Thin Reaction Zones Research Articles

Inner Flame Front Structures and Burning Velocities of Premixed Turbulent Planar Ammonia/Air and Methane/Air Flames

Ammonia (mathrm {NH_3}) has attracted interest as a future carbon-free synthetic fuel due to its economic storage and transportation. In this study, quasi direct numerical simulations (quasi-DNS) with detailed-chemistry have been performed in 3-D to examine the flame thickness and assess the validity of Damköhler’s first hypothesis for premixed turbulent planar ammonia/air and methane/air flames under different turbulence levels. The Karlovitz number is systematically changed from 4.26 to 12.06 indicating that all the test conditions are located within the thin reaction zones combustion regime. Results indicate that the ensemble average values of the preheat zone thickness deviate slightly from the thin laminar flamelet assumption, while the reaction zone regions remain relatively intact. Following the balance equation of reaction progress variable gradient, normal strain rate and the tangential diffusion component of flame displacement speed variation in the normal direction to the flame surface are found to be responsible for thickening the flame. However, the sum of reaction and normal diffusion components of flame displacement speed variation in the normal direction to the flame surface is in charge of flame thinning for ammonia/air and methane/air flames. In addition, the validity of Damköhler’s first hypothesis is confirmed by indicating that the ratio of the turbulent burning velocity to the unstrained premixed laminar burning velocity is relatively equal to the ratio of the wrinkled to the unwrinkled flame surface area. Furthermore, the probability density functions of the density-weighted flame displacement speed show that the bulk of flame elements propagate identical to the unstrained premixed laminar flame.

Computational assessment of effects of throat diameter on combustion and turbulence characteristics in a pre-chamber engine

Towards fundamental investigation of key physical aspects of pre-chamber combustion, the current work utilizes computational fluid dynamics to comprehend the effect of the throat diameter in an engine operated with methane. Previous studies showed that this parameter is dominant in pressure build-up and flow pattern inside the pre-chamber, suggesting that a detailed characterization is necessary. This pre-chamber type is composed of an upper conical part that lodges the spark plug and fuel injector, followed by a straight and tubular region called throat, which tip accommodates the nozzles responsible for the charge exchange between pre and main chambers. Two types of pre-chamber having distinct throat diameters are investigated, while utilizing consistent experimental data for validation of the model. The combustion process is modeled with the G-Equation model; the laminar flame speed was tabulated from a methane oxidation mechanism reduced from the GRI 3.0; the turbulent flame speed was computed using Peters' relation. The simulations were run for a full cycle, starting at exhaust valve opening. A homogeneous charge of methane is considered at the intake port, maintaining a global λ = 1.8, while 3% of total energy fuel is added through the pre-chamber. The results show that the throat changes the flow field inside the pre-chamber, impacts the air-fuel ratio, stratification, turbulence, jet dynamics, and ultimately the pre and main chambers combustion processes and heat fluxes. The combustion regime according to the Borghi-Peters diagram were found to lay in the thin reaction zone and in the flamelet regime.

Assessment of Damköhler's hypotheses in the thin reaction zone regime using multi-step chemistry direct numerical simulations of statistically planar turbulent premixed flames

The effects of the definition of the reaction progress variable and equivalence ratio on the validity of Damköhler's hypotheses for turbulent premixed flames belonging to the thin reaction zone regime have been studied using multi-step chemistry direct numerical simulations of statistically planar CH4–air premixed flames with equivalence ratios of 0.8 and 1.0. Although CH4–air premixed flames with equivalence ratios of 0.8 and 1.0 have effective Lewis numbers close to unity, local differential diffusion effects can play a non-negligible role in determining the turbulent burning velocity and flame surface area in all cases. However, the augmentations of burning rate and flame surface area under turbulence do not occur in equal proportion, but their ratio remains of the order of unity. This conclusion holds irrespective of the definition of the reaction progress variable for the cases considered here. Damköhler's second hypothesis, which relates the ratio of turbulent burning velocity and the unstretched laminar burning velocity to the ratio of turbulent diffusivity and molecular diffusivity, has been found not to hold in the sense of equality, but it is valid in an order of magnitude sense for all choices of reaction progress variable definition. The findings of the current analysis indicate that Damköhler's first and second hypotheses should only be interpreted in an order of magnitude sense in the thin reaction zone regime even when the effective Lewis number remains close to unity.

