Experimental Investigations of Flow Boiling Heat Transfer under Near-Critical Pressure for Selected Working Fluids

Abstract

This paper deals with experimental investigations of flow boiling in tubular ducts of selected refrigerants—R134a, R507A, and R600a—under near-critical pressures. Near-critical boiling is characterised by low specific enthalpy of evaporation. The positive effect of this feature is the fact that only a small amount of heat consumed by Organic Rankine Cycles is at a constant temperature. This allows a lower terminal temperature of the heating fluid and more effective utilisation of heat sources, especially of low-grade heat sources. The experimental investigations covered a heat flux density of 0.4 to 10 kW/m2 and a mass velocity of 60 to 200 kg/(m2·s). The results of the experimental data were compared to the modified heat transfer correlation of Gungor and Winterton, which provices the best fit for the obtained experimental data. The maximum heat transfer coefficient occurred at the two-phase quality—approximately 0.4 for all the tested fluids under high pressure conditions—which may be thought of as a characteristic feature of the boiling process under near-critical conditions. A modified Gungor–Winterton correlation improves prediction accuracy, especially under the lowest (up to 3 kW/m2) and highest (over 7 kW/m2) heat flux densities for all the tested fluids.