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Article
Publication date: 6 February 2017

Alain Fossi and Alain DeChamplain

Safety improvement and pollutant reduction in many practical combustion systems and especially in aero-gas turbine engines require an adequate understanding of flame…

Abstract

Purpose

Safety improvement and pollutant reduction in many practical combustion systems and especially in aero-gas turbine engines require an adequate understanding of flame ignition and stabilization mechanisms. Improved software and hardware have opened up greater possibilities for translating basic knowledge and the results of experiments into better designs. The present study deals with the large eddy simulation (LES) of an ignition sequence in a conical shaped bluff-body stabilized burner involving a turbulent non-premixed flame. The purpose of this paper is to investigate the impact of spark location on ignition success. Particular attention is paid to the ease of handling of the numerical tool, the computational cost and the accuracy of the results.

Design/methodology/approach

The discrete particle ignition kernel (DPIK) model is used to capture the ignition kernel dynamics in its early stage of growth after the breakdown period. The ignition model is coupled with two combustion models based on the mixture fraction-progress variable formulation. An infinitely fast chemistry assumption is first done, and the turbulent fluctuations of the progress variable are captured with a bimodal probability density function (PDF) in the line of the Bray–Moss–Libby (BML) model. Thereafter, a finite rate chemistry assumption is considered through the flamelet-generated manifold (FGM) method. In these two assumptions, the classical beta-PDF is used to model the temporal fluctuations of the mixture fraction in the turbulent flow. To model subgrid scale stresses and residual scalars fluxes, the wall-adapting local eddy (WALE) and the eddy diffusivity models are, respectively, used under the low-Mach number assumption.

Findings

Numerical results of velocity and mixing fields, as well as the ignition sequences, are validated through a comparison with their experimental counterparts. It is found that by coupling the DPIK model with each of the two combustion models implemented in a LES-based solver, the ignition event is reasonably predicted with further improvements provided by the finite rate chemistry assumption. Finally, the spark locations most likely to lead to a complete ignition of the burner are found to be around the shear layer delimiting the central recirculation zone, owing to the presence of a mixture within flammability limits.

Research limitations/implications

Some discrepancies are found in the radial profiles of the radial velocity and consequently in those of the mixture fraction, owing to a mismatch of the radial velocity at the inlet section of the computational domain. Also, unlike FGM methods, the BML model predicts the overall ignition earlier than suggested by the experiment; this may be related to the overestimation of the reaction rate, especially in the zones such as flame holder wakes which feature high strain rate due to fuel-air mixing.

Practical implications

This work is adding a contribution for ignition modeling, which is a crucial issue in various combustion systems and especially in aircraft engines. The exclusive use of a commercial computational fluid dynamics (CFD) code widely used by combustion system manufacturers allows a direct application of this simulation approach to other configurations while keeping computing costs at an affordable level.

Originality/value

This study provides a robust and simple way to address some ignition issues in various spark ignition-based engines, namely, the optimization of engines ignition with affordable computational costs. Based on the promising results obtained in the current work, it would be relevant to extend this simulation approach to spray combustion that is required for aircraft engines because of storage volume constraints. From this standpoint, the simulation approach formulated in the present work is useful to engineers interested in optimizing the engines ignition at the design stage.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 27 no. 2
Type: Research Article
ISSN: 0961-5539

Keywords

Article
Publication date: 6 November 2020

Alain Fossi, Alain DeChamplain, Benjamin Akih-Kumgeh and Jeffrey Bergthorson

This study aims to deal with the large eddy simulation (LES) of an ignition sequence and the resulting steady combustion in a swirl-stabilized liquid-fueled combustor…

Abstract

Purpose

This study aims to deal with the large eddy simulation (LES) of an ignition sequence and the resulting steady combustion in a swirl-stabilized liquid-fueled combustor. Particular attention is paid to the ease of handling the numerical tool, the accuracy of the results and the reasonable computational cost involved. The primary aim of the study is to appraise the ability of the newly developed computational fluid dynamics (CFD) methodology to retrieve the spark-based flame kernel initiation, its propagation until the full ignition of the combustion chamber, the flame stabilization and the combustion processes governing the steady combustion regime.

