Candidate concepts capable of generating dynamic temperatures were identified and analyzed for use in verifying experimentally the frequency response of the dynamic gas temperature measurement system. A rotating wheel concept and one other concept will be selected for this purpose. Modifications to the data reduction code algorithms developed were identified and evaluated to reduce substantially the data reduction execution time. These modifications will be incorporated in a new data reduction program to be written in FORTRAN IV

Elmore, D. L.

Robinson, W. W.

Watkins, W. B.

English

NASA Technical Reports Server

N87-11185
FURTHER DEVELOPMENT OF THE
DYNAMIC GAS TEMPERATURE MEASUREMENT SYSTEM
D. L. Elmore, W. W. Robinson, and W. B. Watkins
Pratt & Whitney Engineering Division
SUMMARY
Candidate concepts capable of generating dynamic temperatures have been
identified and analyzed for use in verifying experimentally the frequency response
of the dynamic gas temperature measurement system of reference i. A "rotating
wheel" concept and one other concept will be selected for this purpose.
Modifications to the data reduction code algorithms developed in the reference I
program have been identified and evaluated to reduce substantially the data
reduction execution time. These modifications will be incorporated in a new data
reduction program to be written in Fortran IV.
INTRODUCTION
The measurement system developed in reference 1 uses a compensated two element
thermocouple probe. The compensation technique uses the ratio of the signal
amplitudes from the two thermocouples' passive responses to gas temperature
variations. Comparisons with a numerical heat transfer model allows compensation of
temperature fluctuations to above 1Khz.
The objectives (Figure I) of the present program, "Further Development of the
Dynamic Gas Temperature Measurement System", (Contract NAS3-24228) are; i) to verify
experimentally the frequency response of the dynamic gas temperature measurement
system developed under a previous contract (Reference i); 2) to optimize the
computer compensation method for execution speed; and 3) to implement the reference
1 computer code in Fortran IV for use on generally available computers.
The program is organized into four basic tasks including (I) frequency response
experimental verification; (2) compensation code execution streamlining; (3)
implementation of the compensation code in Fortran IV; and (4) data acquisition at
NASA Lewis Research Center. Efforts to data have been on Tasks 1 and 2 and the
preliminary results are discussed below.
FREQUENCY RESPONSE EXPERIMENTAL VERIFICATION
Task I (Figure 2) involves designing and conducting two experiments which can
be used to verify the frequency response of the dynamic gas temperature measurement
system. The first experiment is defined in the contract requirements and consists
of a "rotating wheel" concept. The second experiment to be designed under Task I
involves the identification and evaluation of several candidate concepts and the
selection of the most promising one. During the identification and evaluation of
the candidate concepts consultation was provided by Dr. Robert J. Moffat of Stanford
University. The experimental test conditions required for the frequency response
verification are for a working fluid of air or combustion gases at a pressure of
atmospheric or greater, minimum peak-to-peak temperature fluctuations of 278K
p_E_ED_qG PAGE BLANK NOT FILMED 41
https://ntrs.nasa.gov/search.jsp?R=19870001752 2020-03-23T15:03:15+00:00Z
(500°F), mach number of 0. i or greater, and temperature fluctuations of 250 Hz or
greater.
Methods identified so far fall into two general categories, the first using an
atmospheric pressure laboratory-scale combustor, and the second using one of a
number of candidate techniques for generating periodic or step-impulse temperature
signals using air as a working fluid.
An available atmospheric pressure combustor produces a dynamic temperature
signal with peak-to-peak temperature variations in excess of 278K (500°F) (reference
I). The advantages of such a combustor as a temperature source are i) the
temperature spectrum qualitatively resembles the desired application (i.e. a turbine
engine combustor), and 2) the assumptions of the analysis are fulfilled, namely low
correlation between temperature and velocity fluctuations. Various test concepts
defined for use with the laboratory burner are listed in figure 3 and discussed in
the following paragraphs.
Impulse Actuated Two Wire Probe - Two gas streams would be established
having the desired high and low temperature. The probe would be moved from
one to the other stream by a mechanical actuator. A small fine wire or
other temperature standard would be used with the two wire probe. It is
recognized that it may be difficult to attain the required 250 Hz frequency
response due to the actuator speed limitations.
Two Wire Probe and Aspirating Hot Film Probe - In this concept, the natural
frequency fluctuations of the laboratory burner would be the only source of
fluctuations, and a reference probe of high frequency response, such as an
aspirating (TSI Model 1735) probe (figure 4) would be used as the standard
measurement of the gas temperature fluctuations. The referenced TSI probe
has a frequency response of up to 100K Hz and is capable of operation at
3500°C (6330°F), 152 m/s (500 f/s), and 1 atmosphere. However, the
accuracy of this probe may be limited due to conduction errors.
TWo Wire Probe and Anemometer Probe - In this concept, the temperature
standard would be a hot wire anemometer probe replacing the previously
discussed aspirating hot film probe. The anemometer would be a high
temperature probe such as a TSI Series A4 probe (figure 5) operated as a
constant current temperature sensor.
Three Wire Probe - In this concept, again the natural frequency
fluctuations occurring in the combustor would be the source. An additional
very small third thermocouple element would be added to the two-wire probe.
This thermocouple element would provide additional verification by
compensating data in three combinations: very small-small, very
