Showing papers in "International Journal of Refrigeration-revue Internationale Du Froid in 2011"
TL;DR: In this paper, the authors reviewed the existing numerical modeling of room temperature active magnetic regenerator (AMR) to date and concluded that more detailed modeling of these phenomena is required to obtain a better understanding of the AMR cycle.
Abstract: The active magnetic regenerator (AMR) is an alternative refrigeration cycle with a potential gain of energy efficiency compared to conventional refrigeration techniques. The AMR poses a complex problem of heat transfer, fluid dynamics and magnetic field, which requires detailed and robust modeling. This paper reviews the existing numerical modeling of room temperature AMR to date. The governing equations, implementation of the magnetocaloric effect (MCE), fluid flow and magnetic field profiles, thermal conduction etc. are discussed in detail as is their impact on the AMR cycle. Flow channeling effects, hysteresis, thermal losses and demagnetizing fields are discussed and it is concluded that more detailed modeling of these phenomena is required to obtain a better understanding of the AMR cycle.
197 citations
TL;DR: In this paper, the authors reviewed the major research on refrigerant injection techniques in detail and compared them with liquid and vapor refrigerant injections and compared the performance of different types of compressors and variable speed compressors.
Abstract: This paper reviews the major research on refrigerant injection techniques in detail. Liquid and vapor refrigerant injection techniques are discussed and compared. The current research on refrigerant injection techniques falls into two categories: system level research and component level research. The system level research is focused on low ambient temperature heating, heat pump water heating, high ambient temperature cooling, cycle comparison, and control strategy development. Internal heat exchanger and flash tank cycles are the two typical cycles for refrigerant injection. These two cycles are discussed and compared in detail. The component level research is focused on employing different types of compressors, variable speed compressors, the injection process, and the flash tank. Different types of compressors employing refrigerant injection are presented. Based on the literature study, the potential future research directions are presented and discussed. The flash tank cycle control strategy and refrigerant charge management strategy are worth further research efforts. Compressor design can be improved in order to optimize the performance with refrigerant injection. The appropriate design of flash tanks plays a vital role in achieving appropriate two-phase flow patterns in the flash tank. Computational Fluid Dynamics (CFD) modeling can be a useful tool to facilitate the design of the flash tank.
192 citations
TL;DR: In this article, the authors present the latest analytical and experimental results of a comprehensive study carried out to investigate possible performance improvements of transcritical R744 two-phase ejector systems.
Abstract: This paper gives an overview of historical and present developments on how ejectors can be utilized to improve the performance of air-conditioning and refrigeration systems. Research on ejector refrigeration cycles that utilize low-grade energy sources to produce cooling is summarized. Another major class of ejector refrigeration cycles that is described tries to recover expansion work by means of a two-phase ejector. This particular approach appears to be very promising for transcritical carbon dioxide (CO 2 , R744) systems with inherently large throttling losses. The paper further presents the latest analytical and experimental results of a comprehensive study carried out to investigate possible performance improvements of transcritical R744 two-phase ejector systems. Relevant operational parameters were varied and effects on performance resulting from different ejector geometries were studied as well. Two-phase mixing shock waves inside the ejector were detected by recoding static wall pressure distributions.
171 citations
TL;DR: Magnetic refrigeration (MR) using permanent magnets is investigated for near-room temperature applications as mentioned in this paper, and an MR prototype is described and results using gadolinium as refrigerant are presented.
Abstract: Magnetic refrigeration (MR) using permanent magnets is being investigated for near-room temperature applications. An MR prototype is described and results using gadolinium as refrigerant are presented. Important design features are simple sealing, compactness, high operating frequencies, and ease of use. Using a total of 110 g of refrigerant, the device produces a maximum temperature span under no thermal load of 29 °C, and 10 °C under 50 W. The overall COP determined by using the power to the drive motor is between 0.3 and 0.8 under most operating conditions and temperature spans. The maximum COP measured is 1.6 with a span of 2.5 °C when operating at 1.4 Hz and utilization of 1.28. If the inefficiency of the motor is removed the maximum COP is 2.2, and, when the magnet drive losses are excluded, the maximum COP becomes 10. The peak specific exergetic cooling power is 0.085 W T−1 cm−3.
