final report 2014.pdf1

I

PRIMARY BREAKUP OF LIQUID SHEET UTILIZING AIR-

ASSISTED TWIN-FLUID ATOMIZER

A PROJECT REPORT

SUBMITTED BY

by

AVINASH KUMAR RAY – CB.EN.U4MEE10012

SAMMETTA SAMBASIVA RAO – CB.EN.U4MEE10044

L VENAKTESH MUTHURAMAN – CB.EN.U4MEE10056

PUNNAMARAJU V GIRITEJA – CB.EN.U4MEE10233

In partial fulfulment for the award of the degree

of

BACHELOR OF TECHNOLOGY

IN

MECHANICAL ENGINEERING

DEPARTMENT OF MECHANICAL ENGINEERING

AMRITA SCHOOL OF ENGINEERING

AMRITA VISHWA VIDYAPEETHAM COIMBATORE – 641112

II

May, 2014


AMRITA VISHWA VIDYAPEETHAM, COIMBATORE 641112

BONAFIDE CERTIFICATE

This is to certify that the thesis entitled “Experimental characterization of liquid

sheet break up using acoustic perturbations” submitted by AVINASH KUMAR

RAY, SAMMETTA SAMBASIVA RAO, L VENKATESH

MUTHURAMAN, PUNNAMARAJU VENKATA GIRITEJA for the award

of the Degree of Bachelor of Technology in Mechanical Engineering is a bonafide

record of the work carried out under our guidance and supervision at Amrita School of

Engineering, Coimbatore.

Mr. K. Balaji

Project guide

Dept. of Mechanical Engineering,

Amrita School of Engineering.

Dr. V. Sivadas

Project Co - guide

Dept. of Aerospace Engineering,


Dr. S. Thirumalini

Chairman

Dept. of Mechanical Engineering

Amrita School of Engineering

This report was examined and the candidates underwent Viva-Voce examination on

1st May 2014

Internal Examiner External Examiner

III


AMRITA VISHWA VIDYAPEETHAM, COIMBATORE 641112.

DEPARTMENT OF MECHANICAL ENGINEERING

DECLARATION

We, AVINASH KUMAR RAY (CB.EN.U4MEE10012), SAMMETTA

SAMBASIVA RAO (CB.EN.U4MEE10044), L VENKATESH MUTHURAMAN

(CB.EN.U4MEE10056), PUNNAMARAJU VENKATA GIRITEJA

(CB.EN.U4MEE10233) hereby declare that project work entitled “Primary breakup

of liquid sheet utilizing Air-assisted Twin-fluid atomizer”, is the record of the original

work done by us under the guidance of Mr. K. BALAJI, Assistant Professor,

Department of Mechanical Engineering and Dr. V. SIVADAS, Associate

Professor, Department of Aerospace Engineering, Amrita School of Engineering,

Coimbatore. To the best of my knowledge this work has not formed the basis for the award

of any degree/diploma/associateship/ fellowship or a similar award to any candidate in any

University.

AVINASH KUMAR RAY – CB.EN.U4MEE10014

SAMMETTA SAMBASIVA RAO - CB.EN.U4MEE10044

L VENAKATESH MUTHURAMAN – CB. EN.U4MEE10056

PUNNAMARAJU VENKATA GIRITEJA– CB.EN.U4MEE10233

Place: Coimbatore 641112.

Date:

COUNTERSIGNED

Mr. K. BALAJI

Project guide

Dept. of Mechanical Engineering,


Dr. V. SIVADAS

Project Co-guide

Dept. of Aerospace Engineering,

Amrita School of Engineering

4

TABLE OF CONTENTS

ACKNOWLEDGEMENT..................................................................................................5

LIST OF FIGURES..........................................................................................................6

ABSTRACT.....................................................................................................................7

1. INTRODUCTION........................................................................................................8

2. LITERATURE SURVEY................................................................................................10

3.EXPERIMENTAL SETUP AND TECHNIQUES................................................................18

3.1 Experimental setup................................................................................................19

3.2 Air supply system...................................................................................................21

3.3 Fuel supply system.................................................................................................22

3.4 Image acquisition and processing..........................................................................24

3.5 Calibration of Fuel supply system..........................................................................27

3.6 Calibration of Air line.............................................................................................29

3.7 Breakup length and Spray angle measurements...................................................30

4. RESULTS AND DISCUSSIONS.....................................................................................32

4.1 Qualitative Analysis...............................................................................................32

4.2 Quantitative Analysis.............................................................................................36

5. CONCLUSION...........................................................................................................44

6. REFERENCES............................................................................................................45

5

ACKNOWLEDGEMENT

It gives us great pleasure in acknowledging our project guide Mr. K. Balaji, Assistant

Professor, Department of Mechanical Engineering and our co-guide Dr. V. Sivadas,

Associate Professor, Department of Aerospace Engineering for technical guidance, moral

support and friendly discussions throughout this project. Without their support this

project wouldn’t have grown to this extent. A sincere thanks won’t be sufficient to the

discussions and advices that they have contributed over the course of project. Their

aptitude in engineering and research was inspirational and continuously motivating us to

get into the field and contribute to the scientific community.

