Computational fluid dynamics analysis of flammable refrigerant leakage through a microcrack

Abstract

Hydrofluorocarbons(HFCs) are potent greenhouse gases in refrigeration systems, and their contribution will accelerate climate change. In the context of global warming, low global warming refrigerants have to be reconsidered, these refrigerants are commonly flammable, so systems that use them can pose a risk. Thus, an analysis of potential pipe leaks in refrigeration systems is urgently needed. In existing research on refrigerant leak, most studies have examined holes as leak points. However, the true morphology of leak points is commonly long and extremely narrow. Therefore, a round-pipe microcrack leak model with crack opening displacement (COD) ranging from 10 to 50 µm were built to perform a simulation and experiment. The Fluent (v.20, ANSYS Inc.) software performed computational fluid dynamics analysis. The R290, R32, and R1234yf refrigerants were simulated, and the orifice flow equation was introduced to calculate leakage from the microcrack. An axial and circumferential microcrack model was constructed, and the experiment validated the simulation with largest relative error of 2.9%. The simulation results indicated that for side flow through the microcrack, using the conventional discharge coefficient (Cd) in the orifice equation may be unsuitable. Accordingly, based on the simulation results, empirical equations for predicting the Cd of axial microcrack when R290, R32, and R1234yf are leaking are obtained. Additionally, the effect of length-to-diameter ratio and orifice-to-pipe-diameter ratio on Cd are also discussed. Compared with existing studies, a smaller infinity discharge coefficient value (Cd∞) was observed when COD < 30 µm.