Parametric Analysis of Non-Adiabatic Transcritical Flow in Capillary Tubes for a New Refrigeration Cycle

This paper presents a non-adiabatic homogeneous model of carbon dioxide flow in a novel integrated accumulator-expander-heat exchanger, which integrates the functions of refrigerant storage, expansion and heat transfer, and can lower weight and cost of the system. The model is based on the 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 choking situation at the capillary outlet is evaluated by local sonic velocity judgment. Relationships between cooling pressure, evaporating temperature, capillary size, and other parameters are presented and analyzed in detail. It can be seen that the heat transfer changes with different kinds of capillary tubes under different conditions. The present model can be used for both system design and performance evaluation. It is also very helpful in understanding the transcritical flow behaviour inside capillary tubes.