© 2011 ANSYS, Inc. 8/29/111

ANSYS Advanced Solutions for Gas Turbine Combustion

Gilles EggenspielerANSYS, Inc.

© 2011 ANSYS, Inc. 8/29/112

AgendaSteady State: New and Existing

Capabilities• Reduced Order Combustion Models• Finite-Rate Chemistry Models• Chemistry Acceleration Methods

Scale Resolving Simulation (SRS)• LES and Scale Adaptive Simulation• Embedded and Zonal LES

Innovative Combustion and Pollutant Models

• Thickened Flame Model• G-Equation Model• CO Pollutant Modeling

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© 2011 ANSYS, Inc. 8/29/113

An ANSYS Solution for every Simulation Challenge

High Quality Fuel/Air Mixing

Liquid Fuel Injection

Complex Chemistry

Emission Predictions

Heat Transfer Computation

Configuration Optimization

Lifing

Advanced Turbulence Models (RANS, SAS, LES)

DPM tracking, Advanced Break-Up Models

Complete Array of Turbulent Chemistry Models

Post-Processing and Coupled Pollutant Models

Advanced Wall Functions and Turbulence Models

Parametric Simulation, Design Exploration

Fluid-Structure Interaction, ANSYS FEA, nCode

© 2011 ANSYS, Inc. 8/29/114

A complete Portfolio of Reduced Order Combustion Models

Non-Premixed Combustion

Mixture Fraction

Chemistry Tabulation - Equilibrium Chemistry - Non-Equilibrium Flamelets

Compressibility EffectsNon-Adiabatic Systems

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Partially-Premixed Combustion

Mixture Fraction-Progress Variable ApproachesChemistry Tabulation - Equilibrium Chemistry - Non-Equilibrium FlameletsFlame Speed Models - Zimont Flame Speed - Peters Flame Speed

Compressibility EffectsNon-Adiabatic Systems

Premixed Combustion

Progress Variable

Flame Speed Models - Zimont Flame Speed - Peters Flame Speed

Enhanced Coherent Flame Model

Compressibility EffectsNon-Adiabatic Systems

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Post-Processing Pollutant Models - NOx - SOx - SootSteady and Unsteady Post-Processing

Decoupled Detailed Chemistry - Pollutant Finite-Rate Chemistry added on-top of the existing simulation - Pollutants and Minor Species onlySteady and Unsteady Post-Processing

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Finite-Rate Chemistry Models: An Extensive Offering

Laminar Finite Rate (Chemistry Only) Eddy-Dissipation (Turbulence Only)

Laminar Finite Rate/Eddy-Dissipation (Chemistry/Turbulence Interactions)

Eddy-Dissipation Concept (Chemistry/Turbulence Interactions)

Premixed

Non-Premixed

Partially Premixed

Post-Processing Pollutant Models - NOx - SOx - SootSteady and Unsteady Post-Processing

Decoupled Detailed Chemistry - Pollutant Finite-Rate Chemistry added on-top of the existing simulation - Pollutants and Minor Species onlySteady and Unsteady Post-Processing

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Composition PDF Transport (Chemistry/Turbulence Interactions)

KEY TECHNOLOGY: CHEMISTRY ACCELERATION

© 2011 ANSYS, Inc. 8/29/116

Efficient Chemistry Acceleration

From 2 to 10’s of Species

Stiff Reaction Rates

Minor Species and RadicalsChallenge

Chemistry Computation Cost >> Fluid Computation Cost

Chemistry Agglomeration

In-Situ Adaptive Tabulation (ISAT)

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Dimension Reduction

Decoupled Detailed Chemistry

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Solution

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Efficient Chemistry Acceleration

IN-SITU ADAPTIVE TABULATION

Store Reaction Mappings in an ISAT table

Retrieve Reaction rates when needed Up to 100 Speed-Up Factor

CHEMISTRY AGGLOMERATION

Agglomerate cells of similar Composition

Call ISAT on Agglomerated Cells

Map Reaction Step back to Original Cells

DIMENSION REDUCTION

User selects the transported Species

Calculate the remaining unrepresented species using constrained chemical equilibrium

Allows 50+ species in the full mechanism

DECOUPLED DETAILED CHEMISTRY

Slow chemistry (pollutants) - NO - CO Compute Chemistry (Minor Species) on a frozen (Fluid/Major Species) Field

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Accurate Emission Prediction: GE LM 1600

Geometry

Mesh

Temperature

NO Predictions

• Challenge– GE LM-1600– Non-Premixed/Air-natural Gas– Prediction of NO Emission– Annular combustion chamber– 18 nozzles

• ANSYS Solution– High Quality Mesh– Laminar Flamelet model– 22 species, 104 reactions reduced

GRI-MECH 1.22 mechanism– Differential diffusion included

• Results– Accurate Prediction of the

Combustion Processes– Accurate Prediction of the NO

(Pollutant) Emissions

Courtesy of Nova Research and Technology Corp.

