Irreversible refrigerators with isothermal heat exchanges: Réfrigérateurs irréversibles avecéchanges thermiques isotherms

A maximum for the coefficient of performance (COP_f) is verified for a temperature difference between reservoirs and the hot isotherm of a closed irreversible cyclical refrigerator working at steady-state conditions. A maximum also exists when the COP_f is considered as a function only of a parameter depending on the thermal characteristics of the heat exchangers. The influence of the parameter and entropy generation on the COP_f maxima is described.     

Propagation of stochastic temperature fluctuations in refrigerated fruits: Propagation des fluctuations de température stochastiques dans des fruits réfrigérés

A steady state variance propagation algorithm is derived to investigate the effect of stochastic air temperature disturbances on the variability of the temperature field inside refrigerated fruits during cold storage conditions. The disturbances are modelled by continuous-time autoregressive processes of variable order. The algorithm is based on the finite element formulation of the heat conduction equation and involves the numerical solution of matrix equations of the Lyapunov and Sylvester type. As an example, the cold storage of pineapple has been considered. It is shown that disturbances of different order but with the same scale of fluctuation result in comparable centre temperature variances.     

Modeling of integral-type Stirling refrigerator using system dynamics approachModélisation d''un réfrigérateur Stirling de type intégré à l''aide d''une approche par dynamique du système

A modeling of the integral-type Stirling refrigerator (ITSR) using a system dynamics approach is developed in the present study. The system dynamics models are derived to describe the input/output relation of each component of an ITSR. Connecting the equivalent circuits of the components together, we obtain a flow network diagram and derive a transfer function to represent system dynamics behavior of an ITSR. The performance analysis of an ITSR is then easily carried out by use of the transfer function with sinusoidal signal assumption. The performance calculation for a test refrigerator has shown that the analysis is accurate. An empirical correlation for a correction coefficient r in the expansion space was also derived for better results. The present study has demonstrated how a linear system theory or system dynamics approach can be applied to the design of an ITSR.     

Testing of propane/isobutane mixture in domestic refrigeratorsEtude sur un mélange propane/isobutane dans les réfrigérateurs domestiques

The performance of a propane/isobutane (R290/R600a) mixture was examined for domestic refrigerators. A thermodynamic cycle analysis indicated that the propane/isobutane mixture in the composition range of 0.2 to 0.6 mass fraction of propane yields an increase in the coefficient of performance (COP) of up to 2.3% as compared to CFC12. For the actual tests, two commercial refrigerators of 299 and 465 l were used. For both units, all refrigeration components remained the same throughout the tests, except that the length of the capillary tube and amount of charge were changed for the mixture. Each refrigerator was fully instrumented with more than 20 thermocouples, two pressure transducers, and a digital watt/watt-h meter. For each unit, both ‘energy consumption test’ and ‘no load pull-down test’ were conducted under the same condition. The experimental results obtained with the same compressor indicated that the propane/isobutane mixture at 0.6 mass fraction of propane has a 3–4% higher energy efficiency and a somewhat faster cooling rate than CFC12. The mixture showed a shorter compressor on-time and lower compressor dome temperatures than CFC12. In conclusion, the proposed hydrocarbon mixture seems to be an appropriate long term candidate to replace CFC12/HFC134a from the viewpoint of energy conservation requiring minimal changes in the existing refrigerators.     

Thermodynamic properties of ammonia in the critical regionPropriétés thermodynamiques de l'ammoniac dans la région critique

A theoretically based crossover model, which incorporates a crossover from singular thermodynamic behavior at the critical point to regular thermodynamic behavior far away from the critical point, is presented for the thermodynamic properties of ammonia. The equation is capable of representing the thermodynamic properties of ammonia between 398 and 500 K in an appreciable range of densities around the critical density. The crossover equation of state was constructed so that it can be used in conjunction with the classical equation of state developed by Tillner-Roth et al. (TR EOS), which works well outside the critical region. We have also compared our crossover equation of state with the widely used equation of state for ammonia developed earlier by Haar and Gallagher (HG EOS).     

