Gradient free regime identification (GFRI) is applied to 1D Raman/Rayleigh/LIF measurements of temperature and major species from the intermediate velocity case of the Sydney piloted inhomogeneous jet flame series to better understand the structure of reaction zones and the downstream evolution of multi-regime characteristics. The GFRI approach allows local reaction zones to be detected and characterized as premixed, dominantly premixed, multi-regime, dominantly non-premixed, or non-premixed flame structures, based on flame markers (mixture fraction, chemical mode, and heat release rate) derived from the experimental data. The statistics of chemical mode zero-crossings, which mark premixed reaction zones, and the relative populations of flame structures are shown to be sensitive to the state of mixing in the near field of the flame and to the level of local extinction farther downstream. Multi-regime structures, where premixed and non-premixed reaction zones occur in close proximity and both contribute to overall heat release, account for nearly half the total population at streamwise locations within the first several jet diameters. There is a rapid transition within the near field whereby the relative population of non-premixed and dominantly non-premixed structures grows from 0.05 to nearly 0.5, and the population of premixed and dominantly premixed structures decreases correspondingly as fluid entering the reaction zone becomes progressively fuel-rich. Local extinction and re-ignition bring a resurgence in premixed-type structures, many of which occur at fuel-lean conditions. There are also modest populations of multi-regime structures, having chemical mode zero-crossings at lean conditions, which would not exist in a fully burning jet flame.
This paper presents new measurements of species concentrations, temperature and mixture fraction in selected regions of a turbulent ethanol spray flame. The line-Raman-LIF-CO-OH setup developed at the Sandia's Combustion Research Facility is utilised to probe regions of a spray flame where laser breakdown of liquid droplets is avoided and the remaining interferences can be corrected. The spray flame is stabilised on the piloted Sydney needle spray burner, where axial translation of the liquid injecting needle in the air-blast stream can transition the spray from dilute to dense. The solution to obtaining successful measurements is found to be multifaceted and includes: the appropriate selection of flame conditions; high sensitivity of the Raman detection system permitting reduced laser energies; development of a pre-processing algorithm to reject strong droplet interferences; and application of the hybrid matrix inversion method combined with wavelet denoising to account for interference corrections and noise at the very low signal levels obtained. Unique and necessary for the successful measurements reported in this paper, a pre-processing algorithm is outlined that removes data points corrupted with strong interferences from droplets. These interferences arise from a range of sources, but the most intense are due to the laser interaction with surrounding mist or liquid fragments, such that measurements near the jet centreline are corrupted and hence discarded. Reliable measurements of mixture fraction, temperature obtained from the sum of the species number densities, and species mole fractions are reported for regions in the flames sufficiently far from the centreline. The paper demonstrates the feasibility of the judicious use of Raman scattering in turbulent spray flames, the results of which will be extremely useful for validating numerical simulations.
Obtaining information about burning characteristics and flame structures by analyzing experimental data is an important issue for understanding combustion processes and pursuing combustion modeling approaches. It has been shown that Raman/Rayleigh measurements of major species and temperature can be used to approximate the local heat release rate and the chemical explosive mode, and that these results are sufficiently accurate for a qualitative assessment of the relative importance of different heat release zones within the same overall flame structure in laminar and mildly turbulent partially premixed flames [1,2]. The present study uses data from direct numerical simulation (DNS) to extend and quantify the understanding of the approximation method with respect to premixed and stratified-premixed flames with significant turbulence–chemistry interaction (high Karlovitz number). The accuracy of the approximation procedure is assessed as previously applied, using just major species and temperature, as well as with the OH radical included as an additional experimentally accessible species. The accuracy of the local chemical explosive mode and the local heat release rate results from the approximation are significantly improved with OH included, yielding quantitative agreement with the DNS results. Further, a global sensitivity analysis is applied to identify the sensitivity of the heat release rate and chemical explosive mode to experimental uncertainties imprinted upon the DNS data prior to the approximation procedure.
