temperature oscillations in loop heat pipes – a … temp oscillation ku - 2018 • ku, j., “high...

LHP Temp Oscillation Ku - 2018

Temperature Oscillations in Loop Heat Pipes –A Revisit

Jentung KuNASA Goddard Space Flight Center

Greenbelt, Maryland

2018 Spacecraft Thermal Control WorkshopEl Segundo, California, March 20-22, 2018

https://ntrs.nasa.gov/search.jsp?R=20180002076 2018-06-25T16:54:54+00:00Z


• Introduction/Background– LHP natural operating temperature – CC temperature and vapor front movement

• 3 Types of Temperature Oscillation• High Frequency Low Amplitude Temperature Oscillation

– Governing equations– Factors affecting temperature oscillation

• Low Frequency High Amplitude Temperature Oscillation– Governing equations– Necessary conditions– Driving mechanism for temperature oscillation– Sequence of events– Factors affecting temperature oscillation

• Summary and Conclusions

Outline

2


LHP Operating Temperature without Attached Thermal Mass

3

Ql,a

Evaporator SectionReservoir

Condenser Sink

Vapor Line

Liquid Line

TIN

TCC

Te

Qe

m.

m.

Tsink

Tcond

Qsub Qe,vap

Lvap

• The LHP operating temperature (CC temperature) is a function of– Evaporator power– Condenser sink temperature– Ambient temperature

• As the operating condition changes, the CC temperature will go through a transient period and eventually reach a new steady state temperature.

Net Evaporator Power, Qe

CC

Tem

pera

ture T1

T2

T3

T4

Q2 Q3 Q4 Q5

Region IIRegion I

1

2

34

5

Q1


Governing Equations for Operation of an LHP without Attached Thermal Mass

Ql,a

Evaporator SectionReservoir

Condenser Sink

Vapor Line

Liquid Line

TIN

TCC

Te

Qe

m.

m.

Tsink

Tcond

Qc,sub Qe,vap

Lvap

4

Qe = Qe,cc + Qe,vap

Qe,cc = Ge,cc (Te - Tcc)

Qe,vap = m

Lvap = Qe,vap /[hD (Tcc - Tsink)]

Tin – Tcond = Ql,a /(mCp)

Qsub = mCp (Tcc -Tin)

Qe,cc = Qsub

P = (Te -Tcc)/ (Tcc v)


LHP Operating Temperature without Attached Thermal Mass

5

• The CC temperature of an LHP is well defined for a given evaporator power and sink temperature.

• Each point on the temperature curve also corresponds to a vapor front location inside the condenser.

• Under certain conditions, the CC temperature never reaches a true steady state, and displays oscillatory behaviors.

Condenser Sink

Vapor

Condenser

Liquid

Liquid/Vapor Interface

2 14 3

5


CC

Tem

pera

ture T1

T2

T3

T4

Q2 Q3 Q4 Q5

Region IIRegion I

1

2

34

5

Q1


• Ultra High Frequency Temperature Oscillation– Periods less than 1 second; Amplitude is a tiny fraction of one Kelvin– Induced by vapor generation in evaporator and vapor condensation in

condenser – No publications specifically dealing with this subject

• High Frequency, Low Amplitude Temperature Oscillation– Periods on the order of seconds to minutes; Amplitudes on the order of

one Kelvin– Caused by vapor front movement near condenser inlet or exit– No large thermal mass attached to the evaporator

• Low Frequency, High Amplitude Temperature Oscillation– Periods on the order of hours– Amplitudes on the order of tens of Kelvin– Can be caused under various operating conditions

Three Types of LHP Temperature Oscillation

6


• Ku, J., “High Frequency Low Amplitude Temperature Oscillations in Loop Heat PipeOperation,” Paper No. 2003-01-2387, 33rd International Conference on EnvironmentalSystems, Vancouver, British Columbia, Canada, July 7-10, 2003.

• Ku, J., and J. Rodriguez, “Low Frequency High Amplitude Temperature Oscillations inLoop Heat Pipe Operation,” Paper No. 2003-01-2388, 33rd International Conferenceon Environmental Systems, Vancouver, British Columbia, Canada, July 7-10, 2003.

