authors presented two numerical models to simulate the transient and steady state behavior of a vapor compression refrigeration system, where the condenser and the evaporator were divided into a number of control volumes.
Abstract: This work presents two numerical models to simulate the transient and steady state behavior of a vapor compression refrigeration system. The condenser and the evaporator were divided into a number of control volumes. Time dependent partial differential equations system was obtained from the mass, energy and momentum balances for each control volume. As the expansion valve and the compressor both have very small thermal inertia, the steady state models were applied for these components. Transient and steady state models numerical predictions were compared and good agreement was found. Further simulations were performed with the objective of verifying the possibility of controlling the refrigeration system and the superheating of the refrigerant in the evaporator outlet by varying the compressor speed and the throttling valve sectional area. The results indicate that the proposed models can be used to formulate an algorithm for controlling a refrigeration system.
202 citations
TL;DR: In this article, the possibilities and problems of using hydrocarbons as working fluids in refrigerating equipment are discussed, and an overview of safety standards is given, and different hydrocarbon alternatives are listed and characteristics in terms of thermodynamic cycles and heat transfer are shown.
Abstract: Possibilities and problems of using hydrocarbons as working fluids in refrigerating equipment are discussed. An overview of safety standards is given. Different hydrocarbon alternatives are listed and characteristics in terms of thermodynamic cycles as well as heat transfer are shown. The general conclusion is that hydrocarbons offer interesting refrigerant alternatives for energy efficient and environmentally friendly refrigerating equipment and heat pumps. However, safety precautions due to flammability must be seriously taken into account. For some applications this can be done without adding noticeably to the total installation cost, but not in the general case.
194 citations
TL;DR: In this paper, a study of two-phase flow and heat transfer in a small tube of 1 mm internal diameter has been conducted experimentally as part of a wider study of boiling in small channels.
Abstract: A study of two-phase flow and heat transfer in a small tube of 1 mm internal diameter has been conducted experimentally as part of a wider study of boiling in small channels. R141b has been used as the working fluid. The boiling heat transfer in the small tube has been measured over a mass flux range of 300–2000 kg/m2 s and heat flux range of 10–1150 kW/m2. In this paper the boiling map for a mass velocity of 510 kg/m2 s and heat flux of 18–72 kW/m2 is discussed and the problems of determining heat transfer coefficients in small channels are highlighted.
182 citations
TL;DR: In this paper, a new concept is presented, in which a solar air collector is integrated into the desiccant cooling cycle as the only heat source and a comparative study is presented which compares system performance for different system configurations and different climatic situations.
Abstract: The use of heat produced by solar thermal collectors is an interesting option for thermal driven air conditioning processes A thermal driven cooling technique which fits well to non-tracking solar collectors is the desiccant cooling technique Recently several projects have been carried out which focus on the connection of desiccant cooling systems with solar thermal energy for regeneration of the sorbents This communication deals with three main topics: (1) experiences achieved in a realized system which is coupled to a solar collector are discussed, (2) a new concept is presented, in which a solar air collector is integrated into the desiccant cooling cycle as the only heat source and (3) a comparative study is presented which compares system performance for different system configurations and different climatic situations
171 citations
TL;DR: In this paper, heat transfer coefficient and pressure drop were measured for condensation and evaporation of R410A and HCFC22 inside internally grooved tubes inside a conventional spiral groove tube of 8.01 mm o.d. and 7.30 mm mean i.d., respectively.
Abstract: Heat transfer coefficient and pressure drop were measured for condensation and evaporation of R410A and HCFC22 inside internally grooved tubes. The experiments were performed for a conventional spiral groove tube of 8.01 mm o.d. and 7.30 mm mean i.d., and a herring-born groove tube of 8.00 mm o.d. and 7.24 mm mean i.d. To measure the local heat transfer coefficients and pressure drop, the test section was subdivided into four small sections having 2 m working length. The ranges of refrigerant mass flow density was from 200 to 340 kg/(m2 s) for both condensation and evaporation of R410A and HCFC22, and the vapour pressure was 2.41 MPa for condensation and 1.09 MPa for the evaporation of R410A. The obtained heat transfer data for R410A and HCFC22 indicate that the values of the local heat transfer coefficients of the herring-bone grooved tube are about twice as large as those of spiral one for condensation and are slightly larger than those of spiral one for the evaporation. The measured local pressure drop in both condensation and evaporation is well correlated with the empirical equation proposed by the authors.
