VAPOR DYNAMICS OF HEAT PIPE START-UP

Vapor dynamics of heat pipes during the start-up phase of operation is analyzed. The vapor flow is modeled by a two-dimensional, compressible viscous flow in an enclosure with inflow and outflow boundary conditions. For high-input heat fluxes, a compression wave is created in the evaporator early in the operation. A nonlinear filtering technique, along with the centered difference scheme, is used to capture the shocklike wave and overcome the cell Reynolds number problem. Multiple wave reflections are observed in the evaporation and adiabatic regions. These wave reflections cause a significant increase in the local pressure and flow circulations, which grow with time. It is shown that the maximum and maximum-averaged pressure drops oscillate periodically because of the wave reflections. Although the pressure drops converge to a constant value at steady state, they are significantly higher than their steady-state value at the initiation of the process. The time for the vapor core to reach a steady-state condition was found to be on the order of seconds.