Effective Emissivity of Nonisothermal Isogrid for Spacecraft Radiator Applications - PowerPoint Presentation

1. Effective Emissivity of Nonisothermal Isogrid for Spacecraft Radiator ApplicationsSeth AbramczykNovember 19th, 2021

2. AbstractAccurate knowledge of the optical properties of spacecraft components, especially external components, is critical for proper spacecraft thermal design. The effective emissivity of isogrid (an array of equilateral triangular cavities) is not well understood, which poses a challenge for spacecraft thermal management. In this thesis the effective emissivity of isogrid with a prescribed base temperature and nonisothermal walls is examined. The temperature profile of the cavity’s walls and the overall effective emissivity of the cavity are found using Thermal Desktop with Monte Carlo ray tracing. The effective emissivity’s dependence on the wall height, wall thickness, wall resistance, and surface emissivity are examined. The existence of a critical wall height, and the contributing factors to this critical height, are discussed. Comparisons between isogrid and cavities with different base geometries are made. A variable emissivity isogrid spacecraft radiator concept employing the cavity effect is proposed, and mechanisms for achieving this are discussed. 11/19/20212

3. ContentsAbout meBackgroundEffective-ε and Cavity effectSpacecraft Thermal Control OverviewSpacecraft RadiatorsRoman Space TelescopeMethod Thermal Desktop, RadCAD, and Finite Difference MethodProblem DescriptionModel DescriptionModel ParametersResults/Conclusion View FactorsTemperature Profile & Gradient vs Wall HeightCritical Wall HeightIsothermal vs Nonisothermal WallsWall NodalizationWall ThicknessWall ResistanceEdge EffectsIsogrid vs Other GeometriesVariable-e Isogrid Radiator ConceptSummaryFuture Work 11/19/20213

4. About meEducationM.S. Mechanical Engineering – Expected 2021University of Maryland Baltimore CountyB.S. Mechanical Engineering – 2019The University of Texas at DallasNASA Goddard Space Flight Center Pathways Co-op (2018 – Present)Thermal Engineering Branch (2019 – Present)Roman Space Telescope (2019 - Present)SNOOPI & Dione CubeSats (2020 - Present)MOMA (2019-2020)Operations & Maintenance Branch (2018)11/19/20214

5. Radiant emissionEmissivity ()Ratio of radiant emission compared to that of a blackbody, The Cavity EffectEffective emissivity () Emissivity from a cavity compared to a blackbody with the same cavity opening area Effective Emissivity & The Cavity Effect11/19/20215

6. Spacecraft Thermal Control OverviewThermal design goal of is to keep all components within temperature limits during worst case extreme hot & cold casesDesign for hot case and cold case extremesAll components have hot & cold temperature limitsSpacecraft steady-state energy balanceHeat is transported from dissipating components (electronics boxes, instruments, etc.) to radiators which reject the waste heat to space 11/19/20216Internal generationDirect SolarAlbedo (reflected solar)Earth IRRejected to Space

7. Spacecraft Radiator SizingHot case: Radiators sized large enough to reject all waste heat without breaking hot temperature limitCold case: Heaters used to prevent breaking the cold temperature limitRadiator Sizing ExampleElectronics box on radiator dissipation varies 10 to 50 ; radiator temperature limits of -10°C to 40°C; ; no environmental backloading; assume Area for 50 box dissipation at 40°C: 0.102 Heater power to for 10box dissipation at -10°C: 14.9 11/19/20217

8. Variable Emissivity RadiatorsThermal control device which vary emissivity with temperature and are only variables that can be variedLouversLow-ε vanes cover a high-ε radiator surfaceVanes open/close via bi-metallic strips, covering and exposing high-ε surfaceLow temperatureVanes closed, low , less heat dissipatedHigh temperatureVanes open, high , more heat dissipatedAble to maintain a setpoint temperature by tuning of bi-metallic strips 11/19/20218D. G. Gilmore et al., Spacecraft Thermal Control Handbook Volume I: Fundamental Technologies, 2nd ed. The Aerospace Press, 2002.

