HEAT PIPE.. It is used to transfer heat from high temperature area to low temperature area.

Heat pipe

1. HEAT PIPE Presented by, Abhijith Siji [email protected]

2. INTRODUCTION A heat pipe heat exchanger is a simple device which is made use of to transfer heat from one location to another by 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. It utilizes the latent heat of the vaporized working fluid instead of the sensible heat. 05-08-2024 Department of Mechanical Engineeering 2

3. Passive two-phase heat transfer device operating in a closed system -Heat/Power causes working fluid to vaporize -Vapor flows to cooler end where it condenses -Condensed liquid returns to evaporator by gravity or capillary force Typically a 2-5 °C ΔT across the length of the pipe k eff ranges from 10,000 to 200,000 W/m-K Heat Pipes can operate with heat flux up to 50-75W/cm 2 -Custom wicks to 500W/cm 2 05-08-2024 Department of Mechanical Engineeering 3

4. 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. 05-08-2024 Department of Mechanical Engineeering 4

5. IDEAL THERMODYNAMIC CYCLE 05-08-2024 Department of Mechanical Engineeering 5

6. 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 non-phase changing gas (such as air). Due to gravity, the fluid rests at the bottom of the pipe in the heat source region or evaporator. it will be heated and boiled under the action of the heat crossing the pipe walls through conduction. 05-08-2024 Department of Mechanical Engineeering 6

7. 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. 05-08-2024 Department of Mechanical Engineeering 7

8. COMPONENTS Container Working fluid Wick or Capillary structure 05-08-2024 Department of Mechanical Engineeering 8

9. 1. Container The function of the container is to isolate the working fluid from the outside environment. 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. 05-08-2024 Department of Mechanical Engineeering 9

10. Selection of the container material depends on, • 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 05-08-2024 Department of Mechanical Engineeering 10

11. 2. 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. The 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 05-08-2024 Department of Mechanical Engineeering 11

12. Examples of Working Fluid 05-08-2024 Department of Mechanical Engineeering 12

13. 3. Wick Structure It is a porous structure made of materials like steel,alumunium , 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. 05-08-2024 Department of Mechanical Engineeering 13

14. 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. 05-08-2024 Department of Mechanical Engineeering 14

15. Purpose:- Transports working fluid from the Condenser to the Evaporator. Provides liquid flow even against gravity. Working:- 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. 05-08-2024 Department of Mechanical Engineeering 15

16. There are Two main types of wicks design:- 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. 05-08-2024 Department of Mechanical Engineeering 16

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18. Axial Groove Created by carving out grooves on the interior core of the Heat Pipe. 05-08-2024 Department of Mechanical Engineeering 18

19. Screen Mesh Wick Utilizes multiple wire layers to create a porous wick. Sintering can be used. 05-08-2024 Department of Mechanical Engineeering 19

20. Sintered Powder Wick Utilizes densely packed metal spheres. Sintering must be used to solidify the spheres. 05-08-2024 Department of Mechanical Engineeering 20

21. Properties effect wick designs:- 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. 05-08-2024 Department of Mechanical Engineeering 21

22. TYPES OF HEAT PIPE Constant Conductance Heat Pipes Heat Pipes Vapour Chambers Gas-Loaded Heat Pipes Variable Conductance Heat Pipes (VCHPs) Pressure Controlled Heat Pipes (PCHPs) Gas Trap Diode Heat Pipes Interrupted Wick Liquid Trap Diode Heat Pipes Heat Pipe and VCHP Heat Exchangers Alternate Means of Liquid Return Thermosyphons Rotating Heat Pipes 05-08-2024 Department of Mechanical Engineeering 22

23. Vapor Chamber 05-08-2024 Department of Mechanical Engineeering 23 Constant Conductance Heat Pipe. Vapor chambers are planar heat pipes for heat spreading/ isothermalizing.

24. Benefits:- Multi-component mounting Thickness from 0.12” (3 mm) Excellent Heat Spreading Resistance < 0.15 °C/W, < 0.08 °C/W for special wicks Excellent Isothermalization High heat flux to low heat flux Ideal for high heat flux/high performance applications Heat flux > 60 W/cm2, up to 750 W/cm2 for special wicks 05-08-2024 Department of Mechanical Engineeering 24

25. Gas-Charged Heat Pipes Standard heat pipes are evacuated, then filled with the working fluid Vapor space contains only working fluid vapor In Gas Charged Heat Pipes, a Non-Condensable Gas (NCG) is added Noble Gas Typically argon for horizontal heat pipes, helium for vertical NCG is driven to the end of the pipe Gas-Charged Heat Pipes are Variable Conductance Heat Pipes (VCHP) Pressure Controlled Heat Pipes (PCHP) Gas-Charged Diodes 05-08-2024 Department of Mechanical Engineeering 25

