Numerical simulation on enhancement of natural convection heat transfer by acoustic cavitation in a square enclosure

Natural convection heat transfer of liquid in a square enclosure with and without acoustic cavitation is numerically investigated. In order to model the effect of sound field, a prescribed periodic change pressure is imposed on liquid in the region with ultrasonic beam. The cavitation model which takes into consideration such effects as phase change, bubble dynamics, and noncondensable gases is established. In order to capture the cavitation phenomenon and obtain a realistic pressure field and cavitation zones, the time step is set to quarter period of sound pressure and a relatively small relaxation factor for momentum equation and near zero relaxation factors for vaporization mass and density are used. The simulation results show that both liquid temperature uniformity and heat transfer coefficient of heating surface are greatly improved by means of acoustic cavitation. This phenomenon can be explained by jet-flow bundle formed by the asymmetry collapse of cavitation bubbles on the heating surface. The jet-flow bundle cannot only enhance fluid mixing around the heating surface and thus improve the uniformity of liquid temperature in the square enclosure, but also directly impact the heating surface and thus make thermal boundary layer become thin. The field synergy principle is introduced to analyze the heat transfer enhancement by acoustic cavitation.