Flow boiling heat transfer and pressure drop of R32 and R410A inside a horizontal small-diameter microfin tube.

This study experimentally investigated the flow-boiling heat transfer and pressure drop characteristics of R32 and R410A in a horizontal small-diameter microfin tube with an outer diameter of 3.0 mm and an equivalent diameter of 2.6 mm. The flow-boiling heat transfer and pressure drop characteristics were measured at mass velocities of 200 and 400 kg/(m2s); heat fluxes of 5, 20, and 40 kW/m2; and a saturation temperature of 15°C. The heat transfer coefficients of both refrigerants increased with increasing vapour quality in the long pre-dryout region. The heat transfer coefficient of R32 was higher than that of R410A in the regions dominated by the forced-convection heat transfer and thin liquid-film evaporation. However, the heat transfer coefficients of the two refrigerants in the region dominated by nucleate-boiling heat transfer exhibited little difference. The frictional pressure drop increased with increasing vapour quality in the range of x < 0.8 (where x is the vapour quality) and slightly decreased with increasing vapour quality in the range of x > 0.8. The frictional pressure drop of R32 was 1.3–1.4 times higher than that of R410A at the same mass velocity and vapour quality.