A Polar-Coordinate Survey Method for Determining Jet-Engine Combustion-Chamber Performance Page: 4 of 30
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NA-A TN 3566
Tm mass-weighted mean total temperature
Tt absolute total temperature (in numerical examples given in OF rather
than in absolute units)
,V velocity
w mass-flow rate
r ratio of specific heats
p density
DESCRIPTION OF SURVEY SYSTEM
Turbojet combustor-exhaust passages are commonly annular in shape.
While previous traversing systems for these ducts have appeared in the
o literature (refs. 1 and 5), the survey method described herein offers
Sa means of obtaining a more complete traverse by moving a single-point
H probe circumferentially across the duct passage at a number of radial
Stations. A polar-coordinate probe movement allows the probe to be
positioned from a fixed pivot point outside the duct, simplifying the
probe actuation.
Although the survey method could be adapted to a number of differ-
ent flow-passage geometries, the one considered here was designed for
use with a 900 sector of an annular combustor, frequently used in lab-
oratory installations to conserve air and fuel supplies. The polar-
coordinate survey section constructed at the Lewis laboratory is shown
in figure 1. The probe enters the gas passage through a wall-enclosed
slot below the duct. The probe is swept circumferentia3ly by an
angular drive hub, held in place by bearing blocks below the cooled duct
walls, and driven radially through a shaft below the hub.
A cross-sectional view of the probe installation is shown in fig-
ure 2. The passage between the probe and the radial-drive lead screw
is pressurized by outside air for cooling. This air escapes through
holes drilled in the clamping collet which holds the bottom of the probe
to the lead screw. Escape of the:cooled air into the combustor-exhaust
duct through the probe access slot is minimized by drawing the cooling
air out through the vac~um gone4tion just below the angular-drive hub.
The vacuum outlet may be&'ot4ted for combustor operation at high
pressures.
The probe chosen for this example consists of two elements: a
total-pressure tube and a platinum-13-percent-rhodium - platinum thermo-
couple. The principles of the sonic-aspirated thermocouple are described
in references 2, 4, and 6 to 8. This type has the advantages of constant
recovery factor and negligible radiation and conduction errors (ref. 9).
A vacuum connection allows the exhaust gases to be drawn through the noz-
zle past the thermocouple junction at sonic velocities.
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Friedman, Robert & Carlson, Edward R. A Polar-Coordinate Survey Method for Determining Jet-Engine Combustion-Chamber Performance, report, September 1955; (https://digital.library.unt.edu/ark:/67531/metadc57811/m1/4/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.