Experimental analysis of high temperature flow boiling heat transfer and pressure drop in a plate heat exchanger

Organic Rankine cycle technology has gained worldwide acceptance as an efficient way to utilize low-grade heat sources. Plate heat exchangers are the most common type of heat exchanger employed as evaporators in small-scale organic Rankine cycle units, in which a high saturation temperature is the prevailing working condition. However, there is a lack of research on high temperature flow boiling in plate heat exchangers. This paper presents an experimental analysis on flow boiling heat transfer and pressure drop characteristics in a plate heat exchanger, and the development of prediction methods for the heat transfer coefficient and frictional pressure drop. Seven working fluids, R134a, R236fa, R245fa, R1234ze(E), R1233zd(E), propane and isobutane, were tested at the reduced pressures of 0.45, 0.55 and 0.65, corresponding to saturation temperatures ranging from 55 °C to 141 °C, and various mass fluxes. Two heat transfer mechanisms, nucleate boiling and thin-film evaporation, were identified in the heat transfer processes of the different working fluids, due to the diversity in their thermo-physical properties. Moreover, the results indicate that propane and isobutane have higher heat transfer coefficients than the other working fluids, while R236fa has the lowest heat transfer coefficient. The frictional pressure drops show the same characteristics for all the working fluids, increasing with the increase of the vapor quality and mass flux and the decrease of the saturation temperature. A superposition model presented in the paper achieves a good prediction for the heat transfer data, with a 12.8% mean absolute percentage deviation. A correlation developed in a previous work by the authors enables a prediction with an 11.1% mean absolute percentage deviation for the pressure drop data. The prediction methods presented in the paper will facilitate the modelling and design of plate heat exchanger evaporators in organic Rankine cycle units.