Scalar length scales and spatial averaging effects in turbulent piloted methane/air jet flames

The effects of spatial averaging in measurements of scalar variance and scalar dissipation in three piloted methane/air jet flames (Sandia flames C, D, and E) are investigated. Line imaging of Raman scattering, Rayleigh scattering, and laser-induced CO fluorescence is applied to obtain simultaneous single-shot measurements of temperature, the mass fractions of all major species, and mixture fraction, ξ, along 7-mm segments. Spatial filters are applied to ensembles of instantaneous profiles to quantify effects of spatial averaging on the Favre mean and variance of mixture fraction and scalar dissipation at several locations in the three flames. The radial contribution to scalar dissipation, χ_r = 2D_ξ (∂ξ/∂r)², is calculated from the filtered instantaneous profiles. The variance of mixture fraction tends to decrease linearly with increasing filter width, while the mean and variance of scalar dissipation are observed to follow an exponential dependence. In each case, the observed functional dependence is used to extrapolate to zero filter width, yielding estimates of the “fully resolved” profiles of measured quantities. Length scales for resolution of scalar variance and scalar dissipation are also extracted from the spatial filtering analysis and compared with length scales obtained from spatial autocorrelations. These results provide new insights on the small scale structure of turbulent jet flames and on the spatial resolution requirements for measurements of scalar variance and scalar dissipation.     

Flame front analysis of high-pressure turbulent lean premixed methane–air flames

An experimental study on lean turbulent premixed methane–air flames at high pressure is conducted by using a turbulent Bunsen flame configuration. A single equivalence ratio flame at Φ = 0.6 is explored for pressures ranging from atmospheric pressure to 0.9 MPa. LDA measurements of the cold flow indicate that turbulence intensities and the integral length scale are not sensitive to pressure. Due to the decreased kinematic viscosity with increasing pressure, the turbulent Reynolds numbers increase, and isotropic turbulence scaling relations indicate a large decrease of the smallest turbulence scales. Available experimental results and PREMIX code computations indicate a decrease in laminar flame propagation velocities with increasing pressure, essentially between the atmospheric pressure and 0.5 MPa. The u′/S_L ratio increases therefore accordingly. Instantaneous flame images are obtained by Mie scattering tomography. The images and their analysis show that pressure increase generates small scale flame structures. In an attempt to generalize these results, the variance of the flamelet curvatures, the standard deviation of the flamelet orientation angle, and the flamelet crossing lengths have been plotted against which is proportional to the ratio between the integral and Taylor length scales, and which increases with pressure. These three parameters vary linearly with the ratio between large and small turbulence scales and clearly indicate the strong effect of this parameter on premixed turbulent flame dynamics and structure. An obvious consequence is the increase in flame surface density and hence burning rate with pressure, as confirmed by its direct determination from 2D tomographic images.     

Burning velocity correlation of methane/air turbulent premixed flames at high pressure and high temperature

Turbulent burning velocities for methane/air mixtures at pressures ranging from atmospheric pressure up to 1.0 MPa and mixture temperatures of 300 and 573 K were measured, which covers the typical operating conditions of premixed-type gas-turbine combustors. A bunsen-type flame stabilized in a high-pressure chamber was used, and OH-PLIF visualization was performed with the pressure and mixture temperature being kept constant. In addition to a burner with an outlet diameter of 20 mm for the high-pressure experiments, a large-scale burner with an outlet diameter of 60 mm was used at atmospheric pressure to extend the turbulence Reynolds number based on the Taylor microscale, R_λ, as a common parameter to compare the pressure and temperature effects. It was confirmed that R_λ over 100 could be attained and that u′/S_L could be extended even at atmospheric pressure. Based on the contours of the mean progress variable c = 0.1 determined using OH-PLIF images, turbulent burning velocity was measured. S_T/S_L was also found to be greatly affected by pressure for preheated mixtures at 573 K. The bending tendency of the S_T/S_L curves with u′/S_L was seen regardless of pressure and mixture temperature and the R_λ region where the bending occurs corresponded well to the region where the smallest scale of flame wrinkling measured as a fractal inner-cutoff approaches the characteristic flame instability scale and becomes almost constant. A power law of S_T/S_L with (P/P₀)(u′/S_L) was clearly seen when S_T was determined using c = 0.1 contours, and the exponent was close to 0.4, indicating agreement with the previous results using the mean flame cone method and the significant pressure effects on turbulent burning velocity.     

