Fast Temperature Sensor for Combustion Control Using H2O Diode Laser Absorption near 1.4 µm

Xin Zhou*, Jay B. Jeffries† and R. K. Hanson‡

High Temperature Gasdynamics Laboratory Department of Mechanical Engineering

Stanford University, Stanford, CA 94305-3032

Guoqiang Li+ and Ephraim J. Gutmark§

Aerospace Engineering & Engineering Mechanics University of Cincinnati Cincinnati, OH 45221

A diode-laser sensor system has been developed for non-intrusive measurements of gas temperature in combustion systems using a scanned-wavelength 2f technique. The sensor is based on a single diode laser (distributed-feedback), operating near 1.4 µm and scanned over a spectral range targeting a pair of H2O absorption transitions (7154.354cm-1 & 7153.748 cm-1). The single-fiber-coupled-laser design makes the system compact, rugged, low cost and simple to operate. Gas temperature is inferred from the ratio of the second harmonic signal of two selected H2O transitions. The sensor design includes software for fast data acquisition and analysis to provide rapid temperature measurements; a temperature readout rate of 2 kHz has been demonstrated for measurements in a swirl-stabilized spray flame at atmospheric pressure. Comparisons are presented with earlier work using a direct-absorption-based sensor which scanned two H2O transitions near 1.8 µm. The combination of scanned-wavelength and wavelength-modulation minimizes interference from emission and provides a robust temperature measurement that should prove useful for combustion control applications.

I. Introduction 2Ocopa

is one of the primary combustion products. Measurements of water vapor are thus generally useful in mbustion and propulsion engineering, since water vapor concentration can be related to performance rameters such as combustion and propulsion efficiency, and heat release1-4. Consequently techniques based

on high-resolution absorption spectroscopy of H2O have been developed to provide fast, sensitive, and nonintrusive means of measuring multiple parameters such as temperature, pressure, velocity, and density1-8. The use of tunable semiconductor diode lasers for application of these spectroscopic techniques is attractive as these lasers are compact, rugged, cost effective, compatible with optical fiber transmission, and simple to operate. By suitable choice of laser wavelength it is possible to measure temperature using a single diode laser.7 The use of a single diode laser can greatly simplify the sensor system and reduce cost compared with wavelength-multiplexing techniques.

H

Previous combustion control work2,6 in our laboratory showed that temperature is a good control variable for complete combustion and reduced emissions in a forced vortex incinerator. Furlong et al.2 used two multiplexed diode lasers to infer temperature at a 2 kHz rate from the peak absorbance ratio. Although this ratio was a good

* Research Assistant, Stanford University, CA 94305-3032, AIAA Student Member + Research Assistant Professor, University of Cincinnati, OH 45221, AIAA Member † Senior Research Engineer, Stanford University, CA 94305-3032, AIAA Associate Fellow § Professor, University of Cincinnati, OH 45221, AIAA Associate Fellow ‡ Professor, Stanford University, CA 94305-3032, AIAA Fellow

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43rd AIAA Aerospace Sciences Meeting and Exhibit10 - 13 January 2005, Reno, Nevada

AIAA 2005-627

Copyright © 2005 by authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

control variable, the ratio was contaminated by optical emission and other interference; thus the inferred absolute temperature was subject to large uncertainty. A scanned-wavelength approach utilizing ratios of integrated absorbance offers significant mitigation of these problems at the cost of significantly more complex data analysis. The laser wavelength is scanned across an absorption feature and the zero absorption transmission baseline must be inferred. The current work illustrates that this data acquisition and processing can be significantly simplified by using a wavelength modulation approach with 2f detection together with scanned wavelength, yielding a temperature measurement readout rate of 2 kHz.

Previous work7 reported a 1.8 µm single-laser temperature sensor based on wavelength-scanned direct-absorption measurement of two adjacent H2O lines. This sensor system has the desired flexibility, sensitivity, speed and accuracy to be a useful tool for fundamental and applied combustion monitoring. However, it is not without disadvantages. A major limitation is that the strong absorption at room temperature of one of these lines makes it sensitive to ambient air interference. Additionally, although this previous sensor could acquire temperature data at kHz rates, the complex data analysis method required post-processing to extract temperature.