150 citations
TL;DR: Artificial cryopreservation achieves intracellular vitrification with rapid cooling, modest osmotic contraction and, often, added cryoprotectants in order to avoid crystallization during warming.
Abstract: Several physical stresses kill cells at low temperatures Intracellular ice is usually fatal, so survival of freezing temperatures involves combinations of dehydration, freezing point depression, supercooling and intracellular vitrification Artificial cryopreservation achieves intracellular vitrification with rapid cooling, modest osmotic contraction and, often, added cryoprotectants High warming rates are required to avoid crystallization during warming Environmental cooling is much slower and temperatures less cold, but environmental freezing damage is important ecologically and agronomically For modest sub-freezing temperatures, supercooling sometimes allows survival At lower temperatures, extracellular water usually freezes and cells may suffer large osmotic contractions This contraction concentrates solutes and thus assists vitrification, but is not necessarily reversible: the rapid osmotic expansion during thawing may rupture membranes Further, membranes and other ultrastructural elements may be damaged by the large, anisotropic mechanical stresses produced when their surfaces interact via hydration forces Solutes reduce these stresses by osmotic, volumetric and other effects
TL;DR: In this article, the authors proposed a new philosophy of excellence based on existing information and on new research results, and five principles are suggested as elements behind this philosophy: better indoor air quality increases productivity and decreases SBS symptoms; unnecessary indoor pollution sources should be avoided; the air should be served cool and dry to the occupants; personalized air, i.e. a small amount of clean air, should be serve gently, close to the breathing zone of each individual; individual control of the thermal environment should be provided.
Abstract: Although air-conditioning has played a positive role for economic development in warm climates, its image is globally mixed. Field studies demonstrate that there are substantial numbers of dissatisfied people in many buildings, among them those suffering from Sick Building Syndrome (SBS) symptoms, even though existing standards and guidelines are met. A paradigm shift from rather mediocre to excellent indoor environments is foreseen in the 21st century. Based on existing information and on new research results, five principles are suggested as elements behind a new philosophy of excellence: better indoor air quality increases productivity and decreases SBS symptoms; unnecessary indoor pollution sources should be avoided; the air should be served cool and dry to the occupants; “personalized air”, i.e. a small amount of clean air, should be served gently, close to the breathing zone of each individual; individual control of the thermal environment should be provided. These principles of excellence are compatible with energy efficiency and sustainability.
TL;DR: In this paper, a first principles-based model was developed for a transcritical CO2 gas cooler, using a finite element method, using published correlations for refrigerant and airside heat transfer and pressure drop.
Abstract: A first principles-based model was developed for a transcritical CO2 gas cooler, using a finite element method. The model uses published correlations for refrigerant and airside heat transfer and pressure drop. Experimental results are presented at 48 operating conditions. The model predicted the gas cooler capacity within ±2% and pressure drop on the R-744 side well within the range of experimental error. The model's usefulness is demonstrated by analyzing alternative circuiting and multi-slab designs.
TL;DR: In this article, a multi-bed regenerative adsorption chiller design is proposed to extract the most enthalpy from the low-grade waste heat before it is purged into the drain.
Abstract: A multi-bed regenerative adsorption chiller design is proposed. The concept aims to extract the most enthalpy from the low-grade waste heat before it is purged into the drain. It is also able to minimise the chilled water temperature fluctuation so that downstream temperature smoothing device may be downsized or even eliminated in applications where tighter temperature control may be required. The design also avoids a master-and-slave configuration so that materials invested are not under-utilised. Because of the nature of low-grade waste heat utilization, the performance of adsorption chillers is measured in terms of the recovery efficiency, η instead of the conventional COP. For the same waste heat source flowrate and inlet temperature, a four-bed chiller generates 70% more cooling capacity than a typical two-bed chiller. A six-bed chiller in turn generates 40% more than that of a four-bed chiller. Since the beds can be triggered into operation sequentially during start-up, the risk of ice formation in the evaporator during start-up is greatly reduced compared with that of a two-bed chiller.
TL;DR: In this article, fundamental and practical influence of liquid refrigerant injection on the performance of a refrigerant scroll compressor has been investigated experimentally and theoretically, and it was found that the injection basically increases the compression power and decreases the compressor efficiency.
