E. Hassel

A detailed investigation of the stabilization point of lifted turbulent diffusion flames

The stabilization point of lifted turbulent hydrogen diffusion flames is investigated by Raman/Rayleigh/laser-induced fluorescence (LIF) spectroscopy. The stabilization point is determined from simultaneously taken planar laser-induced fluorescence (PLIF) images. It is shown from averaged statistics that lift-off height has negligible influence on the flame length and the far region of the jet. Reactants, premixed downstream of the stabilization point, are rapidly consumed over a very short distance. A new method to generate stabilization point conditioned species and temperature data is proposed and applied to the data. With this method it is possible to describe the surrounding of an observer located at the instantaneous stabilization point. The data are presented by constant contour plots of mixture fraction, species, and temperature in a stabilization point fixed coordinate system. The data obtained by this method are used to assess previously proposed theories on the behavior of lifted turbulent diffusion flames. Experimental findings presented are inconsistent with predictions by the concept of premixed flame stabilization as well as with the flamelet concept. The insensitivity of the spatial location of the stabilization point to the variation of the stoichiometric mixture fraction of the fuels investigated suggests a stabilization mechanism through large-scale turbulent structures. Large-scale structures also explain the existence of products upstream of the stabilization point. The conclusion of this analysis is that large-scale turbulent structures play a dominant role in the stabilization mechanisms of the lifted turbulent diffusion flames, subject to this study.

Laser-diagnostic and numerical study of strongly swirling natural gas flames

This study is focused on the experimental and numerical investigation of the reacting flow field of a strongly swirling, unconfined 150-kW natural gas flame. This work is embedded in the TECFLAM swril burner project in which five research groups currently are involved with various tasks. The final goal is the accurate prediction of complex combustion systems with advanced models based on a complete data set of the governing turbulent flow and reaction field. At this point, attention is paid to the influence of the turbulence structure on the mixing process. A two-component Laser-Doppler Velocimetry (LDV) system is used for the determination of mean and fluctuating velocity components. Two-dimensional temperature fields and fuel-gas distributions are measured via Rayleigh scattering. Three-dimensional temperature distributions and flamefront surfaces are obtained via simultaneous measurements of Rayleight scattering and OH Laser-Induced Fluorescence (LIF) in three adjacent planes. An advanced numerical simulation, based on a nonlinear second moment closure is presented to be in good agreement with experimental data. The mean values of axial and circumferential velocity reconfirm a substantial reverse flow surrounded by a curved shear layer. High strain rates yield an intensive turbulent mixing process. It can be concluded from measured temperature fluctuations in this region that reaction takes place in this inner part of the shear layer. The entraining ambient air cannot penetrate through this highly strained area, thus isolating the hot core and providing a stabilizing mechanism to the flame. The 3-D time-resolved measurements of the flamefront give evidence that its structure is disconnected for strongly swirling flames.

Ultraviolet Raman-scattering measurements in flames by the use of a narrow-band XeCl excimer laser.

Spectra with simultaneous peaks of CO(2), O(2), N(2), CH(4), and H(2)O are taken by the use of spontaneous Raman scattering from free jets and a turbulent CH4 diffusion flame. A narrow-band XeCl excimer laser working at 308 nm and an intensified multichannel camera are used to give full information about all major species and the spectral background. Knowledge of the structure of the background is important for data analysis. For O(2), an enhancement that is due to near-resonant Raman scattering is found. Assuming no influence of this enhancement on the temperature dependence of the O(2) Raman-scattering cross section, temperature and concentrations of all major molecules are determined simultaneously from the intensity of the Raman bands. Experimental details and a data reduction scheme based on the analysis of the entire spectral shape are reported. Strong fluorescence backgrounds from OH radicals found in the high-temperature regions of the flame are discussed.

