Enhanced Ambient Heat Rejection in Passive Thermal Management Systems

The combined trends of increasing computing power with the miniaturization of electronic devices brought about new challenges in terms of ambient heat rejection. The most simple and reliable ambient heat rejection method is natural air convection. However, this technique is limited in terms of the cooling power that can be dealt with. This work presents two technologies that can potentially increase the heat rejection rate to ambient air without using any moving part, thus ensuring a high reliability. The first technology considered uses ionic wind to increase the air flow through cooling passages. Ionic wind occurs when a high voltage potential is applied to an electrode with a large curvature – typically a thin wire or a needle. Due to the strong electric potential close to the electrode, a Corona discharge occurs and air molecules are ionized. The resulting ions induce an air flow through collisions with neutral molecules. In this study, the Corona current is characterized experimentally and a numerical procedure is developed to solve the electrohydrodynamics. A custom-built test bench is used to validate the numerical model experimentally. It is shown that ionic wind can increase the heat removal rate by up to 100% as compared to natural convection only. The second cooling enhancement technology considered is the addition of a chimney on top of the heat sink to increase the air flow through the cooling channels. A semi-analytical model based on thermal- and fluid equivalent resistance networks is developed. The model is validated using a commercial CFD package. Finally, a thermo-economic study is performed using genetic algorithms in order to compare the performance of both technologies versus natural convection only. A Pareto front combining the three technologies is constructed, allowing for cost-effective design decisions based on the cooling power requirements.Ph.D