smelt shattering

5/22/2018 Smelt Shattering

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Shattering Kraft Recovery Boiler Smelt by a Steam Jet

by

Anton Taranenko

A thesis submitted in conformity with the requirements

for the degree of Master of Applied Science

Department of Chemical Engineering and Applied ChemistryUniversity of Toronto

Copyright by Anton Taranenko 2013


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Abstract

Shattering Karft Recovery Boiler Smelt by a Steam Jet

Anton Taranenko, M.A.Sc. Thesis 2013

Department of Chemical Engineering and Applied Chemistry

University of Toronto

Kraft recovery boiler smelt is shattered into small droplets by an impinging steam jet to

prevent smelt-water explosions in the dissolving tank. Inadequate shattering increases the

likelihood of dissolving tank explosions. While industry has not dedicated much effort to

smelt shattering, the safety implications require smelt shattering to be studied in detail.

An experimental set-up was constructed to simulate the shattering operation using a

water-glycerine solution and air instead of smelt and steam respectively. The objective

was to examine how physical properties and flow characteristics affect shattering. It was

found that increasing shatter jet velocity greatly reduced droplet mean diameter.

Increasing the liquid flow rate greatly increased droplet size, as expected. Shattering was

not significantly affected by viscosity, unless a weak shatter jet was used on a highly

viscous fluid. Increasing the proximity of the shatter jet nozzle decreased droplet size.


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Acknowledgements

I would like to extend my utmost gratitude to Professor Honghi Tran for his guidance and

supervision. His motivation and encouragement throughout this project has been

invaluable.

I also extend my thanks to Professor Markus Bussmann for his enthusiasm and

helpfulness in my time as a graduate student. I am deeply appreciative of his technical

and non-technical advice.

I am also grateful to all my colleagues and friends at the Pulp and Paper Centre: Tasnuva,

Daniel, Liming, Wei, Eric, Fariba, Monica, Ivan, Babak, Ameya, Mesoumeh, Frida, and

Aino. I thank them for all the fun times and insightful conversations we had.

Special thanks go to Paul Jowlabar, Sue Mao, and Anna Ho. They have always been

more than happy to help me during my time at U of T.

Lastly, I would like to thank my parents. Without their love and support this thesis would

have been insurmountable. Thank you mom and dad!


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Table of Contents

Abstract.............................................................................................................................. ii

Acknowledgements .......................................................................................................... iii

Table of Contents ............................................................................................................. iv

List of Tables ................................................................................................................... vii

List of Figures................................................................................................................. viii

Chapter 1 ........................................................................................................................... 1

1 Introduction............................................................................................................... 1

1.1 Smelt Shattering ................................................................................................ 4

1.2 Smelt Shattering Practices ............................................................................... 8

1.3 Research Objectives........................................................................................ 13

Chapter 2 ......................................................................................................................... 14

2 Literature Review ................................................................................................... 14

2.1 Cross-flow Atomization .................................................................................. 14

2.2 Melt Atomization ............................................................................................ 17

Chapter 3 ......................................................................................................................... 20

3 Experimental Design and Methodology ................................................................ 20

3.1 Experimental Set-up ....................................................................................... 20

3.2 Experimental Methodology ............................................................................ 23

3.2.1 Image Processing..................................................................................... 25


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3.2.2 Droplet Size Data Processing ................................................................. 26

3.3 Air Jet Measurements..................................................................................... 29

3.4 Spout Flow Measurements ............................................................................. 31

Chapter 4 ......................................................................................................................... 36

4 Experimental Results and Discussion ................................................................... 36

4.1 Effect of Air Velocity ...................................................................................... 37

4.2 Effect of Liquid Flow Rate ............................................................................. 41

4.3 Effect of Viscosity............................................................................................ 47

4.4 Effect of Nozzle Proximity.............................................................................. 53

Chapter 5 ......................................................................................................................... 59

5 Practical Implications ............................................................................................. 59

5.1 Effect of Momentum Ratio............................................................................. 59

5.2 Implications ..................................................................................................... 61

Chapter 6 ......................................................................................................................... 62

6 Conclusions and Recommendations ...................................................................... 62

6.1 Conclusions ...................................................................................................... 62

6.2 Future Work Recommendations ................................................................... 63

References ........................................................................................................................ 65

Appendices....................................................................................................................... 67

Appendix A Calculating Smelt Flow Rate ................................................................. 68


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Appendix B Calculating the Weber Number for Liquid Shattering....................... 69

Appendix C Shattering Images ................................................................................... 70


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List of Tables

Table 1-1: Maximum black liquor firing capacities and the number of spouts, for

four recovery boilers [8, 9] ...................................................................... 4

Table 4-1: Range of parameters considered in this thesis ....................................... 36

Table A-1: Volumetric smelt flow rate .................................................................... 68


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List of Figures

Figure 1-1: Kraft recovery cycle [1] ........................................................................... 2

Figure 1-2: Recovery boiler [1, 7]: (a) air supply, (b) black liquor guns,

(c) dissolving tank.................................................................................... 3

Figure 1-3: Typical smelt shattering........................................................................... 5

Figure 1-4: Documented dissolving tank explosions in North America

since 1973 [12]......................................................................................... 6

Figure 1-5: Smelt spout designs: (a) water-cooled, (b) dry spout .............................. 9

Figure 1-6: Typical spout and jet placements........................................................... 10

Figure 1-7: A conical steam shatter jet that covers a large area ............................... 11

Figure 1-8: Shatter jet nozzles: (a) single hole design with a steam jet cleaner, (b)

single hole design with a shovel to prevent salt build-up, (c) multiple

hole design, (d, e) slit hole design.......................................................... 12

Figure 2-1: Cross-flow atomization.......................................................................... 15

Figure 2-2: Melt atomization [21] ............................................................................ 17

Figure 3-1: Water-glycerine shattering by an air jet................................................. 20

Figure 3-2: Full view of the experimental set-up ..................................................... 21

Figure 3-3: Schematic of the experimental set-up.................................................... 21

Figure 3-4: Liquid stream break-up captured by the high speed camera (front view)

................................................................................................................ 23

Figure 3-5: Liquid break-up captured by the high speed camera (low flow, side

view) ...................................................................................................... 24


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Figure 3-6: Liquid break-up captured by the high speed camera (high flow, side

view) ...................................................................................................... 24

Figure 3-7: Image processing: (a) original image, (b) binary form of the original

image, (c) droplet outlines ..................................................................... 25

Figure 3-8: Droplet diameter distributions obtained from 3, 5, and 10 images. Air

velocity = 100 m/s, shatter nozzle to liquid proximity = 7.5 cm, liquid

flow rate = 0.1 L/s, liquid viscosity = 2.5 cP......................................... 27

Figure 3-9: Volume distributions obtained from 3, 5, and 10 images. Air

velocity = 100 m/s, shatter nozzle to liquid proximity = 7.5 cm, liquid

flow rate = 0.1 L/s, liquid viscosity = 2.5 cP......................................... 28

Figure 3-10: Air velocity distribution (horizontal axis) 7.5 cm from the nozzle exit.

