In such cases improving the accuracy of a small

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Unformatted text preview: if the thermal resistance on the other side of the surface is too large. In steady operation, heat transfer from one medium to another depends on the sum of the thermal resistances on the path of heat flow, and a large thermal resistance may overshadow all others and dominate the heat transfer process. In such cases, improving the accuracy of a small resistance (such as one due to condensation or boiling) makes hardly any difference in overall heat transfer calculations. TOPIC OF SPECIAL INTEREST* Heat Pipes A heat pipe is a simple device with no moving parts that can transfer large quantities of heat over fairly large distances essentially at a constant temperature without requiring any power input. A heat pipe is basically a sealed slender tube containing a wick structure lined on the inner surface and a small amount of fluid such as water at the saturated state, as shown in Figure 10–36. It is composed of three sections: the evaporator section at one end, where heat is absorbed and the fluid is vaporized; a condenser section at the other end, where the vapor is condensed and heat is rejected; and the adiabatic section in between, where the vapor and the liquid phases of the fluid flow in opposite directions through the core and the wick, *This section can be skipped without a loss in continuity. cen58933_ch10.qxd 9/4/2002 12:38 PM Page 547 547 CHAPTER 10 Wick Tube wall Liquid flow Copper tube Heat in Heat out Vapor core Vapor flow Wick (liquid flow passage) Cross-section of a heat pipe Evaporation section Adiabatic section Condenser section FIGURE 10–36 Schematic and operation of a heat pipe. respectively, to complete the cycle with no significant heat transfer between the fluid and the surrounding medium. The type of fluid and the operating pressure inside the heat pipe depend on the operating temperature of the heat pipe. For example, the criticaland triple-point temperatures of water are 0.01°C and 374.1°C, respectively. Therefore, water can undergo a liquid-to-vapor or vapor-to-liquid phase change...
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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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