currently underway in both capillary and pump augmented evaporators using water, ammonia and methylamine in the 0-100°C temperature range. Start-Up of Liquid Metal Heat Pipes From time to time, undue concern is expressed by those not in daily contact with heat pipes about the start-up ability of long heat pipes with a frozen liquid metal working fluid. It is, indeed, possible to design a test in which such a device cannot start properly. The important point, however, is that it is almost always possible to design a heat pipe which will start and operate properly in a given application. Two conditions usually must be met: the heat pipe must have sufficient fluid inventory to sustain both the start-up and operating conditions, and the capillary pumping capability of the wick must have sufficient pressure head to sustain liquid flow under a full set of start-up conditions. The first condition implies a modest excess of liquid during normal operation. This is because the volumetric thermal expansion of liquid metals is greater than that of likely heat pipe envelope materials. Thus, the fluid volume to saturate the wick during the cold condition of start-up results in an excess at the operating temperature. If the vapor velocity is moderate during normal operation, the excess working fluid will distribute itself uniformly along the heat pipe, and its presence is generally not observable. If the vapor velocity is high, the excess liquid will be swept to the end of the condenser, filling the vapor space. Accordingly, extra space should be allowed beyond the intended condensing area to accommodate the excess working fluid. The need for the second start-up requirement is less obvious. When heat is added to a frozen heat pipe, the working fluid first melts and later begins to vaporize. Since the vapor pressure of the liquid metals (K, Na and Li) is essentially zero at their melting points, their exists a temperature conduction front extending from the evaporator which melts the frozen working fluid at a rate which exceeds the progression of the vapor viscous front down the length of the heat pipe. Obviously, if the heating rate is greater than the time constant for the conduction front progression, liquid return will be diminished and the evaporator can be emptied of liquid working fluid. In addition to the conduction front effect, the heat of vaporization is much larger than the heat of fusion, so that a given mass flow of vapor will thaw a larger mass of solid material, thus assuring an increasing reservoir of liquid. Accordingly, start-up will progress provided the thermal mass of the system and the radiant losses are not an effectively infinite heat sink—a condition which will always prevent any heat pipe from starting up. Note that this is an unlikely situation in a space power system which is designed for low mass. As discussed earlier, the large pressure gradients seen in a heat pipe on start-up and as it goes through the sonic and viscous conditions also must be considered in designing the heat pipe’s wick structure. Whether a given heat pipe will or will not start-up is a function of the size of the pipe, its wick structure and the operating conditions. In general, these factors must be taken into consideration when designing the pipe in the first place. Figure 4 is a graph of data taken on a 5.5 m long X 5 cm diameter semicircular cross-section titanium/potassium radiator heat pipe, developed jointly by Thermacore and Los Alamos National Laboratory for the precursor to the SP-100. These start-up curves were taken over the course of several days, allowing thermal equilibrium to be achieved and stable operation maintained for at least 2 h at each power level. From these data, one can see that, at the lowest listed power data, more than one-half of the
RkJQdWJsaXNoZXIy MTU5NjU0Mg==