152 citations
TL;DR: In this article, a prototype of a cascade refrigeration system using NH 3 and CO 2 as refrigerants has been designed and built, which is used to supply a 9-kW refrigeration capacity horizontal plate freezer at an evaporating temperature of −50-°C as design conditions.
Abstract: A prototype of a cascade refrigeration system using NH 3 and CO 2 as refrigerants has been designed and built. The prototype is used to supply a 9 kW refrigeration capacity horizontal plate freezer at an evaporating temperature of −50 °C as design conditions. The prototype includes a specific control system and a data acq*uisition system. The experimental evaluation started with the real conditions within the design operating parameters. Subsequently, several tests were performed fixing four CO 2 evaporating temperatures (−50, −45, −40 and −35 °C). At each one of the evaporating temperatures evaluated, the CO 2 condensing temperature was varied from −17.5 to −7.5 °C and an experimental optimum value of CO 2 condensing temperature was determined. The discussions on the experimental results include the influence of the operating parameters on the cascade system’s performance. In addition, the experimental results are compared with two common double stage refrigeration systems using NH 3 as refrigerant.
120 citations
TL;DR: In this paper, the authors measured the heating performance of air-source heat pumps in which novel vapor injection techniques of a combined flash tank and sub-cooler (FTSC) cycle and a double expansion subcooler cycle were applied.
Abstract: The performance degradation of air-source heat pumps cannot be avoided when they operate at both very low and high ambient temperatures. The refrigerant injection technique has rapidly developed in recent years due to its outstanding performance at low ambient temperatures. This study measured the heating performance of air-source heat pumps in which novel vapor injection techniques of a combined flash tank and sub-cooler (FTSC) cycle and a double expansion sub-cooler (DESC) cycle were applied. The performance of these cycles was compared with that of a flash tank (FT) and a sub-cooler (SC) cycle. The average heating capacities of the FT, FTSC, and DESC cycles were higher by 14.4%, 6.0%, and 3.8%, respectively, relative to that of the SC cycle, but the average COPs for the respective cycle options were very similar.
118 citations
TL;DR: In this article, the effect of mixing length on ejector system performance was analyzed experimentally, and the experiments were performed for both ejector and conventional expansion systems with and without internal heat exchanger (IHX) at different operating conditions.
Abstract: In ejector system using the promising natural refrigerant CO 2 , the mixing of high-speed two-phase primary flow and suction vapor is crucial in designing an efficient ejector. In this study, the effect of mixing length on ejector system performance was analyzed experimentally. The mixing lengths used were 5 mm, 15 mm, and 25 mm, with constant rectangular cross-section. The experiments were performed for both ejector and conventional expansion systems with and without internal heat exchanger (IHX) at different operating conditions. Based on the experimental results, mixing length had significant effect on entrainment ratio and on magnitude and profile of pressure recovery. The 5 mm and 15 mm types yielded the lowest and highest ejector efficiency and COP in all of the conditions used in this research, respectively. The use of IHX had net positive effect on system performance which verified the results of our previous study. A COP improvement of up to 26% over conventional system was obtained but improper sizing of mixing length lowered the COP by as much as 10%.
117 citations
TL;DR: In this paper, the performance of three magnetocaloric material candidates for AMRs, La(Fe,Co,Si)13, (La,Ca,Sr)MnO3 and Gd, were compared in an experimental active magnetic regenerator with a parallel plate geometry.
