A CONTROL-VOLUME APPROACH FOR INVESTIGATING FORCES ON A DEPARTING BUBBLE UNDER SUBCOOLED FLOW BOILING

This paper presents the theoretical and experimental work conducted on nucleating vapor bubbles in subcooled flow boiling of water at atmosp heric pressure and 60 degrees C in a 3 mm x 50 mm x 400 mm long rectan gular channel. A new model is developed for analyzing forces acting on the vapor bubble under pseudo-static conditions corresponding to the thermally controlled region of bubble growth. The model considers two Separate control volumes for the front and rear regions of the bubble. The forces due to surface tension, buoyancy, drag, pressure differenc e, and momentum changes are considered and the effects of different up stream and downstream contact angles are included. These angles and th e departure bubble diameters are measured from the top and the side vi ews of bubbles recorded on a video camera through a microscope. The ne w model and the experimental study confirm that the bubble removal in flow boiling for small diameter hobbles investigated in this study (le ss than 500 mu m) is initiated at the front edge of the bubble through a sweep-removal mechanism. Previous models available in the literatur e consider a force balance on the entire bubble, and are therefore una ble to address the effect of a significant reduction in the component of the surface tension force in the flow direction at the leading edge caused by an increase in the upstream contact angle.