Experimental study on ALN/H₂O and AL₂O₃/H₂O nanofluid flow boiling heat transfer and its influence factors in a vertical tube

In this work, AlN/H₂O and Al₂O₃/H₂O 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 AlN and 20nm diameter for γAl₂O₃.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/H₂O nanofluid is improved mostly about 64% compared with deionized water and Al₂O₃/H₂O 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 Al₂O₃/H₂O nanofluid and 31% for Al₂O₃/H₂O 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 AlN is greater than that of Al₂O₃, the heat transfer capacity of AlN/H₂O nanofluid is greater than that of Al₂O₃/H₂O nanofluid under the same conditions.