• Hoang, T. and Baldauff, R., and Maxell, J., “Stability Theory for Loop Heat Pipe Design, Analysis and Operation,” Paper No. ICES-2015-14, 45th International Conference on Environmental Systems, Bellevue, WA, July 12-16, 2015.

• Hoang, T. and Baldauff, R., and Tiu, C., “Verification of Loop Heat Pipe Stability Theory − Part I Low-Frequency/High-Amplitude Oscillations,” Paper No. ICES-2015-39, 45th International Conference on Environmental Systems, Bellevue, WA, July 12-16, 2015.

• Hoang, T. and Baldauff, “Verification of Loop Heat Pipe Stability Theory − Part II High-Frequency/Low-Amplitude Oscillations,” Paper No. ICES-2015-XXX 45th International Conference on Environmental Systems, Bellevue,WA, July 12-16, 2015.

Papers Related to This Presentation on LHP Temperature Oscillations

7


• Refresh on LHP temperature oscillations• Limit the discussion to LHPs with one evaporator and one

condenser• More detailed discussions on LHP temperature oscillations

– Focus on the understanding of underlying physical processes– Interactions among LHP components and external conditions

• The vapor front movement and the rise and fall of the CC temperature

• The role of the attached large thermal mass in the modulation of the evaporator heat load

• Sequence of events during temperature oscillations– Effect of thermal mass on high frequency low amplitude

temperature oscillations– Analytical predictions of LHP temperature oscillations (paper by

Hoang, et. al)

Scope of This Presentation

8


Vapor Front Stays Inside Condenser Stable LHP operating Temperature

• A sharp vapor front is assumed.• For a given heat load, a steady

CC temperature can be obtained as long as the vapor front stays inside the condenser.

• For a given Qe and Tsink, Tcc and Lvap are well-defined.

• When Qe and/or Tsink, changes, a new set of Tcc and Lvap can be found.

• An oscillating Qe and vapor front movement will lead to an oscillating CC temperature.

9

Condenser Sink

Vapor

Condenser

Liquid


2 14 3

5


CC

Tem

pera

ture T1

T2

T3

T4

Q2 Q3 Q4 Q5

Region IIRegion I

1

2

34

5

Q1


Vapor Front Near Condenser Exit

Condenser Sink

Vapor

Condenser

Liquid


2 14 3

5

10

• When Qe ~ Q4, vapor front can pass the condenser exit. Warm liquid feeds into CC. Tcc continues to increase.

• Condenser is gaining higher and higher heat dissipating capability.

• When Tcc reaches a certain value, vapor front begins to recede back into condenser.

• Liquid subcooling increases as fluid flow exits the condenser.

• Cold liquid feeds into CC. Tcc continues to decrease.

• Condenser is losing its heat dissipating capability.

• When Tcc reaches a certain value, vapor front passes condenser exit again.

• The cycle repeats itself.• Tcc and Tcond are nearly 180 degrees out

of phase.Net Evaporator Power, Qe

CC

Tem

pera

ture T1

T2

T3

T4

Q2 Q3 Q4 Q5

Region IIRegion I

1

2

34

5

Q1


Vapor Front Near Condenser Inlet

Condenser Sink

Vapor

Condenser

Liquid


2 14 3

5

11

• At low power/low sink, vapor front moves near condenser inlet.

• Tcc increases due to decreasing subcooling. Vapor front moves into vapor line.

• Vapor line is not an efficient condenser. At some point, vapor line can no longer dissipate the heat load.

• Liquid slug is pushed into condenser rapidly. Tcc decreases. The rapid vapor front movement into condenser overshoots.

• Vapor front recedes afterwards. Tccincreases. Vapor front recedes further into vapor line.

• The cycle repeats itself.• Without thermal mass, vapor front

does not move far from location 1.Net Evaporator Power, Qe

CC

Tem

pera

ture T1

T2

T3

T4

Q2 Q3 Q4 Q5

Region IIRegion I

1

2

34

5

Q1


Factors Affecting High Frequency Low Amplitude Temperature Oscillations

• The high frequency low amplitude temperature oscillation will disappear when the vapor front moves away from the condenser inlet or exit. Conversely, it will appear when the vapor front moves near the condenser inlet or exit.

• Factors that affect the vapor front location– Evaporator power– Condenser sink temperature– Ambient temperature– Body forces– Active control of CC temperature

• Some experimental results from testing of two LHPs are presented. Both LHPs were tested extensively for various studies of LHP operation.

