diode laser-based air mass flux sensor for subsonic aeropropulsion inlets

Diode laser-based air mass flux sensorfor subsonic aeropropulsion inlets

Michael F. Miller, William J. Kessler, and Mark G. Allen

An optical air mass flux sensor based on a compact, room-temperature diode laser in a fiber-coupleddelivery system has been tested on a full-scale gas turbine engine. The sensor is based on simultaneousmeasurements of O2 density and Doppler-shifted velocity along a line of sight across the inlet duct.Extensive tests spanning engine power levels from idle to full afterburner demonstrate accuracy andprecision of the order of 1–2% of full scale in density, velocity, and mass flux. The precision-limitedvelocity at atmospheric pressure was as low as 40 cmys. Multiple data-reduction procedures are quan-titatively compared to suggest optimal strategies for flight sensor packages. © 1996 Optical Society ofAmerica

Key words: Absorption, O2, density, velocity, mass flux, optical sensor.

1. Introduction

Flight qualifiable sensors for inlet air mass flux aredesired for improved control of conventional aeropro-pulsion gas turbine power plants and variable-geom-etry, mixed-compression inlets of advanced flightvehicles. At the present time, air mass flux is in-ferred from limited spatial samples of total or staticpressure and temperature. One can control vari-able-geometry inlets using open-loop altitude–airspeed scheduling without regard to local mass fluxconditions. We describe the development and test-ing, at laboratory and full-scale conditions, of an op-tical in situ air mass flux sensor that providescontinuous, rapid-response mass flux data within theinlet duct over the entire operational range of a pro-duction engine.Optically based techniques are advantageous for

inlet measurements, particularly in compressibleflows, because they do not disturb the flow field down-stream of the measurement location. Fundamen-tally, the mass flux sensor must simultaneouslydetermine the density and velocity product, rV, of agas flow through a known area. Optical densitymeasurements in unseeded air are limited to spec-troscopic methods using either O2 or N2, or total elas-

The authors are with Physical Sciences, Inc., 20 New EnglandBusiness Center, Andover, Massachusetts 01810.Received 27 November 1995; revised manuscript received 21

March 1996.0003-6935y96y244905-08$10.00y0© 1996 Optical Society of America

tic scattering from the air or aerosol particlesembedded in the air. For velocity measurements,the gas velocity is inferred from the Doppler shift ofeither the absorbed or scattered light.Early versions of the familiar laser Doppler veloci-

metry instrument recorded the Doppler shift of elas-tically scattered light from particles embedded in theflow by heterodyne detection of the scattered lightand the unshifted incident field on a common photo-detector. An imaging extension of this technique,known as Doppler global velocimetry or filtered Ray-leigh scattering has recently found widespread appli-cation.1–3 In these techniques, the Doppler shift isrecorded against a narrow-band molecular absorp-tion filter placed before an imaging detector array,providing a two-dimensional image of a component ofthe velocity field. A single-point airmass flux sensorbased on simultaneous density and velocity measure-ments that use filtered Rayleigh scattering has alsobeen investigated in laboratory flows.4 Because theelastically scattered power is low, these techniquestend to require high-power, large-frame lasers thatare not consistent with practical flight sensors.Alternatively, the Doppler shift may be recorded

with respect to a spectroscopic absorption feature of aconstituent species in the gas flow. Imaging ap-proaches based on inelastic scattering, or fluores-cence, have been demonstrated in a variety of seededaerodynamic flows5,6 or unseeded combustion flows,7taking advantage of strong diatomic absorption tran-sitions and high-power pulsed lasers. The in situDoppler shift may be also be recorded in pure absorp-tion, where the line-of-sight averaging may be advan-

