Thermal performance of a vapor chamber with synergistic effects of droplet jumping and pillared-wick capillarity

Abstract

• Thermal performance of jumping-droplet vapor chamber was experimentally studied. • The number of auxiliary wick columns shows nonmonotonic effect on the performance. • Increased inner chamber height reduces thermal resistance and gravity dependence. • Jumping droplets may not directly reach the evaporator surface. • The minimum thermal resistance of the vapor chamber reaches 0.064 K/W. When condensate droplets coalesce on a superhydrophobic surface, the released surface energy may yield the merged droplet to jump off the substrate. Based on such principle, jumping-droplet vapor chamber (JDVC) has been proposed as a novel type of heat spreader for hot spot cooling, while the internal working mechanism is rather complicated and still remains inadequately understood. In the present work, the thermal performance of JDVC with auxiliary capillary pillars were experimentally studied, with particular interest on understanding the synergistic effects of droplet jumping and pillared-wick capillarity. Five JDVC samples were fabricated (with a fixed diameter of 97.6 mm, three different overall chamber heights of 3.4 mm, 3.9 mm and 4.4 mm, and a fixed heating area of 1.5 cm 2 ) to investigate the effect of filling ratio, number of auxiliary wick columns and inner chamber height on performance. Results show that, the thermal resistance shows a non-monotonic variation of “decrease-increase” with increasing filling ratio, with a higher heat flux favoring a higher optimal filling ratio. The temperature homogeneity is improved under bottom heat mode compared to under top heat mode, which could be possibly attributed to the improved efficiency of droplet removal. The number of auxiliary wick columns shows non-monotonic effect of “enhance-suppress” on the thermal performance, suggesting that a moderate number of auxiliary wick columns helps to gather the jumped-off droplets in space and facilitates stable capillary-driven working fluid circulation. Increasing the inner chamber height yields reduced thermal resistance and improved gravity independence, indicating that direct arrival of jumping droplets on the evaporator surface may not be the primary way of working fluid recirculation, otherwise the thermal performance should degrade since less jumping droplets are able to reach a higher evaporator surface. Among the results, the minimum thermal resistance of the vapor chamber reaches 0.064 K/W and the critical heat flux exceeds 253.2 W/cm 2 .