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Loop Heat Pipe Startup BehaviorsLoop Heat Pipe Startup Behaviors
Jentung KuNASA Goddard Space Flight Centerp g
Greenbelt, Maryland, USA301-286-3130
Jentung.Ku-1@nasa.gov
46th International Conference on Environmental SystemsVienna Austria July 10 14 2016Vienna, Austria, July 10-14, 2016
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Outline• Introduction• Introduction• LHP Operation – Background• LHP Startup Scenarios• Fluid Distribution Between Evaporator and Reservoir• Enhancing Start-up Success• Other Startup Issues• Summary and Conclusions
LHP Startup Behaviors – 2016 Ku 2
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Introduction/Background
• LHPs have been used for instrument thermal control on many orbiting spacecraft.
• An LHP must start successfully before it can commence its service.
• The way an LHP starts may affect its subsequent operations.
• The startup of the LHP is one of the most complex transient phenomena in LHP operation.
• This presentation focuses on the issues related to the startup of a single-evaporator LHP servicing a heat source all by itself.
LHP Startup Behaviors – 2016 Ku 3
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Schematic of a Loop Heat PipePrimary
Wi kSecondary
Wi kWick WickVapor Channel Pump Core
Vapor Channel
Bayonet
Reservoir Evaporator
Vapor LineLiquid Line
Condenser
• The reservoir forms an integral part of the evaporator assembly.• The primary wick with fine pore sizes provides the pumping force.
LHP Startup Behaviors – 2016 Ku
• The secondary wick connects the reservoir and evaporator, supplying liquid.
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LHP Startup Scenarios
• Temperature Overshoot and Undershoot during Start-up
• Four Start up Scenarios• Four Start-up Scenarios
• Start-up Success
• Effect of Heat Load on Start-up Success
• Flow Reversal
LHP Startup Behaviors – 2016 Ku 5
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Temperature Overshoot During Start-up
TemperatureOvershoot
No TemperatureOvershoot
Final Tcc
Initial Tcc Initial Tcc
Final Tcc
TemperatureOvershoot
No TemperatureOvershootInitial Tcc
Initial Tcc
Final TccFinal Tcc
LHP Startup Behaviors – 2016 Ku 6
Temperature undershoot can be defined in a similar manner.
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Four Start-up Scenarios for LHP
• Vapor grooves– Liquid filled: T
empe
ratu
re
TeTccStart-up
Tamb
Tem
pera
ture
TeT
Tamb
St tq
superheat is required for nucleate boilingV
Time
p
Time
TccStart-up
– Vapor presence: instant evaporation
(a) Situation 1 (c) Situation 3
• Liquid core– Liquid filled:
low heat leak
Tem
pera
ture
TeTccStart-up
Tamb
Tem
pera
ture
TeTcc
Tamb
Start-uplow heat leak– Vapor presence:
high heat leakTime Time
Start up
LHP Startup Behaviors – 2016 Ku
(b) Situation 2 (d) Situation 4
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Start-up Success
• The beginning of liquid evaporation or nucleate boiling in vapor grooves is characterized by the rise of the vapor line temperature to near the reservoir saturation temperature and th d f th li id li t tthe drop of the liquid line temperature.
• A successful start-up is characterized by: – The vapor line temperature is the same as or close to the reservoir
temperature;– The evaporator temperature is higher than the reservoir
t t b t d t i d b th h t l d d thtemperature by an amount determined by the heat load and the evaporator thermal conductance;
– The liquid line temperature is lower than the reservoir temperature; Temperatures of the reservoir evaporator vapor line and liquid– Temperatures of the reservoir, evaporator, vapor line and liquid line approach their respective steady state temperatures asymptotically.
LHP Startup Behaviors – 2016 Ku 8
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High and Low Power Start-up
• With high power to the evaporator, liquid in the vapor grooves can be vaporized quickly regardless of the initial two-phase status in the grooves and the evaporator core.status in the grooves and the evaporator core.– The required superheat, if any, can be achieved in a short time.– Within the short time, the total heat leak is small.
