# LES Approaches to Combustion

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1 LES Approaches to combustion LES Approaches to combustion LES Approaches to Combustion W P Jones Department of Mechanical Engineering Imperial College London Exhibition Road London SW7 2AZ SIG on Combustion Science, Technology and Applications, Imperial College London 28 th September 2017

2 Turbulent Flows: DNS Inert Flows Combusting Flows Resolve the smallest scales Number of mesh points, N Time step t CPU time Many reactions of type A B C r A at Re Re 3 4 E YY exp( ) RT W n A B Pressure 1 bar, 0.01 mm 3 Pressure 40 bar, mm r r xyx B Re 9 4

3 Modelling Approaches Reynold/Favre Averaged Approaches (RANS) Quasi-steady and quasi-homogeneous assumptions years research Large Eddy Simulation (LES) Assumptions: scale separation, i.e. high turbulence Reynolds numbers Resolve large scale energy containing motions responsible for transport. Model fine scale dissipative motions / replace physical viscosity with sub-grid scale (sgs) viscosity. sgs viscosity provides mechanism for dissipation. Mean profiles, Reynolds stresses, turbulent kinetic energy etc. insensitive to sgs viscosity.

4 Large Eddy Simulation Introduce a spatial filter: 3 x, t x, t G x x, d x x, t,,, x, t 3 x t x t G x x d x where G 3 x x, d x 1 ; G( x x, ) 0 Generalised moments: The filter width is given by: etc. 2 2,, uiu j uiu j xyz 13

5 Filtered Equations Continuity ρ t + ρ u i = 0 x i Momentum ρ u i t + ρ u i u j x j Scalars ρ φ α t + ρ u jφ α x j = p + µ u i + u j 2 u k δ x i x j x j x i 3 x ij k = x j ρd φ α x j + ρ ω α φ, T J sgs α,j x j τ sgs ij + ρg x i j Closures are required for the sub-grid stress τ sgs ij, sub-grid flux J α,j the filtered rate of formation ρ ω α φ, T terms. sgs and with C S determined dynamically i.e. C S = C S (x,t)

6 LES: Justification Large scale energy containing motions responsible for turbulent transport. Energy transfer from large to small scales. Energy Dissipation via viscosity at the smallest scales. Energy dissipation rate and hence turbulent transport independent of viscosity and indeed the precise dissipation mechanism, (Kolmogorov). LES resolve large scale energy containing motions responsible for transport. model fine scale dissipative motions / replace physical viscosity with sub-grid scale (sgs) viscosity. sgs viscosity provides mechanism for dissipation. mean profiles, Reynolds stresses, turbulent kinetic energy etc insensitive to sgs viscosity

7 Numerical Requirements Discretisation:- Spatial Derivatives Convection terms: the use of dissipation free schemes is desirable if excessive CPU times/memory are to be avoided. at least second order accurate central differences. Asymmetric approximation such as QUICK and upwind schemes are too diffusive! Diffusion and Pressure gradient Terms: Second order accurate central approximations yield reasonable results. Time At least second order accurate: Crank-Nicholson, Adams-Bashforth 3-step Runge-Kutta and three-point backward difference approximations have all been used to good effect. Solution Methods If is linked to the mesh spacing then the solution will not be smooth on the mesh high frequency noise

8 Numerical Accuracy Grid independence If is linked to the mesh spacing then the only truly grid independent solution will be a DNS. Resolve 80-90% of turbulence energy mean quantities, Reynolds stresses etc independent of mesh size. Energy Spectrum Mesh Quality Indicators?? There is no reliable single mesh accuracy indicator Mesh size a rule of thumb Equal mesh spacing, i.e. x y z, expansion ratios close to unity; sufficiently fine to resolve mean motion

9 LES of an isothermal Jet

10 Combustion LES: Approaches Turbulent transport, e.g. Reynolds stresses, turbulence kinetic energy, scalar fluxes etc., is likely to remain insensitive to the sgs eddy viscosity. Combustion will take place predominantly within the sub-grid scales. Thus sgs combustion models are likely to exert a more important influence in the LES of turbulent flames. Approaches: One/two Scalar Descriptions Non-premixed Flames Conserved scalar/unstrained flamelets/cmc. Premixed Flames Thin flame approach (flame approximated as a propagating material surface). Models: flame surface density, level set methods ( G equation), scalar dissipation. Partially Premixed/Stratified Flames Thickened Flame Model, FGM, FSD, Scalar Dissipation,CMC, unsteady flamelets Sub-grid scale or Filtered joint scalar pdf transport equation method.

