Methane-air Jet Research Articles

Numerical investigation of methane-air jet flame with hydrogen addition in industrial kiln burners

Emission reduction and decarbonisation in the industry are crucial for low-carbon industry and energy transition at the global level. Replacing traditional fossil fuel with clean energy is an effective approach to reduce carbon emissions and optimize energy efficiency in manufacturing processes. Industrial kiln, which requires high natural gas fuel consumption, typically releases amounts of harmful combustion products. In this paper, the objective is to study the influence of hydrogen addition into methane-air jet flame in industrial kiln burners. The industrial burner analysed in this study is a cylinder vessel with axial orifices and swirl turbulent co-flow air jets in the fuel-air inlet structure. A more recent reduced chemical kinetic mechanism for methane-hydrogen combustion is utilized in the present flame simulation and validated in benchmark flames. The chemical mechanism involves 45 reactions and 18 species. Various methane-hydrogen blending fuels are studied in the jet flame, where flame structure and flame characteristics including chemical species, temperature, and velocity are predicted.

A Comparative Study between FGM and SLF Approach for Turbulent Piloted Flame of Methane

This study validates the RANS simulation results by comparing them with experimental data. Numerical simulations were performed for a piloted methane-air jet flame in an axisymmetric burner. It is noteworthy that RANS simulations have been performed using a Non-premixed model with Steady Laminar Flamelet (SLF) and a partially premixed model with Flamelet Generated Manifold (FGM) of the Ansys-Fluent solver are used to express the chemistry-turbulence interaction, to provide an initial solution to the simulation performed by the Pdf transported, joint two kinetic mechanisms for oxidation of methane, detailed GRI-Mech 3.0 mechanism (53 species, 325 reactions), and CH4-Skeletal mechanism (16 species, 41-step). The case test consists of a rich premixed flame (Sandia Flame D). A comparison between the results of the obtained simulations and experimental data shows good agreement, in particular in the context of RANS/FGM with both mechanisms (GRI 3.0 and CH4-Skel).

Modeling Edge Flame Propagation for Flame Particle Tracking in the Forced Ignition Process of Turbulent Nonpremixed Jet Flame

ABSTRACT To ensure safe and reliable operation, ignition performance in propulsion and power devices needs to be thoroughly assessed. Reduced order models (ROMs) for ignition have gained increasing attention, given that first-principles models are restricted by immense computational costs that preclude rapid assessment for the design purpose. Currently, flame self-propagation is generally neglected in the ROM descriptions of ignition process and the impact on the flame front dynamics as well as the ignition probability map is not clear for low swirl or jet flames which are of interest for industrial applications. In this study, an empirical model for flame self-propagation is proposed to augment the capability of Lagrangian flame particle tracking method, a ROM model to predict ignition probability based on nonreacting flow field. The model adopts the classic scaling law of turbulent flame speed together with fuel gradient correction and flame self-propagation to mimic the edge flame dynamics during the ignition process. The model performances in terms of flame front dynamics and ignition probability map are quantified in a turbulent nonpremixed methane-air jet flame. Results reveal that only with flame self-propagation, the flame particle tracking can well capture the transient evolution of the leading flame front as observed in experiments. It is found that the predicted ignition probability map agrees well with experimental data with high ignition probability around the vicinity of the stoichiometric region at the downstream axial distance of 6 to 40 times the jet diameter. This study illustrates the importance to account for flame self-propagation to correctly predict ignition probability in jet dominant nonpremixed flames.

