Sten Starner

Conserved scalar measurements in turbulent diffusion flames by a Raman and Rayleigh ribbon imaging method

Abstract A new method to obtain images of conserved scalars in turbulent flames is presented and implemented with simultaneous Rayleigh and fuel Raman measurements in a methane/air jet diffusion flame by the use of a single dye laser and two intensified CCD cameras. The laser beam is focused to a line and retroreflected with a slight offset to form a thin ribbon, sufficient to measure gradients in two dimensions. A robust, iterative data reduction technique is used to derive statistics of temperature, fuel mass fraction, mixture fraction ( f ), and scalar dissipation (χ). Results for a flame of Reynolds number 20,600 show that the lower moments, pdfs, and scatter plots of the computed quantities do not differ markedly from published results of point measurements in similar flames, strengthening confidence in this new approach. The computed components of χ show behavior similar to that in nonreacting flows; there is some anisotropy, with the ratio of the radial to the axial component in the shear region around 2.0. The azimuthal component, measured by off-axis laser beam alignment, is roughly equal to the radial component. The correlation between f and χ is small on the flame axis, but the correlation coefficient R fχ rises to around 0.4 near the edge, which is largely consistent with other recent results for cold jets and jet flames.

A simple model for carbon monoxide in laminar and turbulent hydrocarbon diffusion flames

A model has been developed for the composition in diffusion flames. It consists of a flame sheet for the pyrolysis or consumption of fuel on the rich side of stoichiometric. Near stoichiometric the molecular and radical species are assumed to be in partial equilibrium and the burnout of CO is controlled by the rate of recombination of the radicals in three-body reactions. The composition and reaction rates for the excess moles are expressible in terms of two variables, the mixture fraction and the excess moles. Calculations for these two variables have been carried out in laminar and turbulent jet diffusion flames and the predicted composition compared with experimental data. In broad terms the agreement is good and the prediction of CO on the lean and rich sides of both laminar and turbulent flames is particularly encouraging. Application of the model to the prediction of CO emissions in combustors is discussed.

LDA Measurements in a Turbulent Diffusion Flame with Axial Pressure Gradient

Abstract Earlier work on the turbulent diffusion flame of a hydrogen jet issuing into a co-flowing air stream has been extended to include the effects of axial pressure gradients. Pressure gradients with parameters of − 1.1 × 10−3, — 0.41 × 10−3 and +0.09 × 10−3 were produced by converging and diverging the floor and roof of the working section. Laser-Doppler anemometer (LDA) measurements including mean velocities, turbulence intensities and Reynolds stress were made with only the jet seeded so that the measurements are of a turbulent zone average nature. Both favourable (negative) and adverse pressure gradients shorten the flame. A flame of elliptical cross-section was produced but the distortion is fully accounted for by the free stream strain and the results are presented in pseudo-axisymmetric form. For favourable pressure gradient, the mean velocity profiles in the radial direction are closely similar to those found earlier for near-zero pressure gradient. For the adverse pressure gradient, the mean ...

Scalar Dissipation Measurements in Turbulent Jet Diffusion Flames of Air Diluted Methane and Hydrogen

Abstract Simultaneous two-dimensional Rayleigh and fuel Raman images have been collected in air-diluted methane and hydrogen jet diffusion flames. Temperature, fuel mass fraction and mixture fraction images are derived by a two-scalar approach based on one-step chemistry and equal species diffusivities. This enables calculation of two components of the scalar dissipation rate x-The inherently weak Raman signal has been maximised by intra-cavity measurements, using a flashlamp-pumped dye laser. In addition, the Raman signal-to-noise ratio is drastically improved by a novel contour-aligned smoothing technique which exploits the high correlation between the Rayleigh and Raman signals. Quantitative measurements of scalar dissipation are presented, including probability density functions for components of x- Profiles of mean and rms mixture fraction show the usual features already documented in other published results for this type of flame. Probability density functions of ξ are close to Gaussian on the axis,...