A priori assessment of flame surface density modelling for large-eddy simulation of sound generation by turbulent premixed flames

This paper presents an a priori assessment of flame surface density modelling in the context of large-eddy simulation (LES), with a focus on flame-generated sound. A direct numerical simulation (DNS) dataset of a stoichiometric premixed, methane-air, round jet flame in the thin reaction zones (TRZ) regime is used to perform this assessment. Models for different parts of the filtered flame front displacement term (ρSd)|∇c|¯ are studied, including those for the surface averaged density weighted displacement speed (ρSd)¯s and flame wrinkling factor models proposed by Charlette at al. (2002), Fureby (2005) and Muppala et al. (2005).In the first step, the wrinkling factor, the flame surface density and their conditional averages with the filtered progress variable are examined. Locally, large deviations of the modelled wrinkling factor from the DNS data are observed, especially near flame annihilation events. Nevertheless, the conditionally averaged flame surface density is predicted well by the Charlette and Fureby models.In the second step, Lighthill’s acoustic analogy is used to assess the models when the time derivative of the filtered flame front displacement term is used as a source. When the wrinkling factor is calculated with the models and (ρSd)¯s is obtained from the DNS, a reasonable agreement is achieved for the unsteady pressure and sound pressure level (SPL) of the generated noise. However, when the exact (ρSd)¯s is replaced with simple models, a significant underprediction of SPL is observed. These results suggest that flame surface density modelling requires significant improvement, particularly for (ρSd)¯s. Furthermore, special attention needs to be paid to flame annihilation events, which feature large variation of the wrinkling factor and flame displacement speed.

Modeling of turbulent deflagration behaviors of premixed hydrogen-air in closed space with obstacles

The propagation of premixed hydrogen-air deflagration flames in a closed duct with different shapes of obstacles was investigated using large eddy simulation (LES). The turbulent flame wrinkling factor in the LES subgrid turbulent combustion model is dynamically modeled based on Charlette's power-law model. The LES results obtained by the dynamic flame surface density (DFSD) model can accurately match the experimental data quantitatively and qualitatively. Numerical results show that the triangular obstacle induces a higher peak overpressure, 7% and 30% higher than that in the square and circle, respectively. The formation of juxtaposed tulip flames is discovered, and the topological analysis of the velocity vector field reveals that the vortex at the tail of the obstacle is the main inducing factor for its formation. Additionally, the Karlovitz number is used to quantify the degree of turbulence-flame interaction, and the transition of deflagration flame from "wrinkled flame" to "thin reaction zone" is observed. The research helps to understand the mechanism of deflagration flame propagation induced by obstacles and provides critical information for safety planning and explosion protection.

Rate-Ratio Asymptotic Analysis of Strained Premixed Laminar Methane Flame Under Nonadiabatic Conditions

