Coherent anti-Stokes Raman Spectroscopy Thermometry Research Articles

A femtosecond time-resolved coherent anti-Stokes Raman spectroscopy thermometry for steady-state high-temperature flame

A simplified thermometry based on femtosecond coherent anti-Stokes Raman spectroscopy (fs-CARS) was proposed to measure the temperature of steady-state high-temperature flame. Nitrogen molecules were used as a temperature gage and the polarization beats appearing in the initial partial fs-CARS transients were used to access the temperature. As the contribution of all rotational levels to the CARS signal was summarized as the frequency-spread decay, the calculation complexity of extracting temperature from CARS transients can be significantly reduced with this method. In our experiments, the CARS transients of atmospheric N2 from room temperature to 1200 K were well fitted with this simplified principle, which proved the effectiveness of this method. The flame temperature of a butane Bunsen burner was practically measured using this fs-CARS thermometry, and the measured temperature was slightly higher than that given by a platinum-rhodium thermocouple. Since the intrusive thermocouple wire will influence the flame reacting flow, we suggest that the measurement result given by fs-CARS is more reliable. At present, this simplified thermometry is only suitable for the measurement of steady flow. Considering the practical diagnostic requirements of dynamic processes, the possibility of fast thermometry based on this method was also discussed.

Coherent Raman imaging thermometry with in-situ referencing of the impulsive excitation efficiency

Simultaneous detection of resonant and non-resonant femtosecond/picosecond coherent anti-Stokes Raman spectroscopy (CARS) signals has been developed as a viable technique to provide in-situ referencing of the impulsive excitation efficiency for temperature assessments in flames. In the framework of CARS thermometry, the occurrence of both a resonant and a non-resonant contribution to the third-order susceptibility is well known. While the resonant part conceives the useful spectral information for deriving temperature and species concentrations in the probed volume, the non-resonant part is often disregarded. It nonetheless serves the CARS technique as an essential reference to map the finite bandwidth of the laser excitation fields and the transmission characteristics of the signal along the detection path. Hence, the standard protocols for CARS flame measurements include the time-averaged recording of the non-resonant signal, to be performed sequentially to the experiment. In the present work we present the successful single-shot recordings of both the resonant and non-resonant CARS signals, split on the same detector frame, realizing the in-situ referencing of the impulsive excitation efficiency. We demonstrate the use of this technique on one-dimensional CARS imaging spectra, acquired across the flame front of a laminar premixed methane/air flame. The effect of pulse dispersion on the laser excitation fields, while propagating in the participating medium, is proved to result, if not accounted for, in an ∼1.3% systematic bias of the CARS-evaluated temperature in the oxidation region of the flame.

Three-beam rotational coherent anti-Stokes Raman spectroscopy thermometry in scattering environments.

Three-beam rotational coherent anti-Stokes Raman scattering (CARS) measurements performed in highly scattering environments are susceptible to contamination by two-beam CARS signals generated by the pump-probe and Stokes-probe interactions at the measurement volume. If this occurs, differences in the Raman excitation bandwidth between the two-beam and three-beam CARS signals can add significant errors to the spectral analysis. This interference to the best of our knowledge has not been acknowledged in previous three-beam rotational CARS experiments, but may introduce measurement errors up to 25% depending on the temperature, amount of scattering, and differences between the two-beam and three-beam Raman excitation bandwidths. In this work, the presence of two-beam CARS signal contamination was experimentally verified using a femtosecond-picosecond rotational CARS instrument in two scattering environments: (1) a fireball generated by a laboratory-scale explosion that contained particulate matter, metal fragments, and soot, and (2) a flow of air and small liquid droplets. A polarization scheme is presented to overcome this interference. By rotating the pump and Stokes polarizations +55∘ and -55∘ from the probe, respectively, the two-beam and three-beam CARS signals are orthogonally polarized and can be separated using a polarization analyzer. Using this polarization arrangement, the Raman-resonant three-beam CARS signal amplitude is reduced by a factor of 2.3 compared to the case where all polarizations are parallel. This method is successfully demonstrated in both scattering environments. A theoretical model is presented, and the temperature measurement error is studied for different experimental conditions. The criteria for when this interference may be present are discussed.

