Quantum cascade laser absorption sensor for carbon monoxide in high-pressure gases using wavelength modulation spectroscopy R. M. Spearrin,* C. S. Goldenstein, J. B. Jeffries, and R. K. Hanson High Temperature Gasdynamics Laboratory, Department of Mechanical Engineering, Stanford University, Stanford, California 94305, USA *Corresponding author: [email protected] Received 17 December 2013; revised 11 February 2014; accepted 13 February 2014; posted 13 February 2014 (Doc. ID 203116); published 19 March 2014
A tunable quantum cascade laser sensor, based on wavelength modulation absorption spectroscopy near 4.8 μm, was developed to measure CO concentration in harsh, high-pressure combustion gases. The sensor employs a normalized second harmonic detection technique (WMS − 2f ∕1f ) at a modulation frequency of 50 kHz. Wavelength selection at 2059.91 cm−1 targets the P(20) transition within the fundamental vibrational band of CO, chosen for absorption strength and relative isolation from infrared water and carbon dioxide absorption. The CO spectral model is defined by the Voigt line-shape function, and key line-strength and line-broadening spectroscopic parameters were taken from the literature or measured. Sensitivity analysis identified the CO-N2 collisional broadening coefficient as most critical for uncertainty mitigation in hydrocarbon/air combustion exhaust measurements, and this parameter was experimentally derived over a range of combustion temperatures (1100–2600 K) produced in a shock tube. Accuracy of the wavelength-modulation-spectroscopy-based sensor, using the refined spectral model, was validated at pressures greater than 40 atm in nonreactive shock-heated gas mixtures. The laser was then free-space coupled to an indium-fluoride single-mode fiber for remote light delivery. The fiber-coupled sensor was demonstrated on an ethylene/air pulse detonation combustor, providing timeresolved (∼20 kHz), in situ measurements of CO concentration in a harsh flow field. © 2014 Optical Society of America OCIS codes: (060.2390) Fiber optics, infrared; (060.2430) Fibers, single-mode; (280.1740) Combustion diagnostics; (300.1030) Absorption; (300.6340) Spectroscopy, infrared; (300.6360) Spectroscopy, laser. http://dx.doi.org/10.1364/AO.53.001938
1. Introduction
Many energy-conversion systems, including combustion engines, operate at elevated pressures due to thermodynamic advantages. Accurate measurements of species concentration in high-pressure reacting gases can provide an empirical basis to evaluate competing flow field mechanisms including mass transport, heat transfer, and chemical kinetics that characterize system performance. Moreover, the short time scales involved in chemistry and high-speed flows require diagnostics that can provide high-bandwidth 1559-128X/14/091938-09$15.00/0 © 2014 Optical Society of America 1938
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response to resolve transient speciation. Laser absorption sensors have been utilized extensively to provide high time-resolution for in situ measurements of species concentration in reacting gases, particularly for shock tube kinetics studies [1]. Some laser-based measurements have also been conducted in high-pressure industrial environments, including coal gasifiers, internal combustion engines, and pulse detonation combustors [2–5]. More widespread application of laser absorption diagnostics to industrial gas systems and combustion engines has generally been limited by the challenges of high-pressure absorption spectroscopy and harsh thermomechanical environments. This work presents a nonintrusive, mid-infrared laser absorption sensor for carbon monoxide (CO), developed
to address the aforementioned challenges and aimed for harsh high-pressure combustion applications. CO is an intermediate and sometimes major (>1%) product of hydrocarbon combustion. The quantity of CO in lean flames provides a sensitive indication of incomplete combustion and the local fuel-to-oxidizer ratio. Excess CO in combustion exhaust is undesirable due to the correlation with poor engine efficiency and matters of health and safety. The need to minimize CO concentrations in various combustion systems drives the demand for highly sensitive diagnostics. The strong fundamental absorption band of CO near 4.7 μm offers potential for very sensitive species detection, even at short path-lengths (3 μm) has typically required cryogenic cooling systems which render the lasers impractical for most sensing applications beyond the laboratory. Due to readily available roomtemperature diode lasers and optical hardware in the near-infrared wavelength domain (1–3 μm), most previous CO absorption-based sensors have been designed to probe the overtone vibrational bands near 1.55 μm [6–9] and 2.3 μm [2,10,11], which unfortunately are more than four and two orders of magnitude weaker, respectively, than the fundamental band. Weak absorption strength, combined with spectral interference from carbon dioxide and water, renders the near-infrared CO sensors insufficient for most short path-length combustion systems. The recent advent of the quantum cascade laser (QCL) has enabled access to mid-infrared wavelengths previously unavailable with room-temperature lasers [12], and researchers have since exploited the fundamental CO absorption band near 4.7 μm for various applications requiring very sensitive species measurements [13–15]. Here we utilized a distributedfeedback QCL near 4.85 μm to probe the fundamental band and extend CO absorption sensing to high pressures (>40 atm) in hydrocarbon-fueled engines. Field sensing in harsh combustion engines necessitates a measurement technique which is robust against thermomechanical noise sources and conducive to spectral characteristics associated with both high pressures and high temperatures (>1000 K). At elevated pressures, discrete spectral lines are broadened and blended such that a zero-absorption baseline signal cannot be directly recovered. This blending effect is amplified at high temperatures where more transitions are active and the absorption spectra more crowded. Additionally, fluctuations in detected light intensity, unrelated to absorption, are common to engine applications due to problems such as beam steering, emission, misalignment due to vibrations, window fouling, and scattering. In combination, these issues complicate direct absorption sensing strategies and can degrade sensor performance [16,17]. To counter such difficulties, a wavelength modulation spectroscopy (WMS) technique was employed for the present sensor. With WMS, frequency filtering of harmonic signals allows for
substantial noise rejection. Moreover, the ratio of two harmonic signals (WMS-nf ∕mf ), which can be related to the absorption spectra, is independent of absolute optical power or intensity baseline [18]. These characteristics reconcile the above-mentioned challenges and make WMS a well-suited spectroscopic technique for harsh engine applications. High-temperature (1100–2600 K), high-pressure (10–40 atm) absorption measurements, using this WMS sensor, were carried out in shock-heated CO-N2 mixtures in order to validate the sensor and calibrate the CO spectral model. The QCL was then free-space coupled to a single-mode fiber for remote light delivery, and the fiber-coupled sensor was demonstrated on an operating pulse detonation combustor (PDC) at the Naval Postgraduate School in Monterey, California, yielding time-resolved, in-stream CO concentration measurements at conditions exceeding 40 atm and 2700 K. 2. Theory
The theory of absorption spectroscopy is briefly outlined to clarify terminology and units; more detailed theoretical discussions can be found in other works [19–21]. The Beer–Lambert law defines the fundamental physical relation governing narrowband laser absorption spectroscopy. This law relates the measurable quantities of incident light intensity and transmitted light intensity through a gas medium by It exp−αv ; I0 v
(1)
where αv denotes the spectral absorbance at frequency v [cm−1 ]. The spectral absorbance is related to specific gas properties, including mole fraction xabs , by X I αv − ln t Pxabs Si Tϕvi L; (2) I0 v i where P [atm] is the total gas pressure, Si T [cm−2 atm−1 ] is the line-strength of a quantum transition i which varies only with temperature, ϕvi [cm] is the line-shape function, and L is the length across a uniform path. In Eq. (2), the spectral absorbance is expressed as a summation to account for the overlap of neighboring transitions, common at elevated pressures. All else known, it follows that species mole fraction can be directly inferred from a measurement of the transmitted and incident light intensities at a specific wavelength, and direct absorption spectroscopy (DAS) is commonly employed for optical gas sensors due to its simplicity [1]. WMS is a more complex sensing technique than DAS with regards to requisite hardware characterization and data postprocessing, but insensitivity to nonabsorption light intensity fluctuations makes the normalized (WMS-nf ∕mf ) method advantageous for harsh environments [18]. A brief description of 20 March 2014 / Vol. 53, No. 9 / APPLIED OPTICS
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the WMS technique is outlined here, with notation borrowed from previous works [10,22]. For WMS, the laser injection current is modulated rapidly, yielding simultaneous modulation of the laser output intensity and wavelength, or frequency [cm−1 ]. Instantaneous frequency modulation (FM), vt, is given by vt v¯ a cosωt;
(3)
where v¯ [cm−1 ] is the center laser frequency, a [cm−1 ] is the modulation depth, and ω is the angular modulation frequency (ω 2πf ). The simultaneous intensity modulation (IM), I 0 t, can be defined as I 0 t I¯ 0 1 i0 cosωt ψ 1 i2 cos2ωt ψ 2 ;
(4)
where I¯ 0 is the baseline laser intensity at v¯ , i0 is the normalized linear IM amplitude with phase shift ψ 1 , and i2 is the normalized nonlinear IM amplitude with phase shift ψ 2 . The parameters i0 ; i2 ; ψ 1, and ψ 2 are characteristic of the laser source and can be predetermined in the laboratory. A lock-in amplifier combined with a low-pass filter is utilized to extract the harmonic (nf ) signals of the laser modulation frequency which relate directly to the absorbance spectra. The second harmonic signal (WMS-2f ) is used in this work, normalized by the WMS-1f signal to cancel out the absolute optical power term common to both. This normalization negates sensitivity to nonabsorption transmission losses and baseline noise characteristic to engine applications. The resulting 1f -normalized WMS-2f signal (WMS-2f ∕1f ) is a function of predetermined laser parameters (i0 ; i2 ; ψ 1 ; ψ 2 ), and thermodynamic properties (T; P; xabs ) associated with the gas environment. With an independent measurement of temperature and pressure, this technique can be used to measure species concentration, or mole fraction. 3. Wavelength Selection
The primary criteria for wavelength selection include strong absorbance and minimal spectral interference from other combustion products, namely water and carbon dioxide. Figure 1 shows the fundamental CO absorption band from 4.4 to 5.5 μm, plotted as line-strengths at 2000 K, and overlaid with the H2 O and CO2 absorption lines within the same domain. The CO2 fundamental band (∼4.3 μm) exhibits a very dense spectrum that interferes substantially with the R-branch of the CO band under typical combustion conditions where the CO2 :CO ratio is much greater than unity. The P-branch also has significant interference from water, but the overlapping H2 O spectrum is much less crowded compared to the interfering CO2 band, such that the larger spacing between water lines yields some narrow regions that are nearly interference-free. Specifically, the spectral domain near 2060 cm−1 provides an attractive combination of low relative interference and strong absorption. Therefore, we select this wavelength region (4.855 μm) to detect CO. 1940
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Fig. 1. Absorption line-strengths of CO, CO2 , and H2 O at 2000 K (HITEMP [23]).
