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PERFORMANCE INVESTIGATION OF TURNING DIFFUSERS AT VARIOUS
GEOMETRICAL AND OPERATING PARAMETERS
by
NORMAYATI BINTI NORDIN
A Thesis
Submitted to the Postgraduate Studies Programme
as a Requirement for the Degree of
DOCTOR OF PHILOSOPHY
MECHANICAL ENGINEERING
UNIVERSITI TEKNOLOGI PETRONAS
BANDAR SERI ISKANDAR,
PERAK
SEPTEMBER 2016
DEDICATION
To my pillars of strength Muhd Adli Yunus, Muhammad Amir Mirza and
Muhammad Adib Hamza for their infinite love and support
To my parents Nordin Abdul Hamid and Natisah Salleh & parents in-law
Yunus Ismail and Jawiah Ahmad for their constant encouragement and
enlightenment
To all my siblings for their continuous inspiration
ACKNOWLEDGEMENTS
First of all, I am grateful to the Almighty God for establishing me to complete this
research.
I wish to express my sincere thanks to my supervisor, Assoc. Prof. Dr. Zainal
Ambri Abdul Karim for providing me with all the necessary help and pursue me to
finish the thesis.
I am very much thankful to Dr. Safiah Othman, my field supervisor for her
constant guidance, keen interest and encouragement at various stages of my study
period.
I am extremely grateful and indebted to my former supervisor, Prof. Dr. Vijay R.
Raghavan for the expert, sincere and endless guidance extended to me. The
opportunity given by him for me to work on this project is so much appreciated.
Immeasurable appreciation is also extended to all department members,
especially, Dr. Mohd Faizal Mohideen Batcha, Mr. Suzairin Md Seri, Dr. Azian
Hariri, Dr. Bambang Basuno, Mdm. Siti Mariam Basharie, Dr. Norasikin Mat Isa,
Dr. Hamidon Salleh, Assoc. Prof. Dr. Norzelawati Asmuin, Assoc. Prof. Dr. Ahmad
Jais Alimin and Prof. Ir. Dr. Abas Abdul Wahab for their insight sharing and help
given throughout the study.
My sense of gratitude is also devoted to my lab mates, Nur Hazirah Noh@Seth,
Alaeddin Mohamed Ejledi, Noraishah Mohammad Nor, Fairuz Nasri, and Mohd
Hanis Amran for the moment and thought of learning shared.
I also thank to Mr. Cheng Yew Hong (Sales and Application Manager of Sound
and Vibrant Technology Sdn Bhd), Mr. Zaid Suleiman (Manager of Quantum Two
Engineering), Mr. Zainal Abidin Alias (Assistant Engineer of Aerodynamic
Laboratory, UTHM) and Mr. Rosman Tukiman (Assistant Engineer of CFD
Laboratory, UTHM) for the technical supports provided in labs.
I am also indebted to my employer, Faculty of Mechanical and Manufacturing
Engineering, UTHM and Ministry of Higher Education under the Fundamental
Research Grant Scheme (FRGS) whom funded the project wholly.
Last but not least, I also thank to my beloved family and friends for their
unceasing encouragement and support.
ABSTRACT
The performance of the turning diffuser regardless of its expansion type, i.e., two-
dimensional (2-D) or three-dimensional (3-D), has been traditionally rated using the
guidelines established specifically for a 2-D turning diffuser. This has provided
merely an approximation and has often led to an inaccurate prediction of 3-D turning
diffuser performance. On top of that, the existing guidelines have just integrated the
geometrical effect by discounting the effect of operating condition on the turning
diffuser performance. Therefore, the current work aims to experimentally and
numerically investigate the performance of 2-D and 3-D turning diffusers for various
geometrical and operating parameters. The performance indexes (pressure recovery
coefficient, flow uniformity index) as a function of geometrical (inner wall length to
inlet throat width ratio, outlet-inlet configurations) and operating (inflow Reynolds
number) parameters are correlated by means of Asymptotic Computational Fluid
Dynamics technique. Stereoscopic particle image velocimetry was used to examine
the flow characteristics, and a manometer provided the inlet and outlet wall static
pressures. Among all the models tried, the best results were obtained with the
standard k- and enhanced wall treatment of y+ 1.1 – 1.8 was applied for the
intensive simulation. Results showed that there was a potential performance of
applying 3-D turning diffuser relative to 2-D turning diffuser. The 3-D turning
diffuser provided higher pressure recovery at low inflow Reynolds number,
Rein = 5.786 x 104 - 6.382 x 104 and better flow uniformity at high inflow Reynolds
number, Rein = 1.027 x 105 – 1.775 x 105 than the 2-D turning diffuser. Minimal flow
separation occured within the 3-D turning diffuser that was close to the outlet edge,
0.9Lin/W1. While flow separation within the 2-D turning diffuser took place earlier on
half of the inner wall length, 0.5Lin/W1. Secondary flow vortices initially emerged at
Rein = 1.027 x 105 (3-D turning diffuser) and Rein = 1.397 x 105 (2-D turning diffuser).
