Quasi-one-dimensional numerical analysis of choked two-phase flashing flow through millitubes

Abstract

Small-diameter tubes are utilized widely as expansion devices in refrigeration systems. Performance of these tubes is reliant upon critical flashing of the two-phase flow that controls the mass flow rate of the refrigeration system, resulting in a steep reduction in pressure and temperature. Due to the evaporating two-phase flow and the choked-flow condition, numerical analysis of flow through short-tube orifices is challenging. Accordingly, all available numerical analyses are performed as one-dimensional flows. The most comprehensive method for analyzing such flows is the two-fluid model, which does not assume equilibrium between phases. However, in all previous applications of this model, two-phase flow calculations at the entrance and vena contracta regions were eliminated. In the current investigation, two additional steps were used to improve the accuracy of computations: (1) applying the most comprehensive two-fluid model, including the effect of various two-phase flow patterns and the metastability of liquid phase; and (2) performing a two-phase analysis of the evaporating flow through the entrance and vena contracta. The second step involves simulation of this region as a contraction from the up-stream diameter to the throat, and an expansion from the throat diameter to the tube diameter. Results showed more compatibility with experimental data in comparison with those of previous investigations for predicting the critical flow condition of common refrigerants hydrofluorocarbon (HFC)-134a, hydrochlorofluorocarbon (HCFC)-22, and HFC-410a through short-tube orifices. Finally, the developed numerical simulation was applied in order to develop selection charts for short-tube orifices based on the common refrigerant HFC-134a and the alternative newly released refrigerants hydrofluoroolefin (HFO)-1234yf and HFO-1234ze. These charts help industry select new sizes for the expansion tools of their systems in order to run the system with its previous mass flow rates.