Premixed Combustion Model Research Articles

Numerical Investigation of Turbulent Premixed Combustion of Methane / Air in Low Swirl Burner under Elevated Pressures and Temperatures

Turbulent combustion modeling of lean premixed methane/air gas mixture in a low swirl burner is carried out using Large Eddy Simulation (LES). The operating conditions of the experiment as well as simulation are carried out at elevated pressure and temperature. The first case-simulation is a premixed combustion model based on C-equation formulation, the second one is based on species transport – Eddy Dissipation Concept (EDC) model. Numerical results for axial velocity and turbulence intensity along the centerline showed a good agreement against the experimental data. Quantitative results of OH mass fraction contour showing the flame structure are in a plausible agreement compared to the experimental measurement.

Development of a mechanism-dynamic-selection turbulent premixed combustion model with application to gasoline engine combustion and emissions simulation

In order to overcome the drawback (tuning reaction rate coefficients) of using a single mechanism in the chemical-kinetics-based turbulent premixed combustion modelling approach, a mechanism-dynamic-selection turbulent premixed combustion model has been developed, validated, and successfully applied to the combustion and emissions simulation of spark ignition gasoline engines. In this new model, based on the local thermal and turbulent conditions of any preheat cell, a certain chemical kinetic mechanism which can achieve the target ratio of turbulent flame speed to laminar flame speed is selected. To realise this goal, based on the laminar flame speed sensitivity analysis of an improved eTRF (ethanol toluene reference fuel) base chemical kinetic mechanism, a derived mechanism library has been generated. Based on enough experimental and DNS (direct numerical simulation) data of S T/S L, a mechanism selection database (mechanism as a function of Borghi-Peters coordinates) has been constructed and implemented into a CFD (computational fluid dynamics) code, also a classification scheme for identifying unburned, preheat, flame front, and burned zones of a combustion domain has been developed and implemented into the CFD code. The developed mechanism-dynamic-selection turbulent premixed combustion model has been validated using the combustion in a spherical constant volume and applied to the prediction of combustion and emissions of a spark ignition gasoline engine under transient cold start process. Validation results show that, without tuning any model parameters, the base mechanism, the derived mechanism library, the mechanism selection function, the classification scheme, as well as a simulation setup strategy for modelling engine transient cold start process, have good prediction performances and are practical for engine combustion and emissions simulation.

Analysis of the partially premixed combustion in a labscale swirl-stabilized burner fueled by a methane-hydrogen mixture

Nowadays hydrogen is gaining more and more attention by Industry, Academia and Politics. Being a carbon free fuel, it is supposed to have a key role in the future energy scenario, especially if produced by renewable sources. The use of mixtures of hydrogen and conventional hydrocarbons in gas turbines is one of the most promising technical solutions for obtaining a sustainable combustion during the transition toward a full decarbonization. For this reason, it is fundamental to investigate the behaviour of fuels enriched with hydrogen in combustion processes. In this work, a lab-scale swirled premixed burner has been investigated by means of a fully 3D URANS approach. Firstly, a numerical simulation with cold flow has been performed to validate the model against experimental data. Then, reactive flow simulations have been performed. Initially, a combustion with 100% methane was considered. Then, a 30% by volume hydrogen blending has been investigated. The partially premixed combustion model has been implemented to take into account the inhomogeneities of the mixture at the chamber inlet. The variation of the flame structure due to the hydrogen enrichment will be described in terms of the temperature and species concentration distributions.

Numerical simulation of methane explosion suppression by ultrafine water mist in a confined space

This numerical study focused on the heat transfer process between the ultrafine water mist and explosion flame, and the suppression mechanism of ultrafine water mist and the effect of mist parameter on methane explosion. Hence, a three-dimensional numerical model for methane explosion suppression by ultrafine water mist was established. Large eddy simulation and partially premixed combustion models were used to determine the explosion flow field characteristics and methane explosion process, respectively. The Euler-Lagrange equation was used to solve the continuous and discrete phases, and the coupling calculation was realized by alternately solving these two phase models. In addition, the validity of the numerical model and calculation method was verified by the experimental results. Mist vaporization, heat transfer between the gas and liquid phases, and influencing factors (including the mist diameter (d), mist velocity (v), and mist concentration (QMist)) were analyzed quantitatively. The results indicate that heat exchange mainly occurs in the reaction zone, and the heat exchange rate can be affected by the mist parameters, which further affects the temperature inside the vessel. Moreover, the vapor pressure generated from mist vaporization is an important component of the explosion pressure in the closed vessel, and it is comprehensively affected by the mist vaporization rate and temperature. Simultaneously, the mist parameters are also the key influencing factors for the above process.

