Cfd analysis of flow charateristics in a gas turbine a viable approach

INTERNATIONALMechanical Engineering and Technology (IJMET), ISSN 0976 –
International Journal of JOURNAL OF MECHANICAL ENGINEERING
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
AND TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 4, Issue 2, March - April (2013), pp. 39-46
IJMET
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CFD ANALYSIS OF FLOW CHARACTERISTICS IN A GAS TURBINE-
A VIABLE APPROACH TO PREDICT THE TURBULANCE

a b
P.S. Jeyalaxmi , Dr.G.Kalivarathan
a
Research Scholar, CMJ University, Meghalaya, Shillong.
b
Principal/ PSN Institute of Technology and Science, Tirunelveli, Tamilnadu, Supervisor,
CMJ University, Shillong.

ABSTRACT

The demands for best performance in gas turbine engines can be obtained by
increasing combustion temperatures to increase thermal efficiency. Hot combustion
temperatures create a harsh environment which leads to the consideration of the durability
of the combustor and turbine sections. Improvements in durability can be achieved
through understanding the interactions between the combustor and turbine. The flow field
at a combustor exit shows non uniformities in pressure, temperature, and velocity in the
pitch and radial directions. This inlet profile to the turbine can have a considerable effect
on the development of the secondary flows through the vane passage. Presents a
computational study of the flow field generated in a non-reacting gas turbine combustor
and how that flow field convects through the downstream stator vane. Specifically, the
effect that the combustor flow field had on the secondary flow pattern in the turbine was
studied.

1.0 INTRODUCTION

The designs of aircraft gas turbine engines have improved and evolved
tremendously. The first gas turbine powered aircraft reached top speeds of 435
Kilometers per hour; only sixty years later aircraft powered by gas turbine engines are
flying at speeds exceeding 2000 Kilometers per hour. The major contributions to the
advances in gas turbine engine performance have been increased in terms of power
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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –

output, reliability and fuel efficiency. The existing demands for improving higher
performance while maintaining affordability and engine durability can only be reached
through the achievement of hot combustion temperatures and better cooling schemes. The
need for increasingly higher temperatures creates a critical ambiance environment for the
first stage turbine vane and combustor liner. The difficult ambiance in the engine pushes
the design holds for durability in both the combustor and turbine sections. Sincerely is
required for the flow field leaving the combustor and its impact on the heat transfer to the
turbine vane is also be investigated to the maximum possible extend.

2.0 COMBUSTOR MODEL TEST SECTION DESIGN

The first stage of both the experimental and computational portions of the present
study involved the design of a non-reacting combustor test facility necessary to simulate
the geometry and flow conditions of a realistic gas turbine engine combustor. The specific
work done by the author of this thesis included the initial design of the combustor test
section geometry and the computational simulation of the wind tunnel model. The actual
wind tunnel design, including matching engine parameters and the liner panel design, was
primarily completed. Prior to describing the computational modeling, however, it is
important to describe the wind tunnel design and model. The computational combustor
model was based directly on the geometry and flow conditions of the experimental wind
tunnel test section, which was designed to match representative engine combustor
conditions. The combustor, which was simulated, is typical of an annular combustor in a
commercial gas turbine engine. The geometry is characterized by an impingement film
flow cooling scheme which consists of four panels of film-cooling holes and two rows of
dilution jets on both the inner and outer diameters with additional coolant provided by a
slot located at the combustor exit. The combustor has a constant area cross section for the
first half of its length followed by a contraction leading to the downstream turbine. The
first step in the design of the combustor test section was to thoroughly analyze the
combustor engine data provided by industry, and to then determine which engine
parameters needed to be matched in the simulation in order to accurately represent the
combustor flow field. The engine data was then scaled up within the size constraints of an
existing facility described in previous studies.

3.0 LINER PANEL DESIGN

The design of the individual liner panels within the combustor test section was a
complicated task in itself involving several important compromises between the engine
and the model. It was desired to create panels that matched as closely as possible both the
geometric details of the combustor as well as the flow parameters of the film cooling,
dilution, and slot flows.

