Numerical and experimental study of the transient behavior of a domestic vapor compression refrigeration system – Influence of refrigerant charge and ambient temperature

In this work, two transient mathematical models are presented for simulating a vapor compression refrigeration system of domestic refrigerator using R134a. One, the thermal model, is based on the application of the first law of thermodynamics and the second, capacitive model, adds the evaluation of the refrigerant mass distribution in the system. An experimental procedure to calculate the thermal conductance and capacity of system components (compressor, condenser, capillary tube, evaporator, cabinet) is presented. Experimental data describing the transient behavior of the refrigeration system are also obtained to validate the simulation procedures. The domestic refrigerator studied is a vertical two compartments refrigerator. The simulation results follow the measured experimental trends and are very satisfactory when compared to the transient and time averaged experimental data. The maximum relative error obtained for simulated temperatures was about 5.65%. Parametric analyses were conducted to identify the influence of refrigerant charge and ambient temperature on the refrigeration system performance. The optimum refrigerant charge has distinct values regarding the operational condition of the system. In the pull-down operation the highest COP is attained for 105g and 140g of refrigerant for transient and the steady-state regimes, respectively. In the on-off operation the optimum refrigerant charges are 105g for transient operation (placing goods inside the compartments) and around 70g to 95g for stationary condition (without goods). Excepting for small refrigerant charges ~50g, the higher quantity of refrigerant during the entire system operation is in the evaporator. For low refrigerant charges, 50–70g, it is also shown a refrigerant migration between condenser and evaporator under the effects of thermal loads when compressor is turned on. Ambient temperature augmentation by 18°C decreases refrigerator COP without thermal load in 10% and 16% for pull-down and on/off operations, respectively. The presented experimental results are new and can be used by other researchers for validating their own simulation models. The developed models can considerably improve the design of domestic refrigeration systems by manufacturers.