We thank Dr. S. Thirumalini, Chairperson, Department of Mechanical Engineering for

allowing us to use the facilities in college and Dr. Elangovan, Professor, Department of

Mechanical Engineering for helping in this project.

We would also like to acknowledge with much appreciation, the crucial role of the staff

of Mechanical Department Laboratories and workshops for their selfless help –

Mr. Sundararaman, Mr. Vignesh, and Mr. Venkatesh.

6

LIST OF FIGURES

Figure

number

Title of the figure Page

number

2.1 Stages of liquid sheet break up 8

2.2 Effect of Rayleigh and Ohnesorge number on atomization 8

2.3 Mechanism of break-up of liquid sheet in co-axial air

stream

9

2.4 Break up length and Spray angle of air assisted atomizer 11

3.1 Schematic diagram of Experimental Setup 14

3.2 Air nozzle duct 16

3.3 Liquid sheet generator 17

3.4 Arduino circuit for calibrating liquid line 20

3.5 Liquid line calibration 21

3.6 Air supply line calibration graph 22

4.1,4.2,

4.3,4.4 Spray Images at different liquid and air velocities 23

4.6 Non-dimensional breakup length and Weber number 24

4.7 Break-up length as function of liquid at various air

velocities

25

4.8 Spray angle as a function of liquid velocities at various air

velocities

26

7

ABSTRACT

An exhaustive study on the effect of air in the breakup of a liquid sheet of water flowing

forms the essence of this project. Water at different flow velocities is passed through a

rectangular nozzle that makes a liquid sheet, along with the design of the separate air

columns for it to break the liquid sheet. Initially the calibration of the liquid-line and air-

line is done. Qualitative and quantitative analysis is done with simultaneous passage of

liquid and the air line with the help of image processing and high speed camera with

which a quantitative approach was made and a trend line was obtained for non-

dimentionalised curves. The project shows promising future works which can be

subsequently done with the current one as prime basis.

8

1. INTRODUCTION

1.1 ATOMIZATION

Atomization is a process in which a liquid jet or sheet is disintegrated by the kinetic

energy of the liquid itself or by the exposure to high velocity air or gas or externally

applied kinetic energy into a fine spray.

1.2 SPRAY

A spray is a dynamic collection of liquid drops entrained in surrounding gas. The

transformation of bulk liquid into fine droplets and sprays is of great importance in day to

day life and in a variety of industrial processes, agriculture, meteorology, medicine, paint

spraying, spray drying of wet solids, food processing, cooling of nuclear cores and to

disperse the liquids fuels for combustion in gas turbine combustors.

The current gas turbine combustor technology focuses on the reduction of NOx emissions

in order to meet the growing restrictions regarding emissions. The air -fuel mixture

properties and its injection conditions into the combustor are crucial parameters that

determine the emission from the engine. The better the atomization of the fuel, the lower

the emissions. Thus various types of injection techniques are being experimented on to

try and improve fuel atomization and produce a fine fuel spray.

1.3 ATOMIZERS

Atomizers are the devices used to atomize the fluids. The main types of atomizers are

pressure swirl atomizers, plain orifice atomizers and twin fluid atomizers.

Pressure swirl atomizers have a pre-swirl chamber where the fluid is given a swirl thus

leading to turbulence and this induces the sheet breakup when injected into the chamber.

9

Plain orifice atomizers involve the injection of the fluid into the combustion chamber

through a tiny orifice which causes the fluid sheet to break up into droplets.

Twin fluid atomizers involve the injection of fluid into the chamber at a relatively low

pressure with another fluid, normally air being used to excite the fluid leading to its break

up. Twin fluid atomizers are of two types: air blast and air assist. Basically it is the

momentum of the impinging sheet that causes the fluid to break up into droplets.

M = mv where M is the momentum of the impinging fluid (1)

m is the mass of the fluid and v is the velocity.

Air assist atomizers use a small mass of high velocity air to atomize the liquid. Air blast

atomizers on the other hand use a large quantity of low velocity fluid to cause liquid

breakup.

Air assist atomizers utilize high velocity air to excite the fuel and trigger disturbances

which lead eventually to breakup of the liquid sheet. Two primary types of air assist

atomizers exist: internal mixing type and external mixing type.

