On the topic of gas turbine combustion

Transcription

1 On the topic of gas turbine combustion, Answers for energy.

2 Outline Part 1, Overview of gas turbine combustion Part 2, Modeling of gas turbine combustion Page 2

3 Part 1: Overview of gas turbine combustion Page 3

4 The gas turbine cycle Page 4

5 Combustion chamber types Can based systems A number of combustion cans are fitted inside a common air casing Siemens SGT 750 The arrangement combines the ease of overhaul and testing of multiple system with the compactness of annular system No direct interaction between flames One draw back is the connection to the first turbine stage when local zones are very hot and local zones are cold Page 5

6 Combustion chamber types Annular systems The Annular Combustion Chamber consist of a single flame tube, completely annular in form, which is contained in an inner and outer casing SGT 800 Due to the less wall area a reduction of 15% cooling air becomes possible which rise the combustion efficiency and save fuel and reduce air pollution One drawback is that the flame zone boundaries are less defined compared to a can based system which could make the flames interact more with each other Testing of burner configurations can be challenging since the test rigs often are can based Page 6

7 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 Page 7

8 DLE Burner Design Fuel/Air Mixing Axial Swirlers Radial Swirlers Cross injection Flame Stabilization Swirl Stabilized Bluff body stabilized Pilot Flames Page 8

9 Fuels For Industrial Gas Turbines Natural Gas Diesel Ethane Propane Hydrogen Bio Gas/Diesel Coke Oven Gas Syngas Blast Furnace Gas Rape Seed Oil Page 9

10 Part 2: Gas turbine combustion modeling Page 10

11 Features of Industrial Gas Turbine Combustion Cases High Reynolds Numbers Often High Karlowitz Numbers Large Geometries Complex Fuel/Air Mixing Page 11

12 Different Strategies for treatment of Turbulence and Chemistry on industrial cases Turbulence Chemistry RANS URANS URANS SAS LES Transported PDF 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 Page 12

13 Selected Test Case, Siemens Atmospheric Combustion Rig Fitted with an SGT700/800 Burner Page 13

14 Case Summary Flow Properties: Re ~250,000, Ka ~1-100 Pressure: Atmospheric Pre-Heat Temperature: 693K Turbulence: URANS k-ω SST URANS k-ω SST-SAS Chemistry: Presumed PDF/Flamelets + Fractal mean reaction rate model Fuel: 100% CH4 20% CH4 + 80% H2 Page 14

15 Computational Grid: 2 Different Grids Studied Mesh 1: 25M cells Mesh 2: 32M cells Page 15

16 Turbulence models The only difference between k- Ω SST and k-ω SST-SAS is the source term PSAS in the eddy frequency equation Page 16

17 Combustion Model Page 17

18 Mesh dependency Reaction progress for different meshes and turbulence models Very few scales are resolved using Mesh 1, regardless of turbulence model Mesh 2 + the SAS turbulence model resolves larger flow scales Page 18

19 Mesh Level of turbulence eddy frequency increased when using finer grid Page 19

20 Why are more scales resolved then? Changing the turbulent mixing time scale drastically decreases the eddy viscosity in regions where unsteady flow is present and the grid is sufficiently fine to resolve the scales. Page 20

21 Adjustment based on experimental data CR constant reduced in order to match OH PLIF data No change in flame stabilization position when the CR constant is reduced for the SST case. Not sufficient for simulations of Hydrogen enrichment! Page 21

22 Flame Stabilization, the precessing vortex core When the swirling flow is expanded from the burner into the combustion chamber a vortex break down occurs, generating a stagnation point close to the burner exit. One part o the vortex break down is a precessing vortex core forming like a vortex precssing around the burner center axis Page 22

23 Interaction between the PVC and the flame front Contour plots of negative pressure and iso-line of c=0.5, representing the flame front Almost all of the wrinkling of the inner part of the flame front occurs in the shear layers around the PVC. This means that the local flame shape and position will be time dependent and that the time averaged flame shape and location will depend on the flow history and not just the time averaged flow quantities. Page 23

24 Hydrogen enrichment by 80%vol H2 Page 24

25 Interaction of PVC and flame front for different fuels 100% CH4 20% CH4 80% H2 Page 25

26 Pressure drop due to fuel composition The pressure drop over the burner is increasing with the amount of hydrogen in the fuel. The increased reaction rat changes the flame shape and position and thereby the way the flow expands across the flame. this causes the higher pressure drop across the burner Page 26

27 Pressure fluctuations for pure methane CFD Rig Data Page 27

28 Pressure fluctuations for hydrogen enriched methane CFD Rig Data Page 28