The Investigation On Boiling Heat Transfer Of Nano-fluids

The nano-fluids are a uniform, stable and new type heat transfer working fluids prepared by adding metal or non-metallic nano-particles into liquid in some ways. This concept was first proposed in 1995 by Choi et al, in the U.S. Argonne National Laboratory. Comparied with the traditional heat transfer medium, nano-fluids have excellent features at the aspect of enhancing heat transfer. The nano-fluids with good stability and high heat transfer characteristics used in the energy, chemical engineering, metallurgy, microelectronics, aerospace and other fields, will satisfy the high heat load and heat transfer intensity of heat exchanger requirements. In this paper, the Al2O3-water, CNTS-water and SiO2-water nano-fluids were prepared and boiling heat transfer characteristics of these three nano-fluids were studied in our paper.These three nano-fluids were prepared by "two-step method" in this article. The Uniform Design was creatively introduced in the study of the stability of nano-fluids. We took four influence factors including the concentration of nanoparticles (X1), the concentration of surfactant (X2), pH value (X3) and ultrasonic vibration time (X4). Each of these factors was set to 12 levels. Then the mathematical models of these three nano-fluids about zeta potential of the suspensions were found separately by using the quadratic polynomial stepwise regression analysis on the experimental results; These three kinds of nano-fluids on boiling heat transfer were investigated with transient method. The 5cm diameter nickel-plated copper ball was treated as boiling surface and heated to about 500℃through pit furnace. At last we obtained the boiling heat transfer curves of three kinds of nano-fluids by quenching in the nano-fluids with heated nickel-plated copper ball.Through analysis, we obtained the mathematical models of Al2O3-water, CNTS-water and SiO2-water nano-fluid about zeta potential of the suspensions separately and the best formula. (Al2O3-water nano-fluid:X1=0.22%, X2=1.77%, X3=2.15, X4=495mim; CNTS-water nano-fluid:X1=1.54%, X2=0.5%, X3=13, X4=5.25 h; SiO2-water nano-fluids:X1=3%, X2= 0.092%, X3= 10.94, X4=9 h;) The pH value of nano-fluids and ultrasonic vibration time have the greatest impact on the zeta potential of suspensions. The different extent interaction was found among the nano-particle concentration, dispersant concentration, pH value of suspension and ultrasonic vibration time.From the experiments, Al2O3-water, CNTS-water and SiO2-water nano-fluid have different extent of improvements on the boiling heat transfer characteristics. Al2O3-water nano-fluid moved up at the nucleate boiling region, but postponed at the transition boiling region; According to water the critical heat flux of CNTS-water nano-fluid was enhanced by 23% and SiO2-water nano-fluid was enhanced by 95%.