Enclosure Thermal Control

25 August 2003 ATST CoDR Dr. Nathan Dalrymple

Air Force Research LaboratorySpace Vehicles Directorate

Enclosure Thermal Control

• Function: Suppress seeing

If a surface is the same temperature as the surrounding air, that surface introduces no seeing

Seeing is caused by temperature differences

Requirements

1. Suppress enclosure seeing

a. Racine experiment: = 0.15 Ti - Te) 1.2

b. Ford analysis: = 0.012 Ts - Te 1.2

c. IR HB aerodynamic analysis: = TV, d. Bottom line: requirements on surface-air T, interior-

exterior T, and wind flushing

2. Provide passive interior flushing to equalize interior and exterior temperatures and to suppress structure and mirror seeing

Ref: Racine, Rene, “Mirror, dome, and natural seeing at CFHT,”

PASP, v. 103, p. 1020, 1991.

Error Budgets

(nm) Exterior budget Interior budget

500 20 nm 10 nm

1600 0.07 arcsec 0.02 arcsec

1000 0.06 arcsec 0.025 arcsec

IR Handbook Seeing Analysis

Given layer thickness and T, we can estimate .

zlG z

Hd2

0

222

Wavefront variance

Gladstone-Dale parameterFluctuating density Line-of-sight correlation length

Layer thickness

HlT

TGz2

10

22

Phase variance

2.01.0 H

lz

Surface-air temperature difference

)(1)exp(

)(33.3

s2

D

s

aberrationweak

aberrationstronglz

Blur angle

Strong/weak cutoff ~ 2 rad

Ref: Gilbert, Keith G., Otten, L. John, Rose, William C., “Aerodynamic Effects” in The Infrared and Electro-Optical Systems Handbook, v. 2, Frederick G. Smith, Ed., SPIE Optical Engineering Press, 1993.

IR Handbook Seeing Analysis (cont.)

Layer thickness (mks units):

2.0

8.05.05.1

0392.0184.0V

L

V

TLH

L: upstream heated length (m)T: average temperature difference over upstream length (˚C)V: wind speed (m/s)

Buoyancy term Hydrodynamic term

Assume: If T < 0 then buoyancy term does not contribute to layer thickness.

Shell Seeing, Diffraction-Limited Error Budget

Blue contours: rms wavefront error (nm)

Acceptable operating range, assuming no AO correction.

AO correction will extend the “green” area.

= 500 nm

Shell Seeing, Seeing-Limited Error Budget

Blue contours: 50% encircled energy (arcsec)

Acceptable operating range

= 1600 nm

Shell Seeing, Coronal Error Budget

Blue contours: 50% encircled energy (arcsec)

Acceptable operating range

= 1000 nm

Dome Seeing (Inside/Outside Air T)

Correlation by Racine (1991)

Approximate error budget

Approximate T requirement

Need lots of passive flushing!

Ref: Racine, Rene, “Mirror, dome, and natural seeing at CFHT,”

PASP, v. 103, p. 1020, 1991.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

local time

seei

ng (

arcs

ec)

Shell seeing (arcsec)

Interior seeing (arcsec)

Dome seeing (arcsec)

IR Handbook aerodynamic treatment

Correlation of Racine (1991)

IR Handbook aerodynamic treatment

Good seeing from KE test

Ref: Racine, Rene, “Mirror, dome, and natural seeing at CFHT,”

PASP, v. 103, p. 1020, 1991.

BBSO Dome Seeing Experiments

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

local time

seei

ng (

arcs

ec)

Shell seeing (arcsec)

Interior seeing (arcsec)

Dome seeing (arcsec)

Bad seeing from KE test

BBSO Dome Seeing Experiments

A Nighttime Comparison: Gemini Dome

Gemini Thermal Tests: 11 - 14 Mar 2003Dome skin temperature [deg C]

-5

0

5

10

15

20

25

30

35

40

3/11 0:00 3/11 12:00 3/12 0:00 3/12 12:00 3/13 0:00 3/13 12:00 3/14 0:00 3/14 12:00 3/15 0:00

Top left (55 deg)

Middle left (36 deg)

Lower left (22 deg)

Top right (55 deg)

Middle right (36 deg)

Lower right (22 deg)

Air Temperature

1 Duct exhaust fan on, low-moderate wind (3 - 5 m/s)

T = -3 ˚C

Acceptable seeing observed with shell subcooled by 3 ˚C.

