Two-Phase Heat Transfer Behaviors of R-134a Refrigerant and Air-Water Mixture in a 1 mm Internal Diameter Tube

Flow boiling of R-134a refrigerant was experimentally conducted in a test section which is a stainless steel tube having internal diameter of 1 mm. The DC power supply was connected to the test section to provide constant surface heat flux conditions. Flow pattern and heat transfer data were obtained for a mass flux range of 252–820 kg/m²s, a heat flux range of 1–21 kW/m² and a saturation pressure of 8 bar. The flow visualization results showed four different flow patterns including slug flow, throat-annular flow, churn flow, and annular flow. The flow boiling heat transfer behaviors were also compared with those based on non-boiling two-phase air-water flow in the same test section under constant surface heat flux conditions. For non-boiling two-phase flow experiment, an air-water T-shaped mixer was served to introduce fluids smoothly along the test section. The results indicated that based on the same gas and liquid Reynolds numbers, flow boiling tends to have Nusselt number higher than that for non-boiling gas-liquid flow.