11 Non-Premixed Combustion Conserved Scalar Formalism Assumptions: High Reynolds Number Adiabatic Flame Fast Reaction Mixture Fraction Normalized element mass fraction z z,1 air stream : ξ 0, z z,2,1 is a strictly conserved quantity fuel stream : 1 sgs u j t x j x j sgs x j

12 Laminar Flamelet Computations Laminar Flamelets Flame Thickness < Kolmogorov length Scale Instantaneous Dependence of Composition, Density and Temperature on mixture fraction and some measure of Flame Stretch is the same as that prevailing in a laminar flame T T, s ; Y Y, s ;, s A simple model: select a single flamelet at a specified value of e.g. T T 1, s T etc, where s 15s (unstraine d) R s R R

13 Turbulence-Chemistry Interactions Probability Density Function Beta PDF ~ 1 x P (, x) d 0 r1 s r1 ; x, 1 / 1 1 s1 P t d 1 1 where r 1 ; s r sgs 2sgs sgs uj Cd 2 t x j x i sgs xi sgs xi or 2 2 C xi 2 2

14 x d Piloted Turbulent Jet Flames CH 4 H 2 O H 2 O x d U ~ x d T ~ H 2 ~ x d x d

15 Local Extinction In RANs approaches the scalar dissipation,χ is often used to provide a measure of flame stretch. In LES the local value of χ and thus P(ξ,χ) is insufficient to describe extinction. Instantaneous mixture fraction contours 20mm Instantaneous mixture fraction dissipation rate 20mm

16 ; Premixed Flames: Parameters Length Time ; Energy containing t motions: 1 3 Kolmogorov scales: 2 k Chemical reaction scales: L L L SL where S L is the burning velocity of an unstrained laminar flame U S L 1 4 Reynolds number: Damköhler Number: Karlovitz Number: Re U t SL DA L U L L 2 K Re 1 / D A k A

17 Premixed Flames Define a reaction progress variable, e.g. unburnt gas c = 0 c Y HO Y 2 2, x, t H O burnt burnt gas c =1 c c t c Ui x, t t xi xi t xi View Flame as a propagating material surface propagating at a speed equal to the local burning velocity, (Bray, Peters, Candel et al, Bradley et al and others) x,t is obtained from either FSD, G-equation, Scalar Dissipation for which an additional equation is solved.

18 Partially Premixed and Stratified Flames At a minimum both the mixture fraction and reaction progress variable is required. Reaction Progress Variable becomes: and Y( x, t) c with Yb ( x, t ) Yb ( ( x, t )) Y ( x, t) b dc c dy c d dt Y dt dt Resulting equation complex; additional terms sometimes neglected. A solution: solve equations for Y and c(x,t) Reaction rate obtained as for premixed flames with additional assumptions, e.g. P( c, ) P( c) P( )

19 Finite Rate Chemistry Effects Fully coupled flow and chemical reaction required Detailed but reduced chemical mechanism required Currently available approaches: Thickened Flame Model CMC with at least double conditioning PDF transport equation methods Lagrangian stochastic particles Eulerian Stochastic fields

20 Combustion: Sub-Grid Pdf Equation Method Fine grained pdf Sub-grid Pdf sgs F The modelled sub-grid Pdf Equation P ; x, t x ', t ; x ', t G( x x '; ) dx ' N sgs u j Psgs Psgs Psgs P t x j x j x j 1 C N N x Psgs 1 1 i i i sgs d N sgs 1 ; x, t x, t F 1 x N s 2 P x x, t P sgs x i

21 Stochastic Field Solution Method Represent PDF by N stochastic fields Configuration field methods, M. Laso and H.C. Öttinger (1993); Stochastic Fields, Valino (1998), Sabel nikov (2005) Ito formulation n n n n sgs d ui dt dt xi x i sgs xi x, t is advanced from t to t dt according to: 12 n sgs n 1 n n n n 2 dwi t 0.5Cd sgs dt dt sgs xi n n where 1 n N, dw dt and n i i is a -1,+1 dichotomic vector i