ABSTRACT Motivated by the pioneering activation-energy asymptotic analysis of strained laminar premixed flames in counterflow by Libby and his coworkers, a rate-ratio asymptotic analysis is carried out to elucidate the structure and predict the critical conditions of extinction of strained premixed methane flames. Steady, axisymmetric, laminar flow of two counterflowing streams: a reactive mixture stream and a product stream toward a stagnation plane is considered. The temperature of the reactive mixture stream is and it is made up of methane (CH4), oxygen (O2) and nitrogen (N2), while the temperature of the product stream is , and it is made up of O2, carbon dioxide, water vapor and N2. The asymptotic flame structure is presumed to be made up of a thin reaction zone where all chemical reactions take place. On one side of the reaction-zone is an inert, preheat zone containing the reactants and on the other side a post-flame zone made up of products. Analysis of the preheat zone gives matching conditions that is required to analyze the structure of the reaction zone. A four-step, reduced mechanism is used to describe the chemical reactions. The reaction zone is presumed to be made up of an inner layer, where CH4 is consumed. The hydrogen (H2) and carbon monoxide that are formed in this layer are consumed in an oxidation layer that is made up of two layers: an H2-oxidation layer and a CO-oxidation layer. The results of the analysis are used to predict the flame location, , flame temperature, , and the speed of the convective flow, , in the reaction zone as a function of the strain-rate, . Classical C-shaped curves were obtained when , and are plotted as a function of and they were used to predict extinction. A key finding of this work is that is proportional to , where is the crossover temperature predicted by the rate-ratio asymptotic analysis. Whether abrupt extinction will take place or not was found to depend on the value of relative to , which is different from the predictions of activation-energy asymptotic analysis where must be compared with the value of the adiabatic temperature. Similar to the analysis of Libby and his coworkers, the rate-ratio asymptotic analysis predicts the existence of “negative flame speeds,” where the convective flow and the diffusive flow of reactants in the reaction zone, are in opposite directions. The predictions of the rate-ratio asymptotic analysis were found to agree with the results of computations with detailed chemistry and previous experimental data.

Structure and scalar correlation of ammonia/air turbulent premixed flames in the distributed reaction zone regime

Instantaneous structures of turbulent premixed ammonia/air flames on a piloted jet burner were investigated using simultaneous planar laser-induced fluorescence (PLIF) and Rayleigh thermometry measurements. Two-dimensional spatial distributions of temperature were simultaneously measured with those of the NH radical and the NO pollutant, respectively. Experiments were conducted for stoichiometric flames under five jet velocities. All flames are located in the distributed reaction zone regime of the Borghi-Peters diagram with the Karlovitz (Ka) number ranging from 274 to 4720. The NH PLIF images are used to characterize the fuel consumption layer of the reaction zones since the formation of NH is associated with the consumption of ammonia. The NH PLIF results show that under all flame conditions investigated, the NH layer remains thin in the proximity of the burner while it becomes progressively thickened and distorted by turbulence with increasing turbulent intensity and axial distance. For flames with Ka of 274, the NH layer essentially remains thin, while at Ka of 590 or higher, significant broadening of the NH layer is observed. Probability density functions (PDFs) of the NH layer thickness show that the NH layer can be broadened by 3 – 4 times as the flames are developed downstream. The broadening of the NH layer is considered to indicate that the flames are in the distributed reaction zone regime. The boundary between the thin-reaction zone regime and the distributed reaction zone regime occurs at a much larger Ka than that in methane/air flames. The broadening of the NH layer is due to the penetration of the turbulent eddies and the merging of flame branches. The latter occurs mainly near the flame tips. NO is shown to exist in a wide region in space, across the preheat zone, reaction zone, and postflame zone. NO formation occurs mainly in the reaction zone, however, it is transported by turbulence eddies to the preheat zone and by convection to the postflame zone. The temperature measurements indicate that the preheat zone is broadened in all flames investigated. The broadening of the preheat zone is moderately sensitive to the Ka number while it is more sensitive to the integral length scale of the flames.

Cu10Sn to Ti6Al4V bonding mechanisms in laser-based powder bed fusion multiple material additive manufacturing with different build strategies

Additive manufacturing of titanium alloy (TiA) – copper alloy (CuA) multiple material components poses many challenges due to the significant differences in their material properties. No previous studies have been found to investigate bonding characteristics of TiA-CuA interface fabricated in the build order of TiA→CuA (i.e. TiA first) via laser powder bed fusion ( L -PBF). Here we show the interface characteristics of TiA→CuA under different build strategies. We have found that direct bonding causes CuA to delaminate from TiA due to the thin Ti-Cu reaction zone (< 10 µm). Remelting of the interface zone and a few CuA layers close to the interface zone makes the CuA melt pool at the interface to enter a keyhole mode to achieve more Ti-Cu mixing, leading to sound bonding between the materials. The functionally graded material (FGM) causes Ti-Cu brittle phases with a hardness > 1000 Hv and leads to cracks and the separation of CuA part from preprinted TiA part. The bonding mechanisms involved in the different scenarios are proposed.

