Thermodynamics HW Solutions 790

Thermodynamics HW Solutions 790 - burnout occurs. The...

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Chapter 10 Boiling and Condensation 10-7C In the film boiling regime, the heater surface is completely covered by a continuous stable vapor film, and heat transfer is by combined convection and radiation. In the nucleate boiling regime, the heater surface is covered by the liquid. The boiling heat flux in the stable film boiling regime can be higher or lower than that in the nucleate boiling regime, as can be seen from the boiling curve. 10-8C The boiling curve is given in Figure 10-6 in the text. The burnout point in the curve is point C. The burnout during boiling is caused by the heater surface being blanketed by a continuous layer of vapor film at increased heat fluxes, and the resulting rise in heater surface temperature in order to maintain the same heat transfer rate across a low-conducting vapor film. Any attempt to increase the heat flux beyond will cause the operation point on the boiling curve to jump suddenly from point C to point E. However, the surface temperature that corresponds to point E is beyond the melting point of most heater materials, and
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Unformatted text preview: burnout occurs. The burnout point is avoided in the design of boilers in order to avoid the disastrous explosions of the boilers. & max q 10-9C Pool boiling heat transfer can be increased permanently by increasing the number of nucleation sites on the heater surface by coating the surface with a thin layer (much less than 1 mm) of very porous material, or by forming cavities on the surface mechanically to facilitate continuous vapor formation. Such surfaces are reported to enhance heat transfer in the nucleate boiling regime by a factor of up to 10, and the critical heat flux by a factor of 3. The use of finned surfaces is also known to enhance nucleate boiling heat transfer and the critical heat flux. 10-10C The different boiling regimes that occur in a vertical tube during flow boiling are forced convection of liquid, bubbly flow, slug flow, annular flow, transition flow, mist flow, and forced convection of vapor. 10-2...
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This note was uploaded on 01/24/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.

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