Large Eddy Simulation of Premixed Combustion: Sensitivity to Subgrid Scale Velocity Modeling

The turbulent reacting flow in an industrial gas turbine combustor operating at 3 bar is computed using LES paradigm. The subgrid scale (SGS) combustion is modelled using a collection of unstrained premixed flamelets including mixture stratification.The non-premixed combustion mode is also included using a simple closure involving the scalar dissipation rate of the mixture fraction. A close attention is paid to maintain physical consistencies among sub-closure models for combustion and these consistencies are discussed on a physical basis. The importance of non-premixed mode and SGS mixture fraction fluctuations are investigated systematically. The results show that the SGS mixture fraction variance plays an important role and comparisons to measurements improve when contributions from the premixed and non-premixed modes are included. These numerical results and observations are discussed on a physical basis along with potential avenues for further improvements. a Research Associate, il246@cam.ac.uk b Research Associate, zc252@cam.ac.uk c Professor, ns341@cam.ac.uk d Engineer, suresh.sadasivuni@siemens.com 4 combustors. Thus, the ability of reacting flow CFD (Computational Fluid Dynamics) to capture these phenomena is crucial for their use in the design of next-generation combustors [2].Past studies have demonstrated that Large Eddy Simulation (LES) is suitable to capture aerodynamics of swirling flows, and the continuous increase of computing power allows application of LES-based models to practical burners [3][4][5][6][7]. Combustion requires modelling as it is a subgrid phenomenon in LES. Closures developed for the subgrid scale (SGS) reaction rate include: dynamic thickened flame model [8,9], linear eddy model [10], fractal flame-wrinkling model [11], partiallystirred reactor model [12], and Eulerian stochastic fields [13]. Another recently developed flamelet model for LES uses Scalar Dissipation Rate (SDR) closure [14,15], which is shown to be successful in RANS studies of industrial gas turbine combustors [16,17], but this approach has not been tested for LES of reacting flows in these combustors. This provides the motivation for this work.Due to large costs for experiments at realistic operating conditions of gas turbines (i.e. with optical access, high pressure, and preheated air), high quality validation data is rare [18]. One widely studied database is the set of laser-diagnostics obtained for Siemens SGT-100 combustor at 3 bar [19]. Analyses of these measurements and past LES results suggest that the combustion has flamelet-like properties despite the highly turbulent flow [5-7, 20, 21]. Flamelet models assume that the combustion time scale, τ c = s L /δ is shorter than the smallest turbulent scales (this also applies for length scales) implying that the flamelet structure is undisturbed by the turbulence. Thus, the SGS reaction rate can be calculated a priori using laminar flamelets. Hence, this methodology is also known as tabulated chemistry approach.Turbulent eddies can penetrate the flame-front di...

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“…The SGS kinetic energy k is obtained using [32], [31]. The SGS pressure-work term, Π, is neglected based on previous LES study [33].…”

Section: A Governing Equationsmentioning

confidence: 99%

Large-Eddy Simulation of Reacting Flows in Industrial Gas Turbine Combustor

Langella

Chen

Swaminathan

et al. 2018

Journal of Propulsion and Power

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“…In the range of conditions investigated here with relatively low sub-grid fluctuations, the above mixing method is relatively robust in premixed flames [29]. It is in principle possible to include a chemical time-scale in the micro-mixing [35,36], to improve predictions with large sub-grid fluctuations. However, this has not been attempted in the present work The method has been successfully applied to capture flame extinction [21], auto-ignition [12] as well as several swirl flames and gas-turbine configurations [22,30,38] among others.…”

Section: Eulerian Stochastic Fields Formulationmentioning

confidence: 98%

Investigation of the Jet-Flame Interaction by Large Eddy Simulation and Proper Decomposition Method

Noh

Navarro‐Martinez

2019

Combustion Science and Technology

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Large Eddy Simulation (LES) results are presented for a premixed methane/air turbulent flame arising from a confined laboratory scale single-nozzle burner. The jet issuing from an off-centred nozzle facilitates the development of a large-scale, dominant lateral recirculation zone that stabilises the flame. A self-sustained jet oscillation is present, which intermittently causes extreme flame fluctuations such as blow-out and re-light events in the bottom section of the combustion chamber. The combined probability density function transport approach with the Eulerian stochastic fields method are used to numerically investigate the influence of this jet oscillation on combustion stability at the operating condition near lean blowout. The general structure of the flow, including the formation of the recirculation zones depending on the location of the flapping jet, is well reproduced together with the mean and fluctuating velocity profiles. The behaviour of the jet oscillation is investigated using a popular decomposition method known as proper orthogonal decomposition (POD) based on the predicted three-dimensional flow fields. Thanks to POD, the evolution of the simulated flame structure featuring a pronounced flame fluctuation is compared against that experimentally measured according to the phase angles of the low-order modelled jet motion. The absence of the most dominant coherent structure at a single frequency is due to a feedback mechanism between the jet oscillation and combustion process. The simulation shows that a low-frequency jet flapping causes the flame blow-out and flashback in the bottom CONTACT S Navarro-Martinez.

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“…[1]. However, this leads to additional unclosed terms [13,25] and computational cost, and is thus often not the preferred choice in a LES. Models which directly estimate the SGS velocity have been developed in the past mainly for nonreacting flows, and their corresponding model constants were derived assuming homogeneity and isotropy of the flow.…”

Section: B Subgrid Scale Velocity Modellingmentioning

confidence: 99%

Study of subgrid-scale velocity models for reacting and nonreacting flows

et al. 2018

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A study is conducted to identify advantages and limitations of existing large-eddy simulation (LES) closures for the subgrid-scale (SGS) kinetic energy using a database of direct numerical simulations (DNS). The analysis is conducted for both reacting and nonreacting flows, different turbulence conditions, and various filter sizes. A model, based on dissipation and diffusion of momentum (LD-D model), is proposed in this paper based on the observed behavior of four existing models. Our model shows the best overall agreements with DNS statistics. Two main investigations are conducted for both reacting and nonreacting flows: (i) an investigation on the robustness of the model constants, showing that commonly used constants lead to a severe underestimation of the SGS kinetic energy and enlightening their dependence on Reynolds number and filter size; and (ii) an investigation on the statistical behavior of the SGS closures, which suggests that the dissipation of momentum is the key parameter to be considered in such closures and that dilatation effect is important and must be captured correctly in reacting flows. Additional properties of SGS kinetic energy modeling are identified and discussed.

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Large Eddy Simulation of Premixed Combustion: Sensitivity to Subgrid Scale Velocity Modeling

Cited by 22 publications

References 66 publications

Large-Eddy Simulation of Reacting Flows in Industrial Gas Turbine Combustor

Large-Eddy Simulation of Reacting Flows in Industrial Gas Turbine Combustor

Investigation of the Jet-Flame Interaction by Large Eddy Simulation and Proper Decomposition Method

Study of subgrid-scale velocity models for reacting and nonreacting flows

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