These limitations are substantially mitigated in the research reported here utilizing a 1.4 µm single-laser temperature sensor together with a combination of scanned-wavelength and wavelength modulation spectroscopy (WMS) with 2f detection. By using wavelength modulation with 2f detection, the measurement sensitivity is improved by shifting the detection to higher frequencies where laser excess noise and detector thermal noise are both much smaller; in addition noise outside the detection bandwidth is suppressed using phase-sensitive detection.8-

14 Furthermore, the data analysis bandwidth is significantly increased because 2f detection simplifies the baseline analysis. This new sensor also takes advantage of the mature telecommunication laser technology at 1.4 µm where fiber-coupled lasers and fiber components are readily available. Moreover, the selected transitions originate on energy levels with significant internal energy and thus have weak absorption at room temperature, minimizing the ambient air interference. This new sensor system offers significant opportunities and advantages for in situ measurements of temperature for combustion control.

Measurements of temperature require measurements of at least two transitions. In the present work we selected a line pair near 1.4 µm, because the overtone and combination bands of H2O near 1.4 µm are relatively free of interference from the absorption spectra of other major combustion products and are readily accessible using commercially available telecommunication fiber-coupled diode lasers and optics. The present work is aimed at establishing the feasibility of a diode-laser absorption sensor system for temperature that might be used in a practical combustion control system, e.g., for gas turbine combustors.

A multiple-swirl spray combustor is used for sensor evaluation, leading to the first demonstration of TDL thermometry in a liquid-fuel swirl-stabilized spray flame. A real-time temperature readout rate of 2 kHz has been achieved, which provides evidence that TDL sensing is sufficiently fast for use in combustion control applications.

Ramp

Measurementpath length+

Detector signals to lock-in

Diode laser Controller

Computer

Functiongenerator

Diode Laser CollimatorModulate at f

Lock-in amplifier

Reference signalFunctiongenerator

Scanned-2f Harmonic Signal

Ramp

Measurementpath length+

Detector signals to lock-in

Diode laser Controller

Computer

Functiongenerator

Diode Laser CollimatorModulate at f

Lock-in amplifier

Reference signalFunctiongenerator

Scanned-2f Harmonic Signal

Figure 1. A typical arrangement for the measurement by the WMS technique.

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II. Theory A typical arrangement for the measurement by the WMS technique is shown in figure 1. The combination of

slow ramp and a fast sinusoidal modulation is used to drive the diode laser. Thus the laser output frequency can be expressed by:

( )ν( ) ν cos 2 mt a fπ= + t (1) where ν [cm-1] is the laser center frequency, a [cm-1] is the modulation amplitude and fm [Hz] is the modulation frequency.

As is well known, the fractional transmission coefficient τ is described by the Beer-Lambert relation

0

exp( )I P X S LI ν

ν

τ φ⎛ ⎞

= = − ⋅ ⋅ ⋅ ⋅⎜ ⎟⎝ ⎠

(2)

where I and I0 are the transmitted and incident spectral intensities, P [atm] is the total pressure, X is the mole fraction of the absorbing species, S [cm-2atm-1] is the line strength, φν [cm] is the line-shape function of a particular

transition, which is normalized such that ( ) 1i dφ ν ν ≡∫ , and L [cm] is the path length.

The quantity τ can be expanded in a Fourier cosine series:

0( cos(2 )) (ν, )cos( 2 )

n

m nn

a f t H a n fτ ν π π=+∞

=+ = ∑ mt

(3)

where (ν, )nH a is the nth Fourier coefficient of the transmission coefficient, given by

01(ν, ) ( cos )

2H a a dπ

π τ ν θ θπ

+−= +∫

(4)

1(ν, ) ( cos ) cos( )nH a a n dππ τ ν θ θ θ

π+−= + ⋅∫ ⋅

(5)

For weak transitions, i.e. ( ) 0.05S P X Lφ ν⋅ ⋅ ⋅ ⋅ ≤ (6)

the transmission coefficient can be approximated as: ( )

0

( )( ) [1 ( ) ]( )

LI e S P XI

α ν Lντ ν φ νν

−= = ≈ − ⋅ ⋅ ⋅ ⋅

(7)

and the nth harmonic Fourier coefficient simplifies as

(ν, ) ( cos ) cos( )n

S P X LH a aπ

π n dφν θ θπ

+−

⋅ ⋅ ⋅=− + ⋅ ⋅∫ θ

(8)

Second harmonic detection (2f) of the WMS signal is the most frequently applied method in practice. It is of great interest for several reasons: First, the 2f line shape peaks at line center and is symmetric due to the nature of even functions. Second, 2f has the strongest signal in the even-numbered set of frequencies, since the (n+1)th harmonic signal has smaller magnitude than nth harmonic signal. Third, linear background signals can be removed from 2f signal.

