Turbulent Combustion Processes Research Articles

In-Situ Adaptive Manifolds: Enabling computationally efficient simulations of complex turbulent reacting flows

Reduced-order manifold approaches to turbulent combustion modeling traditionally involve precomputation of manifold solutions and pretabulation of the thermochemical database versus a small number of manifold variables. However, additional manifold variables are required as the complexity of turbulent combustion processes increases through consideration of, for example, multi-modal, non-adiabatic, or non-isobaric combustion, or combustion featuring multiple and/or inhomogeneous inlets. This increase in the number of manifold variables comes with an increase in the computational cost of precomputing a greater number of manifold solutions, most of which are never actually utilized in a CFD calculation. The memory required to store the pretabulated high-dimensional thermochemical database also increases, practically limiting the complexity of manifold-based combustion models. In this work, a new In-Situ Adaptive Manifolds (ISAM) approach is developed that overcomes this limitation by combining ‘on-the-fly’ calculation of manifold solutions with In-Situ Adaptive Tabulation (ISAT), enabling the use of more complex manifold-based turbulent combustion models. The performance of ISAM is evaluated via LES of turbulent nonpremixed jet flames with both hydrogen and hydrocarbon fuels. A performance assessment indicates that the computational overhead associated with ISAM compared to pretabulation ranges from negligible up to a factor of two, with most of this overhead associated with convolution of the thermochemical state against a presumed subfilter PDF. In addition, the memory requirements of ISAM are more than two orders of magnitude less than conventional tabulation. These results demonstrate the potential for ISAM to accommodate significantly more complex manifold-based combustion models.

Characterization of Supersonic Turbulent Combustion in a Mach-10 Scramjet Combustor

Turbulent combustion remains an unsolved problem, and the simulation models that have been developed, although accurate for subsonic turbulent combustion, have limited accuracy when applied to supersonic combustion. An experiment has been developed to characterize supersonic turbulent combustion in a geometrically simple flowpath that is capable of accurately reproducing the complex and highly nonuniform flow structures present in flight-candidate scramjet engines. An inlet-fueled scramjet engine was designed with a symmetrical intake to reproduce the correct flow structures. This was tested with an inflow condition of Mach 7, using a Mach-10 flow enthalpy of to reproduce the combustion conditions in a flight vehicle operating at Mach 10. The turbulent flame was directly observed in the flow using planar laser-induced fluorescence of OH radicals. These results have been complemented by large-eddy simulations of the experimental conditions, showing that the combustion process in scramjets is multimode, where neither premixed nor non-premixed combustion dominates, and both substantially contribute to heat release. These modes are present simultaneously inside the engine. For both modes, combustion is confirmed to happen over multiple combustion regimes. This work provides new insight into the supersonic turbulent combustion process and enhances our understanding of combustion in scramjet engines.

Oxy-Combustion Modeling for Direct-Fired Supercritical CO2 Power Cycles

Supercritical CO2 power cycles for fossil energy power generation will likely employ oxy-combustion at very high pressures, possibly exceeding 300 bar. At these high pressures, a direct fired oxy-combustor is more likely to behave like a rocket engine than any type of conventional gas turbine combustor. Issues such as injector design, wall heat transfer, and combustion dynamics may play a challenging role in combustor design. Computational fluid dynamics modeling will not only be useful, but may be a necessity in the combustor design process. To accurately model turbulent reacting flows, combustion submodels appropriate for the conditions of interest as defined by the turbulent time and length scales as well as chemical kinetic time scales are necessary. This paper presents a comparison of various turbulence–chemistry interaction (TCI) modeling approaches on a canonical, single injector, direct-fired sCO2 combustor. Large eddy simulation is used to model the turbulent combustion process with varying levels of injector oxygen concentration while comparing the effect of the combustion submodel on CO emissions and flame shape. While experimental data are not yet available to validate the simulations, the sensitivity of CO production and flame shape can be studied as a function of combustion modeling approach and oxygen concentration in an effort to better understand how to approach combustion modeling at these unique conditions.

