The Study on Precise and Quantitative Measurement of Flame OH Concentration by CRDS-CARS-PLIF Techniques

BAI Bing, CHEN Guo-zhu, YANG Wen-bin, CHE Qing-feng, WANG Lin-sen, SUN Wei-min, CHEN Shuang*

1. College of Physics and Optoelectronic Engineering, Harbin Engineering University, Harbin 150001, China 2. State Key Laboratory of Aerodynamics, China Aerodynamics Research and Development Center, Mianyang 621000, China 3. Facility Design and Instrumentation Institute, China Aerodynamics Research and Development Center, Mianyang 621000, China

Abstract In this paper, a pulsed cavity-ring down spectroscopy (CRDS) is employed to measure the quantitative concentration of the OH radical in a plane flame burner with premixed methane/air. By analyzing the cavity ring-down absorption spectrum theory, we select the P1(2) absorption line spectrum of the electronic transition band OH A2Σ+-X2Π(0,0) and build a set of the pulsed CRDS experimental device with a laser wavelength of 308.6 nm. The device of the pulsed CRDS is composed of a pair of mirrors with a reflectance of 99.7%, the cavity length of the ring-down cavity is 270 cm, and the ring-down time of the empty cavity (without a flame in the optical cavity) is 2.33 μs. By analyzing the experimental parameters that affect the precise measurement of concentration, we use Planar Laser Induced Fluorescence (PLIF), Coherent Anti-Stokes Raman Scattering (CARS), and the pulsed CRDS to measure the effective absorption length of OH, high temperature of the flame, and cavity ring-down time. When the premixed methane (1.1 L·min-1) and air (15 L·min-1) are burned in a flat flame burner, and at the height of 6 mm from the burner surface, the precisely measured effective absorption length by PLIF is 7.1% higher than that of directly choosing the diameter of the burner surface as the absorption length, the measured precision of the temperature by CARS is increased by 45% than that measured by the thermocouple under room temperature, the measured precision of the optical cavity ring-down time with flame in the cavity and non-OH absorption wavelength is improved by 21.6% than that measured time of cavity ring-down without a flame in the cavity. By combining the above measurement techniques to measure all experimental parameters precisely, we obtain that the number density of OH molecules (3.59×1013 molecules·cm-3) can reach the maximum value when the height from the furnace burner is 6 mm, and the precision of OH concentration is 35.6% higher than that of the unmodified OH concentration. Under different equivalence ratios (Φ=0.7～1.1), with the increase of the height from the burner surface, the number of OH particles gradually decreases, and the curve fitting shows that the OH concentration decreases in an e-exponential decay. At the same combustion height, the concentration of OH increases with the increase of equivalent ratios. When the methane flow rate is kept constant, the OH concentration in the oxygen-rich combustion condition is higher than in the low-oxygen combustion condition. In the combustion field, the precise measurement method with the multi-spectral technology (CRDS-CARS-PLIF) can achieve the precise quantitative measurement of OH concentration and provide technical support for the quantitative measurement of the concentration of other combustion product molecules, which plays a crucial role in the study of combustion chemical reactions.

Keywords CRDS (cavity ring-down spectroscopy); Effective absorption length; High-temperature testing;Ring-down time; OH concentration

Introduction

Quantitative measurement of substance concentration in the combustion field is necessary to understand the chemical reaction of combustion. As an important intermediate combustion product, OH plays a vital role in studying the chemical combustion reaction. Laser-induced fluorescence (LIF) has been used to measure the number density of OH particles[1]. However, it is difficult to achieve the quantitative measurement of OH concentration by the LIF technique, and the main reasons are constant calibration, quenching of excited particles, and low one-way absorption intensity. Chen Shuang’s group[2]obtained the real OH fluorescence signal in experience by subtracting the difference between the original LIF signal and pure Rayleigh scattering signal to calibrate the undesired background from scattered laser light. However, the uncertainty of the obtained OH concentration is still as high as ±25%. Among them, the largest error comes from the uncertainty of LIF measurements, the measured temperature, and OH concentration distribution at the edges. In addition, the most serious difficulty in achieving quantitative measurement for LIF is first defining the energy transfer of excited states such as the total impact quenching rate of electronically excited states, and the energy transfer of rotational and vibration[3].