Pure-rotational H2 thermometry by ultrabroadband coherent anti-Stokes Raman spectroscopy.

Coherent anti-Stokes Raman spectroscopy (CARS) is a sensitive technique for probing highly luminous flames in combustion applications to determine temperatures and species concentrations. CARS thermometry has been demonstrated for the vibrational Q-branch and pure-rotational S-branch of several small molecules. Practical advantages of pure-rotational CARS, such as multi-species detection, reduction of coherent line mixing and collisional narrowing even at high pressures, and the potential for more precise thermometry, have motivated experimental and theoretical advances in S-branch CARS of nitrogen (N2), for example, which is a dominant species in air-fed combustion processes. Although hydrogen (H2) is of interest given its prevalence as a reactant and product in many gas-phase reactions, laser bandwidth limitations have precluded the extension of CARS thermometry to the H2 S-branch. We demonstrate H2 thermometry using hybrid femtosecond/picosecond pure-rotational CARS, in which a broadband pump/Stokes pulse enables simultaneous excitation of the set of H2 S-branch transitions populated at flame temperatures over the spectral region of 0-2200 cm-1. We present a pure-rotational H2 CARS spectral model for data fitting and compare extracted temperatures to those from simultaneously collected N2 spectra in two systems of study: a heated flow and a diffusion flame on a Wolfhard-Parker slot burner. From 300 to 650 K in the heated flow, the H2 and N2 CARS extracted temperatures are, on average, within 2% of the set temperature. For flame measurements, the fitted H2 and N2 temperatures are, on average, within 5% of each other from 300 to 1600 K. Our results confirm the viability of pure-rotational H2 CARS thermometry for probing combustion reactions.

Pure rotational Coherent anti‐Stokes Raman spectroscopy of water vapor and its relevance for combustion diagnostics

In this work, we report for the first time the rotational coherent anti‐Stokes Raman spectroscopy (CARS) spectrum of water vapor, recorded through experiments at 400 K and 670 K for Raman shifts up to 600 cm−1. Using the standard dual‐broadband pure rotational CARS approach, the resonant structure was overwhelmed by a strong non‐resonant background. By employing a polarization technique, this background could be suppressed and the resonant structure discerned. The spectral line structure matched spectra from spontaneous rotational Raman scattering found in the literature. The CARS signal intensity at the highest peak from water vapor at 400 K was more than five orders of magnitude weaker than for N2 because of a low Raman cross section and a larger number of transitions. Due to the weak resonant signal, water vapor is an unsuitable candidate for CARS thermometry and relative concentration measurements in combustion. Still, rotational CARS spectra from product gases in combustion will be affected by the presence of water, which is briefly discussed. Copyright © 2013 John Wiley & Sons, Ltd.

Rotational CARS N2 thermometry: validation experiments for the influence of nitrogen spectral line broadening by hydrogen

AbstractRotational coherent anti‐Stokes Raman spectroscopy (CARS) in fuel‐rich hydrocarbon flames, with a large content of hydrogen in the product gases (∼20%), has in previous work shown that evaluated temperatures are raised several tens of Kelvin by taking newly derived N2H2 Raman line widths into account. To validate these results, in this work calibrated temperature measurements at around 300, 500 and 700 K were performed in a cell with binary gas mixtures of nitrogen and hydrogen. The temperature evaluation was made with respect to Raman line widths either from self‐broadened nitrogen only, N2N2 [energy‐corrected‐sudden (ECS)], or by also taking nitrogen broadened by hydrogen, N2H2 [Robert–Bonamy (RB)], Raman line widths into account. With increased amount of hydrogen in the cell at constant temperature, the evaluated CARS temperatures were clearly lowered with the use of Raman line widths from self‐broadened nitrogen only, and the case with inclusion of N2H2 Raman line widths was more successful. The difference in evaluated temperatures between the two different sets increases approximately linearly, reaching 20 K (at T ∼ 300 K), 43 K (at T = 500 K) and 61 K (at T = 700 K) at the highest hydrogen concentration (90%). The results from this work further emphasize the importance of using adequate Raman line widths for accurate rotational CARS thermometry. Copyright © 2010 John Wiley & Sons, Ltd.

