Presented by:KISHOR BALDANIYA

(140320721001 )

What is a Heat Pipe?

Working Principle

Types of Heat Pipes

Components of Heat Pipe.

Experimental procedure

Advantages Of Heat Pipes

Heat Pipe Applications

Review Of Literature And Inference

Conclusion

References.

CONTENTS….

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.

What is a Heat Pipe?

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

Types of Heat PipesThermosyphonLeading edge- Rotating and revolving- Cryogenic pumped loop heat pipe Flat Plate- Micro heat pipes-Variable conductance-Capillary pumped loop 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 ratioThermal conductivityEase of fabrication, including welding, machineability and

ductilityPorosityWettability

Container materialsOf the many materials available for the container, three are by

far the most common in use—name 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.

The prime requirements are:1.compatibility with wick and wall material2.Good thermal stability3.wettability of wick and wall materials4.vapor pressure not too high or low over the operating temperature range5.high latent heat6.high thermal conductivity7.low liquid and vapor viscosities8.high surface tension9.acceptable freezing or pour point

Examples of Working Fluid

Medium Melting  Point (°C)

Boiling Point at Atm.  Pressure(°C)

Useful Range(°C)

Helium -271 -261 -271 to -269Nitrogen -210 -196 -203 to -160Ammonia -78 -33 -60 to 100Acetone -95 57 0 to 120Methanol -98 64 10 to 130Flutec PP2 -50 76 10 to 160Ethanol -112 78 0 to 130Water 0 100 30 to 200Toluene -95 110 50 to 200Mercury -39 361 250 to 650Sodium 98 892 600 to 1200Lithium 179 1340 1000 to 1800Silver 960 2212 1800 to 2300

Using Nanofluids as a Working Fluid

❖ Nanofluids have significantly higher thermal conductivities compared to traditional fluids

❖ Although better performance, imposing nanoparticles increases density and viscosity; hinders the performance of the heat pipe

Nanoparticle Selection

Al2O3

❖ Design for:❖ High thermal conductivity.❖ Optimal nanofluid mass concentration.❖ Small particle size.

❖ Aluminum Oxide is a workable fluid as long as:❖ Range of specific heat flux at the desired

temperature range.❖ Compatibility with the pipe and the wick.

1. It is a porous structure made of materials like steel,alumunium, nickel or copper in various ranges of pore sizes.

2. The prime purpose of the wick is to generate capillary pressure to transport the working fluid from the condenser to the evaporator.

3. 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.

4. 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.

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

6. The wick permeability increases with increasing pore size. 7. 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.

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

Wick DesignTwo main types of wicks: homogeneous and composite.

1.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 .

2.Composite- made of a combination of several types or porosities of materials and/or configurations. Capillary pumping and axial fluid transport are handled independently . Tend to have a higher capillary limit than homogeneous wicks but cost more.

Three properties effect wick design1. High pumping pressure- a small capillary pore radius

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

2. 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.

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

Ideal Thermodynamic Cycle

Thermodynamic Cycle

1-2 Heat applied to evaporator through external sources vaporizes working fluid to a saturated(2’) or superheated (2) vapor.

2-3 Vapor pressure drives vapor through adiabatic section to condenser.

3-4 Vapor condenses, releasing heat to a heat sink.4-1 Capillary pressure created by menisci in wick pumps

condensed fluid into evaporator section. Process starts over.(Faghiri, 1995)

Experimental procedure

❖ The experiment setup consists of resistance heater, watt meter, and variable voltage transformer.

❖ Data acquisition part consists of temperature data logger and PC to record the thermocouple readings at different positions of the heat pipe.

Heat Pipe setupFull experiment setup

 •The experimental system was composed of a cooling system, the test section, a power supply and measurement system, and a data acquisition system.

• A PC was used to monitor and process the experimental data. The cooling system included a constant-temperature thermal reservoir and a cooling chamber.

• The condenser section of the heat pipe was inserted horizontally into the cooling chamber.

• The coolant circulated through the cooling chamber, where heat was removed from the condenser section by forced convection, and then to a constant-temperature reservoir.

•The constant-temperature reservoir was set to the required temperature and held at a constant temperature through the tests.

•The temperature variation of the cooling fluid was held to within 40°C.

Analysis of ExperimentAlthough heat pipes are very efficient heat transfer devices, they are subject to a

number of heat transfer limitations.

There are various parameters that put limitations and constraints on the steady and transient operation of heat pipes.

These limitations determine the maximum heat transfer rate a particular heat pipe can achieve under certain working conditions.

For high heat flux heat pipes operating in a low to moderate temperature range, the capillary and boiling limits are commonly the dominant factors that govern the operation of heat pipes.

Capillary LimitFor a given capillary wick structure and working fluid combination, the pumping

ability of the capillary structure to provide the circulation for a given working medium is limited.

