Dome venting: the path to thermal balance and superior image quality

Transcription

Dome venting: the path to thermal balance and superior image quality
Steven E. Bauman*a, Tom Benedict, Marc Baril, Jean Charles Culiandrea & b, I. Look a,
Grant Matsushigea ,Larry Robertsa, Rene Racinec, Derrick Salmona ,Tom Vermuelena
a
Canada-France-Hawaii-Telescope Corporation, Kamuela, HI, USA 96743
b
CEA-Saclay, DSM/IRFU/SAp/LCEG, 91191 Gif-sur-Yvette, France
c
Département de physique, Université de Montréal, C.P. 6128, Montréal, QC, Canada H3C 3J7
ABSTRACT
The Canada France Hawaii Telescope operates a 3.6m Optical/Infrared telescope on the summit of Mauna Kea. As an
effort to improve delivered image quality in a cost-effective manner, a dome venting project was initiated to eliminate
local contributions to 'seeing' that exist along the optical path and arise to a large extent due to temperature gradients
throughout the dome volume.
The quality of images delivered by the telescope is adversely affected by variations in air temperature within the
telescope dome. Air temperature differences are caused by the air’s contact with large structures. They are different
from ambient as a result of their large thermal inertias and the consequent inability of these structures to follow rapid air
temperature changes.
The dome venting project is an effort to add a series of large openings, “vents”, in the skin of the dome with the purpose
of allowing free stream summit winds to flush out “stagnant air”. The term, “stagnant air”, applies to thermally mixed air
from the inside of the dome environment that, for one reason or another, has been heated or cooled by surfaces in the
dome environment.
The addition of vents to the CFHT dome is intended to facilitate the passive flushing of interior air by the local wind,
thereby greatly reducing air temperature variations, a process that has been successfully demonstrated to improve image
quality at other telescope facilities and supported by recent water tunnel tests conducted by CFHT staff.
Keywords: Dome venting, image quality, optical turbulence, stagnate air, thermal environment, delivered image, Dome
flushing, ventilation
1. INTRODUCTION
The dome vent system consists of twelve (12) identical vent units, six on either side of the dome slit shutter track
centered at angles of 71.25, 86.25, 101.25, 116.25, 131.25 and 146.25 degrees from the dome slit centerline between
vertical structural internal web members and an ice deflector mounted on the exterior dome surface immediately above
each vent. Each vent opening measures approximately 6.5 ft. (2m) wide by 18 ft. (5.5 m) high, each consisting of a
vertically-acting weather “rollup” main door and internal louver units to direct and control the flow of air entering the
vent openings.
The vent width fills the available horizontal separation between adjacent internal vertical structural web members. Vent
height was set by estimates of the total vent area desired from water tunnel testing and computational fluid dynamic
simulations. Once the needed vent size was determined, structural analysis was performed on the dome structure and
skin to verify stability and verify structural integrity of the dome.
The dome vent unit doors use a simple Somfy electric motor and interconnecting slats to form a “mat” door. Position and
feedback are communicated via proximity sensors at various locations, closed, open, 1/3 open and 2/3 open. The door
motor is controlled using an Allen Bradley Micrologix PLC and PBC IEC electrical contactors.
The louver system uses a smaller Somfy electric motor with built in RS-485 communication protocol which allows for
one degree angle adjustment of motor blade configurations. In house software was designed by CFHT’s Tom Vermulen
and installed onto a Raspberry Pi unit which provides the control and feedback of the individual louver systems. The
door motor is controlled using a Micrologix PLC and feedback from the proximity sensors. In house low level software,
communications, and wiring were developed by CFHT’s Grant Matsushige.
The vent unit project was completed with help from M3Engineering; for structural design and analysis work, CAID
Industries for fabrication, assembly, and shipment of the vent units; Nexus steel for steel work and vent erection; Tucson
rolling shutters for door, sensors and louver systems; and Steel Tech for onsite equipment and logistics.
2. BACKGROUND & JUSTIFICATION
2.1 Dome venting – image quality studies
The underlying justification for needed dome venting comes from a number of sources, but is primarily based on the
now well established fact that image quality delivered by MegaCam was considerably poorer than our site on Mauna
Kea actually offers. Localized air temperature differences are known to arise from a variety of sources: in-dome
electrical power consumption, an overly warm or cold observing floor, radiative cooling of the exterior dome skin
leading to skin / interior air temperature gradients and inner shell support structure cooling, and finally and perhaps most
fundamentally, the lag experienced by the massive components in the telescope including the primary mirror and their
ability to keep up with rapid air temperature changes.