Improvement of rotational CARS thermometry in fuel‐rich hydrocarbon flames by inclusion of N2‐H2 Raman line widths

AbstractIn rotational coherent anti‐Stokes Raman spectroscopy (CARS) thermometry applied to air‐fed flames, the temperature sensitivity mainly depends on the intensity distribution of the nitrogen spectral lines. Temperatures are estimated by numerical fitting of theoretical spectra to experimental ones, and one uncertainty in the calculation of theoretical CARS spectra for specific flame conditions is the accuracy in utilized line‐broadening coefficients. In a previous article, self‐broadened N2‐N2 line widths were considered in the spectral calculations as well as those of N2‐CO, N2‐CO2, N2‐H2O, and N2‐O2. In the present article, we also include N2‐H2 line widths calculated from a newly developed model, and it is shown that the evaluated temperature from flame spectra increases with increasing mole fractions of hydrogen. For example, in a very rich flame at Φ = 2.5, the use of available line‐width data for all major species gives a temperature raise of 72 K at a temperature of ∼1700 K, in comparison with using self‐broadened N2‐N2 line widths only. Half of this temperature raise is related to the inclusion of N2‐H2 line widths. This article emphasizes the importance of using adequate line‐broadening models for rotational CARS thermometry in flames. Copyright © 2009 John Wiley & Sons, Ltd.

Simultaneous phosphor and CARS thermometry at the wall–gas interface within a combustor

Phosphor thermometry and vibrational coherent anti-stokes Raman spectroscopy (CARS) were applied simultaneously to examine gas–solid interfaces in a generic combustor. For this purpose, an internally air-cooled obstacle was installed within an optically accessible, pressurized combustion chamber. During the operation of a turbulent, swirled n-heptane flame, the obstacle’s surface temperature and the surface-normal gas temperature distribution were measured. The surface temperature was determined by Thermographic Phosphors, materials whose phosphorescence decay times depend on their temperature. Following a pulsed UV laser excitation (355nm), the 659nm emission band of Mg4FGeO6:Mn was monitored by a photomultiplier tube.Non-invasive temperature measurements in the flue gas region of the n-heptane spray flame near the surface were performed pointwise by vibrational CARS of diatomic nitrogen. Beams from a frequency doubled Nd:YAG laser (532nm) and a modeless broadband dye laser (607nm) were phase-matched within a surface-parallel, planar BOXCARS configuration. This allowed gas phase thermometry as close as 30μm to the surface.The thermal boundary layer and wall temperature measurements were consistent with each other. This demonstrates the potential of spectrocopic techniques to study gas–solid interfaces with high temporal and spatial resolution. Using the interior surface temperature within the cooling channel measured by a thermocouple, the heat flux through the wall and the local heat transfer coefficient at the front side of the obstacle were estimated.

Combined phosphor and CARS thermometry at the wall–gas interface of impinging flame and jet systems

