Michael C. Drake

Superequilibrium and thermal nitric oxide formation in turbulent diffusion flames

Abstract Measurements and modeling of the formation of superequilibrium radicals and nitric oxide in atmospheric pressure turbulent jet diffusion flames are presented which quantify the influence of superequilibrium on thermal NO x formation. Variation of fuel gas compositions (CO/H 2 /N 2 , CO/H 2 /CO 2 , and CO/H 2 /Ar) permits partial separation of chemical and fluid mechanical effects. Superequilibrium OH radical concentrations are measured by single-pulse laser saturated fluorescence and NO and NO 2 concentrations by probe sampling and chemiluminescent detection. Four different types of probes were used to quantify probe sampling effects. In turbulent reaction zones, virtually all of the NO x in the flame occurred in the form of NO but far downstream of the flame nearly half of the NO x occurred as NO 2 . Thermal NO x maximized near stoichiometric flame zones; the rich shift observed by others may be a probe sampling artifact. In turbulent CO/H 2 /N 2 jet diffusion flames, both measurements and a nonequilibrium turbulent combustion model show that superequilibrium decreases average temperatures by 250K, increases average OH concentrations by a factor of 4–6, and increases thermal NO x formation principally by broadening the range of mixture fraction (both rich and lean) where thermal NO x is formed. Calculated increases in thermal NO x due to superequilibrium in turbulent CO/H 2 /N 2 jet diffusion flames are factors of 2.5 at 1 atm and 1.4 at 10 atm. The two-scalar pdf model predicts that thermal NO x yield is independent of Reynolds number in disagreement with previous experimental reports.

Measurements of temperature and concentration fluctuations in turbulent diffusion flames using pulsed raman spectroscopy

Instantaneous values of temperature and major component concentrations have been measured simultaneously using pulsed Raman spectroscopy in hydrogen-air diffusion flames of low turbulence produced in a well-controlled co-flowing fan-induced combustion tunnel. Results are presented in the form of probability density functions for each of these scalar variables as a function of flame position. Simultaneously measured values of temperature and concentration (nitrogen or hydrogen) are compared with adiabatic equilibrium flame calculations of these variables. Close agreement with this simplified escription is obtained in the lean regions of the flame; in the fuel-rich regions, the concentrations of N2 are higher than theoretically predicted and those of H2 are lower. Analysis strongly suggests that these systematic deviations arise from differential diffusion of H2. Initial conditioned sampling measurements are reported for hydrogen-propane-air turbulent diffusion flames and for premixed laminar propane-air flames.

Visualization of Turbulent Flame Fronts with Planar Laser-Induced Fluorescence

This report concerns the quantitative time-resolved visualization of reaction zones in laminar, transitional, and turbulent nonpremixed flames. Two-dimensional OH molecular concentrations were measured with planar laser-induced fluorescence excited by a sheet of light (formed from a single tunable ultraviolet laser pulse) and detected with a two-dimensional, image-intensified photodiode array camera. From the resulting data details of instantaneous flame front structures (including positions, shapes, and widths) were obtained.

Single-pulse, laser-saturated fluorescence measurements of OH in turbulent nonpremixed flames

A single-pulse, laser-saturated fluorescence technique has been developed for absolute OH concentration measurements with a temporal resolution of 2 nsec, a spatial resolution of <0.1 mm(3), and an estimated accuracy of +/-30%. It has been applied in laminar, transitional, and turbulent hydrogen-air diffusion flames, providing the first reported quantitative measurements of average values, rms fluctuations, and probability density functions of OH radical concentration in nonpremixed flames.

Comparison of turbulent diffusion flame measurements of OH by planar fluorescence and saturated fluorescence

Planar laser induced fluorescence imaging (PLIF) is shown to be a quantitative method of measuring average values, rms fluctuations, and probability density functions of OH concentration in laboratory scale, H2-air diffusion flames. When compared with single-pulse laser saturated fluorescence (LSF) data, PLIF data show agreement (within a factor of two) for average and rms values in laminar, transitional, and turbulent flames. The unknown temperature dependence of the H2 quenching cross section introduces a factor of two uncertainty in PLIF measurements in rich flame zones. Extensions of PLIF to other molecules and other combustion systems are discussed.