Melt–rock interactions in a veined mantle: pyroxenite–peridotite reaction experiments at 2 GPa

Abstract. Interaction between peridotite and pyroxenite-derived melts can significantly modify the mineralogy and chemistry of the upper mantle, enhancing its heterogeneity, by creating re-fertilized peridotites and secondary-type pyroxenites. We experimentally investigated the reaction between a fertile lherzolite and MgO-rich basaltic andesite produced by partial melting of an olivine-free pyroxenite at 2 GPa and 1300–1450 ∘C. The aim was to constrain the rate and style of melt–peridotite reaction mostly as a function of temperature, i.e. assuming variable physical status of the host peridotite. Experiments juxtaposed pyroxenite on a synthesized fertile lherzolite to evaluate the modal and mineral compositional changes in the fertile lherzolite resulting from the reaction with pyroxenite-derived melt. At 1300 and 1350 ∘C, the reaction produces a thin orthopyroxene-rich reaction zone confined between partially molten pyroxenite and modally unmodified subsolidus lherzolite. Chemical changes in minerals of the pyroxenite crystal mush suggest that element diffusion across the pyroxenite–peridotite interface, coupled with orthopyroxene precipitation, plays a role in the reactive crystallization of mantle pyroxenite veins. At 1380 and 1400 ∘C, infiltration of pyroxenite-derived melt significantly modifies the mineralogy and chemistry of the host peridotite by creating orthopyroxene-rich websterites and pyroxene-rich lherzolite. At 1450 ∘C, pyroxenitic melt fluxes into molten peridotite, enhancing peridotite melting and creating a melt-bearing dunite associated with a refractory harzburgite. At a given pressure, bulk compositions of hybrid rocks originating through melt–peridotite interaction are mostly controlled by the chemistry of the reacting melt. Interaction between pyroxenitic melt and peridotite causes XMg[XMg=Mg/(Mg+Fetot)] and XCr[XCr=Cr/(Cr+Al)] decrease and TiO2 increase in pyroxenes and spinel across the pyroxenite–peridotite boundary. Similar chemical gradients in minerals are observed in pyroxenite–peridotite associations from natural mantle sequences. The comparison with mineral chemistry variations derived by reaction experiments potentially represents a petrologic tool to discriminate between low- versus high-temperature melt–peridotite reactions.

Multiscale analysis of turbulence-flame interaction based on measurements in premixed flames

Multi-scale analysis of turbulence-flame interaction is performed using experimental data sets from three methane- and propane-fired premixed, turbulent V-flames, at an approach flow turbulent Reynolds number of 450 and a ratio of r.m.s. fluctuating velocity from the mean to laminar flame speed of between 2.1 and 3.0, straddling the border between corrugated flamelets and thin reaction zone in the Borghi-Peters diagram. The measurements were made in the plane of a single laser sheet using stereo particle image velocimetry SPIV and planar laser-induced fluorescence to measure three orthogonal components of velocity and flame OH. Methods to approximate the remaining, unmeasured, out of plane derivatives are described. The instantaneous SPIV images were bandpass filtered at user-specified characteristic length scales Lω and Ls (for vorticity and strain rate, respectively) resulting in instantaneous bandpass-filtered velocity fields, u̲bLω and u̲bLs, which were further analysed to give the bandpass filtered vorticity field, ω̲Lω=∇×u̲bLω, the strain-rate field, eijLs, and the tangential strain rate field aTLs. This work quantifies two aspects of turbulence-flame interaction. The first aspect is that of the flame interaction of eddies of size Ls on the turbulence, as found by the statistics of the alignment of vorticity with strain rate. We find that vortical eddies with scale about Lω=2δth (where δth is the flame thickness) are stretched by Ls structures which are larger than about 2 Lω, with this factor broadly true also for vortical eddies of scales Lω=4δth and Lω=6δth. Within the limitations of the data set, these findings are consistent with those in the literature on reacting and non-reacting flows, suggesting that the premixed flame has had little influence on the vortex stretching mechanism. The second aspect of turbulence-flame interaction examined is that of flame surface-averaged tangential strain rate imparted by eddies. Eddies with length scales Ls smaller than 3 or 4δth are likely to have the strongest individual contribution but eddies of this length scale and smaller may contribute only about 1/5th of the total tangential strain rate. This is to be compared with the value of 10% that has been reported in the literature based on analysis of DNS predictions of premixed flames at turbulent Reynolds numbers up to 110. Eddies with length scale Ls larger than about 20δth contribute a negligible amount to the total tangential strain rate. We have found no evidence that the Lewis number up to about 1.8 has an observable effect, but this may reflect the limitations of the current experiment. In the context of large eddy simulations (LES) of premixed combustion, these results are preliminary experimental evidence supporting the suggestion that resolving turbulence scales down to a few multiples of δth might be adequate to capture much of the flame straining caused by turbulence.

