Abstract

The investigation summarized in this report explores a new efficient computational method for predicting heat and mass transfer near liquid-vapor interface in a micro bubble heat pipe. In the micro bubble heat pipe considered here, net vaporization occurs over heated portions of the interface and net condensation occurs on cooled portions of the interface. A dynamic equilibrium is reached when the condensation just balances the vaporization. The dimensions of the micro heat pipe may be comparable to the mean free path of the inside gas molecules. Since the transport may then fall in the transition regime between continuum and free molecular transport, a stochastic modeling scheme, the direct simulation Monte Carlo (DSMC), is used to simulate the transport by modeling the molecular transport from the evaporation end to the condensation end. A new treatment of the boundary conditions at the bubble interface is used which properly accounts for the energy and mass balance at the interface as well as thermodynamic requirements that must be imposed there. Parallel computing is successfully applied in this simulation by parallelizing the originally sequential DSMC scheme to shorten the otherwise over 10 hours of computation running time. Different domain decomposition strategies are applied to the parallelization. Dynamic load balancing is realized to further improve performance. A nearly linear speedup is found in the tests with up to 64 parallel processors of T3E Cray super computer, despite the huge amount of message communication among processors in this 3-D problem. Simulation predictions of temperature, pressure, molecular density profiles, and molecular flow speed profile from the simulation are shown to agree with the theoretical analysis of the resulting compressible flow. The results provide insight into the mechanisms of transport in the micro bubble heat pipe and parametric effects on its heat transfer capacity.

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