Dual-pump Coherent anti-Stokes Raman Scattering Research Articles

CARS thermometry in laminar sooting ethylene-air co-flow diffusion flames with nitrogen dilution

Temperature measurements were performed in nonpremixed, sooting jet flames using coherent anti-Stokes Raman scattering (CARS). The jet fluid was ethylene diluted with different concentrations of nitrogen, and the co-flow fluid was dry air. The flames were stabilized on a Yale burner, also known as Smooke/Long burner, and the operating conditions were chosen to match previous experiments on the burner by the Yale group and other research groups. The selected flames have also been identified as target flames by the International Sooting Flame Workshop. CARS measurements of temperature provide insight into the structure of these sooting flames and serve as benchmark data for comparison with computations. A dual-pump vibrational CARS system was used here in order to avoid soot interferences. We were able to obtain high quality CARS spectra in all regions of the flame including the highly sooting regions and near the exit of the fuel nozzle, where the nonresonant modulation of the nitrogen CARS spectrum is strong due to the high ethylene concentrations. The measurements are compared with CFD results in literature and discussed in context of measured soot volume fraction, also, from literature.

Dual-pump CARS measurements in a hydrogen diffusion flame in cross-flow with AC dielectric barrier discharge

This paper presents dual-pump coherent anti-Stokes Raman scattering (CARS) measurements for simultaneous detection of flow temperature and relative concentration, applied to the characterization of a discharge-coupled reacting jet in a cross flow. The diagnostic is hydrogen Q-branch based, providing a much wider dynamic range compared to detection in the S-branch. For a previously developed dielectric barrier discharge, aligned co-axially with the fuel jet, OH planar laser induced fluorescence measurements show that the disturbance in the flame boundary leads to mixing enhancement. The H2-N2 dual-pump CARS measurement was used to map two-dimensional temperature distributions. The increase of the maximum temperature was up to 300 K, with 50% more H2 consumption, providing the reason for the decrease in the flame length by 25%. The increase of the relative H2O-H2 fraction was accompanied with a temperature increase, which indicates local equivalence ratios of below 1. The H2-O2 dual-pump measurements confirmed that the fuel-oxidizer ratios remain in the fuel-lean side at most of the probed locations.

Dual-pump coherent anti-Stokes Raman scattering system for temperature and species measurements in an optically accessible high-pressure gas turbine combustor facility

The development and implementation of a dual-pump coherent anti-Stokes Raman scattering (DP-CARS) system employing two optical sub-systems to measure temperature and major species concentrations at multiple locations in the flame zone of a high-pressure, liquid-fueled gas turbine combustor are discussed. An optically accessible gas turbine combustor facility (GTCF) was utilized to perform these experiments. A window assembly has been designed, fabricated, and assembled in the GTCF to allow optical access from three directions using a pair of thin and thick fused silica windows on each side. A lean direct injection (LDI) device consisting of an array of nine integrated air swirlers and fuel injectors was operated using Jet-A fuel at inlet air temperatures up to 725 K and combustor pressures up to 1.03 MPa. The DP-CARS system was used to measure temperature and CO2/N2 concentration ratio on single laser shots. An injection-seeded optical parametric oscillator (OPO) was used as a narrowband pump laser source in order to potentially reduce shot-to-shot fluctuations in the CARS data. Large prisms mounted on computer-controlled translation stages were used to direct the CARS beams either into the main leg optical system for measurements in the GTCF or to a reference leg optical system for measurements of the non-resonant spectrum and for alignment of the CARS system. The spatial maps of temperature and major species concentrations were obtained in high-pressure LDI flames by translating the CARS probe volume in the axial and vertical directions inside the combustor rig without loss of optical alignment.