Understanding and quantifying the relative importance of premixed and non-premixed reaction zones within turbulent partially premixed flames is an important issue for multi-regime combustion. In the present work, the recently-developed method of gradient-free regime identification (GFRI) is applied to instantaneous 1D Raman/Rayleigh measurements of temperature and major species from two turbulent lifted methane/air flames. Local premixed and non-premixed reaction zones are identified using criteria based on the mixture fraction, the chemical explosive mode, and the heat release rate, the latter two being calculated from an approximation of the full thermochemical state of each measured sample. A chemical mode (CM) zero-crossing is a previously documented marker for a premixed reaction zone. Results from the lifted flames show strong correlations among the mixture fraction at the CM zero-crossing, the magnitude of the change in CM at the zero-crossing, and the local heat release rate at the CM zero-crossing compared to the maximum heat release rate. The trends are confirmed through a comparable analysis of numerical simulations of two laminar triple flames. These newly documented trends are associated with the transition from dominantly premixed flame structures to dominantly non-premixed flames structures. The methods introduced for assessing the relative importance of local premixed and non-premixed reactions zones have potential for application to a broad range of turbulent flames.
Straub, C.; Kronenburg, A.; Stein, O.T.; Barlow, R.S.; Geyer, D.
A stochastic sparse particle approach is coupled with an artificial thickening flame (ATF) model for large eddy simulations (LES) to predict a series of turbulent premixed-stratified flames with and without shear and stratification. The thickened reaction progress variable serves as reference variable for the multiple mapping conditioning (MMC) mixing model which emulates turbulent mixing of the stochastic particles. The key feature of MMC is to enforce localness in this reference space when particle pairs are mixed and prevents unphysical mixing of burnt and unburnt fluid across the flame front. We apply MMC-ATF to three flames of a series of turbulent stratified flames and validate the method by comparison with experimental data. The new measurements feature increased accuracy in comparison to previously published data of the same flames due to a better signal-to-noise ratio and a setup which is less prone to beam steering. All flame locations are well predicted by the LES-ATF approach and an analysis of the MMC particle statistics demonstrates that MMC preserves the flamelet-like behaviour in regions where the experiments show low scatter around the flamelet solution. Predicted (local) deviations from the flamelet-solution are comparable to deviations observed in the measurements and variations in the flame structure due to differences in stratification and shear are reasonably well captured by the method.
Schneider, Silvan S.; Geyer, Dirk G.; Magnotti, Gaetano M.; Dunn, Matthew J.; Barlow, R.S.; Dreizler, Andreas D.
To explore the effect of H2 addition (20 percent by volume) on stratified-premixed methane combustion in a turbulent flow, an experimental investigation on a new flame configuration of the Darmstadt stratified burner is conducted here. Major species concentrations and temperature are measured with high spatial resolution by 1D Raman-Rayleigh scattering. A conditioning on local equivalence ratio (range from φ = 0.45 to φ = 1.25) and local stratification is applied to the large dataset and allows to analyze the impact of H2 addition on the flame structure. The local stratification level is determined as Δφ/ΔT at the location of maximum CO mass fraction for each instantaneous flame realization. Due to the H2 addition, preferential diffusion of H2 is different than in pure methane flames. In addition to diffusing out of the reaction zone where it is formed, particularly in rich conditions, H2 also diffuses from the cold reactant mixture into the flame front. For rich conditions (φ = 1.05 to φ = 1.15) H2 mass fractions are significantly elevated within the intermediate temperature range compared to fully-premixed laminar flame simulations. This elevation is attributed to preferential transport of H2 into the rich flame front from adjacent even richer regions of the flow. Additionally, when the local stratification across the flame front is taken into account, it is revealed that the state-space relation of H2 is not only a function of the local stoichiometry but also the local stratification level. In these flames H2 is the only major species showing sensitivity of the state-space relation to an equivalence ratio gradient across the flame front.