Abstract: In this study, fundamental and practical influence of liquid refrigerant injection on the performance of a refrigerant scroll compressor has been investigated experimentally and theoretically. In the theoretical analysis, a compression model of vapor/liquid mixture is developed by taking account of heat transfer from the cylinder wall to suction, compression and injection refrigerant. An experiment has been done under the condition of keeping the oil temperature constant in order to investigate the fundamental influence of the liquid refrigerant injection on the compressor performance, and the results were compared with the theoretical ones. It was found that the injection basically increases the compression power and decreases the compressor efficiency, though the situation depends on the condition of the heat transfer to the injection refrigerant. And furthermore, the performance of the liquid refrigerant injection compressor under practical operating condition without controlling the oil temperature has been investigated. Under this condition, the compressor showed recovery and slight improvement of performance due to the decrease of the oil and cylinder temperatures by the injection. In addition, influence of the refrigerant injection on the oil viscosity and refrigerant solubility in the oil, which relate mechanical loss and reliability of the compressor, have been discussed.
TL;DR: In this paper, the authors analyzed the performance of evaporator coils with complex refrigerant circuitry using a distributed simulation model, which has three elements: branch, tube and control volume.
Abstract: This study attempts to analyze the performances of evaporator coils with complex refrigerant circuitry using a distributed simulation model, which has three elements: branch, tube and control volume. The governing equations for a control volume are presented in the paper together with the computer simulation procedure for branches, tubes and control volumes of a coil. The model predictions on four test coils are validated with experimental data collected under different airflow conditions using R134a as a refrigerant. Using this model, the heat transfer and fluid flow characteristics of the coils are studied. The study shows that while the thermal resistance of refrigerant side is comparable to that of airside, the coil comprehensive performance can be improved by changing the refrigerant mass velocity along the flow path. Compared with a common coil, using a complex refrigerant circuitry arrangement where the refrigerant circuits are properly branched or joined may reduce the heat transfer area by around 5% in coil design. A guideline is proposed for the refrigerant circuitry arrangements to improve the coil performance.
TL;DR: In this article, a research project focused on optimization of an existing industrial refrigeration system for a large two-temperature level cold storage distribution facility located near Milwaukee, Wisconsin is presented.
Abstract: This paper is a result of a research project which focused on optimization of an existing industrial refrigeration system for a large two-temperature level cold storage distribution facility located near Milwaukee, Wisconsin. This system utilized a combination of single-screw and reciprocating compressors (each operating under single-stage compression), an evaporative condenser, and a combination of liquid overfeed and direct expansion evaporators. A mathematical model of the existing system was developed. The model was validated using experimental data recorded from the system. Subsequently, the model served as a tool to evaluate alternative system designs and operating strategies that lead to optimum system performance. The methods, analysis, and results presented in this paper focus on evaporative condenser sizing and head pressure control. Operating system head pressures that minimize the energy costs of the system were found to be a linear function of the outdoor wet-bulb temperature. A methodology for implementing the optimum control strategy is presented. Simulation results for the annual performance of the refrigeration system investigated in this project show a reduction in annual energy consumption by 11% as a result of the recommended design and control changes.
TL;DR: In this paper, a finite element method is employed for the analysis of velocity and temperature distributions in refrigerated open display cabinets, based on the stream function-vorticity formulation, and incorporating a LES turbulence model.
Abstract: The finite element method is employed for the analysis of velocity and temperature distributions in refrigerated open display cabinets. The CFD code is based on the streamfunction-vorticity formulation, and incorporates a LES turbulence model. As an example of application, a vertical multi-deck cabinet is investigated under different operating conditions. The numerical results have been validated by comparison with experimental tests performed in accordance with the EN441 Standard. The influence of various design parameters has been investigated.
TL;DR: Improvements to the method of cycle specification and solution have enhanced considerably the convergence capability with large and complex cycles, and additional units and working fluids have been added, resulting in much-enhanced simulation capability and applicability.
