Enhancement of subcooled boiling in confined space using ultrasonic waves

Abstract

Effects of ultrasonic waves on heat transfer and bubble behavior spanning from nucleate boiling to film boiling or microbubble emission boiling (MEB) in a confined space were investigated. Experimental results show that critical heat flux in the confined space is increased by four times in the ultrasonic field. In transition boiling regime, MEB occurs at subcooling over 20 K in the unconfined space, however it cannot be observed in the confined space even for a subcooling of 60 K. Visualizations show that a large coalesced bubble traps in the confined space, which inhibits the supply of liquid onto the heating surface. Applying the ultrasound will reintroduce MEB at subcooling over 25 K. A comparison of MEB in the confined space with and without the ultrasound reveals that the strong bulk convection introduced by the violent oscillation and collapse of vapor film plays an important role in heat transfer of MEB.