Numerical Simulations of Conjugate Heat Transfer Effects on Single- and Two-Phase Cooling via Multi - Microchannels Heat Sinks

We present a numerical study of flow boiling in microchannels where we investigate the impact of the channel-aspect ratio and heat load on both single- and two-phase hydrodynamics and heat transfer. The numerical model reproduces the unit channel of a multi-microchannel evaporator, which features one fluidic channel and the surrounding evaporator walls. Two-phase simulations are performed using the Volume-Of-Fluid method in OpenFOAM v2106, augmented with self-developed libraries to include liquid-vapour phase-change, improve the surface tension force calculation, and incorporate the conjugate heat transfer between fluid and solid regions. The numerical framework is first validated versus experimental data from the literature. Then, the model is used to study flow boiling in a microchannel of hydraulic diameter of $D_{h}= 229\mu m$, width-to-height aspect-ratio in the range $\epsilon = 0.25 - 4$, and base heat flux $q_{b}= 50 - 200$ kW/ m². The operating fluid is water at atmospheric pressure, whereas the evaporator walls are made of copper. This study emphasises that conjugate heat transfer plays an important role on the temperature distribution on the channel walls. The evaporator base temperature increases monotonically with the aspect-ratio, as the thermal resistance of the system increases. The two-phase Nusselt number exhibits non-motonic trends versus $\epsilon$, with little variation in the range $\epsilon= 0.5 - 2$. Higher heat fluxes lead to higher Nusselt numbers because the bubbles grow faster, thus promoting thicker liquid films that better resist dryout.