According to Eqn. (8), the second harmonic Fourier coefficient is given by

2(ν, ) ( cos ) cos(2 )S P X L

H a aππ dφν θ θ θ

π+−

⋅ ⋅ ⋅=− + ⋅ ⋅∫

(9)

The 2f signal is proportional to 0 2 (ν , )I H a if intensity modulation effects are neglected. It is worth noting that the 2f signal depends not only on the transition parameters such as line strength, etc., but

also on the modulation amplitude “a”. An important dimensionless parameter which is widely used in 2f WMS, the modulation depth m, is defined as:

/ 2amν

=∆

(10)

where a [cm-1] is the modulation amplitude, ∆ν/2 [cm-1] is the half width at half-maximum of the absorption lineshape.

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2f line shapes of a Voigt profile are shown in figure 2 (left). It is clearly shown that the maximum amplitude occurs at line center. The line shape becomes wider as the modulation depth increases. The line center peak heights for different modulation depth “m” are shown in figure 2 (right). Note that the peak height is maximum at m=2.2, which optimizes the signal-to-noise ratio.

Similar to direct absorption techniques, WMS employs the two-line method for temperature measurements. The temperature can be obtained from the fact that the 2f peak height ratio of two transitions is a function of temperature, closely related to the ratio of absorption line strengths.

1 12 1 1

2 2 2 2 2

( cos )cos2( ) ( )( ) ( ) ( cos )cos2

aH S TH S T a

π

ππ

π

φν θ θνν φν θ θ

+

−+

−

+ ⋅=

+ ⋅

∫∫

As can be seen in the above equation, 2f peak height ratio function of species concentration, modulation amplitude and pressobviously complicates the temperature measurement. In order to usually done to provide the relationship between temperature anduse of accurate spectroscopic parameters.

It should be noted that, for simplicity, many hardware-related pto determine temperature from the 2f signal ratio, values for ahardware are required exclusive of measuring the 2f signal itself. signal amplification, lock-in setting, etc. The usual approach is to to eliminate the dependence of hardware-related parameters.

III. Improved SensorUsing the line selection method developed in previous work,7

for temperature measurements in flames. Based on these findings, for the design and development of single laser 2f sensor for measThe needed fundamental spectroscopic parameters, including broadening coefficients, and their temperature dependence, were measured experimentally using low-pressure H2O in a heated cell, and this work will be reported separately.

The new 1.4 µm sensor is based on the lessons learned with a previous 1.8 µm sensor. The calculated spectroscopic features for the water line pairs of the 1.4 µm sensor and the 1.8 µm sensor are shown in figure 3. Both sensors are built on the single-laser two-line concept, which measures two nearby temperature sensitive water transitions with a single laser scan, thus simplify the sensor system and reducing cost.

The new 1.4 µm sensor includes several important improvements over the 1.8 µm sensor. The first improvement is that the 1.4 µm sensor has weak absorption at room temperature, as shown in figure 4, thus minimizing ambient air interference. The existence of ambient air interference can reduce the measurement accuracy and lead to increased measurement uncertainty. It is usually necessary to purge outside the target measurement zone with nitrogen to H

American Institute of Aeronautics

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20x10-3

15

10

5

0

-5

-10

-15

Sig

nal [

a.u.

