Boiling Experiments on Vertically Aligned Carbon Nanotubes and Using Surface Micromachined Thin Film Thermocouples (TFT)
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The objective of this investigation was to study saturated and sub-cooled pool boiling on nano-structured surfaces. Experiments were conducted with horizontal flat surfaces that were coated with vertically aligned multi-walled carbon nano-tubes (MWCNT). MWCNT forests of two distinctly different heights: Type-A and Type-B, with heights of 9 and 25 macrons, respectively, were synthesized on a silicon wafer using chemical vapor deposition. The diameter of the MWCNT varied from 8 to 16 nm. Experimental results were obtained over the nucleate boiling and film boiling regimes under saturated and sub-cooled (5C) boiling conditions. PF-5060 was used as the test fluid. To measure the wall temperature, micromachined array of temperature sensors were fabricated on a silicon wafer. The micromachined thin film thermocouple (TFT) arrays were constructed using metal evaporation and "lift-off" process. The thickness of the deposited metal layers was less than 250nm. The thermocouple junction sizes varied from 30-50 microns. Results showed that the CNT enhanced nucleate boiling by over 300% at a given wall superheat and the heat transfer augmentation was not sensitive to the height of the CNT. During film boiling the height of the CNT below 10 microns did not affect the heat transfer (compared to a bare Silicon wafer). However, for height of CNT greater than 10 microns - up to 70% heat transfer augmentation was observed in film boiling. This is consistent with dynamic models of film boiling reported in the literature.
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9th AIAA/ASME Joint Thermophysics and Heat Transfer Conference