Abstract: Magnetic refrigeration is a potentially environmentally-friendly alternative to vapor compression technology because it has a potentially higher coefficient of performance and does not use a gaseous refrigerant. The active magnetic regenerator refrigerator is currently the most common magnetic refrigeration device for near room temperature applications, and it is driven by the magnetocaloric effect in the regenerator material. Several magnetocaloric materials with potential magnetic refrigeration applications have recently been developed and characterized; however, few of them have been tested in an experimental device. This paper compares the performance of three magnetocaloric material candidates for AMRs, La(Fe,Co,Si)13, (La,Ca,Sr)MnO3 and Gd, in an experimental active magnetic regenerator with a parallel plate geometry. The performance of single-material regenerators of each magnetocaloric material family were compared. In an attempt to improve system performance, graded two-material regenerators were made from two different combinations of La(Fe,Co,Si)13 compounds having different magnetic transition temperatures. One combination of the La(Fe,Co,Si)13 materials yielded a higher performance, while the performance of the other combination was lower than the single-material regenerator. The highest no-load temperature span was achieved by the Gd regenerator.
114 citations
TL;DR: In this article, the performance of an Internal Heat Exchanger (IHX) operating in a CO 2 transcritical refrigeration plant is analyzed, from an energetic point of view, in this work.
Abstract: The performance of an Internal Heat Exchanger (IHX) operating in a CO 2 transcritical refrigeration plant is analysed, from an energetic point of view, in this work. The evaluation is based on experimental data by contrasting the performance of the plant working with (44 tests) and without the IHX (46 tests) at the same operating conditions. The experimental evaluation covers three evaporating levels (−5, −10 and −15 °C), at two different gas-cooler outlet temperatures each (31, 34 °C), for a wide range of gas-cooler operating pressures (74.5–105.9 bar). The thermal effectiveness of the IHX is empirically analysed for the different operating conditions in the first part of the paper. Moreover, the relation of its effectiveness with the operating parameters is presented. The second part is devoted to analyse the modification of the energetic performance of the plant caused by the IHX. The results show a maximum increment on cooling capacity of 12%, an increment of the efficiency of the plant up to 12% and a maximum increase on discharge temperature of 10 °C at −15 °C of evaporating temperature.
109 citations
TL;DR: In this paper, the performance of a combined solid desiccant-indirect evaporative cooler system is analyzed by solving the heat and mass transfer equations for both components simultaneously. But the authors focus on varying the descant wheel supply/regeneration and indirect cooler secondary/primary air-flow ratios.
Abstract: Solar desiccant-based air-conditioning has the potential to significantly reduce cost and/or greenhouse gas emissions associated with cooling of buildings Parasitic energy consumption for the operation of supply fans has been identified as a major hindrance to achieving these savings The cooling performance is governed by the trade-off between supplying larger flow-rates of cool air or lower flow-rates of cold air The performance of a combined solid desiccant-indirect evaporative cooler system is analysed by solving the heat and mass transfer equations for both components simultaneously Focus is placed on varying the desiccant wheel supply/regeneration and indirect cooler secondary/primary air-flow ratios Results show that for an ambient reference condition, and 70 °C regeneration temperature, a supply/regeneration flow ratio of 067 and an indirect cooler secondary/primary flow ratio of 03 gives the best performance with COPe > 20 The proposed cooling system thus has potential to achieve substantial energy and greenhouse gas emission savings
109 citations
TL;DR: In this article, the authors present energy and exergy analyses and sustainability assessment of the novel evaporative air cooling system based on Maisotsenko cycle which allows the product fluid to be cooled in to a dew point temperature of the incoming air.
Abstract: This study presents energy and exergy analyses and sustainability assessment of the novel evaporative air cooling system based on Maisotsenko cycle which allows the product fluid to be cooled in to a dew point temperature of the incoming air. In the energy analysis, Maisotsenko cycle’s wet-bulb and dew point effectiveness, COP and primary energy ratio rates are calculated. Exergy analysis of the system is then carried out for six reference temperatures ranging from 0 °C to 23.88 °C as the incoming air (surrounding) temperature. The specific flow exergy, exergy input, exergy output, exergy destruction, exergy loss, exergy efficiency, exergetic COP, primary exergy ratio and entropy generation rates are determined for various cases. Furthermore, sustainability assessment is obtained using sustainability index method. As a result, maximum exergy efficiency is found to be 19.14% for a reference temperature of 23.88 °C where the optimum operation takes place.