12


LHP1 with Thermocouple Locations

• Evaporator: 12.7 mm diameter and 120 mm length• Vapor line: 2.8 mm I.D. and 160 mm length• Liquid line: 2.8 mm I.D. and 130 mm length• Condenser: 2.8 mm I.D. and 100 mm length

1

2

13

10

11

3614

15

16171819

2021 302928272625242322 31

43

34

35

37

40

41

3244

4

5

7

8

33

3739

3841

4042

Evaporator

Liqu

id L

ine

Vapor Line

CondenserCoolant

CC

13


• An increase of power from 23W to 25W caused the vapor front to move near condenser exit, inducing temperature oscillations.

• Vapor front moved around the condenser exit during oscillation.

LHP1 Operating Temperature vs Heat Load

290

293

296

299

302

305

8:30 8:45 9:00 9:15 9:30 9:45 10:00

Time

Tem

pera

ture

(K)

0

5

10

15

20

25

30

35

40

45

50

Pow

er (W

)

Condenser (26)

Liquid Line (36)

CompensationChamber (41)

Evaporator (8)

Condenser Exit (30)

Power

2 December 1999

14


LHP 1 Operating Temperature vs Sink Temperature

285

290

295

300

305

9:30 10:00 10:30 11:00 11:30 12:00 12:30 13:00 13:30 14:00 14:30 15:00 15:30

Time

Tem

pera

ture

(K)

0

8

16

24

32

Pow

er (W

)

Condenser (26)

Liquid Line (36)


Condenser Exit (30)

Power

Chiller (52)

2 December 1999

• When the sink temperature reduced to 287K or lower, the vapor front moved inside the condenser away from the exit, eliminating temperature oscillations.

15


LHP 1 Operating Temperature vs Spin Rate (100W/273K)

280

285

290

295

300

305

310

315

12:30 12:45 13:00 13:15 13:30 13:45 14:00 14:15 14:30 14:45 15:00 15:15 15:30

Time

Tem

pera

ture

(K)

0

20

40

60

80

100

120

140

Spin

Rat

e (R

PM)

Condenser (26)

Liquid Line (36)


Evaporator (8)

Condenser Exit (30)

Spin Rate

15 September 1999

• The centrifugal force changed the fluid distribution inside the LHP and the vapor front location in the condenser, and hence affected temperature oscillation.

16


LHP2 with Thermocouple Locations

DP

TC 4TC2

TC 1,3,5

TC 9,10,11TC 6,7,8

AP34

35

13

37

21

20 22 32 38

33

312319

18 24 30

292517

16 26 282715

14

12

42

9

10

11

8

7

6

5

3

136

CONDENSER

LIQUID LINE

VAPOR LINE

EVAPORATOR

COMPENSATIONCHAMBER

• Evaporator: 25.4 mm diameter and 300 mm length• Vapor line: 3.34 mm I.D. and 1500 mm length• Liquid line: 1.75 mm I.D. and 1500 mm length• Condenser: 3.86 mm I.D. and 2000 mm length

17


LHP2 Temperature Oscillations(100g/5W/248K)

TC3 - CC, TC7 - Evap., TC14/15- Vapor Line, TC16- Cond, TC33 - Liq Line, TC36 - CC Inlet

NRL LHP 01/12/2001

240

250

260

270

280

290

300

310

8:00 9:00 10:00 11:00 12:00 13:00Time (hours)

Tem

pera

ture

(K)

-500

500

1500

2500

3500

4500

Pres

sure

Dro

p (P

a)

DP

TC3

TC14

TC15

TC33

TC36

100 W5 W

01/12/2001

TC16

• Back and forth movement of the vapor front near the condenser inlet caused temperature oscillation.

18


• High frequency low amplitude temperature oscillation can happen under the following two conditions when no thermal mass is attached to the evaporator:– The vapor front moves back and forth near the condenser exit.

The LHP operates under a fixed conductance mode.– The vapor front moves back and forth near the condenser inlet.

The LHP operates under a variable conductance mode.• The movement of the vapor front in the condenser causes a change in

the flow rate and/or temperature of the liquid flow in the liquid line. This in turn changes the subcooling of the liquid returning to the CC, leading to the CC temperature change.– Neither liquid line or vapor line is an efficient condenser.