Abstract: The computer code ABSIM has been developed for simulation of absorption systems in a flexible and modular form, making it possible to investigate various cycle configurations with different working fluids. Based on a user-supplied cycle diagram, working fluid specification and given operating conditions, the program calculates the temperature, flowrate, concentration, pressure and vapor fraction at each state point in the system and the heat duty at each component. The modular structure of the code is based on unit subroutines containing the governing equations for the system's components. A main program calling these subroutines links the components together according to the cycle diagram. The system of equations for the entire cycle is thus established, and a mathematical solver routine is employed to solve them simultaneously. Property subroutines contained in a separate database serve to provide thermodynamic properties of the working fluids. ABSIM has been employed over the past decade by many users worldwide to simulate a variety of absorption systems in different multi-effect configurations and working fluids. The paper will describe the current capabilities of the program and recent improvements made in it. Improvements to the method of cycle specification and solution have enhanced considerably the convergence capability with large and complex cycles. Additional units and working fluids have been added, resulting in much-enhanced simulation capability and applicability. A Windows version has recently been developed with an improved user-interface, which enhances user-friendliness considerably. It makes it possible to create the cycle diagram on the computer screen, supply the data interactively, observe the results superimposed on the cycle diagram and plot them. The paper describes examples of simulation results for several rather complex cycles, including lithium bromide–water double-, triple- and quadruple-effect cycles and ammonia–water GAX, branched GAX and vapor exchange (VX) cycles.
TL;DR: In this paper, a refrigeration/heat-pump system based on a number of simple tubular adsorption modules is described, consisting of a generator and a receiver/condenser/evaporator.
Abstract: A refrigeration/heat-pump system based on a number of simple tubular adsorption modules is described. A single module is comprised of a generator and a receiver/condenser/evaporator. A single generator consisting of a 12.7 mm stainless steel tube lined with 3 mm of monolithic active carbon has been manufactured. A complete module has been tested in a simple rig, which subjects it to alternating hot and cold airstreams, desorbing and adsorbing ammonia. A complete system, consisting of 32 modules has been modelled in detail and its predicted performance is presented. Key parameters have been varied and their effect on the performance discussed.
TL;DR: In this paper, an extensive investigation about the performance of various fin configurations, carried out in the Luve Contardo experimental facilities and aimed to enhance the heat transfer capabilities of air-cooled condensers and liquid coolers.
Abstract: The present paper discusses the results of an extensive investigation about the performance of various fin configurations, carried out in the Luve Contardo experimental facilities and aimed to enhance the heat transfer capabilities of air-cooled condensers and liquid coolers. Test results here discussed are relative to 15 coil prototypes, having the same tube and fin geometry (25×21.65 mm staggered 5/8” tube banks, 2 mm fin spacing) but different fin surface geometry, from flat to wavy to louvered to “winglet”. Different rates of heat transfer and pressure loss enhancement were obtained, also depending on the quality of the pressing process. General approaches to evaluate the “goodness” of one fin design with respect to another one provided questionable results: pressure loss influence on the air flow cannot be properly evaluated unless the actual fan head curve and the coil dimensions (front area and rows number) are stipulated. The performance of air-cooled condensers was therefore predicted and compared, for various fin design and for coil arrangements of practical interest. The type of fin adopted strongly influences the heat exchanger performance and louvered fins generally provide the best results.
TL;DR: In this article, a steady-state model for predicting the performance of vapour-compression liquid chillers over a wide range of operating conditions is presented, where the model requires only those inputs that are readily available to the user (e.g. condenser inlet water temperature and evaporator water outlet temperature).
Abstract: This paper presents a steady-state model for predicting the performance of vapour-compression liquid chillers over a wide range of operating conditions. The model overcomes the idealisations of previous models with regard to modelling the heat exchangers. In particular, it employs an elemental NTU- e methodology to model both the shell-and-tube condenser and evaporator. The approach allows the change in heat transfer coefficients throughout the heat exchangers to be accounted for, thereby improving both physical realism and the accuracy of the simulation model. The model requires only those inputs that are readily available to the user (e.g. condenser inlet water temperature and evaporator water outlet temperature). The outputs of the model include system performance variables such as the compressor electrical work input and the coefficient of performance ( COP ) as well as states of the refrigerant throughout the refrigeration cycle. The methodology employed within the model also allows the performance of chillers using refrigerant mixtures to be modelled. The model is validated with data from one single screw chiller and one twin-screw chiller where the agreement is found to be within ±10%.
TL;DR: In this article, the conditions for the formation of ice particles were investigated theoretically by the diffusion-controlled evaporation model and the prediction of the model was proved to agree relatively well with experiments in which a droplet of initial temperature 20°C and size 50 μm to change into an ice particle in a chamber of height 1.33 m.