Air velocities at the nozzle exit are 100, 150, 200, 250, and 300 m/s ... 29

Figure 3-11: Air velocity distribution (horizontal axis) 15 cm from the nozzle exit.

Air velocities at the nozzle exit are 100, 150, 200, 250, and 300 m/s ... 30

Figure 3-12: A comparison of a 100 m/s jet at 7.5 cm and a 300 m/s jet at 15 cm .... 31

Figure 3-13: Front view of the spout. The diameter of the spout (d) is 3.75 cm........ 32

Figure 3-14: Water velocity at the spout exit versus spout flow rate, for four spout

inclinations............................................................................................. 33

Figure 3-15: Effect of spout inclination on exit velocity............................................ 34

Figure 3-16: Liquid stream velocity at the spout exit versus spout flow rate, for four

different liquid viscosities at 15 inclination ......................................... 35

Figure 4-1: Spray images at different air velocities. Ql= 0.1 L/s, l= 2.5 cP,

Nls= 7.5 cm............................................................................................ 37


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Figure 4-2: Droplet diameter distributions at uair= 100, 150, 200, 250, and 300 m/s.

Ql= 0.1 L/s, l= 2.5 cP, Nls= 7.5 cm.................................................... 39

Figure 4-3: Volume distribution at uair= 100, 150, 200, 250, and 300 m/s.

Ql= 0.1 L/s, l= 2.5 cP, Nls= 7.5 cm................................................... 39

Figure 4-4: Effect of air velocity on SMD. Ql= 0.1 L/s, l= 2.5 cP, Nls= 7.5 cm.. 41

Figure 4-5: Shattering images at different liquid flow rates. l= 2.5 cP, Nls= 7.5 cm

................................................................................................................ 42

Figure 4-6: Effect of liquid flow rate on droplet distribution at uair= 100 m/s,

l= 2.5 cP, and Nls= 7.5 cm.................................................................. 44

Figure 4-7: Effect of liquid flow rate on volume distribution at uair= 100 m/s,

l= 2.5 cP, and Nls= 7.5 cm.................................................................. 44


l= 2.5 cP, and Nls= 7.5 cm.................................................................. 46

Figure 4-9: Effect of liquid flow rate on SMD. l= 2.5 cP, Nls= 7.5 cm ................ 47

Figure 4-10: Shattering images at different liquid viscosities at uair= 100 m/s,

Ql= 0.1 L/s, and Nls= 7.5 cm................................................................ 48

Figure 4-11: Effect of viscosity on droplet distribution at uair= 100 m/s, Ql= 0.1 L/s,

and Nls= 7.5 cm..................................................................................... 49

Figure 4-12: Effect of viscosity on volume distribution at uair= 100 m/s, Ql= 0.1 L/s,

and Nls= 7.5 cm..................................................................................... 49


and Nls= 7.5 cm..................................................................................... 51


and Nls= 7.5 cm..................................................................................... 51


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Figure 4-15: Effect of viscosity on SMD. Ql= 0.1 L/s, Nls= 7.5 cm......................... 52

Figure 4-16: Shattering images at different nozzle distances. Ql= 0.1 L/s, l= 2.5 cP

................................................................................................................ 53

Figure 4-17: Effect of nozzle proximity on droplet distribution at uair= 100 m/s,

Ql= 0.1 L/s, and l= 2.5 cP................................................................... 54

Figure 4-18: Effect of nozzle proximity on volume distribution at uair= 100 m/s,

Ql= 0.1 L/s, and l= 2.5 cP................................................................... 54

Figure 4-19: Effect of nozzle proximity on SMD. Ql= 0.1 L/s, l= 2.5 cP .............. 55


................................................................................................................ 56

Figure 4-21: Effect of shatter jet span on droplet diameter. Ql= 0.1 L/s, l= 2.5 cP

................................................................................................................ 57

Figure 4-22: Effect of shatter jet span on volume distribution. Ql= 0.1 L/s, l= 2.5 cP

................................................................................................................ 57

Figure 5-1: Plot of liquid-to-gas momentum ratio with respect to normalized droplet

mean diameter........................................................................................ 60

Figure C-6-1: l= 1 cP, Ql= 0.1 L/s, Nls= 7.5 cm....................................................... 71

Figure C-6-2: l= 1 cP, Ql= 0.2 L/s, Nls= 7.5 cm....................................................... 72

Figure C-6-3: l= 1 cP, Ql= 0.1 L/s, Nls= 15 cm........................................................ 73

Figure C-6-4: l= 1 cP, Ql= 0.2 L/s, Nls= 15 cm........................................................ 74

Figure C-6-5: l= 2.5 cP, Ql= 0.1 L/s, Nls= 7.5 cm.................................................... 75

Figure C-6-6: l= 2.5 cP, Ql= 0.2 L/s, Nls= 7.5 cm.................................................... 76


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Figure C-6-7: l= 2.5 cP, Ql= 0.1 L/s, Nls= 15 cm..................................................... 77

Figure C-6-8: l= 2.5 cP, Ql= 0.2 L/s, Nls= 15 cm..................................................... 78

Figure C-6-9: l= 10 cP, Ql= 0.1 L/s, Nls= 7.5 cm..................................................... 79


Figure C-6-11: l= 10 cP, Ql= 0.1 L/s, Nls= 15 cm...................................................... 81







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Chapter 1

1 IntroductionThe kraft pulping process is the most widely used method to produce pulp from wood.

Since it produces high strength pulp and is able to process most wood species, the kraft

pulping process is advantageous to other pulping processes.

In the kraft pulping process, wood chips are converted into wood pulp with the use of

white liquor a mixture of sodium hydroxide (NaOH) and sodium sulfide (Na2S). NaOHand Na2S are known as the pulping chemicals. Wood pulp, in the form of cellulose fibers,

accounts for half of the processed wood, while the rest is dissolved with the spent pulping

chemicals to form a liquid mixture known as weak black liquor. Thus, black liquor is

composed of both organic and inorganic components.

The kraft recovery cycle (Figure 1-1) is a part of the pulping process. It recycles the

aforementioned pulping chemicals, generates steam and power from burning the organic

part of the black liquor, and disposes of dissolved wood substances [1].


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Figure 1-1: Kraft recovery cycle [1]

In kraft pulping, the pulp is separated from weak black liquor through washing, leaving

weak black liquor with 80-85% water content. This liquid stream is then sent through a

series of evaporators to reduce the amount of water to 15-25% [2]. The resultant liquid

stream, referred to as heavy black liquor, is injected into a recovery boiler via spray guns.

