STUDY OF CONVENTIONAL AND MODERN HEAT PIPE

 Guided By,SRI.RAJESH S P(ASSISTANT PROFESSORMECHANICAL DEPARTMENT)

SHEFIN M12402053U7

SREE CHITHRA THIRUNAL COLLEGE OF ENGINEERINGTRIVANDRUM-18

CONTENTS• INTRODUCTION• PRINCIPLE• STRUCTURE• PROPERTIES• TYPES OF HEAT PIPES• ADVANTAGES• DISADVANTAGES• APPLICATION• CONCLUSION• REFERENCE

What is a Heat Pipe?• A heat pipe heat exchanger is a simple device which is made

use of to transfer heat from one location to another, using an evaporation-condensation cycle.

• Heat pipes are referred to as the "superconductors" of heat due to their fast transfer capability with low heat loss.

Working Principle

• The heat input region of the heat pipe is called evaporator, the cooling region is called condenser.

• In between the evaporator and condenser regions, there may be an adiabatic region

Working• A basic Heat Pipe consists of a sealed upright pipe containing a small

portion of phase-changing fluid . • The remainder of the inner volume of the pipe is occupied either by the

thermal fluid vapour or by a mix of vapour and nonphase-changing gas (such as air).

• Due to gravity, the fluid rests at the bottom of the pipe (the heat source region, or evaporator of the HP), where it will be heated and boiled under the action of the heat crossing the pipe walls through conduction.

• The vaporized fluid will eventually condense at the upper part of the pipe wall releasing its heat to the heat sink.

• Once condensed, the liquid droplets will fall back to the bottom of the pipe, completing the cycle and being ready to vaporize and condense over and over again

Components of Heat Pipe• Container• Working Fluid• Wick or Capillary Structure

1.Container The function of the container is to isolate the working fluid from the outside environment. 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

Container materials• Of the many materials available for the container, three are by

far the most common in use- copper, aluminum, and stainless steel.

• Copper is eminently satisfactory for heat pipes operating between 0–200◦C in applications such as electronics

cooling. • While commercially pure copper tube is suitable, the oxygen-

free high conductivity type is preferable. • Like aluminum and stainless steel, the material is readily

available and can be obtained in a wide variety of diameters and wall thicknesses in its tubular form.

2.Working FluidThe prime requirements are:• compatibility with wick and wall material• 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

Examples of Working Fluid

3.Wick Structure• It is a porous structure made of materials like steel,aluminium, nickel or

copper in various ranges of pore sizes.• 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.

• Wicks 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 produce.

• The maximum capillary head generated by a wick increases with decrease in pore size.

• The wick permeability increases with increasing pore size. • Another feature of the wick, which must be optimized, is its

thickness. The heat transport capability of the heat pipe is raised by increasing the wick thickness.

• Other necessary properties of the wick are compatibility with the working fluid and wettability.

Wick working phenomenon

Wick DesignTwo main types of wicks: homogeneous and composite.

• Homogeneous- made from one type of material or machining technique. Tend to have either high capillary pressure and low permeability or the other way around. Simple to design, manufacture, and install.

• Composite- made of a combination of several types of porosities of materials configurations. Capillary pumping and axial fluid transport are handled independently.

Three properties effect wick design• High pumping pressure- a small capillary pore radius (channels

through which the liquid travels in the wick) results in a large pumping (capillary) pressure.

• Permeability - large pore radius results in low liquid pressure drops and low flow resistance. Design choice should be made that balances large capillary pressure with low liquid pressure drop. Composite wicks tend to find a compromise between the two.

• Thermal conductivity - a large value will result in a small temperature difference for high heat fluxes.

Types of Heat Pipes• Thermosyphon• Leading edge• Rotating and revolving• Cryogenic pumped loop heat pipe • Flat Plate• Micro heat pipe• Nano Heat pipe• Variable conductance• Capillary pumped loop heat pipe

Variable Conductance Heat Pipe (VCHP)

• One way of controlling the phase changing temperature of a fluid would be to control its pressure.

• If the pressure could be kept constant at a specified value, then a certain HP operating temperature could be regulated. The use of an expansion tank attached to the top of the HP will enable this outcome.

• Therefore, pressure would not build up during operation as it would in a standard HP and the boiling would not be hampered by an excessive increase of pressure. Such a system is called a Variable Conductance Heat Pipe (VCHP).

MICRO HEAT PIPES• The theory of micro heat pipe was introduced by Cotter in 1984. He

defined a micro heat pipe as "so small that the mean curvature of the liquid-vapor interface is comparable in magnitude to the reciprocal of the hydraulic radius of the total flow channel".

• A MHP is a small-scale device with a hydraulic diameter on the order of 100 § and a length of several centimeters. It differs from a conventional heat pipe in that it is much smaller, 5 § to 500 § in hydraulic diameter.

• In general, it does not contain a wick structure to assist the return of the condensate to the evaporator section. It rather uses capillary forces generated in the sharp edges of the pipe's cross section.