• The interaction of the CC temperature and the liquid subcoolingcauses the temperature oscillation in the loop.

• Without a large thermal mass attached to the evaporator, the CC and the loop can adopt to the change quickly, leading to a high frequency low amplitude temperature oscillations.

High Frequency Low Amplitude Temperature Oscillations – Summary of the Theory

19


• Constant Applied Power and Oscillating Sink Temperature– Observed in flight or simulated thermal vacuum tests.– Results are expected.

• LHP with a Single Evaporator and Two Parallel Condensers/Radiators– One sink is colder than saturation temperature, the other is warmer.– The flow regulator with warm sink dries out periodically.– Oscillations can be eliminated by placing the two flow regulators side by

side.– Not covered in this presentation

• Periodic partial Druout of the Secondary Wick During Power Transient for LHP with a Large Thermal Mass Attached to Evaporator– Not cover in this presentation

• LHP with a Large Thermal Mass Attached to Evaporator– Constant heat load to thermal mass– Constant sink temperature– Focus of this presentation

Low Frequency High Amplitude Temperature Oscillations Observed in LHPs

20


12/8/1999 25W start, 263K sink

258

268

278

288

298

308

60 120 180 240 300 360 420 480 540 600 660

Elapsed Time (min)

Tem

pera

ture

(K)

-5

0

5

10

15

20

25

30

Pow

er (W

atts

)

TC31 TC24 TC06 Q1

POWER

TC8

TC31

TC6

TC24

Example of Low Frequency High Amplitude Temperature Oscillations Observed in TES LHP EDU

21

• Amplitudes of temperature oscillation: CC (TC8) ~ 12K, TM (TC6) ~ 10K• Period of temperature oscillation: ~ 2 hours.


Operation of LHP with Thermal Mass

• The LHP operation is a function of the net load to the evaporator, Qe, not the heat load to the thermal mass, Qapp.

• The large thermal mass can modulate the constant heat load Qapp into an oscillating Qe received by the evaporator under certain conditions.

• An oscillating Qe causes temperature oscillations.

22

Qapp

EvaporatorCC

Condenser

Vapor Line

Liquid Line

Tin

Te

m.

Ql,a

m.Tsink

TcondLvap

Thermal Mass

Qe

Tcc

Qsub Qe,vap


CC

Tem

pera

ture T1

T2

T3

T4

Q2 Q3 Q4 Q5

Region IIRegion I

1

2

34

5

Q1


Heat Load Modulation by Thermal Mass

• TM modulates Qe by absorbing or releasing heat.• Qtm <0, Qe < Qapp when TM is absorbing heat and Ttm is increasing.• Qtm >0, Qe > Qapp when TM is releasing heat and Ttm is decreasing.• Under certain conditions, Qe can oscillate between a maximum and a

minimum, leading to low frequency high amplitude temperature oscillations.

23

Qe = Ge,tm (Ttm -Te)

Lvap = Qe /[hD (Tcc - Tsink)]

Thermal Mass; Ttm

LHP

Qapp

Qe = Qapp + Qtm


CC

Tem

pera

ture T1

T2

T3

T4

Q2 Q3 Q4 Q5

Region IIRegion I

1

2

34

5

Q1



• Low frequency high amplitude temperature oscillations can only happen in Region I under the variable conductance mode. It cannot happen in Region II under the fixed conductance mode.

• In region I, Tcc decreases with an increasing heat load Qe, which amplifies the effect of heat load modulation by thermal mass.

• In region II, Tcc increases with an increasing heat load Qe, which counterbalances the effect of heat load modulation by thermal mass.

24


CC

Tem

pera

ture T1

T2

T3

T4

Q2 Q3 Q4 Q5

Region IIRegion I

1

2

34

5

Q1

Thermal Mass; Ttm

LHP

Qapp

Qe = Qapp + Qtm

Qe = Ge,tm (Ttm -Te)

Lvap = Qe /[hD (Tcc - Tsink)]



25

Condenser Sink

Vapor

Condenser

Liquid


2 134

5

• There is a one-to-one correspondence among Qe, Tcc and Lvap.• Without TM, Qe= Qapp Q2 , Tcc = T2 and the vapor front stays at location 2.• When a large TM is attached to the LHP evaporator, the TM can modulate

the constant heat load Q2 into an oscillating Qe that varies between Q1 and Q3. As a result, Tcc varies between T1 and T3, and the vapor front moves between location 1 and location 3.