Internal mixing atomizers involve the mixing of liquid and air within the atomizer body

itself. In these atomizers the spray cone angle is minimum for maximum airflow and the

spray cone angle increases as the airflow decreases. This is used for high viscosity liquids

that require good atomization at low liquid flow rates.

External mixing atomizers involve the liquid impinging on the air outside the atomizer

body. Thus these are safer as the liquid will not be accidentally forced into the air line at

the exit. However their utilization of air is less efficient and power consumption is high.

10

2. LITERATURE SURVEY

2.1 LIQUID SHEET

The liquid emerging from a twin fluid atomizer may be in the form of liquid sheet or

liquid jets depending upon the geometry of nozzle exit [1]. Liquid sheets are obtained

when the liquid is passed through a thin rectangular orifice of high aspect ratio, where as

a jet is obtained when the liquid is forced out of a circular aperture. Liquid sheet is our

center of interest here. The liquid sheet that is obtained out of the nozzle is unstable and

they break up into drops of a variety of sizes. There are several basic processes that are

associated with atomization including hydraulics of flow inside the atomizer and

turbulence properties of the emerging fuel stream [2]. The development of liquid jet or

sheet and the growth of disturbances which lead to the disintegration of the liquid sheet

into ligaments and then into drops are critical in determining the detailed characteristics

of the spray like breakup length and spray angle. These characteristics are dependent on

the internal geometry of the liquid sheet.

2.2 PHYSICS OF BREAKUP

The cause of atomization is the interaction between the internal pressure of the drops, the

aerodynamic force and the surface tension force acting on the liquid.

stAI p + p = p (2)

Where Ip is the internal pressure of the drop

Ap is the external aerodynamic pressure

And stp is the surface tension pressure

11

When the liquid comes out of the injector at a high speed, the aerodynamic force on the

liquid surface increases. If this can be balanced by an increase in surface tension force,

then the sheet is stable [3]. However when the effect of the aerodynamic force exceeds the

surface tension force, this sheet starts splitting up, in order to increase its overall surface

area until the surface area becomes large enough for the surface tension force to balance

the aerodynamic force.

The disintegration of the sheet begins in the form of a disturbance which propagates

through the fluid, atomizing it. If the disturbance frequency is equal to or greater than the

breakup frequency, atomization occurs [4]. If in an air assisted atomizer high velocity air

is made to impinge on the fuel sheet, it produces a large disturbance which ensures

immediate atomization.

Break-up of the liquid sheet occurs in four stages [8].

Fig 2.1 Stages of liquid sheet breakup

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1) The first stage of sheet breakup, attributed to the Rayleigh instability mode, is

caused by an axisymmetric disturbance related to the surface-tension force that

leads to the growth of the capillary wavelengths in the sheet. When the

wavelength grows larger than the sheet thickness, the fluid breaks off.

2) An increase in sheet velocity leads to the first wind-induced regime. In this

regime, frictional and pressure forces between the sheet surface and the

surrounding gas cause oscillations which produce a gross twisting effect on the

liquid column that later leads to the formation of droplets with sizes on the order

of the sheet orifice area. The distance from the orifice to the initial breakup point

is less than that produced by the Rayleigh mode.

3) A further increase in sheet velocity leads to the second wind-induced regime,

which forms a wide droplet distribution ranging from small droplets obtained

from surface breakup to large droplets approaching the size of the sheet thickness.

The distance to the onset of liquid breakup continues to decrease as sheet velocity

increases.

4) When the onset of liquid breakup begins to occur at the sheet exit, the atomization

regime is attained. The atomization regime consists of fine droplet sizes, most of

which are produced by surface breakup rather than sheet fracture.

13

Fig 2.2 Effect of Rayleigh and Ohnesorge number on atomization

The disintegration of a liquid sheet in a coaxial airstream occurs in a slightly different

way as described below [5].

1) Rayleigh Breakup: In this type of breakup, the liquid sheet disintegration does not

involve liquid membrane or liquid fiber ligament formation. This occurs for the

condition Wea < 25.

2) Sheet disintegration via membrane type ligaments: This occurs at higher air flow

rates where the sheet develops into a thin liquid sheet which then forms Kevin-

Helmholtz waves and breaks up into drops. The diameter of the drops is less than

the sheet thickness. For this 25 < Wea < 70

14

3) Sheet disintegration via fiber type ligaments. At very high air flow rates, liquid

fibers are formed, they peel off the sheet and breakup via Rayleigh mechanism.

The liquid core accelerates and develops waves. The ligaments formed by

Rayleigh breakup then break into smaller drops or ligaments. The drop diameter

is of a magnitude much smaller than the sheet thickness. 70< Wea <500.

