Simulation on flow of carbon dioxide in capillary tubes in a transcritical refrigeration cycle

The flow of carbon dioxide in capillary tubes in a trans-critical cycle was divided into three regions, i.e., supercritical gas region, liquid region and two-phase region, according to its state change. Homogenous flow mathematical models were established for this three regions, the flow of carbon dioxide was simulated with these models. The results show that the flow of carbon dioxide in capillary is mainly single phase flow and the length of two-phase flow is short, and the quality of carbon dioxide in the two-phase region increases rapidly; when the inlet temperature increases, the length of supercritical gas flow increases and the length of liquid flow decreases, even to 0 m, and the length of two-phase flow changes in a small range, the total length of capillary tube needed is nearly proportional to inlet pressure and approximately inverse proportional to inlet temperature when other parameters are fixed. The change of capillary length needed is remarkable when inner diameter or mass flow rate is changed.