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.

Antimicrobial and Functional Properties of Duckweed (Wolffia globosa) Protein and Peptide Extracts Prepared by Ultrasound-Assisted Extraction.

Wolffia globosa is an interesting alternative plant-based protein source containing up to 40% protein dry weight. Dried duckweed protein extract (PE) was obtained using ultrasound-assisted extraction (UAE) before isoelectric precipitation (pH 3.5) to yield protein concentrate (PC) and protein solution (PS). The PC was hydrolyzed using Alcalase enzyme to obtain protein concentrate hydrolysate (PCH). Among all fractions, PCH exhibited antimicrobial properties by decreasing populations of Vibrio parahaemolyticus and Candida albicans at 0.43 ± 1.31 log reduction (66.21%) and 3.70 ± 0.11 log reduction (99.98%), respectively. The PE and PS also showed high solubilities at pH 8 of 90.49% and 86.84%, respectively. The PE demonstrated the highest emulsifying capacity (EC) (71.29%) at pH 4, while the highest emulsifying stability (ES) (~98%) was obtained from the PE and PS at pH 6 and pH 2, respectively. The major molecular weights (Mw) of the PE, PC, PCH and PS were observed at 25, 45, 63 and 100 kDa, with a decrease in the Mw of the PCH (<5 kDa). The PCH contained the highest total amino acids, with aspartic acid and glutamic acid being the major components. The results revealed the antimicrobial and functional properties of duckweed protein and hydrolysate for the first time and showed their potential for further development as functional food ingredients.

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

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.