Experimental investigation on nanofluid flow boiling heat transfer in a vertical tube under different pressure conditions

In this study, the saturated flow boiling heat transfer of γ-Al₂O₃/H₂O nanofluids with 20 nm diameter and 0.1%, 0.5% volume concentration in a vertical tube is experimentally carried out. An ultrasonic oscillation was used to prepare nanofluid. The influences of such important parameters as surface heat flux (50-300 kW m^-2), pressure (0.2-0.8 MPa) and mass flux (350-1100 kg m^-2 s^-1) on boiling characteristics are taken into consideration. It is confirmed that the most enhancement is about 86% for γ-Al₂O₃/H₂O nanofluids saturated flow boiling heat transfer compared with deionized water, and the average value of Nusselt number was enhanced 23% and 45% respectively for 0.1 vol.% and 0.5 vol.% in this study. And the Nusselt number of nanofluid flow boiling increases with increasing the surface heat flux, the volume concentration of nanoparticle and pressure. It is confirmed that nanoparticles deposited on the heating surface by SEM observation and nanoparticles do not change obviously after boiling by TEM observations, they are attribute to the continuous operation of ultrasonic oscillation. In addition, the influence of mass flux on the enhancement rate of nanofluid saturated flow boiling heat transfer is negligible. Furthermore, a dimensionless parameter was proposed for nanofluid saturated flow boiling heat transfer data processing.