Experimental analysis of non-equilibrium two-phase heat transfer in plate heat exchangers for organic Rankine cycle applications

Highly efficient two-phase heat transfer significantly contributes to the compactness and intensification of both heat transfer components and energy conversion systems. This, in turn, leads to reduced carbon dioxide emissions and improved energy efficiency. The focus of this thesis lies on the experimental analysis of non-equilibrium two-phase heat transfer in plate heat exchangers for organic Rankine cycle applications. The primary objectives are to uncover the underlying heat transfer mechanisms and to develop innovative prediction methods tailored to the targeted heat transfer processes.

The study commences with an analysis of prediction methods for non-equilibrium condensation heat transfer. State-of-the-art prediction methods developed for the non-equilibrium condensation are summarized and evaluated utilizing a database compiled from open literature. Based on the assessment, a new prediction method is proposed targeting the subcooled condensation region in accordance with the heat transfer mechanisms. Varying operating conditions experiments are conducted to investigate the non-equilibrium boiling with pure working fluids R1234yf, R1234ze(E), and their binary zeotropic mixtures in a plate heat exchanger. A dedicated test section is designed for quasi-local measurements, consisting of heat transfer coefficients, heat transfer area, and heat duties. The effects on heat transfer performance of critical parameters are investigated. The mixture effects are explored by comparing the experimental results between the pure working fluids and the zeotropic mixtures. A database is established to evaluate the previous heat transfer correlations. Moreover, new prediction methods are proposed for accurately predicting local heat transfer coefficients during non-equilibrium boiling in plate heat exchangers, based on the sensitivity analysis, dimensional analysis, and regression method.

The main results indicate that the newly developed prediction method for the subcooled condensation region achieves a deviation of 12 %. Onset of nucleate boiling is triggered when the heat transfer wall reaches a specific superheat from 0.9 oC to 2.6 oC, which is proportionate to the normal boiling point of a certain working fluid. Furthermore, the subcooled boiling plays a crucial role in the non-equilibrium boiling, occupying up to 30 % of the total heat transfer area at operation conditions of large mass flux. New prediction methods proposed for the non-equilibrium boiling using pure and zeotropic mixture working fluids provide good predictive performance, with deviations remaining below 18 %.