Fig 2.3 Mechanism of breakup of liquid sheet in co-axial airstream

All the three mentioned categories can be sub divided into two sub modes:

Pulsating sheet disruption which is the normal sub- mode of atomization and Super-

pulsating sheet disruption which occurs due to extremely high periodic change between

high and low density regions in the spray [9].

2.3 CHARACTERISTIC PARAMETERS OF LIQUID BREAKUP

The spray angle and the break up length are the two main characteristics of the spray that

determine the quality of atomization.

15

2.3.1 Spray Angle

The spray angle is defined as the angle between the locus of points of wave crests along

the stream wise locations and the centerline of the injector. An increase in spray angle

increases the exposure of the droplets to the surrounding fluid leading to improved

atomization.

2.3.2 Breakup length

The breakup length is a measure of the axial distance from the nozzle outlet plane to the

point where the central part of the sheet ceases to exist as a cohesive entity. A decrease in

break-up length of the liquid sheet or jet will lead to fine spray in a short span of the

combustor.

Fig 2.4 Break up length and spray angle from an air assist atomiser

16

2.4 RELEVANT DIMENSIONLESS NUMBERS

Dimensionless numbers play a major role in the experimental study of twin fluid

atomization. The use of dimensionless numbers likes Weber number and Weber ratio

allows us to easily understand the effect of the liquid and atomizer characteristics on the

break-up mechanism [10].

In low velocity liquids the two parameters that affect the breakup are the surface tension

and aerodynamic force. Thus Weber number which is the ratio of dynamic pressure

(ρAUR2/2) and surface tension force (σ/ d0) is used for this purpose [6].

We =

dva

2

(3)

Where

We - Weber number associated with twin fluid flow in the atomizer

a - Density of air

v - Relative velocity between air and liquid

ts - Sheet thickness

- Surface tension between liquid-air interface

The Weber number describes the likelihood of the liquid to break up, the higher the

Weber number, faster the breakup [7]. Here in order to include the effect of inertial forces

of water a new dimensionless no called weber ratio (Wel/Wea) is used

(4)

Where Wea - Weber number of air

WeL - Weber number of liquid

These dimensionless numbers are used to map the breakup regimes of the liquid sheet.

17

The explanation of weber no and ratios are as follows .The weber ratio is chosen because

aerodynamic weber number alone does not take into account the inertial forces of water

and also the surface tension factor was removed when the ratio was chosen. The weber

ratios are calculated by the following calculation.

Wea =

dva

2

(13)

Where

Wea - Weber number of the air flow

a - Density of air

v - Relative velocity between air and liquid

D - Diameter of jet

- Surface tension between liquid-air interfaces

Wel =

dvll

2

(14)

Where

Wel - Weber number of the air flow

l - Density of the liquid

lv - Velocity of the liquid

ts - Sheet thickness

- Surface tension between liquid-air interfaces

18

3. EXPERIMENTAL SETUP AND MEASUREMENT

TECHNIQUES

The experimental setup of the project comprises of 4 sections: test section, air supply

system, fuel supply system, image capturing and processing section

Fig 3.1 Schematic of the Experimental Setup

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3.1 EXPERIMENTAL SETUP

The experimental setup includes twin-fluid atomizer, bucket and atomizer holder. The

spray characterization experiments are performed in this test section. The atomizer is

placed in such a way that the spray flows in a vertical fashion into a bucket to collect the

water.

3.1.1 Twin Fluid Atomizer

The twin- fluid atomizer is specially designed and fabricated to provide required liquid

spray at various liquid and air injection pressures. The twin fluid atomizer is made up of

3 parts, air nozzle duct, and packing and liquid sheet generator.

3.1.1.1 Air Nozzle Duct

The air nozzle duct is entirely made of aluminum. It was designed as per ASME

standards and manufactured using CNC machining. The duct is manufactured to contain

and house the liquid sheet generator. It provides the pressurized air parallel to the flow of

water.

20

Fig 3.2 Air nozzle duct

3.1.1.2 Liquid Sheet generator

The liquid sheet generator is made up of cast iron . The liquid sheet generator is used to

generate the liquid sheet. It is designed to produce a sheet of thickness 0.3mm. The liquid

sheet generator resembles the fuel nozzle used in aircraft atomizers. The fuel is

substituted here with water

21

Fig 3.3 Liquid sheet generator

3.1.1.3 Packing

The packing is the outer covering of the atomizer. Its primary function is to isolate the

water and air columns from each other. It also prevents leakage from atomizer to the

environment. We use a combination of acrylic sheets and rubber sheets for this purpose.

3.2 AIR SUPPLY SYSTEM

This airline system consists of a compressor, pressure gauge, pneumatic distributor, pipes

and joints.

All Dimensions are in mm

22

A reciprocating air compressor is used to compress the air to the desired pressures. This

consists of a motor which powers it, and a reservoir to store the compressed air.

