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This study presents a new coinjected reactor-assisted Bunsen (CRAB) burner for the experimental investigation of ozonolysis-assisted flame speeds at atmospheric pressure and room temperature. The C...

A novel integrated modified sulfur cycle and ammonia production process was suggested for the co-generation of sulfuric acid. Exergy analysis, heat integration, and safety assessment were conducted to investigate the feasibility and analyze the process. The exergy analysis showed that the highest exergy destruction occurred in the section with the most considerable temperature difference involved with a large flow rate. The heat integration — an economic assessment, confirmed that the total cost was estimated to be reduced by 10.9% at the minimum temperature difference of 39 °C. The failure rate contribution to the overall system was 19%, 11%, 22%, and 47% from the Bunsen section, H2SO4 concentration section, HI decomposition section, ammonia production section explosion, fire, and structural damage contributed 82%, 16%, and 2% to the overall system in terms of accident scenario. The accident cost contributed 84% and 16% of accident injury costs to the overall system, respectively. For the sectional based contribution, section 1 (Bunsen process), SA concentration, section 3, and ammonia production process contributed 45%, 29%, 19%, and 6% to the accident injury cost in the overall system, respectively. As a result of individual section failure to the whole section, failure in Bunsen process and HI decomposition led to failure in production of all the products. Failure in NH3 production section led to production in concentrated H2SO4 and H2. The failure in H2SO4 section leads to production in NH3 and diluted H2SO4 concentration. The failure in H2SO4 concentration, NH3 production, and Bunsen process and HI decomposition contributed to the higher failure rate in ascending order.

Lean premixed combustion is one of the most effective methods to constrain pollutant emissions for modern industrial gas turbines. An experimental study was performed on its propagation speed and internal structure at engine-relevant temperatures. A Bunsen burner was employed for the measurement with an optical schlieren system. The results show that the increase of preheating temperature dramatically accelerates the propagation of methane flames. The numerical results predicted by GRI-Mech 3.0, FFCM-1, and USC Mech II were also compared. The GRI-Mech 3.0 seems to overestimate the laminar flame speed at high operating conditions, while FFCM-1 underestimates the laminar flame speed compared to the present experimental data. The prediction by FFCM-1 shows good agreement with the overall existing data. The USC Mech II seems to overestimate the laminar flame speed at fuel-lean conditions while shows good agreement with present experimental measurements at stoichiometric conditions when the inlet temperature increases. It is also indicated that the flame is thinned at high-temperature conditions and the importance of CO production to the propagation speed increases. Finally, based on the experimental data, an empirical correlation of the laminar flame speed was developed in the range of Tu = 300–800 K and ϕ = 0.7–1.0, the maximum deviation of which was less than 8%. The results of this study may contribute to the optimization of advanced gas turbine combustors.

Jet fuels used in regenerative cooling are often pyrolyzed into gaseous and liquid products, which changes their burning properties. The effects of primary pyrolysis products, namely, hydrogen, methane, ethylene, and ethane, on the laminar flame speeds of n-decane/air were investigated at atmospheric pressure and 405 K using a Bunsen burner and chemical kinetics. The experimental results showed that methane inhibited the laminar flame speeds, whereas hydrogen, ethylene, and ethane promoted them. However, the laminar flame amplitude with ethane was about 40% of that with ethylene. Additionally, the enhanced flame speeds with ethylene and ethane were more evident under fuel-rich conditions than under fuel-deficient conditions. The effect of n-decane pyrolysis conversion on flame speeds can be approximated as the superposition of the separate effects of four gases within 26% conversion. Kinetic analyses indicated that laminar flame speeds are sensitive to the generation and consumption of H and OH, where the total molarity of these species determines the laminar flame speeds. The hydrogen absorption reactions of methane consumed large amounts of H and OH, and generated CH₃ enhanced this consumption. The addition of ethylene significantly increased the mole fraction of HCO, the main precursor of H generation, which then generated OH and HO₂. C₂H₅ generated by ethane produced H through a decomposition reaction, which was the major reason for the increase in laminar flame speeds of n-decane.

Flamelet models of turbulent premixed combustion assume that (a) turbulent-transport and combustion processes can be decoupled and treated independently, suggesting that preheat and reaction zones remain layer-like and do not become highly fragmented and/or distributed. By further assuming (b) that the scalar-structure (e.g. the distribution of thermochemical quantities, like species mass fraction, vs. a control variable, such as a reaction progress variable) of a turbulent flame is akin to that in an associated laminar flame, detailed chemical properties can be introduce into a simulation at low computational cost via pretabulated flamelet tables derived from laminar flame calculations. The authors previously quantified conditions when assumption (a) remains valid. The present work assesses assumption (b) for turbulent Reynolds numbers that exceed those of previous studies by ∼ 7 × . Namely, planar laser-induced fluorescence (PLIF) of formaldehyde (CH2O), hydroxyl (OH), and methylidyne (CH) acquired jointly with Rayleigh scattering are used to produce joint PDFs (scatter plots) of the former vs. a progress variable (CR) derived from the latter. Regardless of the turbulence level the piloted Bunsen flames considered here were subjected to, peak-normalized conditional mean (CM) profiles obtained from such joint PDFs agree well with profiles derived from laminar flame calculations. Additionally, within the near field of modestly turbulent flames, two-term β-PDFs are found to accurately describe CR-distributions. However, agreement between β-PDFs and CR-distributions worsens with increased turbulence and axial distance from the burner. Small discrepancies between the measured CM profiles and those obtained from laminar calculations also emerged at highly turbulent conditions. For instance, under such conditions, laminar profiles of CH over predict the measurements where 0.6

Flame particles (FP) are massless, virtual particles which follow material points on the flame surface. This work presents a tracking algorithm for FPs which utilizes barycentric coordinates. The methodology can be used with any cell shape in the computational mesh and allows computationally fast spatial interpolation as well as efficient determination of the intersection of FP trajectories with iso-surfaces. In contrast to previous flame particle tracking (FPT) approaches, the code is fully parallelized and can therefore be used in-situ during the simulation. It also includes fully parallelized computation of flame consumption speed by integrating reaction rates along a line normal to the flame surface at each FP position. Direct numerical simulations of laminar pulsating premixed hydrogen–air Bunsen flames serve as validation cases and showcase the added value of tracking material points for studying local flame dynamics. Exciting the inlet flow harmonically with frequencies equal to the inverse flame time scale leads to a pulsating mode where the flame front is corrugated. Ten times higher frequencies nearly resemble the steady state solution. The FPs are seeded along the flame surface and are used to track the unsteady diffusive, convective and chemical contributions at arbitrary points on the flame front over time. Their trajectories reveal a phase shift between the unsteady flame stretch rate and local flame speed of the order of 0.1 flame time scales for rich hydrogen flames. This is caused by a time delay between straining and stretch due to curvature. The reason is that diffusive processes follow the time signal of curvature while chemical processes are most strongly affected by the straining rate, which dominates the high Lewis number hydrogen flames investigated. This time history effect may help to explain the large scattering in the correlation of local flame speed with flame stretch found in turbulent flames.