Flame temperature analysis in the oxycut process using acetylene gas: a numerical study

To cut metals, the oxycut process uses a preheating flame, usually composed of oxygen and a combustible gas, in addition to a flame-independent jet of oxygen. In the present work, the partially premixed combustion model of a finite volume-based software was applied to analyze the flame temperature in a cutting tip of acetylene. At the torch inlet boundary, the flow rates of oxygen and acetylene were adjusted to sweep a wide range of fuel-oxygen ratio. The results pointed a peak surrounding the stoichiometric ratio, at 3106 °C for the literature and 2650.93 °C for the numerical model, contrasting a deviation of 14.65%. The maximum temperature occurs at 52.38% of oxygen volume for the literature and 50.03% for the model, a difference of 2.35 percentage points only. It is concluded that the results of the numerical approach attained the ends of the research.

Numerical simulation of the influence of pipe length on explosion flame propagation in open-ended and close-ended pipes.

The pipeline length exerts great influence on flame propagation characteristics, Realizable model and Premixed combustion model were used to study the influence of pipe length on propane-air explosion flame in open-ended and close-ended pipes. Using the numerical model verified by experiments, the changes of flame structure and flame propagation speed are studied. The result showed that the Realizable model was in good agreement with the experimental results. It also proved that the reflected wave produced a strong interference on the flame front, which promoted the formation of tulip flame. Besides, some obvious vortices were usually generated in the burned gas after the tulip flame formed, which will affect the flow field around the flame front and thus exert influence on the flame structure. The formation mechanism of tulip flame as well as the flame self-acceleration is different in open-ended and close-ended pipes. In close-ended pipes, the reflection wave at the pipe end and the reflection-induced countercurrent both promote the formation of tulip flame. As the flame propagates to the pipe end, the flame propagation is inhibited by the compression wave formed by the rapid expansion of combustion products under high temperature. While, in open-ended pipes, the turbulence induced by the opening at the pipe end is the main cause of tulip flame formation. The flame acceleration depends on the combustion reaction of unburned gas, so the velocity of flame propagation continues to increase. Generally, the maximum flame propagation velocity in the open-ended pipe is larger than that in the close-ended pipe.

Scale-Resolving Simulation of a Propane-Fuelled Industrial Gas Turbine Combustor Using Finite-Rate Tabulated Chemistry

The scale-resolving simulation of a practical gas turbine combustor is performed using a partially premixed finite-rate chemistry combustion model. The combustion model assumes finite-rate chemistry by limiting the chemical reaction rate with flame speed. A comparison of the numerical results with the experimental temperature and species mole fraction clearly showed the superiority of the shear stress transport, K-omega, scale adaptive turbulence model (SSTKWSAS). The model outperforms large eddy simulation (LES) in the primary region of the combustor, probably for two reasons. First, the lower amount of mesh employed in the simulation for the industrial-size combustor does not fit the LES’s explicit mesh size dependency requirement, while it is sufficient for the SSTKWSAS simulation. Second, coupling the finite-rate chemistry method with the SSTKWSAS model provides a more reasonable rate of chemical reaction than that predicted by the fast chemistry method used in LES simulation. Other than comparing with the LES data available in the literature, the SSTKWSAS-predicted result is also compared comprehensively with that obtained from the model based on the unsteady Reynolds-averaged Navier–Stokes (URANS) simulation approach. The superiority of the SSTKWSAS model in resolving large eddies is highlighted. Overall, the present study emphasizes the effectiveness and efficiency of coupling a partially premixed combustion model with a scale-resolving simulation method in predicting a swirl-stabilized, multi-jets turbulent flame in a practical, complex gas turbine combustor configuration.