As a direct absorption spectroscopy technique, cavityring-down spectroscopy (CRDS) has gained widespread attention for the quantitative measurement of substance concentration[4]. Compared with the LIF technique, the CRDS has the advantages of high absorption intensity without being affected by laser energy fluctuations and the increased length of the absorption path to improve test sensitivity[5-7]. CRDS can be briefly summarized as a multi-pass absorption technique whereby the sample is placed in a high-finesse cavity, and the intensity decay of the laser is detected at the output mirror when the laser traverses in the cavity. In 1994, Gerard Meijer et al.[8]applied the cavity ring-down spectroscopy technique to combustion diagnosis and measured the absorption spectrum of OH. In recent years, CRDS was also employed in microwave plasma-assisted combustion[9]to measure the absolute concentration of the ground state OH radical at the ignition region. In addition, CRDS was also suitable for the quantitative measurement of other combustion intermediate products, such as CH4[10], CN[11], NO[12], HCO[13], and CO[14]. Although the CRDS technique has been widely used, the analysis of the measured concentration error is relatively rare. At present, Tu Xiaobo et al.[15]have carried out concentration error analyses caused by experimental equipment such as laser linewidth and line-type, laser frequency stability, and the response of photomultiplier tubes. However, by analyzing the CRDS theories, we find that the precise measurement of experimental parameters will also cause great errors in CRDS concentration measurement, such as replacing effective absorption length with burner surface diameter[15], the measured temperature errors, and the influence of flame for cavity ring-down time.

In this paper, the quantitative measurement of OH concentration is carried out by a pulsed CRDS technique on a McKenna burner with premixed CH4/air gases. Due to heat radiation and heat transfer[16], it is difficult for the thermocouple to achieve accurate temperature measurement at room temperature, so the high-precision CARS technique is used to measure the real temperature for the high-temperature flame. The PLIF technique obtains the flame structure distribution, and then the effective absorption length at different heights from the burner surface is measured. The P1(2) absorption line spectrum of the electronic transition band of OH A2Σ+-X2Π(0,0) of OH is selected to build a pulsed CRDS experimental device with the 308.6 nm laser wavelength to measure OH concentration. By analyzing the absorption spectrum theories and using the accurate experiment measurement, we realize the precise and quantitative measurement of OH concentration on the flat flame burner.

1 Theory of CRDS

The ring-down time of the optical cavity is an important parameter for CRDS, and its expression is as follows[17].

(1)

whereLis the cavity length of the optical cavity,cis the speed of light,Ris the reflectivity of the cavity mirror,αis the absorption coefficient of the sample,lis the absorption length,ξis the additional loss of the optical cavity.

When the optical cavity is empty (without flame), the ring-down timeτ0can be expressed as

(2)

By substituting Eq.(2) into Eq.(1), the absorption coefficient expression of the sample can be obtained.

(3)

It can be seen from Eq.(3) that the sample absorption coefficient is mainly determined by the ring-down time and absorption length and is not affected by the laser intensity[5,18].

In addition, the absorption coefficient of combustion products can be expressed as[15]

(4)

WherefBrepresents the Boltzmann coefficient corresponding to the ground state of combustion products,his the Planck constant,νis laser frequency,ηis the overlap factor, andB12is the absorption coefficient of Einstein.

Eq.(3) is substituted into Eq.(4), and the concentration of the combustion product OH can be expressed as

(5)

It can be seen from Eq.(5) that the main factors affecting OH concentration are absorption length, decay time, and Boltzmann coefficient. However, the decay time is also affected by the additional loss of the optical cavity, so keeping the consistency of the combustion state can effectively eliminate the error caused by the additional loss. The LIFBASE database finds the Boltzmann coefficientfBcorresponding to each temperature under the steady-state condition. As shown in Fig.1, the absorption intensity of OH gradually decreases with the increase in temperature, which can indicate that temperature impacts the accuracy of OH concentration measurement.

Fig.1 The absorption intensity of OH at different temperatures

2 Experimental Setup

In the complex spectral cackground, the absorption wavelength of OH is selected to avoid the highly overlapping absorption lines in the spectral bandwidth as much as possible, which can improve the signal-to-noise ratio and reduce the interference of adjacent spectral lines. The laser linewidth in the experiment is 0.3 cm-1, so the P1(2) absorption line in the electron transition band of A2Σ+-X2Π(0,0) of OH is selected as the absorption wavelength of OH.

