R32 flow boiling in horizontal mini channels: Part II Flow-pattern based prediction methods for heat transfer and pressure drop

Abstract The heat transfer coefficient and pressure drop of R32 flow boiling in horizontal mini channels were investigated. The working conditions cover 1 and 2 mm channel inner diameters, 10–20 °C saturation temperatures, 50–600 kg m −2  s −1 mass fluxes, 10–30 kW m −2 heat fluxes and 0–1 vapor qualities. At high mass fluxes, the heat transfer coefficient increases with increasing vapor quality in slug, churn and annular flow, and then sharply decreases after dryout; at low mass fluxes, the heat transfer coefficient gradually decreases with increasing vapor quality in stratified flow. The heat transfer coefficient increases with decreasing channel size and increasing saturation temperature, mass flux or heat flux before dryout. The dryout quality, ranges from 0.7 to 0.9, increases with increasing channel size and decreasing saturation temperature, mass flux or heat flux. The pressure drop firstly increases and then decreases with increasing vapor quality. The pressure drop increases with decreasing channel size or saturation temperature and increasing mass flux or heat flux. Flow-pattern based predict methods were established with 92% of the heat transfer coefficient predictions and 90% of the pressure drop predictions being within ±25% of the experimental data including the previous and present studies.