Numerical simulation of a swirl stabilized methane-air flame with an automatic meshing CFD solver

The reliable prediction of the turbulent combustion processes in lean flames is nowadays of crucial importance in the design of gas turbine combustors. This work presents an assessment of the capabilities of Flamelet Generated Manifold (FGM) in the framework of Large-Eddy Simulation (LES), as implemented in the commercial CFD solver CONVERGE. One of the main characteristic of the code is the Adaptive Mesh Refinement (AMR) technique, namely the use of a dynamic mesh where elements size varies during the simulation. For validation purposes, the TECFLAM swirl burner, consisting of a strongly swirling, unconfined natural gas flame, has been chosen. Results highlight the advantages of AMR in describing turbulent flames, leading to a successful prediction of the main characteristics of the reacting flow field.

Impact of Stretch and Heat Loss on Flame Stabilization in a Lean Premixed Flame approaching Blow-off

The accurate prediction of the turbulent combustion process in lean burn flames is of primary importance in the design of gas turbine low-emission combustors. In this framework, the correct account for high strain levels, combined with the heat loss of the flame, by numerical tools is of high technical relevance in order to improve the operational flexibility while reducing emissions. In fact, in high Reynolds lean combustion modelling, the quenching effects due to flame front distortion are expected to govern flame behaviour. The present work presents an assessment of the modelling strategies to introduce the stretch effects on the flame in Flamelet Generated Manifold (FGM) model, in both the framework of Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES). At this purpose a premixed swirl burner experimentally studied at Cambridge University was chosen, consisting of a strongly swirling, confined natural gas flame. Results highlight that LES-FGM, coupled with an extended Turbulence Flame Closure model (TFC), succeeds in predicting the main characteristics of the flame at different operating conditions approaching blow-off, thus representing a valid tool to investigate lean burn flames in such context.

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

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

A new quasi-dimensional flame tracking combustion model for spark ignition engines

A new quasi-dimensional combustion model based on the flame tracking approach is described and presented in the paper. The new quasi-dimensional flame tracking model is able to simulate the turbulent combustion process in premixed fuel/air/residual gas mixtures. A new method for the description of the geometry of the combustion chamber and flame front was developed enabling the visualisation of flame front movement across the combustion chamber. The control of local turbulence quantities in the flame front near the wall enables that the developed flame tracking model can predict the entire turbulent combustion process after the flame kernel development in spark-ignition engines without case-dependent calibration requirement. The developed quasi-dimensional combustion model was validated with the experimental and results of multidimensional model of a single cylinder spark ignition engine on averaged cycles. The model was integrated with the previously developed ignition, mixture stratification and cyclic variability sub-model that enable the simulation of cyclic combustion variability triggered by the stochastic variations of flow angle at the spark plug, mixture stratification and in-cylinder turbulence level. Due to the novelty of the model which includes the control of local integral length scale and turbulent kinetic energy in the flame segments the predictive capability of quasi-dimensional model is achieved with the application of single set of parameters related to average cycle and cyclic combustion variability. Flame tracking model with the low computational time represents a promising tool to calculate the turbulent combustion process including cyclic combustion variability in modern spark ignition engines.

Large eddy simulation of a homogeneously charged turbulent jet ignition system

Transient jet ignition of a homogeneous methane air mixture in a turbulent jet ignition system is studied computationally using a large eddy simulation turbulence model. The jet discharges from a prechamber into a main combustion chamber via one or more orifice(s) and provides a distributed ignition source in turbulent jet ignition. The effect of orifice size and stoichiometry is studied numerically using the Converge computational fluid dynamics code. A reduced kinetic mechanism is used for combustion along with a Smagorinsky sub-model for turbulence modeling. The computed pressure traces are compared with experimental measurements through rapid compression machine tests. Computational fluid dynamics results are in acceptable agreement with the experimental data during compression and the early stage of combustion; however, an over-prediction of peak pressure was reported. Peak pressure error is in the range of 0.1%–4% for Reynolds-averaged Navier–Stokes simulation estimation compared to the experimental measurements. This error is a function of mixture stoichiometry and unburned gas temperature. The error calculation showed that with the large eddy simulation model, 1% and 12% improvements in peak pressure and burn rate estimations, respectively, were achieved compared to Reynolds-averaged Navier–Stokes results. The reduced large eddy simulation error relative to the Reynolds-averaged Navier–Stokes simulations were considered to be in the acceptable range; however, further improvements could be achieved through validation and testing of additional turbulence models. In addition, computational fluid dynamics temperature contours for various nozzle orifices and air–fuel ratios are compared to achieve deeper insight into the turbulent jet ignition combustion process in the rapid compression machine combustion cylinder. The numerical iso-surface temperature contours were obtained which enabled three-dimensional views of the flame propagation, the jet discharge, ignition and extinction events. The heat release process and regeneration of mid-range temperature iso-surfaces (1200 K) were not visible through the experimental images.

