Qxd 942002 1238 pm page 548 548 heat transfer 1942

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Unformatted text preview: temperature ranges for some fluids used in heat pipes Fluid Temperature Range, °C Helium Hydrogen Neon Nitrogen Methane Ammonia Water Mercury Cesium Sodium Lithium 271 259 248 210 182 78 5 200 400 500 850 to to to to to to to to to to to 268 240 230 150 82 130 230 500 1000 1200 1600 cen58933_ch10.qxd 9/4/2002 12:38 PM Page 548 548 HEAT TRANSFER 1942. However, it did not receive much attention until 1962, when it was suggested for use in space applications. Since then, heat pipes have found a wide range of applications, including the cooling of electronic equipment. The Operation of a Heat Pipe The operation of a heat pipe is based on the following physical principles: • At a specified pressure, a liquid will vaporize or a vapor will condense at a certain temperature, called the saturation temperature. Thus, fixing the pressure inside a heat pipe fixes the temperature at which phase change will occur. • At a specified pressure or temperature, the amount of heat absorbed as a unit mass of liquid vaporizes is equal to the amount of heat rejected as that vapor condenses. • The capillary pressure developed in a wick will move a liquid in the wick even against the gravitational field as a result of the capillary effect. • A fluid in a channel flows in the direction of decreasing pressure. Initially, the wick of the heat pipe is saturated with liquid and the core section is filled with vapor. When the evaporator end of the heat pipe is brought into contact with a hot surface or is placed into a hot environment, heat will transfer into the heat pipe. Being at a saturated state, the liquid in the evaporator end of the heat pipe will vaporize as a result of this heat transfer, causing the vapor pressure there to rise. This resulting pressure difference drives the vapor through the core of the heat pipe from the evaporator toward the condenser section. The condenser end of the heat pipe is in a cooler environment, and thus its surface is slightly cooler. The vapor that comes into contact with this cooler surface condenses, releasing the heat a vaporization, which is rejected to the surrounding med...
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