Page 346 - Mechanical Engineers' Handbook (Volume 4)
P. 346
Mechanical Engineers’ Handbook: Energy and Power, Volume 4, Third Edition.
Edited by Myer Kutz
Copyright 2006 by John Wiley & Sons, Inc.
CHAPTER 9
HEAT PIPES
Hongbin Ma
Department of Mechanical and Aerospace Engineering
University of Missouri
Columbia, Missouri
1 INTRODUCTION 335 4.2 Working Fluid Selections 348
4.3 Cleaning and Charging 351
2 FUNDAMENTALS 337 4.4 Testing 353
2.1 Surface Tension 337
2.2 Contact Angle 337 5 OTHER TYPES OF HEAT PIPES 353
2.3 Laplace-Young Equation 338 5.1 Thermosyphon 353
5.2 Loop Heat Pipes/Capillary
3 HEAT TRANSPORT Pumped Loop 354
LIMITATIONS 338 5.3 Pulsating Heat Pipes 355
3.1 Capillary Limit 338 5.4 Micro Heat Pipes 356
3.2 Boiling Limit 342 5.5 Variable-Conductance Heat Pipes 356
3.3 Entrainment Limit 343 5.6 Rotating Heat Pipes 358
3.4 Viscous Limit 343 5.7 High-Temperature Heat Pipes
3.5 Sonic Limit 344 (Metal Heat Pipes) 358
3.6 Effective Thermal Conductivity 344 5.8 Cryogenic Heat Pipes 358
4 HEAT-PIPE FABRICATION NOMENCLATURE 359
PROCESSES 348
4.1 Wicks 348 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 liquid. When the liquid in the evaporator section receives enough thermal
energy, the liquid vaporizes. The vapor carries the thermal energy through the adiabatic
section to the condenser section, where the vapor is condensed into the liquid and releases
the latent heat of vaporization. The condensate is pumped back from the condenser to the
evaporator by the driving force acting on the liquid.
For a heat pipe to be functional, the liquid in the evaporator must be sufficient to be
vaporized. There are a number of limitations to affect the return of the working fluid. When
335