Enclosure Thermal Control 25 August 2003 ATST CoDR Dr. Nathan Dalrymple Air Force Research Laboratory Space Vehicles Directorate

Enclosure Thermal Control Function: Suppress seeing If a surface is the same temperature as the surrounding air, that surface introduces no seeing Seeing is caused by temperature differences

Requirements 1.Suppress enclosure seeing a.Racine experiment:  = 0.15  T i - T e ) 1.2 b.Ford analysis:  =  T s - T e  1.2 c.IR HB aerodynamic analysis:  =  T  V,  d.Bottom line: requirements on surface-air  T, interior- exterior  T, and wind flushing 2.Provide passive interior flushing to equalize interior and exterior temperatures and to suppress structure and mirror seeing Ref: Racine, Rene, “Mirror, dome, and natural seeing at CFHT,” PASP, v. 103, p. 1020, 1991.

Error Budgets (nm) Exterior budgetInterior budget nm10 nm arcsec0.02 arcsec arcsec0.025 arcsec

IR Handbook Seeing Analysis Given layer thickness and  T, we can estimate . Wavefront variance Gladstone-Dale parameter Fluctuating densityLine-of-sight correlation length Layer thickness Phase variance Surface-air temperature difference Blur angle Strong/weak cutoff ~ 2 rad Ref: Gilbert, Keith G., Otten, L. John, Rose, William C., “Aerodynamic Effects” in The Infrared and Electro- Optical Systems Handbook, v. 2, Frederick G. Smith, Ed., SPIE Optical Engineering Press, 1993.

IR Handbook Seeing Analysis (cont.) Layer thickness (mks units): L: upstream heated length (m)  T: average temperature difference over upstream length (˚C) V: wind speed (m/s) Buoyancy termHydrodynamic term Assume: If  T < 0 then buoyancy term does not contribute to layer thickness.

Shell Seeing, Diffraction- Limited Error Budget Blue contours: rms wavefront error (nm) Acceptable operating range, assuming no AO correction. AO correction will extend the “green” area. = 500 nm

Shell Seeing, Seeing-Limited Error Budget Blue contours: 50% encircled energy (arcsec) Acceptable operating range = 1600 nm

Shell Seeing, Coronal Error Budget Blue contours: 50% encircled energy (arcsec) Acceptable operating range = 1000 nm

Dome Seeing (Inside/Outside Air  T) Correlation by Racine (1991) Approximate error budget Approximate  T requirement Need lots of passive flushing! Ref: Racine, Rene, “Mirror, dome, and natural seeing at CFHT,” PASP, v. 103, p. 1020, 1991.

IR Handbook aerodynamic treatment Correlation of Racine (1991) IR Handbook aerodynamic treatment Good seeing from KE test Ref: Racine, Rene, “Mirror, dome, and natural seeing at CFHT,” PASP, v. 103, p. 1020, BBSO Dome Seeing Experiments

Bad seeing from KE test BBSO Dome Seeing Experiments

A Nighttime Comparison: Gemini Dome 1 Duct exhaust fan on, low-moderate wind (3 - 5 m/s)  T = -3 ˚C Acceptable seeing observed with shell subcooled by 3 ˚C.

Bottom Line Requirements Enclosure skin temperature needs to be subcooled by up to 3 ˚C Interior air temperature needs to be within 0.5 ˚C of ambient outside air Need large passive flowrate to flush interior

Skin Energy Balance We want to use this term to control the skin temperature [~0 W/m 2 ] [377 W/m 2 ] [374 W/m 2 ] [98 W/m 2 ] [~100 W/m 2 ] Quantities vary by location on dome and weather conditions

Skin Thermal Control System Concept Concept Features: 1.White oxide paint a.Large  b.Small  s 2.Chilled skin a.Air b.Liquid (EGW) 3.Insulation prevents interior from being chilled by skin coolant

Shutter: air cooled, optional water cooling on lower end h air ~ 8 W/m 2 -K h H2O ~ 100 W/m 2 -K Enclosure support wall: water cooled if present h H2O ~ 100 W/m 2 -K Oblique skin panels: air cooled, h ~ 5 W/m 2 -K Sun-facing skin panels: air or water cooled h air ~ 5 W/m 2 -K h H2O ~ 100 W/m 2 -K Option: use fins on skin underside to increase effective area Skin Thermal Control System Concept (cont.)

Skin Cooling System Flow Loop Insert diagram here

MuSES Modeling: Validation at Gemini North Validation

Skin Thermal Control System Performance MuSES snapshot at 1430LT, 30 April 2003, Mauna Kea Wind speed = 0.5 m/s Ambient air T e = 7 – 8 ˚C Air Cooling Only on Skin ESW Water Cooled Most of surface is acceptable Sun-facing areas are ~ 5 ˚C hotter than ambient Surfaces that see cold sky subcool

MuSES snapshot at 1430LT, 30 April 2003, Mauna Kea Wind speed = 0.5 m/s Ambient air T e = 7 – 8 ˚C Air & Water Cooling Nearly all of surface is acceptably cool Sun-facing areas cooled with water Surfaces that see cold sky subcool Skin Thermal Control System Performance (cont.)

Cooling Requirements Next steps: Fan and system curves Heat exchanger specs Chiller specs Time response of fluid volume At peak heat load, surface cooling requires: Air-cooled skin: 56 kW Water-cooled skin: 18 kW Lower shutter: 14 kW Air-cooled shutter: 18 kW Total for carousel: 106 kW Enclosure support wall: 104 kW Grand total: 210 kW (60 tons)

Flushing System Concept 42 vent gates 168 m 2 flow area, each side

Flushing System Performance

Active Interior Ventilation Gemini volume flowrate: 10 enclosure volumes/hour (150,000 m 3 /hr) This flowrate on the smaller hybrid gives V ~ 0.2 m/s average Directed flow can give V~0.5 – 1 m/s over much of structure Fans may be mounted remotely or on carousel

Active Ventilation Issues Fan blades heat air  seeing Require homogenizing screens, cooling coils downstream of fans May not be simple to mount all this on carousel  possible to mount remotely

Shell Seeing Performance Blue contours: rms wavefront error (nm) Red: average  T of skin, front skin, shutter, lower shutter, ESW Most of the dome surface will give acceptable seeing Back of shutter subcools. May need to add water cooling there as well.