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Heat Pipe | Working and Application

ABSTRACT

The heat pipe is a device having a high thermal conductance which utilizes the transport of a vapour and rejection of latent heat to achieve efficient thermal energy transport. The theory of heat pipes is well developed. Their use in applications involving temperatures in the cryogenic regime, and with development units running as high as 2000 degrees C, shows that they can function over a large part of the temperature spectrum. Applications in spacecraft, electronics and die casting are but few of the uses for these devices.

Heat Pipe

A heat pipe is a device that efficiently transports thermal energy from its one point to the other. It utilizes the latent heat of the vaporized workingfluid instead of the sensible heat. As a result, the effective thermal conductivity may be several orders of magnitudes higher than that of thegood solid conductors.

INTRODUCTION TO HEAT PIPE


heat pipe construction
Heat Pump


A heat pipe consists of a sealed container, a wick structure, a small amount of working fluid that is just sufficient to saturate the wick and it is in equilibrium with its own vapor. The operating pressure inside the heat pipe is the vapor pressure of its working fluid. The length of the heat pipe can be divided into three parts viz. evaporator section, adiabatic section and condenser section. 

2.DESIGN CONSIDERATIONS


The three basic components of a heat pipe are :

1. The container.
2. The working fluid.
3. The wick or capillary structure. 

CONTAINER

 The function of the container is to isolate the working fluid from the outside environment. It has to therefore be leak-proof, maintain the pressure differential across its walls, and enable transfer of heat to take place from and into the working fluid. 

Selection of the container material depends on many factors. These are
as follows:

• Compatibility (both with working fluid and external environment)
• Strength to weight ratio
• Thermal conductivity
• Ease of fabrication, including welding, machine ability and ductility
• Porosity
• Wettability

Most of the above are self-explanatory. A high strength to weight ratio is more important in spacecraft applications. The material should be non-porous to prevent the diffusion of vapor. A high thermal conductivity ensures minimum temperature drop between the heat source and the
wick.

WORKING FLUID

 A first consideration in the identification of a suitable working fluid is the operating vapour temperature range. Within the approximate temperature band, several possible working fluids may exist, and a  variety of characteristics must be  examined in order to determine the most acceptable of these fluids for the application considered. 

The prime requirements are: 
• compatibility with wick and wall materials
• good thermal stability
• wettability of wick and wall materials
• vapor pressure not too high or low over the operating temperature
range
• high latent heat
• high thermal conductivity
• low liquid and vapor viscosities
• high surface tension
• acceptable freezing or pour point 

The selection of the working fluid must also be based on thermodynamic  considerations which are concerned with the various limitations to heat flow occurring within the heat pipe like, viscous, sonic, capillary,entrainment and nucleate boiling levels.

WICK OR CAPILLARY STRUCTURE

It is a porous structure made of materials like steel, alumunium, nickel or copper in various ranges of pore sizes. They are fabricated using metal foams, and more particularly felts, the latter being more frequently used. By varying the pressure on the felt during assembly, various pore sizes can be produced. By incorporating removable metal mandrels, an arterial structure can also be molded in the felt. Fibrous materials, like ceramics, have also been used widely. They generally have smaller pores. The main disadvantage of ceramic fibres is that, they have little stiffness and usually require a continuos support by  a metal mesh. Thus while the fibre itself may be chemically compatible with the working fluids, the supporting materials may cause problems.More recently, interest has turned to carbon fibres as a wick material. Carbon fibre filaments have many fine longitudinal grooves on their surface, have high capillary pressures and are chemically stable. A number of heat pipes that have been successfully constructed using carbon fibre wicks seem to show a greater heat transport capability.  The prime purpose of the wick is to generate capillary pressure to transport the working fluid from the condenser to the evaporator. It must also be able to distribute the liquid around the evaporator section to any area where heat is likely to be received by the heat pipe. Often these two functions require wicks of different forms. The selection of the wick for  a heat pipe depends on many factors, several of which are closely linked to the properties of the working fluid. 

