EXPERIMENTAL STUDY ON ALN/H_2O AND AL_2O_3/H_2O NANOFLUID FLOW BOILING HEAT TRANSFER AND ITS INFLUENCE FACTORS IN A VERTICAL TUBE

摘要

In this work, AlN/H_2O and Al_2O_3/H_2O nanofluid was prepared by an ultrasonic oscillation. Moreover, saturated flow boiling heat transfer in a vertical tube is experimentally investigated by nanofluid, with 0.1％ volume concentration and 30nm diameter for A1N and 20nm diameter for γAl_2O_3 .Several factors are under consideration, including heat flux on the heating surface (48-289kW·m-2), pressure (0.2-0.8Mpa) and mass flow rate (350-1100 kg·m-2·s-1). The results show that the saturated flow boiling heat transfer of AlN/H_2O nanofluid is improved mostly about 64％ compared with deionized water and Al_2O_3/H_2O nanofluid is improved mostly about 61％ compared with deionized water, and the average Nusselt number enhancement rates of nanofluid compared with deionized water are 23％ for Al_2O_3/H_2O nanofluid and 31％ for Al_2O_3/H_2O nanofluid in the range of this work. Furthermore, the heat transfer capacity of nanofluid increases with increasing heat flux on the heating surface and pressure. It is proved that nanoparticle deposited on the heating surface by SEM observations and TEM observations for nanoparticle confirm that nanoparticle does not change obviously after boiling. In addition, the enhancement rate of nanofluid saturated flow boiling heat transfer capacity increases with increasing pressure, but the influence of mass flow rate is negligible. In this study, as the thermal conductivity of A1N is greater than that of Al_2O_3, the heat transfer capacity of AlN/H_2O nanofluid is greater than that of Al_2O_3/H_2O nanofluid under the same conditions.