The effect of blowing agent choice on energy use and global warming impact of a refrigerator

A study, comparing the effect of blowing agent selection on energy consumption and the life cycle climate performance (LCCP) of a typical European refrigerator is discussed. Energy consumption of prototype European-style refrigerators made with a foam formulation with HFC-245fa as the blowing agent was measured and compared with energy consumption of the same model as currently produced (using a foam with a pentane blend for the blowing agent). Results were used in a LCCP study, considering both direct and indirect climate impacts due to blowing agent emissions and energy consumption in manufacturing processes and over the life cycle of the refrigerator. An assumption is made that the refrigerator is built and used in the European market.     

A simulation model for evaporation of defrosted water in household refrigeratorsSimulation de l''évaporation d''eau de dégivrage dans les réfrigérateurs domestiques

This paper presents a simple dynamic simulation model for predicting the evaporation rates of defrosted water from water trays in domestic refrigerators. The model was successfully tested for one domestic refrigerator. The model was used to investigate the effect of different variables (ambient temperature, relative humidity, air velocity, compressor heat, auxiliary condenser) by comparing eight different arrangements with each other on a consistent basis for effective evaporation of defrosted water. The model results indicate that the evaporation of water can be increased to the necessary rate by raising the water temperature and optimising the velocity of air over the water tray.

Résumé

Cette communication présente un modèle de simulation dynamique pour la prévision de la vitesse d'évaporation d'eau de dégivrage contenue dans les bacs de réfrigérateurs domestiques. On a utilisé ce modèle pour étudier les effets de diverses variables (température ambiante, humidité relative, vitesse d'air, chaleur dégagée par le compresseur et le condenseur auxiliaire) en comparant l'évaporation de l'eau de dégivrage dans huit dispositions différentes. Lés résultats indiquent qu'on peut accélerer la vitesse d'évaporation par l'augmentation de la température de l'eau et l'optimisation de la vitesse de l'air passant au dessus du bac à eau.     

Model for transient behavior of pulse tube cryocooler

This article describes an investigation of the transient behavior of a small (2.0 W at 85 K) pulse tube cryocooler operating at 120 Hz with an average pressure of 3.5 MPa, capable of relatively fast cool-down from ambient to about 60 K. In a series of experiments, the cold end temperature was measured as a function of time in a complete cool-down and subsequent warm-up cycle, with no heat load and different quantities of excess mass at the cold end. A transient heat transfer model was developed, that considers the effects of the cooling power extracted at the cold end and that of the heat gain at the warm end on the cool-down time. The heat gain factor was calculated from warm-up data, and found to be approximately the same for all experiments. Using the same model with cool-down data enables a determination of both the gross and net cooling power as functions of time, but more importantly – as functions of the cold end temperature. An expression was derived for the cold end temperature as a function of time for any amount of excess mass, including zero. The cool-down time of the “lean” cryocooler (with no excess mass) was found to be less than 50 s.This cool-down/warm-up method for evaluating the cooling power of a cryocooler seems simpler than steady-state experiments with a heater simulating load at the cold end. Use of the heat transfer model with data from one or two good experiments conducted in the above manner, can yield both the gross and net cooling powers of a cryocooler as functions of the cold end temperature, and allow the determination of cool-down time with any amount of excess thermal mass. While the net cooling power during cool-down differs somewhat from that under steady-state operation, the former can serve as a good measure for the latter.     

Studies on bubble pump for a water–lithium bromide vapour absorption refrigerator: Etudes sur une pompe à bulles pour réfrigérateur à absorption eau–bromure de lithium

The hydrostatic principle and bubble pump technique are used in the two-fluid pumpless continuous vapour absorption refrigerator to create the circulations of the solution and refrigerant. In this paper, the bubble pump, which is the ‘heart' of this natural circulation type of refrigerator, has been investigated both analytically and experimentally. The bubble pump is modelled for intermittent slug flow of solution and vapour mixture. A test rig is built in glass to evaluate the performance of the bubble pump, to visualize the flow behavior and to validate the analytical model. Bubble pump tube diameter, pump lift, driving head and heat input are varied to analyze the pump performance. Results indicate that pumping ratio is nearly independent of the heat input, but increases with decrease in pump tube diameter, decrease in pump lift and increase in driving head.     