The pulsed CRDS experimental device is shown in Fig.2. The laser system consists of a solid laser, a dye laser, and a frequency-doubling device. The laser system outputs a 308.6 nm UV laser with a pulse width of 10 ns, a repetition frequency of 10 Hz, and a pulse energy of 3 mJ. The plane mirror M1 fully reflects the 617.2 nm laser and transmits the 308.6 nm laser. The wavelength meter monitors the reflected 617.2 nm laser to ensure that the wavelength of the 308.6 nm laser can transmit through M1 without shifting. The transmitter 308.6 nm UV laser first passes through the pinhole to filter stray light and then enters the ring-down cavity. The two mirrors of the ring-down cavity (M4 and M5) use concave mirrors with a curvatuse radius of -6 000 mm and have 99.7% reflectance near 308 nm. The cavity length is 2.7 m to avoid interference between two adjacent pulses in the cavity. A filter plate (M6) is placed behind the optical resonator, which filters the background noise and only lets the light around 308 nm pass through. The filtered ring-down signal is received by a photomultiplier tube, and then the data is collected by a high-speed Oscilloscope.

Fig.2 A pulsed CRDS experimental device

In the experiment, a flowmeter is used to control the flow rates of CH4/air/N2, N2is used to protect the flame stability, and the premixed CH4/air is completed before reaching the flat flame burner. The parameters of experimental cases are shown in Table 1. Five cases with different equivalence ratios are selected in the experiment, which can realize high-temperature combustion[19]. Under each case,the interval of 3 mm height is performed from close to the burner surface, and a total of 9 groups of ring-down signal data are measured.

Table 1 Parameters of testing cases

3 Results and Discussion

3.1 Temperature measured by CARS

The temperature must be precisely measured to get the precise Boltzmann fractionfB. In the combustion environment of case 1, the CARS technique and thermocouple are used to measure the temperature of different heights at the same step, and the temperature difference and the relative error offBare shown in Fig.3. As shown in Fig.3, the larger the temperature difference between CARS and the thermocouple, the larger the relative error offB. With the increased distance from the burner surface, the temperature difference decreases gradually. When the distance from the burner surface is 6 mm, the temperature difference between the measuring point of CARS and the thermocouple is the highest, and the relative error offBcan reach 45%, which indicates that the deviation of the measured temperature by the thermocouple in the high-temperature environment is large. The thermocouple cannot meet the requirements of temperature measurement.

Fig.3 The temperature difference between CARS and the thermocouple (red dot) and fB relative error (blue dot) at different heights for case 1

To illustrate the accuracy of temperature measurement of the CARS system in the laboratory[19], five cases in the DLR[20](Deutsches Zentrum fur Luft-und Raumfahrt) are selected and measured for comparison. The results of the measured temperature by CARS are shown in Fig.4(a), and the results indicate that the measured temperature by CARS in the laboratory is completely consistent with those in the literature. Therefore, the CARS system is used to measure the temperature of the remaining casesat different heights, and the temperature results are shown in Fig.4(b).

Fig.4 The measured temperatures by CARS(a): The relationship between DLR literature temperature and CARS temperature; (b): CARS temperatures of different cases

3.2 Effective absorption length measured by PLIF

To accurately measure the effective absorption length of OH, the PLIF technique is first used to obtain the flame structure under different cases, and then the absorption lengths of OH at different heights are measured. As shown in Fig.5(a), the PLIF technique is used to obtain the flame structure of case 1, and the red line in Fig.5(a) is the effective absorption length of OH when the height from the furnace surface is 15 mm. It can be seen from Fig.5(b) that the effective absorption length of OH decreases with the increase of distance from the burner surface. The relative error of absorption length increases with height increasing. If the effective absorption length is replaced by the furnace face diameter, it will cause a 7.1%～39.5% relative error, and the higher the height, the greater the error. Therefore, the effective absorption length of different cases must be accurately measured by the PLIF technique.

Fig.5 (a) The measured active absorption length (red line) by PLIF at the height of 15 mm; (b) Active absorption lengths and relative errors at different heights for case 1

Figure 6 shows the measured effective absorption lengths by the PLIF technique at different heights from the burner surface for the remaining cases. As can be seen from case 4 in Fig.6, when the gas flow rate is very high, only a part of the effective absorption length near the burner surface is equal to the diameter of the burner surface.

Fig.6 Active absorption lengthsindifferent cases

3.3 OH concentration measured by CRDS

When the optical cavity is empty, the signal of the ring-down cavity is shown in the upper right corner of Fig.7(a). In Fig.7(a), the round-trip propagation time of the optical pulse in the cavity is 18 ns,which is larger than the laser pulse width of 10 ns, and the interference of two adjacent pulses in the cavity can be effectively avoided. As shown in Fig.7(b), the ring-down timeτ0of the empty cavity is 2.33 μs by selecting the maximum value of the ring-down signal and e-index fitting.