Absorbance simulations at target engine conditions provide a more quantitative estimation of signal strength and isolation from interfering species. Here the target application is a hydrocarbonair combustor with pressures ranging from 5 to 40 atm. Expected values of temperature, gas composition, and path-length are based on fuel selection, fuel-to-oxidizer ratio, and engine size. Typical combustion temperatures range from 1000 to 3000 K. CO concentrations can vary dramatically from ppm levels to several percent (∼104 dynamic range) depending primarily on the H:C ratio of the fuel, local equivalence ratio, and combustor residence time. This sensitivity enhances the value of the diagnostic for assessing combustion processes, but can make estimation of expected mole fraction and sensor design challenging. The path-length is engine-specific. In the current research, the CO sensor is intended for utilization on an ethylene-air pulse detonation combustor with a path-length of slightly less than 4 cm. Assuming chemical equilibrium, the concentration of CO and other combustion products were estimated at several conditions [24], and the absorbance spectra were simulated using these values. Figure 2 shows absorbance simulations near 4.855 μm at two expected engine conditions by utilization of the Voigt line-shape function coupled with the HITEMP spectroscopic database [23]. The highpressure CO absorption spectrum near 2059.9 cm−1 is composed of multiple overlapping transitions, but the P(20) line in the υ0 → 1 fundamental band makes the dominant contribution. The P(14) line at 2060.33 cm−1 and the P(8) line at 2059.21 cm−1 , from the respective υ1 → 2 and υ2 → 3 fundamental hot bands, also make a small contribution at high temperatures. Interference from CO2 is negligible, and H2 O interference is relatively minimal, especially at the line-center of the P(20) line (2059.91 cm−1 ). The influence of the presence of CO2 and H2 O on the CO absorption measurement is further discussed in a later section. 4. Sensor Development A. Optimizing Modulation Depth
The WMS harmonic signals vary directly with modulation depth (a), which can be adjusted to optimize
(a)
(b)
Fig. 2. Absorbance spectra simulations of equilibrium concentrations of CO, CO2 , and H2 O (air bath gas) near 4.855 μm at expected PDC conditions (C2 H4 -air, ϕ ≈ 1); L 4 cm. (a) P 10 atm, T 2500 K and (b) P 30 atm, T 3000 K.
signal magnitudes for a given set of gas conditions. We define the optimum modulation depth for this sensor as the value at which the WMS-2f signal is maximized, within constraints of the laser. The laser used in this work is a single-mode distributedfeedback (DFB) QCL (ALPES), centered in wavelength near 4855 nm, with a typical output power of 8 mW. The modulation depth is primarily a function of two laser input parameters: modulation frequency (f ) and amplitude of the modulated injection-current. The modulation depth increases approximately linearly with injection-current amplitude and decreases nonlinearly with modulation frequency. A maximum modulation depth at a given modulation frequency can therefore be measured at the upper limit of injection-current amplitude which is bound by the current threshold, maximum operating current, and mode-hoping regions of the QCL. Figure 3 plots the maximum modulation depth for the QCL, when tuned to the P(20) line-center, at modulation frequencies from 40 to 100 kHz. A high modulation frequency is desirable due to a direct correlation with sensor bandwidth, but this comes at expense of modulation depth. Assessment of the WMS-2f signal as a function of modulation depth requires further knowledge of the aforementioned characteristic laser parameters. The linear and nonlinear IM amplitudes (i0 and i2 ) are coupled with modulation depth, and the relationship between these hardware-related parameters is depicted in Fig. 4. The phase-shift components, ψ 1 and ψ 2 , are found to be approximately independent of modulation depth, with measured values of 1.16π radians and 0.84π radians, respectively. Details on the methods used to determine these parameters are found in other works [10,22]. With the laser parameters defined, the relative magnitude of the WMS-2f signal at line-center can be calculated as a function of modulation depth for various test conditions, as illustrated in Fig. 5.
The maximum WMS-2f signal is highly dependent on gas conditions which vary dramatically in a detonation-based engine. Though temperature and pressure are both transient, the WMS-2f signal is most sensitive to pressure due to sensitivity to line broadening. Figure 5 highlights this sensitivity over a range of pressures expected in the PDC. As pressure increases, the optimum modulation depth increases, to the extent that maximizing the WMS-2f signal at pressures greater than 10 atm becomes impractical within the tuning limitations of the current laser, revealing a requisite compromise between sensor bandwidth and signal magnitude or detectability. A modulation depth and frequency of 0.235 cm−1 and 50 kHz, respectively, were chosen for this sensor to yield sufficient signal over the broadest range of transient conditions, while maintaining the desired high bandwidth capability. With modulation depth specified, all hardware
Fig. 3. Measured modulation depth at maximum injectioncurrent amplitude (120 mA) as a function of laser modulation frequency. 20 March 2014 / Vol. 53, No. 9 / APPLIED OPTICS
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Fig. 4. Measured laser intensity parameters (i0 and i2 ) as a function of modulation depth (f 50 kHz).
Fig. 6. Simulated CO WMS spectra (2f , 1f , 2f ∕1f ) near 2059.9 cm−1 at a typical PDC condition; T 2000 K, P 20 atm, L 4 cm, xco 1%.
parameters are established, and the normalized laser intensity can be expressed as I 0 t∕I¯ 0 1 0.718 cos2πf t 1.16π 0.0106 cos4πf t 0.84π;
(5)
where f is 50 kHz. Figure 6 shows a spectral simulation of the relevant WMS harmonic signals at a typical PDC condition based on these selected laser settings. We note that in practice the IM amplitudes, as defined in Eq. (5), vary slightly between experimental setups, whereas the phase-shift terms and modulation depth are insensitive to such extrinsic changes. Due to the strong dependence of the WMS-2f signal on the linear IM amplitude (i0 ), a calibration of this hardware-related parameter for each unique optical setup improves measurement accuracy. B.
Wavelength Shift Due to Modulation
High-frequency modulation of laser injection-current causes a shift in the center wavelength away from the unmodulated laser wavelength at the same mean
Fig. 5. Simulated WMS-2f (background subtracted) at 2059.91 cm−1 as a function of modulation depth for various pressures at 2000 K. 1942
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current and temperature settings. This wavelength shift results from ohmic heating of the laser cavity that increases with modulation frequency and injection-current amplitude [25]. To maintain line-center in our WMS measurement, a corresponding wavelength correction is required. The magnitude of wavelength shift can be determined by comparing measured WMS spectra using specific modulation settings to spectral simulations. For the 4.85 μm DFB-QCL, the wavelength shift, Δv, was measured to be 0.032 cm−1 at the selected modulation settings (f 50 kHz, a 0.235 cm−1 ). C.
