High-fidelity CFD for the design of aeronautical combustors

1CNRS – UNIVERSITE et INSA de Rouen

V. Moureau, CORIA

High-fidelity CFD for the design of aeronautical combustors

V. Moureau, G. Lartigue, P. Bénard, CORIA, www.coria-cfd.frT. Jaravel, E. Riber, B. Cuénot, CERFACS, www.cerfacs.fr

Courtesy L. Guédot


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Aeronautical engines

CFM56 5B DAC

ARRIEL


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Engine design is driven by two major constraints

4Economic constraints� reduced fuel consumption� reduced CO2 emissions

4Global efficiency of the engine

Pollutant emissionsFuel efficiency

Smoke in the trail of a B-52

Propulsive efficiency Thermal efficiency

High bypass ratio architecture

Ultra-high pressure ratio core engine

Ultra low-NOxcombustion chamber

Ducted fan

Gas turbine (core engine)+Turbofan =

4International regulations� CAEP regulations

4Main pollutants• UHC• Smoke• Carbon Monoxide• Nitrogen Oxides


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The CO / NOx optimization issue

Lefebvre, gas turbine combustion, 2010Pollutant emissions

4 Low-NOx: small residence time, homogeneous mixture4 Low-UHC/CO: long residence time, rich pilot flames


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Prediction of pollutant emissions

4 A highly challenging task• Unsteady, multi-scale and multi-physics flow• Complex geometry

4 Unsteady approaches are mandatory to predict these phenomena


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A short history of combustion modeling in industry

1987 19992D and 3D qualitative simulations to help in the

understanding

3D simulations appliedto real geometries

RANS

4CFD started at the beginning of the 70’s and started to beapplied to combustion in the late 70’s.

4Combustion simulation in the SAFRAN group

2004Unsteady advanced CFDLarge-Eddy Simulation

• The number of prototype engines has been dividedby 5 in 20 years


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Combustion Large-Eddy Simulation

4 LES has been applied to industrial flows for more than a decade

4 LES is now a complement to steady approaches in design loops

Lartigue et al., C&F 2005 LESSCO2, IFP-EN, 2005

CTR, Stanford, 2006Boileau et al., CERFACS, 2007


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Advanced CFD tools developed by the French combustion community and the GIS SUCCESS

Code AVBP YALES2

Intellectual property CERFACS & IFPEN CORIAStarted 1992 2007

Governing equations Compressible Low-Mach number

Grids Unstructured hybrid Unstructured hybridIntegration method Explicit Semi-implicit

Discretisation Central FV & FE Central FVControl volumes Node centered Node centered

Theoretical convergence order

3rd in time4th in space

(but very low dispersion)

4th in time4th in space

Language F77/F90 F90# of developers 15 6

# of users 200 160


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Tabulated chemistry models

IMPACT-AE


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4 Following the idea of Zoller et al., a two-variable tabulated chemistrymodel with both prompt and thermal NOx is designed

Carbon chemistry tablefrom 1D premixed flames

Burnt gases table fromdiluted homogeneousreactors

A NOx model for tabulated chemistry

[Pecquery et al. 2013]

4 Tabulated chemistry principle

⇢, T, Yk, ... = f(Yc, Z, ...)


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Validation of the NOx model

4 Sandia D flame, 350M elements, 8192 cores of Curie (CEA)


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LES of an innovative low-NOx combustor4 Courtesy J. Lamouroux, SAFRAN HE4 376 million cells for a sector of 2 injectors


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LES of an innovative low-NOx combustor4 NOx model based on 2 progress variables

[Pecquery et al., C&F 2013]


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Combustion characteristics4 The flame is lifted4 Hot spots appear inside the

combustion chamber (interactionwith pilot injection)

4 NO peaks seem to be correlatedwith these hot spots

4 Maxima of temperature, mixturefraction and NO seem to be wellcontained inside the primary zone


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Different operating conditions

Investigated condition

Other combustion chamber designs and technologies

Comparison with experimental tests

4 The NOx model has been evaluted against variations of inlettemperature, pressure, and AFR, and technological variations

