The augmentation of boiling heat transfer has been a dynamic domain of research for the past several decades due to wide range of energy intensive applications and, in line, many active and passive methods have been developed. The present study discusses about the effect of ultrasonic field of 31 kHz and 40 kHz on saturated pool boiling of R141b over plain Cu surface in presence of various operating pressures in the range of -20 kPa(g) to +30 kPa(g). It was found that ultrasonic field is more effective with higher operating pressures. The surface superheat was found to be reduced by a maximum value of 2.6°C with 31 kHz of ultrasonic field and +30 kPa(g) operating pressure at lower heat flux level of 113 kW/m2. In similar manner, the maximum augmentation in heat transfer coefficient was calculated as 37.1% and 11.4% for frequency of 31 kHz and 40 kHz respectively at lower heat flux level. Nusselt number was found to be increasing in both sides of 1 atm. pressure conditions and maximum improvement was noted as 25.3% at 31 kHz frequency of ultrasonic field and +30 kPa(g) operating conditions. Thus, the study suggests use of lower frequency range of ultrasonic field in presence of higher operating pressures for higher augmentation in saturated pool boiling.

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