The recovery boiler (Figure 1-2) is the most expensive and important component of the

kraft recovery cycle [3]. It has two main functions, one of which is to produce power (via

steam generation) by burning the organic components of black liquor. [4]. As the black

liquor is injected into the recovery boiler via spray guns, some of the organic matter

burns in flight, while the rest of the black liquor falls onto a porous char bed, which burns

the remaining organic matter. The char bed also carries out the second function of the

recovery boiler by converting sodium along with sulfur into sodium carbonate (Na2CO3)

and Na2S the two major components of what is called smelt [4]. The resulting hot,

molten smelt percolates to the bottom of the bed, flows out of the boiler, is shattered into

droplets by an impinging steam shatter jet, and falls into a dissolving tank, where it

dissolves in water to form so-called green liquor. The green liquor is supplied to the

causticizing plant, where Na2CO3is reacted with lime (CaO) in the presence of water and


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converted to NaOH, while the Na2S remains unchanged [1, 5]. The solution of sodium

sulfide and the newly converted sodium hydroxide is white liquor. It is filtered from

calcium carbonate (CaCO3), precipitated from the caustisizing reaction, and is ready to be

reused in pulping. CaCO3is later calcined back into CaO [5, 6].

Figure 1-2: Recovery boiler [1, 7]: (a) air supply, (b) black liquor guns, (c) dissolving

tank

This thesis studies the atomization process of smelt shattering by an impinging steam jet.

The remainder of this chapter describes in detail the motivation for the smelt shattering

process, some of difficulties associated with it, various smelt shattering practices, and the

thesis objective.


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1.1 Smelt ShatteringSmelt is composed of 2/3 Na2CO3 and 1/3 Na2S with trace amounts of sodium sulfate

(Na2SO4), sodium chloride (NaCl), potassium carbonate (K2CO3), potassium sulfide

(K2S), potassium sulfate (K2SO4), and potassium chloride (KCl) [3]. The temperature of

molten smelt in the recovery boiler is around 800C. When smelt is hot and molten it has

a glowing bright red colour. One way to determine smelt temperature is to look at its

colour. As smelt forms in the recovery boiler, it percolates through the char bed and

collects at the bottom of the boiler, forming a pool. Smelt level is regulated by a series of

openings at the bottom of the boiler, as it continuously flows out of the recovery boiler

through these openings and down a series of spouts attached to the boiler outer wall.

Approximately 40% of the black liquor dry solids mass is converted into smelt [3]. Dry

solids (d.s.) refers to the solid content of black liquor. The rate at which black liquor

enters a boiler is defined for any boiler by the maximum firing capacity. Table 1-1 lists

firing capacities of several recovery boilers.

Table 1-1: Maximum black liquor firing capacities and the number of spouts, for four

recovery boilers [8, 9]

Recovery boiler Max black liquor (BL)

firing capacity (tons BL

d.s./day)

Number of spouts

A 1100 2

B 950 2

C 2050 5

C (resized) 2700 6

D 3500 8


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Based on these maximum black liquor capacities and the number of spouts, one can

estimate that the smelt flow rate is approximately 1 L/s per spout (Appendix A).

From the spout, the smelt falls into a dissolving tank filled with water that is kept at 80C

to 90C. If the smelt stream were allowed to fall directly into the dissolving tank, the

smelt-water interaction would quickly transform liquid water into vapour, which could

lead to a smelt-water explosion. This is a serious plant safety concern. To reduce the risk

of an explosion, and to improve smelt-water solubility, it is common practice to shatter

the smelt stream into droplets with a steam jet (Figure 1-3), placed near to the spout exit.

The atomization of the smelt stream into droplets reduces the likelihood of a smelt-water

explosion and thus promotes a safer dissolving tank operation [10].

Figure 1-3: Typical smelt shattering

Despite the use of shatter jets, serious smelt-water explosions in the dissolving tank

continue to be reported [11]. Figure 1-4 shows the number of documented dissolving

tank explosions in North America.


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Figure 1-4: Documented dissolving tank explosions in North America since 1973 [12]

Exactly why a dissolving tank explodes is not known. However, one possible cause for

explosions is shatter jet failure to sufficiently break up smelt streams exiting the recovery

boiler, allowing lumps of smelt to fall into the dissolving tank. Shatter jets can fail for a

number of reasons:

During the regular operation of a boiler, but more commonly during the start-upof a boiler after a shut-down for cleaning, the char bed will include large lumps of

char floating in molten smelt. Char lumps can plug spout openings. When they are

cleared the accumulated smelt can lead to a sudden flow surge, known as smelt

run-off. The exact flow rate of these surges is not known, but it may be too greatfor the shatter jets to adequately break up.

Sometimes a visibly slower smelt flow is detected on a spout. This may indicatethat smelt is cooler and its viscosity is significantly higher than the 3 to 4 cP that


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is a typical smelt viscosity [13]. The highly viscous flow, known as jelly roll

smelt, is much more difficult for shatter jets to break up.

Boiler start-up, or a partially plugged spout, can produce a diminished smelt flowthat drops straight down. The shatter jets, which are directed at the usual smelt

stream, may completely miss a reduced stream.

Smelt-water explosions may occur because of smelt build up on recovery boilerand dissolving tank walls. It is possible that some of the shattered smelt can

accumulate to form solid deposits on the walls. When these deposits grow large

enough, they break off and fall into the dissolving tank. The deposits are

sometimes referred to as cherry bombs.

Sometimes large lumps of smelt fall into the dissolving tank and splash greenliquor onto the shatter jet nozzles. Once the water evaporates, salt deposits will

form on the nozzles. If this happens on a continuous basis, the shatter jet nozzles

will plug. As a result, the shatter jet ability to fully break up smelt is greatly

reduced.

It should be noted that even individual smelt droplets explode when they fall into the

dissolving tank. Boiler operation continuously produces smelt droplets, and, in turn,

audible droplet explosions in the dissolving tank. Boiler operators can assess the

effectiveness of smelt shattering, and more generally the operation of the dissolving tank,

by the rumbling noise coming from the dissolving tank. In some cases the cumulative

energy released from multiple droplet explosions is enough to shake the recovery boiler

itself. The mechanism for these explosions is not yet fully known, but it has been noted

that if the shatter jet steam flow is increased, the noise from the dissolving tank is

reduced. This is most likely related to smelt droplet size. Intuitively, the use of moreshatter steam will reduce the number of larger droplets and unbroken smelt lumps. As a

result, the noise level will decrease.

Even though smelt shattering and dissolving tank operation are important aspects of

recovery boiler operation, with serious safety implications, the industry has not dedicated


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much effort to optimizing shattering operations. No research on smelt shattering has been

done previously.

1.2 Smelt Shattering PracticesSmelt spout design, shatter jet nozzle design and placement, and shatter jet steam

consumption vary from plant to plant. There appears to be little agreement on a best

practice for smelt shattering. It should be noted that much of the information presented in

this section is based on a number of mill visits by Professor Honghi Tran at the

University of Toronto.