26. Variable Conductance Heat Pipes (VCHP) During operation, the working fluid drives the Non-Condensable Gas (NCG) to the condenser The portion of the condenser blocked by NCG is not available for heat transfer by condensation -Inactive condenser region Remaining condenser is available for heat transfer - Active condenser region Active and inactive length depend on working fluid pressure 05-08-2024 Department of Mechanical Engineeering 26

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28. Applications :- Maintain constant evaporator temperature over varying power and sink temperatures -Standard aerospace application Dual condensers for radioisotope Stirling applications -Radioisotope heat must always be removed -Dump heat to secondary condenser when Stirling is turned off Variable Thermal Links Aid in Start-up and Shut-down from a frozen state -Prevents sublimation in water pipes when frozen -NASA radiator application VCHP Heat Exchangers -Provide constant outlet stream temperature, with varying input stream -Fuel cell application 05-08-2024 Department of Mechanical Engineeering 28

29. Pressure Controlled Heat Pipe (PCHP) Heat pipe pressure and temperature set by vapor pressure of working fluid. -Reservoir is linked to the vapor space so NCG is at the same pressure Vary reservoir volume or amount of gas -Actuator drives bellows to modulate the reservoir volume -Pump/vacuum pump adds/removes gas Used for Precise Temperature Control and Power Switching 05-08-2024 Department of Mechanical Engineeering 29

30. Diode Heat Pipes 05-08-2024 Department of Mechanical Engineeering 30 Diode Heat Pipes designed to act like an electronic diode Evaporator Hotter Than Condenser -Heat Flows Condenser Hotter Than Evaporator -Blocks Heat From Flowing No method to throttle heat in the forward direction VCHPs and LHPs will both function as diode if direct sun heats up the evaporator -VCHP evaporator blocked by gas -LHP condenser fills with vapor Thermosyphon also a diode -No liquid available if heat from the top

31. Gas Diode Heat Pipes During normal operation, functions very similar to a VCHP Gas reservoir at condenser end with NCG NCG blocks of parts of the condenser depending on the thermal load During reverse operation vapor flows in the opposite direction NCG gas moves to the opposite end of the heat pipe due to the change in pressure NCG gas blocks off what would be the condensing end, effectively “shutting off” the HP 05-08-2024 Department of Mechanical Engineeering 31

32. Liquid Trap Diode Heat Pipes Wicked reservoir located at evaporator end of HP Reservoir wick does not communicate with HP wick In normal operation functions like CCHP Liquid evaporates at hot end, condenses at cold end and returns to hot end via HP wick In reverse direction, liquid evaporates at the hot side and condenses in the reservoir and becomes trapped Separate reservoir and heat pipe wick traps the condensate in the reservoir, preventing it from returning to the hot end, effectively “shutting off” the HP 05-08-2024 Department of Mechanical Engineeering 32

33. Liquid trap diode developed by David Bugby to link cameras to a cryoradiator During normal operation, a heat pipe cools the cameras, transferring heat to the cryoradiator at 140 K Periodically, the Cameras must be decontaminated by heating them up Hot-side decontamination temperature: 293 K Need to minimize the hot-side decontamination heater power Need to turn off the heat pipe Solution : Cryogenic heat pipe with thermal switching capability provided by a secondary radiator thermally isolated from the primary radiator, a thermally shunted liquid trap, and small liquid trap heater. D. Bugby et al., “Cryogenic Heat Pipe Thermal Transport and Switching System,” 2010 Spacecraft Thermal Control Workshop” 05-08-2024 Department of Mechanical Engineeering 33

34. Liquid Trap Diode During normal operation, the small liquid trap heater keeps the liquid trap warm enough so that it is filled with vapor only thus the heat pipe is ON. During decontamination, the liquid trap heater is turned off and all the working fluid migrates to the liquid trap turning the heat pipe OFF. 05-08-2024 Department of Mechanical Engineeering 34

35. Heat Pipe Heat Exchangers Heat Pipe Heat Exchangers are used when the heat exchange occurs between two fluid streams that must be kept separate Often streams are physically remote Applications :- Cabinet Coolers Heat pipe Air-to-Air Energy Recovery Heat Exchangers Thermosyphon Wrap-Around Heat Pipe Heat Exchangers for Dehumidification Energy Recovery Thermosyphon 05-08-2024 Department of Mechanical Engineeering 35

36. VCHP Heat Exchangers VCHP heat exchangers are used to provide a nearly constant outlet temperature for one fluid stream, with large variations in inlet flow rate and temperature Navy Fuel Cell Application Inlet Temperature from 120 to 400°C Outlet Temperatures must be maintained within ±30°C Changes in electrical load cause changes in reactant flow rates Turndown ratio of 5:1 or greater Current control system utilizes bypass valve Requires power Requires higher Δ P 05-08-2024 Department of Mechanical Engineeering 36

