335
CHAPTER 9
HEAT PIPES
Hongbin Ma
Department of Mechanical and Aerospace Engineering
University of Missouri
Columbia, Missouri
1
INTRODUCTION
335
2 FUNDAMENTALS
337
2.1 Surface Tension
337
2.2 Contact Angle
337
2.3 Laplace-Young Equation
338
3 HEAT TRANSPORT
LIMITATIONS
338
3.1 Capillary Limit
338
3.2 Boiling Limit
342
3.3 Entrainment Limit
343
3.4 Viscous Limit
343
3.5 Sonic Limit
344
3.6 Effective Thermal Conductivity
344
4 HEAT-PIPE FABRICATION
PROCESSES
348
4.1 Wicks
348
4.2 Working Fluid Selections
348
4.3 Cleaning and Charging
351
4.4 Testing
353
5 OTHER TYPES OF HEAT PIPES
353
5.1 Thermosyphon
353
5.2 Loop Heat Pipes /Capillary
Pumped Loop
354
5.3 Pulsating Heat Pipes
355
5.4 Micro Heat Pipes
356
5.5 Variable-Conductance Heat Pipes 356
5.6 Rotating Heat Pipes
358
5.7 High-Temperature Heat Pipes
(Metal Heat Pipes)
358
5.8 Cryogenic Heat Pipes
358
NOMENCLATURE
359
REFERENCES
360
1
INTRODUCTION
The heat pipe is a device that utilizes the evaporation heat transfer in the evaporator and
condensation heat transfer in the condenser, in which the vapor flow from the evaporator to
the condenser is caused by the vapor pressure difference and the liquid flow from the con-
denser to the evaporator is produced by the capillary force, gravitational force, electrostatic
force, or other forces directly acting on it. The first heat-pipe concept can be traced to the
Perkins tube.1,2 Based on the structure, a heat pipe typically consists of a sealed container
charged with a working fluid. Heat pipes operate on a closed two-phase cycle and only pure
liquid and vapor are present in the cycle. The working fluid remains at saturation conditions
as long as the operating temperature is between the triple point and the critical state. As
illustrated in Fig. 1, a typical heat pipe consists of three sections: an evaporator or heat
addition section, an adiabatic section, and a condenser or heat rejection section. When heat
is added to the evaporator section of the heat pipe, the heat is transferred through the shell
and reaches the