20 August 1996 y Vol. 35, No. 24 y APPLIED OPTICS 4905

tageous in providing a single figure that is morerepresentative of the total flow field, including bound-ary layers and flow nonuniformities. This techniquehas been applied to mass flux measurements in su-personic flows using frequency-doubled ring-dye la-sers8 and visible diode lasers.9–11 The visible diodelaser technique is based on weak O2 absorption near760 nm and is uniquely suited to flight applicationsbecause of the low-cost, lightweight nature of thediode laser sources and the high inherent O2 concen-trations present in air flows.The sensor described here is based on an extension

of the diode laser absorption-based approach de-scribed by Phillipe and Hanson.9,10 We incorporatefiber-optic beam transport and an ultra-low-noise de-tection system to obtain high-precision O2 line-shapedata in a compact sensor architecture that is consis-tent with lightweight, low-power consumption flightapplications. The quality of these data allows us toextend the velocity range of the earlier demonstra-tions to velocities more than an order of magnitudelower, permitting the first demonstrations in sub-sonic air flows. With fiber-optic beam transport andremote citing of the laser and control–data-acquisi-tion electronics, mass flux measurements in a full-scale Pratt & Whitney F-100 engine inlet are shownwith better than 2% precision and accuracy from idleto full-power conditions.

2. Overview of the Fiber-Coupled Sensor Architecture

Figure 1 shows a schematic layout of the fiber-cou-pled diode laser air mass flux sensor. The absorp-tion measurements are obtained by using the outputfrom a high-power AlGaAs Fabry–Perot diode laseroperating near 763 nm. This wavelength region cor-responds to the ~0, 0! band of the weak O2 ~b–X!forbidden electronic transition system, which pos-sesses typical peak absorption strengths at room tem-perature and 1 atm of a few parts in 104 percentimeter of absorption path length. The laser out-

Fig. 1. Schematic layout of diode laser air mass flux sensor.

4906 APPLIED OPTICS y Vol. 35, No. 24 y 20 August 1996

put is coupled through an optical isolator into a sin-gle-mode fiber pigtail that typically transmits up to30mW of single-mode radiation. This output is splitinto four equal intensity paths by using a single-modefused 1 3 4 splitter. This splitter creates two signaland reference channels for the detection system.These fibers may be transported over long distancesto the desired measurement location: In the presentapplication, fiber lengths of 60 m were employed.The signal fibers terminate with integral collimatingoptics that are mounted in adjustable launch mod-ules at ;45° onto the flow duct or engine inlet. Theonly free-space propagation of the laser beams isacross these paths, traversing the measurement flowfield, after which the beams are recaptured in one oftwo ways: ~a! with collimating optics that direct thesignals through fiber-optic cables to the balanced ra-tiometric detectors ~BRD’s! ~termed the fully fiber-coupled system!, or ~b! directly on the photodetectorsof the BRD’s signal channel ~fiber-launch–BRD col-lection system!. Data obtained by using both config-urations are presented. The outputs of the BRD’sare amplified and bandpass filtered before recording.In earlier publications12,13 we showed how the com-

mon-mode noise cancellation properties of the BRDcan be used to obtain near-shot-noise-limited absorp-tion measurements in aerodynamic and propulsionapplications without recourse to frequency-modula-tion techniques. In the present application, thisfundamental absorbance sensitivity noise floor is ap-proximately 3 orders of magnitude below the peakabsorption signals, thereby permitting extremelylow-noise measurements in quiescent environments.In practical flow applications, the limiting measure-ment uncertainty is usually imposed by vibrations,which result in pointing instabilities that randomlymodulate the captured power in the two signal chan-nels. For the fully fiber-coupled configuration, eachcaptured beam is refocused onto a 100-mm core, mul-timode signal return fiber. Although the multimodefiber has a substantially increased core diametercompared with the 5-mm single-mode launch fiber,the coupling efficiency and transport losses are verysensitive to the exact position and entrance angle ofthe measurement beam, rendering the system sus-ceptible to beam steering effects and vibration-in-duced noise. An alternative layout was employed inthe full-scale engine tests described below, in whichthe capture modules are replaced with the BRD’s sothat the launch beam directly illuminates the 6mm 3 6 mm square Si detector, thereby providing alarger target and greatly reducing any pointing in-stability noise.One can coarsely tune the laser wavelength to the

desired absorption feature by adjusting the temper-ature of the laser mount with an ILX Model LDT-5910B thermoelectric temperature controller. Inthe current experiments, the laser is tuned to thePQ~11, 10! transition located at 13086.1 cm21 ~764.2nm! or the PQ~13, 12! transition at 13078.2 cm21

~764.6 nm!. The laser wavelength can be scannedacross the absorption feature by modulating the laser

injection current with a waveform generator ~ILXModel LDX-3207B current controller and HP 33120Awaveform generator!. The laser scan rate variesfrom 50 Hz in the engine tests to 1 kHz in the labo-ratory wind tunnel evaluations. At these rates thelaser exhibits continuous scanning over ;1.5 cm21

between mode hops of ;10 cm21. The bandwidth ofthe overall BRD response has been measured as;100 kHz. The scan rate is optimized for a givenapplication, depending on the overall noise level andthe number of scans that must be averaged to achievethe target signal-to-noise ratio. Typical measure-ment times are of the order of 1 s.

