Transferencia De Calor

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Nu6 = 0.42Ra 5 X 104, NuD = 0.0265 Re£8 Pr1/3 where the Reynolds number ReG = GDI^ is based on the equivalent mass velocity G = Gl + Gv(ptl pv}Q-5. The mass velocity for the liquid Gl and for vapor Gv are calculated as if each occupied the entire flow area alone. The Effect of Noncondensable Gases If noncondensable gas such as air is present in a vapor, even in a small amount,the heat transfer coefficient for condensation may be greatly reduced. It has been found that the presence of a few percent of air by volume in steam reduces the coefficient by 50% or more. Therefore, it is desirable in the condenser design to vent the noncondensable gases as much as possible. 4 . . Heat Pipes 353 Heat pipes are a two-phase heat transfer device that operate on a closed two-phasecycle31 and come in a wide variety of sizes and shapes.31'32 As shown in Fig. 43.31, they typically consist of three distinct regions, the evaporator or heat addition region, the condenser or heat rejection region, and the adiabatic or isothermal region. Heat added to the evaporator region of the container causes the working fluid in the evaporator wicking structure to be vaporized. The hightemperature and corresponding high pressure in this region result in flow of the vapor to the other, cooler end of the container, where the vapor condenses, giving up its latent heat of vaporization. The capillary forces existing in the wicking structure then pump the liquid back to the evaporator section. Other similar devices, referred to as two-phase thermosyphons, have no wick, and utilizegravitational forces to provide the liquid return. Thus the heat pipe functions as a nearly isothermal device, adjusting the evaporation rate to accommodate a wide range of power inputs, while maintaining a relatively constant source temperature. Transport Limitations The transport capacity of a heat pipe is limited by several important mechanisms, including the capillary wicking, viscous, sonic,entrainment, and boiling limits. The capillary wicking limit and viscous limits deal with the pressure drops occurring in the liquid and vapor phases, respectively. The sonic limit results from the occurrence of choked flow in the vapor passage, while the entrainment limit is due to the high liquid vapor shear forces developed when the vapor passes in counter-flow over the liquid saturated wick. The boilinglimit is reached when the heat flux applied in the evap-

Evaporator

Adiabatic Condenser Fig. 43.31 Typical heat pipe construction and operation.33

orator portion is high enough that nucleate boiling occurs in the evaporator wick, creating vapor bubbles that partially block the return of fluid. In order to function properly, the net capillary pressure difference between the condenser andthe evaporator in a heat pipe must be greater than the pressure losses throughout the liquid and vapor flow paths. This relationship can be expressed as
APC > AP+ + AP_ + AP, 4- APV

where APC = net capillary pressure difference AP+ = normal hydrostatic pressure drop AP_ = axial hydrostatic pressure drop AP, = viscous pressure drop occurring in the liquid phase APy = viscous pressure dropoccurring in the vapor phase. If these conditions are not met, the heat pipe is said to have reached the capillary limitation. Expressions for each of these terms have been developed for steady-state operation, and are summarized below. Capillary Pressure
»~ - \rj ' (} "

Values for the effective capillary radius rc can be found theoretically for simple geometries or experimentally for pores orstructures of more complex geometry. Table 43.26 gives values for some common wicking structures. Normal and Axial Hydrostatic Pressure Drop AP+ + Pigdv cos $ AP_ = p{gL sin i/> In a gravitational environment, the axial hydrostatic pressure term may either assist or hinder the capillary pumping process, depending upon whether the tilt of the heat pipe promotes or hinders the flow of liquid back to...