Numerical study on the behavior of vapor bubbles during boiling with surface acoustic wave (SAW)

Abstract

In this study, the dynamic mechanism of vapor bubble during boiling process under the effect of acoustic field was numerically investigated, by coupling the phase field method and thermo-viscous acoustic module. The purpose is to use the surface acoustic wave (SAW) to interfere with vapor bubbles in the boiling process, so as to realize control of boiling on the temporal and spatial scales while improving the boiling heat transfer capability. It is observed that the effect of SAW on bubble behaviors is related to the amplitude of SAW and the initial bubble size. For pool boiling case, at the critical displacement amplitude, tiny vortices form at the root of bubble during the bubble growth stage. The velocity amplitude induced by SAW in the liquid phase around bubble can reach up to 2.1 m/s, which is nearly twofold comparing to that without SAW effect. The vortices at the root of bubble disrupt the thermal boundary layer to a certain extent by creating ‘micro-turbulence’, thereby reducing the thermal resistance. Furthermore, the strong SAW effect for the large amplitude case causes one bubble to collapse into two separate bubbles after the bubble detaches from the heating surface. For flow boiling case, the SAW can break the growing bubbles along the flow direction, and the collapsed bubbles flow downstream with a maximum speed of 5 m/s, which is almost 12.5 times of that without acoustic excitation. The acoustic streaming can interrupt the growth of the boundary layer in the single-phase region, so that the subcooled fluid can rewet the microchannel wall in time. Thus, the growth of bubbles and flow instability are effectively inhibited, and flow boiling heat transfer is promoted. It is revealed that the SAW is an effective approach for the active control and enhancement of boiling heat transfer.