Large Eddy Simulations of gaseous flames in gas turbine combustion chambers

Recent developments in numerical schemes, turbulent combustion models and the regular increase of computing power allow Large Eddy Simulation (LES) to be applied to real industrial burners. In this paper, two types of LES in complex geometry combustors and of specific interest for aeronautical gas turbine burners are reviewed: (1) laboratory-scale combustors, without compressor or turbine, in which advanced measurements are possible and (2) combustion chambers of existing engines operated in realistic operating conditions. Laboratory-scale burners are designed to assess modeling and funda- mental flow aspects in controlled configurations. They are necessary to gauge LES strategies and identify potential limitations. In specific circumstances, they even offer near model-free or DNS-like LES computations. LES in real engines illustrate the potential of the approach in the context of industrial burners but are more difficult to validate due to the limited set of available measurements. Usual approaches for turbulence and combustion sub-grid models including chemistry modeling are first recalled. Limiting cases and range of validity of the models are specifically recalled before a discussion on the numerical breakthrough which have allowed LES to be applied to these complex cases. Specific issues linked to real gas turbine chambers are discussed: multi-perforation, complex acoustic impedances at inlet and outlet, annular chambers.. Examples are provided for mean flow predictions (velocity, temperature and species) as well as unsteady mechanisms (quenching, ignition, combustion instabil- ities). Finally, potential perspectives are proposed to further improve the use of LES for real gas turbine combustor designs

Turbulent mixing in temporally evolving stratified channel flow, an investigation through direct numerical simulations