Table 3.1 Compressor specification

MAKE COMPREZZAIR

POWER 2 hp

MAX WORKING PRESSURE 8.5 BAR

NORMAL WORKING PRESSURE 6 BAR

CAPACITY 50 L

The air line from the compressor is connected to a pneumatic distributor. The pneumatic

distributor consists of 2 inlets and 4 outlets. The distributor helps in distribution of air

through 4 outlets from one inlet. The second inlet is connected to a pressure gauge. The

pneumatic distributor is clamped onto the stand. Four pneumatic lines are connected to

outlets of distributor at one end and onto the 4 air inlets of the nozzle which helps in

circumferential flow of air in the nozzle.

The pressure of the line air is controlled using the valve at the compressor itself. The

pressure of the air in the line is determined using the bourdon pressure gauge at the

distributor.

3.3 FUEL SUPPLY SYSTEM

The fuel line has a 50 litre capacity tank which is fitted with a cutoff outlet near the top.

The tank is constantly fed water from a tap. The combination of a contant water source

23

and a cutoff outlet helps ensure a constant head in the tank. A 0.25 hp Cintu centrifugal

pump is used to pump water from the main tank to the liquid line.

Table 3.2 Main pump specification

MAKE Cintu AC Monoblock Pump

HEAD 9m

LPH 1400

POWER RATING 0.06KW

CURRENT RATING 0.6 A

VOLTAGE RATING 230 V AC

The tank is fitted with a valve to toggle the flow of water. The line pressure is measured

using a pressure transducer. The water is fed into the transducer using a T-joint. The line

pressure is controlled is a ball control valve. The water from the valve is then directly fed

into the liquid sheet generator.

3.3.1 ABB Hart Pressure Transmitter

An ABB pressure transmitter shows the liquid line pressure. The transmitter is connected

to the power source using an AC to DC adapter. A resistor of 1 ohm is connected in

series with the device. Measuring the voltage across the resistor allows us to calculate the

24

current in the circuit. The transmitter is calibrated for the desired pressure range so that

the voltmeter shows 4mV for the lowest pressure (LRV) calibrated and 20mV for the

highest pressure (HRV) calibrated. Thus the voltage can be read off the multimeter for

the various valve openings and the corresponding pressures can be obtained by simple

interpolation.

3.3.1.1 Calibration Procedure

To calibrate the transmitter, the pump is switched on and the exit valve is held in the fully

open condition thus giving the lowest attainable pressure in the setup. The zero button,

present on top of the device is pressed thus setting the Lower Range Value at this

pressure. Then the valve is fully closed thus giving the maximum line pressure. Then the

span button is pressed for two minutes and thus the Higher Range Value is also set. Now

the pressures in that range can be easily measured.

3.4 IMAGE ACQUISTION AND PROCESSING

This comprises of 4 parts: - a diffuser plate, a spotlight, a camera and a computer.

The diffuser plate is a milky white screen which is used to provide proper background

illumination and proper contrast during image capturing. This ensures uniform

distribution of light throughout the image. A 1000W spotlight is used for providing

sufficient background lighting.

The camera used is a high speed Sony CCD camera (XCD-X710) with a maximum

acquisition rate of 30 frames per second. The spatial resolution of the camera is 1024 *

768 pixels. The shutter speed chosen is 1/100000 seconds. A standard C- mount lens of

focal length 25 mm was used throughout this study. We use this to capture the spray

25

images. Focus and aperture adjustments are done on the camera to capture high quality

images.

The camera is connected to the computer which helps us to perform an inline testing of

the experiment. All adjustments of the camera except focusing and aperture adjustment is

done using the software IC Capture. This helps us to control the entire functions of the

camera using the software and also help us to store the acquired images in the computer.

The captured images are processed using Image Processing Toolbox in Matlab.

3.4.1 Image Capturing Method

After the working pressure ranges are obtained for both the liquid and air lines, the next

step is the capturing of the images at different pressures. The working liquid and air

pressures and their corresponding velocities are stated below.

Table 3.3 Liquid and Air Working Pressures

Sl.no

LIQUID LINE AIR LINE

PRESSURE(bar) Velocity(m/s) PRESSURE(bar) Velocity(m/s)

1 0.8 2.98 0.3 12.39

2 0.81 2.5 0.4 16.61

3 0.82 1.8 0.6 23.81

4 0.8 27.68

5 0.9 30.32

The images are captured for the above stated liquid and air pressures.

The CCD camera is set on a tripod stand and its position is adjusted so that it is facing the

nozzle. Then the camera is connected to the computer and the software IC capture is

26

opened. Next before the focus an aperture is adjusted, the lighting has to be appropriate.