Dual-resolution Raman spectroscopy is a novel diagnostics technique for measurements of temperature and species in flames where multiple hydrocarbons are present. A dual-resolution Raman spectroscopy instrument has been developed and optimized for measurements of major species .g. N2 O2 H2O CO2 CO H2 and DME) and major combustion intermediates including CH4 CH2O C2H2 C2H4 and C2H6 in DME-air flames. The temperature dependences of the hydrocarbon Raman spectra over fixed spectral regions were assessed through a series of measurements in laminar Bunsen-burner flames and have been used to extend a library of previously acquired Raman spectra up to flame temperature. The first Raman measurements of up to twelve species in hydrocarbon flames and the first quantitative Raman measurements of formaldehyde in flames are presented. The accuracy and precision of the instrument were determined from measurements in laminar flames and the applicability of the instrument to turbulent DME-air flames is discussed.
The TNF Workshop series was initiated in 1996 to address validation of RANS based models for turbulent nonpremixed flames and partially-premixed flames where combustion occurs mainly in a diffusion flame mode. The emphasis has been on fundamental issues of turbulence-chemistry interactions in flames that are relatively simple in terms of both geometry and chemistry. Although the TNF acronym has been retained, the word nonpremixed has been dropped from the title, and our scope has expanded (since TNF9 Montreal, 2008).
Spontaneous Raman spectra for important hydrocarbon fuels and combustion intermediates were recorded over a range of low-to-moderate flame temperatures using the multiscalar measurement facility located at Sandia/CA. Recorded spectra were extrapolated to higher flame temperatures and then converted into empirical spectral libraries that can readily be incorporated into existing post-processing analysis models that account for crosstalk from overlapping hydrocarbon channel signal. Performance testing of the developed libraries and reduction methods was conducted through an examination of results from well-characterized laminar reference flames, and was found to provide good agreement. The diagnostic development allows for temporally and spatially resolved flame measurements of speciated hydrocarbon concentrations whose parent is more chemically complex than methane. Such data are needed to validate increasingly complex flame simulations.
In this paper, a system model is developed to investigate independent and coupled effects of resolution, noise and data processing algorithms on the accuracy of the scalar gradient and dissipation measurements in turbulent flows. Finite resolution effects are simulated by spectral filtering, noise is modelled as an additive source in the model spectrum and differencing stencils are analysed as digital filters. In the current study, the effective resolution is proposed to be a proper criterion for quantifying the resolution requirement for scalar gradient and dissipation measurement. Both effective resolution and noise-induced apparent dissipation are mainly determined by the system transfer function. The finite resolution results, based upon a model scalar energy spectrum, are shown to agree with non-reacting experimental data. The coupled resolution-noise results show three regions in the mean scalar dissipation rate measurement: noise-dominated region, noise-resolution correlated region and resolution-dominated region. Different noise levels lead to different resolution error curves for the measured mean scalar dissipation rate. Experimental procedures and guidelines to improve the scalar gradient and dissipation experiments are proposed, based on these model study results. Finally, the proposed system approach can also be applied to other derived quantities involving complex transfer functions.
One-dimensional (1-D) line Rayleigh thermometry is used to investigate the effects of spatial resolution and noise on thermal dissipation in turbulent non-premixed CH4/H2/N2 jet flames. The high signal-tonoise ratio and spatial resolution of the measured temperature field enables determination of the cutoff wavenumber in the 1-D temperature dissipation spectrum obtained at each flame location. The local scale inferred from this cutoff is analogous to the Batchelor scale in nonreacting flows. At downstream locations in the flames studied here, it is consistent with estimates of the Batchelor scale based on the scaling laws using local Reynolds numbers. The spectral cutoff information is used to design data analysis schemes for determining mean thermal dissipation. Laminar flame measurements are used to characterize experimental noise and correct for the noise-induced apparent dissipation in the turbulent flame results. These experimentally determined resolution and noise correction techniques are combined to give measurements of the mean thermal dissipation that are essentially fully resolved and noise-free. The prospects of using spectral results from high-resolution 1-D Rayleigh imaging measurements to design filtering schemes for Raman-based measurements of mixture fraction dissipation are also discussed.