Spectral Modeling
The current sensor employs a Voigt line-shape profile, coupled with the HITEMP 2010 database, as the foundation for the CO spectral model. Figure 7 illustrates the suitability of the Voigt line-shape function for modeling the P(20) and P(14) CO lines near 2060 cm−1 based on direct-absorption measurements in a heated static cell. Due to the simple diatomic structure of CO, the CO spectra can be modeled accurately using the Voigt line shape, along with computed HITEMP line parameters, over a broad range of conditions [2,10]. At a given thermodynamic condition, a Voigt line-shape profile is defined by two transition-specific spectroscopic parameters: linestrength, ST, and the collisional line-broadening coefficient, 2γT. The Doppler line width is also needed to fully describe the Voigt profile, but this parameter is a function of wavelength and temperature only and not transition-specific. Calculated linestrength values from HITEMP for the fundamental CO band have been validated previously, and shown to provide a high degree of accuracy (σ ∼ 1%) [13]. Line-broadening parameters for N2, CO2 , and H2 O, the dominant collision partners, are found in the literature and noted in Table 1 [26,27]. Broadening uncertainties of less than 10% are reported for all perturbers. Figure 8 presents a sensitivity analysis of the WMS-based mole fraction measurement to
Fig. 7. Measured absorbance spectra of CO in N2 near 2060 cm−1 with active P-branch lines labeled P(v”,J”) and fit with the Voigt function; T 804 K, P 1 atm, L 20.95 cm, xco 0.005.
uncertainties in these key spectroscopic parameters for the two most influential CO lines near 2059.91 cm−1 at a detonation condition. Such analysis helps identify line parameters that deserve further experimental investigation. As previously discussed and underscored by the sensitivity analysis in Fig. 8, the P(20) line makes the dominant contribution to our CO mole fraction measurement by more than a factor of ten relative to the next most important contributor in the P(14) line of the υ1 → 2 band. Moreover, the CO-N2 broadening coefficient is highlighted as the most important broadening parameter, due to the high concentration of diatomic nitrogen in hydrocarbon-air combustion exhaust. To improve sensor accuracy and mitigate uncertainty, we calibrated the CO-N2 broadening coefficient for the P(20) line at elevated temperatures, and validated the WMS measurement technique at detonation-like conditions in a shock tube. Shock tubes can produce well-defined environments for short time periods (∼ms) at combustion temperatures above that attainable in a static optical cell (>1000 K). Here we used a high-pressure, stainless-steel shock tube to heat nonreactive CO-N2 gas mixtures to a range of temperatures (1000–2700 K) representative of the detonation combustor in order to calibrate collisional broadening of the P(20) line by N2 . The shock tube has been described in previous works [4,28,29], hence only limited details are Table 1.
Fig. 8. Sensitivity analysis of the WMS-2f ∕1f mole fraction measurement near 2059.91 cm−1 to the line-strength and collisional broadening parameters of the P(0,20) and P(1,14) CO lines at a typical PDC condition (see Fig. 7).
outlined here. Measurements are made approximately 1 cm from the shock tube end wall with an optical path-length of 5 cm. Gas properties are assumed uniform in accordance with a planar shock, and temperature and pressure are calculated from normal shock relations based on measured initial conditions and the measured incident shock velocity. Typical temperature uncertainty is 1% for steadystate test times of approximately 1 ms behind the reflected shock wave. Assuming known thermodynamic variables in addition to line-strength, a WMS-2f ∕1f measurement at the P(20) line-center can be used to infer the CO-N2 collisional broadening coefficient, 2γ CO-N2 T, the remaining free Voigt parameter for a dilute mixture of CO in nitrogen. The broadening coefficient was iterated within the CO spectral model to match the simulated WMS-2f ∕1f values to the measured values. Broadening coefficients, as measured in the shock tube, are plotted in Fig. 9 against temperature. The temperature dependence of collisional line broadening is typically modeled as a power law n T 2γT 2γT 0 0 ; T
(6)
where n is the temperature-dependence exponent. Previous power-law characterizations of nitrogen or air broadening of the P(20) line are shown along with the measured data in Fig. 9 [23,26,30,31].
Spectroscopic Line Assignments and Collisional-Broadening Parameters for the CO Lines of Interesta
Collisional Broadening (γ in 10−3 cm−1 ∕atm) [26,27] CO-H2 O
CO-N2 P(v”,J”) P(0,20) P(1,14) a
v0
cm−1
2059.91 2060.33
E00
cm−1
806.4 2543.1
S(296 K)
cm−2 ∕atm 10−2
87.6 × 26.4 × 10−5
CO-CO2
γ1000 K
n
γ300 K
n
γ300 K
n
22.5 (3%) 26.8
0.55 (3%) 0.61
119 125
0.72 0.77
51.8 63.1
0.50 0.53
Uncertainties for broadening measurements made in this work (i.e., CO-N2 ) are shown in parentheses. 20 March 2014 / Vol. 53, No. 9 / APPLIED OPTICS
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Fig. 10. Sensor validation data showing the measured CO mole fraction compared to the known mole fraction over a range of conditions produced in a shock tube (σ ≈ 3%); L 5 cm, xco 0.00495 in N2 .
Fig. 9. Measured nitrogen-broadening coefficient for the P(0,20) line from 1100 to 2600 K including a best-fit power law equation and comparison with broadening equations from other sources; P 10–50 atm.
The HITEMP model exhibits excellent agreement with the data at the lower temperature domain (∼1100 K), but under-predicts broadening at higher temperatures (>2000 K), where the empirical model by Varghese shows best agreement. The disagreements between models and data may be attributed to the general insufficiency of the power law to capture temperature dependence of broadening over a large range of temperature [32,33] and the differing temperature domains for which these models were established. Here we best-fit the measured broadening coefficients over the domain of 1100–2600 K, yielding a more refined broadening model for combustion temperatures. The reference broadening coefficient and temperature-dependence exponent for this refined 2γ CO-N2 T model are shown in Table 1. The authors recommend caution employing these parameters outside of the temperature range of the measurements. With the calibrated spectral model, CO mole fraction was inferred from the measured WMS data produced in the shock tube and compared to the known mole fraction of the test gas mixture. Figure 10 depicts the typical uncertainty of the spectral model (σ ∼ 3%) over a wide range of both temperatures and pressures (10–40 atm) that emulate detonation conditions, validating the accuracy of the CO concentration sensor for the intended PDC application. D.
Optical Engineering
To isolate sensitive laser equipment from the thermomechanically harsh engine environment, a remote light delivery system was developed. Figure 11 depicts a simplified schematic of the optical setup as deployed in field experiments. An indium-fluoride (InF3 ) single-mode fiber (L 5 m) with a core diameter of 17 μm was coupled to the 4.85 μm DFB-QCL using an aspheric zinc-selenide (ZnSe) microlens with a focal length of 6 mm. The fiber is free-space aligned with the lens using a five-axis alignment stage and has a characteristic 1944
loss of ∼0.5 dB∕m at the target wavelength. The single-mode fiber mitigates mode noise associated with larger core-diameter multimode fibers and facilitates improved beam collimation at the fiber output [34]. Here we collimated the output beam with a 12 mm focal length ZnSe aspheric lens to achieve a ∼2 mm diameter beam at a working distance of 40 cm. The collimation lens is integrated with an SMA connector to attach directly to the fiber termination, and a four-axis kinematic mount interfaces with the test hardware to direct the beam through the engine. The light collection and detection components are mounted together in a compact linear cage system (Thorlabs—30 mm). The transmitted light is focused by an antireflection-coated calcium fluoride lens (focal length 20 mm) onto an infrared photovoltaic detector (Vigo PVI-4TE-5) with a 2 mm2 detection area and specific detectivity (D ) of ∼3 × 1011 cm Hz1∕2 W−1. The detector bandwidth is 10 MHz. Two narrow-bandpass spectral filters are stacked between the engine mount and collection lens to yield an effective spectral width of ∼25 nm centered at about 4860 nm. Due to the sensitivity of the infrared detector from ∼2 to 5 μm, spectral filtering was critical for reducing thermal emission from H2 O which is typical of combustion exhaust. An analogous, but more detailed, discussion of optical system design for this application can be found in a previous work by the current authors [4].