• When the vapor front is between location 1 and location 2, TM absorbs heat and Qe < Qapp. When the vapor front is between location 2 and location 3, TM releases heat and Qe > Qapp.

Thermal Mass; Ttm

LHP

Qapp

Qe = Qapp + Qtm


CC

Tem

pera

ture T1

T2

T3

T4

Q2 Q3 Q4 Q5

Region IIRegion I

1

2

34

5

Q1


Heat Load Modulation by Thermal MassVapor Front Moves form Location 2 to Location 3

26

Condenser Sink

Vapor

Condenser

Liquid


2 134

5

• Without TM, Qe= Qapp Q2 , Tcc = T2 and the vapor front stays at location 2.• If a TM with a temperature Ttm > Te is attached to evaporator, additional heat

Qtm will flow into the LHP. Lvap will increase and Tcc will decrease.• The decrease of Tcc leads to an increase in Qtm, Qe, and Lvap.• Ttm will decrease at a slower rate than Tcc, leading to a further increase of

Qtm, Qe, and Lvap. This trend continues until Lvap reaches its maximum and Tcc reaches its minimum at location 3.

• At location 3, (Ttm –Tcc) reaches a maximum and TM cannot release any higher Qtm to support a further increase of Lvap and further decrease of Tcc.

Thermal Mass; Ttm

LHP

Qapp

Qe = Qapp + Qtm


CC

Tem

pera

ture

T1

T2

T3

T4

Q2 Q3 Q4 Q5

Region IIRegion I

1

2

34

5

Q1



27

Condenser Sink

Vapor

Condenser

Liquid


2 134

5

• Tcc begins to increase, which leads to a decrease in Qtm, Qe, and Lvap.• Qtm and Qe are decreasing, and the TM is releasing heat at a slower and

slower rate. The trend continues until the vapor front reaches location 2. Note: Qtm > 0, and Qe > Qapp during this period.

• At location 2, TM reaches its minimum temperature and release no heat. Qtm = 0, and Qe = Qapp. Tcc = T2, the LHP natural operating temperature.

Thermal Mass; Ttm

LHP

Qapp

Qe = Qapp + Qtm


CC

Tem

pera

ture T1

T2

T3

T4

Q2 Q3 Q4 Q5

Region IIRegion I

1

2

34

5

Q1



28

Condenser Sink

Vapor

Condenser

Liquid


2 134

5

• Ttm begins to increase from its minimum. Qtm < 0, Qe< Qapp. • Qapp = Qe – Qtm. Qapp has to support the increase of Ttm and sustain the LHP

operation simultaneously. Both Tcc and Ttm rise very slowly.• Still, Tcc rises at a faster rate (decreasing liquid subcooling) than that of Ttm.• (Ttm – Tcc) continues to decrease. As a result, Qe and Lvap continue to

decrease. This trend continue until the vapor front reaches location 1.• At location 1, Tcc is at its maximum based on energy balance of heat leak and

liquid subcooling. (Ttm – Tcc) and Qe are at their minima. An increase of Ttm will lead to an increase of Qe. Note: Qtm < 0, and Qe < Qapp during this period.

Thermal Mass; Ttm

LHP

Qapp

Qe = Qapp + Qtm


CC

Tem

pera

ture T1

T2

T3

T4

Q2 Q3 Q4 Q5

Region IIRegion I

1

2

34

5

Q1



29

Condenser Sink

Vapor

Condenser

Liquid


2 134

5

• (Ttm – Tcc) begin to increase from its minimum. Qe begin to increase. Ttmincreases at a slower rate (gets less heat from Qapp). Tcc begin to decrease.

• Qtm, Qe, Tcc, and Lvap increase at faster and faster rates while Ttm increases at a slower and slower rate until vapor front reaches location 2.

• At location 2, Qtm = 0, TM reaches its maximum temperature and absorbs no heat. Qe = Qapp. TM has its maximum sensible heat in storage. Tcc = T2, the LHP natural operating temperature.

• The entire temperature oscillation cycle repeats subsequently.