3. Data-Analysis and Data-Reduction Techniques

The attenuation of the laser beam propagatingthrough the air is described by Beer’s law:

I 5 Io exp@2S~T!g~v 2 vo!Nl# (1)

where Io is the initial intensity, I is the measuredintensity after propagation through path length l,S~T! is the temperature-dependent line strength ofthe absorbing transition centered at vo, g~v 2 vo! isthe line-shape function at laser frequency v, andN isthe absorber number density. For typical inlet con-ditions, the absorption line shape is described by aVoigt profile that is a function of thermal ~Doppler!and collisional broadening.Figure 2 shows an absorption line shape of the

PQ~11, 10! transition recorded in laboratory air witha path length of 1.5 m, using the fiber-launch–BRDcollection arrangement. The line shape, which is anaverage of 200 individual laser sweeps, has been cor-rected for a linear baseline variation and is plotted asa function of relative frequency from line center.For a typical setup, the linear baseline variation is ofthe order of 5% of the peak absorption. The fre-quency scale was determined by simultaneously re-cording the transmission through a Fabry–Perotinterferometer, providing a continuous record of thelaser scan rate. For these narrow scan ranges, thescan rate is essentially constant, although it is de-pendent on the laser sweep frequency and range.

Fig. 2. Example line shape of the PQ~11, 10! transition at 13086.1cm21 recorded in laboratory air.

Also shown is a Voigt fit to the line shape and thecorresponding line-shape parameters determinedfrom the fit. For the Voigt fitting procedure, theDoppler width was constrained to 0.029 cm21, corre-sponding to the room-temperature conditions. Themeasured full-width at half-maximum ~FWHM! ofthis line is 0.095 cm21, corresponding to a measuredair collision width of 0.086 cm21. The residual be-tween the data and the fit is generally less than 1% ofthe peak height, indicating the high quality of thedata.As the laser is scanned across the line shape, the

absorption signal with respect to the baseline awayfrom the transition is integrated, producing a singlenumber proportional to the air density and therebyeliminating any temperature and pressure depen-dencies arising from line-broadening mechanisms.This scanned-integral approach also reduces any ar-tifacts caused by particulate or condensate absorp-tion or scattering, which is spectrally flat over thesescan ranges and does not contribute to the integratedline-shape signal. The air density is directly relatedto the integrated area under the line shape becausethe O2 mole fraction is a fixed component of air. Intypical installations, a calibration measurement re-lating the integrated area to the known air density atstatic conditions is used to place all subsequent dataon an absolute density basis. The twomeasurementpaths shown in Fig. 1 represent two separate densitydeterminations that can be averaged together, if de-sired, to improve the signal-to-noise levels. The airvelocity is determined from the frequency shift Dvbetween the absorption line centers recorded in thetwo measurement paths. Assuming nominally one-dimensional flow in the duct, we find that this veloc-ity is given by

V 5Dv

vo

c~cos u1 2 cos u2!

, (2)

where vo is the unshifted line-center frequency, c isthe speed of light, and u1 and u2 are the angles shownin Fig. 1. Possible swirling flow in the duct would besmaller than the main axial flow and would symmet-rically upshift and downshift the apparent absorptionfrequency along the line of sight; thus it would notcontribute to the measured velocity or mass flux.This is desirable because the swirl velocity does notcontribute to the axial mass flow. Inlet tests withknown azimuthal distortions are under way to deter-mine the effect on the sensor’s performance.Determining the frequency shift between the two

