We report the direct measurements of methyl radicals (CH3) in methane/air flames at atmospheric pressure by using coherent microwave Rayleigh scattering (Radar) from Resonance Enhanced Multi-Photon Ionization (REMPI), also known as the Radar REMPI technique. A tunable dye laser was used to selectively induce the (2 + 1) REMPI ionization of methyl radicals (CH3, band) in a near adiabatic and premixed laminar methane/air flame, generated by a Hencken burner. In situ measurements of the REMPI electrons were made by non-intrusively using a microwave homodyne transceiver detection system. The REMPI spectrum of the CH3 radical was obtained and a spatial distribution of the radicals limited by focused laser beam geometry, approximately 20 µm normal to the flame front and 2.4 mm parallel to the flame, was determined. The measured CH3 was in good agreement with numerical simulations performed using the detailed kinetic mechanism of GRI-3.0. To the authors’ knowledge, these experiments represent the first directly-measured spatially-resolved CH3 in a flame at atmospheric pressure.
© 2011 OSA
Radical species are key reaction components in combustion, but the spatially-resolved quantitative measurements of many radicals are still unsatisfactory, especially at elevated pressures. One of the most prevailing radicals and chain carriers in hydrocarbon flames, the methyl radical (CH3) is involved in ignition and flame propagation reactions (via hydrogen abstraction by H atoms and hydroxyl radicals (OH)) . However, detection of CH3 is a significant challenge. The methyl radical is characterized by a strong predissociation of its electronically excited states, preventing fluorescence detection. For this reason, direct measurements of CH3 have mostly been conducted using absorption-based methods [2–4], conventional REMPI [5, 6] and degenerate four wave mixing (DFWM) [7, 8]. Absorption-based methods, either direct absorption of CH3 or cavity ring-down spectroscopy, have the limitation of path integration, leading to limited spatial resolution. Conventional REMPI has a low signal-to-noise ratio for measurement of the CH3 distribution in flames at atmospheric pressure  but a physical probe or electrodes required for the collection of electrons or ions may introduce irrelevant disturbances in the reacting flows. DFWM utilizes third order optics effects, which may have a low signal to noise ratio for applications at low pressures. Indirect measurements have been conducted by photolysis of CH3 into the methylidyne radical (CH) and its subsequent laser induced fluorescence [10–12], which inherently complicates the detection scheme and requires care for the interpretation of the experimental data.
Coherent microwave Rayleigh scattering (Radar) from Resonance Enhanced Multi-Photon Ionization (REMPI) has been demonstrated recently to have the capability to achieve high spatial and temporal resolution measurements, which allow for sensitive nonintrusive diagnostics and accurate determinations of concentration profiles without the use of physical probes or electrodes. It has been applied for the optical detection of species such as argon , xenon, nitric oxide , carbon monoxide  and atomic oxygen  both within enclosed cells and open air. For the first time, we have demonstrated Radar REMPI to in situ, non-intrusively and directly measure the methyl radical in methane/air flames at atmospheric pressure. To the authors’ knowledge, this represents the first directly-measured spatially-resolved CH3 measurement in flames at atmospheric pressure.
Compared to other techniques, the Radar REMPI technique is easy to set-up, robust to perturbations, and possible for standoff detection. It uses only one tunable laser, such as a commercial dye laser or OPO laser, as the excitation source. It requires a simple lens to focus the beam and can be spatially scanned easily. Minimal optical alignment is required for the measurement and necessitates only one optical access port. It has a higher spatial resolution compared to line integrated absorption measurements. In contrast to conventional CARS or DFWM, the signal to noise ratio may even be better at lower pressure due to the slow recombination and/or attachment processes in the REMPI plasma. Furthermore, it does not require a dark environment to perform the measurements. It may also be used as a standoff approach, due to the limited radiation background in the microwave range .
In this paper, the REMPI spectrum of the methyl radical ( band) was obtained in flames at atmospheric pressure. The spatial distribution of the radical with a resolution of approximately 20 µm was measured through the flame. Good agreement has been achieved between the experimental measurements and numerical simulations using detailed combustion kinetics. The method has demonstrated excellent spatial resolution for measurements in flames at atmospheric pressure. It has also shown great promise for the key radical measurements at lower or higher pressures, which may significantly improve our understanding of combustion processes at practical engine or combustor conditions.