The normal lighting in the room is switched off and the 1000W special purpose light is

switched on. This is placed parallel to the diffuser plate so that the lighting obtained at

the test section is uniform and of the required intensity. The only light in the room should

be from the background lighting source. Then the resolution, frame rate and shutter speed

are set using the software. Then the focus and aperture opening are manually set so that a

clean focused view of the test section is seen from the computer. The lighting during the

entire process should be the same as is to be used in testing.

First an image of the background is taken using the camera. This acts as a reference

image and when subtracted from the other images using appropriate software gives us a

clear view of the spray.

Then the air line is set for the first air velocity. The liquid then line velocities are then

varied and the images of the spray are obtained at different air velocities and these are

automatically stored in the designated folders. To capture the images, the start button is

clicked on the screen and then after a few seconds, the stop button is clicked. At a frame

rate of 30 frames per second 100 images of the spray at that condition are obtained. Then

the liquid line condition is changed to obtain the next velocity. After all the liquid

velocities are obtained at that air velocity, we move on to the next air velocity. In this

way images for the designated spray conditions are obtained. These are then subjected to

quantitative analysis.

The images that are captured with the help of high speed camera are processed by using

image processing techniques during quantitative analysis. The images are processed in

order to calculate break up length, a parameter of liquid sheet breakup, at different fuel

line pressures and airline pressures.

27

3.5 Calibration of Fuel Supply System

The nozzle geometry and the instantaneous flow rate of the fuel through the nozzle define

the spray properties of the fuel. Since there is no measuring system available that

measures the instantaneous flow rate through the nozzle directly, one usually uses

secondary information that defines this flow rate. Thus calibration is done for flow rate

versus pressure. The velocity can be calculated at different pressures using calibration

chart.

A volume flow rate measurement is used for the velocity calculation. The volume of

liquid collected in a certain time interval is accurately measured and the time for this is

also noted.

V = Volume of collection tank/ time taken for collection

V = volume flow rate of liquid collected

Liquid exit velocity (v) = V/cross sectional area of nozzle exit

3.5.3 Calibration Procedure

Calibration of water line is done by calculating the volume flow rate. Volume flow rate is

obtained by using an arduino circuit. There are two copper rods which are placed at a

difference of 400 ml in a beaker. The arduino is programmed in such a way that it gives

us the time taken for filling that 400 ml. Time starts when the arduino is switched on. The

time at which water comes in contact with the lower rod is called first interrupt and with

the upper rod, it is called second interrupt. The difference between these interrupts the

time taken for filling 400ml. This gives us the volume flow rate. From the density we can

calculate the velocity of water. Thus at a given pressure in the line the corresponding

velocity at the exit of the nozzle is obtained.

28

Fig.3.4. Arduino circuit for calibrating liquid line

29

0

0.5

1

1.5

2

2.5

3

3.5

0.795 0.8 0.805 0.81 0.815 0.82 0.825

Liq

uid

Ve

loci

ty U

l (m

/s)

Pressure P (bar)

Liquid Line Calibration

Fig 3.5 Liquid Calibration Curve

3.6. Calibration of air line

The air supply line is calibrated so that the air velocities at the corresponding line

pressures indicated on the pressure gauge are known thus making testing easier. This is

done using a static pressure tap near the converging section of the nozzle. For this a 1.5

mm diameter hole is drilled near the entrance of the converging section of the nozzle into

which a hypodermic needle is placed. This relays the static pressure at that point by

connecting the U-tube manometer (using water as the manometric fluid) to the needle

using a tube.The difference in the water level in the manometer is noted down and this

gives the difference between the pressure at the converging section of the nozzle and the

atmosphere. As the static pressure at the exit of the nozzle is atmospheric pressure, the

manometer gives the pressure difference between the converging section and the exit.

30

This along with the continuity and Bernoulli’s equation give the inlet and exit velocity.

The exit velocity is plotted with the line pressure, which is measured using the bourdon

pressure gauge. Thus the velocities of the air during testing can now be obtained if the

line pressure is known. The same procedure can be repeated to get different air velocities.

Fig 3.6 Air Supply line Calibration Graph

3.7 Break-Up Length and Spray Angle Measurement

Image Processing Toolbox in MATLAB is used for this purpose. The image is read into

Matlab using the command “imread”. It is then displayed using , imtool (variable

name)”.The distance tool is used to first measure a reference surface whose dimension is

initially known. Then the spray break up length is measured by getting the distance in

pixels between the nozzle exit and the point where perturbation and breakup in the liquid

31

begins. The break up length is then converted into millimeters by using the length of the

reference surface.