Figure 1. Graphs showing the effect of dome air temperature differences on image quality
*[email protected]; www.cfht.hawaii.edu/~bauman, [email protected], 808-885-3172, 808-896-8996
Figure 2. Images showing representative overly warm and overly cold structures within the 5th floor observing environment
Figure 3, 4, & 5. Graphs showing histograms of temperature difference between outside air and telescope structures of
progressively lower thermal mass (i.e. the Primary mirror, the caisson central, and an upper telescope tube trusses).
Dome venting will not only substantially narrow these distributions, but it will decrease the time air is in contact with the
surfaces and therefore reduce air temperature differences. For more information reference1 .
2.2 Water tunnel tests
Tests were performed to identify the causes of poor seeing in the unvented dome and to find ways to mitigate them. In
particular, investigations of installing vents in the dome to improve air exchange between outside and inside the
enclosure were conducted. A number of vent geometries were tested using water tunnel models at the University of
Washington Aerodynamics Laboratory (UWAL). Relative flushing times for various dome slits were compared for the
different vent geometries (size and #). The general flow characteristics observed were compared with computational
fluid dynamics (CFD) simulations, performed in collaboration with the Thirty Meter Telescope (TMT) project. More
2
information can be found referencing “An aerodynamic study of the dome using water tunnel tests and CFD” .
Figure 6. Flushing time graphs for an unvented dome at four dome slit to wind angles. The oscillation superimposed on the
exponential in the lower right graph is due to a rotating cloud of dye inside the dome. Note the dramatic increase in flushing
time as the slit faces downwind (azimuths 165 and 180).
Figure 7. Comparison of temperature environments for the unvented (left) and vented (right) domes from CFD. Note the
dramatic reduction of the temperature contrasts as the slit faces downwind.
2.3 Venting comparison
The upper curve, in the Subaru figure 8 below, is for a CFHT-type dome but with a back vent. The 3rd curve down is for
the same dome with ~15% porosity side vents open. The bottom curve is for the current Subaru enclosure with all vents
open.
180
venting time to crossing time ratio
160
140
120
100
actual CFHT dome
80
60
40
20
smaller vents
larger vents
0
0
30
60
90
120
slit-to-wind angle ( ° )
150
180
Figure 8. Left: CFHT UW results, Salmon et al. 2011, Right: Subaru water tunnel tests, Ando et al 1991
2.4 The financial value of venting
Venting the CFHT dome will improve the median delivered telescopic image quality (IQ: 500nm, zenith fwhm) by
~15%, from 0.73” (Salmon et al. 2009) to 0.63” (Racine 2010; Cuillandre & Racine 2011). This will result in a ~30%
increase in point source sensitivity for background-limited science or a to ~30% economy of telescope time for the same
signal-to-noise ratio. Enhanced resolution is an additional gain.
The operating budget of the CFHT is $7.3M/yr. (CFHT AR 2009). If the financial cost of venting the dome project was
$1.6M, the expense will be recovered from the additional science done in $1.6M/2.1 years. Over a decade, the financial
value of the science gain is ~$21M (R. Racine, CFHT Annual Report 2009).
3. DOME STRUCTURE
3.1 Dome Geometry
The intent of this section is to provide a brief description of the CFHT dome structure which consists of two concentric
shells and intervening steel trusses. The outer shell is made of vertical strips (gores) of 1/4 inch thick (6 mm) sheet steel.
The inner shell consists of 2 inch (50.8mm) thick foam insulation panels faced on both sides with one mm thick
aluminum sheeting. The two shells are separated by a 24 inch (600 mm) air gap formed by steel truss work, the cavity
between the inner and outer skins. The base of the dome is supported by a massive ring girder sitting on a series of bogie
wheel carriages. The dome shutter and gantry crane are supported by a pair of arch girders that span the diameter of the
dome and are separated by 21.5ft (6.55m); however the clear aperture opening or slit is only 18 ft (5.5m) across.
Figure 9. The CFHT dome showing the arch girders on either side of the dome slit opening and the individual outer vertical
gore sections that combine to make the hyper-hemispheric dome, the gores are alternating ¼ inch thick steel plates which
overlap at their edges.
