Unstrained and strained flamelets for LES of premixed combustion

The unstrained and strained flamelet closures for filtered reaction rate in large eddy simulation (LES) of premixed flames are studied. The required sub-grid scale (SGS) PDF in these closures is presumed using the Beta function. The relative performances of these closures are assessed by comparing numerical results from large eddy simulations of piloted Bunsen flames of stoichiometric methane–air mixture with experimental measurements. The strained flamelets closure is observed to underestimate the burn rate and thus the reactive scalars mass fractions are under-predicted with an over-prediction of fuel mass fraction compared with the unstrained flamelet closure. The physical reasons for this relative behaviour are discussed. The results of unstrained flamelet closure compare well with experimental data. The SGS variance of the progress variable required for the presumed PDF is obtained by solving its transport equation. An order of magnitude analysis of this equation suggests that the commonly used algebraic model obtained by balancing source and sink in this transport equation does not hold. This algebraic model is shown to underestimate the SGS variance substantially and the implications of this variance model for the filtered reaction rate closures are highlighted.The authors express their gratitude to EPSRC, Siemens and Rolls-Royce for their support. This work is funded by the grant numbered EP/I027556/1.This is the final version of the article. It first appeared from Taylor & Francis via https://doi.org/10.1080/13647830.2016.114023

Application of unstrained flamelet SGS closure for multi-regime premixed combustion

Large eddy simulation of turbulent premixed combustion behind a bluff-body is performed using unstrained flamelet model with a presumed probability density function to calculate filtered reaction rate. The subgrid variance of the progress variable required in this approach is calculated using its transport equation to include contributions from reaction, turbulence and molecular diffusive and dissipative processes at sub-grid scales. The dissipation rate of the variance is obtained using an algebraic closure, which maintains physical consistency among turbulence, reaction and molecular diffusion. Various quantities such as mean velocity, temperature and species mass fractions computed for two bluff-body flames experiencing 2% and 24% turbulence intensities are compared to their respective measurements. These comparisons are very good suggesting that the unstrained flamelet SGS closure works well for multi-regime combustion. The demonstrated success of this modelling framework is explained on a physical basis.Engineering and Physical Sciences Research Council (Grant ID: EP/I027556/1), Siemens, Rolls-Royc

A scaling law for the recirculation zone length behind a bluff body in reacting flows

The recirculation zone length behind a bluff body is influenced by the turbulence intensity at the base of the body in isothermal flows and also the heat release and its interaction with turbulence in reacting flows. This relationship is observed to be nonlinear and is controlled by the balance of forces acting on the recirculation zone, which arise from the pressure and turbulence fields. The pressure force is directly influenced by the volumetric expansion resulting from the heat release, whereas the change in the turbulent shear force depends on the nonlinear interaction between turbulence and combustion. This behaviour is elucidated through a control volume analysis. A scaling relation for the recirculation zone length is deduced to relate the turbulence intensity and the amount of heat release. This relation is verified using the large eddy simulation data from 20 computations of isothermal flows and premixed flames that are stabilised behind the bluff body. The application of this scaling to flames in an open environment and behind a backward facing step is also explored. The observations and results are explained on a physical basis.EPSRC DTP studentship (RG80792

Prediction of local extinctions in piloted jet flames with inhomogeneous inlets using unstrained flamelets