The spray angle is similarly obtained by creating a right angle triangle where the

hypotenuse is a straight line from the outlet to the crest of the spray. The corresponding

opposite and adjacent sides are constructed in the image. Using this information the angle

can be measured by simple trigonometry.

32

4. RESULTS AND DISCUSSION

4.1 QUALITATIVE ANALYSIS

The images of the spray at the varied conditions specified are observed and visually

analyzed. It is clearly observed that the liquid sheet when not excited by air breaks up

after penetrating a great distance into ambient air. Only small perturbations are observed

because of the liquid’s inertia and the ambient air’s disturbance. Now as the velocity of

the air is progressively increased for constant liquid velocity, the perturbations in the

liquid sheet are increased because of the increase in the effect of aerodynamic inertia on

the liquid. Thus the break up length of the liquid was seen to increase.

Also at a particular air velocity, it was observed that the droplet size and the break up

length were the smallest and the cone angle over which the liquid is dispersed is the

largest at the smallest liquid velocity. As the liquid velocity increases, the drops become

larger and so does the break up length. Detailed quantitative analysis will reveal the

reasons for such behavior.

33

Fig 4.1 Spray images captured at liquid velocity: 1.7 m/s;

At air velocity: (a)16.81 m/s (b)23.81 m/s (c)27.68 m/s (d)30.32 m/s

(a) (b)

(c) (d)

34

(a) (b)

(c) (d)

Fig.4.3 Spray images captured at liquid velocity: 2.5m/s;

At air velocity: (a) 16.81 m/s (b) 23.81 m/s (c) 27.68 m/s (d) 30.32 m/s

35

(a) (b)

(c) (d)

Fig .4.1 Spray images captured at liquid velocity: 2.98 m/s;

At air velocity: (a) 16.81 m/s (b) 23.81 m/s (c) 27.68 m/s (d) 30.32 m/s

36

4.2 QUANTITATIVE ANALYSIS

Using MATLAB, the parameters for various conditions was determined. Images of all

liquid velocities corresponding to each air velocity were analyzed.

Fig 4.6. Non Dimensional Break up length and Weber Ratio

It is observed that at every air velocity the jet diameter is maximum for the lowest water

velocity. For each air velocity, as the water velocity increases the jet diameter decreases.

It was observed that the maximum value of jet diameter reaches 9mm which corresponds

to the outer wall diameter of the liquid nozzle.The sudden increase in each curve is

attributed to the change in the mechanism of breakup from Rayleigh break up to

aerodynamic forces. This stretching is caused because of the pressure drop between the

air and water exit. Therefore at high water velocity the phenomenon of diameter

stretching was found negligible. This jet diameter is non-dimensionalised and plotted

37

against the Weber ratio (Wel/Wea) in the following graph. In the following plot it is seen

that at very high liquid weber number the jet diameter assumes very low values. This

“stretching” effect occurs because of a pressure gradient that occurs at the partition

between air and water exit .This gradient is low at high Weber ratio and high at low

Weber ratio. As stated before the value never exceeds wall thickness value and never

goes below the diameter of the exit of the water nozzle. The plot between this stretching

factor and the Weber ratio is shown below. Stretching factor is the ratio between the

difference between the jet diameter and the nozzle diameter to the wall thickness of the

water nozzle. This factor is used to define how much the jet diameter expands.

Breakup length calculation:

Using the image processing technique the breakup length of the sheet is also obtained for

all images. The calculations used to get the mean breakup length, the standard deviation

and the error percentage are displayed below.

Mean breakup length l = n

li (9)

Where il is the breakup length of the ith image

n is the number of images

Sample Standard Deviation S =1

)(

n

lli (10)

Where il is the breakup length of the ith image


l is the mean breakup length

38

Percentage error = 100l

S (11)

Where S is the Sample Standard Deviation

l is Mean breakup length

The error bar graph between liquid velocity in the x-axis and the mean breakup length in

the y-axis was plotted. We have multiple curves in the graph where each curve

corresponds to a particular constant air velocity. The standard deviation for each point

has also been plotted on that graph as error bar. The maximum error percentage comes

under 20%. This is attributed to the fact that at high air velocities and low liquid

velocities the break up is a highly dynamic process and thus a variation in the break up

length is expected.

39

Table 4.1 Calculated breakup length at liquid velocity 2.98 m/s


S NO Air Pressure

(bar) Air velocity

(m/s) Mean Breakup Length (mm)

1 0.4 16.81 37.9987

2 0.6 23.81 27.9759

3

4

0.8

0.9

27.68

30.32

23.08981

21.22076

S NO Air Pressure

(bar) Air velocity


1 0.4 16.81 29.8273

2 0.6 23.81 25.68486

3

4

0.8

0.9

27.68

30.32

21.51499 18.86989

40


Fig 4.7 Breakup Length as a function of liquid velocity at various air velocities

S NO Air Pressure

(bar) Air velocity


1

2

0.3

0.4

12.39

16.81

43.52597

24.72531

3 0.6 23.81 20.80688

4

5

0.8

0.9

27.68

30.32

17.57058 16.15706

41

Spray Angle Calculation:

Using the image processing technique the spray angle of the sheet is also obtained for all

images. The calculations used to get the mean spray angle; the standard deviation and the

error percentage are displayed below.