Thermal Mass; Ttm

LHP

Qapp

Qe = Qapp + Qtm


CC

Tem

pera

ture T1

T2

T3

T4

Q2 Q3 Q4 Q5

Region IIRegion I

1

2

34

5

Q1

LHP Temp Oscillation Ku - 2018 30

Vapor Front

Major Events

Point 1 Tcc at maximum; Lvap at minimum; Qtm and Qe at minima; TM absorbs energy at fastest rate; inflection point for Ttm

Point 1 to Point 2a

Tcc decreases at an increasing rate; Qtm and Qeincrease at increasing rates; TM absorbs heat at a decreasing rate; Qe < Qapp

Point 2a

Ttm at maximum; Qtm = 0; Qe = Qapp; inflection point for Tcc

Point 2a to Point 3

Tcc decreases at a decreasing rate; Ttmdecreases at an increasing rate; Qtm and Qeincrease at increasing rates; Qe > Qapp

Point 3 Tcc at minimum; Lvap at maximum; Qtm and Qe at maxima, TM releases heat at fastest rate; inflection point for Ttm; Qe > Qapp

Point 3 to Point 2b

Tcc increases at an increasing rate; Qtm and Qedecrease at decreasing rates ; Ttm decreases at a decreasing rate; Qe > Qapp

Point 2b

Ttm at minimum; Qtm = 0; Qe = Qapp; inflection point for Tcc

Point 2b to Point 1

Tcc increases at a decreasing rate; Qedecreases at a decreasing rate; TM absorbs heat at an increasing rate; Qe < Qapp

Heat Load Modulation by Thermal MassSummary of Sequence of Events


CC

Tem

pera

ture T1

T2

T3

T4

Q2 Q3 Q4 Q5

Region IIRegion I

1

2

34

5

Q1

Condenser Sink

Vapor

Condenser

Liquid


2a 134

5 2b


Temperature Oscillation in TES LHP EDU (25W/263K)

12/8/1999 25W start, 263K sink

258

268

278

288

298

308

60 120 180 240 300 360 420 480 540 600 660

Elapsed Time (min)

Tem

pera

ture

(K)

-5

0

5

10

15

20

25

30

Pow

er (W

atts

)

TC23 TC31 TC22 TC24 TC32 TC06 Q1

POWER

TC8

TC31

TC22

TC32

TC6

TC23 TC24

31

• The above discussions of vapor front movement can start from any vapor front location.

• The results below show that the LHP started with a superheat, underwent an initial transient, then began temperature oscillation that could last indefinitely.


Heat Load Modulation by Thermal MassVapor Front Movement during Temperature Oscillation Cycles

32

Condenser Sink

Vapor

Condenser

Liquid


2 134

5

• The movement of vapor front from location 1 to location 3 (Tcc decreasing) is much faster than that from location 3 to location 1 (Tcc increasing), i.e. the temperature curve is asymmetric.

• When Tcc is decreasing, the TM can accelerate its heat release. When Tcc is rising, Ttm must also rise above Tcc so that it can provide the heat needed to sustain the LHP operation. The large thermal mass and the limited Qapp that is available makes Ttm to rise slowly.

• The curve becomes more asymmetric with a lower power and/or a lower sink temperature.

0

4

8

12

16

20

270

275

280

285

290

295

300

50 150 250 350 450

Pow

er (W

atts

)

Tem

pera

ture

(K)

Time Elapsed (min)

1/24/2000

CC

TM

Power


Heat Load Modulation by Thermal MassVapor Front Movement during Temperature Oscillation Cycles

33

Condenser Sink

Vapor

Condenser

Liquid


2 134

5

Thermal Mass; Ttm

LHP

Qapp

Qe = Qapp + Qtm

• The range of the vapor front movement can be correlated to the amplitude of the temperature oscillation.

• The amplitude and period of temperature oscillation increase with:– Decreasing sink temperature– Decreasing heat load– Increasing thermal mass

• Vapor front can never pass much beyond the condenser exit.


CC

Tem

pera

ture T1

T2

T3

T4

Q2 Q3 Q4 Q5

Region IIRegion I

1

2

34

5

Q1


• With a constant sink, an oscillating heat input to the evaporator will lead to temperature oscillations in the LHP.

