Several examples of miniature probes (null type as well as fixed position) are presented which have proved useful in aircraft and space power systems component testing and are applicable to automotive gas turbine testing. These probes are used to determine component or system performance from the measurement of gas temperature as well as total and static pressure, and flow direction. Detailed drawings of the sensors are presented along with experimental data covering the flow characteristics over the range of intended use

Glawe, G. E.

Krause, L. N.

English

NASA Technical Reports Server

.NASA TECH NICAL NASA TM X-71638
MEMORANDUM
00oo
m (NASA-TM-X-71638) MINIATURE PROBES FOR USE N75-15017
IN GAS TURBINE TESTING (NASA) 15 p BC $3.25
-- i CSCL 14B
Unclas
G3/35 07714
IC
z
MINIATURE PROBES FOR USE IN
GAS TURBINE TESTING
by George E. Glawe and Lloyd N. Krause
Lewis Research Center
Cleveland, Ohio 44135
TECHNICAL PAPER to be presented at
Automative Engineering Conference sponsored
by the Society of Automative Engineers
Detroit, Michigan, February 24-28, 1974
https://ntrs.nasa.gov/search.jsp?R=19750006945 2020-03-23T01:34:01+00:00Z
ABSTRACT
Because of space limitations and system complexity in many applications
associated with gas turbine testing, extremely small flow measuring probes
have sometimes been required. This paper presents several examples of these
miniature probes (null type as well as fixed position) which have proved
useful in aircraft and space power systems component testing and are appli-
cable to automotive gas turbine testing. These probes are used to determine
component or system performance from the measurement of gas temperature as
well as total and static pressure, and flow direction. Detailed drawings of
the sensors are presented along with experimental data covering the flow
characteristics over the range of intended use.
INTRODUCTION
Automotive gas turbine testing requires small probes which measure tem-
perature, pressure, and flow direction to help establish engine performance.
Probes are available to measure a single or a combination of these param-
eters. Furthermore, probes can be used in a fixed position at their measur-
ing station or they can be actuated to rotate and/or translate to various
positions. Compromises must be made in choosing any particular type of probe
and the method in which it is applied. Some considerations involved are cost,
size, accuracy, sensitivity, aerodynamic loading, vibration, and installation
problems. Before application, the probes limits and characteristics must be
established. This present report described various miniature probes along
with their flow characteristics, thus providing a prospective user with in-
formation to choose a given probe and interpret the test data it acquires.
DESCRIPTION OF PROBES
WEDGE-TYPE COMBINATION PROBE - Figure 1 presents the head detail of a
wedge-type combination probe. This probe is a modification of the probe pre-
sented in (1).* The probe head consists of a 600 included angle blunted wedge
with flow direction sensing orifices located on the wedge surface. Projecting
upstream from the wedge are an internally beveled total-pressure tube and a
tubular shielded thermocouple. This sensing head assembly fits into a 4 inch
(.64 cm) diameter support tube (not shown) with the centerline axis of the
inch (.64 cm) tube passing along the blunt leading edge of the wedge. The
probe head is small enough to fall within the projected cross-sectional area
of the support, thus, the probe can be inserted into a test region through a
inch (.64 cm) diameter hole.
Numbers in parenthesis designate References at end of paper.
2TUBE-TYPE COMBINATION PROBE - Figure 2 presents the head detail of a
tube-type combination probe. The probe head consists of a 3 tube bundle
containing a center position total pressure tube with two 450 angled inlet
tubes on either side for sensing flow direction. An unshielded 36 gage wire
thermocouple lies above and parallel to the total pressure tube to complete
the temperature, pressure, and flow angle combination sensor. This sensing
head assembly fits into a 3/16 inch (.48 cm) diameter support tube (not
shown) in such a manner that it falls within the projected cross-sectional
area of the support tube.
TEMPERATURE PROBES - Figure 3 shows a simple bare-wire crossflow therm-
