SWIRL FLOW IN COMBUSTION CHAMBERS AKMAL NIZAM...
Transcript of SWIRL FLOW IN COMBUSTION CHAMBERS AKMAL NIZAM...
SWIRL FLOW IN COMBUSTION CHAMBERS
AKMAL NIZAM BIN MOHAMMED
A thesis submitted in partial fulfillment of the
requirements for the award of the degree of
Master of Engineering (Mechanical)
Faculty of Mechanical Engineering
Universiti Teknologi Malaysia
NOVEMBER 2008
ABSTRACT
Swirling fluid motion in enclosed chambers was studied using two methods,
simulations using Computational Fluid Dynamics, and experimentation using Particle
Image Velocimetry. Using the tangential inlet configuration as the basic design, several
different swirl generator models were created using Computer Aided Design software.
The aim was to see whether a modified design from the original configuration could
provide a reduction in the backflow effect that is constantly present in swirling flows.
Simulations show that swirl generator inlets that are angled to 45 degrees from the
original tangential position results in the backflow being slightly reduced. However,
simulations in which the inlet angle was further increased yield inconclusive results.
Later in the study, a model prototype of the 45 degrees inlet configuration was created for
the purpose of PIV experimentation. From the experiments run, it was found that the
results were comparable to that of the simulations.
ABSTRAK
Pergerakan bendalir secara berpusar dalam ruang tertutup telah dikaji
menggunakan dua kaedah, iaitu secara Dinamik Bendalir Berkomputer (CFD) dan
Penghalajuan Imej Partikel (PIV). Dengan menggunakan turus masuk secara tangen
sebagai asas rekaan, beberapa model ‘penghasil pusaran’ telah dijanakan menggunakan
perisian rekaan berkomputer (CAD). Tujuan penghasilan model-model ini adalah untuk
menilai sama ada penngubahsuaian daripada konfigurasi asal dapat dimanfaatkan untuk
mengurangkan kesan aliran bertentangan yang sering wujud dalam aliran berpusar. Hasil
simulasi menunjukkan bahawa ‘penghasil pusaran’ yang mempunyai turus masuk pada
45 darjah daripada kedudukan tangen dapat mengurangkan sedikit kesan aliran
bertentangan. Walau bagaimanapun, simulasi untuk konfigurasi yang mana sudut turus
masuk telah ditambah tidak menunjukkan hasil yang konklusif. Kemudian, prototaip
model untuk turus masuk 45 darjah telah dibina bagi tujuan menjalankan eksperimen.
Daripada eksperimen yang dilakukan, telah didapati bahawa hasilnya menyerupai hasil
simulasi.
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TABLE OF CONTENTS
CHAPTER TITLE PAGE
TABLE OF CONTENTS i
LIST OF TABLES iv
LIST OF FIGURES vi
LIST OF SYMBOLS xii
CHAPTER 1
INTRODUCTION 1
1.1 Background 1
1.2 Objective 4
1.3 Scope 4
1.4 Numerical Modeling in Turbulent Flow 4
1.5 Past Researches on Swirl Flow 6
1.6 Current Study 7
CHAPTER 2
THEORETICAL BACKGROUND 9
2.1 Swirl Flow Theory 9
2.2 Governing Equations and Major Assumptions 11
2.3 Numerical Models 12
2.3.1 The Standard k-ε Model 13
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2.3.2 The RNG k-ε Model 14
2.3.3 RNG Swirl Modification 16
2.3.4 Wall Treatment 16
2.4 Discretization: The Finite Difference Method 17
2.5 Particle Image Velocimetry 20
CHAPTER 3
METHODOLOGY 22
3.1 Model Parameters 22
3.2 Geometry Specification 23
3.3 Meshing 29
3.4 Simulation 34
3.5 PIV Experimentation 34
3.6 Analysis Procedure 36
CHAPTER 4
SIMULATION RESULTS 37
4.1 Simulation Setup and Case Parameters 37
4.2 Simulation Model 1: Near Perpendicular Inlet 44
4.3 Simulation Model 2: Fully Tangential Inlet 47
4.4 Simulation Model 3: Semi-Tangential Inlet 50
4.5 Comparison between Cases 53
CHAPTER 5
EXPERIMENTAL RESULTS 57
5.1 Experimental Setup and Case Parameters 57
5.2 Image Location A: Back Wall 61
5.3 Image Location B: In Front of the Inlets 66
5.4 Image Location C: Downstream of the Inlets 70
5.5 Image Location D: Model Outlet 74
5.6 Comparison between Image Locations 76
x
5.6 Calibration image of PIV setup at back wall location 62
5.7 Image of PIV experiment at Location A, running at
the lowest speed setting 63
5.8 Software generated vector plot through
cross-correlation for lowest speed setting 64
5.9 Software generated vector plot after applying
moving-average data processing 65
5.10 Software generated vector plot after filtering the data 66
5.11 Vector plot for Location A at medium speed 67
5.12 Vector plot for Location A at high speed 67
5.13 Calibration image of PIV setup downstream right after
the inlets 68
5.14 Image of PIV experiment running at lowest speed setting 69
5.15 Vector plot for Location B at low speed 69
5.16 Vector plot for Location B at medium speed 70
5.17 Vector plot for Location B at high speed 70
5.18 Calibration image of PIV setup further downstream 71
5.19 Image of PIV experiment running at lowest speed setting 72
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5.20 Vector plot for Location C at low speed 72
5.21 Vector plot for Location C at medium speed 73
5.22 Vector plot for Location C at high speed 73
5.23 Calibration image of PIV setup at the model outlet 74
5.24 Vector plot for Location D at low speed 75
5.25 Vector plot for Location D at medium speed 75
5.26 Vector plot for Location D at high speed 76
5.26 Vector plots of cross sections for: a) simulation at a
quarter length of the model downstream from inlet,
b) PIV image at a quarter of the model length
downstream from inlet, c) simulation at the
model outlet, d) PIV image at the model outlet 77
CHAPTER 1
INTRODUCTION
This chapter provides an initial look to the study, outlining its objectives, listing its
scopes and providing an historical background as reference and motivation.
