Fig. 4-74. Heat Pipe Options (right upper) is a round tube with grooves in the wall to enhance return flow of the condensate to the evaporator. The lower two are thin walled tube types whose operation is identical to the first. Under certain design considerations they can provide improved surface contact with the heat source. For example, the lower right design could be attached to a round tube to provide good thermal contact. Both lower types require care in design to ensure that internal pressure will not distort their shape. All types are shown with screen wicks which may consist of one or more layers of screen material. However, with heat pipes employing axial grooves, screen wicks may not be necessary since the grooves serve the purpose of the wick. Screens enhance the capillary pumping pressure of the axial grooves. The second heat pipe option shown appears to be the most attractive design. The axial grooves provide a relatively low resistance path for liquid flow from condenser to evaporator; the fine mesh screen prevents liquid entrainment at higher heat flux rates and provides small radius menisci for effective capillary pumping. 4.10.6.3 Heat Pipe Working Fluids The heat pipe's operating temperature is dictated by the selection of its working fluid. Good working fluids should possess the following characteristics: high latent heat of vaporization, high surface tension and low viscosity. In addition, the fluid must be compatible with the heat pipe envelope and a capillary wick. Candidate working fluids for temperatures under consideration for the radiator are water, mercury, cesium, potassium, sodium and lithium. Figures 4-75 through 4-79 show important characteristics for candidate working fluids. Fig. 4-75. Heat Pipe Fluids: Latent Heat of Vaporization Fig. 4-76. Heat Pipe Fluids: Surface Tension Fig. 4-77. Heat Pipe Fluids: Absolute Viscosity Fig. 4-78. Heat Pipe Fluids: Vapor Pressure
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