Transient three-dimensional modeling of flat heat pipes with discrete heat sources

A stable numerical procedure is developed to analyze the transient performance of flat heat pipes for large input heat fluxes and high wick conductivity in the incompressible flow limit. Computation of flow and heat transfer in a heat pipe is complicated by the strong coupling among the velocity, pressure and temperature fields with phase change at the interface between the vapor and wick. A structured collocated finite volume scheme is used in conjunction with the SIMPLE algorithm to solve the continuity, energy and momentum equations. A numerical scheme is devised to compute system pressurization in the incompressible limit using overall mass balance. The stability of the standard sequential procedure is improved by accounting for the coupling between the evaporator/condenser mass flow rate and the interface temperature and pressure as well as the system pressure. One-dimensional, two-dimensional and three-dimensional models have been developed using the improved and standard numerical scheme to analyze the transient and steady-state performance of flat heat pipes. The model predictions are validated by comparing the heat pipe wall temperatures against experimental measurements. Numerical wall and vapor temperature predictions in a cylindrical heat pipe are also benchmarked with experimental and model results in the literature. Two-dimensional simulations of vapor chambers are performed to analyze the heat spreading characteristics of typical flat heat pipes. Comparisons are made with experimental measurements of the cold and hot wall temperature profiles, as well as with simulations of solid copper spreaders. In the three-dimensional analysis with discrete heat sources, predictions are made of the magnitude of heat flux at which dry-out would occur in a flat heat pipe. The input heat flux and the spacing between the discrete heat sources are studied as parameters. The location in the heat pipe at which dry-out is initiated is found to be different from that of the maximum temperature. The algorithm that has been developed to analyze the transient and steady-state performance of flat heat pipes is implemented into a commercial software package through user defined functions, for ease of use in practical applications