© 2011 ANSYS, Inc. 8/29/119

Innovative Particle Break-Up Model

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New Advanced Droplet Models for Fuel Combustion - Accurate prediction of secondary droplet break-up - Particles characteristics and locations are essential for accurate simulation on of the combustion processes

STOCHASTIC SECONDARY DROPLET (SSD) Break-Up - Valid for High Weber number particles - Break-Up modeled as a discrete random event

- Break-Up Distribution of Diameter over a Range

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ASSUMPTIONS- The probability of break-up is independent of the parent droplet size- Secondary droplet size is sampled from an analytical solution of the Fokker-Planck equation for the probability distribution- Parameters for the size distribution are based on local conditions

© 2011 ANSYS, Inc. 8/29/1110

SSD: Accurate Break-Up Prediction

Visualization of the Droplets Jet

Jet Penetration Results

• Challenge– Simulate accurate Jet Penetration– Hiroyasu tests– Accurate Droplet Break Up is

Required• ANSYS Solution

– Large-Eddy Simulation– SSD Break-Up Model

• Results– Accurate Prediction of the Jet

Penetration at different operating Conditions

© 2011 ANSYS, Inc. 8/29/1111

The Need for Scale Resolving Models

Next generation Combustion Simulations requires to capture unsteady Phenomena• Prediction of Combustion Dynamics• Prediction of Flame instabilities which can

lead to catastrophic phenomena like Blow-Off or Flashback

• Scale Resolving Models proved to be more accurate

State of the Art Scale Resolving Models in ANSYS CFD– Scale Adaptive Simulation– Detached-Eddy Simulation– Delayed Detached-Eddy Simulation– Embedded Large-Eddy Simulation– Large-Eddy Simulation

© 2011 ANSYS, Inc. 8/29/1112

State of the Art Scale Resoling Models

U-RANS (Unsteady RANS)− URANS gives unphysical single mode unsteady behavior

LES (Large Eddy Simulation)− Adequate only for non wall-bounded flows− Too expensive for most industrial flows due to high

resolution requirements in boundary layers

DES (Detached Eddy Simulation)− First industrial-strength model for high-Re with LES-content− Increased complexity (grid sensitivity) due to explicit mix of

two modeling concepts

SAS (Scale-Adaptive Simulation)− Extends URANS to many technical flows− Provides “LES”-content in unsteady regions− A preferred solution for Scale Resolving Simulations of

Engineering applications

U-RANS

SAS

© 2011 ANSYS, Inc. 8/29/1113

A fully Flexible Scale-Resolving Methods Portfolio

Scale Resolving Simulations are computationally expensive

– To capture all relevant turbulent structures, the mesh resolution is finer than typical RANS meshes

– To capture all relevant turbulent structures, the time step is smaller than typical U-RANS time steps

ANSYS Solution: Domain Based Scale Resolving Use – Use Scale Resolving Methods only in Area of

interest– Use typical U-RANS methods in area where the

resolution of unsteady turbulent structure is not needed

– Zonal LES for ANSYS CFX– Embedded LES for ANSYS FLUENT

© 2011 ANSYS, Inc. 8/29/1114

LESU-RANS

Efficient Scale Resolving Simulation:Sydney Flame Example• Challenge

– Simulating the J&R Flame using an Scale Resolving Method

– Reduce the computational costs of the Scale Resolving Simulation

• ANSYS Solution– Use the Embedded LES Method– Use LES in the area of interest:

combustion region– Use U-RANS in regions where

LES cannot be used (swirler)– Use U-RANS in regions where

the LES cost is not justified (inflow pipe and outflow region)

• Results– Results as accurate as a full LES– Reduced Computational cost

when compared to a full LES Simulation

LES

U-RANS

© 2011 ANSYS, Inc. 8/29/1115

Scale Resolving Methods for Combustion Simulations: LES Example (GE – LM6000)

