The effect of fluid properties on development of two-phase flow after an expansion valve based on the comparison of R245fa and R134a

This paper analyzes the developing adiabatic two-phase flow after an expansion valve in an air conditioning system. R245fa and POE 32 mixture is used as the working fluid. The developing two-phase flow patterns are visualized in an 8 mm straight tube after the expansion valve from 15 to 595 mm. To explore the effect of fluid properties on the developing two-phase flow, the experimental results from previous research by the same au-thors are compared with those of R245fa in this paper. An image analysis technique and a one-dimensional developing two-phase flow model proposed in a previous paper by the same authors (Yao & Hrnjak, 2023c) are applied to both R134a and R245fa. The flow patterns near the expansion valve exhibit some similarities when comparing the two refrigerants. However, a significant contrast emerges in the presence of a greater number of droplets suspended within the vapor phase in the case of R245fa flow. This disparity is primarily attributed to R245fa's higher vapor velocity. Furthermore, R245fa exhibits a shorter developing length than R134a. For fully developed flow, R245fa showcases a more pronounced turbulent behavior, and the transition to annular flow happens at a lower vapor quality in comparison to R134a. All of these distinctions can be adequately accounted for by the proposed model. The developed flow patterns of both refrigerants are compared with some existing flow pattern maps. At last, a new flow pattern map is proposed based on the observed flow patterns in this research.