Multi-regime turbulent combustion modelling remains challenging and is explored with occurrence of local extinction in this study. A partially premixed model based on unstrained premixed flamelets is used in this work to investigate a piloted jet flame configuration with inhomogeneous inlets. Three different cases are simulated, which differ in the bulk mean velocity that amounts respectively to about 50\%, 70\% and 90\% of the blow off velocity measured experimentally. As the jet velocity approaches the blow off limit, local extinctions start to occur along the flame surface and thus these flames are challenging from a modelling prospective. Two different numerical approaches, involving scaled and unscaled progress variable respectively, are compared to elucidate their abilities and limitations to predict local extinctions and to deal with the three-stream problem at the pilot/coflow interface. The key modelling details for such predictions are indicated and discussed. LES results are systematically compared to two sets of experimental measurements available in the literature for the three flames. The differences observed in the two experimental datasets are also discussed with the help of LES results. Although both approaches show promising agreement for the flame statistics, the scaled progress variable approach better predicts the local extinctions. The unscaled approach shows to naturally handle the three-stream problem without additional treatment for the pilot/coflow interface, which is required for the scaled approach. Furthermore, computed scalar dissipation rate of mixture fraction is compared with the measurements showing good agreement for the conditions investigated. This further suggests that local extinctions can be predicted using unstrained flamelets if the correct scalar mixing and its dissipation are captured.EPSR

Large-Eddy Simulation of Premixed Combustion in the Corrugated-Flamelet Regime

© 2015, Published with license by Taylor & Francis Group, LLC © 2015, © I. Langella, N. Swaminathan, F. A. Williams, and J. Furukawa. Large-eddy simulation (LES) is applied to a fuel-lean turbulent propane-air Bunsen flame in the corrugated-flamelet regime. The subgrid-scale (SGS) modeling includes a previously developed treatment of the total enthalpy along with three different SGS velocity, (Formula presented.) , models. In addressing the filtered reaction rate, a presumed probability density function (PDF) approach is employed for the reaction-progress variable, closed by a transport equation for its SGS variance. The statistics obtained using the three (Formula presented.) models are in good agreement with the measurements and do not differ significantly from each other for first-order moments suggesting that commonly used SGS modeling may be adequate to get the mean velocities and reaction progress variable. However, all three SGS velocity models fail to reflect a measured bimodality of the PDF of the radial component of the velocity in the central portion of the flame. This emphasizes a need for further development of (Formula presented.) models required at the reaction rate closure level for practical LES of combustion in the corrugated-flamelet regime

Verbi locativi in italiano come varianti di verbi supporto

In questo articolo discuteremo la nozione di "carrier verbs" (secondo la definizione del linguista americano Zellig Harris) o di "light verbs" (come denominata all'inizio del XX secolo da Otto Jespersen). La nostra cornice teorica è rappresentata dal lessico-grammatica di Maurice Gross, una teoria iniziata negli anni settanta a partire dalla sintassi del francese sulle basi matematiche già individuate per la lingua inglese da Zellig Harris. In particolare, discuteremo di come alcuni verbi locativi dell'italiano possano essere usati come verbi supporto (secondo la definizione data da Gross dei "light verbs" harrisiani) e assumano il ruolo sintattico una volta attribuito dai grammatici greci e latini alla copula esse.En aquest article discutirem la noció de "carrier verbs" (segons la definició del lingüista americà Zellig Harris) o de "light verbs" (segons la denominació que Otto Jespersen n'havia donat a principi del segle XX). El nostre marc teòric de referència és el lèxic-gramàtica de Maurice Gross, una teoria començada els anys setanta a partir de la sintaxi del francès en les bases matemàtiques ja individuades per a la llengua anglesa per Zelling Harris. Concretament, tractarem de com alguns verbs locatius de l'italià es poden emprar com a verbs de suport (tal com Gross va definir els "light verbs" harrisians) i de com assumeixen el paper sintàctic que els gramàtics grecs i llatins atribuïen a la còpula esse.In this article we aim at discussing the notion of carrier verbs (according to the definition provided by the American linguist Zellig Harris) or of light verbs (as defined at the beginning of the twentieth century by Otto Jespersen). Our theoretical framework is represented by Maurice Gross' lexicon-grammar, a theory developed during the seventies on the French syntax and directly inspired by Zellig Harris' mathematical grammar of English. Particularly, we are going to discuss of how some Italian locative verbs can be used as support verbs (according to the definition which Maurice Gross has given of the harrisian light verbs) and can play the syntactic role traditionally given to the copula esse by the Greek and Latin grammarians