9. Variable Emissivity Radiator Sizing ExampleSame problem as before, but varies from 0.5 - 0.9Area for 50 box dissipation at 40°C and is same as before: Heater power to for 10box dissipation at -10°C and =0.5: 11.3 W (75.8%) heater power savings from using variable emissivity radiator 11/19/20219Typical RadiatorVariable-ε RadiatorDifferenceEmissivity @ 40°C0.90.90.0Emissivity @ -10°C0.90.50.4Area0.102 0.102 0 Heater Power14.9 3.6 11.3 Typical RadiatorVariable-ε RadiatorDifferenceEmissivity @ 40°C0.90.90.0Emissivity @ -10°C0.90.50.4AreaHeater Power

10. Roman Space Telescope OverviewLaunching mid-2020’s, RST will explore:Dark EnergyUnknown energy causing the universe to expand at an increasing rate. Dark energy makes up 68% of the total energy in the cosmos.Dark MatterUnknown matter that does not interact with normal matterRST will look at weak gravitational lensing caused by dark matterExoplanetsPlanets orbiting other starsWill be detected by RST via microlensing, transiting, and possibly direct imagingCompared to Hubble Space TelescopeSame resolution with 100x the field of view“Over the first five years of observations, Roman will image over 50 times as much sky as Hubble covered in its first 30 years.”11/19/202110NASA, “Nancy Grace Roman Space Telescope,” NASA Goddard Space Flight Center. https://roman.gsfc.nasa.gov/observatory.html.H. L. Peabody, “Tracking Critical Thermal Metrics throughout the Life Cycle of a Large Observatory Thermal Model,” 2020.

11. Thermal Desktop & RadCADThermal DesktopSoftware package made specifically for spacecraft thermal analysisRuns as an application within AutoCADBuilds thermal R-C network and passes to SINDA to solve. Displays and analyses SINDA results.Calculates radiation exchange factors via Monte Carlo ray tracing (RadCAD)RadCADTD’s built-in Monte Carlo ray tracerUsed to calculate radiation exchange factors between surfacesThousands of rays shot from each node in random directions. Deposits energy onto each surface it hits based on the receiving surface’s properties. Each ray is propagated throughout the model until it reaches space or is fully absorbed.11/19/202111T. D. Panczak, S. G. Ring, M. J. Welch, D. Johnson, B. A. Cullimore, and D. P. Bell, Thermal Desktop User’s Manual, 6.0. C&R Technologies, Inc., 2017.

12. Finite Difference MethodGeneral steady state 3D heat diffusion equation without internal heat generationFinite Difference FormulationAssuming only 1 element in the thickness () direction 11/19/202112F. P. Incropera, D. P. Dewitt, T. L. Bergman, and A. S. Lavine, Fundamentals of Heat and Mass Transfer, 6th ed. John Wiley & Sons, 2007.

13. Problem DescriptionIsogridArray of equilateral triangular cavitiesAssumptionsSingle cavity within an arbitrarily large isogrid arrayArray is in space with no radiative backloading from sun, earth, etc.System is at steady stateBase of array is isothermal at prescribed temperature Cavity walls are isothermal through their thicknessMaterial & optical properties independent of temperature 11/19/202113

14. Thermal Desktop Isogrid ModelMade from thin finite difference elements with edge nodesBase of array given boundary node at Array is 22x11 cellsEdge cells in at position are “half cells” (right triangles) 11/19/202114

15. Model ParametersWall Height ()Non-dimensionalis used throughout this thesis because it corresponds to when Wall Thickness ()Non-dimensionalCharacteristic Wall Resistance () 11/19/202115

16. View Factor Across Surface 1 11/19/202116    For η=3.46View factors found with RadCADView factors are highly variable across surface 1 At any given point: 

17. Temperature Profile Across Surface 111/19/202117Highest temperature along At base temperature Lowest temperature at ,  For =0.7, =3.46, =0.010 xz0LH