This paper demonstrates the ability of neural networks to reliably learn the nonlinear flame response of a laminar premixed flame, while carrying out only one unsteady CFD simulation. The system is excited with a broadband, low-pass filtered velocity signal that exhibits a uniform distribution of amplitudes within a predetermined range. The obtained time series of flow velocity upstream of the flame and heat release rate fluctuations are used to train the nonlinear model using a multi-layer perceptron. Several models with varying hyperparameters are trained and the dropout strategy is used as a regularizer to avoid overfitting. The best performing model is subsequently used to compute the flame describing function (FDF) using mono-frequent excitations. In addition to accurately predicting the FDF, the trained neural network model also captures the presence of higher harmonics in the flame response. As a result, when coupled with an acoustic solver, the obtained neural network model is better suited than a classical FDF model to predict limit cycle oscillations characterized by more than one frequency. The latter is demonstrated in the final part of the present study. We show that the RMS value of the predicted acoustic oscillations, together with the associated dominant frequencies are in excellent agreement with CFD reference data.

Internal structure of extremely turbulent flames is investigated experimentally using simultaneous planar laser-induced fluorescence of formaldehyde molecule and hydroxyl radical as well as stereoscopic particle image velocimetry. The mean bulk flow velocity is changed from 5 to 35 m/s. The fuel-air equivalence ratio is 0.7 for all tested conditions. Three different turbulence generating mechanisms leading to a wide range of turbulence intensity with the corresponding Reynolds and Karlovitz numbers ranging from 19 to 2729 and 0.3 to 76.0, respectively, are examined. Preheat and reaction zone thicknesses for the tested flames are calculated and compared with those of the laminar flame to quantify deviation from flamelet behavior. It is shown that the preheat and reaction zone thicknesses increase to values that are about 6.2 and 3.9 times the laminar flame counterparts, respectively. While broadening of the preheat zone is reported in the literature, broadening of the reaction zone for a relatively large diameter Bunsen burner is reported for the first time in this study. The broadening of the reaction zones is related to non-flamelet behavior of the tested flames, and a parameter proposed earlier by the authors is used to quantify this non-flamelet behavior. Swirling strength contours are overlaid on the cold reactants and preheat zones to study the reason for the reported broadening. The results show that positive correlations exist between the preheat/reaction zone thicknesses and the mean value of eddy swirling strength inside the reactants and preheat zone. These correlations as well as the estimated turbulent kinetic energy of these eddies suggest that energetic eddies may potentially penetrate into the preheat and reaction zones causing broadening of these zones.

Interaction of combustible particles with flames occurs in fire scenarios. In this study, numerical investigation of interaction of coal dust or micron-sized particles with lean premixed methane-air flames is presented. A two-dimensional axisymmetric domain is employed to simulate conical premixed flames from lab-scale Bunsen burner. A chemical kinetic mechanism having 25 species and 121 elementary reactions, temperature dependent thermo-physical properties, multi-component diffusion with Soret effect and radiation model accounting for gas and soot radiation are used. Discrete Phase Model (DPM) is used to simulate the transport of coal particles. Coal particles in varies size ranges and concentrations are injected into the premixed reactant mixture at equivalence ratio between 0.75 and 0.85. Multiple species from devolatilization of coal particles are considered to enter the gas-phase. Laminar flame speeds are predicted using numerical shadowgraphs and validated against the experimental data from literature. Injection of coal particles affects the laminar burning velocity and flame structure. The numerical model is able to predict the variation trends in the laminar flame speed data quite reasonably. A detailed analysis of injection of coal particles on the resultant flame dynamics are presented using the fields of temperature, flow, species, net reaction rate, heat release rate and DPM.

Large eddy simulations (LES) have been carried out to calculate turbulent flame propagation in a fan-stirred combustion bomb and a Bunsen-type burner. Objective of the work is to reveal the main mechanism of increased flame wrinkling due to elevated pressure and to assess the ability of the turbulent flame-speed closure (TFC-class) combustion model to reproduce the enhancement of flame wrinkling or burning rate at elevated pressures. The simulations have been performed for a premixed methane/air mixture at equivalence ratio 0.9 and the pressure has been varied from 1 bar to 5 bar. The turbulent kinetic energy is found to increase with pressure in the high frequency range, indicating reinforced small-scale turbulent fluctuations at elevated pressure. The reason is attributed to the increased turbulent Reynolds number with pressure, which shifts the turbulent energy spectra to the higher wave number range. A reinforced flame wrinkling and an increased total burning rate are obtained at elevated pressure, which is in accordance with results from previous high-pressure combustion experiments. In addition, applying the same method to a quiescent flow in the bomb vessel reveals a decrease of the overall burning rate with pressure, which agrees with the behaviour of laminar flame speed at elevated pressures. Therefore, the beneficial effect of increased burning rate or flame wrinkling at elevated pressure can be explained by the enhanced small-scale turbulent fluctuations along with formation of small-scale vortices and flame structures, which over-compensate the reduced local laminar burning velocity at high pressures. The calculated amplification rates of flame wrinkling factor at increased pressures show a reasonable agreement with measured data for both fan-stirred bomb and Bunsen flame configurations, without using any additional adjusting parameters for considering the pressure effect. The results justify the applicability of the current TFC-LES method for high-pressure combustion processes.

The implications of elevated pressure on the statistical behavior of the flame surface density (FSD) transport statistics together with the behavior of selected established sub-models of the unclos...

The statistical behaviour and modelling of the sub-grid variance of reaction progress variable have been analysed based on a priori analysis of direct numerical simulation (DNS) data of turbulent premixed Bunsen burner flames at different pressure levels. An algebraic expression for sub-grid variance, which can be derived based on a presumed bi-modal sub-grid distribution of reaction progress variable with impulses at unburned reactants and fully burned products, has been found to be inadequate for the purpose of prediction of sub-grid variance even for the flames in the wrinkled flamelets/corrugated flamelets regime. Moreover, an algebraic model, which is often used for modelling sub-grid variance of passive scalars, has been found to significantly overpredict the sub-grid variance of reaction progress variable for all the cases considered here. The modelling of the unclosed terms of the sub-grid variance transport equation has been analysed in detail. Suitable model expressions have been identified for the sub-grid flux of variance, reaction rate contribution and scalar dissipation rate based on a priori analysis of DNS data. It has been found that the alternation of pressure does not have any significant impact on the closures of sub-grid flux of variance but a model parameter for the Favre-filtered scalar dissipation rate needs to be modified to account for the variation of pressure.

Performance of representative algebraic flame surface density (FSD) models have been a-priori assessed based on a direct numerical simulation database consisting of four turbulent premixed Bunsen flames at four different pressure levels. Results indicate that for a given resolution of the flame front, the considered algebraic FSD closures perform in a qualitatively similar manner irrespective of pressure variation. However, for a given computational mesh, the performance deteriorates as flame thickness decreases with increasing pressure. Additionally, the contribution of the subgrid scale surface-filtered density-weighted tangential diffusion component of the displacement speed tends to increase with pressure increment, further complicating the modelling of FSD. Further analysis also indicate that the inner cut-off scale normalized by the thermal flame thickness increases with increasing pressure, possibly due to the presence of Darrieus–Landau instability which is more likely to occur at higher pressure due to a larger ratio of hydrodynamic length scale to thermal flame thickness.