The most commonly used spouts are U-shaped (Figure 1-5), although at some mills the

spouts are V-shaped. Typical U-shaped spouts are just under 1 meter in length and about15 cm wide at the top of the U. Most spouts are water-cooled in order to protect the

spout from smelt corrosion and to extend spout life. Spouts that are not water-cooled are

referred to as dry spouts. Spout inclination can vary between 15 and 45, although

most spouts are installed at the lower inclination. The choice of the spout angle affects

smelt shattering. At low inclinations the spout exit is further from the recovery boiler

wall, meaning less shattered smelt will come in contact with the recovery boiler wall.

However, at low spout inclinations and low flow rates the exiting smelt stream falls

straight down as it leaves the spout exit. In cases where shatter jets point straight down

and cannot be pivoted, smelt will not be sufficiently atomized since the shatter jets are

unable to reach the stream. The trajectory of the exiting smelt stream will vary less if the

spout inclination is steep, and so there is less chance that a shatter jet will miss the smelt

stream. The drawback is that more smelt will build up on the recovery boiler wall.


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Figure 1-5: Smelt spout designs: (a) water-cooled, (b) dry spout

The placement of shatter jets varies from mill to mill. Jets can be directed downwards and

placed about 30 cm (1 ft) from the spout lip, so that the vertical distance from the nozzle

to the smelt stream is about 60 cm (2 ft), as shown in Figure 1-6a. Shatter jets may also

be tilted toward the recovery boiler (b) to better accommodate changes in smelt flow

conditions. Most of the time only one shatter jet per spout is utilized, although sometimes

two jets are used (c). The second jet is sometimes only turned on when there is a reduced

smelt flow or a smelt flow surge, when the main shatter jet could fail to sufficiently break

up smelt on its own. The placement and direction of the second jet can vary. Usually it is


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closer to the spout and is sometimes pivoted such that it intersects the exiting smelt

stream (d). In some cases the second jet is directed away from the recovery boiler ( e) in

order to avoid smelt build up on the recovery boiler and dissolving tank walls.

Figure 1-6:Typical spout and jet placements


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Most pulp mills make use of superheated steam for smelt shattering. Steam pressure can

range anywhere from 3.5 bar (50 psi) to 15.5 bar (225 psi), while temperature is between

150C to 250C [14]. The amount of steam used per spout varies from mill to mill. In

fact, most mills only know the total shatter jet steam consumption. Some mills use large

amounts of steam and create large conical shatter jets in order to cover a larger area and

thus compensate for any smelt flow surges or low flows (Figure 1-7). However, the size

of the conical jet is much larger than the size of the smelt stream during normal smelt

flow. This can lead to significant steam wastage.

Figure 1-7: A conical steam shatter jet that covers a large area

Shatter jet steam consumption per nozzle appears to vary widely. Quoted values range

from 180 kg/hour (400 lbs/hour) to 2250 kg/hour (5000 lbs/hour) per nozzle. This is one

order of magnitude difference. This corresponds to a ratio of steam consumption to smelt

production of 0.05 to 0.625 kg of steam per kg of smelt.


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Shatter nozzle designs and sizes also vary greatly. Furthermore, new jets (and new

designs) are occasionally installed during a recovery boiler lifetime, especially if

shattering problems are encountered. Examples of a few shatter jet nozzle designs used in

practice are shown in Figure 1-8. It seems that most pulp mills simply use pipes that

carry steam and modify the pipe outlet to create a nozzle. A shatter nozzle can be circular

(Figure 1-8 a, b, and c) or slit-shaped (d). Some mills use multiple circular holes (c).

Some shatter nozzles have been modified to prevent smelt build up on nozzles. One pulp

mill has an attached shovel-shaped barrier (b), while another directs a small flow of

steam at a shatter nozzle (a) to clean the nozzle.

Figure 1-8: Shatter jet nozzles:(a) single hole design with a steam jet cleaner, (b) single

hole design with a shovel to prevent salt build-up, (c) multiple hole design, (d, e) slit hole

design


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1.3 Research ObjectivesLittle work has been done to study smelt shattering. However the safety implications

dictate that it is absolutely vital that smelt shattering be studied in detail, to prevent

dangerous dissolving tank operation.

The objectives of this project were to:

Construct an experimental apparatus to study smelt shatteringand to use the apparatus to examine how:

Smelt flow rate Smelt viscosity Shatter steam flow rate Nozzle placement and jet shape

affect shattering effectiveness. This thesis documents the apparatus that was built and the

results of the study. It is hoped that these results can be used to guide the development of

safer steam shattering and dissolving tank operation practices.

The remainder of this thesis is divided into five chapters. Chapter 2 introduces some

atomization processes similar to smelt shattering. Chapter 3 presents the experimental

apparatus that was built and the methodology of data acquisition. Chapter 4 presents

experimental results, while Chapter 5 discusses some of their practical application.

Chapter 6concludes the thesis and presents recommendations for future work.


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Chapter 2

2 Literature ReviewAtomization is the break-up of a bulk liquid into numerous smaller particles, usually very

small droplets on the order of micrometers to millimeters in diameter. The break-up

greatly increases the surface area of the liquid [15]. Atomization is an important process

in many applications, including spray combustion in furnaces and engines, crop spraying,

water treatment, spray painting, and powdered metallurgy [16].

Depending on the application, different techniques can be used to break-up a bulk liquid

into droplets. Atomization can be achieved with an atomizer, a device that discharges a

liquid stream through a nozzle, thus creating a liquid jet that quickly disintegrates into

droplets. Bulk liquids can also be reduced to droplets with the use of liquid-liquid jet

impaction, liquid-solid interaction, and gas-liquid interaction. Lastly, atomization can be

achieved with acoustic atomizers, ultrasonic atomizers, and electrostatic

charging [17, 18].

The process, known as smelt shattering, is an example of both cross-flow atomization and

melt atomization. Cross-flow atomization is the process of injection and disintegration of

liquid streams (or jets) in gaseous cross-flows [19]. Melt atomization uses various means

to break up a molten material. One melt atomization method uses a high velocity gas jet

to impact and break-up the metal liquid this is referred to as gas atomization [20]. In

what follows overviews of cross-flow and melt atomization are presented.

2.1 Cross-flow AtomizationThe atomization of liquid jets in subsonic cross-flows has been studied extensively. In

cross-flow atomization a liquid stream is generated and injected normal to an oncoming

flow of gas (Figure 2-1) [19]. The momentum of the moving gas impacts the liquid

column, bends it, shears droplets from the column, and eventually disintegrates the


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column completely into droplets. The gas flow is usually enclosed in a duct so it is

uniform across the cross-section area of the duct, and is constant throughout its length.

Figure 2-1: Cross-flow atomization

The application of cross-flow atomization ranges from Venturi scrubbers to fuel injection

systems and aircraft engines [19, 21]. Venturi scrubbers are used to control particle

emissions. Scrubbers atomize a liquid medium (usually water) into droplets, which can

trap contaminants found in a gas flow [21]. Fuel injector systems use this kind of

injection to improve fuel atomization and vaporization characteristics.

