This form of contamination has been minimized by

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Unformatted text preview: ble 10–6. A major concern about the performance of a heat pipe is degradation with time. Some heat pipes have failed within just a few months after they are put into operation. The major cause of degradation appears to be contamination that occurs during the sealing of the ends of the heat pipe tube and affects the vapor pressure. This form of contamination has been minimized by electron beam welding in clean rooms. Contamination of the wick prior to installation in the tube is another cause of degradation. Cleanliness of the wick is essential for its reliable operation for a long time. Heat pipes usually undergo extensive testing and quality control process before they are put into actual use. An important consideration in the design of heat pipes is the compatibility of the materials used for the tube, wick, and fluid. Otherwise, reaction between the incompatible materials produces noncondensable gases, which degrade the performance of the heat pipe. For example, the reaction between stainless steel and water in some early heat pipes generated hydrogen gas, which destroyed the heat pipe. 180 W EXAMPLE 10–8 Replacing a Heat Pipe by a Copper Rod A 30-cm-long cylindrical heat pipe having a diameter of 0.6 cm is dissipating heat at a rate of 180 W, with a temperature difference of 3°C across the heat pipe, as shown in Figure 10–38. If we were to use a 30-cm-long copper rod in- cen58933_ch10.qxd 9/4/2002 12:38 PM Page 551 551 CHAPTER 10 stead to remove heat at the same rate, determine the diameter and the mass of the copper rod that needs to be installed. SOLUTION A cylindrical heat pipe dissipates heat at a specified rate. The diameter and mass of a copper rod that can conduct heat at the same rate are to be determined. Assumptions Steady operating conditions exist. Properties The properties of copper at room temperature are 8950 kg/m3 and k 386 W/m · °C. · Analysis The rate of heat transfer Q through the copper rod can be expressed as · Q kA T L where k is the therma...
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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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