• Challenge– Simulating Combustion

Processes in the GE LM6000 Gas Turbine Combustion Chamber

– Predict the Flame location and velocity fields

• ANSYS Solution– High Quality Mesh– State of the Art Turbulence

Models (LES)– State of the Art Combustion

Models (Premixed Model)

• Results– Accurate Prediction of the

Combustion Processes– Accurate Prediction of Velocity

Fields

© 2011 ANSYS, Inc. 8/29/1116

Scale Resolving Methods for Combustion Dynamics: SAS Example (Siemens – Dual Fuel DLE) Double Skin Impingement

Cooled CombustorMain Burner

Pilot Burner

PreChamberRadial Swirler

Prediction of Aerodynamic Frequencies in a Gas Turbine Combustor Using Transient CFD - GT2009-59721

• Challenge– Simulating Combustion

Processes in a Gas Turbine Combustion Chamber

– Predict the Combustion Dynamics

• ANSYS Solution– High Quality Mesh– Advanced Turbulence Models

(SAS)

• Results– Accurate Prediction of the

Combustion Processes– Accurate Prediction of the

Acoustics Behavior of the system

© 2011 ANSYS, Inc. 8/29/1117

Newly Implemented Finite-Rate Unsteady Combustion Model: Thickened Flame Model

In Unsteady Mode, the flame structure cannot be resolved on the computational mesh

– When using Finite Rate Chemistry, numerical issues (temperature spikes) can appear because of lack of Flame resolution

– When not resolving flame, flame speed is wrong

ANSYS Solution: Thickened Flame Model– The flame is dynamically thickened to to

limit thickening to flame zone only– An Efficiency Function takes into account

the chemistry/turbulence Interactions

Dynamic Thickening in the reaction zone

Local Thickening Factor as a function of the mesh size

Flame/Turbulence Interaction: Efficiency function

Accurate Flame Representation

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Newly Implemented Premixed Unsteady Combustion Model: G-Equation

In Unsteady Mode, typical Premixed Model can predict a dissipative thick flame surface

– Affect accurate prediction of flame surface/turbulence interaction

– Degrades quality of the results

ANSYS Solution: G-Equation (Level Set)– The distance from the flame front (G) is

tracked – the G-field is re-initialized at every iteration to

ensure that in the entire domain it equals the (singed) distance to the flame front

G-Field: Distance from the flame (Here burnt region where G > 0)

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PRODUCTSREACTANT

REACTANT PRODUCTS

Thin Flame

© 2011 ANSYS, Inc. 8/29/1119

Innovative Pollutant Model: Time-Scale Separation for CO

For typical Lean and Premixed or Partially Premixed Gas Turbine Combustion Chambers:

− Highly non-monotonous evolution of CO− Fast formation of CO at the flame front− Sharp CO peak in the reaction zone− Relatively slow post-flame oxidation

Time-Scale Separation Solution− Separates Flame Front Formation from Post-Flame Oxidation− Data extracted from PDF Chemistry Tables

− Peak CO at the Flame Front− Post-Flame CO Oxidation Rates

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0.00

0.05

0.10

0.15

0.000 0.005 0.010 0.015x, m

Y, -

Y(CH4)

Y(CO)

Y(CO2)

CO formation at the flame front

Oxidation CO CO2

DiffusionSccsYDt

DYCOT

frontCO

CO +⋅+∇=ρρ

© 2011 ANSYS, Inc. 8/29/1120

Demonstration of the CO SST Capabilities

• Challenge– Simulating CO formation and

Oxidation in a Typical Gas Turbine Combustion Chamber

• ANSYS Solution– High Quality Mesh (2.5 M

nodes)– Advanced Turbulence Model

(SST)– Advanced CO Models (TST)

• Results– Fast Simulation– CO Predictions are in

agreement with Experimental Results

CFD Prediction of Partload CO Emissions using a Two-Timescale Combustion Model - GT 2010-22241

Geometry

Accurate Boundary Conditions

Comparison with Experimental Data

© 2011 ANSYS, Inc. 8/29/1121

The Full Power of ANSYS Workbench: Optimization, FSI, etc.

Equivalent elastic strain

Gas temperature

wall temperature

Total deformation1-way coupling

ANSYS CFD

ANSYSMechanical

ANSYS Workbench – Fluid Structure Interaction (FSI)

– Couples CFD and Structural Simulations– Transfer Pressure Loads, Temperature

Loads, CHT data, etc.– 1- and 2-way FSI

Design Optimization – Examples:– Optimize a geometry– Optimize operating conditions

© 2011 ANSYS, Inc. 8/29/1122

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