Blowoff mechanism of airburst blast extinguishment of a methane air jet diffusion flame with micro explosive

In the study, experiments of airburst blast extinguishment of a methane-air jet diffusion flame have been conducted with 20 mg silver azide pellets, and the extinguishing processes have been observed using a high-speed camera and schlieren method. From the experimental results, we have examined the extinguishing mechanisms. When the blast wave interacts with the diffusion flame, vortex pair is generated in the flame due to the baroclinic effect which is caused by the misalignment between the pressure gradient of the blast wave and the density gradient in the combustion region. The vortical motion leads to the local extinction, and finally, the whole flame is blown off. In airburst, a reflected shock wave occurs on the ground surface. Depending on the height of burst and the distance between the explosion location and the extinguishing target, the reflected blast wave merges with the incident blast wave and the Mach stem is formed before the blast wave interferes with the diffusion flame. As a result, it is found that the extinguishing effectiveness is increased by the presence of the Mach Stem. Moreover, the optimal height of burst in the airburst blast extinguishment is clarified.

Surrogate modeling of parameterized multi-dimensional premixed combustion with physics-informed neural networks for rapid exploration of design space

Parametric optimization is a critical component in designing and prototyping combustion systems. However, existing parametric optimization methods often suffer from either insufficient prediction accuracy or computationally prohibitive data generation costs. This study develops and validates an efficient and robust field-resolving surrogate modeling framework for combustion systems based on physics-informed neural networks (PINNs). This study comprises three progressive steps. (1) We propose a PINNs framework for premixed combustion and validate its accuracy with direct numerical simulation (DNS) for one-dimensional and two-dimensional canonical premixed combustion problems. Results show a robust predictive capability of PINNs with R2 generally above 0.99. (2) We continue to develop a field-resolving surrogate modeling framework for parameterized two-dimensional methane-air jet combustion system with five input parameters: inlet equivalence ratio, temperature, velocity, diameter, and coflow temperature. The PINNS surrogate model after unsupervised training is used to predict the combustion fields for 32 input parameter combinations, and the results are compared against corresponding numerical simulations. The surrogate model accurately predicts the temperature fields with R2 over 0.95, except in case of flashback, where the accuracy is slightly lower. Good consistency between PINNs and numerical simulations is also demonstrated for four combustion performance metrics: flame length, maximum temperature, outlet temperature, and outlet CO2 mass fraction. (3) Based on the validated field-resolving PINNs surrogate model, sensitivity analysis of the parameterized combustion system is performed, and its implications for engineering applications are discussed. Most notably, distinct from common data-driven and machine learning approaches, the proposed PINNs models are entirely physics-driven and do not use any training datasets, saving tremendous amount of time in the design of experiments (DOE) and case-by-case simulations. Overall, the results of this study demonstrate PINNs can be an efficient and robust surrogate modeling approach for parameterized combustion system visualization, design, optimization, and active control.

Modelling the response of a turbulent jet flame to acoustic forcing in a linearized framework using an active flame approach

This study performs a linear mean field analysis of a turbulent reacting methane-air jet flame, with the goal of predicting the response of the reacting flow to upstream acoustic actuation. Unlike previous studies, this work develops and applies an active flame approach by taking the heat release oscillations of the flame resulting from the acoustic fluctuations into account. For an active flame approach in the linear mean field analysis, a linearized combustion model is necessary. Linearizing Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS) combustion models leads to closure problems, making their application in this context troublesome, whereas Reynolds-averaged Navier Stokes (RANS) combustion models prove to circumvent this problem making them suitable candidates for this purpose. The RANS combustion models are linearized around the temporal mean state variables of the turbulent jet flame, which is obtained by LES. An a priori analysis shows that a linearized RANS–Eddy Break Up (EBU) model is the best suited among all investigated combustion models for the investigated set-up and reproduces with high accuracy the fluctuations in reaction rate obtained in the LES. Furthermore, the linearized governing equations of the flow including the linearized EBU model for the reaction rate are solved for incoming acoustic perturbations. The response modes show that the reaction rate oscillations are caused by Kelvin–Helmholtz vortex rings, which perturb the jet flame. The results are in good agreement with the LES simulations in terms of the mode shapes of both reaction rate and velocity fluctuations. This study represents a basis for linear mean field analysis of turbulent flames and monolithic modelling approaches for thermoacoustic instabilities in gas turbine combustors.

