Numerical and experimental feasibility study of vapor chambers for LED applications

Abstract

A numerical model is developed to study the heat transport in a vapor chamber assembly and is verified with experiments. The liquid transport in the wick is described with Darcian theory. This is coupled with kinetic theory to describe the interfacial liquid — vapor transport. This model was implemented in a commercially available computational fluid dynamics (CFD) software and solved under transient operation using user-defined functions. Model sensitivity for various input parameters was studied. Results show a significant dependence on the variation of a newly introduced accommodation coefficient. A test vehicle was used to measure surface temperature of heat spreaders under various gravitational orientations. Comparing experimental and numerical results indicates that the vapor chamber model can predict thermal behavior and implies that internal geometry is an important input factor. Preliminary experimental results showed a higher thermal resistance of the vapor chamber than with solid copper, due to greater imperfections in thermal contact. The vapor chamber performed best under a gravity assisted orientation, as opposed to gravity unassisted orientations. Furthermore, at temperatures over 100°C, thermo-mechanical deformation of the vapor chamber container resulted from the increase in internal pressure. Bulk copper, bulk aluminum and vapor chamber heat spreaders were compared using simulation with results showing that the vapor chamber had the lowest thermal resistance as well as reasonably low weight. This suggests that a vapor chamber is a potentially promising technique for LED thermal management, but requires optimization of the thermal path, particularly the interface. Therefore, it is recommended to apply the numerical model in further research of vapor chambers integrated into heat sinks. This would eliminate thermal contact involved in testing between heat spreader and heat sink, exploiting benefits of the investigated heat spreading method.