Simple models for the heat exchange from exhaust gas to super- and sub-critical refrigerant R134a at high temperature differences

The present paper aims at the simple modeling of the heat exchange of the exhaust gas to a super- and sub-critical fluid for heat recovery applications at high temperature differences and the analysis of their influence on the transferred heat flow. Thereto, a simple shell and tube exhaust gas heat exchanger prototype generating super-critical or saturated sub-critical refrigerant R134a was experimentally investigated. Simulation models based on heat transfer correlations from literature are compared to experiments, heat transfer coefficients quantified and some design improvements discussed. The experimental heat flows are reproduced quite well despite the models' simplicity. The influence of the fluid property changes transverse to the flow on the calculated heat flows is significant, while the influence of the sub-critical changes in flow boiling mechanisms is negligible.