• With low power to the evaporator, start-up could be problematic.– Under situation 4, the required superheat for nucleate boiling may
never be achieved.never be achieved.– A reverse flow may occur prior to nucleate boiling.– After the loop starts, a steady state may not be established within
the allowable temperature limit at low powers due to a high heat leak p p gfrom evaporator to CC if the core contains vapor.
LHP Startup Behaviors – 2016 Ku 9
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Unsuccessful Startup Under Situation 4
CC
Evap
T
mpe
ratu
rep
Loop does not start
Tem
• The vapor grooves are filled with liquid, and a superheat is needed to initiate nucleate boiling
TimeTime
needed to initiate nucleate boiling.• Because of the high heat leak from the evaporator to reservoir,
the required superheated was never attained.
LHP Startup Behaviors – 2016 Ku 10
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Schematic of LHP–A and Thermocouple Locations
1 2 3 4 5
6 7 8
9 10 11
6 7 8
6,7,8 9,10,1112
13 134
35
36 ReservoirEvaporator
Lin
e
14
13 15
16
1731
32
33
34
1
Vapor Line
Liqu
id
18
19
20222324252628
29
30
Con
dens
er
Con
dens
er 3
212223242526
27
Condenser 2
LHP Startup Behaviors – 2016 Ku 11
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High Power Startup of LHP-A
• Successful startup with 50W to evaporator 300
301
– Loop started 3 minutes after power application
– 4.5K superheat for nucleate boiling 297
298
299
)
Pump (TC9)
boiling
• This was a situation 3 startup.Th i t t
295
296
297
Tem
pera
ture
(C
Reservoir (TC5)
– The reservoir temperature rose with evaporator because of a heat leak due to heat conduction instead of heat
292
293
294Vapor line (TC15)
pipe effect.291
292
10:15 10:20 10:25 10:30
Time (hr)
Pump liquid inlet (TC36)
LHP Startup Behaviors – 2016 Ku 12
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Low Power Startup of LHP-A
• Successful startup with 5W to evaporator– It took 45 minutes to initiate
nucleate boiling– 2.5 K superheat for nucleate
boilingboiling– 4K temperature overshoot
Thi it ti 4 t t• This was a situation 4 startup.– Reservoir temperature rose
with evaporator (due to heat pipe effect) prior to nucleatepipe effect) prior to nucleate boiling.
LHP Startup Behaviors – 2016 Ku 13
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Ad l ti
Low Power Startup of LHP-A
• Adverse elevation -evaporator and reservoir were 690mm above the condenser
P
Thermal Vacuum Test, Chiller @-10°C, -27” Elevation14060333
• Startup with 10W to evaporator
Pump
Rure
(C)
(W)
80
100
12050
40
30
323
303
313
(K
)
– It took 85 minutes to initiate nucleate boiling
– 2.5 K superheat for nucleate boiling
Res.
Vapor Line
Tem
pera
tu
Pow
er
40
60
8030
20
10283
293
Te
mp
era
ture
boiling– 20K temperature overshoot
• This was a situation 4 startup
Liquid Line
Time(Hours)
20:0019:0018:0017:0016:0015:000
200
-10263
273
• This was a situation 4 startup.– Reservoir temperature rose
with evaporator (due to heat pipe effect) prior to nucleate
Figure 2.9 – NRL Nickel Wick LHP Start-Up
LHP Startup Behaviors – 2016 Ku
pipe effect) prior to nucleate boiling.
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Flow Reversal during Startup Transient
• Flow reversal during startup typically occurs under Situation 4.– Liquid evaporation takes place at the core of the evaporator.– Vapor flow via the liquid line to the condenserVapor flow via the liquid line to the condenser.
• Flow reversal can last from seconds with high power startup to hours with low power startuphours with low power startup.
• After nucleate boiling, forward flow will be established, and LHP will begin its normal operationLHP will begin its normal operation.
LHP Startup Behaviors – 2016 Ku 15
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Schematic of LHP-B
16 26 2827
1511
TC 9,10,11TC 6,7,8
29251714
9
10
8
7
EVAPORATOR
13
312319
18 24 30
12
9
6
5
VAPOR LINE
DP
35
37
21
20 22 32 38
423
136
CONDENSER
LIQUID LINECOMPENSATIONCHAMBER
TC 4TC2
TC 1,3,5
AP34
3533
36
LHP Startup Behaviors – 2016 Ku 16
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Flow Reversal During LHP-B Startup(100 grams/ +6.35mm/ 5W/ 290K)
• Situation 4 startup under an adverse tilt• Flow reversal lasted for 4+ hours with 5W without startup.• Loop started with 100W, then operated at 5W.