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Fig. 11. Simplified mid-infrared light delivery and detection schematic.
5. Sensor Demonstration
Detonation-based engines represent an especially difficult application for diagnostics due to the wide range of thermodynamic properties involved and the rapid transients of those properties. Additionally, pulse detonation engines are mechanically violent, and vibrations or acoustic waves resulting from combustor gas dynamics can easily damage instrumentation which interfaces with the engine. Here we demonstrate the feasibility of the fiber-coupled CO absorption sensor on an operating pulse detonation combustor, highlighting measurement timeresolution and sensor operability range. Discussion regarding combustor analysis has been reserved for other publications and deliberately constrained here. Figure 12 exhibits representative measurements of pressure and WMS-2f ∕1f for two consecutive detonation cycles in the PDC chamber. Detonation waves were produced by ethylene-air gas mixtures, spark-ignited every 50 ms (20 Hz), and the primary detonation event transpired within just a few milliseconds as indicated by the pressure time history. Utilizing simultaneous H2 O thermometry along with pressure data, CO mole fraction was directly inferred from the WMS-2f ∕1f measurement [35]. High-fidelity CO data was resolved over the full detonation cycle and entire range of operating conditions. To reject noise, a 20 kHz low-pass filter was applied to the lock-in amplifier outputs (2f and 1f signals). Such a wide filter, relative to the modulation frequency (50 kHz), was feasible due to the strong harmonic signals associated with the probed CO spectra in the PDC environment. Tighter filters offered minimal further noise reduction, while having the expense of lower measurement bandwidth. The time-variable signal-to-noise ratio (SNR), as noted in the WMS-2f ∕1f time-history in Fig. 12, resulted from the dramatically changing spectral structure of CO during a detonation event. Directly
Fig. 12. Representative time-history data from a pulsed (20 Hz) detonation combustor operating on ethylene/air; L 7.62 cm. Pressure and WMS-2f ∕2f measurements are coplanar in combustion chamber.
behind the detonation wave, at the most extreme conditions (P > 30 atm, T > 2500 K), a detection limit of approximately 0.5% CO was inferred from an SNR of ∼5 and 2.5% measured CO. In the more benign tail at later times of the detonation cycle (P ∼ 10 atm, T ∼ 1200 K), a detection limit of ∼500 ppm was achieved (SNR ∼ 10, xco ∼ 0.5%). The temporal resolution or measurement bandwidth is effectively equated to the width of the low-pass filter on the harmonic signals, or 20 kHz. Other environments may require different frequency filtering schemes to minimize noise, thus yielding different bandwidths. 6. Summary
We report the development of an in situ, high-bandwidth CO concentration sensor for high-pressure combustion flows based on a fixed-wavelength WMS2f ∕1f technique. The sensor probes the fundamental vibrational band of CO near 4.8 μm which provides orders of magnitude greater absorption and detectability than the overtone bands at shorter wavelengths. Wavelength selection at 2059.9 cm−1 exploits the strong absorption strength of the P(20) rovibrational transition and relative spectral isolation from water interference. A distributed-feedback QCL centered near 4.855 μm was modulated in wavelength and intensity at 50 kHz, with the modulation depth maximized for high-pressure applications (>10 atm). The wavelength shift due to laser injection-current modulation was accounted for in the fixed-wavelength, line-center measurement. Shock tube experiments refined and validated the CO spectral model, which utilizes the Voigt line-shape function, at conditions expected in detonation environments (up to 40 atm). Collisional broadening coefficients for CO in N2 were determined for the P(20) line over a range of elevated temperatures (1100–2600 K) behind reflected shock waves and fit with a power law to reduce measurement uncertainty ( 10), time-resolved measurements of CO over the complete detonation cycle at a bandwidth of approximately 20 kHz. Broadly, this research extends the pressure capabilities (∼40 atm) of laser-based absorption sensing of CO in harsh environments, with applicability to numerous high-temperature combustion systems. Field experiments on the PDC were conducted at the Naval Postgraduate School (NPS) in Monterey, California. Support for these diagnostic studies was provided by Innovative Scientific Solutions, Inc., with John Hoke as project manager, and the AFOSR, with Chiping Li as technical monitor. The authors would like to thank Professor Chris Brophy and Dave Dausen at NPS for technical support and operation of the PDC for sensor demonstration. 20 March 2014 / Vol. 53, No. 9 / APPLIED OPTICS
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References 1. R. K. Hanson, “Applications of quantitative laser sensors to kinetics, propulsion and practical energy systems,” Proc. Combust. Inst. 33, 1–40 (2011). 2. R. Sur, K. Sun, J. B. Jeffries, and R. K. Hanson, “Multi-species laser absorption sensors for in situ monitoring of syngas composition,” Appl. Phys. B, doi:10.1007/s00340-013-5567-2 (to be published). 3. D. W. Mattison, J. B. Jeffries, R. K. Hanson, R. R. Steeper, S. De Zilwa, J. E. Dec, M. Sjoberg, and W. Hwang, “In-cylinder gas temperature and water concentration measurements in HCCI engines using a multiplexed-wavelength diode-laser system: sensor development and initial demonstration,” Proc. Combust. Inst. 31, 791–798 (2007). 4. R. M. Spearrin, C. S. Goldenstein, J. B. Jeffries, and R. K. Hanson, “Fiber-coupled 2.7 μm laser absorption sensor for CO2 in harsh combustion environments,” Meas. Sci. Technol. 24, 055107 (2013). 5. A. W. Caswell, S. Roy, X. An, S. T. Sanders, J. Hoke, F. Schauer, and J. R. Gord, “High-bandwidth H2O absorption sensor for measuring pressure, enthalpy, and mass flux in a pulseddetonation combustor,” in 50th AIAA Aerospace Sciences Meeting, Nashville, Tennessee (2012), pp. 1–10. 6. Q. V. Nguyen, B. L. Edgar, R. W. Dibble, and A. Gulati, “Experimental and numerical comparison of extractive and in situ laser measurements of non-equilibrium carbon monoxide in lean-premixed natural gas combustion,” Combust. Flame 100, 395–406 (1995). 7. B. L. Upschulte, D. M. Sonnenfroh, and M. G. Allen, “Measurements of CO, CO2, OH, and H2O in room-temperature and combustion gases by use of a broadly current-tuned multisection InGaAsP diode laser,” Appl. Opt. 38, 1506–1512 (1999). 8. D. T. Cassidy and L. J. Bonnell, “Trace gas detection with short-external-cavity InGaAsP diode laser transmitter modules operating at 1.58 μm,” Appl. Opt. 27, 2688–2693 (1988). 9. R. M. Mihalcea, D. S. Baer, and R. K. Hanson, “A diode-laser absorption sensor system for combustion emission measurements,” Meas. Sci. Technol. 9, 327–338 (1998). 10. X. Chao, J. B. Jeffries, and R. K. Hanson, “Absorption sensor for CO in combustion gases using 2.3 μm tunable diode lasers,” Meas. Sci. Technol. 20, 115201 (2009). 11. J. Wang, M. Maiorov, D. S. Baer, D. Z. Garbuzov, J. C. Connolly, and R. K. Hanson, “In situ combustion measurements of CO with diode-laser absorption near 2.3 μm,” Appl. Opt. 39, 5579–5589 (2000). 12. K. Namjou, S. Cai, E. A. Whittaker, J. Faist, C. Gmachl, F. Capasso, D. L. Sivco, and A. Y. Cho, “Sensitive absorption spectroscopy with a room-temperature distributed-feedback quantum-cascade laser,” Opt. Lett. 23, 219–221 (1998). 13. W. Ren, A. Farooq, D. F. Davidson, and R. K. Hanson, “CO concentration and temperature sensor for combustion gases using quantum-cascade laser absorption near 4.7 μm,” Appl. Phys. B 107, 849–860 (2012). 14. P. Stefanski, R. Lewicki, J. Tarka, Y. Ma, M. Jahjah, and F. K. Tittel, “Sensitive detection of carbon monoxide using a compact high power CW DFB-QCL based QEPAS sensor,” in CLEO (Optical Society of America, 2013), paper JW2A.68. 15. J. Vanderover, W. Wang, and M. A. Oehlschlaeger, “A carbon monoxide and thermometry sensor based on mid-IR quantumcascade laser wavelength-modulation absorption spectroscopy,” Appl. Phys. B 103, 959–966 (2011). 16. G. Rieker, J. Jeffries, R. Hanson, T. Mathur, M. Gruber, and C. Carter, “Diode laser-based detection of combustor instabilities with application to a scramjet engine,” Proc. Combust. Inst. 32, 831–838 (2009). 17. A. E. Klingbeil, J. B. Jeffries, and R. K. Hanson, “Design of a fiber-coupled mid-infrared fuel sensor for pulse detonation engines,” AIAA J. 45, 772–778 (2007).