]

-4 0 4Normalized Frequency (ν−ν0)/∆ν

54321m

m=1 m=2.2 m=3 m=4

Peak at m=2.2

Figure 2. Second harmonic line shape and line-center peakheight for different “m”.

d

d

θ

θ

⋅

⋅

(11)

is not only a function of temperature, but also a ure through the effects of line shape function. This obtain the temperature, a numerical simulation is 2f peak height ratio. This often necessitates the

arameters are not taken into account yet. In order number of parameters related to the instrument These include the laser intensity, detector setting, calibrate the WMS sensor at a reference condition

Design several promising water line pairs were identified a pair of H2O transitions near 1.4 µm was targeted uring temperature in atmospheric-pressure flames. line strengths, self-broadening coefficients, air-

0.010

0.008

0.006

0.004

0.002

0.000

Abs

orpt

ion

coef

ficie

nt [c

m-1

]

7155.07154.57154.07153.57153.0Frequency [cm

-1]

0.010

0.008

0.006

0.004

0.002

0.000

5555.05554.55554.05553.55553.0

T = 296 K T = 1000 K T = 2000 K

P = 1 atmX = 10%

1.8 µm sensor

1.4 µm sensor

Figure 3. Calculated spectroscopic features for water linepairs in the 1.4 µm and 1.8 µm sensors based on HITRAN;X = 10%.

2O

and Astronautics

eliminate the ambient water interference. The amount of nitrogen necessary to attain a desired low humidity set-point depends not only on the ambient humidity level, but also on the absorption strength of the transition. Due to overlap with a cold water transition, the 1.8 µm sensor has a very strong water absorption at room temperature, and therefore the purging process of the 1.8 µm sensor requires significant care. This difficulty is mitigated by the 1.4 µm sensor.

The second improvement is that the 1.4 µm sensor employs a sensitive wavelength-modulation technique, thus achieving better SNR and detection limits. Modulation spectroscopy is a widely used technique for sensitive trace-species detection. It can significantly reduce the dominating 1/f noise by shifting detection to higher frequencies,14 and provides a substantial sensitivity enhancement compared to the direct absorption methods used in the 1.8 µm sensor. Therefore, the 1.4 µm sensor can be applied to noisy environments where the 1.8 µm sensor showed significant uncertainty.

The third improvement is that the 1.4 µm sensor has a simpler data reduction strategy and thus can achieve better real-time performance. The data reduction strategies used in the 1.4 µm sensor and 1.8 µm sensor are illustrated in figure 4. The 1.8 µm sensor employs the ratio of integrated areas to determine temperature; hence the non-linear and time-consuming Voigt fitting of the lineshape usually requires post-data reduction, thus making it impractical to determine temperature in real-time. The 1.4 µm sensor uses the ratio of the 2f peak heights to avoid time-consuming fits to the line shape, and can reach a 2 kHz real-time measurement rate (2 kHz scan repetition rate). Hence, the 1.4 µm sensor could be utilized in many time-critical applications such as real-time monitoring and control of combustion.

The fourth improvement is that the 1.4 µm sensor takes advantage of the mature telecommunication laser techniques at 1.4 µm where fiber-coupled lasers and fiber components are readily available. The fiber-coupled lasers and fiber optics at 1.4 µm have many attractive features that are superior to the free-space lasers and optics at 1.8 µm. These include advanced laser performance, simple installation, easy laser beam alignment, improved ruggedness and flexibility, and reduced overall system cost.

A comparison of the new 1.4 µm sensor and previous 1.8 µm sensor follows below.

3x10-3

2

1

0

-1

2f s

igna

l [a.

b. u

nits

]

7155.07154.57154.07153.57153.0Frequency [cm

-1]

0.010

0.008

0.006

0.004

0.002

0.000Abs

orpt

ion

coef

ficie

nt [c

m-1

]

5555.05554.55554.05553.55553.0

T = 1000 K T = 2000 K

P = 1 atmX = 10%

1.8 µm sensor

1.4 µm sensor

Figure 4. Comparison of the measurement strategy of the 1.4 µm sensor and 1.8 µm sensor.

IV. Combustion Demonstration The 1.4 µm sensor and 1.8 µm sensor were both

applied to an atmospheric-pressure swirl-stabilized spray combustor.

