Evaluation of the Uniform Conditional State Method for Turbulence-Chemistry Interaction Modelling of Swirl-Stabilized Flames

2020 ◽  
Author(s):  
Stefanie De Graaf ◽  
Ludovic de Guillebon ◽  
Marco Konle ◽  
W. Kendal Bushe
Keyword(s):  
Alternative Fuels ◽  
Initial Conditions ◽  
Mixture Fraction ◽  
Moment Closure ◽  
Inner Product ◽  
Conditional Moment ◽  
Local Composition ◽  
Conditional State ◽  
Stabilized Flames ◽  
Aero Engines

Abstract This paper considers a variation on Conditional Moment Closure (CMC) modelling for turbulence-chemistry interaction called the Uniform Conditional State (UCS) model and its application to the prediction of swirl-stabilized flames. UCS is essentially a zero-spatial dimensional, multi-condition CMC method. Unlike conventional CMC methods, for flames that are in (statistically) steady flows, the chemistry can be solved a priori in conditional space only. The reactive scalars are then mapped into real space by taking the inner product of the resulting conditional averages with the joint probability density function of the conditioning variables, here taken to have a presumed form that is a function of the mean and variance of the conditioning variables. Two conditioning variables are used, mixture fraction and progress variable. The combination of these allows for the resulting chemistry table to be applicable to both premixed and non-premixed combustion but also in the partially-premixed regime. In doing so, this new approach is promising to be highly suitable for simulating industrial applications and complex geometries. Another promising aspect is the universal applicability to different fuels and kinetic mechanisms providing great flexibility to the user of this method. Ultimately it is intended to aid the development of industrial burners by providing detailed information about the local composition and emission production, while keeping computational costs significantly low. Not only does this provide additional insight into global emissions and fuel consumption of a new design, but it allows for variability between different stages of mixedness as well as the testing of, for example, alternative fuels in established burner configurations. In this present study a comparison of different fuels and initial conditions is being conducted to analyze their effect on the resulting UCS solution — meaning the chemical source-terms, composition and thermodynamic state in conditional space. Furthermore the use of the UCS solutions as a predictive method in a RANS simulation is being presented here. The paper illustrates the UCS predictions and compares them to experimental data, as well as previously published simulation results of more established modelling approaches. The experimental test case chosen is a model combustor with a swirl-stabilized flame and high technical relevance which demonstrates the applicability of the UCS method to industrial designs for aero engines. Further investigations have begun including the application of this new tool to a real industrial combustor within the framework of this collaboration with MTU Aero Engines AG.

scholarly journals Flow Inhomogeneities in a Realistic Aeronautical Gas-Turbine Combustor: Formation, Evolution, and Indirect Noise

2018 ◽  
Vol 141 (1) ◽  
Author(s):  
Andrea Giusti ◽  
Luca Magri ◽  
Marco Zedda
Keyword(s):  
Gas Turbines ◽  
Transfer Functions ◽  
Guide Vane ◽  
Flame Structure ◽  
Combustion Noise ◽  
Moment Closure ◽  
Combustion Model ◽  
Conditional Moment ◽  
Eddy Simulation ◽  
Aero Engines

Indirect noise generated by the acceleration of combustion inhomogeneities is an important aspect in the design of aero-engines because of its impact on the overall noise emitted by an aircraft and the possible contribution to combustion instabilities. In this study, a realistic rich-quench-lean (RQL) combustor is numerically investigated, with the objective of quantitatively analyzing the formation and evolution of flow inhomogeneities and determining the level of indirect combustion noise in the nozzle guide vane (NGV). Both entropy and compositional noise are calculated in this work. A high-fidelity numerical simulation of the combustion chamber, based on the large-eddy simulation (LES) approach with the conditional moment closure (CMC) combustion model, is performed. The contributions of the different air streams to the formation of flow inhomogeneities are pinned down and separated with seven dedicated passive scalars. LES-CMC results are then used to determine the acoustic sources to feed an NGV aeroacoustic model, which outputs the noise generated by entropy and compositional inhomogeneities. Results show that non-negligible fluctuations of temperature and composition reach the combustor's exit. Combustion inhomogeneities originate both from finite-rate chemistry effects and incomplete mixing. In particular, the role of mixing with dilution and liner air flows on the level of combustion inhomogeneities at the combustor's exit is highlighted. The species that most contribute to indirect noise are identified and the transfer functions of a realistic NGV are computed. The noise level indicates that indirect noise generated by temperature fluctuations is larger than the indirect noise generated by compositional inhomogeneities, although the latter is not negligible and is expected to become louder in supersonic nozzles. It is also shown that relatively small fluctuations of the local flame structure can lead to significant variations of the nozzle transfer function, whose gain increases with the Mach number. This highlights the necessity of an on-line solution of the local flame structure, which is performed in this paper by CMC, for an accurate prediction of the level of compositional noise. This study opens new possibilities for the identification, separation, and calculation of the sources of indirect combustion noise in realistic aeronautical gas turbines.

