Therefore the liquid is slightly superheated in this

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Unformatted text preview: ove the saturation temperature (about 2 to 6°C for water). Therefore, the liquid is slightly superheated in this case (a metastable condition) and evaporates when it rises to the free surface. The fluid motion in this mode of boiling is governed by natural convection currents, and heat transfer from the heating surface to the fluid is by natural convection. Nucleate Boiling (between Points A and C ) The first bubbles start forming at point A of the boiling curve at various preferential sites on the heating surface. The bubbles form at an increasing rate at an increasing number of nucleation sites as we move along the boiling curve toward point C. The nucleate boiling regime can be separated into two distinct regions. In region A–B, isolated bubbles are formed at various preferential nucleation sites on the heated surface. But these bubbles are dissipated in the liquid shortly after they separate from the surface. The space vacated by the rising bubbles is filled by the liquid in the vicinity of the heater surface, and the process is repeated. The stirring and agitation caused by the entrainment of the liquid to the heater surface is primarily responsible for the increased heat transfer coefficient and heat flux in this region of nucleate boiling. In region B–C, the heater temperature is further increased, and bubbles form at such great rates at such a large number of nucleation sites that they form numerous continuous columns of vapor in the liquid. These bubbles move all the way up to the free surface, where they break up and release their vapor content. The large heat fluxes obtainable in this region are caused by the combined effect of liquid entrainment and evaporation. FIGURE 10–6 Typical boiling curve for water at 1 atm pressure. cen58933_ch10.qxd 9/4/2002 12:38 PM Page 520 520 HEAT TRANSFER At large values of Texcess, the rate of evaporation at the heater surface reaches such high values that a large fraction of the heater surface is covered by bubbles, making it diff...
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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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