A. 1.8 µm sensor

Figure 5 illustrates the general arrangement employed for the 1.8 µm sensor. The output from the DFB laser near 1.8 µm is directed across the flame on the swirl-stabilized spray combustor using appropriate mirrors. Although infrared-sensitive cards are commercially available near 1.8 µm, the laser beam is not easily observed due to the low power of the laser. The laser beam is coaligned with a “red” HeNe visible laser beam for alignment purposes, via a flat flip mirror. The diode laser is temperature and current controlled (ILX Lightwave LDC-3900) and injection current tuned (SRS DS345) across the two absorption transitions. The beam path is purged to avoid interference from ambient water vapor.

Diode laser controller

Functiongenerator

Computer

FilterN2 purge N2 purge

SpeakerSpeaker

Mirror

Mirror

HeNelaser

FlipMirror

1.8 µmDFB laser

Collimator

Detector

Diode laser controller

Functiongenerator

Computer

FilterN2 purge N2 purge

SpeakerSpeaker

Mirror

Mirror

HeNelaser

FlipMirror

1.8 µmDFB laser

Collimator

Detector

Figure 5. Schematic diagram of the measurement system applied to the swirl-stabilized spray combustor.

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A detailed description of the swirl-stabilized spray combustor utilized in the present experiment is given elsewhere.16 The combustion chamber is 100mm square and 450 mm long, with flat quartz windows to provide optical access. The laser beam is angled ~10o horizontally to avoid the etalons from multiple reflections within the quartz windows. The total path length is 102 mm.

The laser is scanned at 2 kHz across the H2O line pair to record spectrally resolved absorption line shapes. The transmitted signal is sampled at 1MHz. A laboratory-code (written in labview) was used for post-processing data. A 4-scan average is used to improve SNR, which reduces the temperature data rate to 500 Hz. Temperature is measured for a single axial location at 50 mm downstream of the nozzle for gas fuel (propane) and liquid fuel (ethanol). Figure 6 shows the reduced pair of experimental profiles corresponding to the absorption features using gas and liquid fuel. Figure 6 (left) is for an air flow rate of 10 SCFM and propane flow rate of 10 SCFM. Figure 6 (right) is for an air flow rate of 57.5 SCFM and ethanol flow rate of 0.1 kg/min. As expected, SNR is reduced in liquid fuel experiments due to beamsteering effects and unburned liquid droplet interference. Although 4-scan averaging works for the current condition, additional averaging is required to obtain sufficient SNR under more noisy conditions. For very noisy conditions, the 1.8 µm sensor based on direct absorption may not be feasible for temperature measurements.

To illustrate the use of the sensor to identify acoustic combustion instabilities, a fluctuation was introduced in the flame by modulating the air flow with two speakers attached to the fuel flow line, as shown in figure 5. Measurements were made for an air flow rate of 10 SCFM and propane flow rate of 10 SCFM. Figure 7(a) shows the measured temperature and its power spectrum when the speaker is off. This unforced flame shows no dominant instability at this condition. The speakers are driven with a 100 Hz sine wave, thus producing an oscillating gas temperature. The dominant mode of the temperature fluctuations is clearly shown in figure 7(b), which demonstrates the utility of this sensor for quantitative characterization of acoustic disturbances.

To investigate the best location to observe the temperature fluctuations, four locations at 50 mm, 100 mm, 150 mm downstream of the nozzle and diagnostic direction are examined. Location 1 is near the flame tip; Location 2 is in the center of the flame; Location 3 is above the flame; Location 4 is diagonal through the flame and its tip. It is clear from figure 8 that location 1 is the most sensitive place to monitor temperature fluctuation.

0.08

0.06

0.04

0.02

0.00

Abs

orba

nce

(kL)

2.22.01.81.61.41.2Relative Frequency [cm-1]

2.22.01.81.61.41.2

Raw Data Voigt fit

EthanolPropane

Raw Data Voigt fit

Figure 6. Reduced H2O line shape (4-scan average) recorded in gas fuel (propane) and liquid fuel (ethanol).

200

150

100

50

0

T2 rm

s [K

2 ]

25020015010050

Frequency [Hz]

2400

2000

1600

1200

Tem

pera

ture

[K]

0.50.40.30.20.1Time [s]

Propane Speaker Off

Laser Scan Rate = 2000Hz4-scan average

(a) Unforced flow

200

150

100

50

0

T2 rm

s [K

2 ]

250200150100500

Frequency [Hz]

(b) Forced flow Figure 7. Measured temperature and its power spectrum.

