Page 364 - Mechanical Engineers' Handbook (Volume 4)

5 Other Types of Heat Pipes 353

droplets within the ductwork can be observed and their vaporization under vacuum noted.
Stainless steel has obvious strength benefits. If plastic materials are chosen, the working fluid
used in the charging process would have to not react with the plastic materials.

4.4 Testing
A test facility needs to be established to test the heat-transfer performance and heat-transport
limitations. Both transient and steady-state tests should be conducted for a heat pipe. For
low-temperature heat pipes, however, the steady-state test is the most important. A typical
experimental system for low-temperature heat pipes similar to the one shown in Fig. 10
would normally be used. The test facility shown in Fig. 10 consists of the heat pipe, a heat
power supply and measuring unit, a cooling unit, and a data-acquisition unit for the tem-
perature measurements. The operating temperature of heat pipe can be controlled by a cool-
ing block connected to a cooling bath, where the temperature of the coolant is maintained
at a constant temperature of the designated operating temperature. The heat source is directly
connected to the evaporator. Power input can be supplied by an ac or dc power supply and
recorded by multimeters with signals sent directly to a personal computer, which can be
used to control the entire system. The heat source should be well insulated to reduce con-
vective losses. A number of temperature sensors are attached to the heat-pipe surfaces to
measure the temperature distribution on the heat pipe and temperature variation with the
power input. All of the measured data are sent to the data-acquisition system controlled by
a personal computer. Prior to the start of the experiment, the system is allowed to equilibrate
and reach steady-state such that the temperatures of the cooling media and the heat pipe are
constant. When the desired steady-state condition has been obtained, the input power is
increased in small increments. Previous tests indicate that a time of approximately 5–30
minutes is necessary to reach steady state. To obtain the data for the next successive power
level, the power is incremented every 5–30 minutes. During the tests, the power input and
the temperature data are simultaneously recorded using a data-acquisition system controlled
by a personal computer.

5 OTHER TYPES OF HEAT PIPES
5.1 Thermosyphon
One of the simplest heat pipes is a thermosyphon. As shown in Fig. 11, a thermosyphon is
a vertically oriented, wickless heat pipe with a liquid pool at the bottom. A typical ther-
mosyphon consists of three sections, similar to the convention heat pipe. As heat is added
on the evaporator section where a liquid pool exists, the liquid vaporizes into vapor. The
vapor rises and passes through the adiabatic section to the condenser section, where the
vapor gives up its latent heat and condenses into liquid. The condensate is then pumped
from the condenser to the evaporator by the gravitational force. In the conventional ther-
mosyphon, the evaporator must be located below the condenser for satisfactory operation
because the device has to rely on gravity for condensate return. Therefore, thermosyphons
are ineffective in zero gravity or microgravity. The thermosyphon has been widely used in
the preservation of permafrost, electronics cooling, and heat-pipe exchangers due to its highly
efficient heat-transfer performance, high level of temperature distribution, simplicity/relia-
bility, and cost-effectiveness. Heat-transfer limitations, such as the dryout limit, flooding and
entraniment limit, and boiling limit, should be considered during the design of thermosy-
phons. The dryout limitation might occur when the amount of working fluid in the evaporator
is not sufficient and the heat added on the evaproator cannot be removed which would result