ocouple probe design using a 1/8 inch (.32 cm) diameter cylindrical support.
A related type of probe has been previously reported in (2, 3, and 4).
Figure 4 shows a semi-shielded high-recovery probe. This probe uses
a 3/32 inch (.24 cm) diameter support tube with a notched end which places
the crossflow thermocouple in the stagnation region of the notched end. Other
probes of this type have been described in (3, 4, and 5).
Sheath and wire materials for these two thermocouple probes (figs. 3
and 4) may be chosen to meet the requirements for cold-flow component tests
as well as high temperature testing of the complete engine assembly.
PROBE CHARACTERISTICS
Characteristics of the probes were determined for flow conditions where
the probe is alined with the flow as well as for the case where the probe is
unalined with the flow. The probes can therefore be used with actuators or
in fixed position. Flow orientation nomenclature is presented in figure 5.
TEMPERATURE MEASUREMENTS - In using thermocouple probes, it is conven-
ient to correct for an aerodynamic recovery error by using a recovery-
correction factor A.
3Tt - TindS:= Tt (1)
where Tt is total temperature and Tind is the indicated thermocouple
junction temperature. All temperatures are absolute. In an application
in which the junction has responded to the aerodynamic flow, and where con-
ductive and radiative heat exchange is not present, the quantity Tind is
equal to the adiabatic junction temperature, and Tt can be calculated from
the indicated junction temperature and the value of A.
Tind
Tt =1- A (2a)
and since A << 1
Tt,, Tind(l + A) (2b)
The recovery-correction factor of a thermocouple probe, in alined flow,
varies primarily with stream Mach number, with a secondary effect of stream
pressure. The recovery-correction factor for alined flow, at 1 atmosphere
(1 x 105 N/m 2 ) stream total pressure, will be termed the reference recovery-
correction factor AO.
The variation of A0 with Mach number for the probes reported herein is
presented in figure 6. The curves presented here represent the average cal-
ibration for several probes. In general, deviations of ±10 percent of the
average given value for AO can occur due to material and probe manufactur-
ing variances. For maximum accuracy, probes should be individually calibra-
ted before and periodically during application. Shielded probes are more in-
clined to change calibration during usage because of their higher suscepti-
bility to flow passage area change from stream particles and thermocouple
"grounding" to the close proximity shields. The combination wedge A0  curve
is higher valued than that usually expected for a shielded probe. The reason
4is that its shield geometry allows straight-through flow with only a small
amount of stagnation at the wire bend.
The variation of the recovery-correction factor with pressure, for
alined flow, is shown in figure 7. Conductive heat exchange from the sup-
port and/or shield is probably the largest contributing factor to this ef-
fect, with the greater sensitivity felt in the low pressure (lower density)
regions. As seen in the figure, the high-recovery shielded probe is effected
to a greater extent by this condition.
Variation of recovery-correction factor-ratio with yaw angle (see fig. 5
for flow angle orientation) is presented in figure 8. It is seen from these
curves that the variation of A with yaw for the combination wedge, combin-
ation, tube, and high recovery probe is only slight with small angles of yaw
(+100). The crossflow design, however, is not only highly sensitive to change
in A with change in yaw (up to 10 percent change at 100) but is also sub-
ject to a wider variation with Mach number than the other probes. Thus, the
data is presented as a cross-hatched area. The use of this probe should be
restricted to applications where such uncertainties in A with flow angle
are not significant. No data is presented for deviation from zero pitch angle
since in the majority of applications the probes can be inserted into a test
region, so that, the variation in flow in one plane (pitch) is negligible (say
less than 50) and only the variation in the yaw plane must be accounted for.
An approximation of radiation losses for the crossflow and high-recovery
thermocouple probes (which are applicable to the hot sections of the engines)