Liquid jet trajectory and droplet size depend on fluid and geometric variables. This

dependence is usually expressed in terms of non-dimensional parameters. Using the

Buckingham theorem and applying some mathematical simplifications, a function (C)

for liquid jet trajectory and droplet size in cross-flow atomization can be found [19]:

C=f(q, Weg,Rel) 2-1

where qis the liquid-to-gas momentum ratio, Wegis the Weber number, which is the ratio

of gas inertia to the liquid-gas surface tension force, and Rel is the Reynolds number,


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which is the ratio of inertial to viscous forces. Momentum ratio can be expressed in the

following manner:

2

2

gg

ll

u

u

q

= 2-2

where land g are liquid and gas densities, and uland ug are liquid and gas velocities

respectively. The Weber number can be expressed as:

gl

lgg

g

DuWe

/

2

= 2-3

whereDlis the column (or nozzle) diameter and l/gis the surface tension of the atomized

liquid. Lastly, the Reynolds number can be expressed as follows:

l

lll

l

Du

=Re 2-4

where lis the viscosity of the atomized liquid.

Most research has focused on the liquid jet trajectory and deformation as it is injected

into a cross-flow. Numerous models exist to describe the penetration of the liquid jet into

an oncoming gas stream [19]. For the purposes of this thesis, the break-up process of the

liquid jet is of primary interest. The break-up processes in cross-flow atomization are

divided into three categories primary break-up, column break-up, and secondary break-

up. Primary break-up involves the stripping of ligaments and drops from the main liquid

jet column. Column break-up occurs when a jet disintegrates into a series of droplets.

Secondary break-up refers to the reduction of large droplets formed during column break-

up, into smaller particles. According to the literature [19], if We is on the order of 100,

90% of the liquid column mass is taken away due to stripping. This mass loss coupled

with the liquid column instabilities initiated by gas flow leads to column break-up [19].

Intuitively, higher Weber numbers should instigate column break-up faster.


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It is somewhat difficult to calculate Wefor the shattering process due to the non-uniform

nature of the shatter jet. However, We can reach values on the order of 1000 at the

centerline of the shatter jet (Appendix B). This ensures that liquid column break-up is at

the point of gas-liquid impingement. There is no distinct liquid stream beyond that point

and the liquid column disintegrates into a spray almost instantaneously.

2.2 Melt AtomizationMelt atomization is widely used to produce metal and alloy powders, and to facilitate

metal spray deposition. Metals such as Al, Cu, Fe, and alloys such as steel are melted,

forced through an orifice forming a liquid stream, and subsequently atomized by a high-

velocity liquid or gas jet (Figure 2-2). The end result, after heat transfer, is a solid

powder or surface coating [20, 22].

Figure 2-2: Melt atomization [21]

Melt atomization has many uses in industry. Melt powders can be used in solid rocket

fuel, metallic paints, filters, and batteries. Some components in the aerospace industry are


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manufactured from metallic powders. Depending on the application, particle size can

vary. Atomization is a viable and yet challenging option for producing metal powders.

Due to its complexity, the atomization process is difficult to predict theoretically. Thus, it

is often difficult to achieve droplets within a certain size range. Research and

development is often needed when modifications to atomizer design, melt properties, and

melt flow rate are required [23].

Melt atomization is a two-fluid process, where a molten metal fluid is impacted by a

high-velocity secondary (or atomizing) fluid, a liquid or gas jet. The secondary fluid flow

is created and accelerated by a stream injector (if the fluid is a liquid) or generated by a

pressuring reservoir (if the fluid is a gas). The atomizing fluid exits via a nozzle as a jet,

which is focused on the atomized fluid to facilitate molten metal break-up. This results inthe production of droplets that have a much greater surface-to-volume ratio when

compared to the original molten metal stream. Part of the kinetic energy that is

transferred from the atomizing fluid to the molten metal stream overcomes viscous forces

that resist deformation; the other part overcomes the molten metal surface energy forces

that resist free surface creation. Thus, the kinetic energy that is transferred from the liquid

or gas jet to the molten metal is retained in the form of surface energy [20].

In general terms, melt atomization can be divided into three categories: water

atomization, oil atomization, and gas atomization. Momentum transfer is the main

mechanism for water and oil melt atomization. Due to the high heat transfer rate between

the atomizing liquid and melt, the morphology of melt powders is irregular in comparison

the gas atomized powders, which are more spherical in shape. Water and oil atomization

is most commonly used in instances where melt flow rate is high. Gas atomization is

dependent on the velocity of the impinging gas. The kinetic energy of an impacting gas

jet, which can be subsonic, supersonic, or ultrasonic, forces the melt stream to break up

into droplets. Particle size is governed by the gas-to-melt momentum ratio [20].

Atomization is controlled by both viscous and surface tension forces of the atomized

liquid. The relative importance of each of those forces can be represented in the

following manner [22]:


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U

E

El

= 2-5

where l is the viscosity of the atomized liquid, U (relative velocity) is the difference

between the velocity of the liquid and the velocity of the surrounding gas, and is the

surface tension of the liquid. For a typical relative velocity of 50 m/s, the ratio of viscous

to surface tension forces for molten metals such as steel, zinc, and magnesium would be

around 0.1 to 0.15. Therefore, the effect of viscosity on atomization would be negligible

compared to the effect of surface tension. However, this says little about the direct effect

of viscosity on droplet size, and only that surface tension is more dominant. Furthermore,

empirical evidence and models suggest that viscosity has an effect on droplet size [22]. A

widely used expression that predicts droplet mass median diameter (Dm,0.5) as a function

of liquid viscosity and other liquid and gas parameters is [24]:

2/1

1

5.0, )/1(

+=

gm

g

m

m mmWev

vKD 2-6

where Kis a constant between 40 and 50, is a length scale equivalent to the melt orifice

diameter, vm is the melt kinematic viscosity, vg is the gas kinematic viscosity, mm is the

melt mass flow rate, and mgis the gas mass flow rate. The increase in melt viscosity and

surface tension produces larger droplets. Some further effects on droplet size can be seen

in equation2-6. Increases in melt orifice diameter, velocity, and mass flow rate produce

larger drops. Gas velocity and mass flow rate increases lead to smaller droplets.


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Chapter 3

3 Experimental Design and MethodologyThis section presents the design of the experimental set-up and the methodology used to

run and record the shattering experiments. Shatter jet and spout flow velocities are

characterized as well.

3.1 Experimental Set-upA quarter scale experimental set-up was constructed to mimic a typical smelt shattering

operation. For reasons of safety, a water-glycerine mixture was used in place of smelt,

and air was used instead of steam. A simple illustration of the experimental set-up is

shown in Figure 3-1.

Figure 3-1: Water-glycerine shattering by an air jet


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The constructed experimental set-up is shown in detail in Figure 3-2.

