Flow boiling characteristics of R-245fa in a minichannel at medium saturation temperatures

Abstract

This paper presents new experimental results concerning thermohydraulic characteristics of flow boiling at medium saturation temperatures. The experimental results were obtained in a horizontal 3.0mm inner tube diameter with R-245fa as working fluid. The mass velocity ranged from 100 to 1500kg/m2s, the heat flux from 10kW/m2 to 90kW/m2, the saturation temperature from 60°C to 80°C and the vapor quality from 0 to 1. Four flow patterns are likely to appear in these conditions: intermittent flow, annular flow, dry-out flow and mist flow. Heat transfer coefficients were measured and parametric analyses were performed on the effect of heat flux, mass velocity and saturation temperature on the local heat transfer coefficient. At 60°C and 80°C, the heat transfer coefficient is independent from vapor quality and mass velocity during intermittent flow whereas it is sensitive to heat flux. In annular flow, the heat transfer coefficient increases with increasing vapor quality and/or mass velocity. As a result, nucleate and convective boiling are both present. The pre-dryout database of 3070 data points is compared against several predictive methods from the literature. These comparisons are an extrapolation to test the validity of the prediction methods at medium saturation temperature in minichannels.