1.1 Background
Swirl is a three-dimensional phenomenon in fluids flowing in a vessel with a
circular cross-section at speed, where some of the particles follow a spiraling trajectory
around the longitudinal axis of the vessel as they move along in the flow. Combustion
systems that operate based on the principles of swirling fluid motion to facilitate the
combustion process have increasingly been in extensive use. Swirl flows are now utilized
in modern combustion machinery such as refinery or power station burners, gas turbine
combustors, and internal combustion engines [1].
Swirling fluid motion has been introduced in the combustion engine of modern
cars as part of an effort to reduce the emission of harmful gases, specifically Nitrogen
Oxide (NOx). Through the implementation of newly developed technologies in areas such
as catalytic converters, fuel injection systems, and electronic engine management, the
combustion process in these engines have become increasingly efficient. As a result, the
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emission of carbon monoxides, unburned hydrocarbons and other products of combustion
in automotive vehicles have been greatly reduced. However, efficient combustion usually
involves high combustion temperatures brought about by near stoichiometric fuel-air
ratios in the chamber; such conditions have the negative impact of causing high thermal
NOx emissions [2]. This problem would need to be mitigated primarily through
improvements in combustor design [3].
Thus, combustion systems that used alternating cycles of lean and rich burning
mixtures were introduced. To achieve exact ratios of fuel-air mixture, these components
of combustion would need to blend thoroughly before being ignited in the combustion
chamber; hence the use of swirl generators. There have been many different ways to
generate swirling flows being proposed according to their specific applications, for
example using guide vanes, or rotating blades at the flow inlet. Regardless of the methods
used, the generation of swirl flows enables the creation of a lean premixed reaction zone
in the combustion chamber, which is required for lean mixture combustion. Furthermore,
swirling flow helps in flame stabilization by creating flow reversal through vortex
breakdown.
An interesting phenomenon of swirling flows in enclosed boundaries is the
existence of a precessing vortex core along the longitudinal axis of the flow. The core of
the swirl flow is visually distinct from the rest of the fluid movement, and can be
described as having the shape of a continuous corkscrew spinning in the opposite
direction of the general flow, as shown in Figure 1.1.
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Figure 1.1 Precessing Vortex Core (PVC) [1]
The behavior of this vortex core is a significant occurrence in high velocity flows,
as it affects the aerodynamic and thermal performance of the downstream turbine. In the
combustion process, the existence of the vortex core in the combustion mixture flow will
cause the flame itself to have the form of a vortex, as shown in Figure 1.2.
Figure 1.2 Precessing Vortex Core Flame [1]
Precession
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1.2 Previous Study
The current study is principally based on a conference paper entitled
‘Computational Analysis of Turbulent Swirling Flows for Gas Turbine Combustor
Applications’ by Benim et al. (2007) [4]. Consistent with the title of their presentation, the
authors’ work centers on investigating the existence of highly rotating vortex core in the
flow within the combustion chamber of a modern gas turbine combustor and the impact
of this phenomenon on the nozzle guide vanes at the combustor/turbine interface. They
firstly sought out to validate the predictive capability of current modeling procedures for
turbulent swirling flows, especially near the sub/supercritical vortex core transition. They
then analyzed the effectiveness of these predictions as compared to experimental results
done on two different laboratory test rigs with appropriately arranged parameters. All of
the turbulent models used in the study were picked based on their capability of resolving
three-dimensional transient motion of coherent flow structures without assuming a scalar
turbulent viscosity at all scales; they are the URANS-RSM, LES and DES modeling
approaches.
