Loop Heat Pipes-ashwin Thottumkara Full Ppt

LOOP HEAT PIPES

10/12/09 11:25 PM

1

OUTLINE            

Introduction What are heat pipes? What are loop heat pipes? Basic components of loop heat pipes Operating principles of LHP Conditions for the operation of LHP LHP design Types of LHP’s Limitations of loop heat pipes Applications of LHP Conclusions References

10/12/09 11:25 PM

2

INTRODUCTION  



 

The loop heat pipe (LHP) was invented in Russia in the early 1980’s Thermal management is an important factor .All applications generate high concentrated heat so need to control this heat The LHP is known for its high pumping capability and robust operation because it uses fine-pored metal wicks and the integral evaporator/hydro-accumulator design two-phase heat transfer devices that utilize the evaporation and condensation of working fluid to transfer heat capillary forces developed in fine porous wicks to circulate the fluid

10/12/09 11:25 PM

3

HEAT PIPES ?

     



Offer 100 to a 1000 times more effective thermal conductivity than that offered by a solid copper rod. Hollow cylindrical channel lined with a wick structure. Channel evacuated and a working fluid is injected. One end heated so phase change of the working fluid occurs. Vapor flows through the hollow middle section. Fluid is wicked back through the wick structure. Used as an efficient heat path between a heat source and a heat sink.

10/12/09 11:25 PM

4

LOOP HEAT PIPES?? 

   



serious constraint on conventional heat pipes is the reduction of transport capabilities when condenser is located below evaporator section in a gravitational field. Loop heat pipes are the solution Can perform at any orientation in a gravitational field over long distances Phase change from liquid to vapor state by absorbing latent heat vapor is transported to the cooling sink where it cools down and change phase to the liquid form utilizes the thermodynamic pressure difference developed between the evaporator and condenser to circulate a working fluid through a closed loop

10/12/09 11:25 PM

5

BASIC COMPONENTS OF A LOOP HEAT PIPE  





The evaporator The working fluid The wick or capillary structure condenser

10/12/09 11:25 PM

6

EVAPORATOR   

Three main layers Top layer “ vapor chamber ” Middle layer “ Wick material “ Bottom layer “compensation chamber “

10/12/09 11:25 PM

Three modes of Heat transfer in evaporator •conducted through the walls of the pores in axial and lateral direction. •heat lost in compensation chamber by convection •Evaporation in meniscus of pores. 7

WORKING FLUID Requirements  Compatibility with wick and wall materials  Good thermal stability  Wettability of wick and wall materials  High latent heat  High thermal conductivity  Low liquid and vapor viscosities  High surface tension Some working fluids  Liquid Ammonia  Liquid Nitrogen  Water 10/12/09 11:25 PM

8

WICK

function of the wicking material

•Hold the meniscus using capillary pressures •Surface meniscus to withstand the back pressure effects of the gas Types condenser of wicks •Provide liquid return from •Primary wick to evaporator primary wick will not be totally wetted • Secondary wick The primary function of secondary wick is to wet the primary CPS wick

10/12/09 11:25 PM

There are sintered ceramic or metal wicks (nickel, titanium, stainless steel and copper) working fluid initially is pumped through the secondary wick9 into

CONDENSER 

  

dual core condenser was designed and developed inner core with working fluid outer core with cooling water opposite directions for efficient heat transfer

10/12/09 11:25 PM

10

 

  





OPERATING PRINCIPLES OF LHP

Heat applied into evaporator liquid is vaporized and the menisci formed at the liquid/vapor interface in the evaporator wick develop capillary forces to push the vapor through the vapor line to the condenser Vapor condenses in the condenser the capillary forces continue to push liquid back to the evaporator The waste heat from the heat source provides the driving force for the circulation of the working fluid and no external pumping power is required The two-phase compensation chamber stores excess liquid and controls the operating temperature of the loop In order for the loop to continue to function, the wick in the evaporator must develop a capillary pressure to overcome the total pressure drop in the loop.

10/12/09 11:25 PM

11





CONDITIONS OF OPERATION Total pressure drop =frictional pressure of(evaporator

Total pressure drop =frictional pressure of(evaporator grooves +vapor line +condenser +the liquid line evaporator wick) +any static pressure drop due to gravity Capillary pressure rise= 2σ cos α /R   





σ is the surface tension of the working fluid, R is the radius of curvature of the meniscus in the wick, α is the contact angle between the liquid and the wick

The radius of curvature will continue to decrease with increasing heat loads until it is equal to the pore radius of the wick, Rp. Under this condition, the wick has reached its maximum capillary pumping capability Total pressure drop ≤Capillary pressure rise

10/12/09 11:25 PM

12

LHP DESIGN         



the LHP consists of sealed tubes (liquid line and evaporator line) connecting an evaporator (heat source) with a condenser (heat sink) The evaporator consists of a top cap, coherent porous silicon wick (CPS) and the compensation chamber which acts as a reservoir for the working fluid. The CPS wick as shown is an array of micron-range silicon dioxide capillaries micro machined using KOH through ordinary (100) electronic quality silicon wafers. main concerns in LHP is heat leak from the vapor side to the liquid side Ideally the pump core is always fully primed with liquid and the only heat leak is through the wick material presence of two phases can arise in the core section due to heat leak. extra volume is provided in the form of a reservoir Major components of the evaporator package like top cap, CPS wick and compensation chamber gets heated up before the water in the wick due to their thermal capacitance. The CPS wick as shown an array of micron-range silicon dioxide capillaries

10/12/09 11:25 PM

13

TYPES OF LHPS



0TH generation LHP  



top-cap CPS wick and the bottom reservoir were epoxied together condenser was not designed

1ST generation LHP 

   

A new compensation chamber was included in this design replacing the bottom chamber. allowing the wick not to dry out condenser was also designed Quartz wool was chosen to have good wetting properties to be a secondary wick secondary wick in a LHP will consist of a reticulated structure similar to the primary wick. The main difference will be the larger size pores to allow for a lowpressure drop and sufficient capillary force to bring the liquid back to wet the primary wick.

