Page 365 - Mechanical Engineers' Handbook (Volume 4)

354 Heat Pipes

Adiabatic
evaporator section condenser

Thermocouples
Power Supply &
Measurement
System
heater Heat Pipe Cooling Block

Data Acquisition Cooling Bath
System PC

Figure 10 A heat-pipe test apparatus.

in an significant temeprature increases. This is similar to the critical heat flux in pool boiling.
Flooding and entrainment limits involve the interfacial shear stress between the countercur-
rent vapor and liquid flows. Boiling limitation can be found in a thermosyphon with large
fill volumes and high radial heat flux in the evaporator.

5.2 Loop Heat Pipes/Capillary Pumped Loop
The loop heat pipe (LHP) and capillary pumped loop (CPL) utilize the capillary pressure
developed in a fine-pore wick to circulate the working fluid in a closed loop system. Figure
12 shows the schematic of the CPL. The liquid phase flows through the liquid line from the
condenser to the evaporator and the vapor flows from the evaporator to the condenser through
the vapor line. The LHP or CPL can significantly reduce or eliminate the liquid pressure
drop and vapor flow effect on the liquid flow, resulting in a significant increase in the
capillary pumping ability. Because the evaporator in a LHP or CPL is used not only as the
heat sink to remove the heat, but also as the source to provide the total capillary pumping
pressure, a highly efficient evaporator is a key to a LHP or CPL. Compared with the con-

Heat
output Condenser

Vapor
flow
Adiabatic
Liquid section
flow

Heat Evaporator
input

Pool Figure 11 Schematic of a thermosyphon.