ATST Thermal Design21 Oct 2002
ATST Thermal Design21 Oct 2002
Dr. Nathan DalrympleSpace Vehicles Directorate
Air Force Research Laboratory
Dr. Nathan DalrympleSpace Vehicles Directorate
Air Force Research Laboratory
2
Problem: Seeing
T. Rimmele & BBSO
Other solar telescopes:•Helium backfill•Evacuated optics
ATST must be open-air.Surfaces must be individuallytemperature-controlled.
3
Approach
• Define thermal requirements
– Flow down from SRD, error budgets, interfaces
– Connect image quality to surface temperature with modeling and empirical correlations
– Continue refining until early 2003
• Explore concepts
– Examine prior work
– Assemble short list of concepts
– Model/analyze concepts, list pros/cons
– Can we meet requirements?
– Examine interfaces/trades
– Select baseline concept (~CoDR, spring 2003)
Two concurrent tasks:
4
Big Picture
Thermal Problem Areas
5
Thermal Concerns
20280M6
Absorbed heat (W)Absorbed flux (W/m2)surface
150,000300Exterior shell
22694M5
25271M4
272034M3
3060M2
1866 + 10,271 = 12,137
303,000Heat stop(reflective)
1382110M1
6
Heat Stop Seeing
See Beckers and Melnick [1994] and Zago [1995 & 1997]
Not very restrictive on ∆T. Shoot for ∆T = 10 – 20 °C.
7
Heat Stop Concepts
(LEST)
(CLEAR)
(R. Coulter)
(ATST baseline)
8
Jet Cooling Scheme
Array of normal jets gives very large heat transfer coefficient:h = 30 kW/m2-K or larger not unreasonable.
Large h allows coolant to be near ambient temperature—no complex control systems needed.
9
Heat Stop Design Curves
Jet-cooled cone: d jets = 3 mm, N jets = 40, L jets = 13.5 cm, water coolant
0
50
100
150
200
250
300
0 5 10 15 20 25 30 35 40
∆T (K)
flow
rate
(gp
m)
or f
rict
ion
head
(ft
)
0
5
10
15
pum
p po
wer
(hp
)
Q (gpm)
head (ft)
power (hp)
Looks like we can achieve our goalof 10 – 20 K ∆T with ~50 gpmand less than 1 hp
10
Heat Stop Summary
• Seeing contribution expected to be low, although it is unclear how small-scale plume turbulence affects AO system performance
• Baseline concept: jet-cooled reflective cone
• 10 – 20 K ∆T with 50 gpm water coolant near ambient temperature
• May implement plume suction with larger ∆T
• Shape influences scattered light
11
Primary Mirror Seeing
Convection Regimes for D = 4m,T = 270K
0
1
2
3
4
5
0 1 2 3 4
V (m/s)
∆T(K
)
Mixed Convection
Nat
ural
Con
vect
ion
Fr =
10
Fr =
1.0
Fr =
0.1
Forced Convection
(a) Natural Convection
(b) Mixed Convection
(c) Forced Convection
Thin layers = good seeing.We want to be in forced convection:
low ∆T, high V.
GrRe
gLV
Fr22
=∆
≡ρ
ρ
12
M1 Thermal Requirements
Composite 4m mirror seeing estimateRacine [1991] used for natural convection; Zago [1995] used for mixed convection;
Gilbert et al. [1993] used for forced convection
0.00
0.05
0.10
0.15
0.20
0 1 2 3 4 5 6 7 8
V (m/s)
mirr
or s
eein
g (a
rcse
c)
0.2 K
0.5 K1.0 K
2.0 K5.0 K
GEMINI (0.2 K)
M1 error budget = 0.15 arcsec
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M1 Thermal Models
h1
h2
q"abs (t).
q"rad.
ρ1,k1,cp1,T1(t)
ρ2,k2,cp2,T2(t)
ρ,k,c,T(r,t)
Tr(t)
M1
cold plate
•1D finite-difference on spreadsheet•3D FEM package (TMG)
Would like to use simplest, yet stillphysically realistic model.
14
1D Model Validation
Validation Case 7: Frontside solar loading, backside radiative cooling, convection both sides; h = 5 W/m^2-K, h r = 4.49 W/m2-K
-5
0
5
10
15
20
0 6 12 18 24
t (hours)
T (
K)
1D, back side1D, middle1D, front side3D, front side, center
1D and 3D models agree to within 0.3 °C.
