The present study is to enhance the critical heat flux (CHF) in pool boiling from a flat square heater immersed in nanofluid (water mixed with extremely small amount of nanosized particles). The test results show that the enhancement of CHF was drastic when nanofluid is used as a cooling liquid instead of pure water. The experiment was performed to measure and compare pool boiling curves of pure water and nanofluid at the pressure of 2.89 psia (Tsat=60 °C) using 1×1 cm2 polished copper surfaces as a boiling surface. The tested nanofluid contains alumina (Al2O3) nanoparticles dispersed in distilled and deionized water. Tested concentrations of nanoparticles range from 0 g/l to 0.05 g/l. The measured pool boiling curves of nanofluids saturated at 60 °C have demonstrated that the CHF increases dramatically (∼200% increase) compared to pure water case; however, the nucleate boiling heat transfer coefficients appear to be about the same.
The present study is an experimental investigation of the nucleate pool boiling heat transfer enhancement mechanism of microporous surfaces immersed in saturated FC-72. Measurements of bubble size, frequency, and vapor flow rate from a plain and microporous coated 390 μm diameter platinum wire using the consecutive-photo method were taken to determine the effects of the coating on the convective and latent heat transfer mechanisms. Results of the study showed that the microporous coating augments nucleate boiling performance through increased latent heat transfer in the low heat flux region and through increased convection heat transfer in the high heat flux region. The critical heat flux for the microporous coated surface is significantly enhanced over the plain surface due to decreased latent heat transfer (decreased vapor generation rate) and/or increased hydrodynamic stability from increased vapor inertia; both of which are a direct result of increased nucleation site density.
Experimental investigations were performed to understand the fundamentals of pool boiling heat transfer in nanofluids. The pool boiling curves of water and nanofluids at the pressure of 2.89 psia (Tsat = 60°C) were obtained and compared using a flat square (1 × 1 cm) heater. The tested nanofluids contain aluminum oxide (Al2O3) nanoparticles dispersed in distilled-deionized water. The concentrations of nanofluids range from 0 gram/liter to 0.05 gram/liter. The results show that the boiling heat transfer coefficient is independent of concentrations of nanofluids. Remarkable enhancement (~200%) of CHF was achieved for low concentrations of nanofluids (above 0.01 gram/liter). The boiling parameters, such as bubble size and departure frequency, were measured and analyzed using a 390-μm-diameter platinum wire. The measurement reveals that the size of bubbles increases and the bubble frequency decreases significantly in saturated nanofluids as compared to those in pure saturated water. The surface orientation effects on boiling heat transfer in nanofluids are also investigated. The results show that CHF enhancement of nanofluids is more effective as the boiling surface faces downward.
The positron work function of 6H-SiC was determined to be −2.1±0.1 eV from an analysis of the energy spectrum of positrons reemitted from the surface. The positron reemission yield, highest in the sample inserted into vacuum after atmospheric exposure and cleaning with ethanol, was significantly reduced after sputtering with 3 keV, 125 μA min Ne+ ions. The yield was not recovered even after annealing at 900 °C, presumably due to the stability of sputter induced defects. Sputtering at lower energies caused a smaller decrease in the reemission yield that was largely recovered after annealing at 850 °C. Analysis using electron induced Auger electron spectroscopy and positron-annihilation-induced Auger electron spectroscopy indicated that the surface was Si enriched after sputtering and C enriched after subsequent annealing. Values of positron diffusion length and mobility in the unsputtered material were extracted from the dependence of the reemission yield on the beam energy. The application of SiC as a field-assisted positron moderator is discussed.
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