cen58933_ch10 - cen58933_ch10.qxd 12:37 PM Page 515 CHAPTER...

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BOILING AND CONDENSATION W e know from thermodynamics that when the temperature of a liquid at a specified pressure is raised to the saturation temperature T sat at that pressure, boiling occurs. Likewise, when the temperature of a vapor is lowered to T sat , condensation occurs. In this chapter we study the rates of heat transfer during such liquid-to-vapor and vapor-to-liquid phase transformations. Although boiling and condensation exhibit some unique features, they are considered to be forms of convection heat transfer since they involve fluid motion (such as the rise of the bubbles to the top and the flow of condensate to the bottom). Boiling and condensation differ from other forms of convec- tion in that they depend on the latent heat of vaporization h fg of the fluid and the surface tension at the liquid–vapor interface, in addition to the proper- ties of the fluid in each phase. Noting that under equilibrium conditions the temperature remains constant during a phase-change process at a fixed pres- sure, large amounts of heat (due to the large latent heat of vaporization re- leased or absorbed) can be transferred during boiling and condensation essentially at constant temperature. In practice, however, it is necessary to maintain some difference between the surface temperature T s and T sat for ef- fective heat transfer. Heat transfer coefficients h associated with boiling and condensation are typically much higher than those encountered in other forms of convection processes that involve a single phase. We start this chapter with a discussion of the boiling curve and the modes of pool boiling such as free convection boiling, nucleate boiling, and film boil- ing. We then discuss boiling in the presence of forced convection. In the second part of this chapter, we describe the physical mechanism of film con- densation and discuss condensation heat transfer in several geometrical arrangements and orientations. Finally, we introduce dropwise condensation and discuss ways of maintaining it. 515 CHAPTER 10 CONTENTS 10–1 Boiling Heat Transfer 516 10–2 Pool Boiling 518 10–3 Flow Boiling 530 10–4 Condensation Heat Transfer 532 10–5 Film Condensation 532 10–6 Film Condensation Inside Hori- zontal Tubes 545 10–7 Dropwise Condensation 545 Topic of Special Interest: Heat Pipes 546 cen58933_ch10.qxd 9/4/2002 12:37 PM Page 515
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10–1 BOILING HEAT TRANSFER Many familiar engineering applications involve condensation and boiling heat transfer. In a household refrigerator, for example, the refrigerant absorbs heat from the refrigerated space by boiling in the evaporator section and rejects heat to the kitchen air by condensing in the condenser section (the long coils behind the refrigerator). Also, in steam power plants, heat is transferred to the steam in the boiler where water is vaporized, and the waste heat is rejected from the steam in the condenser where the steam is condensed. Some elec- tronic components are cooled by boiling by immersing them in a fluid with an appropriate boiling temperature.
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