Prediction and measurement of a non-equilibrium turbulent diffusion flame

Superequilibrium radical concentrations in a turbulent CO/H2/N2 jet diffusion flame are computed using a two-scalar pdf model and directly measured using single pulse laser saturated OH fluorescence. The model is based on the averaged Navier-Stokes equations and the k∈l turbulence model. Non-equilibrium chemistry is accounted for by including CO in the partially equilibrated oxyhydrogen radical pool. Two scalars (mixture fraction and eaction progress suffice to describe the thermochemical system. Laser saturated fluorescence is used to directly measure the mean and fluctuating components of OH concentrations and thus the radical pool. Measurements and model both find mean OH concentrations which are four to six times larger than equilibrium with rms values of OH concentration also reasonably predicted. Superequilibrium effects are predicted to lower the mean temperature by as much as 250 K in agreement with experiments. Evidence of the breakdown of partial equilibrium was found in cool fuel-rich zones where predictions of temperature and OH concentration were too high. Extensions of the model to predict thermal NO formation and CO burnout are discussed.

Intermittency and Conditional Averaging in a Turbulent Nonpremixed Flame by Raman Scattering

Pulsed Raman scattering is used to determine zonal averages, intermittency, and conditional probability density functions (pdFs) for temperature, density, conserved scalar, and molecular composition in a turbulent, hydrogen jet diffusion flame. The conditional mean and rms values provide a data base for flame intermittency models. Both conventional and Favre-averaged turbulent zone pdfs of the conserved scalar are highly nonGaussian in the intermittent regions. Turbulent combustion models that assume a clipped Gaussian turbulent pdf in this region could give erroneous results for flame processes sensitive to pdf shapes such as NOX formation. Similar pdf shapes have been found in nonreacting wake flows and correlations obtained by Effelsberg and Peters using a three-zone (laminar, turbulent, and superlayer) model indicate that the superlayer can contribute up to 60°7o of the pdf.

Measurements of superequilibrium hydroxyl concentrations in turbulent nonpremixed flames using staturated fluorescence

The first quantitative, time- and space-resolved measurements have been obtained for probability density functions of OH concentration in nonpremixed flames. Measurements using single-pulse, laser-saturated fluorescence in laminar, transitional and turbulent nonpremixed H 2 -air flames provide unambiguous evidence for substantial OH superequilibrium concentrations, in qualitative agreement with predictions of laminar and turbulent combustion models. The average degree of superequilibrium (¯OH/¯OH AE ) is typically 4–5 near the jet exit and approaches unity far downstream. The maximum instantaneous OH concentration measured in transitional and turbulent H 2 -air flames is ∼6×10 16 molecules/cc in accord with the maximum determined by partial equilibrium thermodynamic calculations and with the maximum OH concentrations measured in premixed H 2 -air flames.

Rotational Raman intensity-correction factors due to vibrational anharmonicity: their effect on temperature measurements

Temperatures can be measured from relative intensities of rotational Raman or rotational coherent anti-Stokes Raman scattering (CARS) from ground and vibrationally excited molecules, provided that relative Raman line strengths are known. Here rotational Raman line strengths are calculated and corrected for vibrational anharmonicity, for N(2), O(2), CO, and H(2). These line-strength-correction factors for vibrational anharmonicity have a pronounced effect on temperature calculations; they lower temperatures previously calculated from O(2) rotational CARS intensities at approximately 3000 K by more than 300 K.

Nitric oxide formation from thermal and fuel-bound nitrogen sources in a turbulent nonpremixed syngas flame

Nitric oxide formation from thermal and FBN sources is studied in a turbulent nonpremixed syngas flame using laser diagnostic techniques and turbulent combustion modeling. The well-characterized syngas flame (produced by a central jet of CO/H2/N2/CH4/NH3 fuel in a co-flowing air stream) is experimentally analyzed with laser velocimetry, pulsed Raman scattering and NO probe sampling. Data for the mean and rms fluctuations of axial velocity, temperature, major species concentrations, density and mixture fraction are presented, along with yields of nitric oxide from thermal and fuel-bound nitrogen sources. A turbulent combustion model based upon time-averaged Navier-Stokes equations with k-∈ turbulence closure gives results in reasonable agreement with experiment for axial velocity and mixture fraction. This model is based upon equilibrium chemistry with an assumed shape mixture fraction pdf. It overpredicts the average temperature by as much as 250 K and underpredicts the yield of thermal NOx by a factor of 3. A simplified kinetic mechanism involving the production and destruction of NO and NH2 successfully predicts the experimentally observed trends in NOx yield from NH3 added to the syngas fuel, although the yield of NOx from NH3 is overpredicted by about 50%. Most of the theory/data discrepancies may be a result of neglecting superequilibrium radical concentrations in the theory.