An analytical and numerical investigation of asymmetrical flat-shaped heat pipes

The present study centers around a comprehensive modeling and theoretical investigation of the transient and steady-state operation and performance characteristics of asymmetrical flat-shaped heat pipes. These consist of a disk-shaped and a flat-plate heat pipes. A comprehensive analytical model based on an in-depth integral analysis and the method of matched asymptotic expansions has been developed to simulate the steady-state vapor and liquid flow for these types of heat pipes. This model incorporates the incompressible vapor flow in the core region of each channel, the liquid flow in the top, bottom and vertical wicks, the liquid-vapor and liquid-liquid hydrodynamic coupling, and the phase change at the vapor-liquid interfaces. The effects of non-Darcian transport through the porous wicks, vapor flow reversal, and the gravitational effects are also included in the model. The heat transfer capability of the flat-shaped heat pipes has been studied extensively and the criteria for the capillary limitation and the boiling limitation have been established. The influence of design parameters on the heat transfer capability of the flat-shaped heat pipes has been investigated comprehensively based on the capillary limitation. An optimization method for the flat-shaped heat pipe design has been proposed based on the parametric study and the boiling limitation. An optimized flat-shaped heat pipe design was obtained for removing high heat fluxes generated during the proton bombardment of the lithium target in a BNCT (Boron Neutron Capture Therapy) device.; The complete problem of the three-dimensional vapor flow in the asymmetrical flat-shaped heat pipe has also been studied numerically using a finite element scheme based on the Galerkin method of weighted residuals. The nonlinear differential elliptical equations of motion are solved over the entire vapor flow channel in order to allow all of the features of the incompressible laminar vapor flow to be taken into account. The three-dimensional effects and the effects of the secondary flow formation have been investigated. A very good agreement was found between the numerical results and the analytical results.; The analytical model has been extended to simulate the startup operation of the flat-shaped heat pipes. The conjugate heat transfer through the heat pipe wall, the liquid-wick structure, and the vapor region as well as the phase change at the vapor-liquid interfaces have been incorporated in the transient model. Analytical results covering the entire startup transient and the steady-state operation have been obtained.