Turbulent flows affected by stable density stratification occur ubiquitously in wide range of geophysical and environmental flows such as the ocean, the atmosphere or rivers and lakes. Within such flows the process of turbulent mixing plays a leading order role in numerous physical and ecological processes such as the vertical transport of heat, salt and nutrients as well as being vital to accurately predict global energy circulation models. A better understanding of the flow dynamics and the mechanisms that govern turbulent mixing as well as its accurate prediction in stratified flows is therefore crucial to accurately resolve such processes. If stratified flow is constrained by physical boundaries perpendicular to the gravitational vector, then the flow develops into a distinctly vertically inhomogeneous state that adds an additional layer of complexity into the mixing dynamics of the flow. Motivated by the stratified river flows of Australia, this thesis aims to enhance our knowledge and understanding of turbulent mixing in vertically inhomogeneous stratified flows through an extensive set of direct numerical simulations (DNS) of stratified channel flow. By performing a canonical ‘sunrise’ DNS of initially isothermal turbulent channel flow subject to sudden radiative heating we demonstrate that the flow undergoes an initial ‘rapid’ suppression of turbulence due the reduction of the fluctuating vertical velocity component w′ from the sudden introduction of stable stratification. The flow ‘slowly’ recovers towards a stationary state as the flow accelerates and the mean shear develops such that equilibrium is achieved in the turbulent kinetic energy and momentum flux balances. We demonstrate that for the temporally evolving flow, the global suppression of turbulent mixing defined by bulk measures of the eddy diffusivity and viscosity is well predicted by the mixed bulk parameter Ri−1 τ Reτ , where Riτ and Reτ are the friction Richardson and Reynolds numbers respectively such that mixing within the flow becomes strongly suppressed for Ri−1 τ Reτ ≲ 10 and approaches neutral conditions for Ri−1 τ Reτ ≳ 100. We find that the convergence of the flow towards stationarity is a globally parabolic process such that the flow at all depths simultaneously obtains equilibrium in the buoyancy and momentum fluxes. Scaling arguments are presented to demonstrate that this process may be parameterized through bulk flow properties such that the flow achieves equilibrium at Ri−1/2 τ (t/Tτ ) ≈ 2 provided that Ri−1 τ Reτ ≲ 100, where t is the measured time from its initial isothermal state and Tτ is the bulk friction time scale. We propose that the bulk scaling presented could lend itself as a useful forecasting tool for the onset of suppressed mixing in real stratified river flows. By considering instantaneous horizontal planar averages of the temporally evolving flow we observe three distinctly different mixing regimes separated by transitional values of turbulent Froude number Fr: a weakly stratified regime for Fr > 1, an intermediate regime for 0.3 < Fr < 1 and a saturated regime for Fr < 0.3. The mixing coefficient Γ is well predicted by the parametrization schemes of Maffioli et al. (2016) and Garanaik and Venayagamoorthy (2019) across all three regimes through instantaneous measurements of Fr and the ratio LE/LO, where LE and LO are the Ellison and Ozmidov length scales respectively. The flux Richardon number Rf shows linear dependence on the gradient Richardson number Rig up to a transitional value of Rig = 0.25 past which it saturates again to a constant value independent of Fr or Rig. By examining the flow as a balance of inertial, shear and buoyancy forces, we derive physically based scaling relationships to demonstrate that Rig ∼ Fr−2 and Rig ∼ Fr−1 in the weakly and moderately stratified regimes and that Rig becomes independent of Fr in the saturated regime. Our scaling analysis and results suggest that an extended range of the LE/LO ∼ Fr−1 scaling of Garanaik and Venayagamoorthy (2019) in the intermediate regime manifests due to the influence of mean shear. Hence we directly reconcile the Fr,Rig and LE/LO frameworks across all three mixing regimes for our shear driven flow. By adapting the density inversion criterion method of Portwood et al. (2016) for our flow, we demon- 1 strate that the flow may be robustly separated into regions of active turbulence for which ReB ≳ O(10) and quiescent fluid where ReB ≲ O(10), where ReB is the buoyancy Reynolds number. The intermittency in the surface heated channel flow spontaneously manifests as a deformed horizontal interface between the upper quiescent and lower turbulent flow. We find the region just below the interface is characterized by vigorous and efficient energetic mixing from Kelvin-Helmholtz type overturning instabilities, with the thickness of the interfacial layer being proportional to the Ellison length LE. The resulting vertical intermittency profile quantified through a depth varying turbulent volume fraction is accurately predicted by a local Monin-Obukhov length normalized in viscous wall units Λ+ such that the flow begins to display intermittency within the parameter range of 2.5 ≲ Λ+ ≲ 260. We find the ‘turbulent’ flow within this region to be described by constant critical gradient Richardson and turbulent Froude numbers of Rig,c ≈ 0.2, Frc ≈ 0.3 and Γc ≈ 0.25, suggesting that for our flow, critical mixing conditions arise from the intermittency resulting from stratification. By considering conditional averages of both the ‘turbulent’ and ‘quiescent’ flow separately within this critical regime, we find that the ‘turbulent’ flow continues to display a Γ ∼ Fr−1 relationship in the limit of Fr < Frc, while the quiescent flow shows no correlation between Γ and Fr. We demonstrate that for stratified open channel flow, the emergence of an asymptotic ‘saturated’ Γ regime in the limit of a low ‘global’ Fr occurs directly due to intermittency and increasing contributions to measurements of Γ from the quiescent flow

Direct Numerical Simulation Of Turbulent Flows Using An Implicit Finite Volume Algorithm