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18. G. B. Rieker, J. B. Jeffries, and R. K. Hanson, “Calibration-free wavelength-modulation spectroscopy for measurements of gas temperature and concentration in harsh environments,” Appl. Opt. 48, 5546–5560 (2009). 19. U. Platt and J. Stutz, Differential Optical Absorption Spectroscopy: Principles and Applications (Springer, 2008), p. 597. 20. P. Kluczynski and O. Axner, “Theoretical description based on Fourier analysis of wavelength-modulation spectrometry in terms of analytical and background signals,” Appl. Opt. 38, 5803–5815 (1999). 21. D. S. Bomse, A. C. Stanton, and J. A. Silver, “Frequency modulation and wavelength modulation spectroscopies: comparison of experimental methods using a lead-salt diode laser,” Appl. Opt. 31, 718–731 (1992). 22. H. Li, G. B. Rieker, X. Liu, J. B. Jeffries, and R. K. Hanson, “Extension of wavelength-modulation spectroscopy to large modulation depth for diode laser absorption measurements in high-pressure gases,” Appl. Opt. 45, 1052–1061 (2006). 23. L. S. Rothman, I. E. Gordon, R. J. Barber, H. Dothe, R. R. Gamache, A. Goldman, V. I. Perevalov, S. A. Tashkun, and J. Tennyson, “HITEMP, the high-temperature molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transfer 111, 2139–2150 (2010). 24. W. C. Reynolds, “The Element Potential Method for Chemical Equilibrium Analysis: Implementation in the Interactive Program STANJAN” (Stanford University, 1986). 25. A. Farooq, J. B. Jeffries, and R. K. Hanson, “Measurements of CO2 concentration and temperature at high pressures using 1f-normalized wavelength modulation spectroscopy with second harmonic detection near 2.7 micron,” Appl. Opt. 48, 6740–6753 (2009). 26. A. Predoi-Cross, C. Luo, P. Sinclair, J. Drummond, and A. May, “Line broadening and the temperature exponent of the fundamental band in CO-N2 mixtures,” J. Mol. Spectrosc. 198, 291–303 (1999). 27. J. M. Hartmann, L. Rosenmann, M. Y. Perrin, and J. Taine, “Accurate calculated tabulations of CO line broadening by H2O, N2, O2, and CO2 in the 200–3000 K temperature range,” Appl. Opt. 27, 3063–3065 (1988). 28. E. L. Petersen, M. Röhrig, D. F. Davidson, R. K. Hanson, and C. T. Bowman, “High-pressure methane oxidation behind reflected shock waves,” Symp. Combust. 26, 799–806 (1996). 29. D. F. Davidson, E. L. Petersen, R. K. Hanson, and R. Bates, “Shock tube measurements of the equation of state of argon,” Int. J. Thermophys. 19, 1585–1594 (1998). 30. P. L. Varghese and R. K. Hanson, “Tunable infrared diode laser measurements of line strengths and collision widths of 12C16O at room temperature,” J. Quant. Spectrosc. Radiat. Transfer 24, 479–489 (1980). 31. P. L. Varghese and R. K. Hanson, “Collision width measurements of CO in combustion gases using a tunable diode laser,” J. Quant. Spectrosc. Radiat. Transfer 26, 339–347 (1981). 32. J. Bonamy, D. Robert, and C. Boulet, “Simplified models for the temperature dependence of linewidths at elevated temperatures and applications to CO broadened by Ar and N2,” J. Quant. Spectrosc. Radiat. Transfer 31, 23–34 (1984). 33. R. T. Pack, “Pressure broadening of the dipole and Raman lines of CO2 by He and Ar. Temperature dependence,” J. Chem. Phys. 70, 3424 (1979). 34. J. Hecht, Understanding Fiber Optics (Pearson/Prentice Hall, 2006), p. 790. 35. C. S. Goldenstein, I. A. Schultz, J. B. Jeffries, and R. K. Hanson, “TDL absorption sensor for temperature measurements in high-pressure and high-temperature gases,” in 50th AIAA Aerospace Sciences Meeting, Nashville, Tennessee (2012), p. 1061.
A tunable quantum cascade laser sensor, based on wavelength modulation absorption spectroscopy near 4.8 μm, was developed to measure CO concentration in harsh, high-pressure combustion gases. The sensor employs a normalized second harmonic detection technique (WMS − 2f ∕1f ) at a modulation frequency of 50 kHz. Wavelength selection at 2059.91 cm−1 targets the P(20) transition within the fundamental vibrational band of CO, chosen for absorption strength and relative isolation from infrared water and carbon dioxide absorption. The CO spectral model is defined by the Voigt line-shape function, and key line-strength and line-broadening spectroscopic parameters were taken from the literature or measured. Sensitivity analysis identified the CO-N2 collisional broadening coefficient as most critical for uncertainty mitigation in hydrocarbon/air combustion exhaust measurements, and this parameter was experimentally derived over a range of combustion temperatures (1100–2600 K) produced in a shock tube. Accuracy of the wavelength-modulation-spectroscopy-based sensor, using the refined spectral model, was validated at pressures greater than 40 atm in nonreactive shock-heated gas mixtures. The laser was then free-space coupled to an indium-fluoride single-mode fiber for remote light delivery. The fiber-coupled sensor was demonstrated on an ethylene/air pulse detonation combustor, providing timeresolved (∼20 kHz), in situ measurements of CO concentration in a harsh flow field. © 2014 Optical Society of America OCIS codes: (060.2390) Fiber optics, infrared; (060.2430) Fibers, single-mode; (280.1740) Combustion diagnostics; (300.1030) Absorption; (300.6340) Spectroscopy, infrared; (300.6360) Spectroscopy, laser. http://dx.doi.org/10.1364/AO.53.001938
1. Introduction
Many energy-conversion systems, including combustion engines, operate at elevated pressures due to thermodynamic advantages. Accurate measurements of species concentration in high-pressure reacting gases can provide an empirical basis to evaluate competing flow field mechanisms including mass transport, heat transfer, and chemical kinetics that characterize system performance. Moreover, the short time scales involved in chemistry and high-speed flows require diagnostics that can provide high-bandwidth 1559-128X/14/091938-09$15.00/0 © 2014 Optical Society of America 1938
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response to resolve transient speciation. Laser absorption sensors have been utilized extensively to provide high time-resolution for in situ measurements of species concentration in reacting gases, particularly for shock tube kinetics studies [1]. Some laser-based measurements have also been conducted in high-pressure industrial environments, including coal gasifiers, internal combustion engines, and pulse detonation combustors [2–5]. More widespread application of laser absorption diagnostics to industrial gas systems and combustion engines has generally been limited by the challenges of high-pressure absorption spectroscopy and harsh thermomechanical environments. This work presents a nonintrusive, mid-infrared laser absorption sensor for carbon monoxide (CO), developed
to address the aforementioned challenges and aimed for harsh high-pressure combustion applications. CO is an intermediate and sometimes major (>1%) product of hydrocarbon combustion. The quantity of CO in lean flames provides a sensitive indication of incomplete combustion and the local fuel-to-oxidizer ratio. Excess CO in combustion exhaust is undesirable due to the correlation with poor engine efficiency and matters of health and safety. The need to minimize CO concentrations in various combustion systems drives the demand for highly sensitive diagnostics. The strong fundamental absorption band of CO near 4.7 μm offers potential for very sensitive species detection, even at short path-lengths (3 μm) has typically required cryogenic cooling systems which render the lasers impractical for most sensing applications beyond the laboratory. Due to readily available roomtemperature diode lasers and optical hardware in the near-infrared wavelength domain (1–3 μm), most previous CO absorption-based sensors have been designed to probe the overtone vibrational bands near 1.55 μm [6–9] and 2.3 μm [2,10,11], which unfortunately are more than four and two orders of magnitude weaker, respectively, than the fundamental band. Weak absorption strength, combined with spectral interference from carbon dioxide and water, renders the near-infrared CO sensors insufficient for most short path-length combustion systems. The recent advent of the quantum cascade laser (QCL) has enabled access to mid-infrared wavelengths previously unavailable with room-temperature lasers [12], and researchers have since exploited the fundamental CO absorption band near 4.7 μm for various applications requiring very sensitive species measurements [13–15]. Here we utilized a distributedfeedback QCL near 4.85 μm to probe the fundamental band and extend CO absorption sensing to high pressures (>40 atm) in hydrocarbon-fueled engines. Field sensing in harsh combustion engines necessitates a measurement technique which is robust against thermomechanical noise sources and conducive to spectral characteristics associated with both high pressures and high temperatures (>1000 K). At elevated pressures, discrete spectral lines are broadened and blended such that a zero-absorption baseline signal cannot be directly recovered. This blending effect is amplified at high temperatures where more transitions are active and the absorption spectra more crowded. Additionally, fluctuations in detected light intensity, unrelated to absorption, are common to engine applications due to problems such as beam steering, emission, misalignment due to vibrations, window fouling, and scattering. In combination, these issues complicate direct absorption sensing strategies and can degrade sensor performance [16,17]. To counter such difficulties, a wavelength modulation spectroscopy (WMS) technique was employed for the present sensor. With WMS, frequency filtering of harmonic signals allows for