Methane swirl-stabilized lean burn flames: assessment of scale-resolving simulations

The reliable prediction of the turbulent combustion process in lean flames is of paramount importance in the design of gas turbine combustors. The present work presents an assessment of the capabilities of Flamelet Generated Manifold (FGM) in the framework of Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES) At this purpose the TECFLAM swirl burner consisting of a strongly swirling, unconfined natural gas flame was chosen. Results highlight that RANS-FGM succeeds in predicting the main characteristics of the reacting flow field and species concentrations. However, only LES is capable of reproducing the actual turbulent mixing between swirling flow and co-flow, thus leading to appreciable enhancements with respect to RANS results.

Dynamic Mode Decomposition of Turbulent Combustion Process in DLR Scramjet Combustor

AbstractLarge eddy simulations (LES) are employed to investigate instabilities in the combustion process of a model scramjet combustor at the German Aerospace Center (DLR). Dynamic mode decompositi...

Control of methane flame properties by hydrogen fuel addition: Application to power plant combustion chamber

This study presents a numerical investigation of the effects of mixing methane/hydrogen on turbulent combustion processes taking place in a burner similar to that integrated in gas turbine power plants. Thereby, in comparison to the reference case where the burner is fuelled by 100% of methane, the variations of the axial velocity field, temperature field and mass fraction of carbon monoxide field are examined for different percentages of hydrogen fuel injection. The computed results, obtained by using the software Fluent-CFD, are compared and validated against experimental reference data. Results show that the hydrogen addition to the methane has an impact on all physical and chemical parameters of the reactive system.

Large-Eddy Simulation of Soot Formation in a Model Gas Turbine Combustor

The computational modeling of soot in aircraft engines is a formidable challenge, not only due to the multiscale interactions with the turbulent combustion process but the equally complex physical and chemical processes that drive the conversion of gas-phase fuel molecules into solid-phase particles. In particular, soot formation is highly sensitive to the gas-phase composition and temporal fluctuations in a turbulent background flow. In this work, a large-eddy simulation (LES) framework is used to study the soot formation in a model aircraft combustor with swirl-based fuel and air injection. Two different configurations are simulated: one with and one without secondary oxidation jets. Specific attention is paid to the LES numerical implementation such that the discrete solver minimizes the dissipation of kinetic energy. Simulation of the model combustor shows that the LES approach captures the two recirculation zones necessary for flame stabilization very accurately. Further, the model reasonably predicts the temperature profiles inside the combustor. The model also captures variation in soot volume fraction with global equivalence ratio. The structure of the soot field suggests that when secondary oxidation jets are present, the inner recirculation region becomes fuel lean, and soot generation is completely suppressed. Further, the soot field is highly intermittent suggesting that a very restrictive set of gas-phase conditions promotes soot generation.

Laminar Flamelets in Turbulent Combustion Modeling

ABSTRACTA brief review is presented of so-called laminar flamelet models of turbulent combustion processes, using premixed combustion as an example. Circumstances where this simple modeling strategy can fail are explored.