Analysis of Thermal Radiation Emitted from Partially Premixed Methane–Air Jet Flames

Analysis of Thermal Radiation Emitted from Partially Premixed Methane–Air Jet Flames

Numerical study on the extinction dynamics of partially premixed meso-scale methane-air jet flames with hydrogen addition

In this paper, a model of partially premixed jet flames that sustained above a meso-scale short tube was established for an individual flame port of domestic gas stoves. The effects of hydrogen addition (volume ratio β = 0%, 10%, 20%, 30%) on the extinction dynamics of CH4-air jet flames were numerically investigated. It is found that flame oscillation occurs once (β = 10% and 20%) or twice (β = 30%) in the extinction process. Moreover, the larger of β, the longer the extinction process can sustain. Analysis was performed in terms of both chemical effect and thermal effect. As to the chemical effect, in the first place, the reaction rate decreases as the inlet velocity is reduced. As a result, the consumption rate of O2 will be less than the supply rate from the incoming mixture, which makes the O2 concentration in the flame center increase. On the other hand, the amount of H radicals increases with the increase of β, and when the O2 content at the flame center reaches a “critical point”, the key elementary reaction “H + O2 ↔ O + OH” will be enhanced and consequently the total reaction rate will also be intensified. After that, the consumption rate of O2 will be larger than the supply rate due to the reduced flow rate of incoming mixture. The total heat release rate will decrease sharply and extinction occurs. As regards the thermal effect, it is revealed that heat recirculation effect (indirect preheating effect) lags behind the variation of the reaction zone (i.e., flame), thus, it has a negligible impact on flame oscillation. In contrast, the preheating temperature in the vicinity of flame front (named as “direct preheating effect”) exhibits a similar variation tendency with the total heat release rate of the flame. And the larger of β, the more remarkable of the direct preheating effect can be. In summary, due to the chemical effect and thermal effect caused by hydrogen addition, the flame can survive for a longer time with fluctuation during the extinction process.

Impact of scalar mixing uncertainty on the predictions of reactor-based closures: Application to a lifted methane/air jet flame

This work is devoted to quantify the predictive uncertainty in RANS simulation of a non-premixed lifted flame due to uncertainty in the model parameters of the scalar dissipation rate transport equation. The uncertainty propagation and the global sensitivity analysis of the effect of such parameters on the quantities of interest (QoIs) is performed employing Polynomial Chaos Expansions as surrogate models of the uncertain response. This approach is applied on a lifted methane-air jet flame in vitiated coflow, already experimentally investigated by Cabra et al [1]. The results show the effectiveness of the approach to provide predictions with estimates of uncertainty. It is shown that the the uncertainty in the mixture fraction and temperature is negligible as long as only pure mixing happens, then it becomes significant in the regions where ignition begins, starting from z/D=30. Of the four parameters considered, i.e., CD1, CD2, CP1 and CP2, main and total effect sensitivity indices show that the largest contribution to the uncertainty in the flame temperature is given by the two dissipation parameters CD1 and CD2, while the production parameter CP2 has almost negligible impact on the predictions. Lastly, the surrogate models are used to determine an optimum set of parameters that minimizes the distance with the experimental measures, leading to improved predictions of the QoIs.

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.

Preheating and premixing effects on NOx emissions in a high-pressure axially staged combustor