This limit is usually called the capillary or hydrodynamic limit. In order to maintain the continuity of the interfacial evaporation, the capillary pressure head must be greater than or equal to the sum of pressure losses along the vapor-liquid path.

The pressure balance can be expressed as

gcevlcap PPPPPP ,,max,

Boiling LimitNucleation within the capillary wick is undesirable for wicked heat pipe

operation since the bubbles can obstruct the liquid circulation and cause hot spots on the heated wall.

If the boiling is severe it dries out the pipe wall, which is defined as the boiling limit.

However, under a moderate heat flux, low intensity stable boiling is possible without causing dry out.

It should be noted that the boiling limitation is a radial heat flux limitation as compared to an axial heat flux limitation for the other heat pipe limits.

S.No Title Author/Journal/Year

Inference

(1)Heat Transfer Performance in Heat Pipe Using Al2O3 –DI Water Nanofluid.

R. Reji Kumar, K. Sridhar, M. Narasimha /2014

Heat pipes are two-phase heat transfer devices with high effective thermal conductivity. This study presents the improvement of thermal performance of heat pipe using Al2O3 nanofluid. The nanofluids kept in the suspension of conventional fluids have the potential of superior heat transfer capability compared to the conventional fluids due to their improved thermal conductivity. The performance of the heat pipe greatly depends on the filling ratio of the working fluid.

(2)A Study On The Heat Transfer of Nanofluids in Pipes

Koh Kai Liang Peter /2014

This paper aims to investigate whether the use of nanofluids as a working fluid, as opposed to using water/oil, will reduce the pipe dimensions in an industrial set up. An understanding of the thermal conductivity in nanofluids is discussed before a suitable heat analysis method is developed to give relationships for the Nusselt number.

Review of Literature and Inference…

S.No Title Author/Journal/Year

Inference

(3)Experimental Investigationof Convective Heat Transfer Coefficient of CNTs Nanofluid under Constant Heat Flux. .

F. Rashidi, N. Mosavari Nezamabad /2013

This study is concerned with the heat transfer behavior of aqueous suspension of multi-walled carbon nanotubes flowing through a horizontal tube under constant heat flux. We measured heat transfer coefficient of CNTs nanofluid in laminar flow regime.

(4)Performance Analysis of Heat Pipe Using Copper Nanofluid with Aqueous Solution of n-Butanol.

Senthilkumar R, Vaidyanathan S, Sivaraman B/2011

The nanofluids kept in the suspension ofconventional fluids have the potential of superior heat transfer capability than the conventional fluids due to their improved thermal conductivity. In this work, the copper nanofluid which has a 40 nm size with a concentration of 100 mg/lit is kept in the suspension of the de-ionized (DI) water and an aqueous solution of n-Butanol and these fluids are used as a working medium in the heat pipe.

Review of Literature and Inference contd…

S.No Title Author/Journal/Year

Inference

(5)Experimental investigation of silver nano-fluid on heat pipe thermal performance.

Shung-Wen Kang, Wei-Chiang Wei, Sheng-Hong Tsai, Shih-Yu Yang /2009

Nano-fluid is employed as the working medium for a conventional 211 µm wide 217 µm deep grooved circular heat pipe. The nanofluid used in this study is an aqueous solution of 35 nm diameter silver nano-particles. The experiment was performed to measure the temperature distribution and to compare the heat pipe thermal resistance using nano-fluid and DI-water. The tested nano-particle concentrations ranged from 1 mg/l to 100 mg/l.

(6)Experimental study of silver nanofluidOn flat heat pipe thermal performance

Yu-Tang Chen/2010

This study utilizes silver nano-fluid filled flat heat pipe. It is aimed at effect of various concentrations on flat heat pipe thermal performance by air-cooling testing equipment. The particles used in these experiments were silver particles 35 nm in size. The base working fluid was pure-water. Nano-fluids were prepared using a two-step method. In the experiment, the thickness and length of the heat pipe are 3 mm and 200 cm, respectively.

Review of Literature and Inference…

Advantages Of Heat PipesMay reduce or eliminate the need fir 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 retrofiting

existing A/C units andAre environmentally safe.

Heat Pipe ApplicationsElectronics 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.

ConclusionHeat transfer device that dissipates heat by the use of a

working fluid, wick, evaporator, and condenserUsed in space applications and small technological devicesNanofluids increase thermal conductivity of working fluid,

enhances thermal performance

 REFERENCES

Andrews, J; Akbarzadeh, A; Sauciue, I.: Heat Pipe Technology, Pergammon, 1997.

Dunn, P.D.; Reay, D.A.: Heat Pipes, Pergammon, 1994.www.heatpipe.com.www.cheresources.com.www.indek.comwww.wikipedia.org

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