Turbulence Intensity and Length Scale Effects on Premixed Turbulent Flame Propagation

The effects of varying turbulence intensity and turbulence length scale on premixed turbulent flame propagation are investigated using Direct Numerical Simulation (DNS). The DNS dataset contains the results of a set of turbulent flame simulations based on separate and systematic changes in either turbulence intensity or turbulence integral length scale while keeping all other parameters constant. All flames considered are in the thin reaction zones regime. Several aspects of flame behaviour are analysed and compared, either by varying the turbulence intensity at constant integral length scale, or by varying the integral length scale at constant turbulence intensity. The turbulent flame speed is found to increase with increasing turbulence intensity and also with increasing integral length scale. Changes in the turbulent flame speed are generally accounted for by changes in the flame surface area, but some deviation is observed at high values of turbulence intensity. The probability density functions (pdfs) of tangential strain rate and mean flame curvature are found to broaden with increasing turbulence intensity and also with decreasing integral length scale. The response of the correlation between tangential strain rate and mean flame curvature is also investigated. The statistics of displacement speed and its components are analysed, and the findings indicate that changes in response to decreasing integral length scale are broadly similar to those observed for increasing turbulence intensity, although there are some interesting differences. These findings serve to improve current understanding of the role of turbulence length scales in flame propagation.

Enhanced burning rates in hydrogen-enriched turbulent premixed flames by diffusion of molecular and atomic hydrogen

Direct numerical simulations (DNS) of fuel-lean turbulent premixed hydrogen-enriched flames are analyzed to improve understanding of local effects of molecular diffusion of fast-diffusing, low-Lewis-number hydrogen species (H2 and H) on the overall burning rates of reactant mixtures. Although it is often assumed that the importance of molecular diffusion decreases with increasing turbulence intensity, our analysis reveals that, on the contrary, diffusion of molecular and atomic hydrogen can remain the rate-controlling processes even at high Karlovitz numbers (Ka≫1). Three-dimensional DNS with detailed chemical kinetics and species transport of turbulent premixed hydrogen-air and hydrogen/ammonia-air flames in the regimes of thin and distributed reaction zones are performed and analyzed at different reactant temperatures and pressure levels. The DNS data reveal a significant impact of H2 and H diffusion in all flames analyzed. In particular, the magnitude of H2 diffusion, occurring from low progress variable near the flame elements of the turbulent reaction front that exhibit convex (positive) curvature towards the fresh mixture, is greatly enhanced at high pressure through increased spatial gradients, resulting in localized equivalence-ratio enrichment and super-adiabatic conditions that accelerate and strengthen the flame front. Moreover, for higher reactant temperatures, back diffusion of atomic hydrogen, transported from the reaction zone into unburnt regions from negatively curved reaction layers surrounding them, facilitates the occurrence of localized spontaneous ignition events at the trailing edges of reaction fronts. For certain conditions these events, induced by differential diffusion, can become the dominant process controlling the overall rate of reactants consumption.

