Effects of noncondensable gas and ultrasonic vibration on vapor bubble condensing and collapsing

Under the condition of high liquid subcooling, boiling is sometimes accompanied by the emission of microbubbles and the collapse of vapor film. This is termed as Microbubble Emission Boiling (MEB) by some researchers and realizes a better heat transfer performance with its heat flux even higher than CHF. In order to simulate the interaction between vapor bubble and cold bulk in a complex boiling, vapor bubbles were issued into a water tank from a circular injection tube with inner diameter of 2 mm. In this work, the process of bubble condensing and collapsing in water with high liquid subcooling is focused and the effects of noncondensable gas and ultrasonic vibration on MEB are researched. Experimental results show that the condensation of vapor/noncondensable gas bubble presents different scenarios with the increase in initial mole fraction of noncondensable gas: collapse to many microbubbles suddenly (x(0) < 2.5%); split up into several tiny bubbles (2.5% < x(0) < 7.5%); keep infrangible (x(0) > 7.5%). When an ultrasonic vibration is applied, the deformation and breakage of the vapor/noncondensable gas bubble become more furious and easier. Noncondensable gas could weaken the bubble condensation, reducing the inertial shock of the liquid on the bubble and making the bubble more stable. This can explain that the presence of noncondensable gas can inhibit the collapse of bubble and deteriorate the heat transfer performance consequently in MEB region. However, ultrasonic vibration will intensify the bubble condensation and enhance the inertial shock of liquid, which will enhance the instability of bubble surface. This, combined with the function of Bjerknes force, would lead the bubble to be more unstable to be broken up easily in the ultrasonic field. Hence, the occurrence of MEB for liquid with noncondensable gas may be intensified by the ultrasonic vibration. (C) 2014 Elsevier Inc. All rights reserved.