NOx emissions remain a primary concern for modern and future gas turbines, particularly as we progress towards a net-zero carbon future. It is critical to identify novel strategies to mitigate environmental pollutants for both modern turbines operated on natural gas and future turbines projected to use carbon-free fuels (such as hydrogen). In the current study, NOx emissions of a reacting methane-air jet in a vitiated crossflow are experimentally investigated in a model axially staged combustor at 5 atm. The combustion products from the main stage combustor flow into the axial stage at temperatures ranging from 1580 to 1650 °C. The axial stage contains a reacting jet in crossflow, which provides an overall temperature rise ranging from 50 to 220 °C to generate exit temperatures similar to gas turbine engines. The focus of this paper is to explore the effects of flame liftoff and ignition timescales on the NOx contribution of this secondary jet. This is done by varying the premixed level and preheat temperature of the axial jet. The effects are explored at different momentum flux ratios (J), jet equivalence ratios (φjet), exit temperatures (Texit), and main stage (headend) temperatures (THE). High-speed CH* chemiluminescence imaging is employed for each test case to determine the flame stabilization point. For a given temperature rise and combustor exit temperature, an increase in ignition delay was observed with a decrease in jet temperature. This provided a NOx benefit, particularly for jets that are injected at or close to an ignitable mixture fraction. Similarly, the coaxial injector caused a significant ignition delay compared to the fully premixed injector; this also led to a NOx benefit. Overall, the results show that a thermally driven delay in the axial stage provides a greater NOx reduction than a mixing delay.

Laser-based measurement and numerical simulation of methane-air jet flame suppression with water mist

Fire incidents caused by leaks from natural gas pipelines should be quickly controlled by using clean and efficient approaches, such as water mist. Previous studies on the methane-air jet fire suppression by water mist have paid not only little attention to the OH radical distribution characterizing the combustion intensity, but also critical extinguishing conditions of jet fires. Therefore, planar laser-induced fluorescence of the OH radical is obtained to visualize OH radical distribution in fire suppression. Fire Dynamics Simulator v 6.7.4 is applied to simulate the flame-spray interaction and detail the instantaneous fire-extinguishing process by the flow field, gas temperature, reaction methane mass fraction and radiation attenuation in non-lift-off jet flame scenarios. The results show that, considering variation of the jet flame tilt angle, the jet fires can be effectively suppressed when the gas-spray momentum ratio is below 0.0068 in non-tilted cases and normalized Reynold number is less than 2.56 in tilted cases. In the terms of water mist system optimization, the installation location of 1.45 times the pipe diameter was considered as the maximum nozzle horizontal position for fire suppression in the flame Froude number range of 0.2∼1.5. Besides, the OH-PLIF measurement revealed that fire suppression resulting from the kinetics effect depends on whether the OH* flame structure is quickly broken by the downward spray thrust. This study may provide suggestions for natural gas fire suppression with water mist system, and give help for the reduction of greenhouse gas emissions under Paris Agreement.

Analysis of experimental flame shapes and blowout velocities of partially-premixed methane-air jet flames

Air/fuel partially-premixed flames may contribute to the suppression of the soot formation and thus, to reduction in the level of pollutant emissions. However, the combustion process may no longer be supported, depending the air/fuel ratio, hence leading to the blowout. This phenomenon can entail several industrial risks with severe consequences. This paper reports an experimental study of partially-premixed methane-air jet flames. Using broadband imaging, the flame-geometry descriptors and blowout occurrence are measured for different ranges of fuel flow rates and air-to-fuel volume ratios. Dimensionless correlations are developed for the geometrical features and the air/methane fuel ratios leading to blowout.

A dual timescale model for micro-mixing and its application in LES-TPDF simulations of turbulent nonpremixed flames

