An experimental study of flow boiling heat transfer enhancement in minichannels with porous mesh heating wall Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/s4655k186

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  • A unique channel surface enhancement technique via diffusion-bonding a layer of conductive fine wire mesh onto the heating wall was developed and used to experimentally study flow boiling enhancement in parallel microchannels. Each channel was 1000 μm wide and 510 μm high. A dielectric working fluid, HFE 7000, was used during the study. Two fine meshes as well as two mesh materials were investigated and compared. According to the flow boiling curves for each channel, the amount of wall superheat was greatly reduced for all the mesh channels at four stream-wise locations; and the critical heat fluxes (CHF) for mesh channels were significantly higher than that for a bare channel in the low vapor quality region. According to the plots of local flow boiling heat transfer coefficient h versus vapor quality, a consistent increasing trend for h with vapor quality was observed for all the tested channels until the vapor quality reached approximately 0.4. However, the three mesh channels showed much higher values of h than the bare channel, with the 100 mesh copper performing the best. Visualization using a high-speed camera was performed thereafter to provide some insights to this enhancement mechanism. A significant increase in nucleation sites and bubble generation was observed, and departure rates inside the mesh channels were attributed to the flow boiling enhancement. A sudden increase of h for mesh channels can also be attributed to the characteristics of nucleate boiling and indicates that nucleate boiling was the dominant heat transfer mode. Another interesting point observed was that the 100 mesh bronze outperformed the 200 mesh bronze for most of the studies. This suggests that nucleations happened inside the mesh openings, instead of on the mesh openings. In addition, an optimal mesh size should exist for HFE 7000 flow boiling.
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