R32 flow boiling in horizontal mini channels: Part I. Two-phase flow patterns

Abstract The two-phase flow patterns 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. Slug flow, churn flow, annular flow, dryout, mist flow and stratified flow were observed. The slug flow region is smaller with increasing mass flux and is larger with increasing tube diameter, saturation temperature or heat flux; the annular flow region is smaller with increasing tube diameter, saturation temperature, mass flux or heat flux; the mist flow region is larger with increasing saturation temperature, mass flux or heat flux and is smaller with increasing tube diameter. The existing flow-pattern maps cannot well predict the flow patterns of R32 flow boiling in mini channels. By adopting the existing correlations and introducing Bo number, a new flow-pattern map was established and it agrees 97.6% of the experimental data in this study.