Air-side heat transfer enhancement of a refrigerator evaporator using vortex generation

In most domestic and commercial refrigeration systems, frost forms on the air-side surface of the air-to-refrigerant heat exchanger. Frost-tolerant designs typically employ a large fin spacing in order to delay the need for a defrost cycle. Unfortunately, this approach does not allow for a very high air-side heat transfer coefficient, and the performance of these heat exchangers is often air-side limited. Longitudinal vortex generation is a proven and effective technique for thinning the thermal boundary layer and enhancing heat transfer, but its efficacy in a frosting environment is essentially unknown. In this study, an array of delta-wing vortex generators is applied to a plain-fin-and-tube heat exchanger with a fin spacing of 8.5 mm. Heat transfer and pressure drop performance are measured to determine the effectiveness of the vortex generator under frosting conditions. For air-side Reynolds numbers between 500 and 1300, the air-side thermal resistance is reduced by 35–42% when vortex generation is used. Correspondingly, the heat transfer coefficient is observed to range from 33 to 53 W m⁻² K⁻¹ for the enhanced heat exchanger and from 18 to 26 W m⁻² K⁻¹ for the baseline heat exchanger.     

Liquid–gas heat exchanger for household refrigerator

This paper considers the influence of heat exchangers to the efficiency of a household refrigerating system. A steady state mathematical model is used to compare three most commonly used heat exchanger designs. For each design, an optimal inner diameter of the heat exchanger, subject to the compressor's capacity and the heat exchanger's length is found. The influence of operating conditions, such as condensation and evaporation temperatures, superheat in the evaporator and sub-cooling in the condenser, to optimal dimensions of the heat exchanger is also analyzed in the paper. Presented guidelines and recommendations can be used for design and modernization of household refrigerators and freezers.     

Performance characteristics of a small-capacity directly cooled refrigerator using R290/R600a (55/45)

In this study, the performance of a small-capacity directly cooled refrigerator was evaluated by using the mixture of R290 and R600a with mass fraction of 55:45 as an alternative to R134a. The compressor displacement volume of the alternative system with R290/R600a (55/45) was modified from that of the original system with R134a to match the refrigeration capacity. Both systems with R290/R600a (55/45) and R134a were tested, and then optimized by varying the refrigerant charge and capillary tube length under experimental conditions for both the pull-down test and the power consumption test. The refrigerant charge of the optimized R290/R600a system was approximately 50% of that of the optimized R134a system. The capillary tube lengths for each evaporator in the optimized R290/R600a system were 500 mm longer than those in the optimized R134a system. The power consumption of the optimized R134a system was 12.3% higher than that of the optimized R290/R600a system. The cooling speed of the optimized R290/R600a (55/45) system at the in-case setting temperature of −15 °C was improved by 28.8% over that of the optimized R134a system.     

Study of domestic refrigerator temperature and analysis of factors affecting temperature: a French survey

A survey was carried out in France from April to June 1999. Temperatures were recorded at three levels (top, middle and bottom) of the refrigerator compartment using a data logger. A questionnaire was filled in, enabling the following information to be obtained: characteristics of the family, characteristics of the refrigerator and the use conditions. One hundred and forty-three domestic refrigerators were surveyed, but due to various technical problems, only 119 sets of recorded temperatures were exploitable. The temperatures of the surveyed refrigerators were: average 6.6 °C, minimum 0.9 °C and maximum 11.4 °C. Statistical analysis such as clustering and segmentation were used. It was found that the heterogeneousness of temperature inside each refrigerator seems to be dependent on the type. This study shows the influence of the use conditions on the temperature. No one factor exerts a single direct effect; a combination of the effects of all factors is observed.     

A numerical simulation model for plate-type, roll-bond evaporators

This study presents a first-principles mathematical model developed to investigate the thermal behavior of a plate-type, roll-bond evaporator. The refrigerated cabinet was also taken into account in order to supply the proper boundary conditions to the evaporator model. The mathematical model was based on the mass, momentum and energy conservation principles applied to each of the following domains: (i) refrigerant flow through the evaporator channels; (ii) heat diffusion in the evaporator plate; and (iii) heat transmission to the refrigerated cabinet. Empirical correlations were also required to estimate the shear stresses, and the internal and external heat transfer rates. The governing partial differential equations were discretized through the finite-volume approach and the resulting set of algebraic equations was solved by successive iterations. Validation of the model against experimental steady-state data showed a reasonable level of agreement: the cabinet air temperature and the evaporator cooling capacity were predicted within error bands of ±1.5 °C and ±6%, respectively.     

Efficiency of free cooling using latent heat storage integrated into the ventilation system of a low energy building

This article presents the results of an investigation into the free cooling efficiency in a heavyweight and lightweight low energy building using a mechanical ventilation system with two latent heat thermal energy storages (LHTESs), one for cooling the fresh supply air and the other for cooling the re-circulated indoor air. Both LHTESs contain sphere encapsulated PCM (paraffin RT20). Using a developed and experimentally verified numerical model of the LHTES, the temperature response functions, based on the heat storage size, the air flow rates and the PCM's thermal properties, are established in the form of a Fourier series and empirical equations and used in the TRNSYS building thermal response model. Several mechanical ventilation, night cooling and free cooling operation modes were analysed and compared. It was found that the free cooling technique enables a reduction in the size of the mechanical ventilation system, provides more favourable temperatures and therefore enables better thermal comfort conditions, and in our studied case also fresh air for the occupants.     