• Under certain conditions, the large thermal mass attached to the evaporator can modulate a constant applied heat load into an oscillating heat input to the evaporator, causing the loop temperature to oscillate. – The thermal mass absorbs energy when its temperature is rising

and releases energy when its temperature is falling.– The net heat load to the evaporator oscillates between a

maximum that is higher than the applied power, and a minimum that is lower than the applied power.

• The temperature oscillation, once started, can continue indefinitely until the operating condition changes.

Low Frequency High Amplitude Temperature Oscillation – Summary of the Theory

34


• In order to sustain a low frequency high amplitude temperature oscillation, all of the following three conditions must prevail:– A large thermal mass is attached to the evaporator.– A small power is applied to the thermal mass.– The condenser has a cold sink (colder than the surrounding

temperature)• The terms “large thermal mass,” “small power,” and “cold

sink” are all relative. – No absolute values can be assigned.– Analytical tool exists (Hoang & Baldauff).

Low Frequency High Amplitude Temperature Oscillations – Necessary Conditions

35


Temperature Oscillation in TES LHP EDU

36


Temperature Oscillation in TES LHP EDU (263K, 15W/ 20W/ 30W)

2/14/2000

258

263

268

273

278

283

288

293

298

360 600 840 1080 1320 1560

Time Elapsed (min)

Tem

pera

ture

(K)

0

5

10

15

20

25

30

35

40

Pow

er (W

atts

)

TC08 TC23 TC31 TC24 TC32 Q1

POWER

TC8

TC23

TC24 TC31

TC32

• Lower power leads to a large amplitude and a longer period of temperature oscillation.

• The periods for temperature rise and fall become more asymmetric with a decreasing power.

37


Temperature Oscillation in TES LHP EDU(50W, 273K/ 263K/ 253K/ 243K)

• Lower sink temperature leads to a large amplitude and a longer period of temperature oscillation.

• The periods for temperature rise and fall become more asymmetric with a decreasing sink temperature.

1/11/2000

243

248

253

258

263

268

273

278

283

288

293

0 120 240 360 480 600 720 840

Time Elapsed (min)

Tem

pera

ture

(K)

-10

0

10

20

30

40

50

60

Pow

er (W

atts

)


POWER

TC8

TC23

TC32

TC31

TC24

38


• Each curve represents the natural operating temperature at the given sink temperature.

• Lower sink temperature allows TM to give out more sensible heat when its temperature is decreasing (T1min < T2min) and to store more sensible heat when its temperature is rising (T1max > T2max). This translates to a larger range for vapor front movement and a higher amplitude and a longer period for temperature oscillation.

• No temperature oscillation will occur when the sink temperature is higher than ambient temperature (no heat load modulation by TM)

Effect of Sink Temperature on CC Temperature

39

Qe, Net Evaporator Power

CC

Tem

pera

ture

Q2 Q1

T2min

T1min

T1max

T2max


Temperature Oscillation in TES LHP EDU (15W/253K, 20W/293K, 20W/253K)

2/8/2000

248

253

258

263

268

273

278

283

288

293

298

0 240 480 720 960 1200 1440 1680

Time Elapsed (min)

Tem

pera

ture

(K)

-5

0

5

10

15

20

25

30

35

Pow

er (W

atts

)


POWER

TC8TC23

TC24 TC31

TC32

• No temperature oscillation when the sink temperature is higher than ambient temperature.

• At low sink temperature, lower power lead to higher amplitude and longer period of temperature oscillation.

40


Temperature Oscillation in TES LHP EDU (50W/268K)

12/15/1999

256

261

266

271

276

281

286

291

296

420 450 480 510 540

Time Elapsed (min)

Tem

pera

ture

(K)

-10

0

10

20

30

40

50

60

Pow

er (W

atts

)

TC08 TC23 TC31 TC26 TC25 TC32 Q1

TC25

TC8

TC23

TC26TC31

TC32

Power

41

• A relatively high power and a high sink temperature lead to a smaller amplitude and a shorter period of temperature oscillation.

• The periods for temperature rise and fall become less asymmetric.


