CARS Temperature Measurements Research Articles

Five kHz thermometry in turbulent spray flames using chirped-probe pulse femtosecond CARS, part I: Processing and interference analysis

We have applied chirped-probe-pulse (CPP) femtosecond (fs) coherent anti-Stokes Raman scattering for 5 kHz temperature measurements in turbulent spray flames. The CPP fs CARS technique has previously been used to perform spectroscopic temperature measurements in highly turbulent laboratory burners with excellent accuracy, precision, temporal resolution, and spatial resolution. In this paper, ultrafast CARS measurements in spray flames are presented as part of a larger effort to provide spatially and temporally resolved temperature fields in harsh spray environments. The Sydney Needle Spray Burner (SYNSBURNTM) was used to stabilize turbulent spray flames of acetone and ethanol. The burner features a retractable fuel injector so that the droplet density at the nozzle exit could be systematically varied. Results from selected regions of the turbulent spray flames are discussed in detail to highlight the challenges of CPP fs CARS temperature measurements. Sources of spectral distortion due to interaction with droplets are discussed along with an uncertainty analysis. The passage of fuel through the probe volume caused varying levels of signal degradation and resulted in complete signal loss on approximately 10% of the laser shots for dense spray conditions. The interferences are attributed to two separate phenomena and are categorized based on the probable phase of the fuel – liquid or gas. Interference caused by liquid fuel was unavoidable in certain regions at certain operating conditions, but easily identified and removed. Interference from vapor fuel was more problematic as the nitrogen signal was only moderately corrupted in the high-frequency portion of the spectrum, and the temperature was generally biased to higher values. Rejecting individual signal spectra, based on a fitting error threshold, was shown to be effective in excluding shots with significant interference from fuel droplets, but shots with only minor interference require a more-advanced rejection criterion. Analysis of the temperature fields for a few selected conditions is presented showing trends with the atomization quality of the liquid fuel. Fourier analysis revealed hydrodynamic instabilities in the shear layer and relatively weak thermoacoustic instabilities in the reaction zone.

Development of temperature evaluation of pure Rotational Coherent Anti-Stokes Raman Scattering (RCARS) spectra influenced by spatial averaging effects

The issue of spatial averaging effect arises in CARS temperature measurements when the probe volume contains temperature gradients, a situation frequently encountered in turbulent reacting flows where flow patterns are irregular and temperature gradients are typically steep. In present work, a new dual-temperature fitting model is developed to deal with the spatial averaging effect and then applied to evaluate the measured pure rotational CARS spectra performed on a premixed flame of a Wolfhard–Parker slot burner. Comparing with the traditional single-temperature fitting model, an improved spectral fitting and therefore, temperature and concentration evaluation by using the dual-temperature fitting model, is demonstrated. Finally, the feasibility of the dual-temperature model is shown in turbulent spray flame.

移动式CARS系统测量超声速燃烧室出口温度

移动式CARS系统测量超声速燃烧室出口温度

New approach to single‐shot CARS thermometry of high‐pressure, high‐temperature hydrocarbon flames

AbstractWe demonstrate the feasibility of a proposed new approach to CARS thermometry with simultaneous measurements of the intensities of the hydrogen Q‐branch lines and their linewidths during a single laser shot by means of a spectrograph combined with an interferometer having orthogonal directions of dispersions. This allows correct temperature evaluation that takes into account the directly measured linewidths. The experiments were carried out in a H2:N2 = 1:10 mixture in a heated cell at a temperature of 1050 K. The values of the gas mixture temperature, which were defined from dual‐broad band (DBB) CARS spectra with linewidth corrections in a pressure range of 2–9 MPa, coincide with thermocouple measurements within ∼2%, while without such a correction, the discrepancies increase up to 5%. The proposed approach can be applied to CARS temperature measurements in investigations of H2 and hydrocarbon combustions without the necessity of species concentration measurements and not requiring the knowledge of line‐broadening coefficients. Copyright © 2007 John Wiley & Sons, Ltd.

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.

Studies of Temperature Elevation Due to the Pre-flame Reaction in a Spark-ignition Engine with CARS Temperature Measurements Using Fuels of Various Octane Numbers.

The unburned end-gas temperatures in a combustion chamber of a conventional 4-cylinder DOHC spark-ignition engine were measured using the broadband CARS temperature measurement technique. The test engine was fueled with primary reference fuel 80 and gasoline with research octane numbers of 70.9, 83.4, 91.5 and 100.4. The measured CARS temperatures were compared with the adiabatic core temperatures calculated from the measured pressures. Significant heating by pre-flame reaction in the end gas zone was observed in the late part of compression stroke under both knocking and non-knocking conditions. The measured CARS temperatures when the cylinder pressures were above 1400kPa were higher than the calculated adiabatic core temperatures. These results indicate that some exothermic reactions exist in relatively low pressure and temperature regions. The CARS temperatures began to be higher than the adiabatic core temperature when the end-gas temperatures reached 700K. The temperature elevation due to the pre-flame reaction correlated well with the unburned gas CARS temperature for different research octane number fuels tested.