Measurements of the air flow field in the freezer compartment of a top-mount no-frost refrigerator: the effect of temperature

The energy consumption of a household refrigerator is closely related to the distributions of temperature inside its compartments. Since, such distributions are dependent on the chilled air circulating inside the refrigerator, its energy consumption can be reduced and its efficiency improved through optimization of the air flow fields inside the compartments. In this work, particle image velocimetry (PIV) was used to measure velocity fields inside the freezer compartment of a commercial top-mount no-frost refrigerator at two critical vertical sections at three different temperatures levels. To perform the tests, a commercial unit was equipped with two specially constructed insulated windows which enabled clear visualization of the flow field under real operating conditions. The results show that significant changes in the air flow distribution occur when the temperature is reduced.     

Heat transfer enhancement by winglet-type vortex generator arrays in compact plain-fin-and-tube heat exchangers

The potential of winglet type vortex generator (VG) arrays for air-side heat transfer enhancement is experimentally evaluated by full-scale wind-tunnel testing of a compact plain-fin-and-tube heat exchanger. The effectiveness of a 3VG alternate-tube inline array of vortex generators is compared to a single-row vortex generator design and the baseline configuration. The winglets are placed in a common-flow-up orientation for improved tube wake management. The overall heat transfer and pressure drop performance are assessed under dry-surface conditions over a Reynolds number range based on hydraulic diameter of 220 ≤ Re ≤ 960. It is found that the air-side heat transfer coefficient increases from 16.5% to 44% for the single-row winglet arrangement with an increase in pressure drop of less than 12%. For the three-row vortex generator array, the enhancement in heat transfer coefficient increases with Reynolds number from 29.9% to 68.8% with a pressure drop penalty from 26% at Re = 960 to 87.5% at Re = 220. The results indicate that vortex generator arrays can significantly enhance the performance of fin-tube heat exchangers with flow depths and fin densities typical to those used in air-cooling and refrigeration applications.     

The effect of radiation shields around the air condenser and compressor of a refrigerator on the temperature distribution inside it

In almost all domestic refrigerators–freezers all components are assembled in the same relative position since several years ago. It is also known that the condenser releases heat at high temperatures (first law of thermodynamics) as well as the compressor. This heat is rejected to the environment in almost all practical situations partially by natural air convection. However, part of it is due to thermal radiation that causes an overheating of the refrigerator–freezer surfaces adjacent to those equipments. As a consequence there are more heat gains to the refrigerator–freezer through these surfaces and hence higher air temperatures inside. This paper describes how a simple technique can be very useful in order to minimize that part of heat transfer by radiation. The improvement is achieved by placing a radiation shield – a sheet of aluminium foil – over the surfaces close to the condenser and the compressor. For validating this technique a refrigerator–freezer was monitored with thermocouples for the measurements of the inside air temperatures in two situations: with and without the radiation shield. Results show that with this practice the average inside air temperatures in the refrigerator–freezer could decrease to about 2 K. An available commercial code was used in order to simulate the air temperature distribution and air velocities inside the refrigerator cabinet in both situations. Results from the experimental apparatus and from simulations show that there is a good agreement between them which validates the experiments carried out.Also an available commercial code, the Fluent, was used to simulate the internal air temperature in both situations.     

Performance of a domestic refrigerator under influence of varied expansion device capacity, refrigerant charge and ambient temperature

This paper reports experimental results of an on/off cycling domestic refrigerator at varied expansion device capacity (EDC), quantity of charge and ambient temperature. It was found that the energy consumption is insensitive to varied EDC and charge within a wide range of settings. For the charge this is explained by the low side accumulator, which buffers over- and undercharge. It was also found that the optimum charge increased at lower ambient temperature. The paper describes an experimental procedure on how to determine the capillary tube length and the quantity of charge for a domestic refrigerator/freezer. This procedure is recommended since it takes different thermal masses and loads into consideration and since the potential for energy saving with a more sophisticated method appears to be limited.     

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.     

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.     

Thawing and freezing of selected meat products in household refrigerators

Recently, several manufacturers of domestic refrigerators have introduced models with “quick thaw” and “quick freeze” capabilities. In this study, the time required for freezing and thawing different meat products was determined for five different models of household refrigerators. Two refrigerators had “quick thaw” compartments and three refrigerators had “quick freeze” capabilities. It was found that some refrigerator models froze and thawed foods significantly faster than others (P<0.05). The refrigerators with the fastest freezing and thawing times were found to be those with “quick thaw” and “quick freeze” capabilities. Heat transfer coefficients ranged from 8 to 15 Wm⁻²K⁻¹ during freezing, and the overall heat transfer coefficients ranged from 5 to 7 Wm⁻² K⁻¹ during thawing. Mathematical predictions for freezing and thawing time in the refrigerators gave results similar to those obtained in experiments. With the results described, manufacturers can improve their design of refrigerators with quick thawing and freezing functions.     

