An analytical model has been developed to predict the length of adiabatic capillary tubes used in domestic refrigerators and low-capacity residential air conditioners. The model predicts the length of two types of tubes-straight and spiral adiabatic capillary tubes. The proposed model is based on the homogenous two-phase flow model, which predicts the length of the adiabatic capillary tubes as a function of refrigerant mass flow rate, capillary tube diameter, degree of subcooling at capillary inlet, internal surface roughness, and the pitch of the Archimedean spiral. The existence of subcooled liquid at the entry of the capillary tube requires the computation of single-phase and then two-phase lengths of the tube. The McAdams et al. (1942) viscosity correlation has been used to evaluate the two-phase viscosity of the expanding refrigerant in the latter part of the capillary tube. All the thermophysical and transport properties of the refrigerants are evaluated by the REFPROP Version 7 database, which is based on the Carnahan-Starling-DeSantis equation of state. The simulation results are validated with the experimental findings of previous researchers. The performance of the above two geometries of adiabatic capillary tube is compared, and it is established that for the same state of refrigerants at the inlet and exit of the adiabatic capillary, spiral capillary is found to have a shorter length. Parametric study of the adiabatic capillary tubes is also carried out. Further, the effect of geometric and physical parameters has been presented in detail. A much smaller capillary tube is required when the eco-friendly refrigerants R-134a and R-I52a are used in place of refrigerant R-12for similar conditions across the adiabatic spiral capillary tube.
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