line shapes involves locating the line center of thepeak amplitude point of the line-shape waveformsand then differencing the positions. Several meth-ods of determining this line center have been ex-plored to find those that give the maximum accuracywith a minimal scan resolution ~number of digitizedpoints per line shape! and minimal computationalcomplexity. In order of computational complexity,the three candidate methods are the FWHM, deriv-ative, and Voigt methods. The FWHM method in-


volves determining the position of the two FWHMpoints of the line and averaging their separation tofind the line center. This method has the advantagethat it can be implemented by using all-analogthreshold discriminators, achieving the maximumthroughput of velocity measurements. In the deriv-ative method, the line shape is differentiated, andlinear interpolation is used to determine the zerocrossing of the differentiated waveform, correspond-ing to the peak center. This method is also poten-tially amenable to all analog processing. In theVoigt method, a Voigt function is fit to the full lineshape, with the line-center frequency being a param-eter of the fitting routine. This method is the mostcomputationally intense, requiring digitization of theline shapes, but it uses the information from the en-tire line shape to determine the line-center fre-quency. This is advantageous in that random signalfluctuations that occur with a characteristic periodless than the linewidth are effectively averaged.Longer period undulations in the baseline, however,may actually degrade the accuracy of the line-centerestimation because they tend to distort the wing be-havior of the line shape. The derivative and FWHMmethods do not use the wing behavior to estimate theline-center frequency.

4. Wind Tunnel and Full-Scale Engine Test Results

Initial laboratory testing of the sensor was conductedin a wind tunnel with a 10 cm 3 5 cm rectangularcross section 40.6 cm in length. The tunnel is anopen-loop suction design with a maximum velocity of120 mys. Mean and fluctuating velocity profileswere recorded with a Pitot-static probe and a con-stant-temperature hot-wire anemometer, respec-tively, at several axial locations in the test section.At typical flow conditions, the center-line velocity isuniform to within 5% and accelerates less than 10%along the test section length. The boundary layerdisplacement thickness was measured to be less than1 mm thick, corresponding to less than 2% of theoptical line of sight of the absorption measurement.The rms velocity fluctuations from dc to 10 kHz wereless than 1.5% of the mean velocity. Flow measure-ments were made at a position 4 cm from the testsection entrance through antireflection-coated win-dows. The two laser beams crossed the 10-cm widthof the test section at 45° angles, resulting in absorp-tion path lengths of approximately 15 cm each.Figure 3 shows twoPQ~13, 12! line shapes recorded

at the maximum tunnel air velocity of 118 mys, asdetermined by the Pitot probe. The peak absorptionin these line shapes is 0.24% with a noise-equivalentabsorbance of 2 3 1026 Hz21y2. For this individualmeasurement, the FWHMmethod resulted in a shiftof 0.00709 cm21, for a velocity of 115 mys. The de-rivative method gave 126 mys and the Voigt methodgave 110 mys.Hundreds of individual measurements of density

and velocity, gathered over the course of several daysand with various optical setups, were examined todetermine the statistical precision and absolute ac-


curacy of the mass flux sensor in this laboratory con-figuration. For the velocity measurements, rmsstandard deviations ~2s! for each data-reductionmethod were ;10 mys, corresponding to a statisticalshift-to-width precision of less than 1% of the atmo-spherically broadened transitions. Histograms ofdensity determinations yielded a precision of 2.4%.The limiting precision of the mass flux sensor in thisconfiguration is therefore driven by the velocity pre-cision and is ;10% of the full range value. Theseprecision values are imposed by weak spurious etaloneffects in the baseline subtraction procedure. Ex-pressed in terms of equivalent absorbance, the rmsvariation in the density determination corresponds toa precision-limited absorbance of approximately 1025

