Numerical investigation on flow boiling instability in a large length-to-diameter ratio microchannel with reservoir

Flow boiling instability in large length-to-diameter ratio microchannels poses significant challenges for large power and high heat flux dissipation in spacecraft. This study presents a two-dimensional numerical model based on the Volume of Fluid (VOF) approach to investigate flow boiling instability in such microchannels with integrated reservoirs. Key factors, including the compressible volume fraction in the reservoir, saturation pressure, and the diverging angle of the microchannel, are analyzed to understand their impact on pressure drop, velocity, and temperature instability. The results reveal that flow boiling instability arises from the complex interplay of bubble generation, growth, and coalescence, rather than being solely induced by compressible volume. Increasing the compressible volume fraction in the reservoir mitigates pressure drop fluctuations but amplifies oscillations in streamwise velocity and inner wall temperature. Lower saturation pressures intensify fluctuations in pressure drop, velocity, and wall temperature, resulting in greater instability. Conversely, diverging microchannel geometries reduce instability by facilitating smoother flow, delaying flow pattern transitions, and preventing local dryout. These findings provide critical insights for the design and optimization of microchannel systems to enhance flow boiling heat transfer performance. A balanced consideration of compressible volume, saturation pressure, and diverging angle is essential for achieving stable and efficient thermal management in practical applications.