EMAS Stirling Cooler OperationsJune 29, 2011

Mike Watson

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E-MAS CryoTel GT Stirling Cooler

• Characteristics:• Does not have over/under (80C/-40C)

temperature protection in this controller model• Next generation controller (available October of

2011) should have this feature• Cooling capacity diminishes by .06W/C when

above 35C• Model installed on EMAS has been upgraded

with a dual frequency absorber to minimize vibration. This absorber can be tuned once it is installed on system with guidance from SunPower.

• 120,000 hours MTBF

• For heat load during operation:• While cooling at full capacity, the device

will dump 240W plus the heat lifted from the system. Therefore, a maximum of 285W (at 240K) will need to be dumped

• At steady state, the device will dump 240W X (% of full load capacity at that temperature) + heat lifted from system.

• For example, if we controlled to lift 10W at 90K, the heat dumped would be 240*(10/19)+10 = 136W.

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E-MAS CryoTel GT Stirling Cooler• Operational modes

• Vacuum chamber during TVAC testing

• Cryocooler fins replaced with bolt-on LN2 heat exchanger

• Cooling manually throttled at external dewar as necessary

• Benchtop during lab testing/operation• Either use LN2 HX or fins• No analysis conducted for this

method.

• Flight• Ram air injected through cryocooler

fins• Low altitude

• Sea level conditions• Mach .35

• High altitude• Mach .7• Temperature and pressure

from ER2 flight data from MidLats and Tropics

Copper collar clamps: same for all setups

LN2 in / out:

VCR Fittings

Ram air /

Chilled airCover removed

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E-MAS CryoTel GT Stirling Cooler• Low altitude flight

• Altitude = Sea level• Temp = 38C (100F)• Pressure = 1 atm• Mach .35

• Using maximum heat load 285W (only generated during portions of cooldown)

• Using turbulent flow correlations for Nusselt / heat transfer coefficient calculations

• Results• Average collar temperature approaching cryocoller

high temperature limit• Operational impacts:

• May need to throttle back on cooling set point• May need to wait until plane achieves a

certain altitude before restarting cryocooler

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E-MAS CryoTel GT Stirling Cooler• High altitude flight

• Altitude = 65000 ft• Temp = -60C• Pressure = 60mbar (.06 atm)• Mach .7

• Using maximum heat load of 285W (only generated during portions of cooldown)

• Heat transfer coefficient difficult to estimate• Limited data on air properties (specifically thermal

conductivity) • Fin geometry and input air conditions indicate that

flow will be turbulent throughout heat exchanger• Using laminar flow correlations for simplicity and

conservativeness• NOTE: thermal conductivity of air begins to drop rapidly

above an altitude of 80K feet (or 10-50 Torr). This will significantly reduce the ability to cool the cryocooler.

• Results• Gradient mostly through copper clamps• May see ~1-2W of reduced cooling capacity this

operating temperature• Possible that might reach lower temperature limits

during lower steady state heat loads and higher conductivities than calculated. This risk is avoided by lowering the setpoint and driving the heatload up.