Performance of Compact Heat Exchanger in Non-Perpendicular Cooling Airflows

Doktorsavhandling, 2015

During recent years the main focus in the vehicle industry has been on cutting the fuel consumption and emissions as well as improving the vehicle performance. To be able to meet these demands additional systems are being introduced into the vehicle. These implementations do not only affect the engine power and the emission levels, they also tend to increase the operating temperature in the engine bay, which in turn increases the cooling demand. Therefore, a trend toward increased cooling performance for vehicles is also seen. There are a number of solutions to solve this demand and for heavy vehicles the most suitable way is to install additional heat exchangers positioned at other locations in the vehicle, due to the limited underhood space. These extra heat exchangers may not be located in the most appropriate position and it is not unusual that the airflow is not perpendicular to the heat exchanger core. For some vehicles today these types of installations can already be seen. Therefore, it is important to evaluate the effects on cooling airflows. Since both heat transfer and pressure drop over the heat exchanger will be affected by the angled airflow these parameters as well as the flow field characteristics, have to be analysed and evaluated. This thesis presents the evaluation of the performance of standard automotive compact heat exchangers and their performance in non-perpendicular airflows. Four angles have been tested to predict variances in pressure drop, heat transfer rates and airflow characteristics. Laboratory experiments and 3D Computational Fluid Dynamics (CFD) simulations have been conducted to study the effects. Methods have been developed to simulate heat exchangers in angled conditions as well as internal parts of the core. The results have been correlated with the experiments to find similarities and deviations. The results showed that the additional loss due to the angling of the heat exchanger, is due to the forced re-direction of the airflow into the core. This loss is increased with the magnitude of the angling. Neither the static pressure drop nor the heat transfer rate was significantly affected by the inclination angle of the heat exchanger relative to the airflow. To reduce the pressure drop within the installation the surrounding geometries had to be considered to prevent areas of losses. If a specific installation is going to be evaluated the information presented in this thesis is of great importance and could be used to find an optimal design for the system.