We present ultrabroadband two-beam femtosecond/picosecond coherent Raman spectroscopy on the ro-vibrational spectra of CO2 and O2, applied for multispecies thermometry and relative concentration measurements in a standard laminar premixed hydrocarbon flame. The experimental system employs fs-laser-induced filamentation to generate the compressed supercontinuum in-situ, resulting in a ∼24 fs full-width-at-half-maximum pump/Stokes pulse with sufficient bandwidth to excite all the ro-vibrational Raman transitions up to 1600 cm-1. We report the simultaneous recording of the ro-vibrational CO2 Q-branch and the ro-vibrational O2 O-, Q- and S-branch coherent Stokes Raman spectra (CSRS) on the basis of a single-laser-shot. The use of filamentation as the supercontinuum generation mechanism has the advantage of greatly simplifying the experimental setup, as it avoids the use of hollow-core fibres and chirped mirrors to deliver a near-transform-limited ultrabroadband pulse at the measurement location. Time-domain models for the ro-vibrational Q-branch spectrum of CO2 and the ro-vibrational O-, Q- and S-branch spectra of O2 were developed. The modelling of the CO2 Q-branch spectrum accounts for up to 180 vibrational bands and for their interaction in Fermi polyads, and is based on recently available, comprehensive calculations of the vibrational transition dipole moments of the CO2 molecule: the availability of spectroscopic data for these many vibrational bands is crucial to model the high-temperature spectra acquired in the flue gases of hydrocarbon flames, where the temperature can exceed 2000 K. The numerical code was employed to evaluate the CSRS spectra acquired in the products of a laminar premixed methane/air flame provided on a Bunsen burner, for varying equivalence ratio in the range 0.6–1.05. The performance of the CO2 spectral model is assessed by extracting temperatures from 40-laser-shots averaged spectra, resulting in thermometry accuracy and precision of ∼5% and ∼1%, respectively, at temperatures as high as 2220 K. Validerad;2022;Nivå 2;2022-03-02 (hanlid);Funder: Netherlands Organization for Scientific Research (15690)

Transported probability density function (TPDF) simulations have been performed in conjunction with DNS data to investigate the mixing characteristics of reactive scalars in two turbulent lean premixed methane-air Bunsen flames with Case A being close to the corrugated flamelet regime and Case C being close to the broken reaction zones regime. The study shows that with an accurate mixing timescale of progress variable being provided, TPDF simulations using the EMST mixing model predict scalar mixing and flame characteristics reasonably well. Modeling reactive scalar mixing rate remains one key challenge. For turbulent flames close to the flamelet regime, i.e. Case A, the turbulent flame structure represented by the scatter of OH, as well as the resemblance of the flame induced dissipation rate to the actual dissipation rate, highlights the necessity to account for flame structure when modeling reactive scalar mixing in flamelet region. A posteriori tests show that the hybrid mixing timescale model, which accounts for both turbulence and flame structure effects on the scalar mixing timescale, yields better performance than the constant mechanical-to-scalar timescale model for turbulent premixed flames close to the flamelet regime. Moreover, the hybrid model shows potential for modeling differential mixing rates of intermediate species featuring their own characteristic timescales. The effects of progress variable definition and turbulence modeling on the computed flame characteristics are investigated, and the significance of turbulence modeling in RANS-TPDF simulation is illustrated.

The physical mechanisms underlying the curvature evolution in turbulent premixed Bunsen flames at different thermodynamic pressures are investigated using a three-dimensional Direct Numerical Simulation database. It is found that, due to the occurrence of the Darrieus-Landau instability, the high-pressure flame exhibits higher probability of developing large negative curvature values and saddle concave topologies than the low pressure cases. The terms in the curvature transport equation due to normal strain rate gradients and curl of vorticity arising from both turbulent flow and flame normal propagation play pivotal roles in the curvature evolution. The mean value of the net contribution of the flame propagation terms dominates over the net contributions arising from the background fluid motion. The net contribution of the source/sink terms tries to reduce the convexity of the flame surface in the positively curved locations. By contrast, the net contribution of the source/sink terms promotes concavity of the flame surface towards the reactants in the negatively curved regions and this effect is particularly strong for the high pressure flame, where the effects of the Darrieus-Landau instability are prominent. This also gives rise to large negative skewness of the probability density functions of curvature in the high-pressure flame with the Darrieus-Landau instability.

Flame brush thickness is a significant parameter that demonstrates the evolution of turbulent premixed flame. In this paper, the flame height and flame brush thickness of lean turbulent premixed Bunsen CH4/air and C3H8/air flames were investigated under low turbulence intensities. The turbulence flow field is generated and controlled by changing the outlet velocity and perforated plates, and it is measured using a hot wire anemometry system. The turbulent flame fronts are detected by OH-PLIF technique. Results show that under fuel-lean conditions, the characteristic turbulent Bunsen flame height and centerline flame brush thickness decrease with increasing equivalence ratio, while they increase with the augment of outlet velocity. Turbulence intensity has marginal effect on these two parameters. The variations of flame brush thickness indicate that there are three regions during the whole development of turbulent Bunsen flames under low turbulence intensity. The first is the turbulence dominating region, where the variations of horizontal flame brush thickness follow the turbulence diffusion theory. The second is the non-local effect manifesting region, where the equivalence ratio, which controls the flame instability wavelength, influences the horizontal flame brush thickness. The third is the flame tip region where the flamelets intersect and make the radial wrinkles offset while the axial wrinkles superimpose. This unique tip region of turbulent Bunsen flame can be demonstrated by the linear relationship between its flame height and centerline flame brush thickness. Turbulent premixed Bunsen flames suffer different non-local effects during the flame development to downstream. To minimize the non-local effect on the statistics of turbulent flame front structure, a new method based on the development time period is proposed. Compared with statistical analysis based on the whole development time, this method can better demonstrate the flame-turbulence interaction.

The impact of combustion heat release and bulk compressive strain on local alignment between the flame front normal and strain rate eigenvectors (principal strain rates) are investigated using simultaneous laser-induced fluorescence imaging of OH and tomographic particle image velocimetry in turbulent premixed CH4/O2/N2 counterflow flames with Karlovitz numbers (Ka) of 1.3 and 2.7. The bulk strain rate imposed by the counterflow introduces a preferential alignment of the most compressive principal strain rate, s3, with the flame normal, n. Dilatation induced by heat release acts as a competing mechanism that promotes the alignment of the most extensive principal strain rate, s1, with n. In the counterflow flames, the preferential s3-n alignment prevails and remains dominant across the entire flame. This alignment stands in stark contrast to observations from previous studies in turbulent Bunsen flames or flames in isotropic turbulence, indicating the significance of bulk strain rate in determining local strain-flame alignment. The effects of increasing turbulence intensity on strain rate-flame front alignment are twofold; on the one hand, turbulence diminishes the s3-n preferential alignment that is associated with the bulk strain field by increasing flame surface wrinkling and reducing the tendency of the flame front normal and s3-eigenvectors to align with the axis of the counterflow. On the other hand, turbulence reduces the impact of heat release and enhances preferential s3-n alignment approximately 1 mm ahead of the flame front, reflecting the characteristic alignment of compressive strain and scalar gradients in turbulent non-reacting flows. The effects of bulk strain are also observed in strain rate alignment statistics based on the fluctuating velocity fields, although the impact is less pronounced than for the statistics based on the full velocity fields. As a result of this complex interplay between heat release, turbulence, and bulk strain rate, the flame-tangential strain rate is on average extensive, and the flame-normal strain rate is dominantly compressive except for an approximately 0.8 mm wide region near the flame front where it is extensive due to dilatation. The compressive bulk strain in the counterflow was also shown to compress the length scales over which the strain rate and its alignment are affected by flame heat release. The present finding is important for developing turbulent flame models to accommodate the effect of bulk strain rate that is inherently associated with practical burner geometries, and the length scale dependence of the bulk strain effect could be a consideration for determining the cutoff scale in the context of large-eddy simulations.

