Numerical and Experimental Characterization of a Plate Compact Multipass Counter Flow and Locally Cross-Flow Recuperator

Abstract

A compact and efficient heat exchanger for exhaust gas recovery energy was needed to raise the total efficiency of a thermo-photovoltaic system TPV (Thermo-Photo-Voltaic) for automotive applications (see [1]). In order to respect the strict condition of a high heat transfer surface to volume ratio, a heat exchanger configuration with a plate compact multi-pass counter flow and locally cross-flow recuperator has been chosen. The goal of this work is the understanding of the behaviour of the heat exchanger with numerical and experimental analysis for different geometrical and operating conditions. A high number of dimensions and manufacturing constraints was evaluated before reaching a definite design of a compact and efficient heat exchanger to be tested in the lab for initial experiments. The experimental work was needed in order to validate the numerical model. As the material needed for the real application could not be easily manufactured and instrumented in a workshop, a simplified real model, made of brass, was built, in order to compare numerical results and experimental findings. It was supposed that results obtained in this way would be sufficient to be considered valid when extrapolated in the real heat exchanger high temperature operating conditions and manufacturing material. The experimental results have been successfully compared with numerical ones obtained with the Fluent CFD code (release 6.2.16) Curves of performance (ε-NTU diagram plotted as a function of the ratio between the minimum and the maximum thermal capacities of the flows and pressure drop -mass flowrate diagram as a function of the average temperature) have been obtained and were useful to choose the adequate configuration for different applications, depending on the requested heat transfer and maximum allowable pressure drop. The output of the investigation was: heat transfer, outlet temperatures for both air flows, heat exchanger efficiency, differential pressure drop for both hot and cold sides. After this validation final numerical simulations have been carried out in order to understand the dependence of the heat exchanger efficiency on other geometrical parameters and operating conditions such as plates dimensions, numbers and height of vanes, operating pressure and so on.