The area where the outer dome skins overlap and meet is the location where the vertical structure members for the dome
exist, established the width of the vents. Alterations to the dome skeletal structure in this area would not be possible as it
would compromise the structural integrity of the dome. Therefore it was decided that any vent units added to the dome
would need to fit between the vertical members.
The first gore sections immediately next to the arch girders were eliminated because an access door to the outside of the
dome is located in this position as well as many electrical enclosures.
Figure 10. The CFHT dome showing the arch girders supporting the dome shutter and exterior access doors on either
side of the arch girders. The horizontal structural members requiring removal for vent installation are shown at each dome
skin vent opening.
Each gore section spans 7.5°. The first gore section that interfaces with the Arch girder has 10 gauge Northwest joist F
cord sections stitch welded to the exterior skin. The horizontal F chord section members begin at 10° below the spring
line of the dome and are arrayed upward at evenly spaced 10° increments. They are arrayed at every other gore section
around until the other arch girder is met.
Figure 11. The CFHT dome showing the arch girders on either side of the dome slit opening and the individual outer
vertical gore sections that combine to make the hyper-hemispheric dome, the gores are alternating ¼ inch thick steel plates
which overlap at their edges.
The second and third gore sections from either side of the arch girder have inner and outer chords with 1.5” diameter
tube web frames welded between the chords. These sections are joined by a bolted connection to the vertical frame
members on either side of the gore section; in addition they are stitch welded to the exterior gore skin. The triangular
horizontal sections repeat from the third gore section every other gore until the opposite arch girder is met, the other side
on the dome is a mirror image about the centerline of the dome shutter.
Figure 12. Void between the outer dome skin (left) and inner foam insulation panels (right).
The bottom of the vent apertures opening (~2m wide by 5.5m high) begins 84” above the 5th floor catwalk. There are a
couple of reasons for this: The external door is present in the first gore section, various electrical enclosures are present
from the 5th floor catwalk to about head height, and for safety reasons it eliminates the possibility for someone to fall
through a vent opening.
External
access door
84”
Electrical
enclosures
Figure 13. Rendering of the inside of the dome showing the external access door and existing electoral enclosures.
3.2 Structural analysis
The structural analysis of the dome was performed by M3 engineering. The proposed locations of the vent units and
existing geometry set the requirements for the structural analysis to be performed. The analysis included the skeletal rib
members of the dome, connections to the existing steel dome outer skin, and the vent unit enclosures combined with the
present dome geometry.
Figure 14. Finite element analysis model of the dome structure, shown with structural skeletal members before analysis and
after, courtesy of M3 engineering.
It just so happens that the weight of the skin removed (1,400lbs) to achieve a cut opening for the new vent unit
enclosures (2,600lbs) was only about a 1,200lb difference from the initial configuration. Therefore the effect of
additional weight per vent unit added to the dome/ring girder unit is negligible under 1997 Uniform building code, Vol 2
Structural Engineering Design Provisions.
The vent unit was checked for wind load, seismic load, dead loads, live loads, and ice loads. None of these loading cases
exceeded the maximum or allowable stress or deflection in any design case, all load were within tolerable limits.
4. VENT DESIGN OVERVIEW
4.1 Vent unit enclosure
Each identical vent unit configuration consists of a steel frame weldment assembly fabricated using ¼ inch steel plate.
As discussed earlier the frame width is designed to fit between the vertical structural members of the dome. The exterior
mounting flange that interfaces with the outer dome skin overlaps the cutout opening 4 inches of the sides and 6 inches
on the top and bottom.
Figure 15. Vent unit frame enclosure missing motors, slats, louvers, conduit, etc.
Each vent unit has a dual control system to allow additional air flow into the dome. An external roll-up door
opens/closed to allow flushing of the dome, the outer door can be configured all the way open, 2/3 open, and 1/3 open to
facilitate flushing control and reduce telescope shake during high winds. The other method uses internal louvers to
throttle the flow or control its direction.
4.2 External door
The outer roll-up door is located on the interior of the steel framed enclosure and consists of interconnected slats that
span the width of the vent unit opening. The slats travel up/down the outer radius of the enclosure in between guide rails
on either side of the opening. The guide rails have felt seals with one seal on the front and one in the back for a total of
four seals per vent unit. The slats are made of extruded aluminum with a double wall extruded 55mm profile with wind
locks. The wind locks mount to the side of the slats and travel inside the guide rail to keep the slat for movement back
and forth with in the track. They have a wind rating of 120 lbs per square foot (psf) and are approved for Dade County
Florida hurricanes.
