Three-dimensional modeling of heat transfer and fluid flow in a flat-grooved heat pipe

Abstract

Flat-grooved heat pipes (FGHP) are widely used in many applications from thermal management of electronic devices to space industry due to their robustness and ability of dissipating heat from the system effectively and reliably. FGHP is basically a container with micro grooves on the inner surfaces, and essentially a bridge that can transfer large amount of thermal energy between a heat source and a sink with small temperature differences by utilizing the phase change mechanism of the working uid. Heat source evaporates the working uid in the one end of the grooves, and due to the pressure difference, the composed vapor ows to the heat sink region in the other end. Then the vapor condenses back into the grooves before it ows to the evaporation region by the capillary force and repeat the cycle. Mathematical modeling of heat transfer and uid ow of FGHP's is crucial to understand the effects of many parameters (dimensions, groove shape, working uid filling ratio, material types) on their operational limits in order to design case-specific heat pipes. In the literature, many models are presented with some simplifications and assumptions. In this thesis, a computational methodology is proposed that models the heat transfer and uid ow fully in 3D for the first time, by using COMSOL Multiphysics R via LiveLinkTM for MATLABR

interface. Combining the exibility of script environment of MATLAB with the benefits of using energy and momentum solvers of a commercial software gives a powerful and practical tool that can overcome great difficulties if this modeling was to be done in a CFD software or an in-house code alone. In the presented model, radius of curvature (R) variation of the working uid in the groove, temperature gradient of the groove wall (Tw), and vapor temperature (Tv) are the essential working parameters of a heat pipe that re ects the efficiency. In this methodology, these variables are estimated initially, and are calculated by a set of inter-bedded and subsequent iterations. The momentum equations are solved for the iteration of R, Tw is iterated by solving the energy equations, and lastly Tv is calculated by the secant method using the conservation of mass. Depending on the values of the variables, the solution domain is regenerated and the phase change boundary conditions are recalculated at each iteration. The presented model is compared with the literature for validation. Then, a parametric study for investigating the effect of groove depth on the performance of a at-grooved heat pipe is conducted. Different power of heat sources are used for determining the dry-out in the grooves.