35000
40000
310
312NRL LHP 01/08/2001
01/08/2001
20000
25000
30000
304
306
308
op (P
a)
re (K
)
Evap (7)
Liq Line (34)
5000
10000
15000
298
300
302
Pre
ssur
e D
ro
Tem
pera
tur
CC (3)
-5000
0
5000
292
294
296
8:30 9:30 10:30 11:30 12:30 13:30 14:30 15:30100 W
DP
Vap Line (15)
CC Inlet (36)
5 W 5 W
LHP Startup Behaviors – 2016 Ku
Time (hours)
17
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V id f ti i th t t l ff t th
Fluid Distribution in Evaporator and Reservoir
• Vapor void fraction in the evaporator core strongly affects the LHP startup and low power operation.
• Vapor void fraction depends upon the fluid distribution in the evaporator and reservoir.
• Factors affecting the fluid distribution– Fluid inventory– Pre-conditioning of the loop prior to startup– Body forces
• Evaporator/reservoir assembly design• Tilt between evaporator and reservoirp• Elevation between evaporator and condenser
• Startup is affected by combinations of factors.
LHP Startup Behaviors – 2016 Ku
p y
18
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Evaporator Assembly and Gravity Effect on Fluid Distribution
LiquidVapor
Liquid
fromCondenser Liquid
Evaporator
Gravity
Vapor
Reservoir
Liquid
from
Vapor
LiquidGravity
Condenser Reservoir Evaporator
LHP Startup Behaviors – 2016 Ku 19
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LHP-B Startup Tests
• Fluid inventories: 83 grams, 100 grams, and 113 grams• Tilts: +6.35mm, 0 mm, and -6.35mm (evaporator end to reservoir end)• Successful startups with 100W or higher under all conditions
St t hi hl d d tilt d i t ith 100W• Startup highly depends on tilts and inventory with
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LHP-B Startup - Reverse Flow(100 grams/ 0mm/ 50W/ 270K)
• Situation 4 startup• Flow reversal lasted for 15 minutes with 50W!• 20K temperature overshoot
310
315NRL LHP 01/18/2001
01/18/01
295
300
305
K)
Evap (7)
Vap Line (15)
280
285
290
Tem
pera
ture
(K
CC (3)
CC Inlet (36)
Liq Line (34)
265
270
275
280
Liq Line (33)
50 WATTS0W
q ( )
LHP Startup Behaviors – 2016 Ku
2657:30 8:00 8:30 9:00 9:30 10:00
Time (hours)
50 WATTS0W
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Enhancing Start-up Success
• Superheat is required for nucleate boiling for Situation 3 and Situation 4 startups– Situation 3: Loop will start, but may take a long time with low powers.Situation 3: Loop will start, but may take a long time with low powers.– Situation 4: Loop may not start with low powers.
• Methods to enhance startup successMethods to enhance startup success– Start-up heater– Thermoelectric converter (TEC)
LHP Startup Behaviors – 2016 Ku 22
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U t t d h t l li d th
Startup Heaters
• Use a concentrated heat source over a localized area on the evaporator.
• The high heat flux will quickly raise the temperature of liquid in the vicinity of the heater while minimizing the heat leak to the reservoir.
• Once nucleate boiling starts and first bubbles are generated, no superheat is required for liquid evaporation.
• The startup heater has proven to be very effective in enhancing the startup success.p
• Many LHPs in flight applications employ such a device because of its simplicity in design and ease in implementation.
LHP Startup Behaviors – 2016 Ku
of its simplicity in design and ease in implementation.