• Micro heat pipe is a small metal pipe with a capillary structure (wick) on the inner wall. The micro heat pipe is a vacuum inside with a small quantity of working fluid. When a section of the micro heat pipe is heated, the heat is quickly transferred to a lower temperature section of the micro heat pipe in the following way

1.The working fluid evaporates at the heated section. 2.The vapor moves to the lower temperature section. 3.The vapor condenses in the lower temperature section.• The condensed working fluid flows back to the heated section

through capillary action. The micro heat pipe is a super conductor of heat for micro device cooling. The micro heat pipe offers increased downsizing and improved performance for all electronic devices.

NANO HEAT PIPE• Heat pipes with Nano fluid as working fluid can be referred as Nano

heat pipe• The heat transfer capability of the all heat transfer devices including

heat pipe is limited by the working fluid transport properties.• To overcome these limitations, the thermo physical properties of the

working fluid have to be improved. • Heat Pipe Using Copper Nano fluid with Aqueous Solution of nButanol is

used.• The heat transfer rate of heat transfer devices can be improved by

adding additives to the working fluids to change the fluid transport properties and flow features.

• One of the methods is to use the aqueous solutions of alcohols, with chain lengths longer than four carbon atoms.

Thermosyphons• Most heat pipes use a wick and capillary action to return the

liquid from the condenser to the evaporator. • The liquid is sucked up to the evaporator, similar to the way

that a sponge sucks up water when an edge is placed in contact with a water pool.

• The wick allows the heat pipe to operate in any orientation, but the maximum adverse elevation (evaporator over condenser) is relatively small, on the order of 25 cm long for a typical water heat pipe.

• Taller heat pipes must be gravity aided. When the evaporator is located below the condenser, the liquid can drain back by gravity instead of requiring a wick. Such a gravity aided heat pipe is known as a thermosyphon

Loop heat pipes• Capillary pumped loops (CPL), and loop heat pipes (LHP), are an

attractive alternative for heat regulation. In the LHP the capillary pumped evaporator is used instead of a boiler.

• Such an evaporator is more flexible from the point of view of its orientation space and is more compact. In the LHP there is a possibility to use an evaporator above the condenser.

• In the LHP the vapour flows through the vapour channels towards the condenser and the liquid goes back the evaporator due to the capillary pressure head of the porous wick.

• In the near future an LHP should be used as thermal control devices in scientific and telecommunication satellites (efficient and flexible thermal link between dissipative elements and radiators.

Spaghetti heat pipes• The small diameter (3 mm) bendable SS spaghetti heat

pipes are similar to pulsating heat pipes, but have a compact condenser and large surface evaporator.

• An example of a spaghetti heat pipe filled with ammonia, shown in is disposed inside the refrigerator chamber in such a way that food can be kept within the refrigerating temperature range as uniformly as possible.

• The ‘‘spaghetti’’ heat pipe is thermally linked with an evaporator of the sorption refrigerator(heat pipe condenser) and has a good thermal contact with this evaporator.

Sorption heat pipe

• The sorption heat pipe (SHP) is a novelty and combines the enhanced heat and mass transfer in conventional heat pipes with sorption phenomena of a sorbent bed.

• Sorption heat pipe could be used as a sorption heat transfer element and be cooled and heated as a heat pipe.

• The sorption heat pipe has a sorbent bed at one end and a condenser and evaporator at the other end.

Advantages Of Heat Pipes• May reduce or eliminate the need for reheat• Allow cost effective manner to accommodate new

ventilation standards• Requires no mechanical or electrical input• Are virtually maintenance free• Provide lower operating costs• Last a very long time• Readily adaptable to new installations and retrofitting

existing A/C units • Are environmentally safe.

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.

Applications

• LAPTOP HEAT PIPE SOLUTION

Heat pipes used in processor

HEAT PIPE IN CPU

CONCLUSION• Heat pipe is a thermal super conductor under certain heat

transfer condition they can transfer the heat energy 100 times more than available best conductive materials, because of negligible temp. Gradient exist in heat pipe.

• The heat pipe has compactness, light weight, reversible in operation and high thermal flux handling capability makes heat pipe to use new modern era and in many wide variety application to overcome critical heat dissipation problem.

REFERENCES

• R.R. Riehl, “Analysis of loop heat pipe behavior using nanofluid” Heat Powered Cycles International Conference (HPC), New Castle, UK, Paper 06102, 2006.

• Midwest Research Institute, Heat Pipes, NASA Report NASA CR-2508, pg. 19, Jan 1, 1975.

• Pressure Controlled Heat Pipe Applications, W. G. Anderson et al., 16th International Heat Pipe Conference, Lyon, France, May 20-24, 2012.

• S.H. Noie, “Heat transfer characteristics of a two-phase closed thermosyphon”, Applied Thermal Engineering vol.25, 2005, pp. 495–506.

• www.heatpipe.com.• www.cheresources.com.• www.indek.com

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