18. Temperature Gradient vs Wall Height11/19/202118Larger temperature gradient with increased wall height () 

19. Characteristic Regions of vs Wall Height 11/19/202119: Surface emissivity: Maximum effective emissivity: Critical wall height for which : Equivalent wall height for which Analogous to critical insulation thickness for a pipe Critical Wall Height

20. Contributing Factors to Critical Wall Height11/19/202120At (wall height = 0) entire contribution is from baseAs increasesdecreasesincreases, reaches a maximum, and begins to decrease 

21. vs Wall Height Results (1/2) 11/19/202121Larger leads to Larger Larger and Because  0.0100.900.952.776.670.700.884.1618.730.500.795.54>200.300.686.93>200.100.4713.86>200.0100.900.952.776.670.700.884.1618.730.500.795.54>200.300.686.93>200.100.4713.86>20

22. vs Wall Height Results (2/2) 11/19/202122Lower → larger relative increase in Lower has more “room to grow” until it reaches the limit still lower for smaller  

23. Isothermal vs Nonisothermal Cavity Walls11/19/202123Isothermal walls at vs nonisothermal wallsIsothermal ≡ nonisothermal for small IsothermalAs , NonisothermalCritical wall height phenomenonAs ,  

24. Varying Wall Nodalization11/19/202124Varied wall nodalization from 5x3, 5x5, 5x7, and 5x9 nodesIncreased Z nodalization better captures nonlinear gradientsMore accurate gradients → more accurate Most plots shown for because good agreement up to that point 

25. Varying Wall Thickness11/19/202125Increased thickness → lower resistance → smaller gradients (more isothermal) → higher  

26. Varying Wall Resistance11/19/202126Plot of vs looks like vs Because  

27. Edge Effects11/19/2021  Slight increase in for edge cavities at but not Because cavities at are right triangles, not equilateralHave a different value for the same heightHigher → higher  27

28. Isogrid vs Other Geometries (1/2)11/19/202128Compared single cell of different geometriesFor small No difference in because For larger  

29. Isogrid vs Other Geometries (2/2)11/19/202129Triangle has largest perimeter/area ratioFor a given Larger P/A → smaller → lower → larger  Cavities shown at  

30. Variable Emissivity Isogrid Radiator Concept11/19/202130Variable-ε radiator using the cavity effect“Propeller” design 3 blades around central mastMast raises/lowers with temperature

31. SummaryMost other studiesCylindrical cavitySingle cavityIsothermal wallsIf nonisothermal, then prescribed temperature profileThis thesisIsogrid cavityArbitrarily large array of cavitiesNonisothermal wallsSolved for both temperature profile and effective emissivityParameters impacting effective emissivityWall heightSurface emissivityWall thicknessWall resistivityCritical wall height peaks at some critical wall height As , Proposed variable-ε isogrid radiator concept 11/19/202131

32. Future WorkVerify results in thermal-vacuum chamberInvestigate Need finer nodalization on walls to capture large gradientsIsogrid on curved surfacesBoth concave and convexVery common in the real-worldEffect of varying base temperatureIf → smaller gradients on wall → higher ?If → larger gradients on wall → lower ?Directionality of emission from cavity openingCavity is likely not a diffuse emitter due to temperature gradients on the wall, even if each surface it itself diffuse 11/19/202132

33. AcknowledgementsUMBCDr. Ruey-Hung ChenConnie BaileyNASA GSFCDr. Vivek DwivediJohn HawkRob ChalmersRoman Space Telescope projectCommittee MembersDr. Ruey-Hung ChenDr. Ronghui MaDr. Liang ZhuDr. Vivek Dwivedi11/19/202133

34. Backup Slides

35. Spacecraft Thermal Control Coatings11/19/202135D. G. Gilmore et al., Spacecraft Thermal Control Handbook Volume I: Fundamental Technologies, 2nd ed. The Aerospace Press, 2002. = IR emissivity = Solar absorptivity  

36. Temperature Profile Across Surface 111/19/202136