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 atmospheric pressure and 60°C in a 3 mm × 50 mm × 400 mm long rectangular 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 difference, and momentum changes are considered, and the effects of different upstream and downstream contact angles are included. These angles and the departure bubble diameters are measured from the top and the side views of bubbles recorded on a video camera through a microscope. The new model and the experimental study confirm that the bubble removal in flow boiling for small diameter bubbles investigated in this study (less than 500 μm) is initiated at the front edge of the bubble through a sweep-removal mechanism. Previous models available in the literature consider a force balance on the entire bubble, and are therefore unable 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.