10/12/09 11:25 PM

14



2ND generation LHP   



3RD generation LHP 



To avoid the thermal mismatch between the top-cap and the compensation chamber, the top-cap was machined in Pyrex glass A condenser was built but was never tested as it was leaking significantly to make any calorimetric measurements. Without insulation around the evaporator package the heat lost to convection dominated the heat transfer out-gassing of the Lexan® used for the evaporator package. Thus it was decided to use a thick piece of borosilicate 7740 (Pyrex®) as of the back plate.

4TH generation LHP   

A new compensation chamber was designed to use gravity to feed the liquid back to the primary wick through the secondary wick. condenser which was capable to perform calorimetric calculations was built in this generation device dual core condenser was designed and developed

10/12/09 11:25 PM

15

LIMITATIONS OF LOOP HEAT PIPES 







The capillary limit for a silicon wick is the pressure that will break the interface and force the interface to leave the wick. So one bad or large pore in the wick will dominate the capillary pressure effect and will be responsible for the vapor-liquid interface to burst through The sonic limit is the maximum allowable mass flow rate or heat transfer that could choke the loop heat pipes. The vapor flow rate will choke, when the vapor reaches the sonic speed. This could happen if the duct crosssectional area decreases while the working fluid is flowing in the pipe. The entrainment limit is the maximum allowable mass flow rate or heat transfer rate that can be used before causing the evaporator to dry out. In general, this could happen in a conventional heat pipe when the vapor shear is able to carry water droplets from the liquid stream flowing back to the condenser. This will cause less flow to go to the evaporator and dry out the evaporator. Superheated liquid limit -Underneath the vapor-liquid interface a superheated liquid exists, since the interface is separates a high-pressure vapor and a low-pressure liquid with heat transfer across the interface. Most interfacial studies support the assumption of no temperature jump across the interface. Assuming no temperature jump across the interface, one can deduct that the liquid underneath the interface has to be superheated liquid and the evaporation process is a non-equilibrium one.

10/12/09 11:25 PM

16

APPLICATIONS 

LOOP HEAT PIPES FOR AVIONICS  Aircraft thermal control applications  Acturator-mounted electronics cooling  Wing and cowl anti-icing using engine waste heat  Avionics Cooling



SATELLITE THERMAL CONTROL TRENDS  waste heat dissipation

 LOOP HEAT PIPES FOR AEROSPACE APPLICATIONS More electronic packages have to be accommodated 

10/12/09 11:25 PM

LOOP THERMOSYPHON TECHNOLOGY Used when length from evaporator to condenser is small 17

CONCLUSION 

   

to have an operating LHP, a buildup pressure should exist between the top side of the evaporator and the bottom side of the evaporator circulating working fluid from the evaporator to condenser back to evaporator buildup pressure depends on the operating temperature LHP design, CPS wick, and the working fluid pressure-temperature gradient depends on operating temperature and the working fluid less circulated mass in the LHP means either the total heat removed decrease or the vapor temperature decrease. LHP with short pumping distance and large tube size is better

10/12/09 11:25 PM

18

REFERENCES 

http://www.thermacore.com/Technologies/loop-heat-pipes.aspx



http://www.1-act.com/lhptech.html



http://www.nlr.nl/smartsite.dws?id=3029



http://altmine.mie.uc.edu/fgerner/public_html/lhp5.pdf



http://www.patentstorm.us/patents/pdfs/patent_id/6892799.html



http://www.patentstorm.us/patents/7347250/fulltext.html



http://altmine.mie.uc.edu/fgerner/public_html/lhp2.pdf



http://encyclopedia.thefreedictionary.com/Loop+Heat+Pipe



http://www2.tku.edu.tw/~tkjse/8-2/8-2-5.pdf



http://etd.ohiolink.edu/send-pdf.cgi/HAMDAN%20MOHAMMAD%20OMAR.pdf? acc_num=ucin1049987207



http://etd.ohiolink.edu/send-pdf.cgi/SHARMA%20MONIKA.pdf?acc_num=ucin1132344889



http://etd.ohiolink.edu/send-pdf.cgi/Medis%20Praveen%20S.pdf?acc_num=ucin1131996727



http://etd.ohiolink.edu/send-pdf.cgi/Suh%20Junwoo.pdf?acc_num=ucin1131033062



http://etd.ohiolink.edu/send-pdf.cgi/Shuja%20Ahmed%20A.pdf?acc_num=ucin1179501051



http://www2.dem.inpe.br/rriehl/lhp.htm

10/12/09 11:25 PM

19

QUESTIONS ???

10/12/09 11:25 PM

20

THANK YOU !!

10/12/09 11:25 PM

21