Abstract: A theoretical and experimental study was performed to examine the water spray method of ice slurry production. First, the conditions for the formation of ice particles were investigated theoretically by the diffusion-controlled evaporation model. The prediction of the model was proved to agree relatively well with experiments in which we examined the conditions for a droplet of initial temperature 20°C and size 50 μm to change into an ice particle in a chamber of height 1.33 m. Second, the production of cold storage heat will increase almost proportionally to the number of spray nozzles because no substantial difference was found in the Sauter Mean Diameter (SMD) of sprays from single and twin nozzle. Third, an ice slurry was experimentally obtained by spraying droplets of 7% ethylene glycol aqueous solution in a vacuum chamber where pressure is maintained below the freezing point of the solution. Finally, based on the theoretical and experimental results, we propose an optimizing chart for providing the operating conditions to make ice slurry using the relations of the staying time of the droplet in the chamber, the injection pressure, the spray droplet size and the chamber pressure.
TL;DR: In this paper, a model for predicting the maximum flow ratio of the ejector was developed to calculate the mass flow ratio, taking into account the performance of the primary nozzle, the flow entrainment and mixing relating to ejector geometry and operating conditions.
Abstract: The occurrence of flow choking in an ejector of an ejector refrigeration system (ERS) was analysed and a model for predicting the maximum flow ratio of the ejector was developed. The multi-parameter equation to calculate the mass flow ratio takes into account the performance of the primary nozzle, the flow entrainment and mixing relating to ejector geometry and operating conditions. We validated the model using the reported experimental data of refrigerant R113, R141b and steam ERS. The present model was shown to provide better accuracy compared with results obtained by applying the existing 1-D ejector theory. We discussed the application of the model and highlighted the significance of the parameters for future work.
TL;DR: In this article, the performance of an innovative evaporatively cooled condenser is compared with that of a conventional air-cooled condenser for a split heat pump system in an environmentally controlled test chamber that was able to simulate test conditions as specified by ASHRAE Standard 116.
Abstract: In this paper, the performance of an innovative evaporatively cooled condenser is compared with that of a conventional air-cooled condenser for a split heat pump system. The system was tested in an environmentally controlled test chamber that was able to simulate test conditions as specified by ASHRAE Standard 116. Tests to optimize refrigerant charge and short tube restrictor size were conducted using refrigerant HCFC-22. The wheel rotation speed of the evaporative condenser was also optimized experimentally to maximize the coefficient of performance. Using these optimum parameters, steady state and cyclic performance tests were conducted. The experimental results showed that the evaporative condenser has a higher capacity than the air-cooled condenser by 1.8 to 8.1%, a higher COP by 11.1 to 21.6%, and a higher SEER by 14.5%.
TL;DR: In this paper, the thickness and surface temperature of the frost layer around a horizontal cylinder were measured by varying the Reynolds number, temperature, and humidity, while the dew point temperature of inlet air was kept below the freezing point throughout the experiment.
Abstract: Frost layers formed on the front and rear surfaces of a horizontal cylinder during cross flow are found to be thicker than those at the top and bottom surfaces where the flow separation is nearly initiated. This observation was obtained in an experimental study carried out to examine frost formation on a horizontal cylinder given a cross flow condition. The thickness of the frost layer and the temperature distribution in the cylinder were measured for various experimental conditions. The local heat flux around the cylinder and the effective thermal conductivity of the frost layer were likewise evaluated, while thickness and surface temperature of the frost layer around the cylinder were measured periodically. These measurements were obtained by varying the Reynolds number, temperature, and humidity. The dew point temperature of the inlet air, however, was kept below the freezing point throughout the experiment. Results also reveal that inlet air velocity, temperature, and humidity affect thickness and thermal conductivity of the frost layer.
TL;DR: In this article, a combined cycle refrigeration system (CCRS) that comprises a conventional refrigeration and air-conditioning system using mechanical compressor (RAC/MC) and an ejector cooling cycle (EJC) is proposed and studied.