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TEC and Thermal Strap
Q
QTEC H
QTEC, L
QTEC, AppTECTTEC, L
Thermal StrapQTEC L
QTEC, H
QTEC, AppTEC
TTEC, H
Thermal Strap
-Qsub Qleak
QTEC, H
CC, Tset Evap, TE-Qsub Qleak
TEC, L
CC, Tset Evap, TE
• Heat Flow When TEC Is Cooling the Reservoir
• Heat Flow When TEC Is Heating the Reservoir
LHP Startup Behaviors – 2016 Ku 24
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Enhancing LHP Startup Using Thermoelectric Converter (TEC)Situation 4 startup
• Without TEC (Figure A)– CC temperature rises with evaporator temperature due to heat leaks.– Required superheat may never be attained at low powers.
• With TEC (Figures B and C)– TEC can maintain a constant CC temperature to achieve the required
superheat, resulting in a successful start-up.TEC l l th CC t t th i d h t– TEC can also cool the CC to create the required superheat.
– Startup heaters can be eliminated.
e Evape
CC
EvapTem
pera
ture
Tem
pera
ture
T
Tem
pera
ture
CC
Evap
TCC
EvapTem
pera
ture
T
Time
Evap
Loop does not start
T
Loop does not start
T
Time
Loop starts
Time
Loop starts
Fi B Fi CFigure A Figure B Figure C
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LHP-C with TEC and Startup Heaters
• TEC was installed on evaporator and connected to CC via a thermal strap.
• An electric heater was also installed on evaporator to serve asAn electric heater was also installed on evaporator to serve as the startup heater
101112141516 27123
713
Vapor Line CCEvaporator
32
Fill TubeAluminum Saddle
Compensation
Chamber
28
1729
18
192030
456
89
36 Ambient33 34 35
Thermal Mass Vapor Line
Evaporator
Copper Strap
TEC Saddle
Slot for TEC
21 22 23 24 25 26
Liquid Line31
Liquid Line
LHP Startup Behaviors – 2016 Ku 26
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LHP-C Startup with TEC and Startup Heater
• TEC provided required heating and cooling during startup to maintain the CC set point.
• By comparison, a higher temperature overshoot when an electric heater was used for CC temperature control.
12/14/2009; 253K sink; 350 g mass;TEC set point@303K on the CC - TC2; PID 12/16/2009; 253K sink; 350 g mass;EH set point@303K on the CC - TC02; PID
25
30
35
300
305
310
)
12/14/2009; 253K sink; 350 g mass;TEC set point@303K on the CC TC2; PID
Evap (5)CC (2)
40
50
300
310
320
)
Evap (5)
CC (2)
5
10
15
20
280
285
290
295
Po
wer
(W)
Tem
per
atu
re (K
)
Evap Power
CC In (27)
Vap Line (11)
20
30
280
290
300
Pow
er (W
)
Tem
pera
ture
(K)
E P
CC In (27)
Vap Line (11)
-5
0
5
270
275
280
9:45 10:00 10:15 10:30
Time (HH:MM)
TEC Power
0
10
260
270
14:00 14:15 14:30 14:45 15:00
Time (HH:MM)
Evap Power
EH Power
LHP Startup Behaviors – 2016 Ku 27
( ) ( )
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• No heat load to the evaporator TEC was used to cool the reservoir
Using TEC to Enhance Start-up SuccessNo heat load to the evaporator. TEC was used to cool the reservoir.
• The loop started at 8:17 with a superheat of 2K.• The heat input to the evaporators came from the power applied to the
TECs, and the heat pumped out of the reservoirs.TECs, and the heat pumped out of the reservoirs.• Once started, the loop continued to operate. Additional heat came from the
sensible heat released from the thermal mass attached to the evaporatorStart-up Test August 12, 2009
50
60
276
278
280
Start up Test August 12, 2009
Mass
30
40
272
274
276
Pow
er (
W)
Tem
pera
ture
(K
)
Vapor Line
0
10
20
266
268
270
CC
Evaporator
Liquid Line
LHP Startup Behaviors – 2016 Ku 28
02668:00 8:15 8:30 8:45 9:00 9:15
Time (HH:MM)
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Other Start-up Issues
• Pressure Spike
• Pressure Surge• Pressure Surge
• Reservoir Temperature Undershoot
• Repeated Cycles of Loop Start-up and Shutdown
LHP Startup Behaviors – 2016 Ku 29
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Pressure Spike
• The required superheat for nucleate boiling can be higher than 10K.