Tez (Doktora) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2012Thesis (PhD) -- İstanbul Technical University, Institute of Science and Technology, 2012Bu doktora tezi bir Doğrudan Sayısal Benzetim (DNS) uygulama çalışmasını içermektedir. Bu kapsamda, son dönemde önerilen bir çözüm algoritması seçilmiş, bunu temel alan bir paralel DNS çözücüsü geliştirilmiş ve türbülansa geçiş ve hidrodinamik kararsızlık içeren akış problemlerine uygulanmıştır. Bu çalışmanın ana amacı, sıkıştırılamaz ve sıkıştırılabilir türbülanslı akışların DNS metodu ile yapılan modellemesinde araç olarak kullanılabilecek bir çözücü geliştirmek ve bunu araştırmacıların kullanımına sunmak suretiyle bu alana katkıda bulunmaktır. Bir başka amaç da, sözkonusu algoritmayı aynı anda uzaysal ve zamansal olarak çok ölçekli fiziksel mekanizmalar içeren problemlere (örneğin laminer, geçişli ve türbülanslı rejimleri ve değişken Mach sayılarını bir arada içeren hidrodinamik kararsızlıklar) uygulamak yoluyla ileri testlerini yapmak ve üzerinde bazı iyileştirmeler geçekleştirmektir. Bu amaçla, Taylor-Green Vortex (TGV) akışındaki türbülansa geçiş problemi, türbülanslı kayma tabakasındaki (TSL) hıza bağlı karışım problemi ve Rayleigh-Taylor kararsızlığındaki (RTI) yerçekimine bağlı karışım problemi özellikle seçilmiştir. Sıkıştırılabilirlik etkileri de ayrıca incelenmiştir. Sonuçlar, daha önce gerçekleştirilmiş olan teorik, deneysel ve sayısal çalışmaların verileriyle uyum içerisindedir. Sözkonusu karmaşık fiziksel mekanizmalar, seçilen sayısal algoritma ve bunu esas alarak geliştirilen geliştirilen çözücü tarafından doğru bir biçimde elde edilmiştir.This Phd thesis study includes an application of Direct Numerical Simulasyon (DNS). A recently proposed solution algorithm was chosen, a DNS solver based on the algorithm has been developed and applied to the flow problems including transition to turbulence and hydrodynamic instability. The primary objective of this study is to make contributions into this area of modelling incompressible and compressible turbulent flows using DNS by developing an in-house, open source, parallel, implicit solver and presenting it as an open source research tool. Another goal is to perform the assessments of the algorithm and to make some improvements by applying it to the problems including spatial and temporal multi-scale physics simultaneously such as hydrodynamic instabilities where laminar, transitional, and turbulent regimes are found together with varying Mach number regimes. For this purpose, the transition to turbulence problem in Taylor-Green Vortex (TGV), the velocity-induced mixing problem in turbulent shear layer (TSL), and the gravity-induced mixing problem in Rayleigh-Taylor Instability (RTI) were carefully chosen as test cases. The effects of the compressibility were also analyzed. Our results are in agreement with the findings of theoretical results and the previous experimental and numerical studies. The complicated physical mechanisms mentioned above were succesfully captured by the algorithm and the solver as well.DoktoraPh

Does the choice of the forcing term affect flow statistics in DNS of turbulent channel flow?

We seek possible statistical consequences of the way a forcing term is added to the Navier--Stokes equations in the Direct Numerical Simulation (DNS) of incompressible channel flow. Simulations driven by constant flow rate, constant pressure gradient and constant power input are used to build large databases, and in particular to store the complete temporal trace of the wall-shear stress for later analysis. As these approaches correspond to different dynamical systems, it can in principle be envisaged that these differences are reflect by certain statistics of the turbulent flow field. The instantaneous realizations of the flow in the various simulations are obviously different, but, as expected, the usual one-point, one-time statistics do not show any appreciable difference. However, the PDF for the fluctuations of the streamwise component of wall friction reveals that the simulation with constant flow rate presents lower probabilities for extreme events of large positive friction. The low probability value of such events explains their negligible contribution to the commonly computed statistics; however, the very existence of a difference in the PDF demonstrates that the forcing term is not entirely uninfluential. Other statistics for wall-based quantities (the two components of friction and pressure) are examined; in particular spatio-temporal autocorrelations show small differences at large temporal separations, where unfortunately the residual statistical uncertainty is still of the same order of the observed difference. Hence we suggest that the specific choice of the forcing term does not produce important statistical consequences, unless one is interested in the strongest events of high wall friction, that are underestimated by a simulation run at constant flow rate