The result of the simulations is as shown in Figure 1.3. The figure is a
temperature contour in a longitudinal plane of a combustion chamber; the core of the
vortex is the hottest part of the flow. The vortex core extends from the inlet to the outlet
of the chamber, interacting intensively with the nozzle guide vanes as a result. This
interaction not only transfer massive amounts of heat to the guide vanes, but also disturbs
the cooling aerodynamics of these vanes, causing them to overheat.
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Figure 1.3 Result of CFD simulation on swirl flow in combustion chamber [4]
1.3 Problem Statement
As exhibited from the previous study, the formation of secondary flows that
moves in the reverse direction from the primary swirling flow brings a negative effect to
components in the combustion chamber. Therefore, this phenomenon, also known as
backflow, needs to be eliminated, or at least reduced.
1.4 Objective
The objective of this study is to reduce the backflow that occurs in swirling flows,
specifically swirls that are produced by tangential inlets that are commonly found in
thermo fluid equipments such as combustion chambers, plenum chambers, and fluidized
beds. As an attempt to improve on the current design, the inlets will be tilted from the
tangent position up to certain angles, the result of which will be studied through
simulations and experiments. To determine whether the backflow has reduced or
increased, the velocity profile at several points in the flow region will be produced.
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1.5 Scope
The scope of the study includes:
• Development of swirl generator model
• Determination of the swirl flow pattern using both Computational Fluid
Dynamics (CFD) and Particle Image Velocimetry (PIV)
• Simulation and experimentation using air as the working fluid at
atmospheric temperature (cold flow), thus without involving heat transfer
• Comparison of results between CFD and PIV
1.6 Past Researches on Swirl Flow
Some of the most important research papers concerning this topic are listed in
Table 1.1. These researches are focused on the flow behavior in the chamber and not the
actual combustion process.
Table 1.1 Past researches on swirl flow
Date Occurrence
1983 Escudier and Keller propose the idea that a vortex breakdown along the swirl
axis can be thought of as an abrupt transition between an upstream supercritical
flow and a downstream subcritical flow. A supercritical condition occurs if the
axial flow velocity exceeds the relative phase velocity of upstream-directed
inertia waves, while a subcritical condition is the opposite.
1986 Weber et al. produces computation results of near-field aerodynamics of swirling
expanding flows.
1988 Hogg et al. provide computation results of highly swirling confined flows with
using the Reynolds Stress Turbulence Model.
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1990 Benim publishes a finite element analysis of confined swirling flows
1997 Xia et al. publish a study on the effects of three-dimensionality on swirling flows
with or without combustion.
2004 Turell et al. CFD simulation of the flow within and downstream of a high-swirl
lean premixed gas turbine combustor.
1.7 History of Numerical Modeling in Turbulent Flow
The complexity of fluid flow in general makes it almost impossible to obtain
complete analytical solutions for most real-world applications of fluid dynamics.
However, without the ability to explain and predict the flow of fluids in the form of
mathematical expressions, one cannot conceivably design any machine or apparatus that
deals with fluids to work effectively under wide ranging circumstances of fluid
movement. Therefore, scientist and researchers have long been involved in creating
models that approximate the actual behavior of fluid flow to varying degrees of accuracy
depending on the situation. In order to understand the correct method of simulating fluid
flow structures, sufficient knowledge of fluid dynamics, computational methods, and
numerical modeling is required. All these fields are encompassed in the study of turbulent
flow modeling.
Turbulent modeling has been around since the late 1800s, when the analytical
approach to solve for turbulent fluid flow has been all but exhausted. Some of the
prominent figures in fluid mechanics began to introduce various concepts to describe the
interactions between fluid particles and the factors that come into play at various
conditions. The following Table 1.2 gives an overview of turbulent modeling
development in chronological order.
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REFERENCES
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[2] S. L. Yang, Y. K. Siow, B. D. Peschke and R. R. Tacina (2003). Numerical Study of Non-Reacting Gas Turbine Combustor Swirl Flow Using Reynolds Stress Model. ASME J. Eng. Gas Turbines Power Vol. 125, 804-811.
[3] B. L. Koff (1994). Aircraft Gas Turbine Emission Challenge. ASME J. Eng. Gas Turbines Power Vol. 116, 474-477.
[4] A. C. Benim, M. P. Escudier, P. J. Stopford, and K. J. Syed (2007). Computational Analysis of Turbulent Swirling Flows for Gas Turbine Combustor Applications. The 9th Asian International Conference on Fluid Machinery NO. AICFM9-IL06.
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