Performance characteristics of pulse tube refrigerators

In the present study experiments were carried out to investigate the performance characteristics of pulse tube refrigerators. It was found that the cool-down time t_c during the transient or start-up period is dominated by the time constant of the pulse tube wall τ_pt and that the dynamics of a basic pulse tube (BPT) refrigerator approaches that of a first-order system. For steady state operation, the cold-end temperature T_L was found to vary with τ_pt, and the cooling load Q_L increases monotonically with increasing τ_pt. This indicates that heat pumped by the gas from the cold to the hot end increases with decreasing h_pt (i.e. less energy exchange between the gas and wall). The process of heat storage or release of the pulse tube wall is thus shown to have a negative effect on the performance of a BPT refrigerator. It was thus found experimentally that the gas compression/expansion process inside the pulse tube, which is similar to a Brayton cycle but lies between isothermal and adiabatic, can explain the performance of BPT refrigerators. The present experiment also shows that the performance of a pulse tube refrigerator at transient and steady states is mainly dominated by the time constant of the pulse tube wall τ_pt.     

Accounting for the real properties of the heat transfer fluid in heat-regenerative adsorption cycles for refrigerationPrise en compte des propriétés du réelles fluide caloporteur dans les cycle frigorifiques à adsoption avec régénération de chaleur

Adsorption cycles for refrigeration or heat pumping can be environmentally friendly. Moreover, when they use the process of heat-regeneration, they can be energetically efficient provided the adsorbers are correctly designed, which requires correct modelling. In such a model, all the heat exchanges undergone by the heat transfer fluid are considered. The temperature changes of this fluid induce variations in its physical properties. The influence of these variations on the cycle performance are numerically investigated and thermodynamically analysed. The effects of the changes in density are negligible, but those of the changes in heat capacity cannot be neglected. The analysis shows that this result is due to the interdependence of all the heat quantities involved in the cycle via the first principle of thermodynamics.     

Modeling of a domestic frost-free refrigerator

In the present study, a comprehensive thermo-fluidic model is developed for a domestic frost-free refrigerator. The governing equations, coupled with pertinent boundary conditions, are solved by employing a conservative control volume formulation, in the environment of a three-dimensional unstructured mesh. Experiments are also conducted to validate the results predicted by the present computational model. It is found that the computational and experimental results qualitatively agree with each other, although certain discrepancies can be observed in terms of the exact numerical values obtained. For the freezer compartment, the computationally predicted temperatures are somewhat higher than the experimental ones, whereas for the refrigerating compartment, the computed temperatures are lower than the corresponding experimental observations. The difference between experimental and computational results may be attributed to the lack of precise data on the airflow rates and the unaccounted heat transfer rates through the door gaskets and the compressor. From the heat transfer and fluid flow analysis, certain modifications in the design are also suggested, so as to improve the performance of the refrigerator.     

微型压缩机驱动的微型混合工质J-T制冷器实验研究

为研究微型混合工质节流制冷器的降温性能,搭建了微型压缩机驱动微型混合工质节流制冷器的制冷系统,并得到了初步实验研究结果。采用Aspen微型压缩机驱动微型J-T节流制冷器,应用混合制冷剂实现深度制冷。微型J-T节流制冷器采用微小通径的不锈钢毛细管制作,其通道特征尺寸为0.3mm。初步实验表明,微型J-T节流制冷器达到了180K温区。由于采用微型压缩机驱动,系统结构紧凑,可在便携生物储存设备、低温医疗以及电子器件冷却等领域应用。     

Modélisation des données thermodynamiques du mélange ammoniac/eauModelling of the thermodnyamic properties of the ammonia/water mixture

The present work deals with the modeling of the thermodynamic properties of the ammonia/water mixture using the Gibbs free energy function. For the liquid phase, a three constant Margules model of the excess free enthalpy is formulated. The vapour phase is considered as a perfect mixture of real gases, each pure gas being described by a virial equation state in pressure truncated after the third term. The model developed describes with a good accuracy the mixture in the three states, subcooled liquid, superheated vapour and liquid-vapour saturation for temperatures from 200 to 500 K and pressures up to 100 bar.     

The temperature performance of domestic refrigerators

Chilled foods are stored for periods of between a few hours and many days in domestic refrigerators. However, there are little published data on the temperature performance of domestic refrigerators within the home. A survey has been taken in 252 households in the UK and some of the results are presented in this paper. The refrigerators investigated in the survey were found to have an overall mean temperature of approximately 6°C, which ranged from 11.4 to −0.9°C. Temperature ranges over the whole refrigerator varied from 4.5 to 30.5°C with 3.7% of the total being warmer than 20°C. On average 29.9% of refrigerators operated below 5°C and 66.7% operated below 7°C. Few refrigerators (7.3%) ran, on average, above 9°C. No refrigerator characteristic (apart from type) could be related to temperatures or temperature distribution in the refrigerators investigated.     