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Figure 1. Schematic of the wind tunnel facility with combustor simulator
test section in the open loop configuration

Figure 2 Schematic of engine combustor geometry with dimensions in cm

The two most important goals in the process were to determine the necessary numbers of
film-cooling holes and the sizes of dilution to properly match the desired mass flow rate
distribution and momentum flux ratios, and then lay out the holes in the liner matching the
engine geometry as closely as possible. A third consideration was the ability to adapt the test
section to several geometries; with and without dilution flow, with and without slot flow, and
the possibility of a different liner geometry altogether.

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Table1. Stator Vane Geometry in the Engine and Model

Engine Wind Tunnel

True Chord 6.45 cm 59.15 cm

Pitch/Chord 0.79 0.79

Span /Chord 0.95 0.95

Flow inlet angle 0° 0°

Flow exit angle 77° 77°

Table 2 Mass Flow Distributions in Combustor Based on Percentage of Exit Mass Flow

Engine Wind Tunnel
Inlet 37.6% 42.6
Panel 1 (ID/OD) 1.43%/2.55% 1.43%/1.45%
Panel 2 (ID/OD) 3.6%/4.65% 3.4%/3.4%
Panel 3 (ID/OD) 3.3%/4.55% 3.3%/3.3%
Panel 4 (ID/OD) 1.4%/2.65% 1.4%/1.4%
Dilution row 1 16.5% 16.5%
Dilution row 2 16.5% 16.5%
Slot 1.38% 1.38%

4.0 GAMBIT MESHING

All of the models were meshed using GAMBIT, a program offered with the Fluent
software package. The mesh was a critical element of the problem setup since the accuracy of
the solution will certainly be limited by the quality of the mesh which is used to calculate it.
The process of mesh generation was quite involved and required several steps, which will be
outlined in detail in this section. First, the combustor and vane geometry was created using
Gambit’s solid modeling capabilities. Then, line and surface mesh spacing layouts were
attached to the geometry. Finally, the internal solid mesh was created from the line and
surface meshes and was refined to reduce cell skewness. The geometry and mesh was then
exported and read into Fluent. The basic geometry may be defined in Gambit in several ways.
For the combustor section, volumes were created corresponding to the cooling holes, dilution
holes, and main combustor geometry. Using Boolean operations, these were combined to
create the complete combustor geometry. In order to create the vane geometry, points were
defined along the contour of the vane and fitted with splines to create the vane surface.

42


Figure 3. Schematics of computational domain for combustor model
5.0 case 2 with no vane, no slot and (b) case 5 with vane and slot.

Figure 4. Domain, mesh and boundary conditions for single hole Case I.

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Figure 5. Velocity profile (U/Uave) at hole inlets for each of the four liner panel axial single
hole cases.

Figure 6. Velocity profiles (U/U∞, ave) cooling hole diameter downstream of the hole trailing
edge for each of the four axial hole panel cases.

44


Figure 7. Adiabatic effectiveness comparison for (a) axial and (b) compound angle cooling
holes at I = 13.1 and M = 3.6.

5.0 CONCLUSION

In this investigation it is observed that the RNG k-ε turbulence model used in the
cases is viable and it is seen that the RSM model remarkably exhibits the turbulence level and
the dilution jet mixing at the combustor outlet. But still, It is easier to predict the variations
through CFD in the mean flow field results, In addition to this the turbulence levels of the
approaching flow may affect the secondary flow field in the vane passage. Because of these
reasons more efforts has to be made towards testing additional turbulence models to access
one more suitable means for predicting this type of swirling, highly turbulent and flow
situation in a generic manner through viable CFD tool.

45


REFERENCES

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Cfd analysis of flow charateristics in a gas turbine a viable approach

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Cfd analysis of flow charateristics in a gas turbine a viable approach