4 Results are mostly inside experimental error margins


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Finite-rate chemistry models


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Finite-rate chemistry models

4 Navier-Stokes equations with species and sensible enthalpy

4 A few mechanisms for kerosene/air combustion• Dagaut, detailed, 2006 : 209 species, 1673 reactions• El Bakali, Ristori, detailed, 2004 : 225 species, 1800 reactions• Luche, skeletal, 2003 : 91 species, 991 reactions• Franzelli et al., fitted, 2010 : 6 species, 2 reactions (no pollutants)

4 The lack of intermediate mechanism with limited species and good accuracy led modelers to starting developing reduced schemes• ARC (Cuénot et al., CERFACS)• ORCH (Vervisch et al., CORIA)

• VOM (Fiorina et al., EM2C)


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Analytically Reduced Chemistry (ARC)

4 Removal of unimportant species/reactions4 Identification with Directed Relation Graph

methods [1,2]

Skeletal reduction

Quasi-steady state approximation (QSS)

= Cost reduction → Less species to transport

ARCDetailed chemistry

= Cost reduction → Less species to transport= Stiffness reduction → Small temporal/spatial scales are removed

[1] Lu and Law, PCI, 2005[2] Pepiot, C&F, 2008

Systematic reduction process

4 Specific treatment for highly reacting intermediate species


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4 Experimental measurements4 PIV4 1D Raman4 OH PLIF4 Exhaust pollutant concentrations

[1] Stopper et al C&F 2013[2] Bulat et al C&F 2014

Case A Case B

Pressure 3 bars 6 bars

Temperature 680 K 680 K

Air mass flow rate 183.8 g/s 338 g/s

Fuel mass flow rate 6.24 g/s 12.8 g/s

Global equivalence ratio 0.52 0.59

Operating Points

The SGT-100 configuration

4 Siemens burner experimented in high pressure DLR test-rig [1]

4 Previous LES study [2]


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Instantaneous results: Case A

Temperature [K]

ɸ

YOH

20

Axial velocity [m/s]


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ZeroIsocontour O Exp

— LES

TemperatureMean

RMS

Mean

RMS

Axial velocity

LES results validation: Case A


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Pollutant prediction

4 Pollutants are directly obtainedas they are transported in themechanism

4 NO• Satisfactory prediction• Slight under-prediction• Trend correctly recovered

4 CO• Significant over-prediction• Better prediction with heat losses

(not shown)

Comparison of exhaust levels with measurements


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Application to a real kerosene/air burner

4Strong influence of stratification on pollutant formation4Intermittent, non-equilibrium CO concentrations at the outlet4Sensitivity to numerical and physical parameters

• Spray description• Uncertainty on chemistry• Heat losses

23

Satisfactory agreement forNOx and CO emission


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Conclusions and perspectives


V. Moureau, CORIA

Conclusions

4 Large-Eddy Simulation is fully integrated in the design cycle foraeronautical burners along with 1D and RANS tools. This multi-fidelity approach is the most efficient.

4 With increasing HPC power, more realistic chemistry descriptionsbecome affordable in the design loop.

4 A strong collaboration between CFD, HPC and chemistry experts ismandatory to reach our objectives.

4 SAFRAN and research labs have been key players in structuringthe community for aeronautical combustion.


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Perspectives

4 Multi-physics is the next step• Spray combustion, soot, radiative and conjugate heat transfer, …

4 Mesh adaptation• LES is highly sensitive to the cell/filter size• Static and dynamic mesh adaptation becomes mandatory


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Perspectives4 Courtesy R. Mercier, SAFRAN TECH4 Dynamic mesh adaptation of a turbulent flame4 1.8B tets, 8400 cores of Cobalt

Longitudinal cut of the Cambridge burner (Mercier, 2015)

Case

SwB5 1.0 0.5 0 8.31 18.7 0.4

Documents

High-fidelity CFD for the design of aeronautical combustors