Interaction of turbulence and radiation in confined diffusion flames

A new approach for modeling turbulence-radiation interaction in confined diffusion flames is proposed. In addition to the balance equations of velocity and mixture fraction f, equations of mean, variance, and covariance of the heat-release rate g and mixture fraction are solved. Coupling with the chemistry model is achieved by means of a two-dimensional PDF of mixture fraction and heat-release rate. The proposed approach is open for improvement by more sophisticated submodels. Validity of this approach is illustrated by simulation of a 20-kW axisymmetric confined swirling combustion system. The results are compared to experimental data from LDV and Raman/Rayleigh measurements obtained from the same combustion system. Particularly, investigations of the influence of different coupling models on the local source terms of the radiation and of the nitrogen oxide production are discussed. The main results of this study are (1) the thin eddy assumption is valid for this system; (2) there are considerable fluctuations of the heat-release rate g with constant mixture fraction f at all locations because of the individual history of the burned gas eddies; (3) the coupling model has only negligible influence on the spatial temperature- velocity- and mixture-fraction fields and on the overall integrated radiation-transfer power; (4) the coupling model has strong influence on the local nitrogen oxide production and on the total nitrogen oxide emission. The present study demonstrates that for middle-sized enclosed diffusion flames, the complex phenomenon of turbulence-radiation interaction can be simulated efficiently without use of tedious stochastic approaches.

Turbulence modulation in jet diffusion flames: Modeling and experiments

Abstract Turbulence in a diffusion flame is modulated by thermal expansion, buoyancy effects and lift-off. Aside from simple shear-generated sources density-velocity correlations, π′u ″ i represent additional source terms in the Reynolds-stress equations that distort the shear-generated turbulence anisotropy. A modeled transport equation and several zero-order models of π′u ″ i are analyzed and their effect on flame predictions with a Favre averaged second-order moment closure for velocity and scalar transport is investigated. The chemical reaction is described by chemical equilibrium and laminar flamelet modeling. The latter is shown to have limitations in application due to differential diffusion. Two attached, vertical H 2 /N 2 -air flames with the same Reynolds numbers but different Froude numbers are investigated numerically and experimentally. The desired data base for an overall comparison is provided by comprehensive 3D-LDV, coherent anti-Stokes Raman spectroscopy and spontaneous Raman spectroscopy measurements. The calculations yield correct results in all measured profiles of velocity, temperature and species concentrations. It is shown that only one zero-order model and the transport equation of π′u ″ i are adequate. The neglect of those terms will falsify the prediction of decay rates, fluctuations and flame shapes. The magnitude of errors depends on the local Froude number which decreases downstream. The increase in the influence of buoyancy leads to smaller decay rates of axial velocity and to enhanced scalar mixing. Furthermore, turbulence intensities are reduced, and scalar fluctuations and anisotropy are enlarged. The experimentally observed visible flame length shortening with decreasing, density weighted Froude number is reproduced by the presented model.

Finite Rate Chemistry and NO Molefraction in Non-Premixed Turbulent Flames

Finite rate chemistry is investigated in turbulent N{sub 2}-diluted H{sub 2} diffusion flames by means of laser spectroscopic methods and a numerical model of combustion. The major species occurring in these hydrogen flames or the temperature are measured with SRS (Spontaneous Raman Spectroscopy) or Rayleigh spectroscopy. Some minor species, the radical OH and the pollutant NO, are simultaneously measured with linear LiF (Laser Induced Fluorescence). Multidimensional pdfs (probability density function) can be deduced from these measurements. Use is made of a numerical model with two principal variables, mixture fraction {zeta} and reaction progress variable {eta}, as a basis for discussion of the experimental results. A {kappa}-{var_epsilon}-turbulence model together with a two-dimensional presumed pdf for the coupling of turbulence and chemistry are applied. So, experimental two-dimensional pdfs as well as mean values of {zeta} and {eta} as functions of the position in the flame are deduced from the simultaneous measurements. The experimental and theoretical spatial maxima of the mean OH molefraction agree well in magnitude, despite the correlation coefficient between {zeta} and {eta} of the measured pdf can be as high as 0.5. The neglect of this covariance for the calculation of the presumed pdf is quantified. It results in clearmorexa0» deviations for the OH molefraction. The experimental NO and OH molefractions are better simulated by flame calculations carried out with the presented combustion model than by the also shown calculations based on a single variable for description of chemistry.«xa0less

Experimental and numerical study of a highly diluted turbulent diffusion flame close to blowout