Morphology and structure of spherically propagating premixed turbulent hydrogen - air flames

Three-dimensional direct numerical simulations of spherically propagating premixed turbulent stoichiometric hydrogen-air flames with detailed chemistry and detailed diffusion are employed to clarify the influence of turbulence-flame interactions with respect to flame structure and morphology. Four cases are considered within the corrugated flamelets and the thin reaction zone regimes. The most significant fuel consumption and heat release rates occur at the negatively-curved flamelets. Furthermore, the increments in burning velocity are lower than the increments in flame surface area, which is due to the reduction in the local burning intensity at the positively-curved flamelets. The morphology of intense reaction zones is quantified using Minkowski functionals and their shapes include ”tubes”, ”pancakes” and more complex shapes, which are compared to their counterparts in planar flames. As turbulence level increases, the number of locally defined intense reaction zones increases, and their boundaries expand to cover more extensive parts of the flame front. However, intense reaction zones’ geometrical dimensions do not significantly differ for each flame as it propagates. Local turbulence properties are obtained for each intense reaction zone. The conditional averages of local Taylor microscale and local Kolmogorov scale, conditioned based on the shapefinders, are investigated. The conditional averages of the local Taylor microscale scale correlate with the planarity and filamentarity of intense reaction zones. Therefore, turbulent motions at Taylor microscale size have a significant role in characterizing turbulence-flame interactions relevant to flame morphology and relevant to the local flame thickness and reaction layers of developing-flames. On the other hand, local Kolmogorov scales’ turbulent motions show weaker or no such correlations. There is a dissidence between global turbulence statistics and local ones representing the interactions at the flame front. Local turbulence-flame interactions of Taylor microscale sizes occur at specific length scales, depending on the flame size and irrelevant of eddies with other length scales.

The explicit filtering method for large eddy simulations of a turbulent premixed flame

This paper reports large eddy simulations (LES) of a premixed turbulent round methane-air jet flame at an equivalence ratio of 0.8 and unburnt gas temperature of 800 K using the explicit filtering LES (EFLES) method. The nominal 1D premixed flame speed, sL=2.05ms−1 and thermal thickness, δL=300μm. EFLES is formally derived from the approximate deconvolution method for LES computations. The nominal turbulent Reynolds number Ret=34 and the turbulent Karlovitz number Ka=25 places the present flame in the thin reaction zones regime of turbulent premixed combustion. LES and DNS results are obtained by solving species transport equations using a reduced 13-species chemical mechanism. Chemical source terms in the LES are computed using resolved species and temperature fields in all simulations. Conditional means of net heat release rate predicted by the LES and DNS show excellent agreement. The LES results show a marginally thicker flame brush and a shorter mean flame height when compared to the DNS results. Good agreement between LES and DNS for mean quantities is seen upstream of the tip. Larger quantitative differences are seen at the tip due to the difference in flame height between LES and DNS cases, as may be expected. Even at these positions, qualitative features of the DNS results and peak mean values are recovered correctly. Agreement between conditional mean statistics from both LES and DNS is good. RMS fluctuation amplitudes show larger quantitative differences between LES and DNS when compared to mean statistics while still capturing the correct qualitative features of the DNS profiles. The radial profiles of unconditioned RMS fluctuations are broader by O(δL) in the LES results, with increasing downstream distance. The smaller range of resolved scales in the LES causes the resolved flow motions to be more energetic resulting in increased turbulent transport rates for mean and RMS quantities. This results in the observed broadening of flow statistics in the LES when compared to the DNS. With mesh refinement, differences between LES and DNS results for all statistics are found to diminish smoothly.

Direct Numerical Simulations of high Karlovitz number premixed flames for the analysis and modeling of the displacement speed.

