Mechanistic model to predict oscillating frequency of flow boiling in large length to diameter ratio micro-channel heat sinks

Abstract

This study presents a new mechanistic model for density wave oscillation (DWO) of flow boiling in horizontal micro-channel heat sinks that enables prediction of oscillation frequency, utilizing both flow visualization results and transient experimental data. The proposed model is based on the hypothesis that the period of oscillation consists of two sub-periods, flow surge and reversal, and establishes a set of 1D transient conservation equations. The flow surge period is associated with liquid rapidly entering the channels, momentarily establishing high pressure gradient between inlet and outlet plenums before the incoming flow is decelerated by a combination of increased flow resistance and bubble expansion within the channels. The initial location of the bubble nucleation is estimated using a criterion for onset of nucleate boiling (ONB) that governs bubble formation from cavities in superheated near-wall liquid. The flow reversal period is associated with back flow into the inlet and estimated by time elapsed before the reversed flow returns to its original location where bubble expansion and onset of annular flow are initiated. Unlike prior models, the proposed model simulates a full cycle of flow oscillation. In this model, a new mass conservation criterion is introduced to ensure agreement between predicted and experimental mass outflows during one cycle. Also developed is a detailed spatial and temporal description of the high density wave front (HDWF) largely responsible for both pressure drop and mass velocity fluctuations during the oscillation period. Finally, a periodic flow boundary condition is imposed on flow velocities and locations to achieve same flow behavior at the start and end of cycle. The proposed model shows good agreement with measured oscillation frequencies.