Loop Heat Pipe Startup Behaviors

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Loop Heat Pipe Startup Behaviors

Jentung Ku NASA/GSFC

Code 545 301-286-3130

[email protected]

18th Workshop on Thermophysics in Microgravity El Segundo California March 24 2014

El Segundo, California, March 24, 2014

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Outline

• Introduction/Background

• LHP Operating Temperature

• LHP Startup Scenarios

• Fluid Distribution Between Evaporator and Reservoir

• Effect of Evaporator Power on Startup Success

• Enhancing Start-up Success

• Other Startup Issues

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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 itsAn 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 of a single-evaporator LHP servicing a heat source all by itself.

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Schematic of an LHP

Primary

Wick

Secondary

Wick Evaporator Vapor Channel

Pump Core Vapor Channel R i Bayonet Reservoir Evaporator Vapor Transport Line Liquid Transport Line Condenser

• Main design Features

• Operating Principles

Condenser

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LHP Operating Temperature

• LHP Natural Operating Temperature

• LHP Operating Temperature Control

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LHP Natural Operating Temperature

• The LHP operating temperature is governed by the CC temperature.

• The CC temperature is a function of

– Evaporator power

– Condenser sink temperature – Ambient temperature

– Evaporator/CC assembly design

• As the operating condition changes, the CC temperature will change during the transient, but eventually reaches a new steady temperature.

-Q_sub Q_leak CC, T_CC T em p er at ure Natural Operating Temperature T2 CC T T1 Q_sub= Q_leak

Net Evaporator Power

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LHP Operating Temperature Control

CC Temperature Controlled at T_set

Q_CC e Natural Operating Temperature -Q_sub Q_leak CC, TCC Tset C C T em p er atur e

Fixed Operating Temperature Temperature Tset C Heating Required Q_CC= Q_sub– Q_leak CC i ld bi d d l t i l h t l d t i t i T t T

Net Evaporator Power Cooling

Req’d

Q_Low Q_High

• CC is cold biased, and electrical heaters are commonly used to maintain T_cc at T_set.

• Overall thermal conductance decreases.

• Q_CC varies with Q_sc, which in turns varies with evaporator power, condenser sink temperature, ambient temperature and number of coupling blocks.

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Thermodynamic Constraint in Two-Phase Systems

The following relation must be

Saturation Curve

The following relation must be satisfied between any

components where liquid and vapor phases coexist in

P_A u re P_B thermodynamic equilibrium. T_B – T_A = (P_B – P_A)/(dP/dT) A Vapor Liquid Press u Or, T T = (P P ) (T 'v/O) T_B Vapor Temperature T_A T_B – T_A = (P_B – P_A) (T_B 'v/O)

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LHP Startup Scenarios

• Superheat for Nucleate Boiling

• 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

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P t d th li id i t f f th b bbl

Superheat for Nucleate Boiling

• Pressure exerted on the vapor-liquid interface of the vapor bubble:

'P = PV- PL = 'Pcap = 2V cosT /R • Clausius-Calpeyron relation: T_V, P_V T_L, P_L Liquid dP/dT = O/(T'v)

• Using ideal gas law:

Heating Surface

• Thus,

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Temperature Overshoot During Start-up

Temperature Overshoot No Temperature Overshoot Final T I iti l T Final T_cc Final T_cc Initial T_cc Initial T_cc Temperature Overshoot No Temperature Initial T Initial T_cc Overshoot Initial T_cc Final T_cc Final T_cc

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Four Start-up Scenarios for LHP

• Vapor grooves

– Liquid filled: Temperature _Start-up TTecc

Tamb Temperature T_Te Tamb St t superheat is required for nucleate boiling – Vapor presence: Time p Time Tcc Start-up instant evaporation • Liquid core

(a) Situation 1 (c) Situation 3

– Liquid filled: low

heat leak

– Vapor presence:

high heat leak

Temperature Te Tcc Start-up Tamb Temperature Te Tcc Tamb Start-up

high heat leak

Time Time

Start up

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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 t

the 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

temperature 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 line approach their respective steady state temperatures asymptotically.

