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Bubble behaviour in subcooled flow boiling at low pressures and flow rates Prodanovic, Vladan


Bubble behaviour in subcooled flow boiling of water at pressures ranging from 1.05 to 3 bar, bulk liquid velocities from 0.08 to 0.8 m/s, heat fluxes from 0.2 to 1 MW/m² and subcoolings from 10 to 30 K was investigated experimentally and analytically. Experiments were carried out on a vertical, annular test section with an inner heating surface and upward flow. High-speed photography at rates of 6000-8000 frames/s captured bubbles from inception to collapse, revealing variations of bubble shapes and sizes, as well as bubble sliding and detachment from the wall. Bubble growth and condensation rates, sliding velocities, variation of bubble lifetime and bubble size with flow rate, subcooling, heat flux and pressure were further examined. New correlations were proposed for maximum and detachment bubble diameters, bubble growth rate, bubble growth time, detachment time and condensation time. High-speed photographic results showed changes in typical bubble behaviour with increasing heat flux, from the appearance of the first bubble toward the onset of significant void. In the low heat flux region nearly spherical bubbles slid long distances without changing significantly in size and shape, occasionally detaching from the surface and typically reattaching soon after. At higher heat fluxes sliding distances were reduced to about a couple of diameters, and detachments from the surface became typical bubble behaviour. After detachment, bubbles traveled in a direction normal to the heater into the subcooled liquid core, where they collapsed rapidly. Further increasing the heat flux resulted in significant bubble coalescence before the onset of significant void was reached. The abrupt change in bubble behaviour between the low and high heat flux regions indicated changes in the heat transfer mode. Based on these observations, a new model was proposed for the transition from partially developed to fully developed boiling. It was observed that bubble shapes, particularly at detachment, deviated significantly from spherical. Therefore, a new ellipsoidal, rather than a spherical model, was used to describe bubble geometry. The analysis of forces acting on ellipsoidal bubbles led to a new bubble detachment model. The model suggests that the role of surface tension is to promote bubble detachment instead of opposing it.

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