WORKING PRINCIPLE OF HEAT PIPE :


Heat pump working

A metal cylinder is sealed with a fluid within it creating a closed system. One end of the tube is heated and the other is cooled. The heat source (the evaporator) causes the fluid to boil and turn to vapor (this is absorbing energy as heat). When that hap latent heat of vaporization. The gas, which then has a higher pressure, moves inside the sealed container to a colder location where it  condenses. Once the vapor reaches the cold end of the tube (the condenser), the fluid changes phase again from vapor back to a liquid. Thus, the gas gives up the latent heat of vaporization and moves heat from the input to the output end of the heat pipe. This liquid returns to the hot (evaporator) end by means of a wick so that the liquid can  repeat the process. This process is capable of transporting heat from a
hot region to a colder region. It requires no addition of external energy.

Heat Pipe Applications

Electronics cooling- Small high performance components cause high heat fluxes and high heat dissipation demands. Used to cool transistors and high density semiconductors.

Aerospace-Cool satellite solar array, as well as shuttle leading edge during reentry.

Heat exchangers- Power industries use heat pipe heat exchangers as air heaters on boilers.

Other applications- Production tools, medicine and human body temperature control, engines and automotive industry.

Types of Heat Pipes

Thermosyphon- Gravity assisted wickless heat pipe. Gravity is used to force the condensate back into the evaporator. Therefore, condenser must be above the evaporator in a gravity field.

Leading edge-Placed in the leading edge of hypersonic vehicles to cool high heat fluxes near the wing leading edge

Rotating and revolving- Condensate returned to the evaporator through centrifugal  Force.No capillary wicks required. Used to cool turbine component and armatures for electric motors. 

Cryogenic- Low temperature heat pipe. Used to cool optical instruments in space.


Flat Plate- Much like traditional cylindrical heat pipes but are rectangular. Used to cool and flatten temperatures of semiconductor or transistor packages assembled in arrays on the top of the heat pipe. 

Capillary pumped loop heat pipe- For systems where the heat fluxes are very high or where the heat from the heat source needs to be moved far away. In the loop heat pipe, the vapor travels around in a loop where it condenses and returns to the evaporator. Used in electronics cooling.


Main Heat Transfer Limitations

Capillary limit- Occurs when the capillary pressure is too low to provide enough liquid to the evaporator from the condenser. Leads to dryout in the evaporator. Dryout prevents the thermodynamic cycle from continuing and the heat pipe no longer functions properly.

Boiling Limit- Occurs when the radial heat flux into the heat pipe causes the liquid in the wick to boil and evaporate causing dryout.


Entrainment Limit- 
At high vapor velocities, droplets of liquid in the wick are torn from the wick and sent into the vapor. Results in dryout.

Sonic limit- occurs when the vapor velocity reaches sonic speed at the evaporator and any increase in pressure difference will not speed up the flow; like choked flow in converging diverging  nozzle. Usually occurs during startup of heat pipe.


Viscous Limit-  at low temperatures the vapor pressure difference between the  condenser and the evaporator may not be enough to overcome  viscous forces. The vapor from the evaporator doesn’t move to the  condenser and the thermodynamic cycle doesn’t occur.

HEAT PIPES IN DIFFERENT SIZES AND SHAPES

MEGA FLATS
Flat heat pipes are typically used for cooling printed circuit boards or for heat leveling to produce an isothermal plane. Mega flats are several flat heat pipes sandwiched together. 

Some of the flat heat pipes manufactured are:

XY Mega Flats: Surface maintained within .01° F isothermal with concentrated load centers. 
6" X 6" Mega Flat: Dissipated 850 watts from a printed circuit board. 

DIFFERENT MEGA FLATS
Weight Reduction Mega Flats:
Standard - aluminum construction. 
Lightweight - ½ the weight of aluminum. 
Very light weight - 1/3 the weight of aluminum.  





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