may be obtained from information found in (2 and 4) where:
Radiation correction, OR C Tind 1 Td (3)
Mp 1000 Tind
where C is the radiation constant for a particular probe, Es is total hem-
ispherical emittance of the thermocouple wire, M is stream Mach number, p
is stream static pressure in atmospheres, Tind is indicated thermocouple
temperature OR, and Td is enclosure duct temperature oR.
The "C" values for the two probes are:
bare-wire crossflow = 3.8
high recovery probe = 5.2
Values of emittance for type K thermocouples may be estimated from (6)
where the values range from 0.2 to 0.3 for a clean bright wire and 0.7 to 0.9
for a highly oxidized wire. Emittances of 0.7 to 0.8 were found for bare
type K thermocouples (4) run for to 1 hour in the exhaust gas of a gasolene-
air combustor.
YAW ANGLE MEASUREMENT - Although the combination probes presented herein
are primarily intended to be used with an actuator that supplies linear and/or
rotational positioning, they may be used to a limited extent in a fixed posi-
tion. The value for the yaw angle 0 can be obtained from probe measurables
using figure 9, where Pt,ind is indicated total pressure and pl and p2
are the pressures measured by each of the directional sensing taps. As with
the thermocouple sensors, deviations of _10 percent of the average values of
figure 9 can occur due to manufacturing variances; so, for maximum accuracy,
individual probe calibrations are required.
TOTAL PRESSURE MEASUREMENT - The variation in indicated total pressure
with yaw angle for the two combination probes used in a fixed position is
presented in figure 10, where pt is total pressure. This figure is used
to obtain stream total pressure from probe measurables after determining yaw
angle through use of figure 9. Additional flow-angle characteristics of
several miniature total-pressure tubes are given in (7).
STATIC PRESSURE MEASUREMENT - For accurate static-pressure measurement
it is desirable that the sensing element be many diameters upstream from the
probe support, as in the Prandtl-type tube. Because the probes must usually
6pass through a small hole and have the sensing elements near the centerline
of rotation, most small combination probes do not include the ability to
measure stream static pressure. However, if it is required that the com-
bination probes have the ability to measure the static pressure, one of the
angle-sensing holes (or the average pressure of the two holes) is normally
used as an indication of this pressure (1). Of the two combination probes
presented here, the wedge type is preferred for static pressure because its
angle ports indicate a pressure much closer to stream static pressure than
the tube type. The uncertainty in static pressure resulting from using such
a technique is due to probe blockage and the variation in blockage as the
probe traverses the passage. An alternate approach to the measurement is to
use static pressure obtained from wall static pressure taps along with the
calculated values obtained from other measurables.
PROBE ACTUATOR SYSTEM
A typical probe actuator system is use at the Lewis Research Center is
shown in figure 11. The actuator is capable of both linear and angular mo-
tion. Typical ranges of motion are 6 inches (15 cm) linear travel and ±90
degrees rotational movement. A block diagram of the complete actuation sys-
tem is shown in figure 12.
The position command for linear travel may be manually operated or com-
puter programmed to stop at pre-determined points. Linear positioning accur-
acy can be acquired to about 0.15 percent of full travel.
The flow angle sensing ports of the probe are connected to a differen-
tial pressure transducer which converts pressure unbalance (misalinement
with flow) to an electrical signal. This electrical unbalance signals the
rotational system to automatically bring the probe back to zero flow angle.
Sensitivity of the flow angle system varies approximately linearly with
7pressure and with the square of the flow velocity. The overall flow angle
measurement uncertainty for nominally mid-range subsonic Mach number flows,
at one atmosphere pressure, is about one degree.
SUMMARY
Design details, general characteristics, and discussions of applica-
tions and limits for several miniature fluid flow probes were presented.
The probes which measure temperature, pressure and flow direction can be