Design/methodology/approach

The CFD model consists of an LES-based spray module coupled to a subgrid-scale ignition model to capture the flame kernel initiation and the early stage of the flame kernel growth, and a combustion model based on the mixture fraction-progress variable formulation in the line of the flamelet generated manifold (FGM) method to retrieve the subsequent flame propagation and combustion properties. The LES-spray module is based on an Eulerian-Lagrangian approach and includes a fully two-way coupling at each time step to account for the interactions between the liquid and the gaseous phases. The Wall-Adapting Local Eddy-viscosity (WALE) model is used for the flow field while the eddy diffusivity model is used for the scalar fluxes. The fuel is liquid kerosene, injected in the form of a polydisperse spray of droplets. The spray dynamics are tracked using the Lagrangian procedure, and the phase transition of droplets is calculated using a non-equilibrium evaporation model. The oxidation mechanism of the Jet A-1 surrogate is described through a reduced reaction mechanism derived from a detailed mechanism using a species sensitivity method.

Findings

By comparing the numerical results with a set of published data for a swirl-stabilized spray flame, the proposed CFD methodology is found capable of capturing the whole spark-based ignition sequence in a liquid-fueled combustion chamber and the main flame characteristics in the steady combustion regime with reasonable computing costs.

Research limitations/implications

The proposed CFD methodology simulates the whole ignition sequence, namely, the flame kernel initiation, its propagation to fully ignite the combustion chamber, and the global flame stabilization. Due to the lack of experimental ignition data on this liquid-fueled configuration, the ability of the proposed CFD methodology to accurately predict ignition timing was not quantitatively assessed. It would, therefore, be interesting to apply this CFD methodology to other configurations that have experimental ignition data, to quantitatively assess its ability to predict the ignition timing and the flame characteristics during the ignition sequence. Such further investigations will not only provide further validation of the proposed methodology but also will potentially identify its shortfalls for better improvement.

Practical implications

This CFD methodology is developed by customizing a commercial CFD code widely used in the industry. It is, therefore, directly applicable to practical configurations, and provides not only a relatively straightforward approach to predict an ignition sequence in liquid-fueled combustion chambers but also a robust way to predict the flame characteristics in the steady combustion regime as significant improvements are noticed on the prediction of slow species.

Originality/value

The incorporation of the subgrid ignition model paired with a combustion model based on tabulated chemistry allows reducing computational costs involved in the simulation of the ignition phase. The incorporation of the FGM-based tabulated chemistry provides a drastic reduction of computing resources with reasonable accuracy. The CFD methodology is developed using the platform of a commercial CFD code widely used in the industry for relatively straightforward applicability.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 31 no. 6
Type: Research Article
ISSN: 0961-5539

Keywords

Article
Publication date: 26 November 2020

Mostafa Esmaeili and Asghar Afshari

This study aims to numerically investigate the flow features and mixing/combustion efficiencies in a turbulent reacting jet in cross-flow by a hybrid Eulerian-Lagrangian…

Abstract

Purpose

This study aims to numerically investigate the flow features and mixing/combustion efficiencies in a turbulent reacting jet in cross-flow by a hybrid Eulerian-Lagrangian methodology.

Design/methodology/approach

A high-order hybrid solver is employed where, the velocity field is obtained by solving the Eulerian filtered compressible transport equations while the species are simulated by using the filtered mass density function (FMDF) method.

Findings

The main features of a reacting JICF flame are reproduced by the large-eddy simulation (LES)/FMDF method. The computed mean and root-mean-square values of velocity and mean temperature field are in good agreement with experimental data. Reacting JICF’s with different momentum ratios are considered. The jet penetrates deeper for higher momentum ratios. Mixing and combustion efficiency are improved by increasing the momentum ratio.

Originality/value

The authors investigate the flow and combustion characteristics in subsonic reacting JICFs for which very limited studies are reported in the literature.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 31 no. 7
Type: Research Article
ISSN: 0961-5539

Keywords

Article
Publication date: 1 June 2005

W.G. Weng, W.C. Fan and Y. Hasemi

To investigate the fluid structure of gravity current in backdraft consisted of the hot gas and the ambient air, to predict the ignition time for backdraft and to study…

Abstract

Purpose

To investigate the fluid structure of gravity current in backdraft consisted of the hot gas and the ambient air, to predict the ignition time for backdraft and to study the effect of opening geometries on the ignition time.

Design/methodology/approach

Numerical models based on large eddy simulation in fire dynamics simulator are adopted to study the ignition time.

Findings

The density (temperature) profiles and velocity fields from the numerical simulation show the typical fluid structure of gravity current, i.e. the slightly raised head, the billows formed behind the head and the lobes and clefts at the leading edge. The increased mixing of gravity current by the ceiling opening geometries comparing to the mixing by the end opening geometries is a result of the three‐dimensional flow. The non‐dimensional velocity presented here is independent of the different normalized density differences, and only depends on the different opening geometries. From this result, it is feasible to predict the ignition time for backdraft in a compartment.

