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Fluid-Structure Interaction for Combustion Systems

Artur PozarlikJim Kok

FLUISTCOMSIEMENS, MUELHEIM, 14 JUNE 2006

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Work performed

Numerical investigation of a cold flow within plenum and combustor chamber and reacting

flow within combustion chamber with the use of commercial CFX code

Reacting flow calculations by using computational code developed at the University of

Twente (CFI)

Design more flexible liner for better fluid-structure interaction (structural Ansys code)

One-way fluid-structure interaction from fluid to structure with the use of CFX and Ansys

(static and dynamic analysis)

Two-way fluid-structure interaction

Backward Facing Step with heat transfer

Participations in DESIRE fire experiment

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Flexible liner

The most dangerous frequencies occur in real gas turbines are below 500 Hz

Present test rig has first eigenfrequency around 200 Hz and low vibration amplitude

To improve response walls on changes in pressure field inside combustion chamber new model of liner with first eigenfrequency below 100 Hz and elevated vibration amplitude was design

To obtain prescribed eigenfrequency

several models of liner with different

in shape, thickness, length of flexible

section was investigated

Fig. 1. Liner configuration

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Flexible liner

ShapeRectangular (50 x 150 mm)

Square (150 x 150 mm)

Thickness 4.0 mm

Flexible part thickness

1.2; 1.0; 0.8 mm

Flexible section length

200; 400; 600; 680 mm

Investigated temperature

Cold case 25 OC

Hot case 760 OC

Investigated models

Structural

Structural with combustion chamber

Structural with combustion chamber and cooling passage

Material Stainless steal 310

Element types

Fluid – Fluid 30

Fluid-Structure – Fluid 30

Solid – Shell 63

Fig. 2. Different shapes of investigated liner

Fig. 3. Model of the numerical connection between structure and cavities

Fluid

Fluid – Structure

Structure

Connection between structural parts

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Flexible liner

Fig. 4. Eigenfrequencies [Hz] in case of a different a) shape, b) length, c) thickness, d) surrounding cavities, e) temperature

Fig. 5. Mode shapes in case of a square cross-section, 0.8mm thickness and 680mm length liner in high temperature, without air cavities

a) b) c) d)

e)

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One – way interaction

Fig.6. Implementing results from CFX to Ansys

One – way interaction is a sequential process of the fluid and the solid physics coupling. The surface pressure and the shear from the flow in the combustion chamber were computed by using CFX CFD simulation. The normal and tangential components of mechanical load are later transferred to the mechanical analysis in the Ansys code. The stress and deformation of the flexible walls are predicted.

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One way interactionAbs. pressure 1.5 bar

Air factor 1.8

Total mass flow rate

90.64 g/s

Number of elements

632 000; mostly in fire zone

ShapeQuarter section with periodic boundaries

Turbulent model k-Combustion model

Eddy Dissipation and Finite Rate Chemistry

Initial conditionsInitial velocity and turbulences are taken from previous full model calculation

Pulsation5% pulsation of equivalence ratio with frequency 100 Hz

Fig. 7. Data for CFX reacting calculation Fig. 8. Velocity and temperature profiles

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One – way interaction

Fig. 9. Liner boundary conditions 7 500 equally distributed elements

One wall taken into account

Simplified geometry (without holes, modular parts together, etc)

Wall treated as clamped on each side

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Damping mechanisms

Frictional damping

Damping of vibration energy in metallic structure itself

Damping by induced flow/acoustic radiation by the liner

Ansys – calculation done with coefficients, which determinate damping matrix [C] as:

[C]= ,

where [M] and [K] are mass matrix and stiffness matrix, respectively

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One – way interaction

Fig. 10. Numerical results of the total deformation and the reduced stress pattern in the case of static analysis in structural code

Numerical calculations was done for two different cases:

transient calculations in CFX and static in Ansys – pressure field exported from CFX to Ansys transient calculations in CFX and dynamic in Ansys – pressure field exported from CFX to Ansys

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One – way interaction

Fig. 11. Deformation shapes obtained during one-way analysis (case dynamic analysis in Ansys)

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Two – way interaction

Two – way interaction is a sequential or simultaneously combined of the fluid and solid physics analysis. In opposite to one – way interaction both codes: Ansys and CFX serve and receive information from numerical calculation.

Master (Ansys) created socket

Slave (CFX) connect to master

Get code infoServe global control info

Serve code infoGet global control info

Get interface meshesDo mapping

Get initial load and restart loads

Serve interface meshesServe initial and restart loads

Serve time step begin and stagger begin

Get

Load transfer Do solve

Load transfer

Load transferDo solve

Load transfer

Get slave local convergenceServe global convergenceServe time convergence

Serve local convergenceGet global convergenceGet time convergence

Numerical codes used:

Ansys 10

Ansys CFX 10

MFX Ansys

All boundary conditions in CFX and Ansys the some as during one – way interaction

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Two – way interaction

Fig. 12. Deformation pattern

Fig. 13. Temperature profile Fig. 14. Pressure distribution

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Two – way interaction

Fig. 15. Pressure distributions along centerline Fig. 16. Max and min pressure at the flexible liner

Fig. 17. Pressure distributions at midpoint near wall Fig. 18. Liner displacement profile at midpoint

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Backward Facing StepWhere:H1 – upstream channel H – step Xr – reattachment length

u/u0

y/H

x/H=4 x/H=6 x/H=10

x/H

Cf*

100

0

Legend:O Jovic data□ SSTx k-

Fig. 19. Flow over backward facing step Fig. 21. Skin friction distribution over bottom wallFig. 20. Axial velocity profiles

x

y

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Backward Facing Step with heat transfer

q

Fig. 22. Flow over BFS with heat transfer Fig. 23. Skin friction coefficient over bottom wall

Fig. 24. Temperature profiles Fig. 25. Stanton number profiles

x

y

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Backward Facing Step with heat transfer

q

uPulsation: sin amplitude 0.2 frequency 10, 100,400, 1000 Hz SST

Fig. 26. Pulsating flow over BFS with heat transfer

Fig. 27. Mean skin friction coefficient Fig. 28. Mean Stanton number

x

y

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Backward Facing Step with heat transfera) b)

c) d)

Fig.29. Axial velocity profiles in case of perturbation with frequency: a) 10 Hz, b) 100 Hz, c) 400 Hz, d) 1000 Hz

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Conclusions The shape, temperature, and the liner flexible section thickness and length have a major influence

on the walls eigenfrequency, minor influence air cavities was observed

Model of the liner with 680 mm length and 0,8 mm thickness appears to be the appropriate one for cases of the FLUISTCOM Project

One-way interaction gives only insight into real system behavior when both analyses are transient.

Two-way interaction shows significant distribution in pressure pattern as a case of vibrating walls

Both, one- and two-way interactions, predicted similar wall deformation and stress

BFS with heat transfer is matched well with experimental results, especially SST turbulence model.

Significant influence of pulsation frequency on flow pattern was noticed

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Future work

Further numerical investigation of one – way interaction from vibrating wall to fluid inside combustion chamber

Flame transfer function analysis

Experimental work at test rig