اصول اولیه مدل شبیه سازی برای راه اندازی و چرخش گذرا در یخچال و فریزر خانگی

A first-principles model for simulating the transient behavior of household refrigerators is presented in this study. The model was employed to simulate a typical frost-free 440-l top-mount refrigerator, in which the compressor is on–off controlled by the freezer temperature, while a thermo-mechanical damper is used to set the fresh-food compartment temperature. Innovative modeling approaches were introduced for each of the refrigerator components: heat exchangers (condenser and evaporator), non-adiabatic capillary tube, reciprocating compressor, and refrigerated compartments. Numerical predictions were compared to experimental data showing a reasonable level of agreement for the whole range of operating conditions, including the start-up and cycling regimes. The system energy consumption was found to be within ±10% agreement with the experimental data, while the air temperatures of the compartments were predicted with a maximum deviation of ±1 °C.

A household refrigerator is basically composed of a thermally insulated cabinet and a vapor–compression refrigeration loop, as illustrated in Fig. 1. The energy consumption of a typical refrigerator is around 1 kWh/day, which is equivalent to the energy consumption of a 40 W light-bulb continuously running. Although the energy consumption of a unitary refrigerator is reasonably low, commercial and household refrigeration appliances are responsible for 11% of the total energy produced annually in Brazil (PROCEL, 1998), which amounts to 2.86 TWh/year. Such a high energy consumption may be easily accounted for considering that there is a large amount of household refrigerators currently in use, and their thermodynamic efficiency is intrinsically low, barely reaching 15% of Carnot's COP. The major part of the energy is wasted by the system components (compressor, condenser, evaporator and capillary tube) due to irreversible losses. Studies carried out to understand such thermodynamic losses shall lead to the development of higher efficiency products.

A great potential of the model is the analysis of the refrigerant migration during the cycling operation, which is complex to be carried out experimentally. Just after the compressor is switched off, the condenser holds most of the refrigerant (∼55 g), as illustrated in Fig. 13. During the pressure equalization process, refrigerant migrates from the condenser to the evaporator. The amount of refrigerant within the evaporator rapidly increases to 55 g, decreasing slightly thereafter due to its migration to the compressor shell. The amount of free refrigerant in the compressor shell or dissolved in the oil increases continuously with the evaporating pressure until the compressor is on again. Just before compressor restarts, there is approximately 10 g of refrigerant in the condenser, 50 g in the evaporator, and 25 g in the compressor. After compressor restarts, the refrigerant rapidly moves from the evaporator and from the compressor shell to the condenser. Over time, the refrigerant returns to the evaporator through the capillary tube. The amount in the compressor decreases continuously with decreasing evaporating pressure and increasing shell temperature, as both tend to reduce the amount of refrigerant dissolved in the lubricant oil.