Originality/value

This paper provides a method for predicting the ignition time for backdraft, and offers helps for people, especially firefighters, avoid the hazard from backdraft.

Details

Engineering Computations, vol. 22 no. 4
Type: Research Article
ISSN: 0264-4401

Keywords

Article
Publication date: 3 January 2017

Seyi F. Olatoyinbo, Sarma L. Rani and Abdelkader Frendi

The purpose of this study is to investigate the accuracy and applicability of the Flowfield Dependent Variation (FDV) method for large-eddy simulations (LES) of decaying…

Abstract

Purpose

The purpose of this study is to investigate the accuracy and applicability of the Flowfield Dependent Variation (FDV) method for large-eddy simulations (LES) of decaying isotropic turbulence.

Design/methodology/approach

In an earlier paper, the FDV method was successfully demonstrated for simulations of laminar flows with speeds varying from low subsonic to high supersonic Mach numbers. In the current study, the FDV method, implemented in a finite element framework, is used to perform LESs of decaying isotropic turbulence. The FDV method is fundamentally derived from the Lax–Wendroff Scheme (LWS) by replacing the explicit time derivatives in LWS with a weighted combination of explicit and implicit time derivatives. The increased implicitness and the inherent numerical dissipation of FDV contribute to the scheme’s numerical stability and monotonicity. Understanding the role of numerical dissipation that is inherent to the FDV method is essential for the maturation of FDV into a robust scheme for LES of turbulent flows. Accordingly, three types of LES of decaying isotropic turbulence were performed. The first two types of LES utilized explicit subgrid scale (SGS) models, namely, the constant-coefficient Smagorinsky and dynamic Smagorinsky models. In the third, no explicit SGS model was employed; instead, the numerical dissipation inherent to FDV was used to emulate the role played by explicit SGS models. Such an approach is commonly known as Implicit LES (ILES). A new formulation was also developed for quantifying the FDV numerical viscosity that principally arises from the convective terms of the filtered Navier–Stokes equations.

Findings

The temporal variation of the turbulent kinetic energy and enstrophy and the energy spectra are presented and analyzed. At all grid resolutions, the temporal profiles of kinetic energy showed good agreement with t(−1.43) theoretical scaling in the fully developed turbulent flow regime, where t represents time. The energy spectra also showed reasonable agreement with the Kolmogorov’s k(−5/3) power law in the inertial subrange, with the spectra moving closer to the Kolmogorov scaling at higher-grid resolutions. The intrinsic numerical viscosity and the dissipation rate of the FDV scheme are quantified, both in physical and spectral spaces, and compared with those of the two SGS LES runs. Furthermore, at a finite number of flow realizations, the numerical viscosities of FDV and of the Streamline Upwind/Petrov–Galerkin (SUPG) finite element method are compared. In the initial stages of turbulence development, all three LES cases have similar viscosities. But, once the turbulence is fully developed, implicit LES is less dissipative compared to the two SGS LES runs. It was also observed that the SUPG method is significantly more dissipative than the three LES approaches.

Research limitations/implications

Just as any computational method, the limitations are based on the available computational resources.

Practical implications

Solving problems involving turbulent flows is by far the biggest challenge facing engineers and scientists in the twenty-first century, this is the road that the authors have embarked upon in this paper and the road ahead of is very long.

Social implications

Understanding turbulence is a very lofty goal and a challenging one as well; however, if the authors succeed, the rewards are limitless.

Originality/value

The derivation of an explicit expression for the numerical viscosity tensor of FDV is an important contribution of this study, and is a crucial step forward in elucidating the fundamental properties of the FDV method. The comparison of viscosities for the three LES cases and the SUPG method has important implications for the application of ILES approach for turbulent flow simulations.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 27 no. 1
Type: Research Article
ISSN: 0961-5539

Keywords

Article
Publication date: 8 November 2021

Ender Hepkaya and Nuri Yucel

This study aims to methodologically investigate heat transfer effects on reacting flow inside a liquid-fueled, swirl-stabilized burner. Furthermore, particular attention…

Abstract

Purpose

This study aims to methodologically investigate heat transfer effects on reacting flow inside a liquid-fueled, swirl-stabilized burner. Furthermore, particular attention is paid to turbulence modeling and the results of Reynolds-averaged Navier–Stokes and large eddy simulation approaches are compared in terms of velocity field and flame temperature.

