Page 347 - Mechanical Engineers' Handbook (Volume 4)
P. 347

336   Heat Pipes

                                        Heat input                        Heat output
                                                        wick







                                                        Vapor flow







                                       Evaporator    Adiabatic section   Condenser

                                                 Figure 1 Schematic of a heat pipe.



                          the pumping pressure produced by the surface tension cannot overcome the summation of
                          the total pressures, the heat transport occurring in the heat pipe reaches a limit known as
                          the capillary limit. There are several other limitations disconnecting the return of the working
                          fluid from the evaporator to the condenser or from the condenser to the evaporator. Among
                          these are the boiling limit, sonic limit, entrainment limit, and viscous limit. When the heat
                          flux added to the evaporator is sufficiently high, nucleate boiling occurs. The bubble formed
                          in the wick significantly increases the thermal resistance, causing the heat-transfer perform-
                          ance to be significantly reduced. More importantly, when the heat flux is so high, the bubbles
                          block the return of the working fluid and lead to a dryout of the evaporator. The boiling
                          limit plays a key role in a high heat flux heat pipe. When the vapor velocity is high and the
                          cross-sectional area variation of the vapor space in a heat pipe cannot meet the flow con-
                          dition, chocked flow occurs and the vapor flow rate will not respond with the amount of
                          heat added in the evaporator. This will lead to a sonic limit. The entrainment limit is due to
                          the frictional shear stresses caused by the vapor flow at the vapor–liquid interface. The
                          viscous limit occurs in a low heat flux heat pipe, where the vapor pressure difference in the
                          vapor phase cannot overcome the vapor pressure drop in the vapor phase.
                             From a thermodynamics point of view, the thermal energy added to the evaporator in a
                          functional heat pipe produces the mechanical work to pump the working fluid. No external
                          power is needed for a typical heat pipe. The phase-change heat transfer occurs almost in the
                          quasi-equilibrium state. The heat pipe has a very high efficiency to transfer the thermal
                          energy from a higher-temperature heat source to a lower-temperature heat source. An oper-
                          ational heat pipe can provide an extra-high effective thermal conductivity and reach a higher
                          level of temperature uniformity. The working fluid medium in a heat pipe can be selected
                          from a variety of fluids, depending on the operating temperature and compatibility with the
                          shell material. The heat pipe can be operated from a temperature lower than4Ktoa high
                          temperature up to 3000 K. Because the evaporator and condenser of a heat pipe function
                          independently, the heat pipe can be made into any shape, depending on the design require-
                          ment. Due to these unique features, the heat pipe has been widely used in a wide range of
                          applications.
   342   343   344   345   346   347   348   349   350   351   352