This paper presents a joint experimental and numerical study on premixed laminar ammonia/methane/air flames, aiming to characterize the flame structures and NO formation and determine the laminar flame speed under different pressure, equivalence ratio, and ammonia fraction in the fuel. The experiments were carried out in a lab-scale pressurized vessel with a Bunsen burner installed with a concentric co-flow of air. Measurements of NH and NO distributions in the flames were made using planar laser-induced fluorescence. A novel method was presented for determination of the laminar flame speed from Bunsen-burner flame measurements, which takes into account the non-uniform flow in the unburned mixture and local flame stretch. NH profiles were chosen as flame front markers. Direct numerical simulation of the flames and one-dimensional chemical kinetic modeling were performed to enhance the understanding of flame structures and evaluate three chemical kinetic mechanisms recently reported in the literature. The stoichiometric and fuel-rich flames exhibit a dual-flame structure, with an inner premixed flame and an outer diffusion flame. The two flames interact, which affects the NO emissions. The impact of the diffusion flame on the laminar flame speed of the inner premixed flame is however minor. At elevated pressures or higher ammonia/methane ratios, the emission of NO is suppressed as a result of the reduced radical mass fraction and promoted NO reduction reactions. It is found that the laminar flame speed measured in the present experiments can be captured by the investigated mechanisms, but quantitative predictions of the NO distribution require further model development.

Previous studies have revealed the existence of distinct regimes of DL-stable and unstable turbulent flames, depending on whether the DL instability could be minimized or not. The mechanism of how mixture composition, flame macroscale and turbulence intensity, etc. act on DL instability has been extensively studied. Even though, the role played by turbulent length scale on DL instability is still unclear. The present study has been experimentally focused on the effect of turbulent integral scale on DL instability of premixed turbulent Bunsen flames. Flame fronts of C3H8/air mixture (ϕ = 0.8) under different turbulence conditions are captured by OH-PLIF technique. It is observed that the DL-unstable flames, obtained at relatively large integral scale and with cusps observed on the flame fronts, transform to be DL-stable flames with planar or disorderly wrinkled flame fronts as the integral scale decreases, indicating the indispensable role of integral scale on DL instability. These cusps on DL-unstable flame fronts decrease flame surface density while enlarge flame volume. Turbulent burning velocities of DL-unstable flames are augmented compared to DL-stable flames, leading to a similar dual-propagation model as observed in literatures. The threshold where turbulence shadows DL instability is analyzed by upgrading a previous model of [Phys. Rev. E 84 (2011) 026322]. The modified model is more reliable to demonstrate the DL-stable and unstable regimes in the Peters-Borghi’s diagram.

The morphological development of wrinkles along the surface of a Bunsen flame in a weakly-turbulent flow is investigated, with turbulence added solely to disturb the flame front. The resulting flame-flow interactions are examined using a hybrid Navier–Stokes/front-tracking methodology within the context of the hydrodynamic theory. Topological markers based on the skewness of curvature are introduced to distinguish between sub- and super-critical conditions, or the absence/presence of the Darrieus–Landau instability, respectively. We show that for sub-critical conditions disturbances created along the flame surface are dampened when convected downstream along the flame front, and the flame surface is only weakly perturbed. For super-critical conditions, on the other hand, disturbances of the flame front are amplified when advected downstream leading to a highly corrugated surface and a flame brush of increasing thickness. A measure of these dramatic changes is included in the mean local stretch rate which, when properly modulated by a Markstein length, directly affects the turbulent flame speed.

Simultaneous particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) utilizing R-branch transitions in the C-X (0,0) band were performed at a 10-kHz repetition-rate in a turbulent premixed flame. The CH lines at 310.690 nm (from the R-branch of the C-X band) used here have greater efficiency than A-X and B-X transitions, which allows for high-framerate imaging with low laser pulse energy. Most importantly, the simultaneous imaging of both CH PLIF and PIV is enabled by the use of a custom edge filter, which blocks scattering at the laser wavelength (below ∼311 nm) while efficiently transmitting fluorescence at longer wavelengths. The Hi-Pilot Bunsen burner operated with a turbulent Reynolds number of 7900 was used to demonstrate simultaneous PIV and CH PLIF utilizing this filtered detection scheme. Instances where pockets of products were observed well upstream of the mean flame brush are found to be the result of out-of-plane motion of the flame sheet. Such instances can lead to ambiguous results when interpreting the thickness of reaction layers. However, the temporally resolved nature of the present diagnostics facilitate the identification and proper treatment of such situations. The strategy demonstrated here can yield important information in the study of turbulent flames by providing temporally resolved flame dynamics in terms of flame sheet visualization and velocity fields.

This study investigates the consequences of flame geometry for the linear response of laminar premixed flames to acoustic perturbations, as expressed by the flame transfer function (FTF). Analytical G-equation-based response models are derived for Slit, Bunsen and Wedge type flames; their respective characteristics are analyzed and validated against data obtained from high fidelity numerical simulations. Motivated by the poor agreement between numerical and analytical flame response predictions, particularly for Slit flames, an extension to the well-established incompressible-convective velocity model is proposed, which employs a Gaussian kernel function. Such a kernel disperses the flame response in time and leads to very good agreement with high fidelity numerical simulations. The validity of the model is further confirmed by comparing model predictions with experimental data taken from the literature. Analyzing the analytical flame response modeling concepts in detail, we find that the surface integration, which is required to compute the change of the global heat-release rate from the instantaneous flame front deflections, constitutes the most significant geometry-related property affecting the FTF. The linearized global heat-release rate of stiffly anchored Slit flames reacts only to movements of the flame tip and, hence, these flames respond time delayed to imposed flame front perturbations. Bunsen flames continuously transform convected flame front deflections to changes in the heat-release rate and, therefore, show a more pronounced low-pass behavior than Slit flames. The heat-release rate of Wedge flames reacts to both movements of the flame tip and the integral of instantaneous flame front deflections. Hence, perturbations of the flame front initially have a rather weak effect until they reach the flame tip, where a sudden and strong response is provoked. This leads to the occurrence of high peak gain values in the corresponding FTF.

The Flame Surface Density (FSD) based mean reaction rate closure proposed by Prof. K.N.C. Bray and co-workers in the context of Reynolds Averaged Navier-Stokes simulations has been a-priori analysed using a Direct Numerical Simulation (DNS) database of turbulent premixed Bunsen flames with different characteristic Lewis numbers representing the strict flamelet regime (i.e. high Damkohler number and low Karlovitz number combustion). The statistical behaviours of stretch factor, orientation factor and wrinkling length scale have been assessed for non-unity Lewis number conditions to identify their Lewis number dependencies. The assumption of presumed bimodal distribution has been found to be rendered invalid close to the nozzle exit where the unresolved wrinkling remains relatively small even when the flow parameters at the nozzle exit represent high Damkohler number and low Karlovitz number conditions. Although the PDF of reaction progress variable shows some resemblance to a bimodal distribution away from the nozzle exit, the Bray-Moss-Libby expressions which can be derived for infinitely large values of Damkohler number have been found to show considerable deviations from the Reynolds averaged reaction progress variable and reaction progress variable variance extracted from DNS data even though these cases represent the wrinkled/corrugated flamelets regime combustion based on nozzle exit conditions. This suggests that it might be necessary to solve a modelled scalar variance transport equation even in the strict flamelet regime instead of using the algebraic relation for the scalar variance. It has been found that the characteristic Lewis number has a major influence on the orientation factor, wrinkling length scale, and the stretch factor. Furthermore, these parameters are found to be (sometimes strong) functions of the axial distance from the nozzle exit. Known parameterisations for the wrinkling length scale and the stretch factor have been shown to be unable to capture the correct variation across the flame brush based on a-priori analysis of DNS data. The variability of the orientation factor and the inadequacy of existing relations to approximate other quantities such as the stretch factor, wrinkling factor and the Reynolds averaged reaction progress variable have the potential to severely limit the performance of algebraic FSD based mean reaction rate closures in turbulent premixed flames within the flamelet regime.