Assessment of LES sub-grid models for turbulent reacting flows

Imperial Users onl

Large eddy simulation of separated boundary layer transition under free-stream turbulence

Physics of laminar-to-turbulent transition in a separated-reattached flow subjected to two free-stream turbulence levels have been explored using Large-Eddy Simulation (LES). Separation of the laminar boundary layer occurs at a curvature change over a flat plate with a semi-circular leading edge. A numerical trip has been used to generate the targeted free-stream turbulence levels. A dynamic Sub-grid-scale (SGS) model has been employed and excellent agreement has been achieved between the LES results and the experimental data. Detailed investigation of the LES data has been carried out to explore the primary instability mechanism at low (< 0.2%) and high free-stream turbulence (5.6%). The flow visualisations and spectral analysis of the separated shear layer reveal that the two-dimensional Kelvin-Helmholtz instability mode, well known to occur at low free-stream turbulence levels, is bypassed at a higher level leading to earlier breakdown to turbulence. The whole transition process leading to breakdown to turbulence has been revealed clearly by the flow visualisations and the differences between the low and high free-stream turbulence cases are clearly evident. Coherent structures are also visualised using iso-surfaces of the Q-criterion and for the high free-stream turbulence case the spanwise oriented two-dimensional rolls, which are clearly apparent in the low free-stream turbulence case, are not visible anymore. Detailed quantitative comparisons between the present LES results against experimental data and the previous LES results at low free-stream turbulence using a staggered grid have been done and a good agreement has been obtained, indicating that the current LES using a co-located grid with pressure smoothing can predict transitional flows accurately. Comprehensive spectral analysis of the separated shear layer at two free-stream turbulence levels has been performed. Under very low free-stream turbulence condition, a distinct regular vortex shedding and trace of the low-frequency flapping phenomena were detected. Under the higher free-stream turbulence however, a mild high-frequency activity was observed. No low frequency oscillations could be detected

