همبستگی انتقال حرارت مبتنی بر جریان سامان یافته برای R245fa در لوله 3 میلی متری Flow regime based heat transfer correlation for R245fa in a 3 mm tube

1. Introduction The use of Organic Rankine Cycles (ORCs) can help us to reduce the global CO2-emissions. ORCs convert low-grade heat into mechanical energy. Some typical applications of ORCs are waste heat recovery and producing electricity of geothermal energy, solar energy or biomass [1,2]. An ORC is a Rankine cycle which uses an organic fluid instead of steam. A typical example of an organic fluid used in ORCs is R245fa [3,4]. In order to size the evaporator of an ORC correctly, a good heat transfer correlation is needed. If the evaporator is not sized correctly, this will either impair the ORC’s performance or increase the construction cost [5]. In literature a large variety of heat transfer correlations exist [6–9]. Each correlation is based on experimental data within a certain range of working conditions. Hence, it is not certain if the correlation will work outside this range. This work focuses on ORC working conditions in small tube sizes. The small tube sizes are used in small-scale ORC applications e.g. heat recovery of a truck’s exhaust [10–12]. Charnay et al. [13] compared his experimental results (R245fa) with existing heat transfer correlations. None of the tested existing heat transfer correlations could predict the heat transfer coefficient very well at the high saturation temperatures relevant for ORCs according to the authors. The aim of this work is to present unpublished experimental heat transfer measurements of R245fa in a tube with an inner diameter of 3 mm. These measurements expand the dataset of Charnay et al. [14]. Further, the influence of the saturation temperature, mass flux, heat flux and vapour quality on the heat transfer coefficient was analysed. Finally a new heat transfer correlations was proposed.