Augmentation of natural convection heat transfer in enclosures via ultrasound: Effects of power, frequency and temperature

The integration of ultrasound to enhance the natural convection heat transfer in an enclosure was experimentally investigated. The effects of ultrasonic power, ultrasonic frequency and surface temperature on convective heat transfer coefficient were analyzed to optimize the ultrasonic input. A novel dimensionless number is defined that indicates the ultrasound-induced convective heat transfer enhancement per added unit of delivered ultrasonic power. To determine and justify the effectiveness of incorporating ultrasound from an energy-savings point of view, the total energy to the system to increase the temperature to a certain degree in non-ultrasonic and ultrasound-assisted experiments was compared. With respect to ultrasonic power, application of ultrasound at any power level enhances the convective heat transfer coefficient by as much as 77% while the effect of power heightens with an increase in frequency. Amongst the three examined frequencies, integration of ultrasound was observed to enhance heat transfer more at higher frequencies. The surface temperature appears to have a negative effect on the observed ultrasound-enhanced heat transfer where higher temperatures abate the enhancement. Per unit ultrasonic power added to the system, higher frequencies show greater improvement in convective heat transfer. Comparing the trends in variation of the augmented heat transfer, the kinematic momentum resulting from acoustic streaming and the acoustic cavitation critical bubble radius with the ultrasonic frequency revealed that at lower frequencies acoustic cavitation is the augmenting mechanism and at higher frequencies acoustic streaming induces the enhancement with acoustic streaming having a higher impact in augmenting natural convection heat transfer. In terms of energy savings, a novel analysis focusing on the energy intensity of utilizing ultrasound unveiled that only at higher frequencies does integration of ultrasound result in lower total energy consumption.