15
M1 surface-air temperature excess for 100 mm thick ULE
-1.5
-1.0
-0.5
0.0
0.5
1.0
0 3 6 9 12
t (hours)
∆T (
K)
V = 0V = 5 m/sV = 10 m/sV = 19.2 m/s
M1 Thermal Model Results
Input Profiles
-30
-25
-20
-15
-10
- 5
0
5
10
15
20
0 6 12 18 24
t (hours)
T -
Ti (
K)
0
50
100
150
abso
rbed
so
lar
load
(W
/m2)
u1 (K)u2 (K)uR (K)qabs (W/m^2)
100 mm thick ULE
Results: •< 1 K ∆T over most of the day.•Mirror flushing assists temperature control.
16
M1 Thermal Model Results II
80 mm thick ULEInput Profiles
-20
-10
0
10
20
0 6 12 18 24
t (hours)
T -
Ti (
K)
0
50
100
150ab
sorb
ed s
olar
load
(W
/m2)
u1 (K)u2 (K)qabs (W/m^2)
Result: < 1 K ∆T over most of the dayEasier cooling than 100 mm case
M1 surface temperature excess, 80 mm thick ULE
-1.5
-1.0
-0.5
0.0
0.5
1.0
0 3 6 9 12
t (hours)
∆T (
K)
V = 0V = 5 m/sV = 10 m/sV = 19.2 m/s
17
M1 surface temperature excess, 200 mm thick ULE
-1.5
-1.0
-0.5
0.0
0.5
1.0
0 3 6 9 12
t (hours)
∆T (
K)
V = 0V = 5 m/sV = 10 m/sV = 19.2 m/s
M1 Thermal Model Results III
200 mm thick ULEInput Profiles
-50
-40
-30
-20
-10
0
10
20
0 6 12 18 24
t (hours)
T -
Ti (
K)
0
50
100
150
abso
rbed
sol
ar lo
ad (
W/m
2)
u1 (K)u2 (K)qabs (W/m^2)
Result: < 1 K ∆T over most of the day.Cooling more difficult.
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M1 Summary
• Surface temperature requirement is a strong function of wind speed:
– ∆T < 0.5 K for V < 0.5 m/s
– ∆T < 1 K for 0.5 < V < 2 m/s
– ∆T < 2 K for V > 2 m/s
• ∆T < 1 K is achievable with 80 – 200 mm thick ULE mirrors
• Cooling difficulty increases with M1 thickness
• Wind flushing assists M1 temperature control
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Enclosure Seeing
wind
ground layer uplift effect
natural convectionfrom heated shell
turbulentboundary
layer
hot air plume
turbulent shear layer
natural convectionfrom dome floor
apertureedge
vorticity
Ventilated Dome
breeze
internal b.l.'s
Variety of sources:•Shell•Ground layer•Internal nat. conv.•Shutter plume•Shear layer•Aperture edges
Passive ventilationreduces internal seeing sources
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Other Concepts
wind
natural convectionfrom heated shell
turbulentboundary
layer
ground layer uplift effect
wind
ground layer uplift effect
apertureedge
vorticity
natural convectionfrom heated shell
hot air plume
turbulent shear layer
internalrecirculation
natural convectionfrom dome floor
turbulentboundary
layer
Retractable:•Eliminates many seeing sources•Still have shell seeing (reduced)
CLEAR concept:•Reduction in heated shell area•Snout may extend past shell layer•Internal ventilation more difficult
21
Shell Seeing
Convection Regimes for D = 20m, T = 270K
0
1
2
3
4
5
0 1 2 3 4
V (m/s)
∆T
(K)
Nat
ural
Con
vect
ion
Mixed Convection
Fr = 10
Fr =
1.0
Fr =
0.1
Forced Convection
Thin layers = good seeing.We want to be in forced convection:
low ∆T, high V. Dome shells veryoften in mixed convection.
GrRe
gLV
Fr22
=∆
≡ρ
ρ
22
Shell Seeing Requirement
0.00
0.10
0.20
0.30
0.40
0.50
0 1 2 3 4 5 6 7 8
V (m/s)
shel
l see
ing
(arc
sec)
0.2 K0.5 K
1.0 K
2.0 K
1.0 K (Ford)
2.0 K (Ford)0.5 K (Racine)
1.0 K (Racine)
2.0 K (Racine)
shell seeing budget 0.15 arcsec
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Enclosure Summary
• Major trade in process
• All concepts suffer from shell seeing
• Wide spread in shell temperature requirement:
1 – 8 K ∆T for 0.15 arcsec, depending on source
• Guess: ~2 K sun-facing ∆T will be adequate (must refine)
• Must provide for internal flushing
• Performance: future work, must model/measure
24
Future Work (to CoDR)
• For every surface:
– Refine requirements
– Define thermal concepts
– Model/measure to predict performance
– Define interfaces/trades
• Complete major trades, e.g.
– Enclosure style
– M1, M2, etc thickness/material/thermal control
25
Discussion