Performance characteristics of an irreversible magnetic Brayton refrigeration cycle

Based on the thermodynamic properties of a paramagnetic salt, an irreversible model of the magnetic Brayton refrigeration cycle is established, in which the working substance is a special paramagnetic material. The expressions of the important performance parameters, such as the coefficient of performance, refrigeration load and work input, are derived. Moreover, the optimal performance parameters are obtained at the maximum coefficient of performance. The results obtained here may include the ones of the magnetic Brayton refrigeration cycle using the magnetic material obeyed the Curie law as the working substance, the magnetic Brayton refrigeration cycle without regeneration and the eversible magnetic Brayton refrigeration cycle. Therefore, the results obtained here have general significance and will be helpful to deeply understand the performance of a magnetic Brayton refrigeration cycle.     

Developing new test procedures for domestic refrigerators: harmonisation issues and future R&D needs—a review

This paper highlights the salient differences among various test standards for household refrigerators/freezers and identifies the main parameters that play important role in the overall energy consumption of a refrigerating appliance. Further the paper examines the merits and demerits of current test standards for refrigerator-freezers and proposes new guidelines that should be considered to improve them. There is a need to develop a new test procedure that harmonises the basic features of various standards and is simple, repeatable and reproducible. Such a procedure should also represent realistic “real” world energy use, encourage product innovation, capture both the software and hardware developments and facilitate ‘free trade’ among various economies. In order to achieve this, the paper highlights areas where concerted efforts are required for carrying out the requisite research and development work.     

The effects of non-condensable gases in domestic appliances

It is well known that the presence of non-condensable gases inside a compression vapour refrigerating circuit introduces an additional thermal resistance at the condenser, which can significantly decrease the energy efficiency of the system. However, this problem so far has been investigated mainly for shell and tube condensers of large capacity and limited information is available on small systems, as is the case for household appliances where the internal volumes are extremely reduced and therefore a very small amount of non-condensable gas has large effect. Moreover, non-condensable gas behaves differently when condensation takes place outside tubes (shell and tube condensers) or inside tubes (condensers of small appliances); in the first case all heat transfer area is wrapped by a gas layer, whereas in the second case non-condensable gas is collected at the end of the tube. The effect of non-condensable gas in this work is experimentally investigated by injecting controlled amounts of air into a refrigerating circuit and by recording the thermal and electric variables during different modes of operation (steady state and cyclic running). The tested refrigerating circuits are part of two appliances on the market, a household refrigerator and a vertical freezer. The presence of non-condensable gas was found to spoil energy efficiency, since it brings about an increase in condensing pressure and a concomitant decrease in evaporating temperature, although larger liquid subcooling partially compensates for the first negative effects: the reason for this behaviour is the clogging action of bubbles of gaseous mixture (air and refrigerant vapour) that enter the capillary tube.     

A study of accurate latent heat measurement for a PCM with a low melting temperature using T-history method

When the latent heat of a phase change material (PCM) with a lower melting point than ambient temperature was assessed according to the standard T-history method using a vertically oriented test tube, a temperature gradient occurred in the longitudinal direction of the tube due to natural convection. This led to a decrease in the accuracy of the latent heat of fusion measurement. In this study, the accuracy of the measurement with the original T-history method was improved without decreasing the test's simplicity and convenience by setting the test tube horizontally. The heat transfer to the vapor-layer of the tube under volume change during melting was assumed to be negligible and the results were calculated using the two inflection points of temperature as the start and end of latent heat period. Under these assumptions, the results agree closely with other reference data. And, the new method proposed in this study showed a remarkable reduction in data scattering.     

An experimental study on minimizing frost deposition on a cold surface under natural convection conditions by use of a novel anti-frosting paint. Part I. Anti-frosting performance and comparison with the uncoated metallic surface

In this study, an experimental investigation is made of the anti-frosting performance of our newly developed anti-frosting paint. By coating the paint on a cold metal surface the onset of the frost formation may be delayed at least 15 min and the thickness of the deposited frost layer may be reduced by at least 40% and thus the weight of the deposited frost may be reduced by more than 40% compared with that on the uncoated copper surface. Under some preferable conditions (air relative humidity <60%, cold plate surface temperature >−10 °C), the coating surface can be kept free of frost at least for 3 h.     

An analysis of the feasibility and characteristics of photoelectric technique applied in defrost-control

An innovative photoelectric technology is presented in this paper for detecting frost in the field of defrost-control of refrigerator or freezer applications. Experiments were conducted with a small-scale laboratory test section under natural convection conditions. Two agreeable properties of the new technology, “on–off” and “linear”, have been demonstrated by the experiments. The first property provides an effective judgment of the defrost-control strategy while the second one is suitable for developing an accurate measurement of the frost height. The characteristics (electric current, environment temperature, metal surface temperature, light intensity and sensor location) that affect the properties were investigated for the development of this technology.     

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.