small-large, and small-large. Previous test experience (reference I)
suggests that very small wires (25_m or less) will survive for many
minutes in this environment.
Oscillating Splitter Plate and Two Wire Probe - This concept is similar to
the previously described test where a splitter plate is used having two
different temperature gas streams in the combustor. However, instead of
moving the probe from the cold to the hot stream the splitter plate would
be oscillated mechanically to divert the streams over a stationary probe
assembly and temperature standard. The principal difficulties in this
42
concept is in making certain that the flow stays attached to both faces of
the splitter plate and in the complexmechanical system required.
The second category of verification experiments involves hardware
configurations which use air as the working fluid and which suggest an obvious
temperature variation. The configurations are:
Rotating Wheel - The original rotating wheel experiment which has been
tested at Pratt & Whitney is shown in Figure 6. It consists of two
rotating wheels which have circles of holes which turn the air flows (hot
and cold) on and off. The hole patterns are phased to alternately heat and
cool the test probe which is positioned between the wheels. Phasing of the
two hole patterns suggests a temperature variation, i.e, if the holes in
pattern "A" are 180° out of phase with the holes in pattern "B", an
approximate square-wave temperature variation will result as the wheels are
rotated. The primary frequency is the number of holes in one pattern
multiplied by the rotation rate, and peak-to-peak variation in temperature
is the difference in stream temperatures. Figure 7 presents typical test
data for the temperature and velocity profiles.
Modified Rotating Wheel - One of the drawbacks to the previously described
rotating wheel is that the streams flow in opposite directions. Based on
recommendations from Dr. Robert Moffat the rotating wheel experiment was
modified (Figure 8) to use one wheel with two hole patterns and
collector/transition duct which blends the two gas streams through a single
nozzle at the probe. Proper design of the phasing of the holes and using a
screen in the transition duct should make it possible to achieve a nearly
sinusoidal temperature waveform, which is preferred.
Blow DownTube - This concept (figure 9) would use a burst diaphragm and a
capacitive heater to provide a step input. The disadvantages to this
system are problems with probe damage due to particles from the burst
diaphragm and inaccuracy resulting from not generating a sharp
instantaneous step function.
Pulsatile Pressure System - This system, shown in Figure i0, consists of a
flow channel with two orifices. The first orifice is operated critically
so the flow is constant. The secondorifice would be periodically occluded
by a rotating disk valve which would cause the pressure in the volume
between the orifices to fluctuate. This would result in temperature
fluctuations due to isentropic compression. The primary disadvantage is
the large variations in flow required to produce the temperature
fluctuations.
Piston In Cylinder - This system, shown in Figure ii, again uses two
orifices but uses a piston to compress the gas between the orifices and
thus obtain a temperature fluctuation. This system still has a flow
variation, however, it is smaller than the pulsatile system.
Sprinkler Head- This concept, shownin figure 12, consists of a wheel made
from two halves containing milled passages. The passages are curved so
that the air flow will provide the momentumto rotate the assembly. The
passages would also be canted slightly in the axial direction so that the
flow from each stream would impinge on the test probe located near the
43
wheel. This configuration has the advantage of providing temperature
fluctuations with a relatively uniform and constant velocity. The primary
disadvantages would be associated with the large amount of air required and
the complexity of designing the rotating seals and bearings.
Fluidic Oscillator - This concept, shown in figure 13, is based on a
modification of a fluidic switch to oscillate two streams (hot and cold).
Switching of the two flow streams (hot and cold) would be achieved by
alternately applying pressure pulses to the control ports. The pressure
pulses could be supplied by a rotating valve assembly. The probe would be
positioned in the area shown in figure 13.
The principal difficulty with the above methods concerns the degree of
correlation between velocity fluctuations and temperature fluctuations, and
consequent applicability of the compensation method.
The concepts will be evaluated for technical merit and subsequent selection of
the most promising method.
COMPUTER CODE OPTIMIZATION
Task II, Compensation Code Execution Streamlining, involves developing a more
efficient numerical compensation method. In the reference 1 numerical method
(Figure 14) thermocouple response is calculated at each of several individual
frequencies, and the compensation spectrum is derived by cumulating individual
frequency results. The calculation time may be shortened through the following
approach (shown in Figure 15). Unit amplitude input frequency over the bandwidth of
interest is first inverse Fourier transformed to yield an impulse function, which
then can be used as time-dependent boundary condition for the thermocouple response
calculation. Fourier transform of thermocouple response then yields the
compensation spectrum directly. Calculation time is greatly reduced with the
impulse function approach. The remainder of the compensation method involving
ratios of the two thermocouple outputs remain unchanged.
i.
REFERENCE
Elm.re, D. L., Robinson, W. W., and W. B. Watkins, "Dynamic Gas Temperature
Measurement System Final Report, Vol. I, Technical Efforts", prepared for
NASA-Lewis Research Center by United Technologies, Pratt & Whitney, Contract
No. NAS3-23154, Report NASA CR-168267, i0 May 1983.
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Furthur development of the dynamic gas temperature measurement system

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