Computations of turbulent lean premixed combustion using conditional moment closure

Conditional Moment Closure (CMC) is a suitable method for predicting scalars such as carbon monoxide with slow chemical time scales in turbulent combustion. Although this method has been successfully applied to non-premixed combustion, its application to lean premixed combustion is rare. In this study the CMC method is used to compute piloted lean premixed combustion in a distributed combustion regime. The conditional scalar dissipation rate of the conditioning scalar, the progress variable, is closed using an algebraic model and turbulence is modelled using the standard k–? model. The conditional mean reaction rate is closed using a first order CMC closure with the GRI-3.0 chemical mechanism to represent the chemical kinetics of methane oxidation. The PDF of the progress variable is obtained using a presumed shape with the Beta function. The computed results are compared with the experimental measurements and earlier computations using the transported PDF approach. The results show reasonable agreement with the experimental measurements and are consistent with the transported PDF computations. When the compounded effects of shear-turbulence and flame are strong, second order closures may be required for the CMC.     

CO2 addition and pressure effects on laminar and turbulent lean premixed CH4 air flames

An experimental study on CH₄–CO₂–air flames at various pressures is conducted by using both laminar and turbulent Bunsen flame configurations. The aim of this research is to contribute to the characterization of fuel lean methane/carbon dioxide/air premixed laminar and turbulent flames at different pressures, by studying laminar and turbulent flame propagation velocities, the flame surface density and the instantaneous flame front wrinkling parameters. PREMIX computations and experimental results indicate a decrease of the laminar flame propagation velocities with increasing CO₂ dilution rate. Instantaneous flame images are obtained by Mie scattering tomography. The image analysis shows that although the height of the turbulent flame increases with the CO₂ addition rate, the flame structure is quite similar. This implies that the flame wrinkling parameters and flame surface density are indifferent to the CO₂ addition. However, the pressure increase has a drastic effect on both parameters. This is also confirmed by a fractal analysis of instantaneous images. It is also observed that the combustion intensity S_T/S_L increases both with pressure and the CO₂ rate. Finally, the mean fuel consumption rate decreases with the CO₂ addition rate but increases with the pressure.     

The compositional structure of highly turbulent piloted premixed flames issuing into a hot coflow

Simultaneous line measurements of major species and temperature by the Raman–Rayleigh technique, combined with CO two-photon laser-induced fluorescence and crossed-plane OH planar laser-induced fluorescence have been applied to a series of flames in the Piloted Premixed Jet Burner (PPJB). The PPJB is capable of stabilizing highly turbulent premixed jet flames through the use of a stoichiometric pilot and a large coflow of hot combustion products. Four flames with increasing jet velocities and constant jet equivalence ratios are examined in this paper. The characteristics of these four flames range from stable flame brushes with reaction zones that can be described as thin and “flamelet-like” to flames that have thickened reaction zones and exhibit extinction re-ignition behaviour. Radial profiles of the mean temperature are reported, indicating the mean thermal extent of the pilot and spatial location of the mean flame brush. Measurements of carbon monoxide (CO) and the hydroxyl radical (OH) reveal a gradual decrease in the conditional mean as the jet velocity is increased and the flame approaches extinction. Experimental results for the conditional mean temperature gradient show a progressive trend of reaction zone thickening with increasing jet velocities, indicating the increased interaction of turbulence with the reaction zone at higher turbulence levels. For the compositions examined, the product of CO and OH mole fractions ([CO][OH]) is shown to be a good qualitative indicator for the net rate of production of carbon dioxide. The axial variation of [CO][OH] is shown to correlate well with the mean chemi-luminescence of the flames including the extinction re-ignition regions. The experimental findings reported in this paper further support the hypothesis of an initial ignition region followed by extinction and re-ignition regions for certain PPJB flames.     