The nature of stability, finite rate chemistry, and differential diffusion inherent in a highly diluted turbulent non-premixed hydrogen flame close to blowout is investigated. Major-species concentrations and temperature are measured by Raman/Rayleigh spectroscopy. OH concentration is simultaneously measured by laser induced fluorescence (LIF), and laser Doppler velocimetry (LDV) is used to determine velocities. The experimental data are used to assess the performance of a probability density function (PDF) simulation with a five-step reduced chemistry mechanism and differential diffusion effects. Comparison is made in terms of averaged quantities, scatter plots, and conditional averaged data. First- and second-order statistics of predictions and experimental data are found in reasonably good agreement. The significant amount of finite rate chemistry observed in the flame as well as the relaxation to equilibrium with downstream distance is well reproduced by the PDF calculation. PDF model predictions of differential diffusion compare favorable with experimentally determined values. Numerical results reveal that differential diffusion may have a major impact on the flame stability, although the average deviation from the equal diffusivity limit is small. Computed laminar extinction limits of laminar flame calculations with multicomponent diffusion and equal diffusivity are in agreement with this observation.

Study of swirling recirculating hydrogen diffusion flame using UV raman spectroscopy

Raman/Rayleigh scattering is used to measure temperature and major species concentrations in a swirling recirculating hydrogen diffusion flame in air and a corresponding nonreacting flow. Mean and fluctuation statistics are reported. Laser-Doppler Velocimetry (LDV) is used to determine properties of the flow field. The main difference between reacting and nonreacting flow is the decreased centerline decay of mixture fraction in the former case, a shortened and broadened recirculation zone, and higher root mean spare (rms) fluctuations of the mixture fraction. Conclusions drawn from the results confirm that the effect of combustion heat release on the turbulent mixing process is considerable. The width of the flow structure based on mixture fraction is found to be approximately linear with axial distance up to the second stagnation point. Results demonstrate the significant influence of buoyancy in the recovery region down-stream of the recirculation zone. Finite-rate chemistry effects result in a temperature well below the adiabatic equilibrium value where the calculated Damkohler number is less than unity at the upstream end of the recirculation zone. A progress variable is defined to describe the departure from chemical equilibrium. The mean progress variable at radial locations with identical mixture fraction is found to be different, depending on its relative position to the recirculation zone.

Experimental data base for numerical simulations of turbulent diffusion flames

The objective of the presented measurements is to provide an experimental data base for comparison with numerical simulation results of turbulent H2-air diffusion flames. Additionally, the date base may also be used for a proof of new measurement techniques, when the same flame conditions are applied. The data base contains time and spatial resolved data on all three velocity components, all Reynolds-stress tensor components, temperature, mixture fraction, species concentrations, higher statistical moments of these quantities and probability density functions for three different flames. The data are given as original measurement data in dependence on flame conditions and location in the flame, as absolute and normalized data and as evaluated data, like anisotropy tensor. The measurements are made to improve the understanding of turbulent transport processes under the influence of combustion and to help the effort to couple the turbulence and combustion model. A Laser-Doppler-Velocimeter was used to obtain three velocity components simultaneously. Temperature was measured with spontaneous Raman-Rayleigh spectroscopy and Coherent Anti-Stokes Raman spectroscopy, separately, while species concentrations and mixture fraction are measured with spontaneous Raman-Rayleigh spectroscopy. Measurements are done from nozzle exit into the self-preserving region up to x/d=100 so that the whole flow field including all boundary conditions are quantified for numerical prediction. A mixture of hydrogen and nitrogen with a mole ratio of 1:1 is used as fuel. Reynolds number and Froude number are set at different values. This complete data set is available upon request.

Measurement of temperature and concentration in oxy-fuel flames by Raman/Rayleigh spectroscopy

For temperatures above ~ 2500 K systematic errors arise in temperature and, depending on the algorithm used for data evaluation, in species concentration measured by Raman or Raman/Rayleigh spectroscopy. These systematic errors are independent of the specific experimental setup and excitation wavelength, but are caused by significant dissociation of the Raman active species into atoms and radicals. The aim of this paper is to develop a basic understanding of high temperature Raman/Rayleigh spectroscopy and to present a matched data post-processing. Methods are developed for an appropriate post-processing of data gained from oxy-fuel flames and can be viewed as an extension of conventional data evaluation strategies. These post-processing schemes presume the knowledge of the chemical performance of the system investigated, whereas in this study chemical equilibrium is used exemplarily. The focus is on single-shot measurement important for the investigation of turbulent flows. Experimental procedures using a KrF* excimer laser at 248.4 nm are scrutinized, and the measurement of the Rayleigh cross-section of OH at 248.4 nm necessary for data evaluation is presented.