Turbulent CombustionModelling and Simulation

Sustainable CombustionLaboratory

Studying Turbulent Combustion Physics with DNS, LES, RANS

HeaRT ‘s key features

Numerical Combustion TeamN.M.Arcidiacono, G.Calchetti, D.Cecere, A. Di Nardo, E.Giacomazzi, F.R. Picchia

CFD Codes : HeaRT (in house) for DNS/LES, ANSYS FLUENT for LES/RANS

Turbulent Combustion Physics Scenario

Both LES and DNS require high spatial resolution, order of 10-4-10-5 m at least, to capture large spatial gradients and small scales of turbulence. Besides, unsteady simulations require small time steps, ranging from 10-6 s down to 10-9 s depending on the integration scheme (implicit or explicit, mainly) and on the inclusion of acoustics. Hence, several millions of grid points and time steps are needed to solve a problem. These make the time to solution large and supercomputing absolutely necessary. The simulations reported here required nearly three months of computation each, even using supercomputing.

Fluid dynamics – Turbulence

Chemical kinetics

Radiant transfer of energy

Acoustics

Multi-phase flows

Development team: E.Giacomazzi, F.R. Picchia, D.Cecere,F.Donato, N.M.Arcidiacono

Implementation Fortran 95 with MPI parallelization. Genetic algorithm for domain decomposition.

Numerics structured grids with possibility to use local Mesh Refinement (in phase of validation); conservative, compressible, density based, staggered, (non-uniform) FD formulation

[S. Nagarajan, S.K. Lele, J.H. Ferziger, Journal of Computational Physics, 191:392-419, 2003]; 3rd order Runge-Kutta (Shu-Osher) scheme in time; 2nd order centered spatial scheme; 6th order centered spatial scheme for convective terms (in progress); 6th and 10th order compact spatial schemes; 3rd order upwind-biased AUSM spatial scheme for convective terms; 5th-3rd order WENO spatial scheme for convective terms for supersonic flows (S-HeaRT); finite volume 2nd order upwind spatial scheme for dispersed phases (HeaRT-MPh); explicit filtering of momentum variables (e.g., 3D Gaussian every 10000 time-steps); selective artificial wiggles-damping for momentum, energy and species equations; extended NSCBC technique at boundaries considering source terms effect; synthetic turbulence generator at inlet boundaries

[Klein M., Sadiki A., Janicka J., Journal of Computational Physics, 186:652-665, 2003].

Complex Geometries Immersed Boundary and Immersed Volume Methods (3rd order for the time being).

IV is IB rearranged in finite volume formulation in the staggered compressible approach.

Diffusive Transports Heat: Fourier, species enthalpy transport due to species diffusion; Mass diffusion: differential diffusion according to Hirschfelder and Curtiss law; Radiant transfer of energy: M1 diffusive model from CTR [Ripoll and Pitsch, 2002].

Molecular Properties kinetic theory calculation and tabulation (200-5000 K, T=100 K) of single species

Cpi, i, i (20% saving in calculation time with respect to NASA polynomials); Wilke’s law for mix; Mathur’s law for mix; Hirschfelder and Curtiss’ law for Di,mix with binary diffusion Di,j estimated by means of stored single species Sci or via kinetic theory; supercritical transport properties and real gas equation.

Turbulence and Combustion Models subgrid kinetic energy transport equation; Smagorinsky model; Fractal Model (modified) for both turbulence and combustion closures; flamelets - progress variable - mixture fraction - flame surface density - pdf approaches; Germano’s dynamic procedure to estimate models’ constants locally; Eulerian Mesoscopic model for multi-phase flows.

Chemical Approach single species transport equation; progress variable and its variance transport equations; reading of chemical mechanisms also in CHEMKIN format.

Acoustic Analysis in a TVC[D. Cecere et al., in progress]

Combustion Dynamics in VOLVO FligMotorC3H8/Air Premixed Combustor

[E. Giacomazzi et al., Comb. and Flame, 2004]

H2 Supersonic Combustionin HyShot II SCRAMJET

[D. Cecere et al., Int. J. of Hydrogen Energy, 2011 Shock Waves, 2012]

SANDIA Syngas Jet Flame “A”

[E. Giacomazzi et al., Comb. Theory & Modelling, 2007 Comb. Theory & Modelling, 2008]

CH4/Air Premixed Comb.in DG15-CON [ENEA]

[D. Cecere et al., Flow Turbul. and Comb., 2011]

Mesh Refinementin LES Compressible Solvers

[G. Rossi et al., in progress]

Immersed Volume Methodfor Complex Geometry TreatmentUsing Structured Cartesian Meshesand a Staggered Approach

[D. Cecere et al., submitted to Computer Methods in Applied Mechanics and Engineering, 2013]

Thermo-Acoustic Instabilities in thePRECCINSTA Combustor

[D. Cecere et al., in progress]

PSI Pressurized Syngas/Air PremixedCombustor

[E. Giacomazzi et al., in progress]

Alternative fuels

CCS

Power2Gas

H2-blends

Renewables

Clean and efficient power generation

Safe operation

Availability and reliability

Lack of a gas quality harmonization code

Electricity grid fluctuations

EU Energy RoadMap

2050

Decarbonization

Security of energy supply

Fuel-flexibility Load-flexibility

ENHANCED COMBUSTION DYNAMICS

Importance of Combustion Dynamics

Temperature (top) and O2 mole fraction (bottom) radial profiles

Instantaneous (left) and mean (right) temperature (a) and OH mass fraction (b).

