Considerations on Optimum Design of Micro Heat Pipe Sinks Using Water as Working Fluid

Abstract

The successful operation of RF wideband gap devices dissipating high levels of heat flux requires effective cooling techniques. One of the most promising thermal management strategies is the use of micro heat pipes (MHP). These devices are very thin profile heat spreaders that can be directly attached to the GaAs or GaN substrate whose function is to allow the spreading of the heat flux almost laterally within a 300 µm thickness. The objective of the work presented was to estimate the convective heat transfer coefficient of a micro-channel heat sink corresponding to a maximum amount of heat removed from a heat source placed on the top surface of the sink. This approach taken used an optimal control technique in which the solution of the heat equation is controlled by the convective boundary condition by taking the heat transfer coefficient as the control parameter. A conjugate gradient method was used to solve the optimal control problem. The results show that the temperature distributions corresponding to the controlled solution are lower than those corresponding to the uncontrolled solution. The difference between the controlled and uncontrolled temperatures (4K -10K) is smaller than in the case of a planar spreader. This suggests that the convective coefficient corresponding to a finned spreader approaches by design an optimum distribution. The effect of liquid charge on the performance of MHP sinks is important. A too large amount of fluid leads to a condenser flooding, while a too low fluid charge leads to an evaporator dry-out and an increase in channel wall temperature. An iterative scheme was devised to compute the liquid charge corresponding to the maximum heat transport capacity of the pipe. This study can provide guidance in designing MHP sinks, which have emerged as an effective technique for cooling electronic components.