Cen58933ch10qxd 942002 1238 pm page 518 518 heat

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Unformatted text preview: on the basis of the presence of bulk liquid temperature. cen58933_ch10.qxd 9/4/2002 12:38 PM Page 518 518 HEAT TRANSFER and boiling in this case is called the bulk or saturated boiling. Next, we consider different boiling regimes in detail. 10–2 I POOL BOILING So far we presented some general discussions on boiling. Now we turn our attention to the physical mechanisms involved in pool boiling, that is, the boiling of stationary fluids. In pool boiling, the fluid is not forced to flow by a mover such as a pump, and any motion of the fluid is due to natural convection currents and the motion of the bubbles under the influence of buoyancy. As a familiar example of pool boiling, consider the boiling of tap water in a pan on top of a stove. The water will initially be at about 15°C, far below the saturation temperature of 100°C at standard atmospheric pressure. At the early stages of boiling, you will not notice anything significant except some bubbles that stick to the surface of the pan. These bubbles are caused by the release of air molecules dissolved in liquid water and should not be confused with vapor bubbles. As the water temperature rises, you will notice chunks of liquid water rolling up and down as a result of natural convection currents, followed by the first vapor bubbles forming at the bottom surface of the pan. These bubbles get smaller as they detach from the surface and start rising, and eventually collapse in the cooler water above. This is subcooled boiling since the bulk of the liquid water has not reached saturation temperature yet. The intensity of bubble formation increases as the water temperature rises further, and you will notice waves of vapor bubbles coming from the bottom and rising to the top when the water temperature reaches the saturation temperature (100°C at standard atmospheric conditions). This full scale boiling is the saturated boiling. 100°C 100°C 103°C 110°C Heating (a) Natural convection boiling Heating (b) Nucleate boiling Vapor film Vapor pockets 100°C 100°C 180°C 400°C Heating (c) Transition boiling Heating (d) Film boi...
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This note was uploaded on 01/28/2010 for the course HEAT ENG taught by Professor Ghaz during the Spring '10 term at University of Guelph.

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