Synchronized visualization and thermal measurements of flow boiling in high-aspect-ratio microchannel with backflow controller

Microchannel flow boiling technology faces significant challenges due to the limited understanding of heat transfer mechanisms during various boiling regimes and lack of transient heat transfer analysis in literature. This study introduces a backflow control mechanism positioned before the inlet of a microchannel test section to suppress flow boiling instability. The aim is to elucidate the effect of the backflow control mechanism on flow boiling heat transfer by synchronizing high-speed flow visualizations with transient temperature data. Experiments are conducted on a microchannel heat sink (MCHS) with forty-four parallel microchannels, each having a hydraulic diameter of 315 mu m. The flow visualizations capture vapor belts, vapor cluster slugs, nucleate and bubbly flow boiling regimes, rewetting phenomena, and their temporal durations within the MCHS. Surface temperature measurements of the MCHS and fluid inlet and outlet temperatures are used to calculate the heat transfer coefficient. Significant alterations in hydrodynamics, including the duration of dominant flow boiling regimes and rewetting periods, are observed when the backflow controller is employed. While high-frequency transient heat transfer coefficient peaks are typical in conventional microchannel flow boiling systems, the use of a backflow controller reduces the amplitude of these peaks and prolongs their duration. The analysis demonstrates the backflow controller's effectiveness in mitigating oscillations and enhancing heat transfer.