Heat Transfer and Velocity Measurement of Laminar Pipe Flow Induced by Ultrasound Released along Mainstream Direction

In this paper, heat transfer characteristics of laminar pipe flow using low frequency ultrasound released along the mainstream direction were investigated experimentally. The test section was a square duct with an inner cross-sectional area of 60 mm2 and a length of 1500 mm. The Reynolds number ranged between 400 and 1,600. A heater with a power of 400 W was installed at the bottom wall to heat the water at 23 °C. Thermocouples were used to measure the wall temperature at a distance of 0.16-0.58 m with an interval of 0.07 m. The ultrasonic transducer with a frequency of 28-80 kHz was set at the entrance to release the waves in a streamwise direction. In addition, the flow behaviour of the water flow induced by ultrasound was illustrated by Particle Image Velocimetry (PIV). The results showed that the heat transfer enhancement factor (HTEF) was increased when the heating wall was close to the ultrasonic transducer position. In particular, acoustic streaming was found to convect the heat transfer by swerving from the entrance to the heating wall. The maximum HTEF of 163.04% was achieved using 28 kHz ultrasonic waves at Reynold number of 400. These results would clearly demonstrate the potential of ultrasonic waves to improve heat transfer in a thermal system in the future.