Design/methodology/approach

Simulations consist liquid fuel distribution using Eulerian–Lagrangian approach. Flamelet-Generated Manifold combustion model, which is a mixture fraction-progress variable formulation, is used to obtain reacting flow field. Discrete ordinates method is also added for modeling radiation heat transfer effect inside the burner. As a parametric study, different thermal boundary conditions namely: adiabatic wall, constant temperature and heat transfer coefficient are applied. Because of the fact that the burner is designed for operating with different materials, the effects of burner material on heat transfer and combustion processes are investigated. Additionally, material temperatures have been calculated using 1 D method. Finally, soot particles, which are source of luminous radiation in gas turbine combustors, are calculated using Moss-Brookes model.

Findings

The results show that the flow behavior is obviously different in recirculation region for both turbulence modeling approach, and this difference causes change on flame temperature distribution, particularly in the outer recirculation zone and region close to swirler. In thermal assessment of the burner, it is predicted that material of the burner walls and the applied thermal boundary conditions have significant influence on flame temperature, wall temperature and flow field. The radiation heat transfer also makes a strong impact on combustion inside the burner; however, luminous radiation arising from soot particles is negligible for the current case.

Originality/value

These types of burners are widely used in research of gas turbine combustion, and it can be seen that the heat transfer effects are generally neglected or oversimplified in the literature. This parametric study provides a basic understanding and methodology of the heat transfer effects on combustion to the researchers.

Details

Aircraft Engineering and Aerospace Technology, vol. 94 no. 4
Type: Research Article
ISSN: 1748-8842

Keywords

Article
Publication date: 7 June 2013

Qi Zhang and Lei Pang

Explosions are the main type of accident causing casualties in underground coal mines. Little attention has been devoted to investigating the flame propagations for…

Abstract

Purpose

Explosions are the main type of accident causing casualties in underground coal mines. Little attention has been devoted to investigating the flame propagations for methane‐air explosion in a tunnel with full scale. This paper seeks to address this topic.

Design/methodology/approach

Based on the numerical simulation and the analysis, the propagation rule of flame and temperature waves inside and outside the space occupied by methane/air mixture at the various concentrations in a tunnel were obtained in this work.

Findings

The original interface of methane‐air mixture and air moves forward in the explosion and the original mixture area extends. For the methane‐air mixture with rich fuel concentration, the flame speed increases with distance within a range beyond the original position of the interface between the mixture and air. The flame speed reaches maximum value outside the original area of methane‐air mixture with rich fuel concentration.

Originality/value

Based on the numerical simulation and the analysis, the propagation rule of flame and temperature wave inside and outside the space occupied by methane/air mixture at the various concentrations in a tunnel were obtained.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 23 no. 5
Type: Research Article
ISSN: 0961-5539

Keywords

Article
Publication date: 11 October 2018

Salman Arshad, Bo Kong, Alan Kerstein and Michael Oevermann

The purpose of this numerical work is to present and test a new approach for large-scale scalar advection (splicing) in large eddy simulations (LES) that use the linear…

Abstract

Purpose

The purpose of this numerical work is to present and test a new approach for large-scale scalar advection (splicing) in large eddy simulations (LES) that use the linear eddy sub-grid mixing model (LEM) called the LES-LEM.

Design/methodology/approach

The new splicing strategy is based on an ordered flux of spliced LEM segments. The principle is that low-flux segments have less momentum than high-flux segments and, therefore, are displaced less than high-flux segments. This strategy affects the order of both inflowing and outflowing LEM segments of an LES cell. The new splicing approach is implemented in a pressure-based fluid solver and tested by simulation of passive scalar transport in a co-flowing turbulent rectangular jet, instead of combustion simulation, to perform an isolated investigation of splicing. Comparison of the new splicing with a previous splicing approach is also done.

Findings

The simulation results show that the velocity statistics and passive scalar mixing are correctly predicted using the new splicing approach for the LES-LEM. It is argued that modeling of large-scale advection in the LES-LEM via splicing is reasonable, and the new splicing approach potentially captures the physics better than the old approach. The standard LES sub-grid mixing models do not represent turbulent mixing in a proper way because they do not adequately represent molecular diffusion processes and counter gradient effects. Scalar mixing in turbulent flow consists of two different processes, i.e. turbulent mixing that increases the interface between unmixed species and molecular diffusion. It is crucial to model these two processes individually at their respective time scales. The LEM explicitly includes both of these processes and has been used successfully as a sub-grid scalar mixing model (McMurtry et al., 1992; Sone and Menon, 2003). Here, the turbulent mixing capabilities of the LES-LEM with a modified splicing treatment are examined.

Originality/value

The splicing strategy proposed for the LES-LEM is original and has not been investigated before. Also, it is the first LES-LEM implementation using unstructured grids.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 28 no. 10
Type: Research Article
ISSN: 0961-5539

Keywords

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