Dual-Pump CARS Measurements of Temperature and Oxygen in a Turbulent Methanol-Fueled Pool Fire

We present broadband, dual-pump coherent anti-Stokes Raman scattering (CARS) measurements of N2 and O2 in 2 m diameter methanol pool fires. The design of the fiber-optically coupled CARS instrument for fire measurements is described. Single-shot temperatures and O2/N2 ratios were obtained simultaneously from a single measurement point at the center of the fire plume, and the measured temperature–oxygen statistics are compared to results of a time-domain-filtered Reynolds-averaged Navier-Stokes simulation. The measured and simulated mean fire temperatures agree to within 2–4%, with larger turbulent fluctuations observed in the measured temperatures. The behavior of the mean temperature conditioned on the O2/N2 ratio is similar for both simulation and experiment, but with simulated temperatures that are up to 10% lower than measured values for O2/N2 below 0.18. The uncertainty in the CARS measurements is described. A single-shot detection limit of O2/N2 = 0.06 was determined from the observed signal to noise ratio in the measured O2 Q-branch spectra. Comparison of the CARS and thermocouple-measured temperatures in tube-furnace-heated air allowed characterization of the CARS instrument accuracy and precision and it was determined that between 500–1400 K the instrument accuracy is ∼1% and the measurements are reproducible to within 5–7%.

Dual-pump CARS temperature and major species concentration measurements in counter-flow methane flames using narrowband pump and broadband Stokes lasers

Dual-pump coherent anti-Stokes Raman scattering (CARS) is used to measure temperature and species profiles in representative non-premixed and partially-premixed CH 4/O 2/N 2 flames. A new laser system has been developed to generate a tunable single-frequency beam for the second pump beam in the dual-pump N 2–CO 2 CARS process. The second harmonic output (∼532 nm) from an injection-seeded Nd:YAG laser is used as one of the narrowband pump beams. The second single-longitudinal-mode pump beam centered near 561 nm is generated using an injection-seeded optical parametric oscillator, consisting of two non-linear β-BBO crystals, pumped using the third harmonic output (∼355 nm) of the same Nd:YAG laser. A broadband dye laser (BBDL), pumped using the second harmonic output of an unseeded Nd:YAG laser, is employed to produce the Stokes beam centered near 607 nm with full-width-at-half-maximum of ∼250 cm −1. The three beams are focused between two opposing nozzles of a counter-flow burner facility to measure temperature and major species concentrations in a variety of CH 4/O 2/N 2 non-premixed and partially-premixed flames stabilized at a global strain rate of 20 s −1 at atmospheric-pressure. For the non-premixed flames, excellent agreement is observed between the measured profiles of temperature and CO 2/N 2 concentration ratios with those calculated using an opposed-flow flame code with detailed chemistry and molecular transport submodels. For partially-premixed flames, with the rich side premixing level beyond the stable premixed flame limit, the calculations overestimate the distance between the premixed and the non-premixed flamefronts. Consequently, the calculated temperatures near the rich, premixed flame are higher than those measured. Accurate prediction of the distance between the premixed and the non-premixed flames provides an interesting challenge for future computations.

Evaluation of temperature and concentration in H2N2 dual‐pump CARS spectra using the Keilson and Storer three‐dimensional model for H2 Q‐branch

AbstractCoherent anti‐Stokes Raman scattering (CARS) is a widely accepted standard for thermometry in combustion environments. Accurate lineshape models are of crucial importance for the computation of CARS spectra used for temperature and concentration evaluations. Therefore, a new lineshape model for the H2 molecule is developed and tested for ambient pressure applications. Measurements were taken in a heated cell and in a partially premixed flame with dual‐pump CARS techniques, respectively. Line broadening coefficients for H2N2 mixture are calculated using the new speed memory effects model. The experimental vibrational‐CARS (VCARS) spectra were evaluated with this new lineshape model and the temperature and relative H2 concentration results were compared with an already existing model. Copyright © 2009 John Wiley & Sons, Ltd.

Development of a simplified dual-pump dual-broadband coherent anti-Stokes Raman scattering system

A dual-pump dual-broadband coherent anti-Stokes Raman scattering (CARS) setup for simultaneous pure rotational and vibrational CARS is further developed by reducing the experimental efforts due to the use of only one detection system. With this system dual-pump CARS for probing ro-vibrational transitions of N(2)/CO/H(2) and dual-broadband CARS for pure rotational transitions of N(2)/O(2)/CO/CO(2) are applicable simultaneously. In this work the improvements to the setup and the data evaluation are described. First simultaneous temperature and concentration results from cell measurements and from a partially premixed propane flame are presented. Interference effects due to smeared vibrational CARS are investigated. It is shown that with similar experimental effort to a conventional dual-pump CARS setup considerably more information can be gained.

