Experimental and Numerical Investigationof CH* and OH* Chemiluminescencein Acetylene Combustion behind ReflectedShock Waves

M. Bozkurt, M. Fikri, and C. Schulz

1 Introduction

Spontaneous light radiation from chemical excited species is responsible for thewell-known emission of flames. Blue light emission due to the CH(A2Δ-X2Π) andUV emission from the OH(A2Δ-X2Π) transitions are specific features in hydro-carbon combustion and are most frequently employed in various applications. Thesimplicity of the detection of the spontaneous emission compared to laser-baseddiagnostics make chemiluminescence spectroscopy very attractive for the investi-gation of flames and potentially for combustion control. This, however, requires afundamental understanding of the coupling of light intensities and spectral featuresto the underlying chemical processes in a practical combustion situation. Charac-terizing the main reaction pathways for the chemiluminescence and determine theirrate coefficients have proven to be a challenging issue.

H+O+M ↔ OH∗+M (1)

CH+O2 ↔ OH∗+CO (2)

OH* chemiluminescence formation is mainly attributed to (1) and (2) in hydrocar-bons [1] - [5]. In a recent study [6] we investigated the OH* formation by means ofshock-tube experiments combined with a numerical approach for different H2/O2/Armixtures. A model-based calibration procedure for high temperatures has been ap-plied to quantify the OH* chemiluminescence in terms of absolute concentrations.It was found that (1) is the main reaction channel for T < 2800 K and its rate wasmeasured and validated against chemiluminescence data from shock tubes and flamevelocities.

M. Bozkurt · M. Fikri · C. SchulzIVG, University of Duisburg-Essen, 47048 Duisburg, Germanye-mail: metehan.bozkurt@uni-due.de

422 M. Bozkurt, M. Fikri, and C. Schulz

Although Broida et al. [7, 8] identified CH* chemiluminescence almost half acentury ago ambiguity about the key reactions for its formation still remains. Theformation reaction producing electronically-excited CH* was suggested by Broidaand Gaydon [8, 9]:

C2 +OH ↔ CO+CH∗. (3)

Glass et al. [10] rejected the relevance of (3) and suggested the reaction of ethynylradicals C2H with atomic oxygen. Also Brennen and Carrington [11] and Grebe andHomann [12] identified the following reaction as the main channel:

C2H+O ↔ CO+CH∗. (4)

A third formation step yielding CH* was proposed in the early sixties by Hand andKistiakowsky [13] and supported by Matsuda et al. [14, 15] where C2H reacts withmolecular oxygen:

C2H+O2 ↔ CO+CH∗. (5)

Despite the number of the studies on hydrocarbon chemiluminescence, there is alarge disagreement in determining the major channel leading to CH* in hydrocar-bon system but also in providing recommendations of the rates coefficients. There-fore, the aim of our work was to improve the knowledge of the formation pathwaysleading to CH* via linking shock-tube experiments and numerical modeling of thechemiluminescence formation.

2 Experimental

The experiments were carried out in a stainless-steel shock tube with a constantinner diameter of 79 mm. An aluminum diaphragm (thickness: 50 μm) divides theshock tube into the driver section (length: 3.5 m) and the driven section (length:5.7 m). Hydrogen was used as driver gas and argon as bath gas. The shock-tube facil-ity is designed for ultra-high vacuum (UHV) purposes. The driver section is pumpeddown to 10−2 mbar. Prior to each experiment final pressures below 3×10−8 mbar areachieved in the driven section by a turbo-molecular pump.

Gas mixtures were prepared manometrically in a stainless steel UHV storagecylinder, which also was evacuated by a separate turbo-molecular pump. Argonand oxygen used in the present study were of the highest commercial available pu-rity (≥99.9999% for Argon, ≥99.998% for O2). Acetylene had a purity better than99.6%. The C2H2 concentration was held constant at 1000 ppm and the oxygenconcentration was varied with regard to the desired equivalence ratios (0.50 ≤ Φ ≤1.00).

The shock speed was measured over three intervals using four piezo-electric pres-sure transducers with a time resolution of 0.4 μs. The temperature T5 and pressurep5 behind the reflected shock wave were determined from measured incident shock-wave velocity and speed attenuation using a one-dimensional shock model. Theestimated uncertainty in reflected shock temperature is less than ±10 K in the tem-perature and time range of our measurements.

Experimental and Numerical Investigation of CH* and OH* 423

The shock tube is equipped with two separate units for chemiluminescence de-tection. OH* and CH* emission were recorded from the sidewall of the shock tube,respectively. After the passage of the reflected shock wave the chemiluminescenceemission passes a combination of two slits and a narrow-band interference filter(λ OH∗ = 308±5 nm, λ CH∗ = 430±5 nm) and is detected by two photomultipliers or-thogonal to each other. This setup provides a time resolution of 1 μs as determinedfrom the light collection angle and the reflected shock wave velocity. To ensurelinearity of the output voltage with respect to the input itensity within a sufficienttime resolution (±2 μs), the PMT/resistor combinations were optimized to providea compromise between time resolution and signal-to-noise ratio. The optical align-ment was kept without changes during the experimental series.

3 Numerical Modeling

The ground-state chemistry significantly affects the prediction of the chemilumines-cence signals. Therefore, a ground-state model must be chosen that is validated forthe relevant conditions.

The base model of this work describes the high temperature oxidation of acety-lene and ethylene. There are 75 species and 529 reactions involved. The entiremechanism was adopted from Wang and Laskin [16], which was validated for awide range of conditions with regard to shock-tube experiments, laminar burningvelocity and burner-stabilized flames.