For the determination of surface normal temperature gradients, a generic system was built up consisting of two opposed, vertical nozzles impinging onto a flat, horizontal copper plate. From below, the plate was heated by non-reacting, turbulent air jets (Re = 5,000) and by a laminar flame (? = 0.7, Re = 350), respectively. For well-defined boundary conditions, the plate was cooled by a turbulent cold jet from above in both cases. Wall temperature as well as gas temperature distributions within and outside of the thermal boundary layer of the hot side of the system were determined. The radial surface temperature profile of the plate was measured by coating it with thermographic phosphors (TP), materials whose phosphorescence decay time is dependent on their temperature. The TP was excited electronically by a frequency-tripled Nd:YAG laser (355 nm). The temporal decay of the phosphorescence intensity was measured pointwise by a photomultiplier tube. In this case, the 659 nm emission line of Mg4FGeO6:Mn was monitored. Non-intrusive point measurements of the gas temperature close to the surface were performed by rovibrational coherent anti-Stokes Raman spectroscopy (CARS) of diatomic nitrogen. Beams from a seeded, frequency-doubled Nd:YAG laser (532 nm) and from a modeless broadband dye laser (607 nm) were phase-matched into a surface-parallel, planar-boxcars configuration. The temperature data could be collected as close as 300 ?m to the surface. Thermographic phosphors as well as CARS proved to be consistent for wall temperature and boundary layer measurements in all test cases. The results and challenges of this approach are discussed.

Spectral synthesis of coherent anti‐Stokes Raman spectra with broadband Stokes lasers of arbitrary line shapes

AbstractAn accurate analysis of coherent anti‐Stokes Raman spectroscopy (CARS) relies on spectral convolutions over the relevant bandwidths. Although analytical solutions of CARS signals generated by pump and Stokes lasers with standard (i.e. Gaussian or Lorentzian) line shapes are well known, broadband Stokes lasers can deviate from the assumed profiles. To minimize the influence of such a deviation, it is customary to normalize the data to the previously measured background CARS spectrum reproducing the Stokes laser profile. However, no one has so far quantified the possible changes in the final outcome of a CARS experiment when the Stokes profile undergoes a distortion from standard line shapes. In an attempt to include this issue in the general treatment of CARS signals, the calculation of CARS spectra with arbitrary Stokes line shapes is described in this paper. To that end, the complexity of such problem is solved by linearizing the smooth profile of the broadband laser. Advantages of this new strategy are discussed in regard to an application to molecular hydrogen and signal corrections are found to be as high as 10% of the standard CARS intensities calculated for a Gaussian Stokes profile. For such signal variations, imprecision of standard CARS thermometry is expected to be about 3%. Comparison with background‐corrected standard intensities reveals, instead, an agreement on the spectral line strengths only; CARS line shapes and off‐resonance signal levels are in disagreement. Copyright © 2007 John Wiley & Sons, Ltd.

Dual-broadband rotational CARS thermometry in the product gas of hydrocarbon flames

Coherent anti-Stokes Raman spectroscopy (CARS) is an established technique for gas-phase thermometry in combustion. In this work, rotational CARS in the dual-broadband approach has been investigated for flame thermometry in the product gas of ethylene/air flames in a wide range of equivalence ratios (0.5 < ϕ < 2.5). The nitrogen lines dominated the rotational CARS spectra in the whole equivalence range, but for fuel-lean flames also oxygen lines gave significant spectral contributions. Therefore, the temperature evaluation was based on spectral fitting of both nitrogen and oxygen. Product gas species such as carbon dioxide, water, carbon monoxide, and hydrogen contribute to the non-resonant susceptibility of the gas and influence the nitrogen and oxygen linewidths through collisions. In this work, the main focus was on the influence of these collisions on the nitrogen and oxygen Raman linewidths and consequently the evaluated temperature in the product gas of flames. In the range of studied equivalence ratios with various amounts of different product gas species, the evaluated temperature was raised up to 45 K when including broadening from CO 2, H 2O, and CO on the measured nitrogen and oxygen lines. It was also concluded that H 2O was the main contributor in all non-sooting flames. The influence of the non-resonant susceptibility in the spectral evaluation is discussed, and its coupling to the inclusion of the line broadening from product gas species as well. It has thus been shown that line-broadening effects from product gas species are highly important for quantitative rotational CARS thermometry in flames.