The simplification of the dynamic model of aviation kerosene oxidation is one of the important problems in numerical simulation of combustion chamber. It plays a decisive role in CFD numerical calculation and for reducing the stiffness of convergence, as well as for introducing chemical reactions to more complex turbulent combustion problems. This study aims to present a two-component surrogate fuel model for chemical reactions. Furthermore, the simplified mechanism of the optimization of the ignition delay time and experimental value is adopted using the direct relation graph method (DRG). As the primary originality for the paper, the eclectic simplified mechanisms with certain precision and fewer components and elementary reactions were concluded. And the simplified mechanism was introduced into FLUENT to simulate the precast evaporation flame of the Bunsen burner, which is a RP-3 fuel. Numerical results are in good agreement with the experimental results. The existing simplified mechanism for the two-component RP-3 aviation kerosene alternative fuel can be applied to the practical engineering problems of the numerical forecast.

Accurate measurement and prediction of laminar burning velocity is important for characterization of premixed combustion properties of a fuel, development and validation of new kinetic models, and calibration of turbulent combustion models. Understanding the variation of laminar burning velocity with thermodynamic conditions is important from the perspective of practical applications in industrial furnaces, gas turbine combustors and rocket engines as operating temperatures and pressures are significantly higher than ambient conditions. With this perspective, a brief review of spherical flame propagation method, counterflow/stagnation burner method, heat-flux method, annular stepwise method, externally heated diverging channel method, and Bunsen method is presented. A direct comparison of power exponents for temperature (α) and pressure (β) obtained from different experiments and derived from various kinetic mechanisms is reported to provide an independent tool for detailed validation of kinetic schemes. Accurate prediction of laminar burning velocities at higher temperatures and pressures for individual fuels will help in closer scrutiny of the existing experimental data for various uncertainties due to inherent challenges in individual measurement techniques. Laminar burning velocity data for hydrogen (H2), gaseous alkane fuels (methane, ethane, propane, n-butane, n-pentane), liquid alkane fuels (n-heptane, isooctane, n-decane), alcohols (CH3OH, C2H5OH, n-propanol, n-butanol, n-pentanol) and di-methyl ether (DME) are obtained from literature of last three decades for a wide range of pressures (1–10 bar), temperatures (300–700 K), equivalence ratios and mixture dilutions. The available experimental and numerical data for H2 and methane fuels compares well for various pressures and temperatures. However, more experimental and kinetic model development studies are required for other fuels. Comparison of laminar burning velocity data obtained from different measurement techniques at higher initial pressures and temperatures showed significant deviations for all fuels. This suggests to conduct focused measurements at elevated pressure and temperature conditions for different fuels to enable the development of accurate kinetic models for wider range of mixtures and thermodynamic conditions.

The flame curvature statistics of turbulent premixed Bunsen flames have been analysed in this paper using a Direct Numerical Simulation (DNS) database of turbulent Bunsen flames at ambient and elevated pressures. In order to be able to perform a large parametric study in terms of pressure, heat release parameter, turbulence conditions and nozzle diameter, a single step Arrhenius type irreversible chemistry has been used for the purpose of computational economy, where thermo-chemical parameters are adjusted to match the behavior of stoichiometric methane-air flames. This analysis focuses on the characterization of the local flame geometry in response to turbulence and hydro-dynamic instability. The shape of the flame front is found to be consistent with existing experimental data. Although the Darrieus Landau instability promotes cusp formation, a qualitatively similar flame morphology can be observed for hydro-dynamically stable flames. A criterion has been suggested for the curvature PDF to become negatively skewed.

Thermometry using two-band near-infrared (NIR) emissions less than 1.1 μm has been implemented for combustion gases formed on a flat flame burner. The gas temperature was calibrated using a SiC thin filament pyrometry (TFP) above 1500 K for H2/air and CH4/air mixtures. The emission spectra were measured with a spectrometer and a monochrome CMOS sensor with an image doubler and two bandpass filters. Two pairs of bandpass filters (850 nm/925 nm and 975 nm/925 nm) were tested to measure temperature. Results indicated that strong H2O emissions existed in the NIR region, and they became stronger with an increase in the temperature. The CO2 emission spectra in the NIR region were negligible compared to that of H2O. Calibration temperature lines were obtained as a function of the intensity ratio of the two-band emissions in the NIR region. Using a CMOS camera, the uncertainties of temperature were 59 K for 850 nm/925 nm and 145 K for 975 nm/925 nm. The resolutions were 0.7 K for 850 nm/925 nm and 1 K for 975 nm/925 nm. Using the calibration line, two-dimensional temperature measurements were performed for H2/air flames, CH4/air flames and Bunsen flames of 13A city gas. For the high temperature region above 1500 K, a quantitative two-dimensional temperature field was measured for lean and rich combustion. In addition, temperature measurement at high pressure was conducted for combustion gas of propane/air mixture with the equivalence ratio of 0.7 using a rapid compression machine. Temperature could be estimated using the calibration line obtained for the flat flame burner. We have demonstrated the performance and the possibility of thermometry two-band H2O emissions in the NIR region.

The iodine–sulfur (IS) process is one of the most promising thermochemical water-splitting processes for nuclear hydrogen production. The Bunsen reaction, which produces sulfuric and hydriodic acids for two decomposition reactions, plays a crucial role in the IS process. Insufficient kinetics data and models of the Bunsen reaction have caused difficulties for designing a Bunsen reactor and optimizing and improving the efficiency of the process. The mass transfer and kinetics mechanism of the Bunsen reaction, which is a complicated gas–liquid slurry process, were first analyzed and proposed on the basis of double-film theory and thermodynamics calculation, and intrinsic reaction rate equation models were deduced with different hypothesized reaction mechanisms. Then, the models were further improved, and the experimental kinetics data were used to verify the models. Finally, a set of reaction rate equations was developed, thereby confirming its reliability for calculating the reaction kinetics data. The bui...

The measurements on flame structure and burning velocities of C3H8/air, CH4/air and CO/H2/air turbulent premixed flames were conducted. Various experimental conditions for these mixtures were considered to understand the molecular and thermal diffusion process in turbulent flame, which was referred to the fuel effects. The fuel effects are interpreted as local burning velocity variation caused by molecular and thermal diffusivity with stretch effect. Leading points concept is a promising scaling approach to incorporate the fuel effects in turbulent flames recently. In this article, data across a variety of equivalence ratio and hydrogen fractions were obtained using Bunsen burner. Flame front was detected with OH-PLIF technique and turbulent burning velocity ST referred to leading edge was derived. A new leading points characteristic speed SL,LP was presented invoking both negative and positive Markstein number mixtures, revising the previous model in literature which is only suitable for negative...