used in a fixed position or in an actuator system. Information on a typ-
ical probe actuator system was also presented.
REFERENCES
1. G. E. Glawe, L. N. Krause, and T. J. Dudzinski, "A Small Combination
Sensing Probe for Measurement of Temperature, Pressure, and Flow
Direction". NASA TN D-4816, October 1968.
2. M. D. Scadron and I. Warshawsky, "Experimental Determination of Time
Constants and Nusselt Numbers for Bare-Wire Thermocouples in High-
Velocity Air Streams and Analytic Approximation of Conduction and
Radiation Errors". NACA TN 2599, January 1952.
3. T. M. Stickney, "Recovery and Time-Response Characteristics of Six
Thermocouple Probes in Subsonic and Supersonic Flow". NACA TN 3455,
July 1955.
4. G. E. Glawe, F. S. Simmons, and T. M. Stickney, "Radiation and Recovery
Corrections and Time Constants of Several Chromel-Alumel Thermocouple
Probes In High-Temperature, High-Velocity Gas Streams'. NACA TN 3766,
October 1956.
5. S. J. Markowski and E. M. Moffatt, "Components Involving Fluid Flow".
SAE Quarterly Transactions, Vol. 2, Jan. 1948, pp. 104-116.
6. W..D. Harvey, L. E. Forrest, and F. L. Clark, "Measurements of Total
Hemispherical Emittance for Chromel and for Alumel Wires". NASA
TMX-2359, September 1971.
7. T. J. Dudzinski and L. N. Krause, "Effect of Inlet Geometry on Flow-
Angle Characteristics of Miniature Total-Pressure Tubes". NASA TN
D-6406, July 1971.
-13 (.33) 0.D.
SECTION VIEW7  x .11(.27) I.D. TUBE
.02 (.05) HOLE 7,
rSWAGED THERMO-
COUPLE
.06(.15)0.D. SHEATH
.11 28GAGE WIRE
04 (.27) MgO INSULATION
-.03 (.07) 0.0.
x.024 (.06)1. D.
.3 IrSECTION
(.75) TUBE, ILINE
0 (.1)
11 .22) 17(.43
(: 117 (. 22) - .21(. 53)
450 CHAMBER
.06
Figure 1. - Wedge-type combination probe.
co
-.085 (. 21) 0. D.
x.065 (.16) I.D.
TUBE SHAPED TO
ACCEPT 5 CYL-
INDER CLUSTER
,-.02 (.05) DIAMETER
SUPPORT ROD
.30 .75)
WELD JUNCTION
.03 R REMOVE EX-
(.08) CESS BEAD---
.02(.05)
t-.17 (.42) SWAGED TC ASSEMBLY
.03(.08)0.D. SHEATH
S02 (.05) O.D. 36 GAGE WIRE
x .015(.04)I. D. TUBES Mg INSULATION
MgO INSULATION
.06(.15)
-r 450
Figure 2. - Tube-type combination probe.
OW
FLOW
FLOW
(.15) 115)-
/-WELD JUNCTION
" AND REMOVE
EXCESS BEAD WELD JUNC- .15(.38)
TION AND
0.30 REMOVE EX-
(0.75) CESS BEAD-- I
-SWAGED TC CON-
STRUCTION .125 (3.1) .090 1.22) 0.D .15(.38)
O.D. METAL SHEATH x .013(.032)
24 GAGE TYPE K WIRE WALL LL .1
MgO INSULATION NOTCHED .062(.155)METAL TUBE I O.D. METAL I
SHIELD .- I SHEATH 28
THERMO- GAGE TYPE
COUPLE IN- K WIRE MgO
Figure 3. - Bare wire - crossflow SERT OF INSULATION
thermocouple. SWAGED CON?'
STRUCTION-
Figure 4. - High recovery thermocouple.
00
PITCH
PLANE A
ALINED FLOW
YAW
PLANE
9988-12
Figure 5. - Flow orientation nomenclature.
.03
CROSS FLOW-\
COMBINATION
S .02- WEDGE
-COMBINATION
L>TUBE
.01-
HIGH
RECOVERY-
0 .2 .4 .6 .8 1.0
MACH NUMBER, M
Figure 6. - Variation of reference recovery-correction factor
with Mach number. A0 is the recovery-correction factor
at a total pressure of 1 atmosphere (1x10 5 Nlm 2). Alined
flow.
S 3-
S 2 -HIGH RECOVERY
0 To ' 2-
'CROSS FLOW, COM-
C BINATION WEDGE, AND COMBINATION TU
0 .4 .8 1.2 1.6 2.0
TOTAL PRESSURE, Pt, ATM (OR x105 NIm 2 )
Figure 7. - Variation of recovery-correction-factor ratio
with pressure. Alined flow. 0.6 <M <0.9.
HIGH RECOVERy,
1.2
o CROSS FLOW- r OMBINATION
S.-_ TUBE AND COM-
< BINATION WEDGE
0 ±10 ±20 130
YAW ANGLE, 0, DEG
Figure 8. - Variation of recovery-
correction-factor ratio with yaw
angle. 0.3 <M<0.9.
30 COMB INATION
WEDGE,,
20
10- '-COMBINATION
STUBE
-0 -0
-20 -
o-
I> -301
-3 -2 -1 0 1 2 3co
YAW PRESSURE DIFFERENCE (pl - P2)I[Pt, ind - 112 ( 1 + P2 )]
Figure 9. - Variation in yaw pressure difference with yaw angle.
0.3<M <0.9.
Z+ 0
.1 -1 '-COMBINATION
COMBINATION WEDGE1 -.2.- TUBE-- 
.
c , -. 3
e0 110 +20 ±30
YAW ANGLE, , DEG
Figure 10. - Variation in total-pressure
difference with yaw angle. 0.3<M <0.9.
AP PRESSURE /-LINEAR
TRANSDUCER1 / DRIVE
MECHANISM
R /
LANGLE
DRIVE PROBE
MECHANISM SENSING ,.
HEAD--
C.-
oO Figure 11. -Actuator, AP pressure transducer, and probe assembly.
I
POSITION
RECORDER
LINEAR POSITION FEEDBACK
POSITION LINEAR
COMMANDS CONTROLLER PROBE PROBE
ANGLE ACTUATOR SENSOR
CONTROLLER
AP PRESSURE
TRANSDUCER
ANGLE ERROR SIGNAL SIGNAL
CONDITIONER
Figure 12. - Probe actuation system.
NASA-Lewis


Miniature probes for use in gas turbine testing

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