Flow boiling heat transfer in minichannels at high saturation temperatures: Part I - Experimental investigation and analysis of the heat transfer mechanisms

This paper presents new experimental data concerning flow boiling heat transfer in minichannel at high saturation temperatures. The experimental data were obtained in a horizontal 3.00 mm inner diameter stainless steel tube with R-245fa as working fluid. The mass velocity ranges from 100 to 1500 kg/m2 s, the heat flux varies from 10 to 50 kW/m2 and the inlet vapor quality from 0 to 1. This experimental work is characterized by a saturation temperature ranging from 100 °C to 120 °C. Flow boiling heat transfer coefficients in these conditions have not been reported in the open literature so far. Four flow patterns are likely to appear in these conditions: intermittent flow, annular flow, dryout flow and mist flow regimes. The kind of flow pattern has a major influence on the heat transfer mechanisms. The influence of the mass velocity and the heat flux was investigated to identify the dominant heat transfer mechanisms. At high saturation temperatures, the experimental results clearly show the dominance of nucleate boiling over a wide range of vapor quality.