Flamelet structure in turbulent premixed swirling oxy-combustion of methane

Two key flame macrostructures in swirling flows have been observed in experiments of oxy-combustion (as well as air-combustion); as the equivalence ratio is raised, the flame moves from being stabilized on just the inner shear layer (Flame III) to getting stabilized on both the inner and outer shear layers (Flame IV). We report results of an LES investigation of two different inlet oxy-fuel mixtures, in a turbulent swirling flow at $\text{Re}=20,000,$ that capture these two macrostructures. Previous work on the effects of heat loss have mostly focused on its impact on macro-scale observations. In this paper, we examine how heat loss impacts the flame microstructures as well for these two macrostructures. For both flames, the flamelet structure, as represented by a scatter plot of the normalized fuel concentration against the normalized temperature, depends on whether the combustor walls are adiabatic or non-adiabatic. For the adiabatic case, the flamelets of both macrostructures behave like strained flames. When wall heat transfer is included, Flame III microstructure is more bimodal. Since this flame extends farther downstream and part of it propagates along the walls, heat transfer has a greater impact on it’s microstructure. These results show that heat loss impacts not just the macro properties of the flame such as its shape or interactions with the wall, but also fundamentally changes its internal structure. Scatter plots of the turbulent flames are constructed and compared to different 1D laminar flame profiles (e.g., strained or with heat loss), and comparisons suggest the important role of the wall thermal boundary conditions in the accurate simulations of combustion dynamics and interpretations of experimental data, including data reduction and scaling.     

Computational study of lean premixed turbulent flames using RANSPDF and LESPDF methods

A computational study is performed on a series of four piloted, lean, premixed turbulent jet flames. These flames use the Sydney Piloted Premixed Jet Burner (PPJB), and with jet velocities of 50, 100, 150 and 200 m/s are denoted PM150, PM1100, PM1150 and PM1200, respectively. Calculations are performed using the RANSPDF and LESPDF methodologies, with different treatments of molecular diffusion, with detailed chemistry and flamelet-based chemistry modelling, and using different imposed boundary conditions. The sensitivities of the calculations to these different aspects of the modelling are compared and discussed. Comparisons are made to experimental data and to previously-performed calculations. It is found that, given suitable boundary conditions and treatment of molecular diffusion, excellent agreement between the calculations and experimental measurements of the mean and variance fields can be achieved for PM150 and PM1100. The application of a recently developed implementation of molecular diffusion results in a large improvement in the computed variance fields in the LESPDF calculations. The inclusion of differential diffusion in the LESPDF calculations provides insight on the behaviour in the near-field region of the jet, but its effects are found to be confined to this region and to the species CO, OH and H₂. A major discrepancy observed in many previous calculations of these flames is an overprediction of reaction progress in PM1150 and PM1200, and this discrepancy is also observed in the LESPDF calculations; however, a parametric study of the LESPDF mixing model reveals that, with a sufficiently large mixing frequency, calculations of these two flames are capable of yielding improved reaction progress in good qualitative agreement with the mean and RMS scalar measurements up to an x/D of 30. Lastly, the merits of each computational methodology are discussed in light of their computational costs.     

Characterization of premixed swirling methane/air diffusion flame through filtered Rayleigh scattering

Characteristics of a premixed, swirl methane/air diffusion flame at atmospheric pressure are measured by filtered Rayleigh scattering (FRS). Three operating conditions are investigated with the equivalence ratios of the methane/air flame covering a range of 0.67—0.83. Under each condition, single-shot and averaged FRS images over a region measured 39.3×65.6 mm² at seven cross sections of the flame are collected to demonstrate the flame behavior. A gradient calculation algorithm is applied to identify reaction zone locations and structures in the instantaneous FRS measurements. Statistical analysis for the mean FRS measurements is performed by means of joint probability density functions. The experimental results indicate that thermochemical state of the swirl flame is strongly influenced by equivalence ratio, leading to varieties of flame structures and temperature distributions. The gradient of the instantaneous FRS images clearly illustrates the characteristics of the reaction zone. The results also demonstrate that FRS can provide detailed insights into the behavior of turbulent flames.     

