Experimental investigation of the heat transfer and friction loss of turbulent flow in circular pipe under low-frequency ultrasound propagation along the mainstream flow.

The characteristics of the heat transfer and friction loss of turbulent water flow in a circular pipe were investigated experimentally at a constant surface temperature of 45 ℃ for 28 kHz ultrasound propagation along the mainstream flow. Transducers were installed in five rows and three columns in the upstream section of the test pipe, and the number of active transducers was varied (1, 3, and 15) for a Reynolds number range of 10,000-25,000. The results indicated that the ultrasonic effects yielded positive results for both the heat transfer and pressure loss of the pipe flow. Under the influence of 15 ultrasonic transducers, the maximum Nusselt number ratio was 1.57 and the greatest reduction in the friction factor was 21.6 % for a Reynolds number of 10,000. The corresponding maximum thermal performance factor was approximately 1.7. However, the thermal efficiency tended to decrease with an increase in the number of transducers. The maximum thermal efficiency values under ultrasonic waves with 1, 3, and 15 transducers were 5.43, 3.37, and 1.95, respectively. When the change in the friction factor per ultrasonic input power was considered, the most suitable number of ultrasonic transducers was three. Finally, predictive formulas were proposed for the Nusselt number ratio and friction factor ratio under low-frequency ultrasound, with deviations from -5.5 % to 5.4 % and -7.4 % to 7.4 %, respectively.