Evolution and scaling of the peak flame surface density in spherical turbulent premixed flames subjected to decaying isotropic turbulence

The peak flame surface density within the turbulent flame brush is central to turbulent premixed combustion models in the flamelet regime. This work investigates the evolution of the peak surface density in spherically expanding turbulent premixed flames with the help of direct numerical simulations at various values of the Reynolds and Karlovitz number. The flames propagate in decaying isotropic turbulence inside a closed vessel. The effects of turbulent transport, transport due to mean velocity gradient, and flame stretch on the peak surface density are identified and characterized with an analysis based on the transport equation for the flame surface density function. The three mechanisms are governed by distinct flow time scales; turbulent transport by the eddy turnover time, mean transport by a time scale related to the pressure rise in the closed chamber, and flame stretch by the Kolmogorov time scale. Appropriate scaling of the terms is proposed and shown to collapse the data despite variations in the dimensionless groups. Overall, the transport terms lead to a reduction in the peak value of the surface density, while flame stretch has the opposite effect. In the present configuration, a small imbalance between the two leads to an exponential decay of the peak surface density in time. The dimensionless decay rate is found to be consistent with the evolution of the wrinkling scale as defined in the Bray-Moss-Libby model.

Statistical Analysis and Verification of a New Premixed Combustion Model with DNS Data

ABSTRACT In this paper, a new analytical premixed flamelet pdf model based on a surrogate reaction source term, is verified with a planar laminar flame and validated using explicitly filtered direct numerical simulation (DNS) data of statistically planar turbulent premixed flames with single-step irreversible Arrhenius chemistry at two different Karlovitz numbers. It is shown that the modeled filtered reaction source term is in excellent agreement with the one using the Arrhenius source term for the planar laminar flame over a wide range of filter widths. The model performs also very well on the filtered turbulent DNS data provided that a flame normalized isosurface area is incorporated. Interestingly, the model performance improves with increasing filter widths. This makes it an ideal candidate for high-pressure premixed combustion where usually the filter widths are much larger than the flame’s thermal reaction zone thickness .

Laminar flame speeds of methane/air mixtures at engine conditions: Performance of different kinetic models and power-law correlations

The laminar flame speed is an important input in turbulent premixed combustion modelling of spark ignition engines. At engine-relevant temperatures and pressures, its measurement is challenging or not possible and thereby it is usually obtained from simulations based on chemical models or power-law correlations. This work aims to investigate the performance of different models and power-law correlations in terms of predicting laminar flame speeds of methane/air at engine conditions. The propagation of spherically expanding laminar flames in a closed chamber was simulated and laminar flame speeds were computed over a broad range of pressures (1-120 atm) and temperatures (300-1100 K) for methane/air mixtures based on seven kinetic models. It was found that at engine conditions, there are notable discrepancies among the predictions. GRI Mech. 3.0 and USC Mech. II respectively predict the largest and smallest values at high pressure conditions. This was explained by the difference in CH3 oxidation and recombination according to reaction pathway analysis. Additionally, laminar flame speeds of methane flames were experimentally determined under engine-relevant conditions. It was shown that the recently developed Foundational Fuel Chemistry Model Version 1.0 model predicts closely the data at high pressures and temperatures. Therefore, it was chosen as the reference model for the comparisons. Thirteen published power-law correlations for laminar flame speeds of CH4/air were implemented, and their performance in predicting the laminar flame speeds at engine conditions was investigated. Most of these correlations have been derived for a narrow range of temperatures and pressures, which are lower than those encountered in engines. A new power-law correlation was derived based on predictions by the Foundational Fuel Chemistry Model Version 1.0. This new correlation is expected to provide reliable predictions at engine conditions for a stoichiometric methane/air mixture and thereby it is recommended to be used in modeling turbulent premixed combustion in spark-ignition engine simulations.

Modelling of a turbulent lean premixed combustor using a Reynolds-averaged Navier–Stokes approach