Performance optimization of a spark-ignition turbocharged VVA engine under knock limited operation

Various solutions are being proposed to improve the performance of spark-ignition internal combustion engines. A very effective approach is the downsizing technique, which allows the reducing of the Brake Specific Fuel Consumption (BSFC) at part load, while maintaining the required performance at high load. On the other hand, the above technique may cause substantial BSFC detrainments at high load because of the onset of knocking combustions.In the present work, a turbocharged spark ignition engine equipped with a fully flexible valve system is numerically investigated by a 1D model (GT-Power™). Proper “user routines” are used to simulate the turbulent combustion process and the knock phenomenon. In a first stage, the engine model is validated against experimental data under both high and part load operations, in terms of overall performance and combustion evolution. The validated model is then integrated in a multipurpose commercial optimizer (modeFRONTIER™) with the aim to identify the engine calibrations that maximize the load and minimize the BSFC under high load knock-limited operations at a speed of 3000rpm. The effects of different intake valve strategies are compared. The optimized operating parameters are the waste-gate valve opening and the air-to-fuel ratio, while the combustion phasing is automatically adjusted to avoid the knock onset. Proper constraints are assigned for the boost pressure, turbocharger speed, and turbine inlet temperature. The adopted optimization process shows the capability to reproduce the experimentally-identified calibration with satisfactory accuracy. In addition, the results underline the BSFC advantages related to an early intake valve closure strategy with respect to a Full Lift one, due to a better combustion phasing and a reduced mixture over-fuelling.The developed automatic procedure allows for a “virtual” engine calibration on a completely theoretical basis and proves to be very helpful in reducing the engine development costs and time-to-market.

About Multi-resolution Techniques for Large Eddy Simulation of Reactive Multi-phase Flows

A numerical technique for mesh refinement in the HeaRT (Heat Release and Transfer) numerical code is presented. In the CFD framework, Large Eddy Simulation (LES) approach is gaining in importance as a tool for simulating turbulent combustion processes, also if this approach has an high computational cost due to the complexity of the turbulent modeling and the high number of grid points necessary to obtain a good numerical solution. In particular, when a numerical simulation of a big domain is performed with a structured grid, the number of grid points can increase so much that the simulation becomes impossible: this problem can be overcomed with a mesh refinement technique. Mesh refinement technique developed for HeaRT numerical code (a staggered finite difference code) is based on an high order reconstruction of the variables at the grid interfaces by means of a least square quasi-eno interpolation: numerical code is written in modern Fortran (2003 standard of newer) and is parallelized using domain decomposition and message passing interface (MPI) standard.

Comparative study of thermal radiation properties models in turbulent non-premixed sooting combustion

ABSTRACTThis work presents a numerical study of radiation heat transfer and its interaction with gray and spectral radiation of combustion products and soot that are formed in a turbulent non-premixed combustion process. To this purpose, weighted sum of gray gases (WSGGM), Rayleigh, narrow-band model, and semi-empirical radiation properties models are considered. Two situations are studied −1-D radiation problem and non-premixed gaseous combustion process. The results demonstrate that the models exhibit discrepancies that are smaller than the available line-by-line or experimental data. A study of the factors that influence such a disagreement is presented, underscoring the role of the chemical kinetics of hydrocarbon species and soot oxidation.

Non-intrusive gas-phase thermometry for industrial oxy-fuel burners

The use of oxy-fuel combustion processes is of large interest for several industrial fields applications since it offers the advantages of low NOx emissions in combination with high combustion temperatures even without additional preheating. For optimization of such processеs a detailed understanding based on precise experimental data is necessary. So far there is still a lack of precise experimental data achieved with high spatial and temporal resolution from industrial relevant turbulent oxy-fuel combustion processes. Beside species concentration information the gas phase temperature is of utmost importance for an improved understanding of the basic chemical reactions and the pollutant formation. The coherent anti-Stokes Raman spectroscopy (CARS) technique is a very well suited laser based tool for a non-intrusive investigation of such turbulent high temperature combustion processes. In this work we analysed an industrial 400 kW oxy-fuel burner with the help of O2 based vibrational CARS system which is integrated in an industrial relevant test furnace. The burner is fed with pure oxygen and natural gas at an equivalence ratio of =0.9. At one downstream position temporal and spatial resolved temperatures were measured along a 600 mm line. Additional air sucked in from the environment seems to influence the gas phase temperature significantly.

