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Emittance & Absorptance for Cryo Testing

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Presentation on theme: "Emittance & Absorptance for Cryo Testing"— Presentation transcript:

1 Emittance & Absorptance for Cryo Testing
Goal: To better understand emittance and absorptance and how they vary at cryo temperatures Sample problem Emittance & absorptance of non-conductors Effects of wavelength (spectral dependencies and trends) Effects of low temperatures Effects of thickness (paints and films) Honeycomb enhancements

2 Example: SIRTF Thermal Testing
SIRTF Cryo Telescope Assembly On orbit, CTA passively cooled to 40 K by radiation to space 40 K well below typical LN2-cooled thermal-vac chambers at 80 K Initial plans for thermal balance test Simulate space environment Add helium-cooled shroud inside existing LN2-cooled thermal-vac chamber Helium-cooled shroud at 4 K Painted honeycomb on shroud for absorptance close to 1.0 Concerns about test Validity (see next chart) Feasibility Cost Time SIRTF CTA (40 K) Helium-cooled shroud (4 K) (painted honeycomb) Nitrogen-cooled coldwall (80 K) Vacuum chamber walls (293 K)

3 Example: Basis for Conclusion
Emittance of paint at 4 K hard to predict At 40 K, epaint = 0.70 ± 0.15 uncertainty (Goddard data) No data at 4 K, but emittance much lower (at 0 K, emittance  0.00) Even with painted honeycomb shroud, emittance at 4 K could be < 0.50 Test vs space: too different Tsink = 4 K vs 2.7 K (OK) esink = 0.48 vs 1.0 (not OK) Heat reflected back to CTA CTA won’t get cold enough Gradients won’t be realistic Heat balance unpredictable Thermal balance in 4 K shroud not meaningful Omit 4 K shroud Cool CTA with direct liquid helium lines Make do with questionable thermal balance Goddard Paint Data Extrapolated From model What’s wrong with this picture?

4 Example: Revised Solution
Helium-cooled shroud gives meaningful test Absorptivity of the paint is relative to 40 K, not 4 K Paint’s absorptivity depends on wavelength distribution of incident radiation Paint’s absorptivity at a given wavelength is independent of paint’s temperature Effective absorptance = emittance of paint at 40 K = 0.70 At 40 K, absorptance of painted honeycomb can be > 0.90 Some variation with paint thickness and paint process Some variation with cell size and honeycomb thickness Use specular paint Calorimeter uncertainties increase at cryo temperatures Helium-cooled shroud could mimic space to within 1% Grow shroud from 2X to 10X the area of SIRTF CTA Additional cost for liquid helium to cool larger shroud Concentric spheres: RadK12 = A1/[1/e1 + (A1/A2)(1/e2 – 1) 98% 99.5% 1/2 1/10

5 Spectral Intensity of a Blackbody
Planck’s Radiation Law I(l,T) = (2phc2/l5)/(ehc/lkT – 1) Flux (Qbb) = area under curve Qbb,T = sT4 s = X 10-8 W/m2-K4 Curves have similar shapes Imax is proportional to T5 lmax is proportional to 1/T 0.004 inches lmax & Imax

6 Spectral Intensity: Log Plot
lT = 1148m-K lmaxT = 2897m-K lT = 22917m-K Everything shifts proportional to 1/T Max power occurs at longer wavelengths at lower temperatures Curve for a lower temperature is less than curve for a higher temperature at all wavelengths At low temperatures, power spreads over wider range of wavelengths 98% of power

7 Absorptance = Emittance: Kirchhoff’s Law
Absorptance = emittance, if the same… Surface Temperature Wavelength Angle of incidence al,T,q,f= el,T,q,f (rest of presentation omits effects of angle of incidence) Total absorptance = total emittance at the same temperature Emittance Total hemispherical emittance Surface at the given temperature Absorptance Surface is at the given temperature Surface is surrounded by blackbody at the same temperature Must be true, else violates the 2nd Law of Thermodynamics a + r + t = 1 a + r = 1 (opaque)

