PowerPoint Presentation by 2i48203


									     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
               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              SIRTF CTA (40 K)
   –   Validity (see next chart)
                                      Helium-cooled shroud (4 K)
   –   Feasibility                       (painted honeycomb)
   –   Cost                                Nitrogen-cooled coldwall (80 K)
   –   Time                                        Vacuum chamber walls (293 K)
                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                     Goddard
                                                     Paint Data
   – Heat balance unpredictable
• Thermal balance in 4 K                          Extrapolated      From
  shroud not meaningful                                             model
   – Omit 4 K shroud
   – Cool CTA with direct liquid
     helium lines
   – Make do with questionable
     thermal balance                   What’s wrong with this picture?
                    Example: Revised Solution
• Helium-cooled shroud gives meaningful test
1. 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
2. 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             98%

3. 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         99.5%
   – Concentric spheres: RadK12 = A1/[1/e1 + (A1/A2)(1/e2 – 1)
                                               1/             1/
                                                    2              10
          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 = 5.6697 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
               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
                                                     • At low temperatures,
                                                       power spreads over
                                                       wider range of

                                      98% of power
     Absorptance = Emittance: Kirchhoff’s Law
• Absorptance = emittance, if the same…
  – Surface
  – Temperature                                                      a + r = 1 (opaque)
  – 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
                        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
                   Emittance of Non-Conductors
•    For non-metals, el and al is essentially independent of temperature
•    2-step absorption process
    1.   Surface reflectance depends on index of refraction
            Reflectance = [( - 1)/( + 1)]2                   (normal)
             = index of refraction = 1/relative light speed ≈ [dielectric constant]½     1
    2.   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
Spectral Emittance of a Paint
                        • Emittance/absorptance
                          at a given wavelength
                          doesn’t vary with
                        • 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
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
                       Honeycomb Blackbodies
• Open, painted honeycomb cells increase emittance or absorptance
   – Cavity offers several chances for absorptance
   – Each cavity approximates a blackbody                       bounces in a
   – Absorptance still equals emittance                         honeycomb
                                                                  hex cell

• 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:          0.9773125 K, 0.929180 K
   – Specular paint:         0.9984125 K, 0.985780 K   Simplified model
                                                       • Same hemispherical emittance
                                                       • 100% diffuse vs 100% specular
       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
               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 …
    1.   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
    2.   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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