The numerical simulation of modern aero-engine combustion chamber needs accurate description of the interaction between turbulence and chemical reaction mechanism. The Large Eddy Simulation (LES) method with the Transported Probability Density Function (TPDF) turbulence combustion model is promising in engineering applications. In flame region, the impact of chemical reaction should be considered in TPDF molecular mixing model. Based on pioneer research, three new TPDF turbulence-chemistry dual time scale molecular mixing models were proposed tentatively by adding the chemistry time scale in molecular mixing model for nonpremixed flame. The Aero-Engine Combustor Simulation Code (AECSC) which is based on LES-TPDF method was combined with the three new models. Then the Sandia laboratory’s methane-air jet flames: Flame D and Flame E were simulated. Transient simulation results show that all the three new models can predict the instantaneous combustion flow pattern of the jet flames. Furthermore, the average scalar statistical results were compared with the experimental data. The simulation result of the new TPDF arithmetic mean modification model is the closest to the experimental data: the average error in Flame D is 7.6% and 6.6% in Flame E. The extinction and re-ignition phenomena of the jet flames especially Flame E were captured. The turbulence time scale and the chemistry time scale are in different order in the whole flow field. The dual time scale TPDF combustion model has ability to deal with both the turbulence effect and the chemistry reaction effect, as well as their interaction more accurately for nonpremixed flames.

Local Exergy Losses of the Sandia Flame D: A Turbulent Piloted Methane–Air Jet Flame

Exergy is a useful tool for quantifying the locations, types, and magnitudes of wastes and losses caused by unavoidable irreversibilities in real processes. However, for the typical and popular turbulent piloted non-premixed methane–air jet flame, the Sandia flame D, no detailed exergy losses have been reported. This study reports the local exergy losses of the Sandia flame D for the first time by demonstrating the local exergy losses from heat transfer, chemical reaction, gas diffusion, and viscous dissipation. The results show that the local exergy losses from heat transfer, chemical reaction, gas diffusion, and viscous dissipation are in the ranges of 0–23210.17, 0–10796.30, 0–6.79, and 0–3.39 kW/m3 in the computational domain, respectively. These make the total local exergy loss of the Sandia flame D vary in the range of 0–23282.45 kW/m3, and it is mainly contributed by heat transfer (71.42%) and chemical reaction (28.56%), followed by gas diffusion (0.01%) and viscous dissipation (0.01%). The results obtained from this study illustrate well what the local exergy losses of the Sandia flame D are as well as how they are caused and contributed.

Experimental and numerical investigation of the influence of laterally sprayed water mist on a methane-air jet flame

This paper investigates the impact of a laterally sprayed water mist on a methane-air jet flame. An experimental apparatus for suppressing a flame using laterally sprayed water mist was developed and used to investigate the effect of the mist on the flame shape, temperature field, velocity field, radiation intensity, and extinguishment. A 3D numerical simulation was then performed to reveal the underlying mechanisms observed in the experiments, with the simulations showing reasonable agreement with the experimental data. Increasing the pressure of the water mist decreased the flame height and increased its tilt angle. For a given water mist pressure, lower–velocity flames exhibited larger tilt angles, because they were more strongly affected by the transverse shear forces exerted by the water mist; additionally, they were more likely to be extinguished by the mist. A relationship between the mean flame tilt angle, and the Froude number was deduced. Water mist was also discovered to substantially decrease the radiation intensity of the flame. Moreover, the influence of gas velocity and water mist pressure on the turbulence intensity, radiation temperature, and velocity field were discussed numerically. This can theoretically help interpret the suppressing effect of water mist on a methane-air jet flame.

Reducing greenhouse gas emissions in Sandia methane-air flame by using a biofuel

Abstract This article aims to study suitability and merits of employing a biofuel in methane-air jet flames for energy security and environmental sustainability purposes. A special biofuel (methyl decanoate, methyl 9-decenoate, and n-heptane) oxidation with 118 species reduced/skeletal mechanism and 837 reactions is combusted instead of methane. The biofuel was fed in the main jet inlet of the well-known Sandia flame D (SFD) while the hot pilot jet is still responsible for ignition. The open-source software OpenFOAM was used for simulating turbulent biofuel-air combustion. To check the accuracy of computational results, the system was initially validated with SFD experimental results and good agreements were found. After ignition, mean temperature distribution and species mean mass fraction at different distances in axial and radial directions were investigated. Results showed that the biofuel can be effectively used as an alternative to SFD not only for generating a reasonable temperature as methane does, but also for significantly reduction in principal greenhouse gas emissions.