A priori investigation of subgrid correlation of mixture fraction and progress variable in partially premixed flames

Subgrid correlation of mixture fraction, Z, and progress variable, c, is investigated using Direct Numerical Simulation (DNS) data of a hydrogen lifted jet flame. Joint subgrid behaviour of these two scalars are obtained using a Gaussian-type filter for a broad range of filter sizes. Joint probability density function (JPDF) constructed using single-snapshot DNS data is compared qualitatively with that computed using two independent β-PDFs and a copula method. Strong negative correlation observed at different streamwise locations in the flame is well captured by the copula method. The subgrid contribution to the Z-c correlation becomes important if the filter is of the size of laminar flame thickness or larger. A priori assessment for the filtered reaction rate using flamelet approach with independent β-PDFs and correlated JPDF is then performed. The comparison with the DNS data shows that both models provide reasonably good results for a range of filter sizes. However, the reaction rate computed using copula JPDF is found to have a better agreement with the DNS data for large filter sizes because the subgrid Z-c correlation effect is included

Entropy and vorticity wave generation in realistic gas turbine combustors

Understanding the nature of the unsteady flow at the combustor exit is required to accurately simulate time dependent phenomena in the turbine entry, such as indirect noise generation. Using Large Eddy Simulations of the combustion process in a realistic geometry, we analyse the flow at its exit. Two realistic, near-ground certification operating conditions are considered. Different mechanisms for large-scale flow and thermal structure generation are described, which are ejected into the turbine. Modal decomposition methods are used to extract the spatial and temporal scales at the turbine entry. We find that, depending on the operating condition, the entropy waves convect as elongated streaks in the core of the combustor annulus or the proximity of the walls. The dominant unsteady character of the fluctuations exhibits different spectral properties, i.e. low-frequency in the core and high-frequency towards walls. At the combustor exit, the vortical field is dominated by the swirl in the air inlet, which is found to have little influence on the entropy perturbations. Further, the importance of considering the interaction of multiple fuel injectors and combustion zones in an annular combustor is investigated. It is shown that pulsating circumferential vorticity modes can occur in multi-sector annular combustors but these, however, do not affect the entropy wave distribution.The authors wish to express their sincere gratitude to Rolls-Royce plc for permission to publish this paper. This work was conducted within the EU Horizon 2020 Framework Research Programme - Clean Sky (CORNET-CORE Noise Technologies), project number: H2020-CS2-CFP01-2014-01/ 686332

Filtered Reaction Rate Modelling in Moderate and High Karlovitz Number Flames: an a Priori Analysis

Abstract: Direct numerical simulations (DNS) of statistically planar flames at moderate and high Karlovitz number (Ka) have been used to perform an a priori evaluation of a presumed-PDF model approach for filtered reaction rate in the framework of large eddy simulation (LES) for different LES filter sizes. The model is statistical and uses a presumed shape, based here on a beta-distribution, for the sub-grid probability density function (PDF) of a reaction progress variable. Flamelet tabulation is used for the unfiltered reaction rate. It is known that presumed PDF with flamelet tabulation may lead to over-prediction of the modelled reaction rate. This is assessed in a methodical way using DNS of varying complexity, including single-step chemistry and complex methane/air chemistry at equivalence ratio 0.6. It is shown that the error is strongly related to the filter size. A correction function is proposed in this work which can reduce the error on the reaction rate modelling at low turbulence intensities by up to 50%, and which is obtained by imposing that the consumption speed based on the modelled reaction rate matches the exact one in the flamelet limit. A second analysis is also conducted to assess the accuracy of the flamelet assumption itself. This analysis is conducted for a wide range of Ka, from 6 to 4100. It is found that at high Ka this assumption is weaker as expected, however results improve with larger filter sizes due to the reduction of the scatter produced by the fluctuations of the exact reaction rate