Abstract: A combined-cycle refrigeration system (CCRS) that comprises a conventional refrigeration and air-conditioning system using mechanical compressor (RAC/MC) and an ejector-cooling cycle (EJC) is proposed and studied. The EJC is driven by the waste heat from the RAC/MC and acts as the bottom cycle of the RAC/MC. A system analysis shows that the COP of a CCRS is significantly higher than a single-stage refrigeration system. Improvement in COP can be as high as 18.4% for evaporating temperature of the RAC/MC Te at −5°C. A prototype of the CCRS was built and tested in the present study. Experimental results show that at Te=−4.5°C, COP is improved by 14% for a CCRS. For Te at 5°C, COP can be improved by 24% for a CCRS with higher condensing temperature of the RAC/MC. The present study shows that the CCRS using the ejector-cooling cycle as the bottom cycle of the RAC/MC is viable. Further improvement in COP is possible since the prototype is not designed and operated at an optimal condition.
TL;DR: In this article, the authors presented pool boiling heat transfer data for 12 different R134a/lubricant mixtures and pure R-134a on a Turbo-BII™-HP surface.
Abstract: This paper presents pool boiling heat transfer data for 12 different R134a/lubricant mixtures and pure R134a on a Turbo-BII™-HP surface. The mixtures were designed to examine the effects of lubricant mass fraction, viscosity, and miscibility on the heat transfer performance of R134a. The magnitude of the effect of each parameter on the heat transfer was quantified with a regression analysis. The mechanistic cause of each effect was given based on new theoretical interpretation and/or one from the literature. The model illustrates that large improvements over pure R134a heat transfer can be obtained for R134a/lubricant mixtures with small lubricant mass fraction, high lubricant viscosity, and a large critical solution temperature (CST). The ratio of the heat flux of the R134a/lubricant mixture to that of the pure R134a for fixed wall superheat was given as a function of pure R134a heat flux for all 12 mixtures. The lubricant that had the largest CST with R134a exhibited the greatest heat transfer: 100%±20% greater than that of pure R134a. By contrast, the heat transfer of the mixture with the lubricant that had the smallest viscosity and the smallest CST with R134a was 55%±9% less than that of pure R134a. High-speed films of the pure and mixture pool boiling were taken to observe the effect of the lubricant on the nucleate boiling.
TL;DR: In this paper, a numerical investigation using a computational fluid dynamics (CFD) code is carried out to predict the turbulent flow field, and heat and moisture transfer in a three-dimensional air-blast chiller with cooked meats of cylindrical and elliptical shapes.
Abstract: A numerical investigation using a computational fluid dynamics (CFD) code is carried out to predict the turbulent flow field, and heat and moisture transfer in a three-dimensional air-blast chiller with cooked meats of cylindrical and elliptical shapes. Three turbulent models [standard, low Reynolds number (LRN) and RNG k-ϵ model] have been used in these simulations. Based on local heat transfer coefficients on the surface of the meat calculated by CFD code, the unsteady heat and mass transfer were simulated which took into account of the effects of conduction within the meat, forced and natural convection, radiation and moisture evaporation on the surface of the cooked meat joint. The model allowed the simultaneous CFD prediction of both temperature distribution and weight loss of the meat throughout the chilling process. Good agreement with experimental results was obtained. The effect of using different models on the accuracy of the simulation of local heat transfer coefficient is presented.
TL;DR: In this paper, the coefficients d'echange locaux dependent fortement du flux impose aux faibles titres and de la vitesse massique aux titres eleves.
Abstract: Resume Une etude experimentale des coefficients d'echange locaux lors de l'ebullition en convection forcee d'un corps pur (R22) et d'un melange non-azeotropique (R407C) est presentee. L'ebullition a lieu a l'interieur de tubes horizontaux, de 12,7 mm de diametre, lisses ou a micro-ailettes. Les essais ont ete realises a densite de flux thermique uniforme sur toute la longueur du tube, variant de 10 a 30 kW m−2, et des vitesses massiques comprises entre 100 et 300 kg m−2 s−1. Les coefficients d'echange locaux dependent fortement du flux impose aux faibles titres et de la vitesse massique aux titres eleves. Par rapport au tube lisse, les tubes micro-ailetes conduisent a une amelioration des echanges thermiques allant jusqu'a 180%. Les coefficients d'echange des tubes lisse et micro-ailetes lors de l'ebullition du R407C sont inferieurs a ceux du R22, respectivement de 15 a 35%. Les meilleures ameliorations sont obtenues aux faibles vitesses massiques et densites de flux.