Study on Conditional Statistics in Two-Dimensional Liquid Jet With the Second-Order Chemical Reaction

2011 ◽  
Author(s):  
Tomoaki Watanabe ◽  
Hiroki Yasuhara ◽  
Yasuhiko Sakai ◽  
Takashi Kubo ◽  
Kouji Nagata ◽  
...  
Keyword(s):  
Turbulent Flow ◽  
Chemical Reaction ◽  
Mixture Fraction ◽  
Spectroscopic Method ◽  
Second Order ◽  
Moment Closure ◽  
Liquid Jet ◽  
Two Dimensional ◽  
Conditional Moment ◽  
Conditional Moment Closure

It is important in engineering to elucidate the mechanism of a chemical reaction in turbulent flow. But there are still few studies on reacting turbulent flow in a liquid phase. In this study, the two-dimensional liquid jet with the second-order reaction (A+B←R) is investigated. The concentrations of the species R and the conserved scalar (which is the concentration of other species independent of the above chemical reaction) are measured simultaneously by the optical fiber probe based on light absorbtion spectroscopic method. The concentrations of species A and B are obtained from the conserved scalar theory. Regarding the velocity field, the streamwise velocity is measured by the hot-film anemometer. The moment closure methods are often used for the prediction of turbulent flow. But it is difficult to apply it to the reacting turbulent flow because of the high non-linearity of the reaction rate terms. It is commonly known that the values of concentrations depend strongly on the mixture fraction (which is a conserved scalar) defined as the normalized concentration of the species which is independent of reaction. Hence, Conditional moment closure (CMC) methods are useful for the prediction of the turbulent flow with chemical reactions. In this study, conditional scalar statistics are investigated by using the conditional moment closure methods and experimental data. It is shown that the conditional averages of concentration of reactant and product species approach the equilibrium limit (which correspond to the limiting case of the fast chemical reaction) in the downstream direction and the value of the conditional scalar (mixture fraction) dissipation decreases and its distribution varies in the downstream direction and comes to show the local minimum value near the point η = ξS (which is the stoichiometric value of the mixture fraction).

scholarly journals Turbulent Combustion Modelling and Experiments: Recent Trends and Developments

2019 ◽  
Vol 103 (4) ◽  
pp. 847-869 ◽  
Author(s):  
A. Giusti ◽  
E. Mastorakos
Keyword(s):  
Turbulent Combustion ◽  
Mixture Fraction ◽  
Moment Closure ◽  
Combustion Model ◽  
Complex Geometries ◽  
Conditional Moment ◽  
Practical Applications ◽  
Spray Flames ◽  
Reaction Zones ◽  
Soot Emissions

AbstractThe development of better laser-based experimental methods and the fast rise in computer power has created an unprecedented shift in turbulent combustion research. The range of species and quantities measured and the advent of kHz-level planar diagnostics are now providing great insights in important phenomena and applications such as local and global extinction, pollutants, and spray combustion that were hitherto unavailable. In simulations, the shift to LES allows better representation of the turbulent flow in complex geometries, but despite the fact that the grid size is smaller than in RANS, the push towards realistic conditions and the need to include more detailed chemistry that includes very fast species and thin reaction zones emphasize the necessity of a sub-grid turbulent combustion model. The paper discusses examples from current research with experiments and modelling that focus on flame transients (self-excited oscillations, local extinction), sprays, soot emissions, and on practical applications. These demonstrate how current models are being validated by experimental data and the concerted efforts the community is taking to promote the modelling tools to industry. In addition, the various coordinated International Workshops on non-premixed, premixed, and spray flames, and on soot are discussed and some of their target flames are explored. These comprise flames that are relatively simple to describe from a fluid mechanics perspective but contain difficult-to-model combustion problems such as extinction, pollutants and multi-mode reaction zones. Recently, swirl spray flames, which are more representative of industrial devices, have been added to the target flames. Typically, good agreement is found with LES and some combustion models such as the progress variable - mixture fraction flamelet model, the Conditional Moment Closure, and the Transported PDF method, but predicting soot emissions and the condition of complete extinction in complex geometries is still elusive.