This work aims to characterize lean premixed turbulent swirling flames representative of gas turbines and jet engines via a numerical study of flame topologies in a laboratory-scale burner. The state of the art of the numerical studies concerning these types of flames is first reviewed, with respect to Reynolds-averaged Navier–Stokes and large eddy simulations. Then, a turbulent, isothermal flow study is performed within the radial swirler. The impact of mesh refinement levels and boundary conditions on the swirl number and overall flow structure is investigated. The mesh refinement level and slip wall boundary condition alter the computed swirl number significantly. The computed swirl number converges to a value of 0.7, which is larger than the geometrical one, 0.4. Furthermore, using Reynolds-averaged Navier–Stokes transport equations, closed by the realizable $$k-\epsilon$$ model, coupled with a two-equation premixed combustion model for methane/air mixtures, two combustion regimes are analysed. These regimes correspond to the outer recirculation zone flame and an unstable regime. The flow structure is characterized in terms of velocity fields, turbulence and combustion properties. A reaction progress variable comparison is also performed, using existing experimental results, yielding qualitatively similar structures for both studied regimes. Some discrepancies between numerical and experimental results concerning the stable regime may be observed: The computed progress variable at the outer recirculation zone, 0.5, is smaller than the experimental value, 0.8, and the average flame brush thickness, 1 mm, is found to be smaller than the measured, 3 mm.

Application of artificial neural network and response surface methodology for predicting and optimizing dual-fuel CI engine characteristics using hydrogen and bio fuel with water injection

In this work, the experiments were conducted to investigate the effect of using hydrogen (H2)andLemon Grass Oil (LGO) as a partial substitute fuel for diesel in a single-cylinder Compression Ignition (CI) engine. Initially, the experiment was conducted with neat diesel and then the engine was modified to operate under dual-fuel mode by inducting H2with intake manifold and LGO as direct injection (DI) fuel. The energy share of H2 was varied from 5 to 10% in this research work. Water (W) was mixed (3% volume) with diesel-LGO blends and its effect on oxides of Nitrogen (NOx) emissions was investigated. An ANN model has been developed to predict the correlation between engine output responses and input factors (load, LGO and hydrogen) using a standard back-propagation algorithm. Response surface methodology is concerned to optimize the engine input parameters in order to minimize the emissions and maximize the thermal efficiency. It was observed that the BTE got increased to 31% for 10%H2 + LGO25 whereas it was 24% for neat LGO and 30% for 10%H2 + LGO25 + 3%W modes. The cylinder pressure increased up to 63 bar for 10%H2mode than neat LGO operation. The HRR, underH2 premixed combustion model was observed to be 79 kJ/CA deg in 10%H2, which is comparatively higher than that of all the LGO operations. The maximum NOx emission was found for 10%H2 + LGO25% at 700 ppm peak load, whereas it reduced to 620 ppm for water-emulsified fuel.

CFD Investigation of Flame and Pressure Wave Propagation through Variable Concentration Methane-Air Mixtures in a Tube Closed at One End

ABSTRACT CFD modeling of methane-air combustion and the subsequent flame and pressure wave propagations from the closed end of a detonation tube is presented, with a focus on propagation through variable concentration mixtures. A partially premixed combustion model that avoids the need to specify the flame speed is developed based upon the Flamelet Generated Manifold (FGM) model and needs no tuning to account for different methane concentrations. The numerical model is extensively validated using the experimental data collected from a large-scale detonation tube. The results show that the pressure wave propagation experiences three sequential stages: i) growth; ii) decoupling; and iii) decay. The peak overpressure is generated in the pressure wave growth stage in which the wave front transiently couples with the flame front, and the confined tube walls induce lateral wave reflections and force the flame front to transit from spherical to planar. Subsequently, the wave front starts decoupling from the flame front, with an almost constant global maximum pressure. After decoupling, the global maximum pressure drops because of the energy loss incurred through the wave propagation. The different methane concentrations introduced initially after the explosion chamber containing a stoichiometric mixture do not affect the peak overpressure or the pressure wave propagation but do affect the profile and propagation of the flame. Exponential acceleration of the flame propagation speed is found in the growth stage of pressure wave propagation, followed by the transition to a linear acceleration stage. For cases with the methane concentration becoming smaller than the stoichiometric concentration, the linear flame acceleration rate is smaller, with more pronounced flame stretching.

