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Frost formation on the cold evaporator surface is a typical problem in household refrigerators. It happens due to humidity present in the ambient air associated with frequent door openings. In addition, mass transfer from stored goods to dry internal cabinet air and moisture infiltration through door gaskets also contribute to frost formation on the evaporator surface. To prevent this, the evaporator has to be defrosted manually or automatically (as in modern refrigerators). To do so, an electrical heater is switched on at a preset time and frequency and defrosts the evaporator. During this time, defrosted water is drained through a pipe into a water tray which is usually placed at the vicinity of the compressor. There the water evaporates mainly due to heat transfer from hot compressor shell. This system of defrosting and evaporation works well in areas with low relative humidity, where the amount of defrosted water is small and the evaporation rate is higher when compared to places with high relative humidity. Moreover, the compressor shell temperatures of high efficient compressors are not usually high enough to provide the minimum evaporation rate to balance the defrosted water flow rate and thus avoid water overflow. The aim of this work is to get a better understanding of the defrosted water evaporation process and to come up with low cost, practical and more efficient designs of water trays. To this end a test facility was designed and constructed to reproduce the compressor operating conditions and the defrosted water flow rate and temperature. The test rig was placed within a climate-controlled room with a strict control of air temperature and relative humidity. A mathematical model was also developed to predict the water evaporation rate based on the fundamental conservation equations of mass and energy. Several tests were carried out under different operating conditions and with different water trays designs. The model predictions were compared to the experimental data base with most of the points falling within an error band of ?10%.

Since the beginning of the “artificial” refrigeration era, a great deal of effort has been applied on the development of high efficient systems. The efforts were primarily focused on the compressor, bringing its efficiency to record-high values. Simultaneously, the compressor noise was brought down to almost inaudible levels. As a consequence, sources of noise that were previously unnoticeable became audible and started to play a major role among refrigerator manufacturers. One of those new sources of noise, generally known as fluid borne noise, is now a growing field of interest in engineering. The aim of this work is to study the relationship between the capillary tube flow and the acoustic behavior of household refrigerators by investigating a particular type of fluid borne noise that only occurs during the freezer mode of dual-evaporator appliances, known as fluctuating noise. To this end a household refrigerator was carefully instrumented and firstly tested in a reverberant chamber. Very heavy sound power level oscillations were found, even with the system in thermodynamic steady-state conditions. Acceleration measurements were also taken at the inlet pipe of the evaporator showing that the refrigerant flow in that region is highly pulsating, i.e., that the refrigerant flow pattern at the discharge of the capillary tube is not steady, in contrast to the indications of the thermodynamic readings. Visualizations studies of the flow pattern at the inlet of the capillary tube were also carried out, showing the presence of vapor bubbles – caused by an unbalance between the compressor and capillary tube mass flow rates – for most of the time. A needle valve was then installed in series with the freezer capillary tube in order to increase restriction and match the compressor and the capillary tube mass flow rates. The restriction provided by the optimum pair needle valve-capillary tube was converted into an equivalent I.D. capillary tube. The original freezer capillary tube was replaced by a same length smaller I.D. capillary tube, and another appliance tests were carried out. It was found that the higher restriction capillary tube attenuate the pulsating characteristic of the refrigerant flow, almost averting the disturbing fluctuating noise.

This paper describes a quasi-steady-state simulation model for predicting the transient behavior of a household refrigerator subjected to periodic door opening. A semi-empirical steady-state sub-model was developed for the refrigeration loop, and a transient sub-model was devised to predict the energy and mass transfer into and within the refrigerated compartments as well as the evaporator frosting. The key empirical heat and mass transfer parameters required by the model were derived from a set of experiments carried out in a climate-controlled chamber. In general, it was found that the model predictions followed closely the experimental trends for the power consumption (deviations within ±10%) and for the compartment temperatures (deviations within ±2 K).

The oscillatory pressure disturbance created by the refrigerant flow through pipes and components is one of the main sources of noise in household refrigerators. This type of excitation is transmitted to the pipes and travels through the structure reaching the cabinet and other parts exposed to the air, where audible airborne sound waves are created. In order to get a better understanding of this type of noise a household refrigerator was especially instrumented with thermocouples, absolute and dynamic pressure transducers and accelerometers. Tests were carried out at different operating conditions, when it was found that the compressor was the main source of excitation, practically affecting the entire refrigeration loop. It was also found that the flow velocity at the outlet of the capillary tube was high enough to induce significant vibrations in the evaporator. Finally, it was found that the entrance of vapor bubbles in the capillary tube produces sporadic and strong excitations. However those excitations do not travel very far along the flow, being attenuated by the condenser and the evaporator, but can nevertheless be transmitted to the pipes. In view of the collected database, comments and suggestions concerning the design of household refrigerators are presented and discussed.

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