Fig.7 Cavity ring-down signal without a flame in the cavity(a): Raw data; (b): Signal fitting

To verify the influence withouta flame in the cavity for the ring-down timeτ0, it is also necessary to measureτ0when there is a flame in the optical cavity. When the flat flame burner is ignited in the cavity, and the laser wavelength is adjusted to the non-absorbing wavelength of OH, the ring-down timeτ0of case 1 is obtained at different heights from the burner surface, as shown in red star in Fig.8(a).It can be seen from Fig.8 (a) thatτ0of the empty cavity is greater than that of the flame inthe cavity, which indicates that the flame in the cavity will affect the value ofτ0by the additional loss of the optical cavity. As shown in Fig.8(b), the relative error of cavity ring-down timeτ0at different heights increases by 12.8%～21.6% compared with the ring-down time with flame in the combustion environment for case 1. It is mainly due to the Rayleigh scattering[15]caused by impurity particles in the flame combustion process, which causes a change in the additional lossξ. Moreover, it can be seen from Eq.(2) that the additional lossξwill affect the value of the ring-down timeτ0, but the ring-down timeτ0of the empty cavity ignores the loss caused by the flame impurity particles.

Fig.8 (a) The influence for τ0 with and without a flame in the cavity, (b) τ0 relative error for case 1

The overlap factorηis 2.188 cm andB12is 2.278×109m2·J-1·s-1, which can be obtained by checking the LIFBASE database. Then the OH concentration will be obtained by substituting the above parameters into Eq. (5). As shown in Fig.9, the OH concentration at different height positions is obtained by accurate measurement of experimental parameters in the combustion environment of case 1. The OH concentration increases by 35.6%～47.9% compared with the uncorrected one.

Fig.9 The concentration of OH and relative error for case 1

The OH concentration and fitting curves at different cases are shown in Fig.10. As can be seen from Fig.10, with the increase of heights from the burner surface, the concentration of OH gradually decreases, which is the same trend as the tested results in the literature[15,21]. It is found that the concentration of OH decreases exponentially with the increase of heights from the burner surface by fitting the curve. By comparing cases 1, 2, 3, and 4 at the same combustion height, we find that the concentration of OH increases with the increase of the equivalent ratio under the condition of oxygen-rich combustion. The comparison of cases 2 and 5 shows that the OH concentration in the oxygen-rich state is higher than that in the low-oxygen state when the CH4flow rate remains constant.

Fig.10 OH concentrations and curve fittings at different cases

4 Conclusion

The experimental results show that CARS, PLIF, and the pulsed CRDS have successfully measured the OH concentration in a flat flame burner with the premixed CH4/air. A set of the pulsed CRDS experimental device with the 308.6 nm laser wavelength is constructed by selecting the P1(2) absorption line spectrum of the electronic transition band OH A2Σ+-X2Π(0,0). By analyzing the spectrum theory,we find that the experimental parameters that affect the precise measurement of concentration are the effective absorption length of OH, temperature, and decay time. Planar laser-induced fluorescence (PLIF) is used to measure the changed effective absorption length of the flame structure. Coherent anti-Stokes Raman scattering (CARS) is used to measure the temperature field of the high-temperature flame accurately. The pulsed CRDS is used to measure the ring-down time by igniting the flame inthe optical cavity, which is affected by the additional loss. When the premixed CH4(1.1 L·min-1) and air (15 L·min-1) are burned on the flat flame burner, and the measured height from the burner surface is 6 mm, the measured accuracy of the effective absorption length by the PLIF technique is 7.1% higher than that by directly choosing the diameter of the burner surface as the absorption length, the measured accuracy of thetemperature by CARS technique is 45% higher than that measured by a thermocouple at room temperature, the measured precision of the cavity ring-down time is improved by 21.6% than that measured ring-down time of the empty cavity when the flame is ignited in the cavity, and non-OH absorption wavelength is selected. By using a combination of the above technologies to achieve precise measuring experiment parameters, we find that the number density of OH molecules (3.59×1013molecules·cm-3) can reach the maximum value when the height from the burner surface is 6 mm, and the precision of OH concentration is 35.6% higher than that of the unmodified OH concentration. Under the different equivalence ratios (Φ=0.7～1.1), with the increase of the heights from the burner surface, the number of OH particles gradually decreases,and the curve fitting shows that the OH concentration decreases in an e-exponential decay. Under the condition of oxygen-rich combustion, the concentration of OH at the same height increases with the increasing equivalent ratios. When the CH4flow rate remains constant, the OH concentration in the oxygen-rich combustion condition will be higher than in the low-oxygen combustion condition.The precise measurement of OH concentration is improved by combining the CRDS-CARS-PLIF multi-technique, which can provide strong data support for chemical combustion analysis.