• Right after nucleate boiling, the vapor bubble will absorb the sensible heat stored in the superheated liquid and grow rapidly.
• The growth of the vapor bubble is similar to an explosion. • Experimental data shows that the pressure differential across the p p
evaporator can be as high as 45 kPa. • Such a high pressure drop may exceed the capillary limit of the
primary wick and cause the vapor to penetrate the wick to reach p y p pthe evaporator core.
• However, the high pressure drop only lasted for fractions of a second.
• Because of the short duration of the pressure spike and the ability of the LHP to tolerate a vapor bubble in the evaporator core, no LHP deprime due to the pressure spike has been observed.
LHP Startup Behaviors – 2016 Ku
p p p
30
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Aft th b ili i i i li id i th li i t i t
Pressure Surge
• After the boiling incipience, liquid in the vapor line is swept into the condenser.
• Liquid moves toward the reservoir at the same volumetric flow t th i b i t d i thrate as the vapor is being generated in the vapor grooves.
• The liquid mass flow rate along the condenser and liquid line can be two orders of magnitude higher than its steady state value at th h t l d d i th l ti f th LHPthe same heat load during the normal operation of the LHP.
• A high flow rate induces a surge of the pressure drop that is imposed on the primary wick until vapor reaches the condenser.
• The magnitude and duration of the pressure surge depend on the working fluid, saturation temperature, heat load, volume of the vapor line and vapor grooves, and initial vapor line temperature.
• The pressure surge is more severe at a low reservoir temperature.• An LHP can usually sustain the pressure surge without any
problem due to its high capillary capability.
LHP Startup Behaviors – 2016 Ku 31
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Repeated Cycles of Loop Startup and Shutdown
• When a severe reservoir temperature undershoot happens, the reservoir control heater will be turned on.
• If the heater power is so large that it raises the reservoir temperature faster than the evaporator can catch up, the loop will be flooded with liquid again by the time the reservoirwill be flooded with liquid again by the time the reservoir reaches its set point temperature.
• The re start will follow the same process as the previous• The re-start will follow the same process as the previous startup. In some cases, this leads to repeated startup and shutdown cycles.
LHP Startup Behaviors – 2016 Ku 32
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C t l l d th th l t i t i it t i t
Repeated Startup/Shutdown Cycles in LHP-C
• Control sensor was placed on the thermal mass to maintain its set point at 313K.
• Control heater (electrical) was attached to the CC.• Repeated startup/shutdown cycles with 10W and 20W to thermal mass• Repeated startup/shutdown cycles with 10W and 20W to thermal mass.• Successful startup with 40W to thermal mass.
140330
12/18/2009; 253K sink; 350 g mass; Pre-heating CC; EH set point@313K on the TM - TC33; PID
100
120
310
320
Evap (5)TM (33)
40
60
80
290
300
Po
wer
(W)
Tem
pera
ture
(K)
CC (2)
CC In (27)
Vap Line (11)
0
20
270
280
Evap Power
EH Power
LHP Startup Behaviors – 2016 Ku 33
-202608:30 8:45 9:00 9:15 9:30 9:45 10:00 10:15 10:30 10:45
Time (HH:MM)
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• LHP startup is one of most complex transient phenomena
Summary and Conclusions• LHP startup is one of most complex transient phenomena.• There are four possible startup scenarios, which are determined by
the initial fluid distribution between evaporator and CC.Se eral factors affect fl id distrib tion bet een e aporator and CC• Several factors affect fluid distribution between evaporator and CC.– Fluid inventory– Pre-conditioning
B d f it– Body forces, e.g. gravity• Evaporator/CC assembly design• Tilt between evaporator and reservoir
El ti b t t d d• Elevation between evaporator and condenser• Startup success is a function of startup scenario, power to
evaporator, and how the CC temperature is controlled.• Using a startup heater or a thermoelectric converter can greatly
enhance startup success.• Repeated startup and shutdown cycles can happen. This can be
LHP Startup Behaviors – 2016 Ku
avoided or mitigated by using a smaller increments for reservoir temperature rise.
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LHP Startup Behaviors – 2016 Ku 35
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