(2-17) Studies of Temperature Elevation Due to the Pre-Flame Reaction in a Spark-Ignition Engine with CARS Temperature Measurements Using Fuels of Various Octane Numbers((SI-5)S. I. Engine Combustion 5-In-Cylinder A/F and Temperatures)

(2-17) Studies of Temperature Elevation Due to the Pre-Flame Reaction in a Spark-Ignition Engine with CARS Temperature Measurements Using Fuels of Various Octane Numbers((SI-5)S. I. Engine Combustion 5-In-Cylinder A/F and Temperatures)

Experimental Observation of Correlation Between the Temperature of a Laminar Flame and Acoustic Oscillations

The use of CARS temperature measurements for investigating interactions of acoustic oscillations with laminar premixed flames is demonstrated. The phase-locked temperature measurements show that the flame temperature periodically changes as a result of modulation of heat exchange between flame and burner surface. In the propane flame with a stoichiometric ratio ϕ≃0.9, temperature variations reach 300 K when driven with only modest acoustical input power.

Two-dimensional mathematical modeling of laminar, premixed, methane-air combustion on an experimental slot burner

A two-dimensional model of the premixed laminar burning of methane-air has been applied successfully to combustion on a slot burner. Computing times could be excessive and a reduced reaction scheme of Peters was found to be satisfactory for nonrich combustion. CARS temperature measurements on such a burner were in good agreement with predictions, as were the flame heights. Heat loss by conduction from the hot gases to the burner surface are shown, both theoretically and experimentally, to be appreciable at low flow rates. Although conditions for flashback could be computed, it was not possible to compute those for liftoff Nevertheless, the conditions that give rise to it can be deduced. The flame curvature and aerodynamic strain contributions to the flame stretch rate provide insights into the liftoff phenomena and these are quantified. Their relationship to local burning velocities enables Markstein lengths also to be quantified for each of the two contributions. As the flow rate increases, negative curvature stretch becomes dominant at the flame tip, whereas at the base of the flame the aerodynamic strain is dominant and negative. The values of Markstein length are such as to induce stability at the tip where the local burning velocity increases with flow rate, and instability at the base where the local burning velocity decreases with flow rate. These effects are reinforced by the diffusion of H 2 toward the tip of the unreacted gases and by heat loss to the burner surface. Consequently, flame liftoff originates at the base.

95/02105 CARS temperature measurements in a Sasol- and methanol-fuelled internal combustion engine

A study is presented of the influence of using mixtures of natural gas and hydrogen in different fractions (0, 25, 50, 75, 100%) on the combustion velocity and cycle-to-cycle variations in a spark ignition engine. The experimental facility consists of a single-cylinder spark ignition engine. The engine rotational speeds are 1000, 1750 and 2500 rpm. Fuel/air equivalence ratio was kept constant equal to 0.7 during the experiments. A two-zone thermodynamic combustion diagnosis model, based on solving the mass and energy conservation equations, is used to analyze the experimentally obtained pressure combustion chamber in the engine. The two-zone model considers a spherical flame front centred at the spark plug, and solves the intersection of the flame front with the piston, cylinder head and cylinder wall, in order to provide the values of the flame radius corresponding to the burned mass volume and the surfaces for heat to the piston and walls. An automatic procedure based on genetic algorithms is used to determine the optimum parameters needed for combustion diagnosis: Angular positioning and pressure offset of the pressure register, dynamic compression ratio, and heat transfer coefficients. The paper focuses on using the values of the burning velocity computed from the pressure register and especially on the analysis of the cycle to cycle variation in the natural gas/hydrogen fuelled engine, quantified through the standard deviation and the coefficient of variation of the burning speed. Increasing the hydrogen content in the mixture with natural gas increases its burning velocity. This effect is linear with hydrogen fraction, except for very high values of the fraction, when the effect of hydrogen dominates combustion. Additionally, and of practical importance, increasing the hydrogen fraction reduces the relative dispersion of combustion. This effect of hydrogen addition on reducing combustion variability is evident from 25% hydrogen content.

Noise in single-shot broadband coherent anti-Stokes Raman spectroscopy that employs a modeless dye laser

The noise in single-shot coherent anti-Stokes Raman (CARS) spectroscopy that employs a broadband modeless dye laser (MDL) is examined and the results are compared with those of a conventional dye laser. The noise of the dye-laser, the nonresonant CARS, and the resonant N(2) CARS signals are determined. The use of a MDL is shown to result in substantially reduced CARS noise when the CARS signal is generated with a single-mode pump laser, but only a marginal reduction of noise is observed with a multimode pump source The noise measurements are compared with theoretical predictions that are based on models that assume modes of random amplitudes and phases in the multimode laser sources. The combination of a MDL and a single-mode pump laser is shown to increase the precision of single-shot N(2) CARS temperature measurements.