Originally, this mechanism does not contain reactions that invole C2 specieswhich are assumed to be potential precursors for CH* formation. Therefore, weadded the C2 sub-mechanism recommended by Williams and Pasternack [17] thatconsists of 18 reactions. We ensured that these modifications did not affect the basemechanism by comparing the original and the modified mechanisms with regard tothe published validation data for the relevant species (OH and CO2) and ignitiondelay times. No notable differences were observed.

To represent the excited-state chemistry a sub-mechanism was implemented,which incorporates OH* and CH* formation pathways as well as their correspond-ing quenching reactions. A detailed description of the OH* subset is published inour recent work [6]. In addition, quenching reactions of OH* for relevant interme-diate species (CO, CO2 and CH4) and reaction (2) are included. The starting mech-anism for CH* consists of the three formation reactions (3) - (5) and 12 quenchingreactions adopted by Tamura et al. [18]. As start values for the simulation, rate co-efficients for CH* formation reactions were originally taken from Hall et al. [19].

4 Results and Discussion

Over 47 experiments were carried out in the shock tube under various conditionsusing acetylene mixtures containing molecular oxygen from stoichiometric to leanmixtures. Figure 1 shows typical time-resolved OH* and CH* emission profiles.Both species are occurring simultaneously and show similar time behavior in terms

424 M. Bozkurt, M. Fikri, and C. Schulz

of the chemiluminescence intensity profile shapes. The profiles are normalized totheir peak maximum intensities.

The values of the measured chemiluminescence signals (CH*) at their peak max-imum are also plotted as function of temperature in an Arrhenius plot (fig. 2) afternormalization to the corresponding value at T = 1800 K. The peak intensities ofCH* chemiluminescence increase exponentially with temperature, while the peaktime (τpeak) - i. e., the time interval between time t = 0 and time tpeak where thechemiluminescence reaches its maximum - shows an opposed effect. The inductiontime decreases with increasing temperature. This findings can be also observed forOH* chemiluminescence (not shown here).

In other experiments it was found that compared to the data with stoichiometricmixtures, a slight decrease in CH* signal was observed when increasing the initialO2 concentration. This depletion effect was also seen in a previous study from Hallet al. [19] for methane/oxygen mixtures. However, the decreasing CH* intensity inthe present study is not as pronounced as in methane and oxygen by [19].

Fig. 1 Chemiluminescence intensity profiles. Measurements: open circles, simulation: lines.(a) OH* and (b) CH*. Experimental conditions behind the reflected shock wave: T5 = 2273K, p5 = 1.32 bar, mixture: 0.10% C2H2 0.33% O2 in Ar. All profiles are normalized to theirpeak maximum

The starting mechanism for the chemiluminescence subset consists of two for-mation reactions (1) and (2) of OH* and three formation reactions (3) - (5) of CH*with their corresponding quenching reactions. Based on this initial subset we per-formed several simulations to reproduce the experimental data in terms of peak timeand relative peak maximum for three equivalence ratios.

Results show that the OH* sub-mechanism accurately describes the experimen-tal data with our basic reaction scheme. The numerical results and experimentaldata is also good with regard to ignition delay, temperature dependence, and profileshapes (for brevity not shown here). It was found that the OH* chemiluminescenceis solely sensitive to (2) in the acetylene/oxygen mixtures. The reaction (1) reversesand consumes OH*. OH* removal is mainly attributed to the quenching collisions

Experimental and Numerical Investigation of CH* and OH* 425

with argon and the reverse reaction of (1). Furthermore, for lean mixtures excessoxygen is also an important quenching collider for the depletion of the chemilumi-nescent species.

When incorporating reaction (5) in the chemiluminescence sub-mechanism witha rate coefficient greater than 1010 cm3mol−1s−1, the numerical results cannot repro-duce the experimentally observed depletion of CH* for lean mixtures. The reducedCH* emission for lean mixtures can exclusively be simulated by CH* formationvia (3). By excluding reaction (5) and optimizing reaction (3) and (4), the simu-lations can accurately predict our experimental data in terms of ignition delay andthe temperature-dependent relative peak maximum (cf. fig. 2) for all three inves-tigated mixtures. We can recommend values for k3 = 2.0×1014 cm3mol−1s−1 andk4 = 3.6×1012exp(-1308K/T) cm3mol−1s−1.

Fig. 2 Experimental (open symbols) and simulated (lines) results of (a) time to peak maxi-mum and (b) peak maximum normalized to the corresponding value at T = 1800 K of CH*chemiluminescence signal for mixture: 0.10% C2H2 0.50% O2 in Ar

5 Conclusions

Shock-tube experiments behind the reflected shock waves have been conducted inC2H2/O2 mixtures. Pathways for the formation of OH* and CH* have been in-vestigated through a combined experimental and numerical analysis. Based on thewell-established basic ground-state model of acetylene oxidation and additionalC2 species chemistry and an implemented chemiluminescence sub-mechanism,ignition delay times and relative intensities of OH* and CH* from shock-tube ex-periments were used to validate the model. From the model the profiles could accu-rately described by (1) and (2) with reaction coefficients k1 = 1.5×1013 exp(-3007K/T) cm3mol−1s−1 and k2 = 8.0×1010 cm3mol−1s−1. CH* chemiluminescence in-tensities were depleted with excess of O2 which is in agreement with previousstudies and was found to be merely sensitive to (3). Our present study reveals theimportance of the following two reactions (3) and (4) with regard to the formationof CH*. With values for k3 = 2.0×1014 cm3mol−1s−1 and k4 = 3.6×1012 exp(-1308K/T) cm3mol−1s−1.

426 M. Bozkurt, M. Fikri, and C. Schulz

Acknowledgements. The authors gratefully acknowledge funding by the German ResearchFoundation, DFG.

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