The characteristics of local extinction on the tip of syngas premixed Bunsen flames with different hydrogen fractions have been investigated experimentally with OH-PLIF technique and numerical simulation. Coupled effects of stretch and diffusion on the syngas Bunsen flame with highly negative curvature were studied. Local Karlovitz number was introduced to indicate the onset of local extinction at the Bunsen flame tip. Results showed that the onset equivelance ratio of local extinction at the syngas Bunsen flame tip does not change with the outlet velocity. The coupled effects of stretch and diffusion result in the decrease of flame reaction rate along the syngas Bunsen flame at fuel-lean condition. When the flame reaction rate decrease to a critical extent, local extinction of Bunsen flame tip occures. The local Karlovitz number can be considered as a parameter of the critical condition. The syngas Bunsen flame tip will extinguish when the local Karlovitz number reaches around 4, irrespective of hydrogen fractions and outlet velocity. This means that Bunsen flame tip starts to appear local extinction when the tangential flow time is comparable to normal chemical reaction time, which is the same as counterflow flames with positive stretch. Furthermore, results also showed that there is no H2 leakage even the syngas Bunsen flame tip extinguishes at the numerical study conditions.

This paper presents high-fidelity flame structure measurements of premixed methane–air Bunsen flames subjected to extreme levels of turbulence. Specifically, 28 cases were studied with longitudinal integral length scales (Lx) as large as 43 mm, turbulence levels (u′/SL) as high as 246, and turbulent Reynolds (ReT,0) and Karlovitz (KaT) numbers up to 99,000 and 533, respectively. Two techniques were employed to measure the preheat and reaction layer thicknesses of these flames. One consisted of planar laser-induced fluorescence (PLIF) imaging of CH radicals, while the other involved taking the product of simultaneously acquired PLIF images of formaldehyde (CH2O) and hydroxyl (OH) to produce “overlap-layers.” The average preheat layer thicknesses are found to increase with increasing u′/SL and with axial distance from the burner (x/D). In contrast, average reaction layer (i.e. CH- and overlap-layer) thicknesses did not increase appreciably even as u′/SL increased by a factor of ∼ 60. Furthermore, the reaction layer thicknesses (based on the CH images only) did not increase with increasing x/D. The reaction layers are also observed to remain continuous; that is, local extinction events are rarely observed. Although based on a sequence of combined CH–OH PLIF images acquired at a rate of 10 kHz, it is apparent that when instances of local extinction do occur they are the result of cool gas entrainment. The results presented here, as well as those from 12 prior experimental and 9 numerical investigations, do not agree with the predicted Klimov–Williams boundary on the theoretical Borghi Diagram. Thus, a new Measured Regime Diagram is proposed wherein the Klimov–Williams criterion is replaced by a metric that relates the turbulent diffusivity ( D T = u ′ L x ) to the molecular diffusivity within the preheat layer ( D * = S L δ F , L ). Justification for this replacement is based on physical reasoning and the fact that the line defined by DT/D* ≈ 180 accurately separates cases with thin flamelets from those with broadened preheat yet thin reaction layers (i.e. BP-TR flames). Additionally, the results suggest that the BP-TR regime extends well beyond what was previously theorized since neither broken nor broadened reaction layers were observed under conditions with Karlovitz numbers as high as 533, which is five times higher than the theoretical boundary.

Investigation of turbulent burning velocity (ST) of methane–air–dust premixed flames with different dust types (coal, sand and sodium bicarbonate) and dust concentrations (λp = 0–75 g/m3) were conducted at three methane–air pre-mixture equivalence ratios (ϕg = 0.8, 1.0 and 1.2) and different turbulent intensities ( u rms ′ = 0.65, 0.72 and 0.88 m/s). Experiments were conducted in a dust Bunsen burner set-up at constant pressure conditions to study stabilized premixed flames. The results indicate that based on the particle type, the variation of turbulent burning velocity with an increase in the particle concentration differs. In general, coal and sodium bicarbonate result in the heterogeneous effect of absorbing heat and releasing volatiles; whereas sand particles just absorb heat from the flame zone. The detailed time scale analysis conducted shows that the presence of particles in the concentration range considered tends to slightly enhance the cold flow turbulence whereas with the presence of flame zone, an increase in the turbulent intensity results in increasing the vaporization rate of the particles. This effects in decreasing the turbulent burning velocity of methane–air mixtures with coal and sodium bicarbonate particles at higher concentrations and turbulent intensities. Out of three dusts examined, sodium bicarbonate addition results in the lowest ST due to the release of CO2 and H2O. Between coal and sand, at fuel lean and stoichiometric conditions, ST values with coal are greater than sand due to the equivalence ratio promotion with the release of CH4. But, as the turbulent intensity increases and for ϕg = 1.0–1.2, ST values with sand becomes comparable to or greater than that of coal. Model coefficients are generated from the experimental data to estimate the turbulent burning velocity in these conditions and the results show a clear distinction in the model coefficients for gaseous and gas–dust mixtures.

The instantaneous flame front structure of high pressure turbulent premixed Bunsen flames has been analyzed for a wide range of characteristic Lewis numbers using a new Direct Numerical Simulation (DNS) database. High pressure turbulent premixed flames of lean-light fuels are likely to feature both thermo-diffusive and Darrieus–Landau instabilities. As the effects of these instabilities are eclipsed by intense turbulence, the present analysis focuses on flames located at the border of the wrinkled and corrugated flamelets regimes. The flame morphology has been characterized by the skewness and kurtosis of the probability density function (PDF) profiles of the mean and Gaussian curvatures. While skewness alone is not sufficient as a marker to distinguish between thermo-diffusive and Darrieus–Landau instabilities, it has been found that excess kurtosis of Gaussian curvature possibly can be used to distinguish between both instabilities. Further, the inner cut-off length and the fractal dimension have been computed for characterization of flame scales and for parameterization of wrinkling factor models. It has been observed that increasing pressure and decreasing Lewis number give rise to flame instabilities which results in an increased fractal dimension and a decreased inner cut-off scale. For high pressure flames with Lewis numbers around unity, the inner cut-off scale scales very well with the critical wavelength for flame instabilities determined from the theoretical analysis by Matalon and Matkowsky (1982). For small sub-unity Lewis numbers, the flames become unconditionally unstable and the critical wavelength loses its meaning, while the inner cut-off scales continues to decrease with Lewis number and the fractal dimension continues to increase up to a limit of about 7/3. The present findings are in excellent agreement with experimental observations from literature and theoretical analysis of flame instabilities.

A high-speed camera system is used to observe the diffusion flame of a Bunsen burner in linear motion. The resultant sequence of instantaneous motion pictures of the flame accelerating at 3.60 m/s2 is processed and used to study the change in the flame area and specific floor area of the flame over different temperature ranges. The results indicate that the total flame area increases in the fuel control zone as the velocity increases over the range of experimental speeds employed (

Due to its low emission and high energy density, iron powder has been proposed as a promising recyclable metal fuel for a future low-carbon society. The comprehensive understanding of combustion behavior of iron particles is crucial for studying fundamental mechanisms, developing suitable combustion technologies, and designing efficient iron powder combustor. In this work, iron particles are combusted in a modified Bunsen burner with a stable metal powder supplying system. As a versatile three-dimensional (3D) imaging technique, high-speed digital in-line holography (DIH) is employed to reconstruct the 3D particle field and simultaneously quantify the size, 3D location and velocity of burning iron particles. The statistical results of three cases with oxygen volume fraction varying from 24% to 40% are obtained and compared at different heights above the burner. Along the height, some typical features of the burning iron particles were observed. The violent combustion of iron particles accelerates the ejection of the particles radially outward from the central region of the flame, resulting in non-uniform spatial distribution of the particles and reducing the particle number density in the measurement volume. Such trend is enhanced with increased oxygen concentration. Besides, the observed particle size enlarged as the height increases, which validates the swelling phenomenon of iron particle oxidation. The results demonstrate that DIH is a powerful tool for in-situ, quantitative characterization of particle dynamics in flames.