Figure 16. The side profile of a aluminum slat and a guide rail (raw alum) with felt seals (gray) on each side of the slat
(white) with wind locks (black) installed.
The motor is a single 120VAC Somfy tubular motor rated for 60,000 cycles; it contains a manual override which can be
cranked by hand or with a manual drill in the case of power failure. The position of the bottom slat of the door is
controlled by using 5 proximity sensors located at the top (open), bottom (closed), 1/3 open, 2/3 open, and compressed
(slats compressed for wind loads). The motor also has built in motor limits in the case of sensor or software failure. The
outer door takes around 1min to open or close.
Minor maintenance of the roll-up door is made by removing a top cover which hides the roll-up drum and motor. All
service work including slate replacement and motor replacement is performed at the top of the shutter door unit’s header
box. Upon removal of the door cover plate and header cover plate, all components can be service and replaced by a
maintenance crew consisting of two people. The heaviest component would be the slat roll, weighing in at 217lbs, or
109lbs per person.
4.3 Internal louvers
The internal louver system consists of three independent motorized louver systems that house either 8 (middle & bottom)
or 7 (top) louver blades mounted in a rectangular frame that mounts to the main steel enclosure. Each louver assembly,
top, middle, or bottom can be removed independently from the enclosure for major repairs and to allow access to the
outer door components if needed. Minor repairs can be made with the framework left in place; each 8 inch aluminum
extruded airfoil blade has mounting shafts incorporated into the blade extrusion on either side which rotate in a bronze
sleeve mounted to the main frame. The bottom louver blade on each framework differs from the other louvers as it
incorporates the motor on one side of the blade. The louver system is controlled by a single 120VAC Somfy tubular
motor rated for 60,000 cycles. The smaller motors are IP addressable and have built in control functionality for infinite
position control and positive position feedback. Communications are established via RS 485 telephone handset cables
which are funneled to a Raspberry PI. Internal high level software created by CFHT’s Tom Vermeulen, allows control of
each louver system independently.
4.4 Proximity sensors
As discussed earlier the position of the bottom slat of the door is controlled by using 5 proximity sensors located at the
top (open), bottom (closed), 1/3 open, 2/3 open, and compressed (slats compressed for wind loads). The sensors provide
feedback to the PLC system for slat position. If a sensor fails there is no backup method for indicating the motor
position, internal motor limits are used to allow the door to be closed or open all the way. The sensors are Omron TLW20ME1 inductive NPN-NO 3-wire unshielded proximity sensors. The sensors are mounted in non metallic conduit
boxes, which will not interfere with the sensor and are chemical and UV resistant. Ferrous indicator plates are mounted
to the slats to indicate slat position.
Figure 19. A proximity sensor and an upper sensor location.
5. DOME VENT INSTALLATION
5.1 Fabrication and assembly
M3 engineering was responsible for the structural design, structural analysis, and conceptual design of the vent unit. The
preliminary design used an enclosure (outer frame) design from M3 with outer shutter door and internal louver
technologies specified and supplied by Tucson rolling shutters (TRS). CAID would perform the fabrication of the vent
unit enclosure and other various big steel, sheet metal parts, and weldments. TRS installed the door and louvers on site at
CAID’s facility. Gilbert electric was sub contracted by CAID to install the electrical conduit, wiring, and junction boxes.
Final assembly and checkout was performed by CAID and TRS.
5.2 Phase 1: Prototype testing
The dome vents were fabricated over two phases. The first phase was to build a prototype according to design
requirements and specification, assemble the prototype and test the vent unit. On site demonstration and acceptance
testing was performed in Tucson, AZ at CAID Industries. Using a requirements traceability matrix, deficiencies in the
operation, reliability, and function of the vent units were recorded for conformance and non-conformance. Problems
were recorded and addressed to eliminate issues on the phase 2 build of the remaining 11 vent units.
Figure 20. John Carney from CAID Industries performing leak tests
5.3 Phase 1: Prototype install
The prototype vent unit was packaged and shipped to Hawaii by means of truck and trailer and barge. The vent unit was
shipped fully assembled and ready for install.
