This study experimentally investigated the effect of low-frequency ultrasonic waves on the heat transfer augmentation of turbulent water flow in a narrow rectangular duct with a width of 5 mm. 25-, 33-, and 40-kHz ultrasonic transducers were set to release waves in a downward direction to disturb the flow, with Reynolds numbers (Re) of 10,000–25,000 at increments of 2500. The results indicated that the ultrasonic waves increased the friction loss by only 0.2–2% over the entire testing Re range, while an 8.1–48.6% enhancement of the heat transfer capability was obtained for the Re range of 10,000–15,000. The maximum Nusselt number occurred at a Re of 12,500 and frequency of 33 kHz. However, beyond Re values of 12,500, the thermal performance tended to decrease with an increase in Re. Consequently, the average Nusselt number ratios at ultrasonic frequencies of 25, 33, and 40 kHz over the tested Re range were 1.123, 1.039, and 1.033, respectively, while the thermal performance values were 1.108, 0.989, and 1.036, respectively. These results confirmed that ultrasound has significant potential for application in heat transfer augmentation of turbulent pipe flow. This paper also provides formulas to predict the friction factor and Nusselt number and discusses the mechanisms of heat transfer enhancement by ultrasonic waves at 25, 33, and 40 kHz.

本研究实验研究了低频超声波对宽度为5 mm的狭窄矩形管道中湍流水流传热增强的影响。将25、33和40 kHz超声换能器设置为向下释放波以扰动流动，雷诺数（Re）为10,000–25,000，以2500为增量。结果表明，超声波增加了通过在整个测试只有0.2-2％摩擦损失重范围内，而对所得的热传递能力的8.1-48.6％的增强的重新的10,000-15,000范围内。最大的Nusselt数出现在Re为12,500，频率为33 kHz的情况下。但是，超越Re的值为12,500，随着Re的增加，热性能趋于下降。因此，在测试的Re范围内，超声频率为25、33和40 kHz时，平均努塞尔数比分别为1.123、1.039和1.033，而热性能值分别为1.108、0.989和1.036。这些结果证实了超声在湍流管道流动的传热增强中具有巨大的潜力。本文还提供了预测摩擦系数和Nusselt数的公式，并讨论了在25、33和40 kHz的超声波下传热增强的机理。