Enhanced Boiling Heat Transfer in Parallel Microchannels With Diffusion Brazed Wire Mesh

Abstract

Flow boiling is an important process in energy conversion applications such as power generation and heating, ventilating, and air conditioning systems. Recently, it has drawn interest in the high heat flux electronics cooling community. Flow boiling enhancement, in addition, has the benefit of early onset of nucleate boiling, potentially lowering wall superheat, and increasing the heat transfer coefficient and critical heat flux (CHF). The objective of this paper was to investigate the use of fine metal wire mesh screens to enhance nucleate boiling in microchannels. Contact resistance between the wire mesh and channel heating surface was essentially eliminated by employing a diffusion brazing process to attach the screen to the wall. The parallel microchannels were 1000 μm in width and 510 μm in depth. A dielectric working fluid, HFE 7000, was investigated during this paper. Flow boiling results were compared for channels with and without wire mesh. According to the flow boiling curves obtained in this investigation for the bare and mesh channels, the amount of wall superheat was substantially reduced for the mesh channels at all four stream-wise locations. This indicated that nucleate boiling in the mesh channel was enhanced due to the increase of nucleation sites the mesh introduced. The mesh channels also displayed a higher flow boiling heat transfer coefficient. Both the nucleate boiling dominated regime and convective evaporation dominated regime were identified, with the overall trend of increasing the flow boiling heat transfer coefficient with respect to vapor quality until this quantity reached approximately 0.4. The CHF for the mesh channel was also significantly higher than that for the bare channel. The flow boiling enhancement was achieved with no apparent pressure drop penalty. The results presented here provide a practical means to implement this surface enhancement technique.