Effect of ultrasonic waves on heat transfer in Al2O3 nanofluid under natural convection and pool boiling

Abstract

Heat transfer in nanofluids due to natural convection and pool boiling (including subcooled boiling and saturated boiling) is analyzed by hot wire method based on the effects of ultrasonic waves in Al2O3 nanofluids. The heat transfer characteristics due to ultrasonic wave propagation in Al2O3 nanofluids under different flow regimes were studied through experiments. In the natural convection state, ultrasonic waves enhance heat transfer in Al2O3 nanofluids, with the highest strengthening heat transfer efficiency reaching 128%. However, the increased heat transfer efficiency showed a downward trend with increased heat flux. Heat transfer in the nanofluid is enhanced by ultrasound when the main body temperature of the fluid is 50 °C, 70 °C and 80 °C under subcooled boiling conditions. However, when the main body temperature is 60 °C, heat transfer in the nanofluid is inhibited by ultrasound, i.e., under saturated boiling condition, ultrasound will still enhance heat transfer in the nanofluids, and the enhanced heat transfer efficiency will decrease with increased heat flux. This paper analyzes experimental results from the aspects of nanoparticle deposition on the heat transfer surface under the influence of ultrasound, the distribution of nanoparticles in liquid, and forces between nanoparticles. In addition, the boiling curves and critical heat flux (CHF) in Al2O3 nanofluids were obtained with and without ultrasound.