Numerical investigation of methanol bluff-body flame using variants of RANS turbulence models with conditional moment closure model

2019 ◽  
pp. 21-30
Author(s):  
R. N. Roy ◽  
S. Sreedhara
Keyword(s):  
Reynolds Stress ◽  
Bluff Body ◽  
Mixture Fraction ◽  
Turbulence Models ◽  
Moment Closure ◽  
Closure Model ◽  
Conditional Moment ◽  
Conditional Moment Closure ◽  
Rans Turbulence Models ◽  
The Mean

In this article, conditional moment closure model (CMC) along with four variants of RANS turbulence models is used for investigating a methanol bluff-body flame. This work attempts to establish the accuracy of turbulence models in predicting the mixing fields, which results in improved predictions of the mean and variance of mixture fraction. This ensures an accurate probability density function (pdf) of the mixture fraction field which is used to obtain unconditional quantities from the conditional quantities calculated from CMC closure. The flow and mixing field are calculated using ANSYS Fluent software by incorporating four different turbulence models viz. standard k-ε (SKE), modified k-ε (MKE), RNG k-ε and Reynolds stress turbulence models. Flow field simulations have been coupled with an in-house CMC solver to obtain the mean flame structure. Profiles of mixture fraction showed an excellent agreement with the experimental data when Reynolds stress turbulence model was used. The unconditional mean temperature and species mass fraction obtained from the CMC model shows improved predictions when coupled with the Reynolds stress turbulence models. Because of inaccurate mixing field and hence the pdf predicted from SKE, MKE and RNG k-ε models, the unconditional quantities showed significant deviations from the experimental results.

scholarly journals Conditional Moment Closure Modelling of a Lifted H2/N2Turbulent Jet Flame Using the Presumed Mapping Function Approach

2015 ◽  
Vol 2015 ◽  
pp. 1-24
Author(s):  
Ahmad El Sayed ◽  
Roydon A. Fraser
Keyword(s):  
Mixture Fraction ◽  
Mapping Function ◽  
Moment Closure ◽  
Scalar Dissipation ◽  
Conditional Moment ◽  
Jet Flame ◽  
Velocity Statistics ◽  
Conditional Moment Closure ◽  
Consistent Manner ◽  
Radical History

A lifted hydrogen/nitrogen turbulent jet flame issuing into a vitiated coflow is investigated using the conditional moment closure (CMC) supplemented by the presumed mapping function (PMF) approach for the modelling of conditional mixing and velocity statistics. Using a prescribed reference field, the PMF approach yields a presumed probability density function (PDF) for the mixture fraction, which is then used in closing the conditional scalar dissipation rate (CSDR) and conditional velocity in a fully consistent manner. These closures are applied to a lifted flame and the findings are compared to previous results obtained usingβ-PDF-based closures over a range of coflow temperatures (Tc). The PMF results are in line with those of theβ-PDF and compare well to measurements. The transport budgets in mixture fraction and physical spaces and the radical history ahead of the stabilisation height indicate that the stabilisation mechanism is susceptible toTc. As in the previousβ-PDF calculations, autoignition around the “most reactive” mixture fraction remains the controlling mechanism for sufficiently highTc. Departure from theβ-PDF predictions is observed whenTcis decreased as PMF predicts stabilisation by means of premixed flame propagation. This conclusion is based on the observation that lean mixtures are heated by downstream burning mixtures in a preheat zone developing ahead of the stabilization height. The spurious sources, which stem from inconsistent CSDR modelling, are further investigated. The findings reveal that their effect is small but nonnegligible, most notably within the flame zone.

scholarly journals Consistent Conditional Moment Closure Modelling of a Lifted Turbulent Jet Flame Using the Presumedβ-PDF Approach

2014 ◽  
Vol 2014 ◽  
pp. 1-25 ◽  
Author(s):  
Ahmad El Sayed ◽  
Roydon A. Fraser
Keyword(s):  
Mixture Fraction ◽  
Turbulent Jet ◽  
Moment Closure ◽  
Scalar Dissipation ◽  
Conditional Moment ◽  
Jet Flame ◽  
Gradient Diffusion ◽  
Conditional Moment Closure ◽  
History Of ◽  
Closure Modelling

