Study on heat transfer characteristics of elliptical heat exchange tubes under acoustic streaming

The elliptical heat exchanger tube at the low-temperature heating surface of a 600 MW unit is taken as an example, and under the action of plane standing wave field, the whole domain solution is carried out inside and outside the boundary layer of the elliptical tube by numerical simulation method. Considering the influence of the parameters such as the aspect ratio (AR), the pulsating Strouhal number St₀ and pulsating Reynolds number Re₀ on the characteristics of the acoustic streaming outside the tube, the intrinsic mechanism of the enhanced heat transfer and the removal of fouling is elucidated by in-depth discussion of the acoustic streaming structure, the strength, the scale and so on. Research results indicate that as the AR increases from 0.2 to 1, the external acoustic streaming structure around the tube transitions from four internal vortices to eight vortices (four internal and four external) under different St₀ and Re₀, with the internal and external vortices rotating in opposite directions. Due to the influence of internal and external vortices, the velocity in the direction of the central angle α exhibits peak-to-trough fluctuations, showcasing rich velocity distribution characteristics at different distances s from the elliptical tube. The larger the AR, the better the heat transfer effect closer to the wall surface. The inner and outer vortex sizes show a similar variation pattern; the smaller the inner vortex size, the better the ash removal effect. The relative Nusselt number NU and relative shear rate SH quantify the effects of heat transfer and ash removal, respectively, and low-frequency high-intensity acoustic streaming is favourable for both. When given Re₀ or St₀, the SH reaches a maximum at AR = 0.4. This is due to drastic changes in the acoustic streaming characteristics for this tube shape, which is reflected in the first-order sound pressure, radial acoustic streaming velocity, and the size of internal and external vortices.