Abstract
• High temperature flow boiling of seven working fluids is experimentally analyzed. • Two heat transfer mechanisms are identified among different working fluids. • A superposition model is developed for the heat transfer coefficient prediction. • The heat transfer coefficient of propane is up to 152% higher than that of R236fa. • Isobutane has the lowest frictional pressure drop among the seven working fluids. 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.
Highlights
Organic Rankine cycle (ORC) technology has gained worldwide acceptance as an efficient way to utilize low-grade heat [1], and the ORC market has seen a near exponential growth in the last decade [2]
The heat transfer process can be classified in three categories: i) a convective boiling-dominated process that is characterized by a strong dependence of heat transfer coefficient on the mass flux and a weak dependence on the heat flux (e.g. Refs. [3,4,5]), ii) a nucleate boiling-dominated process that is characterized by a weak dependence of the heat transfer coeffi cient on the mass flux and a strong dependence on the heat flux (e.g. Refs. [6,7,8,9,10]), and iii) a co-existing process of convective and nucleate boiling where the heat transfer mechanisms vary with the working conditions (e.g. Refs. [11,12,13,14,15])
In addition to the above-mentioned three most common conclusions, Palm and Claesson [16] and Kim et al [17,18] stated that a heat transfer process known as the thin-film evaporation is more appropriate than the nucleate boiling process in order to explain the phenomenon that the heat transfer coefficient is significantly affected by the heat flux
Summary
Organic Rankine cycle (ORC) technology has gained worldwide acceptance as an efficient way to utilize low-grade heat [1], and the ORC market has seen a near exponential growth in the last decade [2]. One example is the diverse results of the main heat transfer mechanisms of flow boiling in PHEs identified by existing research works. The heat transfer process can be classified in three categories: i) a convective boiling-dominated process that is characterized by a strong dependence of heat transfer coefficient on the mass flux and a weak dependence on the heat flux [3,4,5]), ii) a nucleate boiling-dominated process that is characterized by a weak dependence of the heat transfer coeffi cient on the mass flux and a strong dependence on the heat flux [6,7,8,9,10]), and iii) a co-existing process of convective and nucleate boiling where the heat transfer mechanisms vary with the working conditions Due to the compact structure and small size of the PHE channel, a bubble is elongated, deformed and
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