A compatibility principle, analytical formulation, and the G- and Σ-equations in the theory of turbulent premixed combustion

We analyze the nature of the known difficulties that arise when trying to apply the kinematic G-equation and the surface average Σ-equation to the Reynolds-Averaged Navier–Stokes modeling of turbulent premixed combustion, based on the use of the Favre average equations of combustion and hydrodynamics. We consider this issue in the context of an analytical formulation that includes an asymptotically closed infinite system of successively derived unclosed moment combustion and hydrodynamics equations of the problem. We show that the cause of these difficulties is the incompatibility of the G-equation and Σ-equation with the other instantaneous and averaged equations of the problem. This leads us to formulate a compatibility principle, i.e., a rule for the identification of misconceptions about the use of instantaneous and averaged equations that are correct by themselves, due to their incompatibility with the equations of the corresponding analytical formulation. Special attention is devoted to a comparison of two approaches to a description of instantaneous and average premixed combustion, based on the use of the progress variable c and the scalar G. We state that these approaches are equivalent. To prove this, we derive the equations missing from these two approaches and show that the following two groups of instantaneous and Favre-averaged equations, formulated in terms of c and G, respectively, are equivalent. The main result is the proposed compatibility principle. The conclusion following from our analysis is that the concepts of the scalar G and flame surface density Σ useful in applications could be considered superfluous from the theoretical viewpoint.

Non-equilibrium numerical modeling for combustion of LPG within porous media

A numerical model for lean premixed combustion of LPG (70% propane and 30% butane) within a porous inert medium was developed. Experiments were conducted at three different firing rates at the equivalent ratios of 0.4 and 0.6. The model was developed with the thermal non-equilibrium concept between phases and validated with three cases of experimental results. The discussion of model calibration was undertaken by focusing on the effects of the extinction coefficient and convection heat transfer effective area. Comparisons were made of the temperature profile, as well as the peak temperature, with the calculated adiabatic temperature. The model agreed well with experimental results and was robust throughout three firing rates. Moreover, it was found that the two aforementioned thermal parameters had different roles in temperature distribution, which provided insight on flame front location and heat transfer between phases within the porous domain.

A Two-Fluid Conditional Averaging Paradigm for the Theory and Modeling of Turbulent Premixed Combustion

This paper extends a recent theoretical study that was previously presented in the form of a brief communication (Zimont, C&F, 192, 2018, 221-223), in which we proposed a simple splitting method for the derivation of two-fluid conditionally averaged equations of turbulent premixed combustion in the flamelet regime, formulated more conveniently for applications involving unclosed equations without surface-averaged unknowns. This two-fluid conditional averaging paradigm avoids the challenge in the Favre averaging paradigm of modeling the countergradient scalar transport phenomenon and the unusually large velocity fluctuations in a turbulent premixed flame. It is a more suitable conceptual framework that is likely to be more convenient in the long run than the traditional Favre averaging method. In this article, we further develop this paradigm and pay particular attention to the problem of modeling turbulent premixed combustion in the context of a two-fluid approach. We formulate and analyze the unclosed differential equations in terms of the conditions of the Reynolds stresses τij,u, τij,b and the mean chemical source ρW¯, which are the only modeling unknowns required in our alternative conditionally averaged equations. These equations are necessary for the development of model differential equations for the Reynolds stresses and the chemical source in the advanced modeling and simulation of turbulent premixed combustion. We propose a simpler approach to modeling the conditional Reynolds stresses based on the use of the two-fluid conditional equations of the standard “k-ε” turbulence model, which we formulate using the splitting method. The main problem arising here is the appearance in these equations of unknown terms describing the exchange of the turbulent energy k and dissipation rate ε in the unburned and burned gases. We propose an approximate way to avoid this problem. We formulate a simple algebraic expression for the mean chemical source that follows from our previous theoretical analysis of the transient turbulent premixed flame in the intermediate asymptotic stage, in which small-scale wrinkles in the instantaneous flame surface reach statistical equilibrium, while the large-scale wrinkles remain in statistical nonequilibrium.

Effect of Tube Size on Flame and Pressure Wave Propagation in a Tube Closed at One End: A Numerical Study