Dual-pump coherent anti-Stokes Raman scattering thermometry in a sooting turbulent pool fire

We present a dual-pump coherent anti-Stokes Raman scattering (CARS) instrument, which has been constructed for the probing of temperature fluctuations in turbulent pool fires of meter-scale. The measurements were performed at the Fire Laboratory for Accreditation of Models and Experiments (FLAME) facility at Sandia National Laboratories, which provides a canonical fire plume in quiescent wind conditions, with well-characterized boundary conditions and access for modern laser-diagnostic probes. The details of the dual-pump CARS experimental facility for the fire-science application are presented, and single-laser-shot CARS spectra containing information from in-fire N2, O2, H2, and CO2 are provided. Single-shot temperatures are obtained from spectral fitting of the Raman Q-branch signature of N2, from which histograms that estimate the pdf of the enthalpy-averaged temperature fluctuations at the center of the fire plume are presented. Results from two different sooting fire experiments reveal excellent test-to-test repeatability of the fire plume provided by FLAME, as well as the CARS-measured temperatures. The accuracy and precision of the CARS temperatures is assessed from measurements in furnace-heated air, where the temperature can be accurately determined by a thermocouple. At temperatures in excess of 500K, the furnace results show that the CARS measurements are accurate to within 2–3% and precise to within ±3–5% of the measured absolute temperature.

Non-intrusive gas-phase temperature measurements inside a porous burner using dual-pump CARS

The porous burner technology is a modern way of energy conversion with a strong potential to achieve high efficiency, marginal pollutant emissions and low fuel consumption. For a better understanding of the combustion process inside this complex system, accurate measurements of different key parameters are required. In this paper we present to the best of our knowledge the first non-intrusive spatially resolved gas-phase temperature and H 2 concentration measurements taken inside such a porous burner. This is achieved by means of dual-pump vibrational coherent anti-Stokes Raman scattering (CARS) at operating conditions of ϕ = 1 and 1 kW thermal power. Furthermore the combustion of a premixed methane/air mixture (equivalence ratio ϕ = 0.9) has been studied experimentally at thermal powers of 1, 1.5 and 2.5 kW. Due to the strong heat transfer from the gas-phase to the porous solid structure, the maximum gas temperatures are significantly below calculated adiabatic flame temperatures. The temperature gradient in the medium was found to be small compared to conventional premixed flames.

Dual-Pump Coherent Anti-Stokes Raman Scattering Measurements in a Supersonic Combustor

The dual-pump coherent anti-Stokes Raman scattering (CARS) method was used to measure temperature and the mole fractions of N 2 and O 2 in a supersonic combustor. Experiments were conducted in NASA Langley Research Center's Direct-Connect Supersonic Combustion Test Facility. In this facility, H 2 - and oxygen-enriched air burn to increase the enthalpy of the simulated air test gas. This gas is expanded through a Mach 2 nozzle and into a combustor model consisting of a short constant-area section followed by a small rearward-facing step and another constant-area section. At the end of this straight section, H 2 fuel is injected at Mach 2 and at a 30-deg angle with respect to the freestream. One wall of the duct then expands at a 3-deg angle for over 1 m. The ensuing combustion is probed optically through ports in the side of the combustor. Dual-pump CARS measurements were performed at the facility nozzle exit and at four planes downstream of fuel injection. Maps are presented of the mean temperature, as well as N 2 and O 2 mean mole-fraction fields. Correlations between fluctuations of the different measured parameters are also presented.

Dual-pump dual-broadband CARS for exhaust-gas temperature and CO 2–O 2–N 2 mole-fraction measurements in model gas-turbine combustors

Application of dual-pump, dual-broadband (DPDB) coherent anti-Stokes Raman scattering (CARS) for the measurement of temperature and multiple-species mole fractions is presented for the first time in a liquid-fueled combustor of practical interest. In this system pure rotational transitions of O 2–N 2 and the ro-vibrational transitions of CO 2–N 2 are probed using two narrowband pump beams, a broadband pump beam, and a broadband Stokes beam. This technique permits highly accurate temperature measurements at both low and high temperatures as well as mole-fraction measurements of two molecules with respect to N 2 from each laser shot. Single-shot measurements of temperature and mole-fraction ratios of CO 2/N 2 and O 2/N 2 in the exhaust stream of a swirl-stabilized, JP-8-fueled, model gas-turbine combustor are presented for equivalence ratios ranging from 0.45 to 1.0. Agreement between mean rotational and ro-vibrational temperatures is within ∼3%, and mean measurements of CO 2/N 2 and O 2/N 2 mole-fraction ratios are within ∼15% of equilibrium theory. To illustrate the ability of the current measurement system to track multiple scalar statistics in turbulent reacting flows, histograms and scatter plots of temperature and species mole fractions are presented within the potential-core and turbulent-shear-layer regions of the exhaust stream.

