An experimental study for heat transfer coefficient and pressure drop with two-phase flow phase change was conducted with R-1234yf in horizontal circular small tubes. The experimental facilities have been used to take data under various flow conditions with intensive study. The test section is made of stainless steel tubes with inner tube diameters of 1.5 mm and 3.0 mm, the length of 1000 mm and 2000 mm each. The small tubes were uniformly heated by engaging an electric current directly to the single tubes; all components were well insulated to prevent heat losses. Local heat transfer coefficients and pressure drop were obtained for a heat flux of 5 -40 kW m-2, a mass flux of 50-600 kg m-2 s-1, saturation temperatures of 0, 5, and 10°C and quality of up to 1.0. The effects of mass flux, heat flux, saturation temperature, and inner tube diameter on heat transfer coefficient and pressure drop are reported. Nucleate boiling heat transfer contribution was predominant, especially at low quality region, and laminar flow appeared in the evaporative small tubes. A new boiling heat transfer coefficient correlation for R-1234yf was developed.
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