Having said, and shown pictorially, that there is a way to accommodate two-phase flow in wick structures, it is necessary to explain the new understanding. As we all know, radial heat flow into the heat pipe evaporator can be limiting. Every vapor heat transfer device has a maximum input power density beyond which vapor generation rates become dominant and seriously reduce liquid flow into the input area. It was once thought that the onset of nucleate boiling in a wick was the point at which vaporblocking of the liquid flow passage became limiting. This is true in most conventional wick structures, especially the homogeneous ones. However, it is not the case with nonhomogeneous wicks such as sintered powdered metals and metal felts. In these structures, there is a range of pore sizes. At some heat flux, vapor generation commences within the wick. The vapor replaces liquid within the wick, starting with the largest pores. Because the capillary pressure is greatest in the smallest pores, these give up their liquid last. Therefore, in a sintered powdered metal wick with a range of particle (and pore) sizes, it is possible to accommodate simultaneous liquid and vapor flow. Vapor generation within the wick is no longer as limiting as it once was. Accordingly, it now lies in the hands of the theorists to include this wick vapor pressure drop term in the treatment of the pressure balance within the heat pipe. A model of this wick vapor pressure drop term has been generated and is being correlated with the experimental data. These new advances will have two principal effects on system designers. First, the emphasis in thermal system design will tend to be away from the accommodation of heat pipe power-input limits and toward geometries, materials and interfaces that can handle the new power densities without excessive temperature losses and stresses. Second, for heat pipe radiators fed by pumped loops, the loop-to-heat pipe heat exchanger introduces significant mass due to the large heat transfer area dictated by heat pipe power-input limits. By using the new capability to increase the power density in these exchangers, important size and mass reductions may now be possible. This potential mass reduction can be seen by looking at the SP-100 and other advanced radiator systems. Heat to be radiated must be conducted across the contact interface between the heat exchanger loop and the heat pipe evaporators. The current SP-100 radiator heat pipes experience boiling limitations when this interface heat flux exceeds 30 W/cm2. As a result, the heat exchange loop ducts must be sized to accommodate this heat pipe performance limit. In the SP-100 design, over 7 m2 of heat exchanger surface area, along with the associated armor, account for approximately 30% of the total radiator subsystem mass. By increasing the heat flux capabilities of the radiator heat pipes, the required size and mass of these loops can be reduced. This trend is illustrated graphically in Fig. 2(a). Any increase in contact interface heat flux results in a greater temperature drop from the heat exchanger loop to the radiator heat pipes. For example, the SP-100 heat exchange loop operates at 820 K. At a heat flux of 30 W/cm2, the temperature drop is 20 K. This results in the radiator heat pipes operating at 800 K. If the heat flux was doubled to 60 W/cm2, the temperature drop would double to 40 K. With the heat exchange loop operating at 820 K, the radiator heat pipes would then operate at 780 K. The result is a decrease in radiator surface operating temperature. As the radiator surface temperature decreases, a larger surface area (leading to greater mass) is required to radiate equivalent power. The effect of the increased interface heat flux is increased radiator panel mass. This trend is illustrated graphically in Fig. 2(b). The optimum interface heat flux is found by superposing these two effects. Figure 2(c) shows graphically this optimum heat flux at 60 W/cm2. By removing the heat flux
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