2400

2000

1600

1200

Tem

pera

ture

[K]

0.50.40.30.20.1Time [s]

Speaker On: 100Hz

Laser Scan Rate = 2000Hz4-scan average

Propane

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The ability of the 1.8 µm sensor to measure gas temperature and track fluctuations illustrates the potential of TDL temperature measurements for combustion sensing and control.

B. 1.4 µm sensor

The experimental setup for the 1.4 µm, 2f temperature sensor is illustrated in figure 9. The DFB diode laser operating near 1.4 µm is driven by an external modulation, which consists of a 2 kHz saw tooth ramp combined with a faster 500 kHz sinusoidal modulation signal. The laser beam exits the fiber and is collimated with a lens across the flame, and filtered and detected. The fiber optics provides set-up flexibility and ease of alignment, which is advantageous for practical applications under industrial conditions.

A round quartz duct is utilized to generate natural flame instability. Acoustic signals are detected by a Brüel & Kjær microphone (Model 4939-A-011) which is located 0.5 m away from the combustor chamber. The laser beam is intentionally kept away from the centerline of the round quartz duct to minimize etalon interference. The second-harmonic components of the transmitted laser signal are obtained by a Perkin-Elmer lock-in amplifier (Model 7280) with a time constant of 1 µs. The temperature is inferred from the simple ratio of 2f peak height. 2 kHz real-time data processing and reduction is achieved by a fast industrial PC combined with a laboratory code written in C++.

Measurements with the 1.4 µm sensor were carried out at a radial position 15 mm from the spray centerline and an axial position 50.8 mm downstream of the nozzle exit. The total path length is 97 mm. Figure 10 shows the representative 2f lineshape (single scan) corresponding to the absorption features using gas and liquid fuel. Figure 10 (left) is for an air flow rate of 29.0 SCFM and propane flow rate of 40.9 SCFM. Figure 10 (right) is for an air flow rate of 57.5 SCFM and ethanol flow rate of 0.45 kg/min. As seen from figure 10, SNR is about same for liquid fuel and gas fuel. Due to the superior noise properties of the high-frequency detection (1MHz in the present study), the 1.4 µm sensor is seen to work well even in the liquid fuel swirl flame.

PressureTransducer

12

3 4

PressureTransducer

12

3 4

1200800400

0

T2 rm

s [K

2 ]

250200150100500

Frequency [Hz]

80604020

0

600400

200

0

80604020

0250200150100500

21

3 4

Figure 8. Measured positions and temperature power spectrum.

American Institute of Ae

Computer

Lock-in amplifier

Real-time T@ 2000Hz

Round duct (47cm) generate natural flame instability

Collimator

2kHz ramp

Diode laser controller

Functiongenerator

Modulate at f=500 kHz

Functiongenerator

+

Reference signal

Filter

N2 purge

N2 purge

Microphone

Computer

Lock-in amplifier

Real-time T@ 2000Hz

Round duct (47cm) generate natural flame instability

Collimator

2kHz ramp

Diode laser controller

Functiongenerator

Modulate at f=500 kHz

Functiongenerator

+

Reference signal

Filter

N2 purge

N2 purge

Microphone

Figure 9. Schematic diagram of the measurement system applied to the swirl-stabilized spray combustor.

3

2

1

0

-1

-2

2f S

igna

l [ar

b. u

nits

]

500400300200100

Time [us]

500400300200100

Propane Ethanol

Figure 10. Reduced H2O 2f line shape (single scan)recorded in gas fuel (propane) and liquid fuel (ethanol).

ronautics and Astronautics

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Figure 11 shows the measured acoustic signal and its powair flame with an air flow rate of 29.0 SCFM and propantemperature and its power spectrum are depicted under the saresolution of 0.5 ms is achieved. It can be seen that the domifluctuations (sound pressure and temperature), determined transform of the measured time-varying pressures and tempmeasured temperature fluctuation is in good agreement with th

Figure 13 shows the measured acoustic signal and its powflame with an air flow rate of 57.5 SCFM and ethanol flowtemperature and its power spectrum under the same conditionms is achieved. It can be seen that the dominant mode (35(sound pressure and temperature), determined by calculating measured time-varying temperatures from 0 to 0.5 sec, are sensor’s ability to track the temperature fluctuations, as neede

0.12

0.08

0.04

0.00

A. U

. rms

10008006004002000Frequency [Hz]

-1.0

-0.5

0.0

0.5

1.0A

. U.

