An Unsteady Homogeneous Two-Phase Flow Model for Microchannel Oscillating Heat Pipes

Microchannel oscillating heat pipes (OHPs) are promising heat transfer devices for electronic chip cooling due to their wickless construction and high heat transfer capability. Despite the simple geometry of OHPs, their complex transport mechanisms and multiple regimes of operation presents a modeling challenge. The widely accepted slug-plug model is based on a two-phase slug flow pattern, and assumes that evaporation and condensation of a thin liquid film surrounding the vapor bubbles governs the heat transfer process. The flow oscillation of OHPs operating at low power as predicted by the slug-plug model generally agrees with experiments. But as power increases, the flow exhibits a variety of typical bubbly and churn flows which might render the slug-plug model unapplicable. In this work, we use a one-dimensional (1D) unsteady homogenous two-phase model to capture flow instabilities which is one of the mechanisms for the operation of OHPs with more highly dispersed vapor and liquid. The proposed model assumes that the two-phase flow is homogeneous and numerically solves three sets of coupled partial differential mass, momentum, and energy conservation equations. We applied this model to simulate the periodic fluctuation modes of the fluid velocity field inside a portion of OHP which consists of five turns microchannel under a background pressure surge. The fast-oscillating motion of the two-phase mixture transports energy along the microchannel with a time delay between the evaporator heat input and condenser heat rejection. Currently, the model does not consider the compressibility of vapor and other physical mechanisms that are also needed to model the complete closed-loop OHP. Therefore, to model the open-loop response, a unidirectional pressure drop boundary condition is applied as a step function to assure that oscillation of the flow occurs due to intrinsic flow instabilities, rather than an externally driven cyclic pressure drop. Overall, the simulation results provide insights into the physical modeling of flow instabilities inside microchannel OHPs at high heat flux operation and this work will be extended in the future to model a complete closed-loop OHP based on the two-phase homogeneous flow model.