substantial noise rejection. Moreover, the ratio of two harmonic signals (WMS-nf ∕mf ), which can be related to the absorption spectra, is independent of absolute optical power or intensity baseline [18]. These characteristics reconcile the above-mentioned challenges and make WMS a well-suited spectroscopic technique for harsh engine applications. High-temperature (1100–2600 K), high-pressure (10–40 atm) absorption measurements, using this WMS sensor, were carried out in shock-heated CO-N2 mixtures in order to validate the sensor and calibrate the CO spectral model. The QCL was then free-space coupled to a single-mode fiber for remote light delivery, and the fiber-coupled sensor was demonstrated on an operating pulse detonation combustor (PDC) at the Naval Postgraduate School in Monterey, California, yielding time-resolved, in-stream CO concentration measurements at conditions exceeding 40 atm and 2700 K. 2. Theory
The theory of absorption spectroscopy is briefly outlined to clarify terminology and units; more detailed theoretical discussions can be found in other works [19–21]. The Beer–Lambert law defines the fundamental physical relation governing narrowband laser absorption spectroscopy. This law relates the measurable quantities of incident light intensity and transmitted light intensity through a gas medium by It exp−αv ; I0 v
(1)
where αv denotes the spectral absorbance at frequency v [cm−1 ]. The spectral absorbance is related to specific gas properties, including mole fraction xabs , by X I αv − ln t Pxabs Si Tϕvi L; (2) I0 v i where P [atm] is the total gas pressure, Si T [cm−2 atm−1 ] is the line-strength of a quantum transition i which varies only with temperature, ϕvi [cm] is the line-shape function, and L is the length across a uniform path. In Eq. (2), the spectral absorbance is expressed as a summation to account for the overlap of neighboring transitions, common at elevated pressures. All else known, it follows that species mole fraction can be directly inferred from a measurement of the transmitted and incident light intensities at a specific wavelength, and direct absorption spectroscopy (DAS) is commonly employed for optical gas sensors due to its simplicity [1]. WMS is a more complex sensing technique than DAS with regards to requisite hardware characterization and data postprocessing, but insensitivity to nonabsorption light intensity fluctuations makes the normalized (WMS-nf ∕mf ) method advantageous for harsh environments [18]. A brief description of 20 March 2014 / Vol. 53, No. 9 / APPLIED OPTICS
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the WMS technique is outlined here, with notation borrowed from previous works [10,22]. For WMS, the laser injection current is modulated rapidly, yielding simultaneous modulation of the laser output intensity and wavelength, or frequency [cm−1 ]. Instantaneous frequency modulation (FM), vt, is given by vt v¯ a cosωt;
(3)
where v¯ [cm−1 ] is the center laser frequency, a [cm−1 ] is the modulation depth, and ω is the angular modulation frequency (ω 2πf ). The simultaneous intensity modulation (IM), I 0 t, can be defined as I 0 t I¯ 0 1 i0 cosωt ψ 1 i2 cos2ωt ψ 2 ;
(4)
where I¯ 0 is the baseline laser intensity at v¯ , i0 is the normalized linear IM amplitude with phase shift ψ 1 , and i2 is the normalized nonlinear IM amplitude with phase shift ψ 2 . The parameters i0 ; i2 ; ψ 1, and ψ 2 are characteristic of the laser source and can be predetermined in the laboratory. A lock-in amplifier combined with a low-pass filter is utilized to extract the harmonic (nf ) signals of the laser modulation frequency which relate directly to the absorbance spectra. The second harmonic signal (WMS-2f ) is used in this work, normalized by the WMS-1f signal to cancel out the absolute optical power term common to both. This normalization negates sensitivity to nonabsorption transmission losses and baseline noise characteristic to engine applications. The resulting 1f -normalized WMS-2f signal (WMS-2f ∕1f ) is a function of predetermined laser parameters (i0 ; i2 ; ψ 1 ; ψ 2 ), and thermodynamic properties (T; P; xabs ) associated with the gas environment. With an independent measurement of temperature and pressure, this technique can be used to measure species concentration, or mole fraction. 3. Wavelength Selection
The primary criteria for wavelength selection include strong absorbance and minimal spectral interference from other combustion products, namely water and carbon dioxide. Figure 1 shows the fundamental CO absorption band from 4.4 to 5.5 μm, plotted as line-strengths at 2000 K, and overlaid with the H2 O and CO2 absorption lines within the same domain. The CO2 fundamental band (∼4.3 μm) exhibits a very dense spectrum that interferes substantially with the R-branch of the CO band under typical combustion conditions where the CO2 :CO ratio is much greater than unity. The P-branch also has significant interference from water, but the overlapping H2 O spectrum is much less crowded compared to the interfering CO2 band, such that the larger spacing between water lines yields some narrow regions that are nearly interference-free. Specifically, the spectral domain near 2060 cm−1 provides an attractive combination of low relative interference and strong absorption. Therefore, we select this wavelength region (4.855 μm) to detect CO. 1940
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Fig. 1. Absorption line-strengths of CO, CO2 , and H2 O at 2000 K (HITEMP [23]).
Absorbance simulations at target engine conditions provide a more quantitative estimation of signal strength and isolation from interfering species. Here the target application is a hydrocarbonair combustor with pressures ranging from 5 to 40 atm. Expected values of temperature, gas composition, and path-length are based on fuel selection, fuel-to-oxidizer ratio, and engine size. Typical combustion temperatures range from 1000 to 3000 K. CO concentrations can vary dramatically from ppm levels to several percent (∼104 dynamic range) depending primarily on the H:C ratio of the fuel, local equivalence ratio, and combustor residence time. This sensitivity enhances the value of the diagnostic for assessing combustion processes, but can make estimation of expected mole fraction and sensor design challenging. The path-length is engine-specific. In the current research, the CO sensor is intended for utilization on an ethylene-air pulse detonation combustor with a path-length of slightly less than 4 cm. Assuming chemical equilibrium, the concentration of CO and other combustion products were estimated at several conditions [24], and the absorbance spectra were simulated using these values. Figure 2 shows absorbance simulations near 4.855 μm at two expected engine conditions by utilization of the Voigt line-shape function coupled with the HITEMP spectroscopic database [23]. The highpressure CO absorption spectrum near 2059.9 cm−1 is composed of multiple overlapping transitions, but the P(20) line in the υ0 → 1 fundamental band makes the dominant contribution. The P(14) line at 2060.33 cm−1 and the P(8) line at 2059.21 cm−1 , from the respective υ1 → 2 and υ2 → 3 fundamental hot bands, also make a small contribution at high temperatures. Interference from CO2 is negligible, and H2 O interference is relatively minimal, especially at the line-center of the P(20) line (2059.91 cm−1 ). The influence of the presence of CO2 and H2 O on the CO absorption measurement is further discussed in a later section. 4. Sensor Development A. Optimizing Modulation Depth
The WMS harmonic signals vary directly with modulation depth (a), which can be adjusted to optimize
(a)
(b)
Fig. 2. Absorbance spectra simulations of equilibrium concentrations of CO, CO2 , and H2 O (air bath gas) near 4.855 μm at expected PDC conditions (C2 H4 -air, ϕ ≈ 1); L 4 cm. (a) P 10 atm, T 2500 K and (b) P 30 atm, T 3000 K.