• The amplitude of temperature oscillation increases with:– a decreasing power – a decreasing sink temperature

Amplitude of Temperature Oscillationvs Applied Power and Sink Temperature

42

0

5

10

15

20

25

30

35

0 20 40 60 80Applied Power (W)

Tem

pera

ture

Osc

illat

ion

(Pea

k to

Pea

k, K

) 243K Sink 253K Sink 258 K Sink263K Sink 273K Sink


Period of Temperature Oscillation vs Applied Power and Sink Temperature

0

60

120

180

240

300

360

0 10 20 30 40 50 60 70 80

Applied Power (W)

Tem

pera

ture

Osc

illat

ion

Per

iod

(Min

)

243K Sink 253K Sink 258 K Sink263K Sink 273K Sink

43

• The period of temperature oscillation increases with:– a decreasing power – a decreasing sink temperature


Prediction of LHP Temperature OscillationsAnalytical Predictions versus Test Data (Hoang & Baldauff)

44

Pow

er In

put (

W)

Sink Temperature (K) 273 278 283 288

0

10

20

30

40

50

60

70

80

90

293

Test Without OscillationsTest With Oscillations

Predicted Region of Instability (Oscillation)

JPL TES LHP


Heat Load Modulation by Thermal MassLHP Operates in Region II

45

Condenser Sink

Vapor

Condenser

Liquid


2 134

5

• When a heat load of Qapp = Q5 > Q4 is applied to the evaporator, the LHP operate in Region II. Under steady state, the vapor front will stay stably at location 5 inside the condenser and Tcc = T5.

• No high frequency low amplitude or low frequency high amplitude temperature oscillation will occur with or without a large TM attached.– Without a large TM, the transient period will be short– With a large TM, the transient period will be longer.

Thermal Mass; Ttm

LHP

Qapp

Qe = Qapp + Qtm


CC

Tem

pera

ture T1

T2

T3

T4

Q2 Q3 Q4 Q5

Region IIRegion I

1

2

34

5

Q1


Heat Load Modulation by Thermal MassVapor Front Near Condenser Exit

46

Condenser Sink

Vapor

Condenser

Liquid


2 134

5

• When a heat load of Qapp = Q4 is applied to the evaporator and Tcc is near T4, the vapor front will be near location 4 in the condenser.

• Without a large TM attached to the evaporator, high frequency low amplitude temperature oscillations will occur.

• If a large TM is attached to evaporator, the behavior will be similar to high frequency low amplitude temperature oscillations except that the frequency will be lower (period is longer) and the amplitude will be slightly higher.

– Heat load modulation by TM is limited, i.e. ±Qtm will be small and the vapor front cannot move far from location 4. Low frequency high amplitude temperature oscillation will not occur.

Thermal Mass; Ttm

LHP

Qapp

Qe = Qapp + Qtm


CC

Tem

pera

ture

T1

T2

T3

T4 Q2 Q3 Q4 Q5

Region IIRegion I

1

2

34

5

Q1


Summary and Conclusions (1/2)

• The LHP natural operating temperature (non-controlled CC) is well defined with a constant evaporator power and a constant sink temperature.

• Under certain conditions, the CC temperature never reaches a true steady state, and display oscillatory behaviors.

• Two types of temperature oscillations are of concern for spacecraft applications.– High frequency low amplitude temperature oscillation– Low frequency high amplitude temperature oscillation

• High frequency low amplitude temperature oscillation can occur under the following conditions:– The LHP operating temperature coincide with its lowest natural

operating temperature. The LHP may or may not have a large thermal mass attached to its evaporator.

– The LHP has a very low heat load and the vapor front is near the condenser inlet. No large thermal mass is attached to its evaporator.

47


Summary and Conclusions (2/2)

• Low frequency high amplitude temperature oscillation can occur when the LHP operates under the variable conductance mode and all of the following three necessary conditions are satisfied:– A large thermal mass is attached to the evaporator.– A small power is applied to the thermal mass.– The condenser has a cold sink (colder than the surrounding

temperature)• This presentation provides detailed explanations on the underlying

physical processes of these types of temperature oscillation.– Interactions among LHP components and its environment

• Analytical models exist to predict when these types of temperature oscillation will occur.– Heat load, sink temperature, ambient temperature, thermal mass

attached to LHP evaporator• Both types of temperature oscillation disappear when the CC

temperature is actively controlled through cold biasing.

48

temperature oscillations in loop heat pipes – a … temp oscillation ku - 2018 • ku, j., “high...

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