A laminar diffusion flame was measured by X-ray Compton scattering. The temperature distribution was measured from an analysis of Compton scattered X-ray intensity. The chemical state distribution was obtained from a Compton scattered X-ray spectrum analysis (s-parameter analysis). The analysis of intensity and s-parameter of Compton scattered X-ray spectra indicate that the propane molecule emitted from the cylindrical Bunsen burner collapse immediately coincides with soot generation. Furthermore, the temperature increases up to 1500 K and a large amount of CO2 was generated at the combustion field. Our results show that the Compton scattered X-ray analysis can be a novel nondestructive measurement for temperature and chemical states in a combustion reaction.

There is little knowledge about the inhibition efficiency of dimethyl methylphosphonate (DMMP) against turbulent flames, in which vortices of different sizes corrugate and stretch the flame front. This study is to investigate the performance and the mechanism of DMMP in inhibiting turbulent methane/air premixed Bunsen flames. Several important parameters are chosen to characterize the combustion of the flames, including flame brush height, flame brush thickness and turbulent burning velocity. It is found that the flame brush expands both horizontally and vertically with the increase of turbulent intensity. For all the flames, the centerline mean flame brush thickness increases linearly with flame height. Turbulent burning velocity decreases monotonically with the increasing of DMMP addition. However, the normalized turbulent burning velocity increases with the normalized turbulent intensity. Compared with the performance in laminar premixed flames, the inhibition efficiency depends on the turbulent intensity. For low-intensity turbulent flames, the inhibition efficiency is higher than that in laminar premixed flames, but the relative inhibition efficiency decreases with DMMP addition. This can be attributed to the relative size of the vortices and the flame front thickness: when the Kolmogorov length scale is less than the preheat zone thickness but greater than the reaction zone thickness, vortices reduce the burning velocity; when the Kolmogorov length scale is less than the reaction zone thickness, vortices facilitate the combustion. The trend is opposite in high-intensity turbulent flames, which is mainly attributed to the generating of the flame pockets in the flame tips.

The effects of N 2 , CO 2 and H 2 O dilutions on the two-dimensional (2D) temperature and concentration fields of OH in methane Bunsen flames were investigated by employing two-line planar laser-induced fluorescence (PLIF) thermometry and bi-directional PLIF technique, respectively. The spatial resolution of the measured temperature and concentration fields of OH radical was 41.7 μm. It was found that the measured temperature fields in methane Bunsen flames with different diluents exhibited a non-uniform and a symmetrical structure. In terms of decreasing the flame temperature, the CO 2 diluent achieved a good result compared with N 2 and H 2 O diluents. The total uncertainties in PLIF thermometry measurements were estimated to be ±8.44%, ±8.17% and ±8.27% for each diluent, respectively. The OH peak absorption cross sections at different heights were obtained in methane Bunsen flames with different diluents. Finally, the 2D absolute OH concentration filed in methane Bunsen flames diluted with three different diluents were obtained by using bi-directional PLIF technique. The peak absolute OH number densities in the flames with H 2 O, N 2 and CO 2 diluents were 9.84 × 10 17 cm −3 , 8.57 × 10 17 cm −3 and 7.84 × 10 17 cm −3 , with the overall uncertainties of ±20.56%, ±19.04% and ±23.43%, respectively. The numerical simulations of chemical kinetics were performed by using a physical model of one-dimensional laminar premixed flame and a detail methane reaction mechanism of GRI-Mech 3.0 so as to investigate the difference between the simulation results and experimental values. The comparison results for the temperature and OH concentration indicated that the reaction mechanism of GRI-Mech 3.0 might underestimate the chemical effects of CO 2 and H 2 O diluents on the production of OH radical in premixed methane flames.

The general influence of electric fields on flames has been known for many years, the underlying mechanisms, however, are not finally identified yet. The changes observable in the flame structure and the pollutant emission when a flame is exposed to an electric field are mainly attributed to the ionic wind. Yet, especially the resulting flame temperature under the influence of electric fields is still an open research topic. Thus, in the present study the temperature distributions in a laminar premixed Bunsen type flame were measured for different air–fuel-mixtures using point-wise vibrational coherent-anti-Stokes Raman spectroscopy (vibrational-CARS) when a static electric field is activated. Additionally, CH 2 O- and OH-PLIF (planar laser-induced fluorescence) and particle image velocimetry (PIV) were applied to identify the best-suited locations for the temperature measurements and to visualize changes in the flame structure induced by the electric field for clarifying the underlying mechanisms. The results show that the electric field leads to increased fresh gas and maximum flame temperatures and that this effect is most distinct for rich premixed flames. Both PIV/PLIF and CARS experiments confirm that the flame is constricted and pushed towards the burner due to the ionic wind, which explains the increased temperature of the fresh gas and of the exhaust gas. The increased burner and fresh gas temperatures due to the ionic wind were verified by additional thermocouple measurements. The ionic wind is maximal for the fuel-rich flame, which is explained by the lowest electric resistance due to high charge carrier concentration, and larger flame area, which reduces the gap between the flame and the positively charged electrode. Furthermore, a clear broadening of the hot exhaust gas region was measured, which increases the burn out of UHC and CO reported in the literature. Furthermore, this effect increases the residence time that could also contribute to changed pollutant emission for flames under the influence of electric fields. It can be concluded that the change of the flame temperature is mainly the result of the modified velocity field induced by the ionic wind. Consequently, the ionic wind is the dominant mechanism for many effects observed in flames when applying weak electric fields.

Previous measurements of turbulent burning velocity ( S T ) have been reported by Gulder and colleagues for intense levels of turbulence, defined to be u ’⁄ S L values between 12 and 24, and normalized integral scales ( L x ⁄ δ L ) up to 46. The present work extends burning velocity measurements to much higher levels of turbulence than have been considered before: to extreme turbulence defined as u ’⁄ S L values from 25 to 163 and L x ⁄ δ L up to 114. These conditions are argued to be more representative of the turbulence found in certain engines. To do so, a new large, piloted Bunsen burner (called Hi-Pilot) was developed and OH and formaldehyde PLIF images provided the time-averaged contours of progress variable based on OH ( c OH ). The conventional global consumption speed ( S T , GC ,1 ⁄ S L ) is based on the c OH = 0.5 contour and it was found to exceed 25. Two other measured speeds are based on the leading edge ( S T , GC ,2 ) and the component due to flamelet surface density ( S T , F ). Varying the integral scale had a significant effect on S T , GC ,2 but not on the other two burning velocities. The consumption speed S T , GC ,1 curve displayed “bending” in the range of extreme turbulence, while the flamelet surface density contribution ( S T , F ) curve instead flattened out and was independent of turbulence intensity. A possible explanation for these measured trends is based on the observed extensive broadening of the preheat zone. Preheat broadening depends on the integral scale and is believed to attenuate the turbulence that eventually interacts with the reaction zone. Results indicate a breakdown of the laminar flamelet assumption; it appears that preheat broadening may cause thermal diffusivity to dominate over the flame wrinkling mechanism.