Modified friction factor correlation for CICC’s based on a porous media analogy

The first cool-down of the EDIPO (European DIPOle) test facility is foreseen to take place in 2011 by means of the existing 1.2 kW cryoplant at EPFL-CRPP Villigen. In this work, the thermo-hydraulic analysis of the EDIPO cool-down is performed in order both to assess the its duration and to optimize the procedure. The cool-down is driven by the helium flowing in both the outer cooling channel and in the windings connected hydraulically in parallel. We take into account limitations due to the pressure drop in the cooling circuit and the refrigerator capacity as well as heat conduction in the iron yoke. Two schemes of the hydraulic cooling circuit in the EDIPO windings are studied (coils connected in series and coils connected in parallel). The analysis is performed by means of an analytical model complemented by and numerical model. The results indicate that the cool-down to 5 K can be achieved in about 12 days.     

Numerical analysis and experimental investigation into the performance of a wire-on-tube condenser of a retrofitted refrigerator

The present paper discusses (a) the analysis of a wire-on-tube condenser under varying operating conditions of free convection using FEM, and (b) experimental verification of the performance of two wire-on-tube condensers in a retrofitted domestic refrigerator using refrigerant R-134a. The study is motivated by the desire to investigate if the wire-on-tube condensers used in R-12 based refrigerators could be used in a modified refrigerator using R-134a refrigerant. Experiments were conducted in a climate chamber under controlled and varying ambient temperatures and mass flow rates to determine the locations where phase change occurs and the degree of subcooling achieved. In terms of initial and final phase change point locations the predicted results agree with the experimental results to within ±10%. The analysis and the experiments also lead to the information about the adequacy of the number of tubes for complete condensation of the refrigerant vapour under given operating conditions. The methodology can be used as a design tool for the design of wire-on-tube condenser of a small refrigerator as well as the suitability of specific decommissioned condensers for use in a retrofitted refrigerator. It also indicates that R-12 based refrigerators using wire-on-tube condensers retrofitted with R-134a compressor and refrigerant deserve and warrant further studies for adoption.     

A theoretical and experimental study of a small-scale steam jet refrigerator

The paper provides the results of a theoretical and experimental study of a steam jet refrigerator. A small-capacity steam jet refrigerator has been tested with boiler temperatures in the range 120–140°C. The experimental data were found to be within 85% of the theoretical values. The experiments showed that choking of the secondary flow in the mixing chamber of the ejector plays an important role in the system performance. Maximum COP was obtained when the ejector was operated at its critical flow condition. Off-design performance characteristics of the system are provided.     

The cool-down characteristics of a miniature Joule-Thomson refrigerator

Cool-down characteristics of a miniature J-T refrigerator with a pressurized vessel, which has different initial pressures of nitrogen gas, were investigated by experiment. The supply and exhaust pressures, the temperatures on the surface of the cryostat, and the mass flow rate were measured simultaneously to analyze the cool-down characteristics of the J-T refrigerator with a pressurized vessel or reservoir. The results show the effect of the supply pressure on the cold end temperature of the J-T refrigerator. The influence of the supply pressure and the temperature on the mass flow rate during the cool-down stage is discussed in details.     

A contribution to the evaluation of the economic perspectives of absorption chillers: Contribution à l'évaluation des perspectives économiques des refroidisseurs à adsorption

The development of new working pairs and cycles extends the field of application of absorption systems with corresponding environmental benefits. The performance of standard cycles can be enhanced, e.g. by multi-staging. For each application the suitable working pair and cycle can be chosen regarding thermodynamical and economical aspects. Still, the performance strongly depends on the given external conditions. In this paper, basic thermodynamic constraints stemming from those conditions and valid for all sorption cycles are derived using the concept of endo-reversibility. Fundamental economic conclusions can be drawn. Subsequently, real machines are analysed. A comparison with manufacturers data and experimental data is made. The working pairs lithium bromide/water and binary hydroxide solution/water are discussed.