Figure 3-2: Full view of the experimental set-up

The process flow schematic of the experimental set-up is presented in Figure 3-3.

Figure 3-3: Schematic of the experimental set-up


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A heavy duty polyethylene collection tank (75 x 100 x 75 cm) stores the water-glycerine

solution (bottom of Figure 3-2). The solution is pumped out by a cast iron 3/4 hp

centrifugal pump (ain Figure 3-3) with a semi-open impeller (3 diameter). The inlet of

the pump is 2, while the outlet is 1. The outlet of the pump is connected to a tee so that

a recycle line (1 diameter) can be used to divert some of the liquid back into the

collection tank. A gate valve (b) is installed on the recycle line to control the flow. The

second line (1/2 diameter) coming from the tee leads to the inclined tank. A simple

water filter (c) with a clear housing, a gate valve (d), and a water flow meter (e) are

installed on the way to the inclined tank. The flow meter measures water flow rates up to

40 L/min.

A smaller heavy duty polyethylene inclined tank (80 x 32 x 46 cm) is used to collect thewater-glycerine solution. A U-shaped poly(methyl methacrylate) spout (top of

Figure 3-2) is attached to the inclined tank (40 cm above the tank bottom). Spout length

and width are around 30 and 3.75 cm respectively. A rising pool of liquid is generated

once the water-glycerine solution is supplied to the inclined tank. When the water-

glycerine solution reaches the attached spout much like smelt does, it flows down the

spout and exits as a liquid stream back into the collection tank. The large size of the

inclined tank inhibits flow disturbances. As a result, the stream flowing down the spout is

steady.

The liquid stream is shattered and reduced to droplets by an impinging air jet. Air is

supplied by the building air supply station through a 1/2 pipe line. The flow rate is

controlled by a ball valve (f in Figure 3-3) and measured by a flow meter (h) with a

capacity of 15 to 150 scfm (calibrated at 70F and 100 psi). A pressure gauge (g) is

installed to monitor the pressure inside the air line, from 0 to 100 psi. The shatter jet is

generated with a Laval type nozzle (i) with an 11.9 mm outlet diameter and a 7.9 mm

throat diameter; the air flow is always subsonic. The position of the nozzle is adjustable,

and is maintained at 7.5 cm or 15 cm above the liquid stream, pointed straight down. The

horizontal distance between the nozzle and the spout exit is about 10 cm.


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3.2 Experimental MethodologyThe water-glycerine droplet formation is illuminated by a light source and captured by a

high speed camera. The light source is a 300W light bulb with a 5 cm diameter

multifaceted reflector. Translucent sheets are used to diffuse the light intensity. A digital

high speed camera (Mega Speed MS70K S2) fitted with a Sigma 28-300mm f/3.5-6.3

DG macro lens is used to capture droplet formation. The Mega Speed software package is

used with the digital camera.

Since droplets travel at high velocities, the lens exposure time is kept low around 10 to

25 s. The light shines directly into the camera to brighten the images and create dark

contours around the transparent droplets to make them visible (Figure 3-4).

Figure 3-4: Liquid stream break-up captured by the high speed camera (front view)

Although the water-glycerine break-up can be seen in the figure above, it is difficult to

detect single droplets from such an image. Therefore, the high speed camera is focused

on a small area of interest, about 3.5 by 3.5 cm, 50 cm below the point of impingement.

The camera is placed 1 m away from the image area (Figure 3-5).


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Figure 3-5:Liquid break-up captured by the high speed camera (low flow, side view)

The shattered plume of droplets tends to be conical and symmetrical at lower liquid flow

rates and high air velocities. However, larger droplets travel closer to the camera, while

smaller droplets are further away. Increasing the liquid flow rate shifts the plume closer

to the high speed camera (Figure 3-6).

Figure 3-6: Liquid break-up captured by the high speed camera (high flow, side view)


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Since the camera is always focused on the same area, the data is limited to the droplets

that appear in that plane. Thus, the captured images may not fully represent the droplet

size distribution of the break-up.

3.2.1 Image ProcessingOnce the images are obtained, ImageJ software is used to process them and extract

droplet diameter data. The processing of a sample image is illustrated in Figure 3-7:

Figure 3-7: Image processing: (a) original image, (b) binary form of the original image,

(c) droplet outlines


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As can be seen, the original image (a in Figure 3-7) is first converted into binary form

(b), and then droplet sizes can be calculated. It should be noted that only circular drops

are analyzed; large unbroken lumps are discounted, since volume cannot be computed

from such data. Due to the three dimensional nature of the water-glycerine breakup it is

somewhat difficult to predict how close a droplet is to the camera lens. The depth of field

is kept low (on the order of 2 cm) so that more accurate droplet size calculations can be

made. Droplets that are out of focus are not processed. Droplets that are processed are

outlined and numbered in Figure 3-7c.

3.2.2 Droplet Size Data ProcessingThe most common technique to evaluate atomization is to examine droplet diameter,

volume, and mass distributions. In addition, droplet mean diameters are calculated to

assess atomization. A common measure is the Sauter mean diameter (SMD or D32),

which is a ratio of drop volume to surface area [20, 25, 26]:

=

===

N

i

i

N

i

i

D

D

SMDD

1

2

1

3

32 3-1

whereDis droplet diameter andNis the number of droplets. The Sauter mean diameter is

particularly applicable to the shattering process because it characterizes the ratio of

inertial force to the aerodynamic drag [26].

For every shattering experiment, over 100 break-up images were captured, of which a

few were used to obtain distribution and average drop size results. Figure 3-8presents

droplet distribution plots obtained using data from 3, 5, and 10 images of a particular

experiment.


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()

(

%)

Figure 3-8: Droplet diameter distributions obtained from 3, 5, and 10 images. Air

velocity = 100 m/s, shatter nozzle to liquid proximity = 7.5 cm, liquid flow rate = 0.1 L/s,

liquid viscosity = 2.5 cP

These three droplet distribution curves are very similar, but these plots do not adequately

reflect the presence of large droplets (diameter greater than 1 mm), since the number of

small droplets greatly exceeds the number of big ones. A volume distribution curve

presents the liquid volume distribution as a function of droplet diameter, and better

illustrates the significance of bigger drops. The fluid volume distribution data, using 3, 5,

and 10 images, is presented in Figure 3-9:


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()

(

%)

Figure 3-9:Volume distributions obtained from 3, 5, and 10 images. Air

velocity = 100 m/s, shatter nozzle to liquid proximity = 7.5 cm, liquid flow rate = 0.1 L/s,

liquid viscosity = 2.5 cP

The plot suggests that processing 5 images yields data very similar to that obtained from

10 images, and is enough to yield accurate distribution results. Also, the Sauter mean

diameters based on 3, 5, and 10 images are 0.96, 1.24, and 1.3 mm respectively. Thus, 5

images were analyzed to produce all shattering plots presented in Chapter 4.