0.50.40.30.20.10.0Time [s]

FFT

MicrophonePropane

Figure 11. Measured acoustic signal and its power spectrum in the burned region above the propane-air flame.

200

150

100

50

0

Trm

s [K

]

10008006004002000Frequency [Hz]

Figure 12. Measured temperature and its power spectrum in the burned region above the propane-air flame.

4000

3000

2000

1000 Tem

pera

ture

[K]

0.50.40.30.20.10.0Time [s]

FFT

Propane 1.4 µm sensor

V. DiscusIn the present study, we present the first application of TD

combustor. The TDL sensor is appealing since it is noninvasivAlthough the 1.8 µm sensor has the capability to measure

several limitations are encountered. First, extra efforts are reqthe low power of the laser and strong absorption at room

American Institute of Aerona

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1.2

0.8

0.4

0.0

A. U

. rms

10008006004002000Frequency [Hz]

Figure 13. Measured acoustic signal and its powerspectrum in the burned region above the ethanol-airflame.

100

80

60

40

20

0

Trm

s [K

]

10008006004002000Frequency [Hz]

Figure 14. Measured temperature and its powerspectrum in the burned region above the ethanol-airflame.

4000

3000

2000

1000 Tem

pera

ture

[K]

0.50.40.30.20.10.0Time [s]

FFT

Ethanol 1.4 µm sensor

-10

-5

0

5

10

A. U

.

0.50.40.30.20.10.0Time [s]

FFT

MicrophoneEthanol

er spectrum in the burned region above the propane-e flow rate of 40.9 SCFM. In figure 12, real-time me condition. With a laser scan rate of 2 kHz, a time nant mode (232 Hz) and harmonic (464 Hz) modes of by calculating the magnitude of the discrete Fourier eratures from 0 to 0.5 sec, are clearly revealed. The e measured pressure fluctuation from the microphone. er spectrum in the burned region above the ethanol-air rate of 0.45 kg/min. Figure 14 shows the measured . With laser scan rate of 2 kHz a time resolution of 0.5 0 Hz) and harmonic (700 Hz) modes of fluctuations the magnitude of the discrete Fourier transform of the again clearly revealed. These results demonstrate the d for combustion instability control.

sion L thermometry to a liquid-fuel swirl-stabilized spray

e and can be used in harsh environments. temperature of both the gas and liquid swirling flames, uired for the laser alignment and nitrogen purge due to temperature. Second, the 1.8 µm sensor may require

utics and Astronautics

averaging to obtain sufficient SNR under noisy conditions, which limits the measurement bandwidth. Third, complex data reduction renders the method make it impractical for real-time measurements.

The 1.4 µm sensor uses a line pair with relatively weak absorption at room temperature to minimize interference from ambient air in the measurement path. Second, a very compact and robust setup was realized by fiber optics, which has advantages for practical application under industrial conditions. Third, a wavelength modulation spectroscopy technique is employed to shift measurements to high frequency, and significantly reduces the dominating 1/f noise. Fourth, the ratio of integrated area is replaced by the ratio of peak-to-peak 2f signals. We have achieved a real-time repetition rate of 2 kHz with the 1.4 µm sensor. These changes greatly improve the sensor performance and allow us to further demonstrate the utility of in situ temperature sensing for combustion control.

VI. Conclusions A new 1.4 µm sensor, based on wavelength modulation spectroscopy, has been demonstrated. This sensor

improves the design of an earlier 1.8 µm sensor. Both sensors are based on a single-laser two-line concept, thus having the advantage of a simple optical system. They are both applied to a liquid-fuel swirl spray combustor, providing the first demonstration of TDL sensors in such practical flames. The 1.4 µm sensor incorporates several improvements over the 1.8 µm sensor design, enabling a real-time temperature readout rate of 2 kHz, superior performance in harsh environments, and immunity from ambient water vapor interference.