Cars temperature measurements and the cyclic dispersion of knock in spark ignition engines

The use is described of the coherent anti-Stokes Raman spectroscopy (CARS) technique in a single-cylinderresearch engine to measure end-gas temperature in the regime that leads to the onset of knock. The investigations, under nominally the same running conditions, covered a large cyclic dispersion of knock, with knock occurring in about 80% of the cycles. In-cylinder pressures were measured with a transducer, and an ion gap signified flame arrival towards the end of the flame travel and just before autoignition. In general, the temperature increased by over 100 K, both with and without knock, as a result of exothermic preflame reactions. This measured elevation was supported by computations with a simple five-step model of autoignition, calibrated against measured autoignition times in a rapid compression machine. The temperatures measured just prior to knock increased with the severity of the knock. The observed severe cyclic dispersion of knock, under the conditions of the study, is attributed primarily to the cyclic dispersion of flame speeds. An increase in flame speed is associated with an increase in temperature and greater propensity to knock.

Collisional narrowing and spectral shift in coherent anti-stokes Raman spectra of molecular nitrogen up to 2500 bar and 700 K

CARS spectra of the N2 Q-branch up to 2500 bar and 700 K have been measured. Calculated spectra based on theoretical models show significant disagreement with measured spectra above 300 bar so that CARS temperature measurements are in error by 130 K at 700 K and −150 K at 295 K. The spectral shift of the Q-branch reaches an asymptotic value corresponding to that measured in liquid nitrogen.

Parameter studies in practical nitrogen CARS thermometry using standard and advanced fitting codes

A systematic study of the influence of the collisional narrowing and the cross coherence effect on the temperature analysis of N2-Q branch-CARS spectra at atmospheric pressure is presented. A comparison of calculated spectra over a temperature range 300–2000 K reveals that the standard theory neglecting these effects leads to temperature errors of +1.7% under flame conditions, when the nonresonant background is suppressed. This result is supported by the analysis of experimental CARS temperature measurements on a standard laminar diffusion flame. Furthermore, the temperature misreadings originating from erroneous slit function parameters and laser linewidth were investigated.

Effect of detector nonlinearity and image persistence on CARS derived temperatures

The image persistence of self-scanning photodiode arrays (IPDA) incorporating P-20 phosphor-based intensifiers is shown to make them unsuitable for single-pulse CARS temperature measurements in turbulent combustion. Correcting CARS flame spectra for the nonlinear response of the IPDA detectors increases CARS derived temperatures approximately 3-6%. This error is partially offset by correcting for the perturbations in the N(2) vibrational population resulting from stimulated Raman pumping. The effect of these population perturbations on CARS-derived temperatures is determined. CARS flame spectra obtained with uncorrelated pump beams that are corrected for IPDA nonlinearity and stimulated Raman pumping are shown to give temperatures in good agreement with combined thermocouple/sodium line-reversal measurements.

Unburned Gas Temperatures in an Internal Combustion Engine. I:Cars Temperature Measurements

Abstract Broadband,coherent anti-Stokes Raman scattering (CARS) measurements of unburned gas temperatures have been performed in an internal combustion engine.The engine is fueled with n-butane and air and is operated under both knocking and non-knocking conditons.Temperatures are determined by fitting calculated CARS spectra to experimentally obtained,cycle-averaged nitrogen CARS spectra.The accuracy of the fitted temperature depends on a number of factors,including the modeling of collisional narrowing and the response of the intensified diode array detector,the accuracy of nitrogen linewidths and nonresonant susceptibilities used in the CARS fitting code,and the use of the correct laser line shape convolution.The estimated accuracy of the CARS temperatures is 5-9 percent and is presently limited by long-term drift in the broadband dye laser frequency spectrum.The CARS temperatures are compared with the temperature predictions of a heat release model for the engine.The implications of the measurements f...

Precision of multiplex CARS temperatures using both single-mode and multimode pump lasers

The noise level in single-pulse resonant nitrogen CARS spectra is shown to decrease with increasing pump laser bandwidth. This is the reverse of the trend observed for nonresonant CARS spectra. The precision of single-pulse CARS temperature measurements is shown to be dramatically increased by performing a weighted fit of theoretical and experimental CARS spectra using the measured detector noise coefficients as weighting parameters. The inclusion of collisional narrowing and cross-coherence in the CARS theory calculations and their effect on best fit temperatures are discussed. These temperatures, measured in a flat-flame burner, are compared with those obtained by Na line-reversal.