8 Conclusions So Far Emittance varies with wavelength for real surfaces
Some surfaces have a fairly constant emittance over a range of wavelengths Emittance at a given wavelength can also change with temperature The blackbody intensity changes non-linearly with temperature Increases with temperature to the 4th power At lower temperatures, the distribution shifts towards longer wavelengths At lower temperatures, the power spreads out more Therefore, effective emittance changes with temperature, if… Emittance varies with wavelength, or if… Emittance at a given wavelength changes with temperature For the range of wavelengths of importance at the given temperature

9 Emittance of Non-Conductors
For non-metals, el and al is essentially independent of temperature 2-step absorption process Surface reflectance depends on index of refraction Reflectance = [( - 1)/( + 1)]2 (normal)  = index of refraction = 1/relative light speed ≈ [dielectric constant]½ Volumetric absorptance sometimes limited by thickness Dielectrics are partially transparent Absorptance within material increases with thickness: a = 1 – e-kx Free-standing film, or backed by metal layer No significant difference beyond certain thickness (1 to 10 mils typically) At low temperatures, emittance of paints and films decreases Energy shifts to longer wavelengths When wavelengths exceed thickness, paint or film becomes more transparent No decrease for non-conductive substrate—if thick enough Surfaces becomes more specular at low temperatures As more wavelengths exceed roughness of surface and substrate 1 2

10 Spectral Emittance of a Paint
Emittance/absorptance at a given wavelength doesn’t vary with temperature Total emittance may vary with temperature as the range of wavelengths shifts Changing temperature of emitting source may shift the absorptance of an absorbing surface Changing temperature of absorbing surface does not change its absorptance

11 Emittance of Non-Conductors: Films
For non-conductors, radiation transfer is more of a volumetric phenomenon Many thin films are partially transparent Absorptance (and emittance) varies exponentially vs thickness Films are volume-limited At low temperatures, wavelengths are longer and films are more transparent Different paints or films show a decrease in emittance at different temperatures Emittance of FEP Teflon films drops off at higher temperatures than most films or paints Paints or OSRs are better on cryo radiators Painted honeycomb gives highest emittance If material is thick enough, emittance stays constant to much lower temperature Emittance of 35-mil fused silica constant from 25 K to 300 K

12 Honeycomb Blackbodies
Open, painted honeycomb cells increase emittance or absorptance Cavity offers several chances for absorptance Each cavity approximates a blackbody Absorptance still equals emittance Not too sensitive to honeycomb geometry Aspect ratio: cell width versus cell height Aluminum honeycomb minimizes DT to base At cryo temperatures, DT not a factor Obtaining uniform paint may be driver Recommend larger cell honeycomb Allows thicker paint Paint process less critical Specular paint increases effective emittance Diffuse paint: K, K Specular paint: K, K Multiple bounces in a honeycomb hex cell Simplified model Same hemispherical emittance 100% diffuse vs 100% specular

13 Percent Power vs Wavelength for Cryo
40 mils 1% of power at l less than 1448/T Maximum power at l of 2897/T Also 25%/75% split 50% of power either side of l of 7393/T 99% of power at l less than 22,917/T 4 mils Typical paint thickness = 2 to 8 mils Paint have reduced emittance when wavelengths exceed thickness ¼” (honeycomb cell) = 6,350 microns Cell size well beyond significant wavelength effects

14 Conclusions / Recommendations
For radiation between hot and cold surfaces, the hot surface dominates Temperature of hot emitter determines cold non-conductor’s absorptance Absorptance depends on distribution of incident wavelengths Most of the incident radiation originates at the hot surface For non-conductors, al does not vary with temperature Total emittance varies with temperature if … Emittance varies with wavelength For paints and films, emittance drops off at longer wavelengths (cryo temperatures) Thicker substrates of non-conductors will not show this effect Emittance at a given wavelength varies with temperature Typical non-conductors do not show such an effect Thicker paint has higher absorptance at low temperatures Use specular paints for honeycomb or multi-bounce blackbodies

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