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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