A Presentation on Heat Pipes

Submitted to:

Dr.-Ing. Jyotirmay Mathur

Associate Professor

MNIT, Jaipur

Submitted By:

Subhash Patel

(2011PME5264)

Heat Pipe Background

• 1800s – A. M. Perkins and J. Perkins developed Perkins tube

• 1944 – R. S. Gaugler introduced the use of a wicking structure

• 1964 – G. M. Grover published research and coined the “Heat Pipe” name

Heat

Transfer of Heat

Heat PipeHeat Sink

Processor

Heat Added Heat Released

*Drawing is not to scale.

Evaporation Condensation

Heat Absorbed

Heat Absorbed Heat Released

Heat Released

Heat Transfer within a Heat Pipe

*Drawing is not to scale.

Wick Structure

Wick Structure

Container

Container

Component of heat pipeContainer

• Metal Tubing, usually copper or aluminum.

• Provides a medium with high thermal conductivity.

• Shape of tubing can be bent or flattened.

Working Fluid

• Pure liquids such as helium, water and liquid silver

• Impure solutions cause deposits on the interior of the heat pipe reducing its overall performance.

• The type of liquid depends on the temperature range of the application.

  MEDIUM

 MELTING PT. (° C )

BOILING PT. AT ATM. PRESSURE

(° C)

 USEFUL RANGE

(° C)

Helium

Ammonia

Water

Silver

- 271

- 78

0

960

- 261

- 33

100

2212

-271 to -269

-60 to 100

30 to 200

1800 to 2300

Examples of Working Fluid

Choosing the Working Fluid

Chi(1976) developed a parameter of gauging the effectiveness of a working fluid called the liquid transport factor:

Where is the latent heat of vaporization and is the surface tension. Subscript refers to the liquid

l

llN

l

(Peterson, 1994).

The wicking structure

Axial Groove Wick

Created by carving out grooves on the interior core of the Heat Pipe.

Screen Mesh Wick

Utilizes multiple wire layers to create a porous wick.

Sintering can be used.

Sintered Powder Wick

Utilizes densely packed metal spheres.

Sintering must be used to solidify the spheres.

Purpose of the Wick

• Transports working fluid from the Condenser to the Evaporator.

• Provides liquid flow even against gravity.

How the Wick Works

• Liquid flows in a wick due to capillary action.

• Intermolecular forces between the wick and the fluid are stronger than the forces within the fluid.

• A resultant increase in surface tension occurs.

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.

Ideal Thermodynamic Cycle

(Faghiri, 1995)

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. (Faghiri, 1995)

• Rotating and revolving- condensate returned to the evaporator through centrifugal force. No capillary wicks required. Used to cool turbine components and armatures for electric motors.

• Cryogenic- low temperature heat pipe. Used to cool optical instruments in space. (Peterson, 1994)

Types of Heat Pipes

• 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.

(Faghiri,1995)

Types of Heat Pipes

• Micro heat pipes- small heat pipes that are noncircular and use angled corners as liquid arteries. Characterized by the equation: rc /rh1 where rc is the capillary radius, and rh is the hydraulic radius of the flow channel. Employed in cooling semiconductors (improve thermal control), laser diodes, photovoltaic cells, medical devices.

Types of Heat Pipes

• Variable conductance- allows variable heat fluxes into the evaporator while evaporator temperature remains constant by pushing a non- condensable gas into the condenser when heat fluxes are low and moving the gas out of the condenser when heat fluxes are high, thereby, increasing condenser surface area. They come in various forms like excess-liquid or gas-loaded form. The gas-loaded form is shown below. Used in electronics cooling. (Faghiri,1995)

Types of Heat Pipes

• 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. (Faghiri, 1995)

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 dry out in the evaporator. Dry out 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 dry out.

Heat Transfer Limitations

• Entrainment Limit- at high vapor velocities, droplets of liquid in the wick are torn from the wick and sent into the vapor. Results in dry out.

• 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.

The stages in the design :

( i) Select wick and Wall materials

(ii) Select working fluids

Criteria - limitations

- pressure

-priming

-handling

-purity etc .

(iii) Examine wick types :

Homogeneous rejected

Arterial selected

The stages in the design :

(iv) Determine artery sizes

(v) Examine radial resistance to heat flow

(vi) Examine overall pressure balance of proposed design

(vii) Select final configuration on basis of (vi) and such features as manufacturing difficulties etc.

Container Design• Things that should be considered for container

design:– Operating temperature range of the heat pipe.– Internal operating pressure and container structural

integrity.– Evaporator and condenser size and shape.– Possibility of external corrosion.– Prevent leaks.– Compatibility with wick and working fluid.

• Stresses:– Since the heat pipe is like a pressure vessel it must

satisfy ASME pressure vessel codes.

Container Design

• Typical materials:– Aluminum– Stainless steel – Copper– Composite materials – High temperature heat pipes may use refractory

materials or linings to prevent corrosion.

Sample Design A heat pipe is required which will be capable of

transferring a minimum of 15 W at vapour temperatures between 0 and 80 0C over a distance of 1 m in zero gravity (a satellite application) . Restraints on t he design are such that the evaporator and condenser sections are each 8 cm long , located at each end of t he heat pipe , and the maximum permissible temperature drop between the outside wall of the evaporator and the outside Hall of the condenser is 6 0 C. Because of Height and volume limitation, the cross - sectional area of the vapour space should not exceed 0 . 197 cm 2 • The heat pipe must also with stand bonding temperatures.

