W 10 contents 101 boiling heat transfer 516 102 pool

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Unformatted text preview: 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 Horizontal Tubes 545 10–7 Dropwise Condensation 545 Topic of Special Interest: Heat Pipes 546 515 cen58933_ch10.qxd 9/4/2002 12:37 PM Page 516 516 HEAT TRANSFER Evaporation Air 10–1 Water 20°C Boiling Water 100°C Heating FIGURE 10–1 A liquid-to-vapor phase change process is called evaporation if it occurs at a liquid–vapor interface and boiling if it occurs at a solid–liquid interface. P = 1 atm Water Tsat = 100°C Bubbles 110°C Heating element FIGURE 10–2 Boiling occurs when a liquid is brought into contact with a surface at a temperature above the saturation temperature of the liquid. I 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 electronic components are cooled by boiling by immersing them in a fluid with an appropriate boiling temperature. Boiling is a liquid-to-vapor phase change process just like evaporation, but there are significant differences between the two. Evaporation occurs at the liquid–vapor interface when the vapor pressure is less than the saturation pressure of the liquid at a given temperature. Water in a lake at 20°C, for example, will evaporate to air at 20°C and 60 percent relative humidity since the saturation pressure of water at 20°C is 2.3 kPa and the vapor pressure of air at 20°C and 60 percent relative humidity is 1.4 kPa (evaporation rates...
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