TL;DR: The results have shown that the multi-objective design more acceptably satisfies generalized engineering criteria than other two single-objectives optimized designs.
Abstract: A cooling tower assisted vapor compression refrigeration machine has been considered for optimization with multiple criteria. Two objective functions including the total exergy destruction of the system (as a thermodynamic criterion) and the total product cost of the system (as an economic criterion), have been considered simultaneously. A thermodynamic model based on energy and exergy analyses and an economic model according to the Total Revenue Requirement (TRR) method have been developed. Three optimized systems including a single-objective thermodynamic optimized, a single-objective economic optimized and a multi-objective optimized are obtained. In the case of multi-objective optimization, an example of decision-making process for selection of the final solution from the Pareto frontier has been presented. The exergetic and economic results obtained for three optimized systems have been compared and discussed. The results have shown that the multi-objective design more acceptably satisfies generalized engineering criteria than other two single-objective optimized designs.
TL;DR: In this article, a number of new desiccant materials have been proposed which have the potential to improve the performance of desiccants being regenerated at low temperature, such as superadsorbent polymer and zeolite materials.
Abstract: A number of new desiccant materials have been proposed which have the potential to improve the performance of desiccant wheels being regenerated at low temperature. Desiccant wheels containing two such desiccant materials (zeolite and superadsorbent polymer) were compared with a conventional silica gel desiccant wheel. The superadsorbent polymer desiccant wheel achieved greater dehumidification than the silica gel wheel when dehumidifying high relative humidity air with low temperature (50 °C) regeneration air. The temperature of dehumidified air exiting the polymer wheel was also lower. The zeolite desiccant wheel was generally less effective at dehumidifying air and had a higher pressure drop.
TL;DR: This study employs feature selection techniques, such as mutual-information-based filter and genetic-algorithm-based wrapper, to help search for the important sensors in data driven chiller FDD applications, so as to improve FDD performance while saving initial sensor cost.
Abstract: The benefits of applying automated fault detection and diagnosis (AFDD) to chillers include less expensive repairs, timely maintenance, and shorter downtimes. This study employs feature selection (FS) techniques, such as mutual-information-based filter and genetic-algorithm-based wrapper, to help search for the important sensors in data driven chiller FDD applications, so as to improve FDD performance while saving initial sensor cost. The ‘one-against-one’ multi-class support vector machine (SVM) is adopted as a FDD tool. The results show that the eight features/sensors, centered around the core refrigeration cycle and selected by the GA-SVM wrapper from the original 64 features, outperform the other three feature subsets by the GA-LDA (linear discriminant analysis) wrapper, with an overall classification correct rate (CR) as high as 99.53% for the 4000 test samples randomly covering the normal and seven typical faulty modes. The CRs for the four cases with FS are all higher than that without FS (97.45%) and the test time is much less, about 28–36%. The FDD performance for normal or each of the faulty modes is also evaluated in details in terms of hit rate (HR) and false alarm rate (FAR).
TL;DR: In this article, a novel cascaded absorption/vapor-compression cycle with a high temperature lift for a naval ship application was conceptualized and analyzed, and a thermodynamic model was developed to analyze the performance of the cascaded system, and parametric analyses were conducted to estimate the performance over a range of operating conditions.
