Characterization of heat transfer and friction loss of water turbulent flow in a narrow rectangular duct under 25–40 kHz ultrasonic waves

Abstract

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.