Hz21y2, nearly an order of magnitude poorer that thebackground-limited absorbance demonstrated in Fig.2. This degradation may be attributed to the sensi-tivity of the fully fiber-coupled system to small, vi-bration-induced pointing instabilities.After the laboratory experiments, the sensor was

tested on a full-scale Pratt andWhitney F-100 enginein an open-ground test stand at the NASA DrydenFlight Research Facility. A constant area duct ~0.92m in diameter and 1.54 m in length! equipped withstatic and total pressure probes and instrument portspermitting access to the flow was attached to theengine between the compressor entrance face and theinlet bellmouth. Beam launch and capture moduleswere fabricated with 1-in. ~2.54-cm!-diameter win-dows and attached directly to the side of the duct.The laser, control electronics, and data-acquisitionsystems were located in a control bunker 15 m fromthe engine. Both fully fiber-coupled and fiber-launch–BRD collection configurations were investi-gated. The diode laser beams were transported toand from the inlet duct by using 60-m lengths ofsingle-mode ~launch! and 100-mm-diameter multi-mode ~capture! fibers. Although the fully fiber-cou-pled system worked well at engine idle conditions, itbegan to degrade substantially at higher engine pow-ers because of the vibration-induced beam steeringdiscussed above. For the remainder of the tests, weused direct illumination of the BRD’s. In this case,

Fig. 3. Example Doppler-shifted line shapes in a low-speed windtunnel at 118 mys.

the two BRD’s were mounted directly on the engineinlet and their output was transported to the controlroom using a BNC cable.Over the course of a 2-day period, the sensor was

tested over a range of engine power levels. Table 1lists the inlet air settings under which testing wasperformed. The test points are spaced in approxi-mately equal mass flux increments from idle ~testpoint 1! to military specifications ~test point 7, here-after referred to as mil-spec! conditions. Testing atthe full-afterburner power setting was also per-formed. This last condition is essentially the samemass flux as test point 7, although the vibration andacoustic loadings to the sensor are greatly increasedand represent the most extreme conditions experi-enced in flight. The air data values shown are basedon a single-point Pitot-static pressure probe insertedinto the inlet just beyond the boundary layer. Thelast column reports the calibrated digital electronicengine controller ~DEEC! values of mass flux used bythe engine control system. The difference betweenthe air data and DEEC values are illustrative of theconfidence levels of these data.Before the engine was run, the sensor was cali-

brated for absolute density measurements in the qui-escent atmosphere of the test stand. The relativedensity during the test is then inferred by the inte-grated area relative to this calibration condition.Figure 4 shows the line shapes for each channel re-corded at the reference conditions ~307 K, 0.91 atm!along with Voigt fits for each line shape. The signal-to-noise ratio of these data is high and the line-shapeparameters from the two fits agree with one anotherto within 2%. The collision width for this measure-ment is ;5% lower than the sea-level determinationsshown in Fig. 2, reflecting the reduced pressure at thetest stand elevation. The ability to detect these sub-tle differences in the measured line shapes indicatesnot only the quality of the data but also the potentialfor simultaneous temperature and pressuremeasure-ments based on these line shapes.The three methods used to estimate the line-center

Doppler shift were applied to these quiescent lineshapes, and all revealed slight shifts of the order of0.1% of the FWHM or a few meters per second.

Table 1. Summary of Full-Scale Engine Test Conditions

Test Point

Air Data ValuesDEECValue

Density~kgym3!

Velocity~mys!

Mass Flux~lbys!

Mass Flux~lbys!

1 1.050 46.1 65.3 NAa

2 1.040 60.1 84.3 NA3 1.027 78.9 109.3 NA4 1.012 102.4 139.8 130.65 0.995 113.3 152.1 147.16 0.956 139.6 182.2 174.07 0.920 167.0 207.3 201.4

aNot available.

These apparent shifts result from slight errors in thebaseline subtractions procedures and reflect the re-sidual influence of the etalon effects noted above.This zero-velocity offset is subtracted from all subse-quent data and indicates a limitation to the absoluteaccuracy of the velocitymeasurement, which can nev-ertheless be calibrated for any given setup by usingsimple static air measurements. The only other con-figuration-dependent correction applied to the dataarises from the small static air path outside the duct.Because this air absorption was the same throughoutthe engine test, its contribution to the integrated ab-sorbance was subtracted from all the density mea-surements by determining the fractional contributionto the calibration signal ~based on the fractional pathlength outside of the duct!. The correction for thevelocity measurement is more subtle. Because theair outside the inlet was not moving, the contributionof this path length to the total line shape is not Dopp-ler shifted. For the present application, the staticair path corresponded to;10% of the total absorptionpath length. Modeling of this effect for stagnant airpaths up to 20% of the total path length and forshift-to-width ratios of the order of 0.1 or less showedthat this potential line-shape distortion is small andacts to decrease the measured shift by an amountproportional to the fraction of the total path lengththat is stagnant. The measured shifts are simplycorrected by dividing them by the ratio of the pathlength inside the inlet to the total path length. Thisresult also illuminated the response of the sensor to