Selection of Material

• The selection of wick and wall material is based in various criteria.

• In this problem mass being an important parameter• So Aluminium alloy (HT30) is chosen for the wall,

and Stainless Steel for the Wick.

Selection of Working Fluid

Working fluid compatible with the wall and wick materials, based on available data, includes:

• Freon 11• Freon 113• Acetone• Ammonia

The limitations on heat transport must now be examined for each working fluid.

Heat pipe Compatibility

• Working fluid/ material

compatibility.

(Faghiri, 1995)

Conclusion on Selection of Working Fluid

• After the various examination like Sonic Limit, Entrainment Limit, Wicking Limit, Priming of the Wick Acetone is Selected.

• Properties of Acetone are shown

ACETONE

Detail Design

Circumferential liquid distribution

• The circumferential wick thickness is limited by the fact that the temperature drop between the vapour space and the outside surface of the heat pipe and vice versa should be 3 0 c maximum Assuming that the temperature drop through the aluminum wall is negligible , the thermal conductivity of the wick may be determined and used in steady state condition.

Final Analysis

Computational Study of Improving theEfficiency of Photovoltaic Panels in the UAE

The efficiency of photovoltaic cells decreases as temperature increases, therefore cooling is essential at elevated illumination situations for instance concentrating

systems, or hot and humid conditions.

•With the average temperature in the UAE reaching up to 42° C in the summer the cell temperature could reach up to 80° C which decreases the output power by up to 0.65%/K, fill factor to 0.2%/K and conversion efficiency to 0.08%/K of the PV module, above the operating temperature .

•a reduction by 20°C will give an increase in efficiency between 0.6 and 1%.•The overall reduction in the highest possible output power (Pmax) of a solar cell decreases as the cell temperature increase, shown in Fig.

Heat Produced by Photovoltaic Cells

•When PV modules are exposed to sunlight it converts only 10% to 15% of the light to electricity the rest is converted to heat.•PV panels are rated at 25°C and isolation of 1 kW/m². The power output of PV cells can be estimated from the expected Nominal Operating Cell Temperature (NOCT), defined as the open circuit temperature of the module at 800 W/m² irradiance (on cell surface), air temperature of 20°C, 1 m/s wind velocity and mounted with an open back.

Ross, R.G. (1980) approximation can be used to

calculate the cell temperature (T cell)

Heat Pipe Selection

•The two main assessments for the heat pipe design is the selection of the heat pipe’s working fluid and envelope (wick) materials for compatibility with the heat pipe. •The second main decision is the designing of the wick to cool the PV panel reliably, under any orientation and environmental conditions.

Heat Pipe Materials

•Falling under the temperature range of -20 to 1000 C, two potential heat pipe wall and wick materials are aluminum and copper. •In this study copper is selected for its higher thermal conductivity as compared to aluminum. •Compatible working fluids for copper according to surveys by Dunn and Reay, Brennan and Kroliczek and Anderson are:

Compatible with copper: Water, Methanol, Ethanol

Incompatible/Unsuitable with copper: Ammonia , Acetone

Heat Pipe Fluid Selection Selection

Typical results of the compatibility of working fluid and wall material are being shown in Fig. and it is shown that the power output of copper/water heat pipe is six times greater than the other fluids.

Description of the Proposed Finned Heat Pipe

• After the choice of heat pipe and working fluid, the next step was the selection of fin arrangements. In this case, the fins were arranged according to the constrained of the need to fit between the rear side of the PV panel and the result of cooling the panel.

• The 3D profile of the proposed arrangement used is shown in Fig.

•the glass provides protection for the solar cells and in some cases anti-reflection coatings are applied for

reduction in light scattering. T•The PV panel is attached to an aluminum frame to be is beneficial for the proposed finned heat pipe arrangement due to the high conductivity that aluminum can achieve. •The proposed finned heat pipe arrangement consist a copper heat pipe with attached aluminum fins and an aluminum saddle acting as a heat sink for the finned heat pipe.

Results

The climatic conditions in the UAE lead to the corresponding cell temperatures given by the Ross, R.G.

Results

Results

Results

Discussions•the use of fins on heat pipe is more efficient as compared to heat pipes alone. •the cooling of PV panels to its maximum operating efficiency by maintaining the solar cell operating temperature under the UAE’s climatic conditions can be obtained with the help of the proposed finned heat pipe.•This study confirms the advantages of a finned heat pipe for practical use, especially in the high-temperature region.•The proposed finned heat pipe can be used to passively

remove the heat, accepting high heat flux by natural

convection, at a much lower heat flux.

REFERENCES

• Computational Study of Improving the Efficiency of Photovoltaic Panels in the UAE: Ben Richard Hughes, Ng Ping Sze Cherisa, and Osman Beg, World Academy of Science, Engineering and Technology 73 2011.

• Fundamentals of Heat Pipes by Widah Saied• Heat Pipes by P.D. Dunn and D.A. Reay• Nptel

Nomenclature

Nomenclature

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