Recurrence Analysis of Combustion Noise

Recurrence analysis of phase space trajectories reconstructed from scalar time series—a relatively new technique in the field of combustion dynamics—is proposed for analyzing combustion noise data. A demonstration based on the implementation of this analysis on combustion noise data acquired in experiments on an open bluff-body stabilized turbulent premixed flame is presented in this article. A combined analysis methodology involving conventional techniques: spectral analysis and proper orthogonal decomposition, together with recurrence analysis is found to be effective in identifying features embedded in combustion noise signals. In particular, the new perspective provides insights into the temporal transitions of pressure fluctuations between noise and periodic dynamics. The recurrence analysis technique, which is found to be instrumental in extracting the dynamical makeup of combustion noise signals in this work, is foreseen to be greatly beneficial for the analysis of experimental and numerical results on turbulent flows and turbulent combustion—processes that often involve an interplay between noise and deterministic dynamics.

Combustion regime classification of HCCI/PCCI combustion using Lagrangian fluid particle tracking

A new combustion regime classification method is proposed for HCCI/PCCI combustion based on Lagrangian fluid particle tracking. The performance of the present method is demonstrated with the time trace of net heat conduction to local mixture which is one of the quantities best representing the phenomena. Local combustion regimes are classified based on magnitudes of the maximum and minimum heat conduction on each fluid particle throughout the combustion process into autoignition, flame propagation and flame enclosing. Direct numerical simulations (DNS) of turbulent HCCI/PCCI combustion are conducted with different initial temperature fluctuations. A detailed kinetic mechanism including 53species and 325elementary reactions is used for methane–air reaction and a reduced kinetic mechanism including 37species and 61reactions is adopted for n-heptane–air reaction. The present DNS results show that the combustion process is highly sensitive to initial temperature fluctuations. The proposed method is applied to DNS results and the characteristics of fluid element classified into each combustion regime are investigated. By adopting appropriate threshold for the maximum and minimum heat conduction to local mixture, the contributions of three combustion regimes to whole fuel consumption can be discussed. The autoignition regime is dominant for lower temperature fluctuation case. However, the flame propagation and flame enclosing regimes turn to be significant for higher temperature fluctuation case where the autoignition regime is less than 20% and only triggers combustion start. Such qualitative tendency is confirmed in two fuel species. However, quantitatively, the transition of dominant combustion regime from autoignition to flame propagation/flame enclosing takes place at lower temperature fluctuation for the n-heptane–air mixture case. The proposed classification method effectively characterizes the turbulent combustion process in HCCI/PCCI engines. This classification method can be extended to quantify reaction mechanisms by tracking representative quantities such as the mole fraction of key intermediates.

Evaluation of scale resolving turbulence generation methods for Large Eddy Simulation of turbulent flows

Large Eddy Simulation (LES) has become an attractive simulation method even for technical processes and it usually provides space and time resolved fluctuations of a significant portion of the spectrum. However, in contrast to a RANS simulation an accurate LES requires the definition of suitable initial and boundary conditions, which includes turbulent structures with physically sound spatial and temporal correlations. Such turbulent structures are usually generated artificially at the boundary. Three different algorithms for generating turbulent fluctuations are evaluated in the present work. The investigated methods are Filtered noise [1], Diffused noise [2] and an Inverse Fourier approach [3,4]. These techniques were developed for generating inflow data for LES and have already been used in published research [5–15], e.g. for investigating turbulent combustion processes. In the present work the turbulent statistics i.e. energy spectra and velocity correlations as well as derived quantities such as turbulent kinetic energy and subgrid scale viscosity are investigated in more detail in a comparative fashion for the generated turbulent velocity fields. As a simple test case, the decay of turbulence in a cubical box, is considered here to provide information on the initially generated turbulence as well as its temporal evolution. The results are analyzed in detail and are compared to experimental data. Turbulence fluctuations generated by Filtered noise and Diffused noise lead to similar results. The resulting energy spectra and velocity correlations agree generally well with experimental data despite some discrepancies at very early times after initialization. The Inverse Fourier approach yielded good agreement at all times, but at increased computational cost. In addition, the implementation of Filtered noise and Diffused noise might be easier for most cases of practical interest. In particular, the Diffused noise approach can be used for the generation of inhomogeneous turbulence on arbitrary grids.