Mean break up length l = n

li (9)

Where il is the spray angle of the ith image


Sample Standard Deviation S =1

)(

n

lli (10)

Where il is the spray angle of the ith image


l is the mean spray angle

Percentage error = 100l

S (11)

Where S is the Sample Standard Deviation

l is Mean spray angle

The error bar graph between liquid velocity in the x-axis and the mean spray angle in the

y-axis was plotted. We have multiple curves in the graph where each curve corresponds

to a particular constant air velocity. The standard deviation for each point has also been

plotted on that graph as error bar. The maximum error percentage comes under 20%. This

42

is attributed to the fact that at high air velocities and low liquid velocities the break up is

a highly dynamic process and thus a variation in the break up length is expected.

Table 4.6 Calculated spray angle at liquid velocity 1.8 m/s

S NO Air Pressure

(bar) Air velocity

(m/s) Mean Spray

angle (degrees)

1

0.4

16.81

30.82324

2 0.6 23.81 33.74066

3 0.8 27.68 40.28074

4 0.9 30.32

41.53691


S NO Air Pressure

(bar) Air velocity

(m/s) Mean Spray

angle (degrees)

1

0.4

16.81

28.42549

2 0.6 23.81 32.02589

3 0.8 27.68 30.54898

4 0.9 30.32

32.63325

43


S NO Air Pressure

(bar) Air velocity

(m/s) Mean Spray

angle (degrees)

1

0.4

16.81

17.5102

2 0.6 23.81 26.83547

3 0.8 27.68 21.23606

4 0.9 30.32

24.24812

Fig 4.8 Spray Angle as a function of liquid velocity at various air velocities

44

5. CONCLUSION

The effect of liquid and air velocities on breakup length of the liquid sheet emerging from

a twin fluid atomizer is experimentally characterized. The parameters such as breakup

length and spray angle was thoroughly studied and its effect on atomization was

discussed this study mainly focuses on how the use of an air assisted atomizer can lead to

improved atomization of the fuel. The breakup and spray angle is observed at various

liquid and air velocities.

45

REFERENCES:

1. F.Durst., M. Ismailov., D. Trimis., 1996, “Measurement of instantaneous flow

operating injection systems”, Experiments in Fluids, Springer-Verlag

2. F. Durst., A. Melling., D. Trimis. and P. Volkholz.,1996, “Development of a flow

meter for instantaneous flow rate measurements of anaesthetic liquids”, Flow

Max Instrum., Vol. 7, No. 314, pp. 215-221

3. May. Y. Leong, Vincent. G. McDonell, and G. Scott. Samuelsen., “Mixing of an

Airblast-Atomized Fuel Spray Injected Into a Crossflow of Air”, NASA/CR—

2000-210467

4. Norman Chigier, William Bachalo, Rolf Reitz.,1994, “Spray Technology Short

Course notes”, Department of Mechanical Engineering Carnegie Mellon

University

5. Pipatpong Watanawanyoo, Sumpun Chaitep. and Hiroyuki Hirahara., 2009,

“Development of an Air Assisted Fuel Atomizer (Liquid Siphon Type) for a

Continuous Combustor”, American Journal of Applied Sciences, Science

Publications

6. R. Jeremy Kenny, Nathan B. Walker., May 2010, “Atomization of Shear Coaxial

Liquid Jets”, Applied Physics Research Vol. 2, No. 1

7. S H Jin, M Brear, H Watson, and S Brewster., June 2008, “An experimental study

of the spray from an air-assisted direct fuel injector, Proc. IMechE Vol. 222 Part

D: J. Automobile Engineering, pp 1-4

8. S. P. Lin, R. D. Reitz., 1998, “Drop and spray formation from a liquid jet”, Annu.

Rev. Fluid Mech. 30:85–105

9. V. Sivadas, M. V. Heitor, Rui Fernandes., February 2007, “A Functional

Correlation for the Primary Breakup Processes of Liquid Sheets Emerging From

Air-Assist Atomizers”, Journal of Fluids Engineering, , Vol. 129 / 193 ,pp. 1-2.

10. Zoltan Farago, Hasan Eroglu and Norman Chigier, October 1990, “Coaxial

atomizer liquid intact lengths”, Phys. Fluids A, Vol. 3, No. 2

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