Figure 21. Left: Prototype vent unit ship method, Right: Phase 2 remaining 11 vent units shipped
Two Steel erecting firms, Steel Tech and Nexus, meet the vent unit at the observatory facility with equipment consisting
of a articulating boom lift and boom truck crane. A picture frame fixture was shipped with the vent unit which was used
as a template to cut the opening for the vent unit. A theodolite was used on the summit ridge to mark hole locations for
top bolt holes that would be used to locate the fixture to the dome for cutting the pattern and continue making the holes
around the perimeter of the opening.
Figure 22. Left: Dome skin removed with handling/cutting template fixture, Right: Dome vent installed into a cut opening
Once the fixture was installed a creative method developed by Nexus’s Gene Feldman was used to cut the opening in
less than an hour. The method used a device called a nibbler which followed the perimeter of the template and by means
of some custom fixturing to hold the device to the frame, cut open the door. The fixture also had built in means to
support the massive 1,400 lb piece of 1/4 inch plate removed from the dome.
The vent unit was lifted from the top with the boom truck crane and installed in the opening with a crew on the inside
and outside of the dome guiding the frame enclosure into place. The prototype vent unit was installed and secured in
place with bolts around the flange into the dome skin and additional anchors installed in the skin cavity attaching to the
vertical members. Once secure, second round acceptance testing was conducted. Unfortunately some damage was
incurred during transit and few more issues discovered. However the issues discovered benefited all as a better method
for supporting the vent units during shipping was developed and improvements to the guide track for the door were
made.
5.4 Phase 2:
The remaining 11 units were fabricated and assembled with the design changes discovered during phase 1 acceptance
testing before and after prototype installation. The phase 2 acceptance testing went without a hitch and previous tests
exploited the weaknesses found in phase 1 which greatly improved the reliability and smooth operation of both vent
systems. As before, the vent units were packaged and shipped to Hawaii where the steel builders met the vent units at the
summit and continued to install them. The prototype unit was reworked to incorporate all changes that were added to the
phase 2 vent units.
Figure 23. View from inside the dome showing all the vents installed before inner skins installed, courtesy of J.C. Culiandre
6. CONCLUSSIONS
The dome vent units were installed successfully and released for night time operations in mid January 2014. Due to poor
weather the vent units were only used 2-3 times until sometime in April. Unfortunately this did not provide much
feedback as to the image quality improvement with the vents or the vent reliability. Since April, the weather has
improved and the vent units are being used every night. No major operational problems have occurred and those close to
the image quality improvement program and dome venting project are anxiously awaiting a data set where the influence
of the vents of dome seeing and image quality can be reported.
Figure24. The CFHT Dome with all vents installed at sunset
ACKNOWLEDGEMENTS
CFHT would like to express our gratitude to the companies involved in the dome venting project, M3 Engineering,
CAID Industries, Tucson Rolling Shutters, Nexus steel Inc., and Steel Tech Inc., for their professional conduct,
dedication, and support to the dome venting project and CFHT. Their contributions are invaluable and continue to keep
CFHT competitive in the astronomy community.
REFERENCES
[1] Salmon, D., Cuillandre, J.-C., Barrick, G., Thomas, J., Ho, K., Matsushige, G., Benedict, T. and Racine, R., "CFHT
Image Quality and the Observing Environment”, PASP 121, 905-921 (2009).
[2] CFHT Annual Report 2009, http://cfht.hawaii.edu/AnnualReports/AR2009_2010e.pdf
[3] Cuillandre J.-C. & Racine, R. 2011, SAC Report, Spring2011
[4] Racine, R. 2010 UM 2010, Taipei
[5] Racine, R., Salmon, D., Cowley, D., Sovka, J., “Mirror, Dome, and Natural Seeing at CFHT”, PASP 103, 10201032 (1991).
[6] Salmon, D., Cuillandre, J.-C., Barrick, G., Thomas, J., Ho, K., Matsushige, G., Benedict, T. and Racine, R., "CFHT
Image Quality and the Observing Environment”, PASP 121, 905-921(2009).
[7] Racine, R., “The Ground Layer Seeing at the CFHT from CFHT DIMM, MKAM MASS/DIMM, MegaCam and
PTP Data”, Canada France Hawaii Telescope Corporation internal report (2011).
[8] Thanjavur, K.G., Ho, K.K., Bauman, S.E., Salmon, D., Gajadhar, S., “Canada-France-Hawaii Telescope image
quality improvement initiative: thermal assay of the observing environment”, Proc. SPIE 8444, (2012).

Dome venting: the path to thermal balance and superior image quality

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