A liftedH2/N2turbulent jet flame issuing into a vitiated coflow is investigated using the conditional moment closure. The conditional velocity (CV) and the conditional scalar dissipation rate (CSDR) submodels are chosen such that they are fully consistent with the moments of the presumedβprobability density function (PDF). The CV is modelled using the PDF-gradient diffusion model. Two CSDR submodels based on the double integration of the homogeneous and inhomogeneous mixture fraction PDF transport equations are implemented. The effect of CSDR modelling is investigated over a range of coflow temperatures (Tc) and the stabilisation mechanism is determined from the analysis of the transport budgets and the history of radical build-up ahead of the stabilisation height. For allTc, the balance between chemistry, axial convection, and micromixing, and the absence of axial diffusion upstream of the stabilisation height indicate that the flame is stabilized by autoignition. This conclusion is confirmed from the rapid build-up ofHO2ahead ofH,O, andOH. The inhomogeneous CSDR modelling yields higher dissipation levels at the most reactive mixture fraction, which results in longer ignition delays and larger liftoff heights. The effect of the spurious sources arising from homogeneous modelling is found to be small but nonnegligible, mostly notably within the flame zone.

scholarly journals Modelling of a Bluff-Body Stabilised Premixed Flames Close to Blow-Off

Computation ◽  
2021 ◽  
Vol 9 (4) ◽  
pp. 43
Author(s):  
Shokri Amzin ◽  
Mohd Fairus Mohd Yasin
Keyword(s):  
Bluff Body ◽  
Fluid Dynamic ◽  
Premixed Flames ◽  
Pollutant Emissions ◽  
Moment Closure ◽  
Recirculation Region ◽  
Scalar Dissipation ◽  
Average Error ◽  
Conditional Moment ◽  
Lean Premixed

As emission legislation becomes more stringent, the modelling of turbulent lean premixed combustion is becoming an essential tool for designing efficient and environmentally friendly combustion systems. However, to predict emissions, reliable predictive models are required. Among the promising methods capable of predicting pollutant emissions with a long chemical time scale, such as nitrogen oxides (NOx), is conditional moment closure (CMC). However, the practical application of this method to turbulent premixed flames depends on the precision of the conditional scalar dissipation rate,. In this study, an alternative closure for this term is implemented in the RANS-CMC method. The method is validated against the velocity, temperature, and gas composition measurements of lean premixed flames close to blow-off, within the limit of computational fluid dynamic (CFD) capability. Acceptable agreement is achieved between the predicted and measured values near the burner, with an average error of 15%. The model reproduces the flame characteristics; some discrepancies are found within the recirculation region due to significant turbulence intensity.

scholarly journals Modelling of Conditional Scalar Dissipation Rate in Turbulent Premixed Combustion

Computation ◽  
2021 ◽  
Vol 9 (3) ◽  
pp. 26 ◽  
Author(s):  
Shokri Amzin ◽  
Mariusz Domagała
Keyword(s):  
Dissipation Rate ◽  
Premixed Flames ◽  
Moment Closure ◽  
Navier Stokes ◽  
Scalar Dissipation ◽  
Flame Surface ◽  
Scalar Dissipation Rate ◽  
Turbulent Premixed Flames ◽  
Conditional Moment ◽  
Turbulent Premixed

In turbulent premixed flames, for the mixing at a molecular level of reactants and products on the flame surface, it is crucial to sustain the combustion. This mixing phenomenon is featured by the scalar dissipation rate, which may be broadly defined as the rate of micro-mixing at small scales. This term, which appears in many turbulent combustion methods, includes the Conditional Moment Closure (CMC) and the Probability Density Function (PDF), requires an accurate model. In this study, a mathematical closure for the conditional mean scalar dissipation rate, <Nc|ζ>, in Reynolds, Averaged Navier–Stokes (RANS) context is proposed and tested against two different Direct Numerical Simulation (DNS) databases having different thermochemical and turbulence conditions. These databases consist of lean turbulent premixed V-flames of the CH4-air mixture and stoichiometric turbulent premixed flames of H2-air. The mathematical model has successfully predicted the peak and the typical profile of <Nc|ζ> with the sample space ζ and its prediction was consistent with an earlier study.

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