Analytical and experimental study of flat plate heat pipes

Abstract

This dissertation is composed of two parts. In the first part, the performance characteristics of a flat plate heat pipe with a composite wick structure were studied both analytically and numerically. The vapor flow in the flat plate heat pipe for the case of non-uniform heat input to the evaporator surface and non-uniform heat removal from the condenser surface was investigated by an analytical method. By neglecting the inertia effect and the axial conduction heat transfer, with a suitable choice of stream function, the steady, incompressible Navier-Stokes equations were simplified to an ordinary, fourth-order, non-linear differential equation. The velocity profiles and the pressure distribution in the vapor region of the flat plate heat pipe were calculated and the results were presented. The results are valid for small radial Reynolds numbers. It may be applied for miniature/micro heat pipes in which the Reynolds numbers are usually small. As for the liquid phase in the flat plate heat pipe, the pressure distribution and velocity field of the working fluid in an anisotropic wick structure were studied analytically under block-heating condition. Using the method of Fourier expansion, the pressure distribution and velocity field for liquid phase were calculated. It was found that the pressure and velocity distributions depended strongly on the anisotropic property of the wick structure. The effects of the anisotropic property and the heater location on the maximum capillary pressure required were also discussed. Furthermore, the pressure distributions and velocity fields in both vapor and liquid phases and the overall performance of the flat plate heat pipe with localized block-heating conditions were presented. A numerical-analytical model for the arbitrarily block-heated flat plate heat pipe was also developed. In the vapor phase, the general three-dimensional Navier-Stokes equations were solved numerically. It was then matched with the analytical solution of the flow in the liquid-wick porous region. The effects of variations of the location of block-heating and anisotropic property of the wick structure were examined and discussed. The results were compared with existing results. Reasonable agreement was obtained.