Precision of single‐shot dual‐broadband rotational CARS thermometry with single‐mode and multi‐mode Nd:YAG lasers

AbstractThe precision of single‐shot coherent anti‐Stokes Raman spectroscopy (CARS) thermometry is an important characteristic for the application of CARS in various combustion devices such as internal combustion engines. It is generally assumed that the precision of CARS is directly related to the spectral noise of the interacting laser fields, and previous studies have presented a theoretical model describing the behaviour of the noise as a function of various experimental parameters. In this work, the aim was to test the results predicted by the previously developed model for dual‐broadband rotational CARS. We studied the precision of evaluated temperatures in pure nitrogen under different experimental conditions, namely the dependence of precision on the gas pressure and on the linewidth of the narrowband laser (the Nd:YAG laser). It was demonstrated that our experiments reproduce the main features of the model, and that a single‐mode Nd:YAG laser leads to better precision than a multi‐mode Nd:YAG laser for dual‐broadband rotational CARS thermometry. Copyright © 2003 John Wiley &amp; Sons, Ltd.

N 2 CARS thermometry and O 2 LIF concentration measurements in a flame under electrically induced microbuoyancy

Microgravity combustion is fundamentally characterized by the absence of buoyancy driven flows. To facilitate a large range of diagnostics on microgravity flames, it is useful to create an equivalent microbuoyant condition in an earth-based laboratory. This experiment simulates microbuoyancy using electric fields to balance local convection in a region of the flame. Previous studies used N 2 coherent anti-Stokes Raman spectroscopy (CARS) temperature measurements to show that a region exists in which the temperature profile corresponds to that of a spherically symmetric diffusion profile, as would be expected in a true microgravity environment. The current study utilizes CARS thermometry and laser induced fluorescence (LIF) to examine the temperature and O 2 concentration profiles in a region below the flame. The results show that the electrically balanced flame apparatus produces a spherically symmetric wedge of microbuoyant flame extending 10° from the vertical axis of the capillary flame arrangement; thereby quantifying the extent of the microbuoyant region.

CARS thermometry in high pressure rocket combustors

Coherent anti-Stokes Raman spectroscopy (CARS) has been used to investigate high pressure cryogenic liquid oxygen/gaseous hydrogen (LOX/GH 2) flames at pressures up to 6.5 MPa. The method employs two synchronized broadband CARS systems to probe simultaneously H 2 and H 2O. Temperature is deduced from the H 2 and H 2O CARS profiles while the H 2O concentration is deduced from the modulation from the resonant the H 2O resonant signal superimposed on the nonresonant background signal. Details of the modeling of the CARS spectra at high pressure and the data reduction are discussed. Problems issued from the presence of liquid phase are also investigated. The results demonstrate the applicability and usefulness of CARS for investigating cryogenic combustion under representative rocket model combustor regimes where the pressure range is above the supercritical condition ( P>5.04 MPa).

Temperature field measurements of small, nonpremixed flames with use of an Abel inversion of holographic interferograms.

Interferometry has been used for many years as a semi-quantitative image-based diagnostic for combustion research. In this paper, we use image-plane, double-pulse holographic interferograms of axisymmetric flames to infer their radial temperature distribution. An Abel inversion is performed on the fringe data to account for line-of-sight integration through the flame. The sensitivity of nonresonant refractive diagnostics decreases inversely with temperature, and the accuracy of the technique is discussed in this context. A small, nonpremixed capillary flame is investigated, and the temperatures inferred from interferometry are compared with those obtained with N2 coherent anti-Stokes Raman spectroscopy thermometry. Additionally, the thermal field of a burning monodisperse methanol droplet stream is investigated interferometrically. Because of their small size, both of these flames challenge the performance limit of temperature interferometery.

Inhomogeneous speed effects on H2 vibrational line profiles in ternary mixtures

A study of speed inhomogeneous broadening of the hydrogen vibrational line profiles in the collisional regime for ternary mixtures is reported. The Q(1) line of H2 in H2–Ar–N2, H2–He–Ar, and H2–He–N2 mixtures is investigated by high resolution stimulated Raman spectroscopy for various concentrations and temperatures. A model, successfully used for binary mixtures, is extended to ternary mixtures. An excellent agreement is obtained between theory and experiment for H2–Ar–N2, by using the collisional parameters previously obtained from binary mixtures study. For H2–He–Ar and H2–He–N2, H2–He collisions play a “hardening” effect in the H2 soft speed memory mechanism for H2–Ar or H2–N2. The present experimental results allow us, via the frame of our model, to determine the “hardness” parameter values for H2–He (inaccessible from binary mixtures data) and to get an accurate description of the spectral line shape. This study should be useful for hydrogen coherent anti-Stokes Raman spectroscopy thermometry in H2/air flames at high pressure through its possible extension to the H2–N2–H2O system.

