A CFD based investigation of two-phase flow behavior of alternative low-GWP refrigerants in an expansion capillary tube

Purpose The purpose of this paper is to investigate two-phase flow behavior of low-GWP alternatives (R600a, R1234yf, R290, R1234ze) in a capillary tube. Design/methodology/approach This study uses a computational fluid dynamics (CFD) framework using the Volume of Fluid (VOF) multiphase model in ANSYS Fluent to simulate two-phase flow in an expansion capillary tube. The phase change during flashing is modeled with the Lee mass transfer model, while turbulence is resolved using the k-ω SST model. The present CFD model is used to visualize flow patterns and analyze the flow characteristics of low-GWP refrigerants in a capillary tube. Findings The validated CFD model shows that two-phase flow evolves from bubbly to annular before becoming vapor-dominated at the exit. Among the alternative refrigerants, R290 exhibits the lowest exit temperature due to early flashing, while R1234yf behaves similarly to R134a with 5% lower mass flow rate and comparable pressure drop. R290 also provides the highest cooling capacity. Sensitivity analysis reveals that capillary tube diameter is the dominant parameter, causing 95–98% mass flow variation, whereas coil diameter has a marginal effect (approximately 2%). Originality/value This study addresses the critical industry transition to low-GWP refrigerants by investigating the two-phase flow behavior of alternative refrigerants (R290, R600a, R1234yf and R1234ze) in capillary tubes. A CFD-based VOF) framework is used to provide high-fidelity visualization of complex two-phase flow patterns and liquid–vapor interfacial dynamics in a helical capillary tube, which has not been explored in previous studies. The study further presents a direct and systematic comparison of flow behavior between next-generation low-GWP refrigerants and the conventional baseline refrigerant R134a. In addition, a comprehensive sensitivity analysis quantifies the influence of key geometric and operating parameters on flow characteristics specific to eco-friendly refrigerants. Overall, the outcomes deliver design-level insights for the selection and optimization of capillary tubes operating with next-generation environmentally sustainable refrigerants.