The pressure recovery was affected mainly by the existence of flow separation and
vortices, whereas the flow uniformity was affected by the dispersion of core and
secondary flows. A high free-stream turbulent intensity imposed on the flow favoured
the overall performance of the turning diffuser by suppressing the separation of the
inner wall boundary layer and mixing to give better uniformity of the flow. Excessive
elongations, Lin/W1 ≥ 20 (2-D turning diffuser) and Lin/W1 ≥ 9 (3-D turning diffuser)
inherently impaired the pressure recovery. The performance correlations as a function
of geometrical and operating parameters for 2-D and 3-D turning diffusers were
successfully developed to satisfy both the CFD and experimental results within ±8%.
In conclusion, the physics of flow particularly within the 3-D turning diffuser have
been grasped with credible performance data have been established as benchmark.
The developed correlations can be used henceforth by one to evaluate the
performance of turning diffusers without necessarily running the full simulation or
experiment. For future work, the same outlined methods particularly via Asymptotic
Computational Fluid Dynamics can be applied to develop performance correlations of
other diffuser types. The current work can be further extended by considering the
variation of turning angles and installation of flow control devices to improve the
performance of turning diffusers. The effects of skin friction and turbulent intensity
should be also looked into more details.
In compliance with the terms of the Copyright Act 1987 and the IP Policy of the
university, the copyright of this thesis has been reassigned by the author to the legal
entity of the university,
Institute of Technology PETRONAS Sdn Bhd.
Due acknowledgement shall always be made of the use of any material contained
in, or derived from, this thesis.
© Normayati binti Nordin, 2016
Institute of Technology PETRONAS Sdn Bhd
All rights reserved.
CHAPTER 1
INTRODUCTION
This chapter introduces the background, problem statement, objectives, scope and
contributions of the research. The important keywords associated with the research
are defined in the research background. In the problem statement, the motivation for
the research being conducted is briefly explained. The objectives and scope are
outlined and the contributions of the research are highlighted.
1.1 Research Background
A diffuser is often introduced in fluid flow systems as (i) an adapter to join the
conduits of different cross-sectional areas or (ii) an ejector to decelerate the flow and
raise the static pressure before discharging to the atmosphere. The basic idea of
introducing diffuser in flow lines is to conserve the energy by having as uniform a
flow as possible. In a circulating fluidised bed (CFB) system, a diffuser is installed to
assemble the lower and upper parts of riser which are at different cross-sectional areas
[1]. In a heating, ventilation and air-conditioning (HVAC) system, a diffuser is used
as a part of room air distribution subsystem [2, 3], while, in aircraft applications,
diffusers are installed to recover the energy by converting the kinetic energy into
presure energy [4-7].
As shown in Figure 1.1, diffusers are commonly classified by their geometry.
Generally, a diffuser that is introduced with no turn is known as a straight diffuser,
whereas a diffuser introduced with certain angle of turn is called a turning diffuser or
a curved diffuser. The cross-sectional area of diffuser expands gradually in either
two - dimensions (2-D) or three-dimensions (3-D) in the direction of flow.
Sometimes, diffusers are also named based on their unique shape such as a pyramidal
Figure 1.1: Several types of diffuser classified by the geometry (a) three-dimensional
straight diffuser [2] (b) S-shaped diffuser [4] (c) two-dimensional turning diffuser
with 55angle of turn [8] (d) two-dimensional turning diffuser with 90 angle
of turn [9]
diffuser (square inlet- outlet cross-sectional shape), a conical diffuser (circular inlet-
outlet cross-sectional shape), a slab diffuser (rectangular inlet-outlet cross-sectional
shape), a S-shaped diffuser (‘S’ shape of turning) and a Y-shaped diffuser (‘Y’ shape
of turning). Study of the performance of various diffusers has been of fundamental
interest to researchers in the area of fluid mechanics since decades and it continues to
grow.