Effects of detailed finite rate chemistry in turbulent combustion

The development of advanced combustion energy-conversion systems requires accurate simulation tools, such as Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES), for capturing and understanding ignition, combustion instability, lean blowout, and emissions. However, the characteristic timescales in combustion systems can range from milliseconds to picoseconds or even lower. This renders the use of detailed finite rate chemistry prohibitive in DNS/LES of turbulent combustion, which requires the calculation of a large number of species and reactions on a large number of grid cells. Due to these high computational costs, DNS and LES typically employ either a flamelet model with detailed chemistry or a simplified/reduced finite rate chemistry with non-stiff reactions. Both approaches, however, are of limited accuracy and may reduce the overall prediction quality. To address this, a framework with high fidelity by incorporating finite rate chemistry, while mitigating additional computational cost, is necessary for the development of advanced combustion systems. In this dissertation, a new numerical framework for DNS and LES of turbulent combustion is established employing correlated dynamic adaptive chemistry (CoDAC), correlated evaluation of transport properties (CoTran), and a point-implicit stiff ODE solver (ODEPIM). CoDAC utilizes a path flux analysis (PFA) method to reduce the large chemical kinetics mechanism to a smaller size for each location and time step. Thermo-chemical correlation zones are introduced and only one PFA calculation is required for each zone, which diminishes the CPU overhead of CoDAC to negligible computation costs. CoTran uses a similar correlation method to accelerate the evaluation of mixture-averaged diffusion (MAD) coefficients. This framework is firstly applied to investigate the non-equilibrium plasma discharge of C2H4/O2/Ar mixtures in a low-temperature flow reactor. The accelerated case has been verified against the benchmark case by both temporal evolution and spatial distribution of several key species and gas temperature. Simulation results show that it accelerates the total computation time by a factor of 3.16, the calculation of chemical kinetics by a factor of 80, and the evaluation of MAD coefficients by a factor of 836. The high accuracy and efficiency of this proposed framework illustrate its promise in the simulation of diverse combustion problems. Secondly, this framework is evaluated for a canonical turbulent premixed flame employing a conventional jet fuel kinetics model. Again, the results show that the new framework provides a significant speed-up of chemical kinetics and transport computation, enabling DNS with large kinetics mechanisms while maintaining high accuracy and good parallel scalability. Detailed diagnostics show that, for this test case, calculation of the chemical source term with ODEPIM is 17 times faster than that of a pure implicit solver. CoDAC further speeds up the calculation of chemical source terms by 2.7 times. CoTran makes the evaluation of MAD coefficients 72 times faster. Comparing to the conventional DNS, the total computation time of this framework in this test is 20 times faster, with that of chemical kinetics 46 times faster, and that of the evaluation of transport properties 72 times faster. Based on the above DNS framework, an efficient finite-rate chemistry (FRC) - LES formulation is developed for numerical modeling of a turbulent jet flame. Comparing to the conventional FRC-LES, this framework provides a speed-up of 8.6 times for the chemistry calculation, and 6.4 times for the total computation, using a 20 species kinetics model. Both the new FRC-LES and flamelet/progress-variable (FPV)-LES are conducted for a piloted partially premixed methane/air flame (Sandia Flame D). The two approaches provide similar predictions in terms of time-averaged flame field and statistics, which agree well with the experimental data. For the instantaneous flame field, FPV-LES predicts significantly smaller regions with high temperature than the FRC-LES case, especially in the downstream region. Near the stoichiometric region, FPV-LES over-predicts the radical generation with respect to the experimental data, but under-predicts the CO generation and heat release, which explains its under-prediction of temperature. In contrast, on the fuel rich side, CO is no longer the bottleneck species, thus the FPV-LES predicts a higher temperature than FRC-LES. With respect to the experimental data, FRC-LES provides overall better predictions than FPV-LES for both temperature and species. Most existing chemical kinetics models offer similar predictions of ignition and extinction in 0D/1D finite-rate simulations of laminar combustion processes. Is it appropriate, therefore, to extend this observation to a 3D turbulent combustion environment? In order to investigate the sensitivity of predictions to chemical kinetics models, two different kinetics models, GRI-Mech 3.0 and an 11-species syngas model, are compared by performing 3D finite-rate kinetics-based DNS of a temporally evolving turbulent non-premixed syngas flame. The framework enables computationally efficient simulation incorporating the detailed GRI-Mech 3.0. Both chemical kinetics models provide comparable qualitative trends, and capture local extinction/re-ignition events. However, significant quantitative discrepancies (e.g. 86~100 K difference in the temperature field) indicate high sensitivity to the chemical kinetics model. The 11-species model predicts a lower radicals-to-products conversion rate, causing more local extinction and less re-ignition. This sensitivity to the chemical kinetics model is amplified relative to a 1D steady laminar simulation by the effects of unsteadiness and turbulence (up to 7 times for temperature, up to 12 times for CO, up to 13 times for H2, up to 7 times for O2, up to 5 times for CO2, and up to 13 times for H2O), with the deviations in species concentrations, temperature, and reaction rates forming a nonlinear positive feedback loop under reacting flow conditions. The differences between the results from the two models are primarily due to: (a) the larger number of species and related kinetic pathways in GRI-Mech 3.0, and (b) the differences in reaction rate coefficients for the same reactions in the two models. Both (a) and (b) are sensitive to unsteadiness and other turbulence effects, but (b) is more pronounced. During local extinction events, the major differences between the results from the two chemical kinetics models are in the peak values and the volume occupied by the peak values, which is dominated by unsteady effects. During re-ignition events, differences are mainly observed in the spatial distribution of the reacting flow field, which is primarily dominated by the complex turbulence-chemistry interaction. Further analysis shows that GRI-Mech 3.0 predicts more net radical production associated with the major global pathways, explaining the prediction of less local extinction and more re-ignition.Ph.D