vapor penetrated the artery through a ‘breached’ overstressed large pore, the vapor was swept to the evaporator end and condensed in the subcooled region before large vapor blocking pockets could coalesce. The ultimate limit was reached when a large number of smaller pores became overstressed and breached, and the ensuing vapor coalesced before it could be condensed. The measured non-subcooled horizontal performance was 1800 W and the measured subcooled performance at an adverse tilt of 10 cm was 3200 W. These translate into a power-length product of 1.04 million Win and 1.8 million Win, respectively, both believed to be records for a pipe of these dimensions. A second, and more subtle, improvement in axial heat transport is being realized because nonhomogeneous wicks, with their boiling tolerant capability and small effective capillary radius, can provide the capillary forces necessary to operate heat pipes at their viscous limit over a wide range of temperatures and, therefore, pressures. The viscous limit of a heat pipe is that non-isothermal and normally not desirable condition when the far end of the condenser is ‘cold’. Accordingly, the pressure drop in the vapor is the difference between the saturated vapor pressures of the working fluid at the evaporator and condenser temperatures. For example, with sodium at an evaporator temperature of 700°C and condenser temperature of 400°C this difference is 0.15 bar. Accordingly, the heat pipe wick structure must be capable of supporting this vapor pressure difference plus all of the liquid pressure drops in the wick for a sodium heat pipe to operate with a 700-400°C temperature profile. Conditions such as these are not normally expected, and one tries to design around them. However, recently in making heat pipes for NASA LeRC to provide heat to a RE-1000 Stirling engine, such a temperature profile was encountered in testing the heat pipes in a water cooled gas gap calorimeter which was used to simulate the oscillating flow of helium gas in the Stirling engine heater head. The heat pipes are 16 in. long and have a nominal inside diameter of 0.75 in.. Normal operation is with the evaporator above the condenser.
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