These pipes were required to operate at two steady state conditions of 2 kW with evaporator temperatures of 600°C and 700°C. Because of size restrictions, the 600°C condition was on the sonic curve and the 700°C condition was pushed into the viscous condition. Accordingly, performance was obtainable only because of the high performance capability of the nonhomogeneous sintered stainless wick structure. To date, these heat pipes are performing as designed in the LeRC test facility [6]. Pumped Two-phase Heat Exchanger Heat pipes are passive devices that can provide for the exchange of heat at relatively high heat flux rates, low delta-T over areas in the tens to hundreds of square centimeters. Pumped convection loops are active systems that can provide for the exchange of heat over larger surface areas at equivalent heat fluxes and delta-Ts but at the expense of electrical pump power, system vibration, higher mass and lower reliability. Properly designed pumped two-phase capillary heat exchangers can offer the advantages of both. Capillary forces are small. If higher heat transfer rates are desired than can be accommodated by the flow rates sustainable with capillary pumps, a capillary evaporator can be aided by external pumping. Such a hybrid system can have appreciable advantages to trade off against the undesirable addition of the pump. The pump returns slightly subcooled condensate from the condenser and feeds a capillary evaporator which assures even distribution of liquid over the heat input surface. By relieving the capillary structure of the need to offset the pressure drop in the liquid return section, more pressure is available to produce local flow for liquid distribution purposes (high power density) or to resist vapor penetration into the wick. Very attractive performance levels have been demonstrated. Because the pump is feeding an evaporative heat transfer system which takes advantage of the latent heat of vaporization, the mass flow is low and, consequently, the pumping power and system vibration are low. Accordingly, the parasitic drain of the pump is low. A general statement on the improvement achievable with pumped capillary evaporators is at best argumentative since, to be correct, one needs to present all of the necessary qualifiers and design parameters along with the data. However, a discussion of what makes for a high Q/A, low delta-T evaporator can be generalized. Capillary evaporators, whether pump augmented or not, will have the highest Q/A at lowest delta-T if the liquid film thickness is held to a minimum. This is well known in heat pipes in the fact that vee grooves generally produce the lowest delta-T at a given Q/A. However it is also known that supplying the grooves with enough liquid to sustain operation is difficult, i.e. bulk liquid distribution. Once the film thickness and Q/A reach the point where boiling, rather than evaporation, is taking place, it has been found that enhanced performance can be obtained and modeled by treating the capillary wick as an extended fin surface [7]. Nucleate boiling takes place on the surface of the wick particles but is entirely driven by capillary forces. Rosenfeld [8] has extended his spreading model [7] to show that heat transfer in wicks in self-pumped and pump-augmented wick structures can behave like extended fin heat transfer surfaces and can be used in series, i.e. a fin on a fin on a fin. Accordingly, high Q/A low delta-T evaporators will be obtained if one can supply the working fluid in multiple closely spaced points to a capillary structure with large surface area that has short heat flow lengths. Tests in the 50-350 W/cm2 range are
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