37. Alternate Means of Liquid Return All of the heat pipes discussed above have a wick to return liquid from the condenser to the evaporator Allows heat pipe to operate in any orientation Typically screen or sintered for terrestrial Grooved for spacecraft applications There are at least 3 other ways to return liquid Gravity Thermosyphons Loop Thermosyphons Centrifugal forces Rotating heat pipes Rotating HiK ™ shafts Electrohydrodynamic Forces 05-08-2024 Department of Mechanical Engineeering 37

38. Thermosyphon Normal Heat Pipes can operate in any orientation Use capillary forces in the wick to return liquid to the evaporator when the evaporator is elevated above the condenser Thermosyphons are gravity-aided heat pipes Evaporator must be located below the condenser Fluid returns to the evaporator by gravity Evaporator normally wicked for start-up Higher powers Essentially unlimited lengths VDT with electric source of energy for snow melting between railway switches 05-08-2024 Department of Mechanical Engineeering 38

39. Long Thermosyphons - Vertical Collect geothermal energy for heating buildings (with heat pump) Propane working fluid 5.3 cm diameter 93 m long Fabricate in place – Insert sections and weld as lower into hole Camera inserted to view behavior After system start-up, takes several minutes for the entire wall to be wetted 05-08-2024 Department of Mechanical Engineeering 39

40. Loop Thermosyphons Loop thermosyphon an improvement over pool boiler designs Have liquid pool in the evaporator and relies on pool boiling for the liquid-to-vapor phase change. The pool boiling mechanism is very inefficient (i.e. large superheat ΔT) and cannot handle high heat fluxes (i.e. low CHF). Evaporator design includes a porous wick structure in evaporator partially fed by circulating two-phase flow, and capillary forces Very high heat fluxes 05-08-2024 Department of Mechanical Engineeering 40

41. Long Thermosyphons - Horizontal Entrainment Limit for thermosyphons decreases as get closer to horizontal No flooding limit data near horizontal “Horizontal Vapordynamic Thermosyphons, Fundamentals and Practical Applications” Horizontal heat transfer for tens of meters Vapor flows in an interior tube – tube has high conductivity Condenses in an outer annular tube Gravity returns fluid from the condenser to the evaporator Demonstrated Flexible Condensers They say that their design functions without the noise, vibrations, and temperature overshoot typical for conventional thermosyphons with smooth walls 05-08-2024 Department of Mechanical Engineeering 41

42. Rotating Heat Pipes Wickless, return fluid by centrifugal forces Typically tapered to provide return force 05-08-2024 Department of Mechanical Engineeering 42

43. Difficult to remove heat from engine shaft Rotating heat pipes removes heat outside engine 05-08-2024 Department of Mechanical Engineeering 43

44. ADVANTAGES 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. 05-08-2024 Department of Mechanical Engineeering 44

45. DISADVANTAGES High cost. Requires that the air streams must be relatively clean and may require filtration. Requires that the two air streams be adjacent to each other. 05-08-2024 Department of Mechanical Engineeering 45

46. 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 re-entry. 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. 05-08-2024 Department of Mechanical Engineeering 46

47. LAPTOP HEAT PIPE SOLUTION 05-08-2024 Department of Mechanical Engineeering 47

48. HEAT PIPES USED IN PROCESSOR 05-08-2024 Department of Mechanical Engineeering 48

49. HEAT PIPE IN CPU 05-08-2024 Department of Mechanical Engineeering 49

50. SPACE CRAFT 05-08-2024 Department of Mechanical Engineeering 50

51. CAMERA 05-08-2024 Department of Mechanical Engineeering 51

52. Flux :- Flux(J) is rate of mass transfer across unit surface area of a barrier and mathematically expressed as: Flux is always positive quantity because it increases continuously during process Fick’s First Law :- Fick’s first law states that the flux is directly proportional to the concentration gradient. J= atoms/area/time α concentration gradient FICK’S LAW OF DIFFUSION 05-08-2024 Department of Mechanical Engineeering 52

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54. Application Of Fick’s First Law Used to explain drug diffusion across bio-membranes with desirable parameters Applied in the design of sustained and controlled release systems 05-08-2024 Department of Mechanical Engineeering 54

55. Fick’s Second Law :- It explains the change in concentration at definite location w.r.p.t x, y & z axes. . Fick’s second law refers to change in concentration of diffusant with time at any distance x . i.e , non steady state flow.. FICK’S LAW OF DIFFUSION 05-08-2024 Department of Mechanical Engineeering 55

56. VEDIOS https://www.youtube.com/watch?v=r1gdQTGNtPg https://www.youtube.com/watch?v=GHTeKHKkItA https://www.youtube.com/watch?v=eKrdJpDSowY https://www.youtube.com/watch?v=51bwzEO8XCw https://www.youtube.com/watch?v=fWfMbmXSAVI https://www.youtube.com/watch?v=fWfMbmXSAVI 05-08-2024 Department of Mechanical Engineeering 56

57. 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 temperature 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. 05-08-2024 Department of Mechanical Engineeering 57

58. THANK YOU 05-08-2024 Department of Mechanical Engineeering 58