On the thermal characteristic of a heating flat surface under low frequency ultrasonic waves

This study investigates the heat transfer capability between the near-wall water and a heating flat surface under the low frequency ultrasonic waves. The experiment was conducted at the main stream velocities of 0.12, 0.14, and 0.17 m/s, corresponding with the local Reynolds number between 60,000 and 200,000. The ultrasound, having a frequency of 25, 33, and 40 kHz was released from a 60 W flat transducer in a downward direction to disturb a near-wall flow over the flat surface having a constant heat flux of 1875 W/m2. The temperature decrease due to the disturbance of ultrasound, detected by 7 thin-leaf thermocouples, led to the increase in the Nusselt number by up to 10%, 16.5%, and 20.8%, at the wave frequency of 25, 33, and 40 kHz, respectively. The use of the thermochromic liquid crystals on the heating surface, disturbed by the 40 kHz wave, revealed that the bound of the wave incident region was shaped like a spiky structure that alternately occurs following with time. This incident region moved further downstream when the mainstream velocity increased and caused a retardation of the near wall flow, leading to a worse heat transfer in the upstream region. Therefore, the inclination of the acoustic beam, occurring as a variation of mainstream velocity, must be considered in order to use the ultrasound to enhance the heat transfer. Finally, the predictive formulas for the local Nusselt number under the 25-40 kHz waves are also provided and the mechanism of heat transfer is discussed.