Heat Pipe Operation

Although there are several limits which apply to heat pipe operation, these generally lend themselves to specific design solutions or occur at sufficiently high levels of performance to permit a wide latitude of practical appHcations. The envelope of these limits is shown generically in Figure 3. 

The UV Region. The results for the relative cross sections of potassium vapor in the energy region of 210-400 nm are shown in Figure 3. Because of strong absorptions in this wavelength region the vapor pressure of potassium in the heat-pipe could not be reduced to the point such that the heat-pipe operation condition could still be sustained. The isothermal zone is thus not established. Therefore, only relative data are measured in the present work. 

While in most applications heat pipes operate in a passive manner, adjusting the heat flow rate to compensate for the temperature difference between the evaporator and condenser [37], several active control schemes have been developed [38]. Most notable among these are (1) gas-loaded heat pipes with some type of feedback system, (2) excess-liquid heat pipes, (3) vapor-flow-modulated heat pipes, and (4) liquid-flow-modulated heat pipes [9], In one such pipe, a temperature-sensing device at the evaporator provides a signal to the reservoir heater, which when activated can heat the gas contained in the reservoir, causing it to expand and thereby reducing the condenser area. 

Excess-liquid heat pipes operate in much the same manner as gas-loaded heat pipes, but utilize excess working fluid to block portions of the pipe and control the condenser size or prevent reversal of heat transfer. Vapor-flow-modulated heat pipes utilize a throttling valve to control the amount of vapor leaving the evaporator. In this type of control scheme, increased evaporator temperatures result in an expansion of the bellows chamber containing the control fluid. This in turn closes down the throttling valve and reduces the flow of vapor to the condenser. This type of device is typically applied in situations where the evaporator temperature varies and a constant condenser temperature is desired. 

Another possibility is that of using molybdenum alloyed heat pipes with lithium as operating fluid to move the heat from the core to the ultimate heat sink outside of the reactor boundaries. For the 500-kW core, a set of 3 3 heat pipes operating will suffice because each heat pipe will carry a load of 15.15 kW, which is well below the proven capacity of 40 kW per heat pipe. This heat pipe approach would only require 33 holes of 1.6-cm diam traversing the 1.5-m-diam reactor s core. With an area of 2 cm per hole, it will take away a total of 66 cm out of 17672 cm (i.e., 0.37% of the core s cross section area). 

At low temperatures, viscous forces are dominant in the vapor flow down the heat pipe. At very low operating temperature, the vapor pressure difference between the closed ends of the evaporator (the high-pressure region) may be extremely small. Because of the small pressure difference, the viscous forces within the vapor region may prove to be dominant and hence limit the heat pipe operation. From the 2-D analysis by Busse,... 

Radiator Heat Pipe Operating Temperature Sensitivity - Design Case.25... 

K maximum radiator heat pipe operating temperature ... 

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