M1 Airflow Lab Tests

Test procedure for mirror test cell airflow and temperature tracking investigation.

Sloan Digital Sky Survey Telescope Technical Note 20011206-01

Larry Carey

Test Sample

A three-cell test section roughly representative of the primary mirror was provided for this test by the Steward Mirror Lab.

Test Goals

Determine the relative time delay for the "mirror" temperature to track the temperature of the environment. This is intended to be a relative comparison, "A to B to X" of different airflow patterns, with each case measured at various airflow rates. Additionally, the tests are intended to determine relative airflow vs. pressure drop characteristics for each of the air flow patterns so the results can be evaluated both as a function of flow rate and of pressure drop along the flow path.

Test Setup (see Photos)

A wooden box, roughly 36" x 36" x 4.5" was used to approximate the plenum of the PSS. Three holes were cut in the upper surface and PVC tubes installed, extending up from the upper surface of the box. The tubes interface with the cell cavities of the test sample, duplicating the cell geometry of the SDSS primary mirror installation.

The test cell was supported above the plenum on blocks (imitation belloframs) and instrumented to take 2 representative temperature measurements on its walls and at 5 locations on the upper surface, plus 1 air temperature reading. The plenum was ported from below to allow measurements of differential pressure to be taken inside the plenum and airflow to be measured as it was drawn from the plenum.

An over sized garbage can was insulated and placed (upside down) over the test sample to form the walls of the test chamber, providing a controlled environment, and directing inlet air onto the test cell only from above. The bottom (top as installed) was cut out so it could close the chamber for heating and be removed for the cooling tests.

An array of different sized exhaust fans were installed to draw air from the plenum. The flow rate was controlled by using different combinations of fans and partially blocking their outlet ports.

Test Process

With the test chamber closed, heat the test cell to 25 degrees F above ambient, (approximately 95 °F). When the cell and chamber have reached equilibrium turn off the heat; document "starting" temperatures and time; open the vent in the top of the chamber; and start the controlled airflow. Record time and temperature readings at 2-minute intervals until all parts of the test cell reach ambient temperature. During this time period take independent sets of measurements of airflow and of differential pressure at 30 minute intervals. The test is to be conducted at each of the following nominal flow rates through the tube: 5 ft./sec., 10 ft./sec., 15 ft./sec. and 20 ft./sec. Each test is to be conducted at least twice.

Test Geometry Conditions

Conduct tests per the above test process with each of the following air flow geometries:

A. Using the existing hardware (with "frisbees" installed)

B. "Frisbees" removed and cups fully lowered.

C. "Frisbees" spaced away from the mirror surface by 0.25 in.

D. Cups crenellated to provide 0.4" clearance from mirror, except at 3 contact points of cup to mirror.

E. All ancillary parts removed from the tops of the tubes (and frisbees removed).

F. Flexible spiral flutes attached to the outside of the PVC tube (and ancillary parts removed).

G. Tube extension lowered to provide 1/2 in. clearance from mirror surface to cups at nominal mirror position (with "frisbees" removed).

Test Results

The results most relevant to the SDSS M1 are presented here, along with the results of actual measurements on the SDSS M1 and a full discussion. A complete table of all test results is available separately.