An a priori model for premixed turbulent flame combustion in the thin reaction zone (TRZ) regime is presented. This a priori model is deduced from the analysis of data from a series of direct numerical simulations (DNS) of stoichiometric (ϕ=1.0) premixed turbulent iso-octane flames, with Karlovitz number ranging from 2.9 to 46.2. For each case two flames are considered: one with unity Lewis numbers to isolate the effect of turbulence on the flame, and one with non-unity Lewis numbers to study the influence of differential diffusion. First the reaction zone is shown to remain thin for each flame, leading to focus this study on a specific iso-surface in the reaction zone and how it is affected by turbulence. Second, the displacement speed Sd on this iso-surface shows a differentiate dependency on tangential strain rate and curvature. This dependency is modeled through an expression of Sd formally similar to the ones used in laminar flame theories, but using two effective turbulent Markstein lengths in place of the laminar ones. These lengths are shown to depend on the Lewis number and to decrease when the Karlovitz number increases, in agreement with previous studies showing a reduction of the effective Lewis number with the Karlovitz. From these DNS, an extension of the coherent flame model (CFM) to the TRZ regime is proposed, using a fine-grained flame surface density (FSD) located in the reaction zone. Models for the displacement speed, the tangential strain rate, and the stretch due to curvature are proposed. The a priori evaluation of these closures shows a significant improvement compared to the flamelet formulations.

난류생성기 판 형상에 의한 난류화염 특성 연구

The cross-shaped fractal grids which have RRBT(reduced ratio of bar thickness) and the hexagonal grid were used to generate various turbulent flow properties for low swirl flames. In the non-reacting flow field with the fixed bulk velocity at 10 m/s, decreasing RRBT induces the increasing turbulent intensity(ISUBU/SUB) with the delay in the location of lowest turbulent mean velocity(Ū) and highest turbulent fluctuation(ú) due to vortex shedding from the bar thickness. This phenomenon was well explained by a wake interaction theory using axial velocity profiles at the centerline. At various equivalence ratios for Hexa. and fractal grids, the characteristics of low swirl flames could be identified to a thin reaction zone regime in the Borghi – Peter regime diagram. Decreasing RRBT increases ISUBU/SUB in front of flame brush with increasing turbulent local displacement speeds (SSUBT,LD/SUB) via a linear correlation. The turbulent Reynolds number, ReSUBT/SUB composed of the ratio of length scale and velocity components induced the proportional trends in non linear relationship showing that the influence of the length scale ratio. The empirical correlation study of SSUBT,LD/SUB as a function of ReSUBT/SUB and ú/SSUP0/SUPSUBL/SUB was investigated to compare the influence of velocity and turbulent length scales ratio. The result of linear correlation equation with SSUBT,LD/SUB indicates the influence of velocity factors greater than ReSUBT/SUB through the exponential factor.

Numerical Investigation of the Turbulent Flame Propagation in Dual Fuel Engines by Means of Large Eddy Simulation

Dual fuel combustion depicts a possible alternative to reduce emissions from large engines and is characterized by injecting a small amount of diesel fuel into a lean natural gas–air mixture. Thereby, the presence of autoignition, diffusive and premixed combustion determine the high complexity of this process. In this work, an Extended Coherent Flame Model was adapted to consider the effect of natural gas on the ignition delay time. This model was afterward utilized to simulate 25 consecutive engine cycles employing LES. In this framework, the ensemble-average flow field was compared to a RANS solution to assess the advantages of LES in terms of the prediction of the in-cylinder flow field. A detailed investigation of the heat release characteristic showed that natural gas already highly contributes to the heat release at the beginning of combustion. Furthermore, a methodology to investigate the turbulent combustion regimes was utilized. It could be ascertained that the combustion mainly occurs in the regime of thin reaction zones. Possible triggers of cycle-to-cycle variations were determined in the velocity fluctuations in the cylinder axis direction and the flame formation in the gaps between the spray plume. The findings support the understanding of dual fuel combustion and serve as a basis for developing future combustion models.

Scaling of reaction progress variable variance in highly turbulent reaction waves

Self-propagation of a reaction wave, which consists of an infinitely thin reaction zone (front) and a thick inert mixing layer adjacent to the front, in constant-density statistically stationary, homogeneous isotropic turbulence unaffected by the wave is analytically studied. In the asymptotic case of a high turbulent Reynolds number, high Karlovitz number, and low Damköhler number Da, the scalar variance c′2¯ is shown to be proportional to Da for the statistically stationary stage of the wave evolution. This scaling is supported by newly analyzed Direct Numerical Simulation data discussed in detail by Sabelnikov et al. [“Thin reaction zones in constant-density turbulent flows at low Damköhler numbers: Theory and simulations,” Phys. Fluids 31, 055104 (2019)]. The obtained analytical results also show that, under conditions of the present study, spatial gradients of reactant concentration non-uniformities due to the reaction and spatial gradients of reactant concentration non-uniformities due to the turbulence are of the same order of magnitude. Accordingly, major statistical characteristics of the scalar field c(x,t), such as the mean area of an iso-scalar surface c(x,t)=const, the mean molecular flux through this surface, etc., can be found adopting results known in the theory of inert and passive turbulent mixing. Nevertheless, the reaction indirectly affects these characteristics by controlling the mean thickness of the reaction wave and, consequently, the spatial gradient of the mean reaction progress variable.