2. Experimental set-up
In order to provide a platform for the detection of CH3 using the Radar REMPI technique, a near adiabatic flat flame at atmospheric pressure with optical access of the detailed structure of the flame would be ideal. An example of such a type of flame is a freely-propagating premixed flame. The structure of a freely-propagating rich methane/air flame at atmospheric pressure is shown in Fig. 1 . The temperature and CH3 number density across the flame front was computed using the PREMIX code from the CHEMKIN package  and the chemical kinetic mechanism of GRI-3.0 . While significant number densities (on the order of 1016 molecules/cm3) of CH3 can be produced, it is present across a thickness of only 0.5 mm in the flame. Furthermore, in experiments, this type of flame is typically close to the burner surface with significant heat loss, such as with a porous plug flat flame burner , complicating the use of detailed optical measurements. Alternatively, a Hencken burner with a 1 inch by 1 inch square exit geometry (Technologies for Research, Model: RT1x1) was used to generate a laminar and near adiabatic methane/air flame with sufficient optical access. This burner has been widely used as a test bed for laser diagnostic techniques [20–26]. The Hencken burner produces a unique, nearly adiabatic flame that is lifted from the burner surface since the fuel and oxidizer streams are separated until the exit of the burner. The fuel tubes have an inner diameter (ID) of approximately 0.54 mm and an outer diameter (OD) of approximately 0.83 mm. The oxidizer honeycomb is a hexagonal pattern with cell size of approximately 0.90 mm. For the current experiments, the air flow rate was fixed at 11 slpm (Standard Liters Per Minute) and the methane flow rate was fixed at 1.35 slpm, which produced a flame with an equivalence ratio of 1.22.
The experimental setup of the Radar REMPI measurements of CH3 is shown in Fig. 2 . A frequency doubled Nd: YAG laser (Continuum Surelite SI-10) was used to pump a tunable dye laser (Continuum ND6000, DCM, [2-[2-[4-(dimethylamino)phenyl]ethenyl]-6-methyl-4H- pyran-4-ylidene]-propanedinitrile, C19H17N3O as the dye). The output of the dye laser was frequency doubled by an automatic frequency doubling and tracking system (Continuum UVT). The output wavelengths were tunable from 332 to 335 nm, with output power ranging from 6 to 8 mJ/pulse. The laser output was then focused by a lens with a focal length of 50 cm to generate the REMPI plasma in the methane/air flame of a Hencken burner in open air. A lens with a long focal length (50 cm) was used to avoid gas breakdown or avalanche ionization in the flame. From Gaussian beam geometry, the focal region was about 20 µm in diameter and 2.4 ± 0.4 mm in length. The length of the interrogation region arose from the microwave signal scattering along the laser beam upstream and downstream of the focus. The length and uncertainty was related to the laser intensity and corresponding microwave scattering signal. At 2.8 mm, the laser intensity falls to about 33% and the REMPI efficiency and microwave scattering drop to about 11%, compared to the center of the laser focus. The beam steering mirrors and focusing lens were placed on a translational stage to move the laser focus through the flame front.
A microwave homodyne transceiver detection system was used to detect the REMPI plasma. A 10 dBm tunable microwave source (HP 8350B sweep oscillator, set at ~10 GHz) was first split into two channels. One channel was used to illuminate the ionization point through a microwave horn (WR75, 15dB gain). Microwave scattering from the plasma was collected by the same microwave horn. The received microwave passed through a microwave circulator and was amplified 30 dB by one preamplifier at approximately 10 GHz. After the frequency was down converted in the mixer, two other amplifiers with bandwidth of 2.5 kHz to 1.0 GHz amplified the signal by another factor of 60 dB. It should be noted that the filter after the mixer can effectively block the scattering background from the environment. Therefore Radar REMPI measurements inside an enclosure will not suffer from surface scattering interference. From the geometry of dipole radiation, the polarization of the microwave was chosen to be along the propagation direction of the laser to maximize the scattering signal.