Acknowledgments We gratefully acknowledge support from the ONR via the University of Cincinnati and the Global Climate

Energy Program at Stanford.

References 1E.R. Furlong, D.S. Baer, and R.K. Hanson, "Real-Time Adaptive Combustion Control Using Diode-Laser Absorption Sensors," 27th Symp. (International) on Combustion, The Combustion Institute, pp. 103-111, (1998). 2E.R. Furlong, R.M. Mihalcea, M.E. Webber, D.S Baer, and R.K. Hanson, “Diode Laser Sensors for Real-Time Control of Pulsed Combustion Systems,” AIAA J. 37, 732-737 (1999). 3R.M. Mihalcea, D.S. Baer, and R.K. Hanson. "Advanced Diode Laser Absorption Sensor for in-situ Combustion Measurements of CO2, H2O, and Gas Temperature." Proc.Comb.Inst., 27, 95 –101 (1998). 4S.T. Sanders, D.W. Mattison, J.B. Jeffries, and R.K. Hanson, "Rapid Temperature-Tuning of a 1.4 µm Diode Laser with Application to High Pressure H2O Absorption Spectroscopy," Opt. Letters 26, 1568-1570 (2001). 5M.E. Webber, J. Wang, S.T. Sanders, D.S. Baer, and R.K.Hanson. "In Situ Combustion Measurements of CO, CO2, H2O and Temperature Using Diode Laser Absorption Sensors. " Proc. Comb. Inst., 28. 407-413(2000). 6M.G. Allen, E.R. Furlong, and R.K. Hanson, "Tunable Diode Laser Sensing and Combustion Control," in Applied Combustion Diagnostics, ed. K. Kohse-Höinghaus and J.B. Jeffries, Taylor and Francis, NY, 2002, pp. 479-798.. 7X. Zhou, X. Liu, J.B. Jeffries and R.K. Hanson, “Development of a Sensor for Temperature and Water Concentration in Combustion Gases Using a Single Tunable Diode Laser,” Meas. Sci. Technol. 14, 1459-1468 (2003). 8J.T.C. Liu, J.B. Jeffries and R.K. Hanson, “Wavelength Modulation Absorption Spectroscopy with 2f Detection using Multiplexed Diode Lasers for Rapid Temperature Measurements in Gaseous Flows,” Applied Physics B, 78, 503-511 (2004) 9T. Fernholz, H. Teichert and V. Ebert, “Digital, Phase-Sensitive Detection for In Situ Diode-Laser Spectroscopy under Rapidly Changing Transmission Conditions”, Appl. Phys. B 75, 229-236 (2002). 10A. N. Dharamsi and A. M. Bullock, “Applications of Wavelength-Modulation Spectroscopy in Resolution of Pressure and Modulation Broadened Spectra,” Appl. Phys. B 63, 283-292 (1996). 11D.C. Hovde, J.T. Hodges, G.E. Scace, and J.A. Silver, “Wavelength-Modulation Laser Hygrometer for Ultrasensitive Detection of Water Vapor in Semiconductor Gases,” Applied Optics, 40, 829-839 (2001). 12T. Aizawa, “Diode-Laser Wavelength-Modulation Absorption Spectroscopy for Quantitative In Situ Measurements of Temperature and OH Radical Concentration in Combustion gases”, Applied Optics, 40, 4894-4903

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13A.M. Bullock, A.N. Dharamsi, W.P. Chu and L.R. Poole, “Measurement of Absorption Line Wing Structure by Wavelength Modulation Spectroscopy,” Appl. Phys. Lett. 70(10), 1195-1197(1997) 14J. Reid and D. Labrie, “Second-Harmonic Detection with Tunable Diode Lasers- Comparison of Experiment and Theory,” Appl. Phys. B, 26, 203-210(1981)]. 15L. C. Philippe and R. K. Hanson, “Laser Diode Wavelength Modulation Spectroscopy for Simultaneous Measurement of Temperature, Pressure, and Velocity in Shock-Heated Oxygen Flows,” Appl. Opt. 32, 6090-6103 (1993) 16Guoqiang, Li. “Combustion Characteristics of a Multiple Swirl Combustor”， Paper number 2003–0489, AIAA 41st Aerospace Sciences Conference, Reno, NV, January 2003.

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