ABSTRACT Deflagration of fire and explosion caused by ignition of a methane-air mixture is a major safety concern in industrial settings, such as the mining and petrochemical industries. The aim of this study is to investigate numerically the effect of tube size on the flame and pressure wave propagations for methane-air deflagration in a tube closed at one end. A partially premixed combustion model that avoids the need to specify the flame speed is deployed based upon the Flamelet Generated Manifold (FGM) model. Good agreement is achieved between the predicted results and the benchmark experimental data collected using a large-scale detonation tube (L/D = 66). Subsequently, the explosion behaviour immediately after the ignition of methane-air mixtures and the propagation characteristics of the flame front and pressure wave through the tube are examined, covering a broad range of L/D, that is from 26 to 526, in which the diameter is changed and the length is kept fixed. The results show that the pressure wave propagates significantly faster in narrower tubes and hence decouples from the flame front shortly after ignition, which in turn results in a low overpressure at the flame front. Moreover, abrupt changes in gas properties are observed in narrow tubes with L/D ≥ 132. The peak overpressure increases as the tube diameter increases; however, the local maximum pressure decreases substantially in large tubes when approaching the tube vent but remains almost constant throughout in the narrow tubes. Similarly, the flame propagates faster in narrower tubes. A correlation that estimates the distance the flame propagates in the exponential acceleration stage is proposed as a function of tube size and time. Deviations of less than 7% are obtained when comparing the predicted results using the correlation against the experimental data. The results provide local information that aids the theoretical interpretation of experimental observations and the understanding of the fuel combustion and explosion phenomena in different sized tubes.

Decomposition of Turbulent Fluxes from Filtered Data and Application to Turbulent Premixed Combustion Modelling

The exact reconstruction of statistical quantities from filtered data is a fundamental question in turbulence research. Filtered data can be either data from a Large Eddy Simulation (LES) or it can be experimental data with insufficient resolution to capture the finest scales. Usually it is assumed that averages of filtered and unfiltered quantities are the same and as a consequence the Reynolds stress or turbulent scalar flux is traditionally split in a resolved part and a mean subfilter part. However, this decomposition holds only true for homogeneous flows without mean gradients as demonstrated recently (Klein and Germano [1]). In this work an exact relation between filtered, respectively Favre filtered, and unfiltered turbulent fluxes will be derived and tested in the context of turbulent premixed combustion, where heat release gives locally rise to strong gradients and consequently the failure of the standard assumption is expected. For demonstration of the problem, filtered data will be considered from a purely mathematical point of view by applying a convolution operation to data representing solutions to the Navier-Stokes Equations. Results will be discussed for the subfilter stress, the turbulent scalar flux and the subfilter scalar variance.

Computational Investigation of Porous Media Combustion Technology in Spark Ignition Engine

In the present work, computational fluid dynamics approach is adopted to study the possibility of using porous media in combustion and emission in a single-cylinder four-stroke spark ignition engine. The combustion behavior inside the engine without and with application of porous media is studied. The analysis with application of PM signifies enhanced combustion behavior. The compression ratio is kept at 7. Two different types of fluid computational domains are considered, namely, without and with PM. The three-dimensional modeling, meshing and simulations of the combustion chambers are performed with the ANSYS 17.1-FLUENT software. The k–e turbulence model, the premixed combustion model and the spark ignition model are used in the study. The parameters to be analyzed are the variation of cylinder pressure, temperature and turbulence at different crank angles. The results of the computational work without PM are validated with the experimental work, which is executed on the research engine setup operated in SI mode. The work is performed at different engine loads by varying the engine torque, viz. 6 Nm (25%), 12 Nm (50%), 18 Nm (75%) and 24 Nm (100%) along with the no-load condition. The analyses of the results indicate that the peak value of total pressure (1,986,868.99 Pa) and average value of total temperature (632.63 K) with PM at 100% load are lower than without PM (1,992,314.50 Pa and 604.81 K). The concentrations of NOX emissions are significantly reduced by the use of PM for all the loads. The percentage reductions in NOX emissions at no load, 25%, 50%, 75% and 100%, are found to be 2.5%, 11.1%, 9.4%, 9% and 2.12%, respectively.

The Effects of Turbulent Burning Velocity Models in a Swirl-Stabilized Lean Premixed Combustor

Abstract The effects of turbulent burning velocities in a turbulent premixed combustion simulation with a G-equation are investigated using the 3D LES technique. Two turbulent burning velocity models – Kobayashi model, which takes into account the burning velocity pressure effect, and the Pitsch model, which considers the flame regions on the premixed flame structure – are implemented. An LM6000 combustor is employed to validate the turbulent premixed combustion model. The results show that the flame structures in front of the injector have different shapes in each model because of the different turbulent burning velocities. These different flame structures induce changes in the entire combustor flow field, including in the recirculation zone. The dynamic mode decomposition (DMD) method and linear acoustic analysis provide the dominant acoustic mode.