22 Pdf Equation/Stochastic Fields: Applications Simulated Flames Auto-ignition Hydrogen and n-heptane Lifted flames : Cabre - Hydrogen & methane Forced ignition: methane, Local extinction Sandia Flames D, E & F Premixed swirl burner (Darmstadt) Darmstadt stratified flame Lean burn (natural gas) industrial combustor Premixed baffle stabilised flame Cambridge/Sandia stratified flames Methanol Spray flames Axisymmetric swirl combustor FAUGA Combustor, Kerosene Sector combustor Genrig combustor spray

23 SGT100 Combustor: Snapshot of Heat Release Rate

24 Stratified Premixed Turbulent Flames The Cambridge Stratified Swirl Burner Ui (m/s) Uo (m/s) Uco (m/s) Ʌ (mm) Re_i Re_o φ_g

25 Computational Setup BOFFIN-LES 13.5M cells & 610 blocks 3.7M cells & 163 blocks 0.8M cells & 27 blocks Synthetic turbulence generation Dynamic Smagorinsky model Constant Prandtl/Schmidt number = 0.7 Reduced GRI 3.0 with 19 species and 15 reactions 8 stochastic fields ~ measurement locations

26 Results SwB5 Velocity Magnitude Methane Temperature

27 Results SwB5 Velocities Mean Velocities RMS Velocities

28 Results SwB5 Scalars - Mean Temperature CH 4 O 2 CO 2 CO H 2 10 mm 30 mm 50 mm 70 mm

29 Results SwB5 Scalars - RMS Temperature CH 4 O 2 CO 2 CO H 2 10 mm 30 mm 50 mm 70 mm

30 Methanol Spray Flame Gas Phase Stochastic Fields 18 species-84 reaction steps for methanol-air combustion Dispersed Phase Lagrangian stochastic particles (droplets) with models for: Sub-grid dispersion Abramson-Sirignano evaporation Stochastic breakup model The instantaneous gas-phase velocity field with droplet motions and (b) a detailed view of breakup processes near the atomiser. Note that the appearance of droplets is scaled according to their size.

31 Methanol Spray: Profiles of SMD and mean and rms droplet axial velocity

32 Reacting Spray: Instantaneous gas-phase temperature

33 Reacting Spray: Profiles of mean and radial Droplet velocity snd SMD

34 Instantaneous OH mass fraction and droplets

35 LES with Conditional Moment Closure: ignition, blow-off, emissions Mastorakos, Cambridge

36 Large Eddy Simulation of Swirl Stabilized Combustor 1 combustor 24 combustors Modelling of fuel injection (DNS) 24 combustors + outer casing E 3a x x t t Dong-hyuk Shin, Edinburgh University

37 High-Hydrogen Content Alternative Fuel Blends for Stationary and Motive Combustion Engines Large eddy simulations of non-premixed syngas jet flames and swirling flames 1. Ranga Dinesh, Jiang, van Oijen et al. Proc. Combust. Inst., Ranga Dinesh, Jiang, van Oijen et al. Int. J. Hydrogen Energy, Ranga Dinesh, Luo et al. Int. J. Hydrogen Energy Ranga Dinesh, van Oijen et al. Int. J. Hydrogen Energy 2015

38 Conclusions Velocity Large Eddy Simulation is capable of reproducing the velocity field in the majority of inert and combusting flows. Providing that the Reynolds number, Re ul is sgslarge and the flow is adequately resolved then results are relatively insensitive to the sub-grid stress and scalar-flux models. Near Wall Flows and the Viscous sublayer models needed if DNS like requirements are to be avoided. no accurate and reliable wall models currently exist. Combustion Thin flame LES combustion models give good results when applied appropriately. The LES Stochastic field pdf method together with detailed but reduced chemistry has been applied to a wide range of flames non-premixed, partially premixed, premixed and spray flames - to good effect. Practical liquid fuel systems limited by chemical reaction mechanisms.

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