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 temperaturesSample problemEmittance & absorptance of non-conductorsEffects of wavelength (spectral dependencies and trends)Effects of low temperaturesEffects of thickness (paints and films)Honeycomb enhancements
2 Example: SIRTF Thermal Testing SIRTF Cryo Telescope AssemblyOn orbit, CTA passively cooled to 40 K by radiation to space40 K well below typical LN2-cooled thermal-vac chambers at 80 KInitial plans for thermal balance testSimulate space environmentAdd helium-cooled shroud inside existing LN2-cooled thermal-vac chamberHelium-cooled shroud at 4 KPainted honeycomb on shroud for absorptance close to 1.0Concerns about testValidity (see next chart)FeasibilityCostTimeSIRTF 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 predictAt 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 < 0.50Test vs space: too differentTsink = 4 K vs 2.7 K (OK)esink = 0.48 vs 1.0 (not OK)Heat reflected back to CTACTA won’t get cold enoughGradients won’t be realisticHeat balance unpredictableThermal balance in 4 K shroud not meaningfulOmit 4 K shroudCool CTA with direct liquid helium linesMake do with questionable thermal balanceGoddard Paint DataExtrapolatedFrom modelWhat’s wrong with this picture?
4 Example: Revised Solution Helium-cooled shroud gives meaningful testAbsorptivity of the paint is relative to 40 K, not 4 KPaint’s absorptivity depends on wavelength distribution of incident radiationPaint’s absorptivity at a given wavelength is independent of paint’s temperatureEffective absorptance = emittance of paint at 40 K = 0.70At 40 K, absorptance of painted honeycomb can be > 0.90Some variation with paint thickness and paint processSome variation with cell size and honeycomb thicknessUse specular paintCalorimeter uncertainties increase at cryo temperaturesHelium-cooled shroud could mimic space to within 1%Grow shroud from 2X to 10X the area of SIRTF CTAAdditional cost for liquid helium to cool larger shroudConcentric spheres: RadK12 = A1/[1/e1 + (A1/A2)(1/e2 – 1)98%99.5%1/21/10
5 Spectral Intensity of a Blackbody Planck’s Radiation LawI(l,T) = (2phc2/l5)/(ehc/lkT – 1)Flux (Qbb) = area under curveQbb,T = sT4s = X 10-8 W/m2-K4Curves have similar shapesImax is proportional to T5lmax is proportional to 1/T0.004 incheslmax & Imax
6 Spectral Intensity: Log Plot lT = 1148m-KlmaxT = 2897m-KlT = 22917m-KEverything shifts proportional to 1/TMax power occurs at longer wavelengths at lower temperaturesCurve for a lower temperature is less than curve for a higher temperature at all wavelengthsAt low temperatures, power spreads over wider range of wavelengths98% of power
7 Absorptance = Emittance: Kirchhoff’s Law Absorptance = emittance, if the same…SurfaceTemperatureWavelengthAngle of incidenceal,T,q,f= el,T,q,f (rest of presentation omits effects of angle of incidence)Total absorptance = total emittance at the same temperatureEmittanceTotal hemispherical emittanceSurface at the given temperatureAbsorptanceSurface is at the given temperatureSurface is surrounded by blackbody at the same temperatureMust be true, else violates the 2nd Law of Thermodynamicsa + r + t = 1a + 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 wavelengthsEmittance at a given wavelength can also change with temperatureThe blackbody intensity changes non-linearly with temperatureIncreases with temperature to the 4th powerAt lower temperatures, the distribution shifts towards longer wavelengthsAt lower temperatures, the power spreads out moreTherefore, effective emittance changes with temperature, if…Emittance varies with wavelength, or if…Emittance at a given wavelength changes with temperatureFor 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 temperature2-step absorption processSurface reflectance depends on index of refractionReflectance = [( - 1)/( + 1)]2 (normal) = index of refraction = 1/relative light speed ≈ [dielectric constant]½Volumetric absorptance sometimes limited by thicknessDielectrics are partially transparentAbsorptance within material increases with thickness: a = 1 – e-kxFree-standing film, or backed by metal layerNo significant difference beyond certain thickness (1 to 10 mils typically)At low temperatures, emittance of paints and films decreasesEnergy shifts to longer wavelengthsWhen wavelengths exceed thickness, paint or film becomes more transparentNo decrease for non-conductive substrate—if thick enoughSurfaces becomes more specular at low temperaturesAs more wavelengths exceed roughness of surface and substrate12
10 Spectral Emittance of a Paint Emittance/absorptance at a given wavelength doesn’t vary with temperatureTotal emittance may vary with temperature as the range of wavelengths shiftsChanging temperature of emitting source may shift the absorptance of an absorbing surfaceChanging 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 phenomenonMany thin films are partially transparentAbsorptance (and emittance) varies exponentially vs thicknessFilms are volume-limitedAt low temperatures, wavelengths are longer and films are more transparentDifferent paints or films show a decrease in emittance at different temperaturesEmittance of FEP Teflon films drops off at higher temperatures than most films or paintsPaints or OSRs are better on cryo radiatorsPainted honeycomb gives highest emittanceIf material is thick enough, emittance stays constant to much lower temperatureEmittance of 35-mil fused silica constant from 25 K to 300 K
12 Honeycomb Blackbodies Open, painted honeycomb cells increase emittance or absorptanceCavity offers several chances for absorptanceEach cavity approximates a blackbodyAbsorptance still equals emittanceNot too sensitive to honeycomb geometryAspect ratio: cell width versus cell heightAluminum honeycomb minimizes DT to baseAt cryo temperatures, DT not a factorObtaining uniform paint may be driverRecommend larger cell honeycombAllows thicker paintPaint process less criticalSpecular paint increases effective emittanceDiffuse paint: K, KSpecular paint: K, KMultiple bounces in a honeycomb hex cellSimplified modelSame hemispherical emittance100% diffuse vs 100% specular
13 Percent Power vs Wavelength for Cryo 40 mils1% of power at l less than 1448/TMaximum power at l of 2897/TAlso 25%/75% split50% of power either side of l of 7393/T99% of power at l less than 22,917/T4 milsTypical paint thickness = 2 to 8 milsPaint have reduced emittance when wavelengths exceed thickness¼” (honeycomb cell) = 6,350 micronsCell size well beyond significant wavelength effects
14 Conclusions / Recommendations For radiation between hot and cold surfaces, the hot surface dominatesTemperature of hot emitter determines cold non-conductor’s absorptanceAbsorptance depends on distribution of incident wavelengthsMost of the incident radiation originates at the hot surfaceFor non-conductors, al does not vary with temperatureTotal emittance varies with temperature if …Emittance varies with wavelengthFor paints and films, emittance drops off at longer wavelengths (cryo temperatures)Thicker substrates of non-conductors will not show this effectEmittance at a given wavelength varies with temperatureTypical non-conductors do not show such an effectThicker paint has higher absorptance at low temperaturesUse specular paints for honeycomb or multi-bounce blackbodies
Your consent to our cookies if you continue to use this website.