Fig. 4. Calibration line shapes recorded at an engine test standprior to tests.


variations in velocity ~or mass flux! along the line ofsight. As long as the total shift-to-width values aresmall ~as they will always be in subsonic inlets nearatmospheric pressure!, the measured velocity is geo-metrically weighted according to the velocity–path-length product, providing an accurate determinationof the path-averaged velocity. Because both of thesecorrections depend only on the geometry of the sensorinstallation, integration of them into the data-reduc-tion algorithms is straightforward and does notchange once the sensor is installed.Figure 5 shows example line shapes recorded

through each channel at idle, mil-spec, and full-after-burner conditions. Each line shape shown repre-sents the average of 200 individual sweeps, whichrequired 4 s to record because of the relatively slow50-Hz laser scan rates employed. With increasingpower, different vibrational resonances of the enginewere excited, resulting in a complex ~though well-

Fig. 5. Comparison of O2 line shapes recorded in full-scale enginetests at ~a! idle, ~b! mil-spec, ~c! full-afterburner conditions.


understood by the engine manufacturer! series ofspectrally well-isolated noise sources. In mostcases, their amplitude was sufficiently small that sin-gle sweep data line shapes still exhibited acceptablesingle-to-noise values, although a small number ofaverages reduced their influence considerably. Inthe full-afterburner demonstrations shown in Fig.5~c! some residual noise is still apparent, particularlyin one of the channels, which appeared to suffer fromgreater vibration-induced beam steering than theother over the entire range of tested engine powers.Dozens of individual measurements were made at

each power condition by using a variety of data-re-cording and data-reduction methods selected to eval-uate potential flight test configurations. In thissection, we show results from the highest perfor-mancemethods and discuss degradations determinedby using simpler data-reduction procedures andlower resolution data-recording schemes. The high-est performance system recorded the data with a two-channel, 1-MHz digital sampling oscilloscope, whichprovided 10,000 data points over each laser sweep.A comparison of the optical density measurements tothe pitot-static air density determinations is shownin Fig. 6. The rms standard deviation ~2s! of theoptical data at each condition is shown by the errorbars, except for the low-speed conditions where theerror bars are smaller than the data symbol. Exactagreement with the air data is indicated by the solidline. The density precision is everywhere less than1% and within 1% of the independent air data values.This represents a precision-equivalent absorbance of1 3 1024 Hz21y2, approximately 2 orders of magni-tude worse than the best single-sample uncertaintyobtained in quiescent laboratory conditions. Inthese measurements, the precision is limited by thevibration-induced fluctuations in the baseline and isessentially independent of engine power. Morecoarsely resolved measurements obtained with 1000points across a line shape ~consistent with rapid and

Fig. 6. Comparison of optical density measurements to Pitot-static air data as a function of engine power.

autonomous flight packages! resulted in a 2% stan-dard deviation.Comparisons of the optical velocity measurements

to the air data values are shown in Fig. 7. Onlyresults from the FWHM and Voigt reduction proce-dures are shown—the derivative data-reductionmethod suffered intolerable uncertainties because ofthe residual vibration-induced noise level in the data.The standard deviations of the measurements areindicated by the error bars on the symbols. For themajority of conditions, the air data velocities liewithin the standard deviation of the optical sensormeasurements. When all power settings are aver-aged over, the percent difference between the sensorand air data velocities is ;3% for both the Voigt andFWHM methods. Thus, these two methods yieldequivalent average measurements; however, theVoigt method results have substantially lower stan-dard deviations—less than 1 mys for most conditionscompared with ;3 mys for the FWHMmethod. Theconfidence level in the air data measurements are ofthe same order as the percent differences realizedbetween these values and the optical sensormeasure-ments; thus, we believe that the measurement preci-sion of the current results ~expressed as rmswidths ofan ensemble of individual measurements! providesthe best measure of the sensor’s performance.For the high-resolution digital oscilloscope data, a

precision of,2%was obtained with the Voigt methodand ,4% with the FWHM method. With the use ofthe coarser data sets, these values increased to ,3%and ,5%, respectively. It is significant to note thatthe precision of the Voigt method was as high as 40cmys at an average mean velocity of 48 mys andcorresponds to a limiting shift-to-width ratio of 3 31024, twice the statistical precision achieved in thelaboratory demonstrations. This improved perfor-mance is due to the increased absorption strength inthe 1-m duct compared with the fixed spurious etalonfringes, the relative insensitivity of the velocity re-duction algorithms to the vibration-induced noise,

