Transient Thermal Buffer for Microchannel Flow Boiling Using Gallium Phase Change Material

Microchannel flow boiling presents an effective thermal management strategy for high heat flux microelectronics due to large surface area to volume ratios, high heat transfer coefficients, high device temperature uniformity, and relatively low flow rates and small fluid inventories compared with liquid cooling. However, devices such as laser diode arrays or power semiconductor devices often operate under highly transient heat loads such as cold startups or pulsed modes and have performance metrics which are sensitive to device temperatures. The low volume of microchannel packages reduces thermal resistance but often at a penalty in thermal capacitance, leading to large temperature fluctuations during transient heating events. Furthermore, flow instabilities present in microchannel flow boiling at steady state may be exacerbated under unsteady heating conditions. Phase change materials (PCMs), such as gallium, which offer high volumetric latent heat and thermal conductivity, may improve the transient thermal response by dampening the heating rate to the fluid when added in parallel to the heat flow path. The current study investigates the transient thermal response of a low thermal resistance silicon microchannel evaporator with 22 µm x 141 µm (hydraulic diameter, Dh = 38 µm) rectangular channels at the onset of boiling in R134a under a range of unsteady heat steps. Large temperature overshoots above the steady state temperature as well as strong vapor backflow into the inlet plenum were observed. The temperature overshoots increased with peak heat flux. Using the open-source finite difference software, ParaPower (DEVCOM Army Research Lab, Adelphi, MD, USA), the performance of gallium and gallium-copper composite PCMs on the device in single-phase operation was simulated using a range of PCM thicknesses and areas. The transient heat rate to the fluid was extracted and then applied to the experimental device for each case. Boiling initiated at lower temperatures with PCMs of higher thermal conductivity or with increased amount of PCM, but the peak temperatures and backflow were not significantly affected by the addition of the simulated PCM.