Study on acoustic streaming characteristics around elliptical heat exchanger tubes at different angles of attack

The distribution of acoustic streaming outside the tube in a planar standing wave field is calculated by numerical simulation using an elliptical heat exchanger tube at the low-temperature heating surface of a 600 MW unit as an example. The study discusses the impact of angle of attack (AOA), Strouhal number (St0), and Reynolds number (Re0) on the external acoustic streaming characteristics of the tube from structural, intensity, and scale perspectives. The results indicate that the strong nonlinearity of the second-order acoustic streaming in the Stokes boundary layer is driven by the first-order acoustic field. As the AOA increases, the external acoustic streaming vortices exhibit a phenomenon of inner and external vortex merging. Along the direction of the inner vortex center, the acoustic streaming velocity shows peak and valley fluctuations influenced by the vortices. In the normal direction, the acoustic streaming is significantly weakened, and the velocity distribution characteristics vary at different distances s. The inner vortices i = 1 and 2 achieve maximum and minimum values around AOA = 60° and 30°, respectively. The relative Nusselt number NU and relative shear rate SH quantify heat transfer and ash removal effects, respectively. Both small St0 and large Re0 enhance the heat transfer effectiveness. The SH shows a similar variation pattern to the scale of the acoustic streaming under the corresponding cases. The conclusion aligns well with the requirements of enhanced heat transfer and ash resistance for low-temperature heating surfaces, providing important reference for the selection of AOA for elliptical heat exchange tubes.