Abstract: A novel cascaded absorption/vapor-compression cycle with a high temperature lift for a naval ship application was conceptualized and analyzed. A single-effect LiBr–H 2 O absorption cycle and a subcritical CO 2 vapor-compression cycle were coupled together to provide low-temperature refrigerant (−40 °C) for high heat flux electronics applications, medium-temperature refrigerant (5 °C) for space conditioning and other low heat flux applications, and as an auxiliary benefit, provide medium-temperature heat rejection (∼48 °C) for water heating applications. A thermodynamic model was developed to analyze the performance of the cascaded system, and parametric analyses were conducted to estimate the performance of the system over a range of operating conditions. The performance of the cascaded system was also compared with an equivalent two-stage vapor-compression cycle. This cycle was found to exhibit very high COPs over a wide range of operating conditions and when compared to an equivalent vapor-compression system, was found to avoid up to 31% electricity demand.
TL;DR: In this article, the authors compared the falling film absorption between ammonia and water with various kinds of nano-particles and found that the effect of nanofluid is more obvious compared to that without nanoparticles.
Abstract: Based on the preparation of Al 2 O 3 , Fe 2 O 3 and ZnFe 2 O 4 nanofluid, the comparative experiments on the falling film absorption between ammonia–water and ammonia–water with various kinds of nano-particles are carried out. Experimental results show that the sorts and mass fraction of nano-particles, the viscosity and stability of nanofluid, as well as the mass fraction of ammonia in the basefluid are considered as the key parameters. The absorption of ammonia is weakened by only adding surfactants or adding poorly dispersed nano-particles. The increase of mass fraction of nano-particles with matched surfactants can improve the absorption rate of ammonia under the condition that the viscosity of nanofluid does not increase remarkably, and there is an optimal mass fraction for each kind of nano-particles and surfactant. With the increase in ammonia mass fraction of initial nanofluid, the absorption potential capacity decline, but the enhancing effect induced by the nanofluid is more obvious compared to that without nano-particles. The effective absorption ratio can be increased by 70% and 50% with Fe 2 O 3 and ZnFe 2 O 4 nanofluid respectively when the initial ammonia mass fraction is 15%. The absorption enhancement by the nanofluid is attributable to the heat transfer enhancement and the decrease in viscosity of nanofluid, which are strongly proved by the temperature differences in cooling water and nanofluids as well as the falling film flowing time.
TL;DR: In this paper, the nanoparticles (i.e., SiO2 and Al2O3 nanoparticles) and methanol are combined into SiO 2/methanol and Al 2O3/mETHanol nanofluids to enhance the CO2 absorption rate of the base fluid (Methanol), and absorption experiments are performed in the bubble type absorber system equipped with mass flow controller (MFC), mass flow meter (MFM), and silica gel (which can remove the methanoline vapor existing in the outlet gases).
Abstract: In this study, the nanoparticles (i.e. SiO2 and Al2O3 nanoparticles) and methanol are combined into SiO2/methanol and Al2O3/methanol nanofluids to enhance the CO2 absorption rate of the base fluid (methanol). The absorption experiments are performed in the bubble type absorber system equipped with mass flow controller (MFC), mass flow meter (MFM) and silica gel (which can remove the methanol vapor existing in the outlet gases). The parametric analysis on the effects of the particle species and concentrations on CO2 bubble absorption rate is carried out. The particle concentration ranges from 0.005 to 0.5 vol%. It is found that the CO2 absorption rate is enhanced up to 4.5% at 0.01 vol% of Al2O3/methanol nanofluids at 20 °C, and 5.6% at 0.01 vol% of SiO2/methanol nanofluids at −20 °C, respectively.
TL;DR: In this paper, a CO 2 air/water heat pump for the production of tap hot water in a residential building is described, which consists of a single-stage piston compressor, a coaxial type gas cooler, an electronic expansion valve, a finned tube evaporator and a low pressure receiver.
Abstract: This paper describes the development of a CO 2 air/water heat pump for the production of tap hot water in a residential building. The basic design consists of a single-stage piston compressor, a coaxial type gas cooler, an electronic expansion valve, a finned tube evaporator and a low pressure receiver. The heat pump is combined with a storage tank designed to maintain internal water stratification. The gas cooler pressure optimisation in the case of fixed water delivery temperature was theoretically analysed. A new control method for the upper cycle pressure was developed to maximise the COP of the heat pump, while the water mass flow was adjusted to maintain the set water temperature at the gas cooler exit. Before commissioning, the heat pump was factory tested to verify its energy performance and to validate the high pressure control logic.