Pressure signal in the plenum and in the chamber

Axial velocity profiles

Φ = 0.7 (25 kW) Reynolds 35000-swirl number 0.6

EXP* 6

mm

+ 10 mm

o 15 mm

< 40 mm

> 60 mm

Lean premixed combustion in gas turbines (GT) is widely used in order to meet stringent low NOx emissions demands. If this technique allows the achievement of a quite homogeneous temperature distribution, thermo-acoustic instabilities are a common problem in gas turbine combustors operating in lean premixed mode. Pulsations, caused by resonant feedback mechanism coupling pressure and heat release, can lead to strong perturbations in the gas turbine. Equivalence ratio fluctuations is one of the major cause of flame instability. In this study the experimental campaign conducted at the German Aerospace Center (DLR) was chosen as test case. The simulations were conducted using the commercial CFD code ANSYS-FLUENT. The computational grid consists of about 4.000.000 of computational cells

Performance evolution of HeaRT from CRESCO2 to CRESCO4 Test CaseThree slot premixed

burners Stoichiometric CH4/Air Central Bunsen flame Flat flames at side burners 2mm side walls separation Computational domain 10 x 7.5 x 5 cm3 (Z x Y x X) BIG case 534x432x207 47752416 nodes Aims Single zone performance

analysis. Validation of a new SGS

turbulent combustion model.

Shaheen(Blue Gene/P)

222 TFlops

16384 Single-Proc 4 cores32-bit

PowerPC 450 850 MHz 65536 4 GB/node64 TB

3D “torus”

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CRESCO2 vs CRESCO3 vs CRESCO4

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CRESCO2 vs CRESCO3 vs CRESCO4 vs SHAHEEN

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Wall Time for time-stepShaheen from 2048 to 32768 cores

HeaRT’s LES APPLICATIONSHere, some examples of HeaRT code simulations are reported. Topics cover both theoretical and applied aspects of turbulent combustion. On the theoretical side, the research group is interested in analysing and modeling turbulence / combustion interaction (e.g., VOLVO FligMotor), and hence in understanding the role and dynamics of turbulent structures in a reactive flow and the effects of chemical reactions on vortices. On the application side, interest is focused on premixed combustion of natural gas and air (e.g., DG15-CON) and on combustion of hydrogen blends (in particular, syngas and hydrogen enriched natural gas) at low (e.g., SANDIA Flame A) and high (e.g., PSI) pressure, in premixed and non-premixed conditions. Some studies aims also at identifying the dynamic behaviour of new combustor concepts (e.g., TVC). Besides, some activities are devoted to the general development of the code, i.e., to the implementation of numerical integration schemes and numerical techniques aiming at enhancing its accuracy, efficiency (e.g., Mesh Refinement), its capability of modeling complex geometries (e.g., IVM and PRECCINSTA) and of simulating supersonic flows (HyShot II).

Direct Numerical Simulation (DNS) of a turbulent premixed slot CH4/H2-Air flame at Re=2586 and equivalence ratio of 0.7. Isosurfaces of x-velocity at -/+ 5 m/s and temperature snapshot.The DNS is performed for studying the effects of H2 on methane flames and obtaining an ENEA turbulent flame database for Large Eddy Simulation model validation.

HeaRT’s DNS APPLICATION

The trapped vortex technology offers several advantages as gas turbines burner and the systems experimented so far have limited mainly this technology at the pilot part of the whole burner. Aim of the work was to design a combustion chamber completely based on that principle, investigating the possibility to establish a MILD combustion regime, in case of syngas as fuel. The simulations, performed with the ANSYS-FLUENT code, were carried out according to a steady RANS approach. The models adopted for chemical reactions and radiation are the EDC , in conjunction with a reduced mechanism and the P1, respectively. NOx were calculated in post-processing. In order to save computational resources, the simulations were conducted only on one sector of the whole prototype, imposing a periodicity condition on side walls. A structured hexahedral grid, with a total number of about 2 million cells, was generated.

Trapped-vortex approach for syngas combustion in gas turbines

Temperature field(K)

ANSYS-FLUENT’s LES/RANS APPLICATIONSThermo-acustic instabilities in a lab-scale burner