Temperature and CO 2 concentration measurements in the exhaust stream of a liquid-fueled combustor using dual-pump coherent anti-Stokes Raman scattering (CARS) spectroscopy

Single-shot, dual-pump coherent anti-Stokes Raman scattering (CARS) measurements of N 2 and CO 2 were performed in the exhaust stream of a swirl-stabilized JP-8-fueled combustor under sooting conditions. The combustor is designed to study particulate formation and particle-size distributions for different flame conditions and therefore is operated at near-stoichiometric overall fuel–air ratios. Various jet fuels and additive concentrations were studied. These conditions pose a significant challenge for temperature measurements using standard N 2 CARS due to strong flame emission and absorption of the CARS signal by the C 2 Swan band. With the dual-pump CARS technique employed in this study, the N 2 CARS signal is generated at a wavelength (496 nm) that is not absorbed by C 2, and concentration measurements of CO 2 can be performed. The standard deviations of the single-shot temperature measurements were approximately 3–4% of the mean values for equivalence ratios ranging from 0.4 to 1.1, whereas those of the single-shot CO 2 concentration measurements were between 9 and 20% of the mean values. Previous single-shot temperature and CO 2 concentration measurements using dual-pump CARS in this liquid-fueled combustor were limited to an equivalence ratio of 0.45, with standard deviations in temperature of about 5–6% of the mean value of 1143 K (Lucht et al., AIAA J. 41 (4) (2003) 679–686). The current study demonstrates a significant improvement in the applicability of single-shot CARS temperature and CO 2 concentration measurements to practical, swirl-stabilized combustors under sooting conditions.

Dual-Pump Coherent Anti-Stokes Raman Scattering Temperature and CO Concentration Measurements

Measurements of temperature and CO2 concentration using dual-pump coherent anti-Stokes Raman scattering, (CARS) are described. The measurements were performed in laboratory flames,in a room-temperature gas cell, and on an engine test stand at the U.S. Air Force Research Laboratory, Wright-Patterson Air Force Base. A modeless dye laser, a single-mode Nd:YAG laser, and an unintensified back-illuminated charge-coupled device digital camera were used for these measurements. The CARS measurements were performed on a single-laser-shot basis. The standard deviations of the temperatures and CO2 mole fractions determined from single-shot dual-pump CARS spectra in steady laminar propane/air flames were approximately 2 and 10% of the mean values of approximately 2000 K and 0.10, respectively. The precision and accuracy of single-shot temperature measurements obtained from the nitrogen part of the dual-pump CARS system were investigated in detail in near-adiabatic hydrogen/air/CO2 flames. The precision of the CARS temperature measurements was found to be comparable to the best results reported in the literature for conventional two-laser, single-pump CARS. The application of dual-pump CARS for single-shot measurements in a swirl-stabilized combustor fueled with JP-8 was also demonstrated.

Dual-Pump Coherent Anti-Stokes Raman Scattering for Simultaneous Pressure/Temperature Measurement

A new dual-pump coherent anti-Stokes Raman scattering (CARS) technique for the simultaneous measurement of pressure and temperature is discussed. For these measurements the wavelengths of the two pump lasers and the Stokes laser are selected so that the vibrational and pure rotational spectra of nitrogen are simultaneously acquired. Because collisional narrowing is an important effect for the vibrational spectrum but not for the isolated pure rotational lines, the relative intensities of the vibrational and pure rotational spectra change markedly as the pressure changes. Consequently, pressure as well as temperature can be determined from the spectral shape of the dual-pump CARS spectrum. The pressure measurement capability of the technique was demonstrated in a roomtemperature gas cell at pressures ranging from 0.1 to 20 atm. The pressures determined from averaged dual-pump CARS spectra are in good agreement with pressure transducer measurements over this entire pressure range, with an average absolute error of less than 8%. Simultaneous pressure and temperature measurements were also performed along the centerline of an underexpanded jet. Results agreed well with theoretical predictions. Single-laser-shot pressure and temperature measurements are discussed.