Experimental Investigations on the Effect of Ultrasonic Field in External Pool Boiling Under Various Operating Pressures

The augmentation of boiling heat transfer has been a dynamic domain of research for the past several decades due to a wide range of energy intensive applications and, in line, many active and passive methods have been developed. The present study discusses the effect of ultrasonic field of 31 kHz and 40 kHz on the saturated pool boiling of R141b over plain Cu surface at different operating pressures. It was found that the ultrasonic field is more effective at higher operating pressures. The surface superheat reduced by a maximum value of 2.6 °C with the application of 31 kHz ultrasonic field in comparison to the pool boiling without ultrasonic field application for +30 kPa(g) operating pressure at lower heat flux level of 113 kW/m². The maximum augmentation in heat transfer coefficient was calculated as 37.1% and 11.4% for frequency of 31 kHz and 40 kHz, respectively, with respect to the no ultrasonic field condition at +30 kPa(g) for lower heat flux level of 113 kW/m². The Nusselt number was found to be increasing in the sub-atmospheric as well as the pressurized operating pressure range. In comparison to the boiling without application of ultrasonic field, the maximum improvement in Nusselt number was noted as 25.3% at 31 kHz frequency of ultrasonic field and +30 kPa(g) operating conditions. This study suggests the use of lower frequency range of ultrasonic field in the presence of higher operating pressures for higher augmentation in saturated pool boiling.