Bottom Line Requirements

• Enclosure skin temperature needs to be subcooled by up to 3 ˚C

• Interior air temperature needs to be within 0.5 ˚C of ambient outside air

• Need large passive flowrate to flush interior

Skin Energy Balance

We want to use this term to control the skin temperature

[~0 W/m2]

[377 W/m2]

[374 W/m2]

[98 W/m2]

[~100 W/m2]Quantities vary by location on dome and weather conditions

Skin Thermal Control System Concept

Concept Features:1. White oxide paint

a. Large b. Small s

2. Chilled skina. Airb. Liquid (EGW)

3. Insulationprevents interior from beingchilled by skin coolant

Shutter: air cooled, optional water cooling on lower endhair ~ 8 W/m2-KhH2O ~ 100 W/m2-K

Enclosure support wall: water cooled if presenthH2O ~ 100 W/m2-K

Oblique skin panels: air cooled, h ~ 5 W/m2-K

Sun-facing skin panels:

air or water cooledhair ~ 5 W/m2-KhH2O ~ 100 W/m2-K

Option: use fins on skin underside to increase effective area

Skin Thermal Control System Concept (cont.)

Skin Cooling System Flow Loop

Insert diagram here

MuSES Model Validation: Measured and Predicted Dome Skin Temperature

30 April 2003

0

5

10

15

20

25

30

9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00

Top left (55 deg)

Lower left (22 deg)

Air temperature

Elem 2749

Elem 2738

MuSES Modeling: Validation at Gemini North

Validation

Skin Thermal Control System Performance

MuSES snapshot at 1430LT, 30 April 2003, Mauna KeaWind speed = 0.5 m/sAmbient air Te = 7 – 8 ˚CAir Cooling Only on SkinESW Water Cooled

Most of surface is acceptable

Sun-facing areasare ~ 5 ˚C hotter than ambient

Surfaces that see cold sky subcool

MuSES snapshot at 1430LT, 30 April 2003, Mauna KeaWind speed = 0.5 m/sAmbient air Te = 7 – 8 ˚CAir & Water Cooling

Nearly all of surface is acceptably cool

Sun-facing areascooled with water

Surfaces that see cold sky subcool

Skin Thermal Control System Performance (cont.)

Cooling Requirements

• Next steps:•Fan and system curves•Heat exchanger specs•Chiller specs•Time response of fluid volume

At peak heat load, surface cooling requires:• Air-cooled skin: 56 kW• Water-cooled skin: 18 kW• Lower shutter: 14 kW• Air-cooled shutter: 18 kW• Total for carousel: 106 kW• Enclosure support wall: 104 kW• Grand total: 210 kW (60 tons)

Flushing System Concept

42 vent gates

168 m2 flow area,each side

Flushing System Performance

Active Interior Ventilation

• Gemini volume flowrate: 10 enclosure volumes/hour (150,000 m3/hr)• This flowrate on the smaller hybrid gives V ~ 0.2 m/s average • Directed flow can give V~0.5 – 1 m/s over much of structure

Fans may be mounted remotely or on carousel

Active Ventilation Issues

• Fan blades heat air seeing• Require homogenizing screens, cooling coils

downstream of fans• May not be simple to mount all this on

carousel possible to mount remotely

Shell Seeing Performance

Blue contours: rms wavefront error (nm)

Red: average T of skin, front skin, shutter, lower shutter, ESW

Most of the dome surface will give acceptable seeing

Back of shutter subcools. May need to add water cooling there as well.