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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.

The required superheat if any can be achieved in a short time

– 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 problematicWith low power to the evaporator, start up could be problematic.

– Under situation 4, the required superheat for nucleate boiling may never be

achieved.

– After the loop starts, a steady state may not be established within the

ll bl t t li it t l d t hi h h t l k f allowable temperature limit at low powers due to a high heat leak from evaporator to CC if the core contains vapor.

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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 298 299 ) Pump (TC9) boiling

• Worst case scenario under

situation 4 295 296 297 T e m p er at u re ( C ) Reservoir (TC5) situation 4

– Reservoir temperature rose

with evaporator prior to

nucleate boiling 292 293 294 Vapor line (TC15) nucleate boiling 291 292 10:15 10:20 10:25 10:30 Tim e (hr) Pump liquid inlet (TC36)

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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 boiling nucleate boiling – 4K temperature overshoot W t i d

situation 4

with evaporator prior to with evaporator prior to nucleate boiling

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Ad El ti

Low Power Startup of LHP-A

• Adverse Elevation

• Evaporator and reservoir

were 690mm above the d

Pump

Thermal Vacuum Test, Chiller @-10°C, -27” Elevation

120 140 60 50 323 333 condenser • Startup with 10W to evaporator Res. p er a tur e ( C ) o we r (W) 60 80 100 40 30 20 303 313 tu re ( K )

– It took 85 minutes to initiate

nucleate boiling

– 2.5 K superheat for nucleate

boiling Vapor Line Liquid Line Tem p P o 20 40 60 20 10 0 273 283 293 T e m per a boiling – 20K temperature overshoot

situation 4 Figure 2.9 – NRL Nickel Wick LHP Start-Up

Time(Hours) 20:00 19:00 18:00 17:00 16:00 15:00 0 -10263 situation 4

with evaporator prior to nucleate boiling

g p

• Flow reversal 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 startup hours with low power startup.

• After nucleate boiling, forward flow will be established, and

LHP will begin its normal operation LHP will begin its normal operation.

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Schematic of LHP-B

16 26 28 27 15 11 8 TC 9,10,11 TC 6,7,8 29 25 17 14 9 10 8 7 EVAPORATOR 13 31 23 19 18 24 30 12 9 6 5 VAPOR LINE

W

N

35 37 21 20 22 32 ₃₈ 4 2 3 1 36 CONDENSER LIQUID LINE COMPENSATION CHAMBER TC 4 TC2 TC 1,3,5 AP 34 35 33 36 Q

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Flow Reversal During LHP-B Startup

(100 grams/ +6.35mm/ 5W/ 290K)

• Situation 4 startup

• Flow reversal lasted for 4+ hours with 5W without startup

• Loop started with 100W, then operated at 5W

35000 40000 310 312 NRL LHP 01/08/2001 01/08/2001 20000 25000 30000 304 306 308 o p ( P a) re ( K ) Evap (7) Liq Line (34) 5000 10000 15000 298 300 302 Pr e s s u re Dr o T e m p er at u r CC (3) -5000 0 5000 292 294 296 8:30 9:30 10:30 11:30 12:30 13:30 14:30 15:30 100 W DP Vap Line (15) CC Inlet (36) 5 W 5 W

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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

– Evaporator/reservoir assembly design and gravity – Pre-conditioning of the loop prior to startup

– Fluid inventory and tilt in ground tests – Body forces

– Tilt between the evaporator/reservoir and condenserp

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Evaporator Assembly and Gravity Effect

on Fluid Distribution

Vapor Liquid from Vapor Liquid Gravity

Condenser _Reservoir _Evaporator

Vapor Liquid from Condenser Vapor Liquid Gravity

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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 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 <100W

• Successful startups with t 5W in all tilts at 100 grams and 113 grams

• Loop could not start with < 25W in all tilts at 83 grams

100 grams 49.5% filled H/D = 0.496 83 grams 32.1% filled H/D = 0.312 113 grams 71.8% filled H/D = 0.73