TL;DR: In this paper, a model of a miniature-scale linear compressor for electronics cooling is presented, which incorporates all of the major components of the linear compressor including dynamics associated with the piston motion.
Abstract: A comprehensive model of a miniature-scale linear compressor for electronics cooling is presented. Linear compressors are appealing for refrigeration applications in electronics cooling. A small number of moving components translate to less theoretical frictional losses and the possibility that this technology could scale to smaller physical sizes better than conventional compressors. The model developed here incorporates all of the major components of the linear compressor including dynamics associated with the piston motion. The results of the compressor model were validated using experimental data from a prototype linear compressor. The prototype compressor has an overall displacement of approximately 3 cm 3 , an average stroke of 0.6 cm. The prototype compressor was custom built for this work and utilizes custom parts with the exception of the mechanical springs and the linear motor. The model results showed good agreement when validated against the experimental results. The piston stroke is predicted within 1.3% MAE. The volumetric and overall isentropic efficiencies are predicted within 24% and 31%, MAE respectively.
TL;DR: In this article, the effect of storage temperature on kinetic parameters was successfully described by the Arrhenius equation, and the results showed significant alterations in broccoli green colour, vitamin C content and drip loss were observed.
Abstract: Studies were undertaken on colour CIE L*a*b* values, vitamin C (ascorbic acid) and drip loss alterations of frozen broccoli (Brassica oleracea L. ssp. Italica) stored at isothermal (−7, −15, and −25 °C) and non-isothermal (accelerated life testing with step-stress methodology; temperature range from −30 to −5 °C) conditions. The storage temperatures were selected according to conditions that occur in the cold chain. Frozen storage at all regimes had significant impact on all quality parameters analysed. Significant alterations in broccoli green colour, vitamin C content and drip loss were observed. Experimental data of h* colour degradation and drip loss (%) at isothermal conditions and a ∗ / a 0 ∗ at non-isothermal conditions could be modelled by zero order kinetics. A first order kinetic model was adequate for the remaining quality factors and temperature regimes. The effect of storage temperature on kinetic parameters was successfully described by the Arrhenius equation.
TL;DR: In this article, the experimental and CFD results for a 5 kW capacity steam ejector with variable primary nozzle geometry are compared and compared and the variable geometry was achieved by applying a movable spindle at the primary nozzle inlet.
Abstract: In the present paper, experimental and CFD results for a 5 kW capacity steam ejector with variable primary nozzle geometry are presented and compared. The variable geometry was achieved by applying a movable spindle at the primary nozzle inlet. Operating conditions were considered in a range that would be suitable for an air-conditioning application, with thermal energy supplied by vacuum tube solar collectors. The CFD model was based on the axi-symmetric representation of the experimental ejector, using water as working fluid. The experimental entrainment ratio varied in the range of 0.1–0.5 depending on operating conditions and spindle tip position. It was found that the primary flow rate can be successfully adjusted by the spindle. CFD and experimental primary flow rates agreed well, with an average relative error of 7.7%. CFD predicted the secondary flow rate and entrainment ratio with good accuracy only in 70% of the cases.
TL;DR: In this paper, the authors investigated the influence of the geometrical parameters on vortex tube refrigeration capacity by using nitrogen as the working fluid and found that there is an optimum angle for obtaining the highest refrigeration performance, and 4° is the optimal candidate under their experimental conditions.
