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


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