3. Results and discussion
The time-accurate microwave scattering signal was monitored by an oscilloscope. Shown in Fig. 3 , the lifetime (at 1/e of the peak) of the REMPI electrons in the flame was about 84 ns, which shows promise for the radical measurements at lower and higher pressures. Since the lifetime of REMPI electrons becomes longer at lower pressures, the signal to noise ratio will be significantly improved. The microwave signal was also input to an automatic gated integrator data acquisition system (gate width = 10 ns), which recorded the REMPI spectrum of the methyl radicals as the laser was tuned. In order to eliminate the perturbations caused by microwave interferences or laser pulse-to-pulse fluctuations, the microwave background and laser pulses were sampled and averaged over 20 times. Subtraction of the microwave background and scaling of the laser pulses were undertaken for the final spectrum.
Fig. 4 shows a typical REMPI spectrum of CH3 in a methane/air flame at atmospheric pressure. The spectrum obtained in the flame was in agreement with previously published spectra [27–29]. The measurement was conducted at 1.45 mm above the burner surface, which corresponded to about the maximum concentration of CH3 in the flame. Only transition from the ground state of CH3 to the state of was obtained in the flame by Radar REMPI. No clear vibrational structure of the transitions was observed. The wings off the peak at 333.6 nm have different values for the measurement conducted at different locations of the flame, which indicated different rotational temperatures of the CH3 radicals in the flame.
The one-dimensional reaction zone structure in Fig. 1 is not applicable to the flames produced by the Hencken burner because the separated fuel and air streams tend to establish small diffusion flamelets. Therefore, two-dimensional simulations were performed using UNICORN (UNsteady Ignition and COmbustion with ReactioNs)  with the detailed chemical kinetics of GRI-3.0 for a single flamelet at atmospheric pressure. Fig. 5 shows the calculated two-dimensional spatial distribution of CH3 across two flamelets. UNICORN is a time-dependent, axisymmetric mathematical model that is used to investigate two-dimensional steady and unsteady reacting flows. It has been tested with experiments designed to predict ignition, extinction, stability limits, and the dynamic and steady-state characteristics of diffusion and premixed flames burning various fuels. Here UNICORN used a detailed chemical kinetics model, GRI-3.0, for methane/air flames at atmospheric pressure. The Hencken burner using methane/air at an equivalence ratio of 1.22 at atmospheric pressure produces a flame front with many flamelets (one flamelet for each fuel tube). This is clearly evident in Fig. 5 with two flamelets shown. The Radar REMPI measurement was 20 μm normal to the flame front and 2.4 mm ± 0.4 mm in length parallel to the top of the burner. Therefore the Radar REMPI measurement crossed through two flamelets (depicted by the black box in Fig.5). Depending upon where the 2.4 mm interrogation area was located across the flamelets, there could be different shaped profiles. Figure 6(a) shows the variation when moving the interrogation area across the two flamelets with each simulation number accounting for a successive translation of 0.25 mm. Simulations 2 – 5 were translated 0.25, 0.5, 0.75 and 1.0 mm from the case of the 2.4 mm length centered over the edge of the flamelets, respectively. The translation produced a significant difference on the upstream side of the CH3 profile (closer to the burner surface) because of the bell shaped CH3 distribution. Furthermore, all of the profiles from the simulations were moved in the streamwise direction (closer to the burner) to match the experimental profile. This movement was not unreasonable since past simulations have also predicted higher flame liftoff heights than experiments . Nevertheless, the flame shape and therefore CH3 profile should remain predominately independent of the changes in flame liftoff height in the current experiment and provide a good means of comparison. Since the REMPI interrogation area covered more than one flamelet, an average of the CH3 concentration with 2.4 ± 0.4 mm along the flame front was sampled from the calculation results and is shown in Fig. 6(b). There is good agreement between the measured and calculated normalized CH3 profiles, including the total width and asymmetric structure of the profile. The relatively low signal to noise ratio from the measurements is most likely due to short time gate, laser fluctuations, and flame instability.
In summary, for the first time, Radar REMPI was demonstrated to in situ, non-intrusively and directly measure the methyl radicals in methane/air flames at atmospheric pressure. The REMPI spectrum of the methyl radical ( band) was obtained in the flame. Good agreement has been achieved between the experimental measurements and calculations for the spatial distribution of methyl radicals in methane/air flames.
Funding for this research at the University of Tennessee Knoxville was provided by NSF CBET-1032523.
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