Experimental Investigation Of The Effect Of Ultrasound On Pool Boiling Heat Transfer

With the development of technology, both energy conservation and high performance equipment heat dissipation require much higher heat transfer enhancement technologies. So we need develop further new principles and heat transfer enhancement technologies. As a new active heat transfer enhancement technology, ultrasonic is paid more and more attentionin in recent years and has a great potential in the coming industrial application. So it is necessary to study heat transfer enhancement technologies by ultrasound.In this paper, the effect of ultrasonic wave on heat transfer of pure water and nanofluids was studied experimentally. The main investigations and conclusions of my work are as follows:(1) We designed and built the pool boiling heat transfer experimental setup and calibrated the setup with the Rohsenow empirical formula.(2) The effect of ultrasound on pool boiling heat transfer at different water subcooling degrees, ultrasonic generator power and ultrasonic emission distance was studied. It was found that the lower subcooled temperature, the larger the ultrasonic power and the shorter the distance to the heat surface, the more noteworthy the enhancement of heat transfer was. In addition, the heat transfer enhancement mechanism was analyzed.(3) By the high-speed photography, the heating surface characteristics under different heat flux in liquid temperature of 60℃,80℃,100℃ were visualized and the mechanism of heat transfer enhancement of ultrasonic was analyzed based on the bubble dynamics.(4) The pool boiling heat transfer performance influenced by different volume concentration of Al2O3 and Ag nanofluids was studied without ultrasound. The results showed the two kinds of nanofliuds had higher effectiveness of coefficient on heat transfer than pure water, especially for Al2O3 nanofluids. Accordingly, they all reduced the temperatures for the onset of boiling. All concentrations of Al2O3 nanofluid and the volume concentration of 0.001% Ag nanofluids had a higher critical heat flux than pure water. Nevertheless, the concentration of 0.0025%,0.005% Ag nanofluids were lower than pure water in critical heat flux.(5) The effect of ultrasound on the Al2O3 nanofluids boiling heat transfer performance showed the boiling curve shifted to the left when the wall superheat was less than about 3℃ for all concentrations of Al2O3 nanofluids. when the wall superheat was less than 8℃, the HTC enhancement by ultrasound was obvious for all the Ag nanofluids. When the boiling was in the fully developed nucleate boiling stage, the influence of ultrasonic on the heat transfer performance of two kinds of nanofluids was relatively weak. Compared with the boiling without ultrasound, the onset of nucleate boiling was reduced by 1℃. However, the influence of ultrasound on the critical heat flux was negligible.(6) By the scanning electron microscope (SEM), the heating surface characteristics of polished surface and those after boiling in nanofluids with and without ultrasound were analyzed. It was found that the surfaces after boiling with nanofluids sintered a nanoparticles adsorption layer and the roughness and cavity density increased a lot than the polished surface. The surface after boiling with Al2O3 nanofluids had a porous structure without ultrasonic. When the ultrasound was applied, we could see many cracks and nanoscale particles on the sintered layer. However, for the surfaces after boiling with Ag nanofluids, the structures of the surfaces were similar whether the ultrasound was applied or not, except that the roughness and cavity number of the heat surface became larger by using ultrasound.