Fig. 7. Comparison of optical velocity measurements obtainedwith Voigt and FWHM data-reduction methods to Pitot-static airdata values.

and the direct BRD illumination geometry employedin the engine tests.Because the uncertainty of the velocity measure-

ments exceeds that of the density measurements, theprecision of the mass flux determinations is domi-nated by the velocity precision. Figure 8 comparesthe standard deviations of the mass flux measure-ments over the engine power range for the Voigt andFWHM methods applied to the high-resolution dataset. With the Voigt fitting method, the average pre-cision and accuracy of the mass flux sensor are betterthan 2% of full scale. With the use of the coarserdata sets and the computationally less intenseFWHMmethod, the precision reduces to less than 4%over the entire range. As shown in Fig. 5, the vibra-tion-induced noise pickup was substantially larger inone channel and could presumably be reducedthrough repair of this mount so that it achieved thestability evidenced by the other optical mount. Weestimate that a 3% uncertainty could be achievedwith this improved mount and note again that theFWHM and line-shape integration data-reductionmethodologies could be implemented in all-analoghardware, thereby reducing the time to report re-duced mass flux values to below 1 s.

5. Conclusions and Summary

These results demonstrate the feasibility of a com-pact diode laser-based air mass flux sensor forfull-scale, subsonic aeropropulsion inlets. In appli-cations in which high accuracy is essential and real-time control is not required, high-resolution line-shape data were shown to yield density, velocity, andmass flux accuracy and precision of the order of 1–2%over the full range of engine operating power. Con-tinuous, real-time sensor systems with bandwidthsapproaching 1 Hz that use simpler processing algo-rithms amenable to all-analog implementationshould achieve 3–4% precision. These results wereall obtained near atmospheric pressure. Assumingthe limiting shift-to-width ratio is an approximatescaling parameter, we can project limiting velocitysensitivities at other pressures. In room-tempera-ture flows, as the pressure drops below 0.1 atm, theO2 line shape becomes predominantly Doppler broad-ened with a limiting FWHM of 0.029 cm21. Theprecision-limited velocity shift demonstrated on thefull-scale engine would then be ;9 3 1026 cm21,

Fig. 8. Comparison of the rms standard deviation of diode laser-based air mass flux measurements on a full-scale Pratt and Whit-ney F-100 engine from idle to mil-spec power levels. Resultsshown were obtained with high-resolution data.


corresponding to a velocity of ;15 cmys. Above ;1atm, the room-temperature FWHM becomes essen-tially linearly proportional to pressure so that thelimiting velocity precision at 10 atm, for example,would increase to ;4 mys.Research is in progress to install the sensor on a

calibrated American Society of Mechanical Engineersduct at the NASA Lewis Research Center to enlargethe database of precision statistics in higher velocityand mass flux ranges. These tests are expected toprovide a calibrated mass flux standard with 0.25%absolute accuracy—approximately an order of mag-nitude better than the engine test stand values usedas a standard in the present research. Further testswith the sensor mounted downstream of a variable-geometry supersonic inlet are also planned.

This research was sponsored by the NASA DrydenFlight Research Facility under contract NAS2-14001.The authors gratefully acknowledge the extensivesupport of the NASA technical monitor, TimConners,in organizing the engine tests, assisting in the setupof the sensor at the test stand, and in obtaining andreducing both the Pitot-static air data and DEECcomparison values. Dan Palombo of Physical Sci-ences, Inc. was instrumental in fabricating and test-ing the BRD’s used for the laboratory and field tests,as well as developing themodels required to interpretquantitatively their output.

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diode laser-based air mass flux sensor for subsonic aeropropulsion inlets

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