Abstract: Energy separation performance of vortex tube can be improved by using a divergent hot tube. Experiments are carried out to investigate the influence of the geometrical parameters on vortex tube refrigeration capacity by using nitrogen as the working fluid. In this work, the parameters are focused on the divergence angle of hot tube, length of divergent hot tube and number of nozzle intakes. Experimental results present that there is an optimum angle for obtaining the highest refrigeration performance, and 4° is the optimal candidate under our experimental conditions. Divergent tube length which exceeds a critical length has slight effect on the refrigeration capacity. The critical length to diameter ratio is L/D = 12 in our experiment. Increasing number of nozzle intakes increases the sensitivity of temperature reduction and can obtain the highest possible temperature reduction. Moreover, similarity relations for the prediction of the cold exit stream are presented and confirmed by the experimental data.
TL;DR: In this article, the effect of internal heat exchanger (IHX) in the performance of ejector refrigeration system was analyzed experimentally and compared with conventional expansion refrigeration systems.
Abstract: The performance of CO2 ejector refrigeration system needs further improvement to make CO2 more viable than traditional harmful refrigerants. In this research, the effect of internal heat exchanger (IHX) in the performance of ejector refrigeration system was analyzed experimentally and compared with conventional expansion refrigeration system. Experiments were performed at different operating pressure and temperature for the cases of without IHX, 30 cm IHX and 60 cm IHX. The results showed that IHX significantly increased the coefficient of performance (COP) of ejector system. At the conditions used in this research, the ejector system with 60 cm IHX provided the maximum COP improvement of up to 27% compared to similar conventional system. The motive nozzle’s inlet condition had significant effect on the performance of ejector system. The results also confirmed the presence of considerable amount of liquid refrigerant at separator’s gas outlet of ejector system which was deemed possible in our previous research.
TL;DR: In this paper, an analytical model for determination of the performance and optimum dimensions of porous fin with consideration of different models of predictions has been presented in a comparative way so that the merit of the models adopting in the present work can easily be understood.
Abstract: Enhancement of heat transfer rate through porous fins is one of the common choices nowadays As the energy equation is highly nonlinear, researchers never concentrated on analyzing porous fins analytically In the present study, an effort has been devoted to develop an analytical model for determination of the performance and optimum dimensions of porous fins with consideration of different models of predictions Every result has been presented in a comparative way so that the merit of the models adopting in the present work can easily be understood The optimum design analysis of porous fins has also been carried out The influences of all the dependent parameters on the performances and optimization conditions have been studied for the selection of a design criterion of porous fins in such applications where the requirement of heat dissipation is essentially high
TL;DR: In this paper, the authors reviewed various crystallization control technologies available to resolve this problem: chemical inhibitors, heat and mass transfer enhancement methods, thermodynamic cycle modifications, and absorption system-control strategies.
Abstract: The key technical barrier to using water/lithium bromide (LiBr) as the working fluid in air-cooled absorption chillers and absorption heat-pump systems is the risk of crystallization when the absorber temperature rises at fixed evaporating pressure. This article reviews various crystallization control technologies available to resolve this problem: chemical inhibitors, heat and mass transfer enhancement methods, thermodynamic cycle modifications, and absorption system-control strategies. Other approaches, such as boosting absorber pressure and J-tube technology, are reviewed as well. This review can help guide future efforts to develop water/LiBr air-cooled absorption chillers and absorption heat-pump systems.
TL;DR: In this article, the heat transfer coefficient of HFO-1234yf inside a smooth small-diameter horizontal tube (inner diameter: 2 mm) was experimentally investigated.
Abstract: The flow boiling heat transfer coefficient of the low-GWP (global warming potential) refrigerant HFO-1234yf inside a smooth small-diameter horizontal tube (inner diameter: 2 mm) was experimentally investigated. The local heat transfer coefficient was measured at heat fluxes of 6–24 kW m -2 , mass fluxes of 100–400 kg m -2 s -1 , evaporating temperature of 288.15 K, and inlet vapor quality of 0–0.25. The results show that the effect of heat flux on the heat transfer was large at low vapor quality, while the effect of mass flux was large at high vapor quality. The heat transfer coefficient of HFO-1234yf was almost the same as that of R-134a. The heat transfer coefficients calculated based on correlations with Saitoh et al. agreed well with the measured values compared to other correlations. The measured pressure drop agreed well with that predicted by the Lockhart-Martinelli correlation.
