Numerical simulation on acoustic streaming characteristics in boiler tube array

Abstract

Strong sound can produce acoustic streaming around the tube, which can change the flow state around the tube, and then affect the heat exchange between the tube and the surrounding medium. In order to obtain the influence law of sound wave on the heat transfer characteristics in the tube array, firstly, we study the acoustic streaming and heat transfer characteristics around a single cylindrical tube by the scheme of theoretical derivation and simulation. On this basis, we study the acoustic streaming and heat transfer characteristics in the periodic tube array. The results show that the acoustic streaming structure outside the tube is axially symmetric, and the flow direction of the inner vortices and the outer vortices are opposite. When the frequency is constant, sound pressure level (SPL) only changes the acoustic streaming velocity, not the acoustic streaming distribution. As the frequency continues to increase, the velocity peak gradually shifts from the inner vortex to the outer vortex, so the outer vortex velocity gradually becomes the main body of flow. Low frequency and high intensity sound waves can form strong acoustic streaming movement and enhance heat transfer outside the tube. In the tube array, the acoustic streaming distribution should consider the effect of different incident directions on crystal plane spacing. For incident angle φ = 0°∼34°, the ratio of maximum velocity of outer vortex to the one of inner vortex an Nusselt number (Nu) are increasing with φ increasing. In this stage, the heat transfer effect changes greatly with the change of angle. For incident angle φ = 35°∼45°, the inner and outer vortex flow velocity and Nu increases together, but the scale of outer vortex decreases. In this stage, with the change of angle, the heat transfer effect changes less due to the reduction of the scale of outer vortex dominating heat transfer between tubes. In tube arrays, the optimal flow field incidence angle is φ = 34 °and the optimal heat transfer incidence angle is φ = 45°. The sound band gap and sound pressure level enhancement exist in the tube array, which affects the acoustic streaming distribution. The results are helpful to reveal the mechanism of acoustic streaming more clearly and provide a basis for the augmentation of heat transfer in tube arrays by acoustic waves.