Heat transfer intensification by low or high frequency ultrasound: Thermal and hydrodynamic phenomenological analysis

The aim of this work is to quantitatively demonstrate the intensification of heat transfer in forced convection by mean of ultrasonic irradiation at low (25 kHz) or high (2 MHz) frequency. High frequency ultrasound induces convective acoustic streaming while low frequency ultrasonic waves produce mainly cavitation effects. These hydrodynamic phenomena are at the origin of strongly different observations in terms of flow pattern modification and thereby in Nusselt number values. A link is tentatively established between hydrodynamic behaviors at both ultrasonic frequencies and corresponding thermal results. Hydrodynamic approach was performed with a 2D-2C PIV device while thermal one was carried out under uniform heat flux conditions. It seems that thermal enhancement effect of acoustic streaming (2 MHz) decreases as flow rate increases. This behavior is consistent with the decrease of turbulent kinetic energy produced by acoustic streaming in the same conditions. In the contrary, thermal enhancement effect produced by acoustic cavitation (25 kHz), increases as flow rate increases. This result could be due to the increase of relative size of acoustic bubbles with respect to laminar boundary layer thickness as flow rate increases. In conclusion, the two different ultrasound frequencies, which lead to two different hydrodynamic effects, also lead to two different thermal and turbulence trend with respect to flow rate modifications.