TL;DR: In this paper, a fast and flexible, time-dependent, one-dimensional numerical model was developed in order to study in detail the operation of an active magnetic regenerator (AMR).
Abstract: A new, fast and flexible, time-dependent, one-dimensional numerical model was developed in order to study in detail the operation of an active magnetic regenerator (AMR). The model is based on a coupled system of equations (for the magnetocaloric material and the heat-transfer fluid) that have been solved simultaneously with the software package MATLAB. The model can be employed to analyze a wide range of different operating conditions (mass-flow rate, operating frequency, magnetic field change), different AMR geometries, different magnetocaloric materials and heat-transfer fluids, layered and single-bed AMRs, etc. This paper also presents an optimization of the AMR’s geometry, where the AMR consists of a packed-bed of grains (spheres) of gadolinium (Gd). The optimization of the mass-flow rate and the operating frequency of the AMR were performed by studying five different diameters of Gd spheres.
TL;DR: In this article, a mathematical model is developed to design R134a ejector and to predict the performance characteristics of a vapor jet refrigeration system over a wide range of the investigated parameters.
Abstract: In the present paper, a mathematical model is developed to design R134a ejector and to predict the performance characteristics of a vapor jet refrigeration system over a wide range of the investigated parameters. These parameters include boiling temperature (65–85 °C), condensing temperature (25–40 °C), evaporating temperature (0–10 °C), degrees of superheat (0–15 °C), nozzle efficiency (0.75–0.95) and diffuser efficiency (0.75–0.95). Simulated results showed that the present model data are in good agreement with experimental data in the literature with an average error of 6%. It is found that the ejector area ratio at boiling temperature of 85 °C is about double that at boiling temperature of 65 °C for various evaporating and condensing temperatures. The present results confirm that waste heat sources of temperature ranging from 65 to 85 °C are adequate to operate vapor jet refrigeration system for air-conditioning applications.
TL;DR: In this paper, an analytical technique based on the decomposition method is presented to determine the temperature distribution and thermal performance parameters of a constructal T-shape porous fin, where the effect of radiation on natural convective heat transfer is considered in the analysis.
Abstract: This paper presents an analytical technique based on the decomposition method to determine the temperature distribution and thermal performance parameters of a constructal T-shape porous fin. The effect of radiation on natural convective heat transfer is considered in the analysis. The governing energy equations of the stem and flange part of this T-shaped porous fin for the aforementioned conditions are highly nonlinear. The adopted decomposition solution gives an explicit expression of temperature distribution in the fin as a function of a coordinate expressed by infinite power series from which fin performance parameters and heat transfer rates can easily be calculated without the need of linearization. The effects of different geometric and thermophysical parameters on the dimensionless temperature distribution and fin performances are studied. Finally, the increase in heat transfer is noticed by selecting porous medium condition in the fin.
TL;DR: In this paper, the R-134a flow distribution was experimentally studied for a round header/ten flat tube test section simulating a brazed aluminum heat exchanger.
Abstract: The refrigerant R-134a flow distribution was experimentally studied for a round header/ten flat tube test section simulating a brazed aluminum heat exchanger. Three different inlet configurations (parallel, normal, vertical) were investigated. Tests were conducted with downward flow for mass flux from 70 to 130 kg m −2 s −1 and quality from 0.2 to 0.6. Tubes were flush-mounted in the test section, with no protrusion into the header. It is shown that normal and vertical inlet yielded similar flow distribution. As mass flux or quality increased, however, better results were obtained for normal inlet configuration. The flow distribution was worst for the parallel inlet configuration. Possible explanation is provided based on flow visualization results. Correlations were developed to predict the fraction of liquid or gas taken off by downstream channel as a function of header gas Reynolds number at immediate upstream.