Cross Sectional View of Evaporator/Reservoir with Various Fluid Inventories Cross-Sectional View of Evaporator/Reservoir with Various Fluid Inventories

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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 310 315 NRL LHP 01/18/2001 01/18/01 295 300 305 K ) Evap (7) Vap Line (15) 280 285 290 T e m p er a tur e ( K CC (3) CC Inlet (36) Liq Line (34) 270 275 280 Liq Line (33) q ( )

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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

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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 inThe 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 nucleates starts and first bubbles are generated noOnce nucleates 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 the startup success.

• Many LHPs in flight applications employ such a device because

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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 can also cool the CC to create the required superheat. – Startup heaters can be eliminated.

re Evap re CC Evap T e mperature T e mperature 'T T e mperatu CC 'T CC Evap T e mperatu r 'T Time

Loop does not start Loop does not start

Time

Loop starts

Time

Loop starts

Fi 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

1 0 11 12 14 1 5 16 2 8 2 7 1 4 5 6 2 3 7 1 3 V ap or Line CC Evapo rator 3 2 1 0 11 12 14 1 5 16 2 8 2 7 1 4 5 6 2 3 7 4 5 6 2 3 7 1 3 V ap or Line CC Evapo rator 3 2 Fill Tube Evapo rator

Aluminum Sad dle

Co mp ensation Ch amber 1 7 29 18 1 9 2 0 30 21 _{2 2} 2 3 ₂₄ 2 5 _{2 6} 8 9 36 Ambi en t 1 7 29 18 1 9 2 0 30 21 _{2 2} 2 3 ₂₄ 2 5 _{2 6} 8 9 8 9 36 Ambi en t Vap or Line Evapo rator Co pp er Strap TEC Sadd le Slot fo r TEC 21 _{2 2} 2 3 ₂₄ 2 5 _{2 6} Liqu id Line 3 3 34 3 5 Thermal M as s 31 21 _{2 2} 2 3 ₂₄ 2 5 _{2 6} Liqu id Line 3 3 34 3 5 Thermal M as s 31

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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.

• A higher temperature overshoot when electric heater was used.

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 Pow e r ( W ) T e m p er a tur e ( K ) Evap Power CC In (27) Vap Line (11) 20 30 280 290 300 Power ( W ) T e m p er at u re ( 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 ( ) ( )

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Other Start-up Issues

• Pressure Spike and Pressure Surge

• Reservoir Temperature Undershoot

• Repeated Cycles of Loop Start up and Shutdown

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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 p

the 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

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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 th

rate 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 LHP

the 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

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Reservoir Temperature Undershoot

• A large reservoir temperature undershoot can happen at any

time during the LHP operation as a result of a sudden influx of a large amount of very cold liquid into the reservoir.

– Most frequently seen during the startup of a fully flooded loop

• Even if the reservoir temperature is regulated, a severe

temperature undershoot can still occur because the heater power is not unlimited

– This may lead to a violation of the instrument minimum allowable

temperature.

If th t t d t t th id i

• If the evaporator cannot adapt to the rapid reservoir

temperature drop, the loop could deprime.

– Reservoir cold shock

• Although the drop of the reservoir temperature itself is not an

issue, its temperature rise when regulated by a heater could lead to repeated loop startup and shutdown.

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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 reservoir

will 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.

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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.

140 330

12/18/2009; 253K sink; 350 g mass; Pre-heating CC; EH set point@313K on the TM - TC33; PID

100 120 140 310 320 330 Evap (5) TM (33) 40 60 80 290 300 Pow e r ( W ) T e m p e ra tu re (K ) CC (2) CC In (27) Vap Line (11) 0 20 270 280 T Evap Power ( ) EH Power -20 260 8:30 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.

– Evaporator/CC assembly design – Tilt in ground tests

B d f

– Body forces – Fluid inventory – Pre-conditioning

• 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