Characteristics of a piloted diffusion flame designed for study of combustion turbulence interactions

Abstract A laboratory burner has been developed for turbulent combustion of nonpremixed gaseous hydrocarbon fuels, with emphasis on producing very high mixing rates well away from the burner nozzle which is placed in a coflowing stream of air. This design has been chosen so that the effects of fast mixing on flame chemistry and extinction can be studied without the influence of flame lift-off or extinction at the nozzle. A concentric premixed pilot flame is used, with C 2 H 2 H 2 fuel mixed in a ratio to make the basic pilot product composition equal to that of the main fuel, so that the flow can be modeled by parabolic partial differential equations as a two-stream mixing problem. Shadowgraph photography reveals rapidly developing turbulence close to the nozzle in which the pilot stream is quickly mixed out. Thermocouple measurements show that a low temperature region exists some 20 fuel jet diameters from the exit plane where extinction eventually occurs as flow rates are increased until mixing rates become excessive. Measurements of electrical conductivity in the flame indicate that it propagates intermittently before becoming extinguished.

Piloted Diffusion Flames of Diluted Methane Near Extinction: Detailed Structure from Laser Measurements

Abstract Simultaneous laser Raman, Rayleigh and fluorescence measurements have been made in diluted turbulent methane diffusion flames. Time and space resolved results for CH4, O2, N2, H2O, CO, CO2, H2, OH, temperature and mixture fraction arc presented as scatter plots and conditional probability density functions for conditions close to extinction. Dilution, whether by air or nitrogen, results in a flame structure with wider reaction zones, and it is shown that the reaction zone width markedly affects the development of bimodality of species and temperature pdfs around stoichiometric composition. The position of the mean stoichiometric contour, relative to the shear layer, is also found to alter the shapes of pdfs of temperature and species. Hydrogen and hydroxyl radical mass fractions are much in excess of predictions for stretched laminar flames. Dilution with air does not seem to alter the chemical composition near extinction, either at stoichiometric mixture or in the richer reactive region of the f...

An improved method of data aquisition and reduction for laser raman-rayleigh and fluorescence scattering from Multispecies

We review the spontaneous Raman scattering process as applied to the determination of species concentration and thermometry in turbulent reactive flows. Comments on design improvements to the apparatus are made. We elaborate on a matrix formulation for data reduction that logically removes the following undesired sources of signal in Raman scattering systems: 1) cross-talk due to Raman scattering from other species, 2) laser induced fluorescence, 3) background flame chemiluminescence, and 4) noise in the data acquisition system.

Mixture fraction imaging in a lifted methane jet flame

This work contributes to the study of the stabilization mechanism at the base of the classical lifted flame formed in a turbulent methane jet. Despite several recent efforts to elucidate the features of this mechanism, there is no consensus on the structural details of the flame base. In an effort to examine these mechanisms, the fine structural details at the flame base need recording. The authors present here high-resolution images of temperature and mixture fraction, using a two-scalar technique, simultaneously recording Rayleigh and fuel Raman images.

Joint Measurements of Radial Velocity and Scalars in a Turbulent Diffusion Flame

Abstract Simultaneous laser Doppler and Mie scattering measurements have been made in a horizontal turbulent hydrogen diffusion flame in a co-flowing stream. The nozzle fluid is seeded with aluminium oxide particles. The resulting light scattering signal is used to obtain scalar time traces which are processed together with the record of radial velocity to yield turbulent fluxes of the main species, temperature and mixture fraction. The measurement technique has been tested by experiments in a known isothermal flow. The profiles and values of normalized Favre mixture fraction fluxes are close to those of non-reacting jets only after burnout, far downstream, and it appears that chemical reaction reduces the upstream flux. The mixture fraction radial gradients and the corresponding radial fluxes show similar profiles throughout the measured region, x/D = 40 to 160, with no significant instance of counter-gradient flux. Radial pressure gradients modify the scalar fluxes significantly; estimates point to augm...

Intracavity technique for improved Raman/Rayleigh imaging in flames

A novel intracavity laser diagnostic has been developed to perform quantitative two-dimensional imaging of major species and temperature by using spontaneous Raman scattering and Rayleigh scattering in a turbulent flame. A flash-lamp-pumped dye laser cavity is modified to include sheet-forming optics and 100% reflecting end mirrors. In a comparison of this system with a laser configuration with the sheet-forming optics outside the cavity, the beam waist is comparable, while the intensity is increased by a factor of 5. This technique has applicability to many systems in which weak scattering must be monitored, such as Rayleigh scattering, Raman scattering, and laser-induced fluorescence of certain combustion radicals.