A new regime of extreme turbulence – defined as the ratio of turbulence intensity to laminar flame speed uʹ/SL from 25 to 243 – was characterized for six premixed flames using a new piloted Bunsen burner (called Hi-Pilot). The flames studied had uʹ/SL values several times larger than those of previous related studies and integral scales and turbulent Reynolds numbers as large as 41 mm and 99,000, respectively. Layer thicknesses were determined from planar laser-induced fluorescence (PLIF) images of OH and formaldehyde. Preheat layer thickness was found to increase to sixteen times the laminar value. Residence time of eddies in the flame appears to be important, since the flame tip had preheat regions that were thicker than at the flame base. Reaction layers were not broadened, remaining below twice the laminar value. Four of the cases were predicted to lie in the Broadened Preheat - Thin Reaction layer (BP-TR) regime and the measurements confirmed that they had a BP-TR structure. However, two cases went far beyond the predicted boundary for the Broken Reactions (BR) regime but measurements showed that they were not broken but retained their BP-TR structure. Thus the regime of BP-TR is measured to persist over a wider range than previously predicted. One explanation is that the turbulent eddies may become weakened by the thick, viscous preheat layer before they arrive at the reaction front. Distributed reactions were not observed in the six cases that were selected.

Aluminum particle dust cloud combustion with oxygen is numerically simulated using an Eulerian–Lagrangian two-phase approach. A single aluminum combustion model containing detailed aluminum combustion phenomena (melting, the heterogeneous surface reaction, alumina shell growth, combustion heat distribution, etc.) is suggested and applied to a three-dimensional, compressible CFD in-house code. High order space discretization and field variable interpolation for particle technics are employed to reduce grid dependency. Flame propagation is simulated in a slip-wall bounded open duct with well distributed aluminum particles in an O 2 –N 2 mixture. The flame structure shows a flame zone that is longer than the pre-heating zone. This is unlike a gas phase flame, and it is due to the long combustion time needed for solid particles. The temperature behaviors of the gas and particle phase are captured reverse as the temperature rises. The dust cloud combustion model is validated by comparing burning velocities with the Bunsen burner experimental data in various oxygen and dust concentrations. Additionally, flame structure analysis is accomplished for better understandings on the behaviors of burning velocity in different conditions.

The effects of combustion on the strain rate field are investigated in turbulent premixed CH4/air Bunsen flames using simultaneous tomographic PIV and OH LIF measurements. Tomographic PIV provides three-dimensional velocity measurements, from which the complete strain rate tensor is determined. The OH LIF measurements are used to determine the position of the flame surface and the flame-normal orientation within the imaging plane. This combination of diagnostic techniques enables quantification of divergence as well as flame-normal and tangential strain rates, which are otherwise biased using only planar measurements. Measurements are compared in three lean-to-stoichiometric flames that have different amounts of heat release and Damkohler numbers greater than unity. The effects of heat release on the principal strain rates and their alignment relative to the local flame normal are analyzed. The extensive strain rate preferentially aligns with the flame normal in the reaction zone, which has been indicated by previous studies. The strength of this alignment increases with increasing heat release and, as a result, the flame-normal strain rate becomes highly extensive. These effects are associated with the gas expansion normal to the flame surface, which is largest for the stoichiometric flame. In the preheat zone, the compressive strain rate has a tendency to align with the flame normal. Away from the flame front, the flame – strain rate alignment is arbitrary in both the reactants and products. The flame-tangential strain rate is on average positive across the flame front, and therefore the turbulent strain rate field contributes to the enhancement of scalar gradients as in passive scalar turbulence. Although increases in heat release result in larger positive values of the divergence as well as flame-normal and tangential strain rates, the tangential strain rate has a weaker dependence on heat release than the flame-normal strain rate and the divergence.

Single-regime turbulent combustion has been the main focus in previous studies. Significant limitations exist in those studies since most practical combustion applications involve multi-regime combustion. Developing and validating multi-regime turbulent combustion models are expected to be an emerging area with significant challenges. To facilitate model assessment across different combustion regimes, we develop a model validation framework by unifying several existing pilot stabilized turbulent jet flames in different combustion regimes. The characteristic similarity and difference of the employed piloted flames are examined, including the Sydney piloted flames L, B, and M, the Sandia piloted flames D, E, and F, a series of piloted premixed Bunsen flames, and the Sydney/Sandia inhomogeneous inlet piloted jet flames. Proper parameterization and a regime diagram are introduced to characterize the pilot stabilized flames covering non-premixed, partially premixed, and premixed flames. A preliminary model assessment is carried out to examine the simultaneous model performance of the large-eddy simulations (LES)/probability density function (PDF) method for the piloted jet flames across different combustion regimes.

To investigate the combustion performance of RP-3 aviation kerosene, n-decane was chosen as a one-component surrogate fuel. Sensitivity analysis and the reaction-path analysis method were used to simplify the detailed reaction mechanism of n-decane, and a simplified mechanism including 36 species and 62 elementary reaction steps was obtained. A Bunsen burner for the combustion of premixed, pre-evaporated RP-3 aviation kerosene was designed to verify the simplified mechanism, and the temperature and gas component concentrations in the axial and radial directions at different heights were measured. The combustion process of the premixed, pre-evaporated RP-3 aviation kerosene in the Bunsen burner was also simulated on the basis of the simplified mechanism, and the numerical results were compared to the experimental data. The results show that the simulated distributions of the temperature and O2 concentration are in good agreement with the experimental data in all cases. In addition, the simulated distributi...

Existing designs of small-scale burners and domestic cookers have relied on open flame, where a large amount of heat is lost with flue gas. Flue Gas Recirculation (FGR) has been an effective technique for energy saving for large-scale industrial furnaces. Against this background, an experimental study was conducted to investigate the thermal, emission and heating performance of CH4-H2/air flames in terms of their dependence on both dilution effect (CO2 and N2) and preheat effect (initial reactant temperature from 20 °C to 100 °C). Testing of flame stability showed detrimental and favorable effects of dilution and preheat, respectively. Examination of flame height showed that under constant Re and Ф, the reaction cone is shorter at higher initial reactant temperature, indicative of enhanced burning velocity caused by preheat. Emission of CO and NOx was monotonically increased as the result of higher flame temperature due to preheat. The key question whether flame heating performance can be promoted or not is answered by detailed analysis of the radial heat flux configurations. The data reveals that the combined effects of dilution and preheat indeed induce higher heat transfer rate, thus theoretical evidence is provided for feasibility of adoption of FGR in domestic cookers.

There exists a large scatter in reported scaling (power) exponents q characterizing response of turbulent flame speed St or burning velocity Ut to changes of various factors such as the root-mean-square turbulent velocity u′, integral length scale L or laminar flame speed SL0, e.g. St∝u′^q. This scatter is often interpreted as lack of generality of the notion of turbulent flame speed or burning velocity in the sense that these quantities may not be extrapolated beyond a particular flame configuration used to evaluate them. The aim of the present paper is to assess this interpretation. For this purpose, a simple model based on the notion of burning velocity Ut as a physically meaningful quantity defined by mixture and turbulence characteristics is used to simulate statistically stationary two-dimensional V-shaped and Bunsen flames. Various St and Ut are evaluated applying five different methods, which were used in earlier measurements and simulations, to numerical solutions obtained for different values of u′, L, or SL0 and three different mean flame surfaces and at three different distances from flame-holder for both flame configurations. So obtained scaling exponents are sensitive both to flame configuration and analysis method. This result (a significant scatter in the scaling exponents can be computed invoking a model based on the notion of Ut) shows that the scatter reported in the literature does not prove that turbulent burning velocity is of minor fundamental value.