During experiments, the amount of total droplet volume detected in shattering images

gradually decreases as the air velocity increases, due to the fact that the shatter jet spreads

the plume of droplets outward, effectively out of the camera depth of field. This is thereason why the droplet data in the volume distribution plots is not presented in terms of

actual liquid volume.


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3.3 Air Jet MeasurementsThe gas flow that exits the shatter nozzle expands outward, and the centerline velocity

decreases with distance from the nozzle exit. A small experimental set-up was used to

measure air jet velocities away from the nozzle exit to assess jet strength at the point of

impingement. A pitot tube was attached to a monometer and used to measure air jet

dynamic pressures at the centerline of the jet. Jet velocities were measured at four

distances 7.5, 10, 12.5, 15 cm from the nozzle exit. After scaling up, these distances

are representative of how shatter nozzles are placed relative to smelt streams in recovery

boilers. The pitot tube was also shifted (perpendicular to the air flow) horizontally,

vertically, and diagonally to obtain dynamic pressures away from the centerline of the jet.

Figure 3-10presents air velocity along the horizontal axis, 7.5 cm from the nozzle exit.

()

(

/)

Figure 3-10: Air velocity distribution (horizontal axis) 7.5 cm from the nozzle exit. Air

velocities at the nozzle exit are 100, 150, 200, 250, and 300 m/s


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As the nozzle exit velocity increases from 100 to 300 m/s, the centerline velocity of the

jet 7.5 cm from the nozzle only rises by about 35 m/s (or 60%.) The maximum span of

the jet (or jet diameter) at that distance increases by 1.5 cm (around 30%). It is apparent

that the jet profile shifted to one side. This is most likely due to the slight misalignment

of the pitot tube and the nozzle exit.

Figure 3-11presents air velocity along the horizontal axis, 15 cm from the nozzle exit.

()

(/)

Figure 3-11: Air velocity distribution (horizontal axis) 15 cm from the nozzle exit. Air

velocities at the nozzle exit are 100, 150, 200, 250, and 300 m/s

As the air velocity at the nozzle exit increases from 100 to 300 m/s, the centerline

velocity of air rises by 20 m/s (or 75%). The diameter of the jet expands by 1.5 cm (about

25%). The centerline velocities are halved as the nozzle-to-pitot tube distance increases

from 7.5 to 15 cm, and the diameter of the jet increases by 25 to 30%. Once again, the

due to the slight misalignment of the pitot tube and the nozzle exit, the profile is shifted.


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Figure 3-12compares air velocity distribution for nozzle distances of 7.5 and 15 cm at

exit velocities of 100 m/s and 300 m/s, respectively. This plot is meant to compare the

diameters of two jets with similar centerline velocities. The effect of jet diameter on

shattering effectiveness will be discussed in Section 4.4.

()

(/)

Figure 3-12:A comparison of a 100 m/s jet at 7.5 cm and a 300 m/s jet at 15 cm

The two jets have similar centerline velocities of about 50 m/s, and these velocities

remain similar within a 5 mm radius of the centerline. However, the 300 m/s jet is

stronger outside of this region, and its diameter is about 60% greater than that of the

100 m/s jet.

3.4 Spout Flow MeasurementsTo understand the relationship between liquid flow rate down a spout and velocity, liquid

velocities were measured at the spout exit. Liquid stream velocities were also measured,

in order to estimate the momentum ratios presented in equation2-2.


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The experimental set-up (described in Figure 3-2) was run at different liquid flow rates

and viscosities to determine liquid levels (hin Figure 3-13) at the spout exit, in order to

calculate the cross-sectional area. A regular digital camera was focused on the spout exit

to capture the liquid heights. Water-glycerin flow rates were then divided by the cross-

sectional areas to determine the average liquid velocities.

Figure 3-13:Front view of the spout. The diameter of the spout (d) is 3.75 cm.

Figure 3-14illustrates the effect of water flow rate on velocity at the spout exit, for spout

inclinations of 0, 10, 15, and 20 degrees.


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(/)

(

/)

Figure 3-14: Water velocity at the spout exit versus spout flow rate, for four spout

inclinations

Average spout exit velocities for 0, 10, 15, and 20 inclinations are 0.45, 1.15, 1.3, and

1.4 m/s respectively. When the spout is horizontal, there is a small rise in velocity as the

liquid flow rate increases, of about 17% as the liquid flow rate is doubled (from 0.1 L/s to

0.2 L/s). Higher spout inclinations 10, 15, and 20 produce higher average

velocities due to gravity, as expected. However, there is no evident velocity rise or drop

when flow rate is varied at those inclinations. This is in part due to the difficulty of

making precise measurements. Higher spout inclinations produce more turbulent flow,

due to the increase in velocity, and much lower water levels (around 4 mm at low flows)

when compared to flow at 0 inclination. Water levels are around 8 mm at low flows

when the spout is horizontal. As a result, velocity measurements obtained for 10, 15,

and 20 have an error that is somewhat higher than that for 0.

The effect of inclination on average spout exit velocity is presented in Figure 3-15:


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()

(/)

Figure 3-15: Effect of spout inclination on exit velocity

As expected, greater spout inclination increases the average exit velocity due to gravity.

Spout exit velocity significantly increases when the spout inclination is changed from 0

to 10, while subsequent inclination increases lead to a steady velocity rise. This occurs

because the liquid in the spout does not accelerate when the spout is horizontal.

To examine the effect of viscosity on liquid flow, Figure 3-16, spout exit velocity data

was obtained for four different liquid viscosities at an inclination of 15.


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(/)

(

/)

Figure 3-16: Liquid stream velocity at the spout exit versus spout flow rate, for four

different liquid viscosities at 15inclination

Spout exit velocities are lower at higher viscosities because resistance to flow is greater.

Average velocities for 1, 2.5, 10, and 50 cP at 15 inclination are 1.3, 1.2, 1.05, and

0.8 m/s respectively. Increasing liquid flow rate has little effect on spout exit velocities at

1 and 2.5 cP. However, results obtained at 10 and 50 cP show a stronger dependence of

velocity on liquid flow rate. An increase of 30% in spout exit velocity was observed

when the flow rate was doubled (from 0.1 L/s to 0.2 L/s) at 50 cP. Once again, it was

difficult to make precise measurements for less viscous fluids due to turbulence and very

low fluid heights. As a result, the error is greater and no strong relationship between flow

rate and velocity could be determined.


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Chapter 4

4 Experimental Results and DiscussionA series of experiments were run to examine the effects of air velocity (uair), liquid flow

rate (Ql), liquid viscosity (l), and nozzle proximity to the liquid stream (Nls). The spout

inclination was kept at 15. The ranges of parameters are summarized in Table 4-1:

Table 4-1:Range of parameters considered in this thesis

Air velocity at the nozzle exit (m/s) 100, 150, 200, 250, 300

Liquid flow rate (L/s) 0.1, 0.2

Liquid viscosity (cP) 1, 2.5, 10, 50

Nozzle proximity to the liquid stream (cm) 7.5, 15

The lower liquid flow rate is representative of the smelt flow encountered under normal

recovery boiler conditions, while 0.2 L/s is more representative of heavy smelt flow. Of

the four liquid viscosities, 2.5 cP is most representative of recovery boiler smelt since its

viscosity is typically between 3 and 4 cP. The nozzle proximity to the liquid stream, if

scaled up, translates into 1 to 2 feet a typical distance between the shatter nozzle and

the smelt stream. Some of the collected shattering mages are shown in Appendix C.

