AN EQUILIBRIUM MODEL AND ASSESSMENT OF THE EFFECT OF METASTABILITY FOR STRAIGHT ADIABATIC CAPILLARY TUBES

Abstract

A numerical two-phase flow model is developed here for adiabatic capillary tubes in small capacity vapour compression refrigeration systems using alternative refrigerant (R-134a). The model is based on the fundamental equations of conservation of mass, energy and momentum that are solved simultaneously for a homogeneous flow under saturated, sub-cooled and two-phase conditions. The model also uses well established empirical correlation for friction factor. The effect of sudden contraction at the capillary entrance
is also considered in the model. To asses the effect of metastability; Chen’s model for the prediction of underpressure of vaporization is used. Using this value of under pressure of vaporization the change in the length of capillary and change in mass flow rate is determined to finally asses the percentage change in these parameter due to metastability.