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

•Ultrasonication improves convective heat transfer in enclosures by as much as 77%.•Effect of ultrasonic power heightens at higher frequencies.•Acoustic streaming observed to contribute the most to the observed enhancement.•Per unit ultrasonic power, the observed enhancement is proportional to frequency.•Only at higher frequencies, integration of ultrasound results in energy-saving.