(a)
(b)
(c) (d)
The performance of a diffuser is primarily measured using the outlet pressure
recovery coefficient (Cp). Cp indicates how much kinetic energy is successfully
converted to pressure energy. The main problem in achieving high pressure recovery
is flow separation, which results in dissipation of energy and non-uniform flow
distribution [2, 3, 10-12]. The problem becomes even worse when a turning diffuser
with sharp 90 angle of turn () as sketched in Figure 1.2 is considered.
Its strong curvature maximally thickens the inner wall boundary layer, increases
the potential flow loadings and reduces the turbulent mixing along the inner wall. As
a result, the fast stream flow deflects much toward the outer wall to produce
unfavourable outlet flow uniformity. The flow uniformity index (out) is used to
measure the dispersion of local velocity from the mean velocity. It is strongly
dependent on the distribution of the core flow and the presence of secondary flow.
The flow is considered uniform with the presence of secondary flow of less than 10%
[6, 7].
Figure 1.2: Geometric layout of 90 turning diffuser
Practical applications always seek a compromise between the maximum
permissible pressure recovery and flow uniformity. This can be achieved by setting
the geometrical and operating parameters, namely, the inner wall length to the inlet
throat width ratio (Lin/W1), area ratio (AR), outlet-inlet configurations (W2/W1, X2/X1)
and inflow Reynolds number (Rein) optimally.
1.2 Problem Statement
Flow through a 90 turning diffuser is complex, apparently due to the expansion and
sharp inflexion introduced along the direction of flow. The inner wall is subjected
to curvature-induced effects where under a strong adverse pressure gradient, the
boundary layer on the inner wall is likely to separate, and the core flow tends to
deflect toward the outer wall region [8, 13-16]. Flow separation is undesirable as it
could adversely affect the overall performance of diffusers [9, 17-19].
Despite extensive literature on diffusers are available, less attention has been
given to 3-D diffuser type [2, 19, 20]. A 3-D turning diffuser is used widely in
industrial flow particularly in HVAC and wind tunnel systems on account of its
design flexibility offering various ranges of outlet-inlet configuration. Nevertheless,
the performance of 3-D turning diffuser has never been scientifically justified and is
commonly estimated using the guideline specifically established by Fox and Kline
[13] for 2-D turning diffuser, or even often without a sound theoretical basis merely
based on rule of thumb.
Prior performance investigations of turning diffusers focused solely on the
geometrical aspects. For instance, Fox and Kline [13] correlated the effect of
geometrical parameters (AR = 1.2 - 4.0, Lin/W1 = 1.5 - 30, = 0 - 90) on the flow
regimes of 2-D turning diffusers. Sagi and Johnson [14] established a design
procedure for 2-D turning diffusers of AR = 1.5 - 2.1, Lin/W1 = 4.0 - 10 and
= 30 – 90. Since the early 1980s, the variation of operating conditions, Rein have
been taken into consideration to affect the performance of diffusers such as 55 2-D
turning diffuser (Rein = 7.8 x105 – 1.29 x 106) [8], annular diffuser (Rein = 6.0 x103 –
6.0 x 105) [21] and combined 90o bend diffuser (Rein = 8.8 x 104 – 1.94 x 105) [3].
However, the works remain unresolved with no guidelines so far available integrating
the effects of both geometrical and operating parameters on the performance of 90
turning diffusers.
Therefore, the current work aims to experimentally and numerically investigate
the performance of 90 2-D and 3-D turning diffusers. Performance correlations are to
be developed to integrate the effects of both geometrical and operating parameters. It
is expected that the geometrical and operating parameters affect significantly the
diffusers’ performance particularly in case of 3-D turning diffuser which possesses
relatively complex flow. It is also anticipated that the developed correlations can
reasonably predict the pressure recovery and flow performances.
1.3 Objectives of Research
The research objectives are specified as follows:
1. To examine the potential performance of 3-D turning diffuser relative to 2-D
turning diffuser.
