Evaluation of Heat Transfer and Pressure Drop for the Heater-Core in an Automotive Heat Pump System

The heat transfer and pressure drop results for a heater-core of an automotive system are presented and discussed in this article. The heater-core is a type of compact heat exchanger that is used as part of an automobile heating-cooling system for heating the passenger cabin on cold seasons. The automotive heating-cooling system in this study includes a standard refrigeration cycle consists of a condenser, an evaporator, a compressor and an expansion valve using the refrigerant R134a as the working fluid. Furthermore, the system uses two separate secondary fluid loops using a 50% glycol-water mixture to exchange energy with the main refrigeration loop. During the cold weather season, the system is operated in the heat pump mode and one of the fluid loops is used to transfer heat from the condenser to the heater-core for heating the passenger cabin. The heat transfer from the heater-core to the passenger cabin is accomplished using air flow through the heater-core openings in an unmixed and cross-flow fashion. The air-side of the heater-core has a unique louver system that is intended to enhance the air-side heat transfer while the glycol-side has a twisted wire inserts to enhance flow turbulence and heat transfer. Semi-empirical correlations for the heat transfer and pressure drop for both glycol-water mixture and air flows in the heater-core are proposed. The flow of the glycol-water mixture in the heater-core is a single-phase flow within a bundle of parallel circular tubes with the twisted wire inserts. The flow of air through the heater-core is approximated as a flow across a finned-tube compact heat exchanger with continuous plate-fins. A modified Wilson plot technique is applied to determine correlations for heat transfer on both glycol-water mixture and air sides. The frictional pressure drop on the glycol-side is calculated from the total measured pressure drop and adjusted for pressure drops within manifolds and inlet/outlet ports. The results for the heat transfer and pressure drop analyses are finally plotted, discussed and compared with the relevant previous studies. These results show that the heat transfer rate is increased in the glycol-side due to the twisted wire inserts, in comparison with the smooth circular tubes. The air-side heat transfer rate is also enhanced due to the louvers in the air passages, as compared to flat-plate fins in compact heat exchangers.