Enhancing Flow Boiling Heat Transfer in Microchannels by Surface Engineering

The continuous miniaturization of electronic chips leads to the generation of a huge amount of heat in small areas, which poses a significant challenge for the existing cooling system. Efficient cooling methods with the capability of dissipating a heat flux on the order of 10 MW/m2 are needed in order to meet the future cooling demand. Flow boiling in microchannels is a promising cooling mechanism to achieve this goal due to the large latent heat associated with the liquid-vapor phase change process, the convective two-phase flow through microchannels, and the large surface-to-volume ratio of microchannels. Over my Ph.D. period, I have explored several surface modification methods to enhance flow boiling heat transfer in microchannels. The underlying mechanisms of the performance enhancement were explained by visualization of flow patterns. ; My first study aims to improve the flow boiling heat transfer coefficient (HTC) through adding expansion areas in microchannels. Flow boiling experiments were conducted on samples with plain microchannels and microchannels with single or three expansion areas for comparison. Our experimental results show that expansion areas in microchannels can enhance the flow boiling HTC by 44.2%, compared to that of plain microchannels, without the obvious increase in pressure drop. As disclosed by the flow boiling curves and visualization of flow patterns, the improved HTC can be attributed to the early onset of nucleate boiling and the delayed dryout due to the low flow velocity near the corners of expansion areas. In addition, as the effective range of single expansion area is limited, increasing the number of expansion areas leads to the further enhancement of the HTC during the boiling process. This study provides an effective approach to enhance the HTC of flow boiling in microchannels through expansion areas. ; My second study is focused on enhancing the flow boiling performance by patterning pin fins in microchannels. Pin fins have been commonly used to improve the heat transfer ...