signal magnitudes for a given set of gas conditions. We define the optimum modulation depth for this sensor as the value at which the WMS-2f signal is maximized, within constraints of the laser. The laser used in this work is a single-mode distributedfeedback (DFB) QCL (ALPES), centered in wavelength near 4855 nm, with a typical output power of 8 mW. The modulation depth is primarily a function of two laser input parameters: modulation frequency (f ) and amplitude of the modulated injection-current. The modulation depth increases approximately linearly with injection-current amplitude and decreases nonlinearly with modulation frequency. A maximum modulation depth at a given modulation frequency can therefore be measured at the upper limit of injection-current amplitude which is bound by the current threshold, maximum operating current, and mode-hoping regions of the QCL. Figure 3 plots the maximum modulation depth for the QCL, when tuned to the P(20) line-center, at modulation frequencies from 40 to 100 kHz. A high modulation frequency is desirable due to a direct correlation with sensor bandwidth, but this comes at expense of modulation depth. Assessment of the WMS-2f signal as a function of modulation depth requires further knowledge of the aforementioned characteristic laser parameters. The linear and nonlinear IM amplitudes (i0 and i2 ) are coupled with modulation depth, and the relationship between these hardware-related parameters is depicted in Fig. 4. The phase-shift components, ψ 1 and ψ 2 , are found to be approximately independent of modulation depth, with measured values of 1.16π radians and 0.84π radians, respectively. Details on the methods used to determine these parameters are found in other works [10,22]. With the laser parameters defined, the relative magnitude of the WMS-2f signal at line-center can be calculated as a function of modulation depth for various test conditions, as illustrated in Fig. 5.
The maximum WMS-2f signal is highly dependent on gas conditions which vary dramatically in a detonation-based engine. Though temperature and pressure are both transient, the WMS-2f signal is most sensitive to pressure due to sensitivity to line broadening. Figure 5 highlights this sensitivity over a range of pressures expected in the PDC. As pressure increases, the optimum modulation depth increases, to the extent that maximizing the WMS-2f signal at pressures greater than 10 atm becomes impractical within the tuning limitations of the current laser, revealing a requisite compromise between sensor bandwidth and signal magnitude or detectability. A modulation depth and frequency of 0.235 cm−1 and 50 kHz, respectively, were chosen for this sensor to yield sufficient signal over the broadest range of transient conditions, while maintaining the desired high bandwidth capability. With modulation depth specified, all hardware
Fig. 3. Measured modulation depth at maximum injectioncurrent amplitude (120 mA) as a function of laser modulation frequency. 20 March 2014 / Vol. 53, No. 9 / APPLIED OPTICS
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Fig. 4. Measured laser intensity parameters (i0 and i2 ) as a function of modulation depth (f 50 kHz).
Fig. 6. Simulated CO WMS spectra (2f , 1f , 2f ∕1f ) near 2059.9 cm−1 at a typical PDC condition; T 2000 K, P 20 atm, L 4 cm, xco 1%.
parameters are established, and the normalized laser intensity can be expressed as I 0 t∕I¯ 0 1 0.718 cos2πf t 1.16π 0.0106 cos4πf t 0.84π;
(5)
where f is 50 kHz. Figure 6 shows a spectral simulation of the relevant WMS harmonic signals at a typical PDC condition based on these selected laser settings. We note that in practice the IM amplitudes, as defined in Eq. (5), vary slightly between experimental setups, whereas the phase-shift terms and modulation depth are insensitive to such extrinsic changes. Due to the strong dependence of the WMS-2f signal on the linear IM amplitude (i0 ), a calibration of this hardware-related parameter for each unique optical setup improves measurement accuracy. B.
Wavelength Shift Due to Modulation
High-frequency modulation of laser injection-current causes a shift in the center wavelength away from the unmodulated laser wavelength at the same mean
Fig. 5. Simulated WMS-2f (background subtracted) at 2059.91 cm−1 as a function of modulation depth for various pressures at 2000 K. 1942
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current and temperature settings. This wavelength shift results from ohmic heating of the laser cavity that increases with modulation frequency and injection-current amplitude [25]. To maintain line-center in our WMS measurement, a corresponding wavelength correction is required. The magnitude of wavelength shift can be determined by comparing measured WMS spectra using specific modulation settings to spectral simulations. For the 4.85 μm DFB-QCL, the wavelength shift, Δv, was measured to be 0.032 cm−1 at the selected modulation settings (f 50 kHz, a 0.235 cm−1 ). C.
Spectral Modeling
The current sensor employs a Voigt line-shape profile, coupled with the HITEMP 2010 database, as the foundation for the CO spectral model. Figure 7 illustrates the suitability of the Voigt line-shape function for modeling the P(20) and P(14) CO lines near 2060 cm−1 based on direct-absorption measurements in a heated static cell. Due to the simple diatomic structure of CO, the CO spectra can be modeled accurately using the Voigt line shape, along with computed HITEMP line parameters, over a broad range of conditions [2,10]. At a given thermodynamic condition, a Voigt line-shape profile is defined by two transition-specific spectroscopic parameters: linestrength, ST, and the collisional line-broadening coefficient, 2γT. The Doppler line width is also needed to fully describe the Voigt profile, but this parameter is a function of wavelength and temperature only and not transition-specific. Calculated linestrength values from HITEMP for the fundamental CO band have been validated previously, and shown to provide a high degree of accuracy (σ ∼ 1%) [13]. Line-broadening parameters for N2, CO2 , and H2 O, the dominant collision partners, are found in the literature and noted in Table 1 [26,27]. Broadening uncertainties of less than 10% are reported for all perturbers. Figure 8 presents a sensitivity analysis of the WMS-based mole fraction measurement to
Fig. 7. Measured absorbance spectra of CO in N2 near 2060 cm−1 with active P-branch lines labeled P(v”,J”) and fit with the Voigt function; T 804 K, P 1 atm, L 20.95 cm, xco 0.005.
uncertainties in these key spectroscopic parameters for the two most influential CO lines near 2059.91 cm−1 at a detonation condition. Such analysis helps identify line parameters that deserve further experimental investigation. As previously discussed and underscored by the sensitivity analysis in Fig. 8, the P(20) line makes the dominant contribution to our CO mole fraction measurement by more than a factor of ten relative to the next most important contributor in the P(14) line of the υ1 → 2 band. Moreover, the CO-N2 broadening coefficient is highlighted as the most important broadening parameter, due to the high concentration of diatomic nitrogen in hydrocarbon-air combustion exhaust. To improve sensor accuracy and mitigate uncertainty, we calibrated the CO-N2 broadening coefficient for the P(20) line at elevated temperatures, and validated the WMS measurement technique at detonation-like conditions in a shock tube. Shock tubes can produce well-defined environments for short time periods (∼ms) at combustion temperatures above that attainable in a static optical cell (>1000 K). Here we used a high-pressure, stainless-steel shock tube to heat nonreactive CO-N2 gas mixtures to a range of temperatures (1000–2700 K) representative of the detonation combustor in order to calibrate collisional broadening of the P(20) line by N2 . The shock tube has been described in previous works [4,28,29], hence only limited details are Table 1.
Fig. 8. Sensitivity analysis of the WMS-2f ∕1f mole fraction measurement near 2059.91 cm−1 to the line-strength and collisional broadening parameters of the P(0,20) and P(1,14) CO lines at a typical PDC condition (see Fig. 7).
outlined here. Measurements are made approximately 1 cm from the shock tube end wall with an optical path-length of 5 cm. Gas properties are assumed uniform in accordance with a planar shock, and temperature and pressure are calculated from normal shock relations based on measured initial conditions and the measured incident shock velocity. Typical temperature uncertainty is 1% for steadystate test times of approximately 1 ms behind the reflected shock wave. Assuming known thermodynamic variables in addition to line-strength, a WMS-2f ∕1f measurement at the P(20) line-center can be used to infer the CO-N2 collisional broadening coefficient, 2γ CO-N2 T, the remaining free Voigt parameter for a dilute mixture of CO in nitrogen. The broadening coefficient was iterated within the CO spectral model to match the simulated WMS-2f ∕1f values to the measured values. Broadening coefficients, as measured in the shock tube, are plotted in Fig. 9 against temperature. The temperature dependence of collisional line broadening is typically modeled as a power law n T 2γT 2γT 0 0 ; T
(6)
where n is the temperature-dependence exponent. Previous power-law characterizations of nitrogen or air broadening of the P(20) line are shown along with the measured data in Fig. 9 [23,26,30,31].
