Enhancement of the subcritical boiling heat transfer in microchannels by a flow-induced vibrating cylinder

The flow boiling heat transfer in microchannels has been extensively used in engineering due to its high heat dissipation with a small temperature difference. This study employs a hybrid method to numerically investigate the effects of a flow-induced vibrating cylinder on enhancing the subcritical boiling heat transfer in microchannels. The hybrid approach integrates the pseudopotential multiphase lattice Boltzmann method for modeling unsteady flows, the finite difference method for solving the heat transfer equation, and the immersed boundary method for handling the boundary condition at the fluid-cylinder interface. Flow boiling simulations in the microchannel are performed for three setups: a smooth vertical channel, a vertical channel with a stationary cylinder, and a vertical channel with a flexibly supported cylinder. Simulations have been conducted by varying the Reynolds number based on the diameter of the cylinder (Re-d) from 35 to 333.3, the dimensionless boiling number (Bo) from 0.001 84 to 0.045 97, and blockage ratio (BR) of 3.0, 4.0, and 5.0. It is found that the vortical wake of the cylinder is important in enhancing the heat transfer in microchannels, which is quantified by the (Re-d). Specifically, when Re-d < 48.0, both stationary and flexibly supported cylinders have almost the same effect on heat transfer during the flow boiling process, as there is no vortex shedding from both cylinders; when 48.0 <= Re-d < 68.2, the flexibly supported cylinder achieved higher enhancement than the stationary cylinder, which is due to the vortical wake generated by the flow-induced vibration in a subcritical Reynolds number regime; when 68.2 <= Re-d, both stationary and flexibly supported cylinders have comparable effect on the rates of heat transfer, because both cylinders generate similar vortical wakes. Flow field analysis indicates that the disturbance due to the vortex wakes on the thermal boundary and/or the vapor insulation layer is the mechanism of the heat transfer enhancement in channels.