LES/PDF Modeling of Turbulent Premixed Flames with Locally Enhanced Mixing by Reaction

Large-eddy simulations (LES) combined with the transported probability density function (PDF) method are carried out for two turbulent piloted premixed methane-air jet flames (flame F1 and flame F3) to assess the capability of LES/PDF for turbulent premixed combustion. The conventionally used model for the sub-filter scale mixing time-scale (or the mixing frequency) fails to capture the premixed flames correctly. This failure is expected to be caused by the lack of the sub-filter scale premixed flame propagation property in the sub-filter scale mixing process when the local flame front is under-resolved. It leads to slower turbulent premixed flame propagation and wider flame front. A new model for specifying the sub-filter scale mixing frequency is developed to account for the effect of sub-filter scale chemical reaction on mixing, based on past development of models for the sub-filter scale scalar dissipation rate in premixed combustion. The new model is assessed in the two turbulent premixed jet flames F1 and F3. Parametric studies are performed to examine the new model and its sensitivity when combined with the different mixing models. Significantly improved performance of the new mixing frequency model is observed to capture the premixed flame propagation reasonably, when compared with the conventional model. The sensitivity of the flame predictions is found be relatively weak to the different mixing models in conjunction with the new mixing frequency model.

Spectral characteristics of optical discharge in a high-speed methane–air jet

Results of gasdynamic and spectroscopic investigations of optical discharge in the subsupersonic flow of a homogeneous fuel–air (CH4 + air) mixtures are presented. The combustion was initiated and maintained by optical discharge created using a CO2 laser. The laser radiation propagated across the flow and was focused by a lens on the axis of the supersonic jet (M = 2). Emission-spectroscopy techniques provided data on the composition of radiating combustion products and the intensity of components emitted in the region of optical discharge. Patterns of Toepler’s visualization of the flow structure in the schlieren scheme are presented. The images were monitored by a high-speed video camera operating at an exposure time of 1.5 μs and a frame frequency of 1000 s–1.

On defining progress variable for Raman/Rayleigh experiments in partially-premixed methane flames

Progress variable is a central concept in the theory and modeling of partially-premixed flames. However, there has been no consensus in the literature on its definition, particularly in the context of experiments. Several possible definitions of progress variable are considered, based on mass fractions of the seven major species that can be measured by combined Raman/Rayleigh scattering methods applied to turbulent methane flames. The behavior of three candidate progress variables is evaluated using laminar opposed-flow flame calculations. The effects of normalizing by the fully-burnt state rather than the more commonly used equilibrium state are considered. A progress variable, cO, defined as the mass of oxygen bound in the products CO2, CO and H2O, divided by that which would be bound if the sample were taken to its fully-burnt state, is argued to offer some advantages over the other definitions considered. It is shown that inclusion of a larger number of species in calculating cO (18 versus 7) makes little difference, such that the seven major species accessible to Raman/Rayleigh laser diagnostics are sufficient to quantify reaction progress in partially-premixed methane flames at atmospheric pressure. The measured joint statistics of mixture fraction, Z, and progress variable, cO, are presented for two turbulent piloted partially-premixed methane–air jet flames that have significantly different mixture fraction profiles at the jet exit. Favre average radial profiles of first and second moments, as well as the correlation coefficient, RZ, c, highlight differences in near-field scalar structure between the two flames. Examples of joint Z, cO histograms, sampled from 0.3-mm segments centered at the radial location where Z˜ = 0.065 in each profile, are presented. This mixture fraction value corresponds roughly to the location of peak heat release rate on the fuel-rich side in the laminar flame calculations. A clear conclusion is that the common modeling assumption of statistical independence of mixture fraction and progress variable is contradicted by the turbulent flame measurements. The proposed progress variable can be easily calculated from simulations as a post-processing step, independent of the modeling approach, allowing for consistent comparison of measured and modeled turbulent flame results.