Heat Chap10-001 - Chapter 10 Boiling and Condensation...

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Chapter 10 Boiling and Condensation Chapter 10 BOILING AND CONDENSATION Boiling Heat Transfer 10-1C Boiling is the liquid-to-vapor phase change process that occurs at a solid-liquid interface when the surface is heated above the saturation temperature of the liquid. The formation and rise of the bubbles and the liquid entrainment coupled with the large amount of heat absorbed during liquid-vapor phase change at essentially constant temperature are responsible for the very high heat transfer coefficients associated with nucleate boiling. 10-2C Yes. Otherwise we can create energy by alternately vaporizing and condensing a substance. 10-3C Both boiling and evaporation are liquid-to-vapor phase change processes, but evaporation occurs at the liquid-vapor interface when the vapor pressure is less than the saturation pressure of the liquid at a given temperature, and it involves no bubble formation or bubble motion. Boiling, on the other hand, occurs at the solid-liquid interface when a liquid is brought into contact with a surface maintained at a temperature T s sufficiently above the saturation temperature T sat of the liquid. 10-4C Boiling is called pool boiling in the absence of bulk fluid flow, and flow boiling (or forced convection boiling ) in the presence of it. In pool boiling, the fluid is stationary, and any motion of the fluid is due to natural convection currents and the motion of the bubbles due to the influence of buoyancy. 10-5C Boiling is said to be subcooled (or local ) when the bulk of the liquid is subcooled (i.e., the temperature of the main body of the liquid is below the saturation temperature T sat ), and saturated (or bulk ) when the bulk of the liquid is saturated (i.e., the temperature of all the liquid is equal to T sat ). 10-6C The boiling curve is given in Figure 10-6 in the text. In the natural convection boiling regime, the fluid motion is governed by natural convection currents, and heat transfer from the heating surface to the fluid is by natural convection. In the nucleate boiling regime, bubbles form at various preferential sites on the heating surface, and rise to the top. In the transition boiling regime, part of the surface is covered by a vapor film. 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. 10-1
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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.
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Heat Chap10-001 - Chapter 10 Boiling and Condensation...

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