ENHANCEMENT OF SPRAY COOLING HEAT TRANSFER USING EXTENDED SURFACES AND NANOFLUIDS

Abstract

Spray cooling is a powerful heat transfer technique in which an atomizing nozzle provides a flow of liquid droplets directed towards a hot surface. This dissertation explores two potentially powerful techniques capable of improving traditional spray cooling: nanofluids and extended surfaces. Nanofluids were experimentally studied in a pool boiling system to elucidate the underlying mechanisms of critical heat flux (CHF) enhancement. Dilute suspensions of nanoparticles were found to have a degrading or no effect on boiling performance. Greater concentrations (≥ 0.5 g/L) lead to modest (up to ~37%) increase in the CHF. The results were highly dependent on the working fluid/substrate combination, specifically wetting characteristics. Poorly wetting systems (e.g. water on copper) could be enhanced by nanofluids, whereas better wetting systems (e.g. ethanol on glass) showed no improvement. This conclusion was re-enforced when nanofouling caused by dryout of nanofluid was found to improve wetting as shown by a reduction in the advancing threephase contact angle. Interestingly, similar CHF enhancement was achieved without nanofluids using an oxidized surface, which is easily wetted with pure fluids. In fact, surface treatment alone resulted in similar CHF enhancement at ~20°C less wall superheat than required using nanofluids. Spray cooling was found to be adversely affected by the addition of nanoparticles due to changing thermophysical properties and/or nozzle clogging due to particle deposition. The addition of high aspect ratio open microchannels to the sprayed surface resulted in significant enhancement at all wall superheats and over 200% enhancement in the low temperature single-phase regime. The two-phase regime began at lower temperatures with microchannels, which lead to heat transfer enhancements of up to 181%. The onset of two-phase effects was found to be a strong function of channel depth. However, the onset of two-phase effects was found to occur at a temperature that was independent of nozzle pressure/mass flow rate. Therefore, nucleation and two-phase effects are likely triggered by the unique liquid distribution caused by the extended structures. Using high aspect ratio open microchannels, these mechanisms resulted in spray efficiencies approaching one, indicating almost complete utilization of the spray's ability to absorb heat.