Collisional Raman linewidths of nitrogen at high temperature (1700–2400 K)

New high-temperature measurements of collisional linewidths of the Raman Q branch of nitrogen have been performed at 1700-2400 K with stimulated Raman spectroscopy in a tungsten filament. We fitted these data together with previous data obtained in the 295-1500-K range to redetermine the parameters of the relaxation models used in coherent anti-Stokes Raman spectroscopy thermometry. The improvement in the accuracy of the temperature measurement has been checked. Semiclassical calculations of linewidths in the 1700-2400-K range agree with the experimental data and have been extended to even higher temperatures.

Effect of the Slit Function of the Detection System and a Fast-Fitting Algorithm on Accuracy of CARS Temperature

The effect of the erroneous functional forms of the slit functions on the best-fit temperature in CARS (coherent anti-Stokes Raman spectroscopy) thermometry has been investigated. When the shape of the slit function is described as a Lorentzian or a Gaussian function and its width adjusted with the room-temperature CARS spectrum, the maximum systematic errors due to the slit functions reach 1.5–2.5% in the temperature range from 300 to 2100 K. A fast-fitting algorithm exponentially interpolating the CARS spectrum at an intermediate temperature with two precalculated spectra has been proposed, and its validity has been assessed by calculating the error due to the fitting algorithm.

Improvement in Temperature Measurement Accuracy of Q-Branch CARS Thermometry. Effects of Spectral Resolution of Detection System.

The temperature measurement accuracy of N2 Q-branch CARS (Coherent Anti-Stokes Raman Spectroscopy) below 1000K has been improved by improving the spectral resolution of the detection system. The total spectral resolution was improved from 1. 3cm-1 to 0. 6cm-1 by dispersion magnification (×3. 6), which was effected by adding a magnifier between a monochromator and an intensified photodiode array detector. The accuracy of temperature measurement in the temperature range of 300-800K for pressures of 0. 1-3. 0MPa has been greatly improved from ±100K, in the case without the magnifier, to ±40K. The method reported in this paper has a potential advantage for practical use because of its convenience.

Temperature characteristics of a combusting droplet stream

This study presents coherent-anti Stokes Raman spectroscopy (CARS) temperature measurements in a combusting droplet stream. The experiment uses a 50 μm diameter nozzle vibrated by a piezoelectric crystal to generate a monodisperse droplet stream with a droplet diameter of about 150 μm and droplet-to-droplet spacing of 10 droplet diameters. The objectives of this study are: (1) to demonstrate CARS thermometry for droplet combustion applications, (2) to describe the flame structure around interacting droplets using the CARS measured temperature field, and (3) to provide experimental data for numerical model comparison. The results show that there is a small thermal wake behind each droplet in the stream. The temperature profile measured radially outward from a droplet has a local minimum near the droplet surface, rises to a maximum about 6.5 droplet diameters away, and then falls to room temperature at a radial distance of 15 mm (100 droplet diameters). The temperature profile measured between two adjacent droplets on the stream axis reveals a thermal boundary layer of 1.5 droplet diameters near the droplet surface. The profile is nearly flat, suggesting that individual flames do not surround each droplet. The local effects due to the presence of individual droplets completely disappear about 15 droplet diameters away from the droplet stream centerline. Temperature contours constructed from the measurements of several radial profiles around two adjacent droplets show a cylindrical flame structure. The local heat transfer Nusselt numbers calculated from CARS measured temperatures agree qualitatively with numerical model predictions.