2. To assess the effects of varying geometrical and operating parameters on the
performance of 2-D and 3-D turning diffusers.
3. To develop performance correlations as a function of geometrical and
operating parameters for 2-D and 3-D turning diffusers.
1.4 Scope of Research
The scope of research is outlined as follows:
1. 90 2-D and 3-D turning diffusers with identical inlet condition were
considered.
2. Performance of turning diffusers was evaluated primarily in terms of Cp and
out.
3. Geometrical and operating parameters for 2-D turning diffuser (AR, W2/W1,
Lin/W1 and Rein) and 3-D turning diffuser (AR, W2/W1, X2/X1, Lin/W1 and Rein)
were considered.
a. Geometrical and operating parameters were varied within 1.2 AR 4.0,
1.5 Lin/W1 30 and 5.786 x 104 Rein 1.775 x 105.
b. The ranges were selected based on common diffuser applications in
HVAC and wind tunnel systems [2, 3, 9, 14, 15].
4. Experiment was conducted to primarily examine the potential performance of
3-D turning diffuser relative to 2-D turning diffuser.
a. 2-D and 3-D stereoscopic particle image velocimetry (PIV) systems were
used to examine the flow characteristics within the turning diffusers.
b. A digital manometer of resolution 1.0 Pa was used to measure the average
static pressures at the inlet (pin) and outlet (pout).
5. Computational fluid dynamics (CFD) was applied to assess the effects of
geometrical and operating parameters on the performance of 2-D and 3-D
turning diffusers.
a. ANSYS 14.5 was employed to perform the CFD works including project
and data management (Workbench), modelling (DesignModeler), grid
generation (ICEM CFD) and flow analysis (Fluent).
b. The applicability of standard (ske), renormalization group (rngke) and
realizable (rke) k- turbulence models to represent the actual cases was
investigated.
c. Appropriate near wall treatments, i.e. standard wall functions, non-
equilibrium wall functions and enhanced near wall treatment were
verified.
6. Asymptotic computational fluid dynamics (ACFD) was performed to develop
performance correlations of 2-D and 3-D turning diffusers (Cp and out) as a
function of geometrical and operating parameters (Lin/W1, W2/W1, X2/X1, Rein).
1.5 Contributions of Research
The significant contributions of the research to the body of knowledge are stated as
follows:
1. The potential performance of 90 3-D turning diffusers has been scientifically
assessed. Credible performance data for 90 3-D turning diffusers have been
established as benchmark.
2. The physics of flow within the turning diffusers how it affects the overall
performance have been grasped. This was mainly by the use of high-end flow
measurement techniques, 3-D stereoscopic PIV and CFD that provided three-
dimensional flow analysis.
3. The performance correlations integrating the effects of both geometrical and
operating parameters for 2-D and 3-D turning diffusers have been developed.
These correlations can be used after this by one to evaluate the performance of
2-D and 3-D turning diffusers without necessarily running the full simulations
or experiments.
1.6 Thesis Outline
The thesis is outlined as follows:
Chapter 1 presents the introduction to the research work. Various diffuser types are
available but the current work focuses on investigating the performance of 90o
turning diffuser which is anticipated to greatly be influenced by the geometrical
and operating parameters.
Chapter 2 reviews the previous reported works on diffusers that are clustered based
on the methodology applied, i.e. experimental, numerical and ACFD. The gap of
knowledge within the field is identified and the objectives, scope and methodology
of the current work are formulated.
Chapter 3 delineates in detail the experimental, numerical and ACFD methods
applied to investigate the performance of 2-D and 3-D turning diffusers. Design
and development of rig to produce fully-developed entrance flow are explained.
The flow visualisation procedures by means of PIV (experimental) and ANSYS
code Fluent (numerical) are elucidated. Taylor series expansion applied via ACFD
to develop correlations is elaborated.
Chapter 4 analyses and discusses explicitly the experimental, numerical and ACFD
results. The experimental results highlight the potential performance of 3-D turning
diffuser relative to 2-D turning diffuser. The onset flow separation of the diffuser is
also determined. The effects of geometrical and operating parameters are assessed
numerically. The efficacy of performance correlations developed is verified.
Chapter 5 concludes the research findings as achievement of research objectives.
Recommendations for future works are given.
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