An LES-DFSD study of transient premixed propane/air flames propagating past obstacles

Simulation of deflagrations requires accurate modelling of transient premixed flames. An initial laminar flame kernel is subjected to progressive stretch by obstacle generated turbulence as it propagates downstream. Large eddy simulation (LES) based flame surface density models may encounter difficulties when a predefined model constant is used for given turbulence characteristics. The reliable description of flame wrinkling considerably determines the transient flame behaviour. In this work, we applied a dynamic flame surface density (DFSD) model to automatically adjust the model parameter based on the instantaneous resolved flame information, whilst recovering laminar flame propagation in the absence of sub-grid turbulence. In the work we have successfully verified the LES-DFSD approach against recent experimental data of stoichiometric propane/air deflagrating flames past obstacles. The overpressure, flame front speed and other flame features have been correctly reproduced by the model at various stages of flame propagation for distinct obstacle configurations. The wrinkling factor was identified as an informative parameter to understand the flame dynamics and pressure build-up mechanisms, and the significant contribution from the sub-grid flame surfaces have been demonstrated. By constructing the combustion regime diagram for LES, we have demonstrated that the flame wrinkling is fully resolved during the early propagation, and the flame lies in the thin reaction zone regime when the pressure peak is reached. Through comprehensive parametric studies, we found that ignition modelling has a considerable impact on the time taken to reach the peak pressure yet a much weaker effect on the peak magnitude. A negative correlation was identified between the Smagorinsky constant and the maximum pressure. Early-stage overpressure and flame evolutions were confirmed to be grid-independent, while the pressure peak is slightly mesh-sensitive. It was found that the magnitude of the dynamic parameter adequately self-adjusts to varying grid and filter widths. The satisfactory agreements with experiments and the robust findings ensured by the sensitivity studies indicate the predictive capabilities and the benefits of the LES-DFSD model.

Efficient emission modelling in lean premixed flames with pre-tabulated formation characteristics

It is practically important to accurately predict NOx and CO emissions in lean premixed flames for the development of fuel-efficient and low-emission combustion systems. In this study, the efficient modelling with pre-tabulated formation characteristics of NOx and CO is systemically investigated. Their formation characteristics for turbulent flames in the flamelet and thin reaction zone regimes are first quantified with one-dimensional adiabatic premixed flames through the incorporation of turbulence induced diffusion. Results show that the formation of NOx and CO can be characterized by two different stages, i.e., a rapid increase of both in flame front and the linear growth for NOx and exponential decay for CO in post-flame zone. An efficient NOx and CO modelling approach is formulated and demonstrated in a full-scale methane gas turbine combustor, in which species NOx and CO are transported and solved, with the source terms being modelled by pre-tabulated formation characteristics from one-dimensional premixed flames with detailed chemical kinetics. Realizable k-epsilon model and finite rate/eddy dissipation model in conjunction with a two-step global mechanism are employed to primarily predict the flow and flame characteristics. Results show that the predicted NOx and CO agree with experimental measurements over a wide range of equivalence ratios. The discrepancy in NOx emission can be accounted by fuel/air unmixedness. For CO, the rapid increase due to the incomplete CO oxidation resulting from its increasing characteristic oxidation time near lean blow-out (LBO) is correctly captured. It is further shown that the predicted CO emission is sensitive to combustion/kinetics model parameters and the accurate prediction of flame shape and dynamics is crucial to capture the trend of CO formation near LBO.