Study of capillary tube applications in a transcritical heat pump system

Capillary tubes have been used in refrigeration systems for many years, but not with a transcritical CO[2] system. The effects of capillary tubes in a transcritical CO[2] refrigerating system have been investigated theoretically. This paper presents a non-adiabatic homogeneous model of carbon dioxide flow through a capillary tube, where the carbon dioxide undergoes a phase change from supercritical fluid flow to subcritical two-phase flow. The model is based on fundamental conservation equations of mass, momentum and energy. These equations are solved simultaneously through iterations. The in-tube flow can be divided into a single-phase region and a two-phase region. The extreme choked situation at the outlet is evaluated by local sonic velocity judgment. Relationships between gas cooler pressure, evaporating temperature, ambient heat transfer coefficient and other parameters are presented and analyzed in detail.