On the topic of gas turbine combustion - Strö · PDF file14.10.2015 · On the...

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On the topic of gas turbinecombustionDaniel Moëll, 2015-10-14

2015-10-14

Restricted © Siemens Industrial Turbomachinery AB 20XX All rights reserved.

Page 2 Daniel Moëll

Outline

§ Part 1, Overview of gas turbine combustion

§ Part 2, Modeling of gas turbine combustion

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Part 1: Overview of gas turbine combustion

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The gas turbine cycle

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Combustion chamber typesCan based systems

§ A number of combustion cans are fitted inside acommon air casing

§ The arrangement combines the ease of overhauland testing of multiple system with thecompactness of annular system

§ No direct interaction between flames

§ One draw back is the connection to the firstturbine stage when local zones are very hot andlocal zones are cold

Siemens SGT 750

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Combustion chamber typesAnnular systems

§ The Annular Combustion Chamberconsist of a single flame tube,completely annular in form, which iscontained in an inner and outer casing

§ Due to the less wall area a reduction of15% cooling air becomes possible whichrise the combustion efficiency and savefuel and reduce air pollution

§ One drawback is that the flame zoneboundaries are less defined compared toa can based system which could makethe flames interact more with eachother

§ Testing of burner configurations can bechallenging since the test rigs often arecan based

SGT 800

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Cooling of combustion chambers

Two basic cooling types

Serial Cooling System§ Often used in pre-mixed

systems§ Air is first used for cooling and

then used for combustion§ Higher pressure drop

compared to parallel cooling§ Lower flame temperature

compared to parallel cooling

Parallel Cooling System§ Often used for “traditional” non-premixed

systems but also for premixed systems§ Air from the compressor is divided between

cooling air and combustion air§ Rather low pressure drop over the combustor§ Often higher flame temperature compared to

serial cooling systems

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DLE Burner Design

Fuel/Air Mixing• Axial Swirlers

• Radial Swirlers

• Cross injection

Flame Stabilization• Swirl Stabilized

• Bluff body stabilized

• Pilot Flames

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Fuels For Industrial Gas Turbines

• Natural Gas• Diesel• Ethane• Propane• Hydrogen• Bio Gas/Diesel• Coke Oven Gas• Syngas• Blast Furnace Gas• Rape Seed Oil

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Part 2: Gas turbine combustion modeling

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Features of Industrial Gas Turbine CombustionCases

• High Reynolds Numbers

• Often High Karlowitz Numbers

• Large Geometries

• Complex Fuel/Air Mixing

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Different Strategies for treatment of Turbulence andChemistry on industrial cases

§ Turbulence• RANS• URANS• URANS SAS• LES• Transported PDF

§ Chemistry• Presumed PDF (Flamelets)

• Flame surface density model• G-Equation

• Finite Rate Chemistry• Thickened flame models• Reactor models

• Transported PDF• Lagranian/Eularian Particle based

monte carlo methods• Eulerian Stochastic Fields

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Selected Test Case, Siemens AtmosphericCombustion Rig Fitted with an SGT700/800 Burner

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Case Summary

Flow Properties:Re ~250,000, Ka ~1-100Pressure: AtmosphericPre-Heat Temperature: 693K

Turbulence:URANS k-Ω SSTURANS k-Ω SST-SAS

Chemistry:Presumed PDF/Flamelets + Fractal mean reaction rate model

Fuel:100% CH420% CH4 + 80% H2

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Computational Grid: 2 Different Grids Studied

Mesh 1: 25M cells

Mesh 2: 32M cells

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Turbulence models

The only difference between k-Ω SST and k-Ω SST-SAS is thesource term PSAS in the eddyfrequency equation

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Combustion Model

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Mesh dependency

Very few scales are resolvedusing Mesh 1, regardless ofturbulence model

Mesh 2 + the SAS turbulencemodel resolves larger flowscales

Reaction progress for different meshes and turbulence models

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Mesh

Level of turbulence eddyfrequency increased whenusing finer grid

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Why are more scales resolved then?

Changing the turbulent mixing time scale drastically decreases the eddy viscosity in regionswhere unsteady flow is present and the grid is sufficiently fine to resolve the scales.

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Adjustment based on experimental data

CR constant reduced inorder to match OH PLIF data

No change in flamestabilization position whenthe CR constant is reducedfor the SST case. Notsufficient for simulations ofHydrogen enrichment!

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Flame Stabilization, the precessing vortex core

When the swirling flow is expanded from the burner into the combustion chamber avortex break down occurs, generating a stagnation point close to the burner exit. Onepart o the vortex break down is a precessing vortex core forming like a vortex precssingaround the burner center axis

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Interaction between the PVC and the flame front

Almost all of the wrinkling of the inner part of the flame front occurs in the shearlayers around the PVC. This means that the local flame shape and position will betime dependent and that the time averaged flame shape and location will depend onthe flow history and not just the time averaged flow quantities.

Contour plots of negative pressure and iso-line of c=0.5, representing the flame front

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Hydrogen enrichment by 80%vol H2

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Interaction of PVC and flame front for different fuels

100% CH4

20% CH480% H2

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Pressure drop due to fuel composition

The pressure drop over theburner is increasing with theamount of hydrogen in thefuel. The increased reactionrat changes the flame shapeand position and thereby theway the flow expands acrossthe flame. this causes thehigher pressure drop acrossthe burner

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Pressure fluctuations for pure methane

CFD Rig Data

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Pressure fluctuations for hydrogen enriched methane

CFD Rig Data

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On the topic of gas turbine combustion - Strö · PDF file14.10.2015 · On the...

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