Comparative analysis of methods for heat losses in turbulent premixed flames using physically-derived reduced-order manifolds

Heat losses have the potential to substantially modify turbulent combustion processes, especially the formation of pollutants such as nitrogen oxides. The chemistry governing these species is strongly temperature sensitive, making heat losses critical for an accurate prediction. To account for the effects of heat loss in large eddy simulation (LES) using a precomputed reduced-order manifold approach, thermochemical states must be precomputed not only for adiabatic conditions but also over a range of reduced enthalpy states. However, there are a number of methods for producing reduced enthalpy states, which invoke different implicit assumptions. In this work, a set of a priori and a posteriori LES studies have been performed for turbulent premixed flames considering heat losses within a precomputed reduced-order manifold approach to determine the sensitivity to the method by which reduced enthalpy states are generated. Two general approaches are explored for generating these reduced enthalpy states and are compared in detail to assess any effects on turbulent flame structure and emissions. In the first approach, the enthalpy is reduced at the boundary of the one-dimensional (1D) premixed flame solution, resulting in a single enthalpy deficit for a single premixed flame solution. In the second approach, a variable heat loss source term is introduced into the 1D flame solutions by mimicking a real heat loss to reduce the post-flame enthalpy. The two approaches are compared in methane–air piloted turbulent premixed planar jet flames with different diluents that maintain a constant adiabatic flame temperature but experience different radiation heat losses. Both a priori and a posteriori results, as well as a chemical pathway analysis, indicate that the manner by which the heat loss is accounted for in the manifold is of secondary importance compared to other model uncertainties such as the chemical mechanism, except in situations where heat loss is unphysically fast compared to the flame time scale. A new theoretical framework to explain this insensitivity is also proposed, and its validity is briefly assessed.     

Direct numerical simulations of rich premixed turbulent n-dodecane/air flames at diesel engine conditions

Rich premixed turbulent n-dodecane/air flames at diesel engine conditions are analyzed using direct numerical simulations. The conditions correspond to a parametric variation of the Engine Combustion Network Spray A (pressure 60 atm; oxidizer oxygen level and temperature 21% and 900 K, respectively; fuel temperature 363 K). Three simulations with equivalence ratios of 3, 5, and 7 are performed with a Karlovitz number (Ka, based on flame time) of order 100 to match the estimated Ka of the rich premixed combustion region in Spray A. At these conditions, the reference laminar flames exhibit a complex structure which involves both low-temperature chemistry (LTC) and high-temperature chemistry over a wide range of length scales. In the presence of turbulence, the flame structure is strongly affected in physical space and the reaction zone exhibits a very complex structure in which broken, distributed, and thin regions co-exist, especially for the leanest case. However, the contribution of the LTC pathway is only weakly affected by turbulence. In progress variable space, the mean flame structure, including the chemical source terms, is found to match remarkably well that of the corresponding unity Lewis number laminar flame, particularly for the $?=$ 3 and 5 cases. This behavior is attributed to the strong turbulent mixing occurring throughout the flames/reaction zones, which suppresses differential diffusion effects. Nevertheless, large conditional fluctuations around the mean chemical source terms are identified. These are found to correlate very well with radical species mass fractions such as OH. In addition, a similar functional dependence is obtained from counterflow laminar flames. As such, it appears from these results that laminar flame models have a potential to be used to represent the thermochemical state of rich premixed turbulent flames under diesel engine conditions.     

Large eddy simulation of a turbulent nonpremixed piloted methane jet flame (Sandia Flame D)

Large eddy simulation (LES) is conducted of the Sandia Flame D [Proc. Combust. Inst. 27 (1998) 1087, Sandia National Laboratories (2004)], which is a turbulent piloted nonpremixed methane jet flame. The subgrid scale (SGS) closure is based on the scalar filtered mass density function (SFMDF) methodology [J. Fluid Mech. 401 (1999) 85]. The SFMDF is basically the mass weighted probability density function (PDF) of the SGS scalar quantities [Turbulent Flows (2000)]. For this flame (which exhibits little local extinction), a simple flamelet model is used to relate the instantaneous composition to the mixture fraction. The modelled SFMDF transport equation is solved by a hybrid finite-difference/Monte Carlo scheme. This is the first LES of a realistic turbulent flame using the transported PDF method as the SGS closure. The results via this method capture important features of the flame as observed experimentally.