Graduate Thesis Or Dissertation
Enhancement of Heat Transfer in Pool and Spray Boiling with Microporous Coatings Public Deposited
The relentless increase in the heat flux dissipation levels in electronic devices has necessitated development of new thermal management methods that can handle such heat flux levels, while maintaining a low device temperature. Direct cooling of the electronic components using dielectric liquid coolants, along with phase-change heat transfer, has the potential to achieve this. In this work, the enhancement of boiling heat transfer by the use of a thermally conductive copper microporous coating with a dielectric coolant (3M Novec HFE-7100) under two configurations is studied: passive pool boiling, and spray impingement boiling.
Pool boiling experiments were performed on microporous surfaces and plain surfaces. The microporous surfaces, with coating thicknesses in the range 100 to 700 μm, and porosity of ~57%, showed a significantly lower boiling incipience temperature, enhanced the heat transfer coefficient by 50 – 270%, and enhanced the critical heat fluxes (CHF) by 33 – 60%, when compared to the plain surface. At low heat flux levels, the surface with a thicker microporous coating showed better performance than the thinner one. However, the thinner microporous coating resulted in higher CHF than the thicker surface. High-speed visualization was used to measure the nucleation site density, bubble diameter at departure, and bubble departure frequency. Based on a simple heat flux partition model, neglecting the heat transfer effects due to bubble coalescence, the individual modes of heat transfer (evaporative and single-phase) were computed. Reasonably good agreement between the partition model and the experimental data was obtained. On the plain surfaces, the evaporative and single-phase components were approximately equal, while on the microporous surfaces, the evaporative component was found to be significantly higher.
We also investigated spray boiling heat transfer performance on the microporous copper surface. Heat transfer data was measured using two full-cone spray nozzles spanning a range of volumetric flow rate from 1.1 cm3/s to 15.8 cm3/s, and liquid subcooling levels from 30 °C to 0 °C. The microporous surface showed an enhancement of 300% – 600% in the heat transfer coefficient at a given wall superheat compared the plain surface. The CHF also increased by up to 80%. Counterintuitively, we observed that the liquid spray at near-saturated temperature (0 °C subcooling) had higher heat transfer coefficient and CHF than the subcooled spray, on both surfaces. This likely results from the limited residence time of the liquid droplets in contact with the heater surface and the much higher efficiency of phase change heat transfer. The near-saturated spray undergoes phase change much faster than the subcooled liquid, removing heat more efficiently than the subcooled liquid. New correlations are proposed for predicting the CHF of spray impingement boiling on both plain and enhanced surfaces.
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