Spectroscopic Line Assignments and Collisional-Broadening Parameters for the CO Lines of Interesta
Collisional Broadening (γ in 10−3 cm−1 ∕atm) [26,27] CO-H2 O
CO-N2 P(v”,J”) P(0,20) P(1,14) a
v0
cm−1
2059.91 2060.33
E00
cm−1
806.4 2543.1
S(296 K)
cm−2 ∕atm 10−2
87.6 × 26.4 × 10−5
CO-CO2
γ1000 K
n
γ300 K
n
γ300 K
n
22.5 (3%) 26.8
0.55 (3%) 0.61
119 125
0.72 0.77
51.8 63.1
0.50 0.53
Uncertainties for broadening measurements made in this work (i.e., CO-N2 ) are shown in parentheses. 20 March 2014 / Vol. 53, No. 9 / APPLIED OPTICS
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Fig. 10. Sensor validation data showing the measured CO mole fraction compared to the known mole fraction over a range of conditions produced in a shock tube (σ ≈ 3%); L 5 cm, xco 0.00495 in N2 .
Fig. 9. Measured nitrogen-broadening coefficient for the P(0,20) line from 1100 to 2600 K including a best-fit power law equation and comparison with broadening equations from other sources; P 10–50 atm.
The HITEMP model exhibits excellent agreement with the data at the lower temperature domain (∼1100 K), but under-predicts broadening at higher temperatures (>2000 K), where the empirical model by Varghese shows best agreement. The disagreements between models and data may be attributed to the general insufficiency of the power law to capture temperature dependence of broadening over a large range of temperature [32,33] and the differing temperature domains for which these models were established. Here we best-fit the measured broadening coefficients over the domain of 1100–2600 K, yielding a more refined broadening model for combustion temperatures. The reference broadening coefficient and temperature-dependence exponent for this refined 2γ CO-N2 T model are shown in Table 1. The authors recommend caution employing these parameters outside of the temperature range of the measurements. With the calibrated spectral model, CO mole fraction was inferred from the measured WMS data produced in the shock tube and compared to the known mole fraction of the test gas mixture. Figure 10 depicts the typical uncertainty of the spectral model (σ ∼ 3%) over a wide range of both temperatures and pressures (10–40 atm) that emulate detonation conditions, validating the accuracy of the CO concentration sensor for the intended PDC application. D.
Optical Engineering
To isolate sensitive laser equipment from the thermomechanically harsh engine environment, a remote light delivery system was developed. Figure 11 depicts a simplified schematic of the optical setup as deployed in field experiments. An indium-fluoride (InF3 ) single-mode fiber (L 5 m) with a core diameter of 17 μm was coupled to the 4.85 μm DFB-QCL using an aspheric zinc-selenide (ZnSe) microlens with a focal length of 6 mm. The fiber is free-space aligned with the lens using a five-axis alignment stage and has a characteristic 1944
loss of ∼0.5 dB∕m at the target wavelength. The single-mode fiber mitigates mode noise associated with larger core-diameter multimode fibers and facilitates improved beam collimation at the fiber output [34]. Here we collimated the output beam with a 12 mm focal length ZnSe aspheric lens to achieve a ∼2 mm diameter beam at a working distance of 40 cm. The collimation lens is integrated with an SMA connector to attach directly to the fiber termination, and a four-axis kinematic mount interfaces with the test hardware to direct the beam through the engine. The light collection and detection components are mounted together in a compact linear cage system (Thorlabs—30 mm). The transmitted light is focused by an antireflection-coated calcium fluoride lens (focal length 20 mm) onto an infrared photovoltaic detector (Vigo PVI-4TE-5) with a 2 mm2 detection area and specific detectivity (D ) of ∼3 × 1011 cm Hz1∕2 W−1. The detector bandwidth is 10 MHz. Two narrow-bandpass spectral filters are stacked between the engine mount and collection lens to yield an effective spectral width of ∼25 nm centered at about 4860 nm. Due to the sensitivity of the infrared detector from ∼2 to 5 μm, spectral filtering was critical for reducing thermal emission from H2 O which is typical of combustion exhaust. An analogous, but more detailed, discussion of optical system design for this application can be found in a previous work by the current authors [4].
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Fig. 11. Simplified mid-infrared light delivery and detection schematic.
5. Sensor Demonstration
Detonation-based engines represent an especially difficult application for diagnostics due to the wide range of thermodynamic properties involved and the rapid transients of those properties. Additionally, pulse detonation engines are mechanically violent, and vibrations or acoustic waves resulting from combustor gas dynamics can easily damage instrumentation which interfaces with the engine. Here we demonstrate the feasibility of the fiber-coupled CO absorption sensor on an operating pulse detonation combustor, highlighting measurement timeresolution and sensor operability range. Discussion regarding combustor analysis has been reserved for other publications and deliberately constrained here. Figure 12 exhibits representative measurements of pressure and WMS-2f ∕1f for two consecutive detonation cycles in the PDC chamber. Detonation waves were produced by ethylene-air gas mixtures, spark-ignited every 50 ms (20 Hz), and the primary detonation event transpired within just a few milliseconds as indicated by the pressure time history. Utilizing simultaneous H2 O thermometry along with pressure data, CO mole fraction was directly inferred from the WMS-2f ∕1f measurement [35]. High-fidelity CO data was resolved over the full detonation cycle and entire range of operating conditions. To reject noise, a 20 kHz low-pass filter was applied to the lock-in amplifier outputs (2f and 1f signals). Such a wide filter, relative to the modulation frequency (50 kHz), was feasible due to the strong harmonic signals associated with the probed CO spectra in the PDC environment. Tighter filters offered minimal further noise reduction, while having the expense of lower measurement bandwidth. The time-variable signal-to-noise ratio (SNR), as noted in the WMS-2f ∕1f time-history in Fig. 12, resulted from the dramatically changing spectral structure of CO during a detonation event. Directly
Fig. 12. Representative time-history data from a pulsed (20 Hz) detonation combustor operating on ethylene/air; L 7.62 cm. Pressure and WMS-2f ∕2f measurements are coplanar in combustion chamber.
behind the detonation wave, at the most extreme conditions (P > 30 atm, T > 2500 K), a detection limit of approximately 0.5% CO was inferred from an SNR of ∼5 and 2.5% measured CO. In the more benign tail at later times of the detonation cycle (P ∼ 10 atm, T ∼ 1200 K), a detection limit of ∼500 ppm was achieved (SNR ∼ 10, xco ∼ 0.5%). The temporal resolution or measurement bandwidth is effectively equated to the width of the low-pass filter on the harmonic signals, or 20 kHz. Other environments may require different frequency filtering schemes to minimize noise, thus yielding different bandwidths. 6. Summary
We report the development of an in situ, high-bandwidth CO concentration sensor for high-pressure combustion flows based on a fixed-wavelength WMS2f ∕1f technique. The sensor probes the fundamental vibrational band of CO near 4.8 μm which provides orders of magnitude greater absorption and detectability than the overtone bands at shorter wavelengths. Wavelength selection at 2059.9 cm−1 exploits the strong absorption strength of the P(20) rovibrational transition and relative spectral isolation from water interference. A distributed-feedback QCL centered near 4.855 μm was modulated in wavelength and intensity at 50 kHz, with the modulation depth maximized for high-pressure applications (>10 atm). The wavelength shift due to laser injection-current modulation was accounted for in the fixed-wavelength, line-center measurement. Shock tube experiments refined and validated the CO spectral model, which utilizes the Voigt line-shape function, at conditions expected in detonation environments (up to 40 atm). Collisional broadening coefficients for CO in N2 were determined for the P(20) line over a range of elevated temperatures (1100–2600 K) behind reflected shock waves and fit with a power law to reduce measurement uncertainty ( 10), time-resolved measurements of CO over the complete detonation cycle at a bandwidth of approximately 20 kHz. Broadly, this research extends the pressure capabilities (∼40 atm) of laser-based absorption sensing of CO in harsh environments, with applicability to numerous high-temperature combustion systems. Field experiments on the PDC were conducted at the Naval Postgraduate School (NPS) in Monterey, California. Support for these diagnostic studies was provided by Innovative Scientific Solutions, Inc., with John Hoke as project manager, and the AFOSR, with Chiping Li as technical monitor. The authors would like to thank Professor Chris Brophy and Dave Dausen at NPS for technical support and operation of the PDC for sensor demonstration. 20 March 2014 / Vol. 53, No. 9 / APPLIED OPTICS
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