Experiments were run for all combinations of the parameters. However, in this chapter, in

order to show the effect of each parameter (uair, Ql, l, or Nls) on shattering effectiveness,

others are kept constant. For example, the effect of air velocity on droplet formation is

presented at a liquid flow rate of 0.1 L/s, viscosity of 2.5 cP, and nozzle to stream

distance of 7.5 cm. The applicability of the experimental results presented in this chapter

to smelt shattering practice is discussed in Chapter 5.


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4.1 Effect of Air VelocityThis section considers the effect of air flow rate on droplet formation. The liquid flow

rate is 0.1 L/s, the liquid viscosity is 2.5 cP, and the nozzle proximity to the liquid stream

is 7.5 cm. Representative snapshots of liquid breakup are presented in Figure 4-1.

Figure 4-1: Spray images at different air velocities. Ql= 0.1 L/s, l= 2.5 cP, Nls= 7.5 cm


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As one would expect, the faster the air jet, the smaller the droplets. This is due to an

increase in aerodynamic drag applied to the liquid stream. The shatter jet exiting at

100 m/s does not have enough momentum to completely atomize the liquid stream into

droplets, since large irregularly shaped lumps of liquid appear in the images. Increasing

the velocity of the shatter jet by 50 m/s largely eliminates the lumps, producing only

circular droplets. Subsequent air velocity increases lead to more smaller droplets.

Figure 4-2 is a droplet distribution curve that compares the effect of air jet velocity on

droplet size. The droplets are sorted into 0.15 mm diameter bins. So, for example, the

percentage of droplets, with a diameter between 0 and 0.15 mm is presented over the

0.15 mm tick-mark in the figure. To view the same data differently, Figure 4-3compares

the liquid volume distribution versus air velocity.


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()

(%)

Figure 4-2: Droplet diameter distributions at uair= 100, 150, 200, 250, and 300 m/s.

Ql= 0.1 L/s, l= 2.5 cP, Nls= 7.5 cm

()

(%

)

Figure 4-3: Volume distribution at uair= 100, 150, 200, 250, and 300 m/s.

Ql= 0.1 L/s, l= 2.5 cP, Nls= 7.5 cm


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The droplet distribution curves show no significant effects of air flow on droplet

formation. In all cases, close to 50% of the droplets are between 0.15 and 0.3 mm in

diameter, while less than 5% are larger than 1 mm. Furthermore, the air velocity does not

affect the peak droplet size.

The volume distribution plot shows a more distinct effect of air velocity. As expected,

more liquid volume is in the form of smaller droplets as the air velocity increases. The

volume percent of very small droplets (0-1 mm in diameter) steadily rises from 20 to

65% as the air velocity increases from 100 to 300 m/s. The volume distribution curve

maximum shifts towards a lower range as the air velocity increases from 100 to 300 m/s.

More than half of the liquid volume is in droplets greater than 1 mm in diameter at all air

velocities except 300 m/s. However, the volume distribution curve maximum shifts to alower droplet size range (0-1 mm from 1-2 mm) as the air velocity increases from 200 to

250 m/s. At its maximum velocity, the shatter jet was effective enough to break up 60%

of the liquid stream into droplets less than 1 mm in diameter.

A significant amount of liquid (20%) is in droplets 3 to 4 mm in diameter when the air

velocity is only 100 m/s. This is due to the presence of one large droplet. Droplets of this

size were uncommon for this air velocity.

Figure 4-4presents droplet SMD versus air velocity.


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(/)

()

Figure 4-4: Effect of air velocity on SMD. Ql= 0.1 L/s, l= 2.5 cP, Nls= 7.5 cm

Mean droplet diameter decreases gradually as the air velocity increases. The mean

droplet diameter is reduced by about 50% when the air velocity is tripled.

4.2 Effect of Liquid Flow RateThis section presents the effect of liquid flow rate on droplet formation and size. All of

the data presented is for a liquid viscosity of 2.5 cP, while the nozzle proximity to the

liquid stream is 7.5 cm. Representative snapshots of liquid breakup at air velocities of

100 and 300 m/s are presented in Figure 4-5.


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Figure 4-5:Shattering images at different liquid flow rates. l= 2.5 cP, Nls= 7.5 cm

As expected, shattering improves as the liquid flow rate decreases. More large drops and

unbroken lumps of liquid form at a higher liquid flow rate. A number of large drops are

out of focus at 0.2 L/s and an air velocity of 100 m/s, which implies that the shatter jet isnot strong enough to break up large lumps of liquid. Instead, these lumps follow the

trajectory of the liquid stream.


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Figure 4-6presents droplet size distribution versus liquid flow rate at an air velocity of

100 m/s. Droplet volume distribution is plotted in Figure 4-7to provide insight into the

liquid flow rate effects on large drop formation.


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()

(%)

Figure 4-6: Effect of liquid flow rate on droplet distribution at uair= 100 m/s,

l= 2.5 cP, and Nls= 7.5 cm

()

(%

)




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No effect of liquid flow rate is seen in the droplet distribution plot. The volume

distribution plot indicates that a higher liquid flow rate is more difficult to shatter. This

confirms the visual inspection of images captured for 0.1 and 0.2 L/s flow rates at

100 m/s in Figure 4-5. At a flow rate of 0.1 L/s, more than 60% of liquid volume is in

droplets that are smaller than 2 mm in diameter. Conversely, only about 25% of liquid

volume is in droplets that are smaller than 2 mm in diameter, at a flow rate of 0.2 L/s. In

the analyzed images, there are 13 drops in the 2-3 mm diameter range for a liquid flow

rate of 0.2 L/s, compared to only 2 for the lower liquid flow rate. Furthermore, only one

drop larger that 3 mm was detected for a flow rate of 0.1 L/s, while there are 3 drops

greater than 3 mm for a higher liquid flow rate. The volume distribution curve maximum

shifts from the 1-2 mm range to the 2-3 mm range with an increase in liquid flow rate.

The effect of liquid flow rate was also examined at a higher air velocity. The droplet

distribution plot, which compared the effect of liquid flow rate at 300 m/s, was found to

be almost identical to the plot in Figure 4-6. No effect of liquid flow rate on shattering

was noticed.

As before, volume distributions were plotted to examine the presence of large droplets.

Figure 4-8presents the effects of liquid flow rate on liquid volume distribution.


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()

(

%)



The increased air velocity shifts the volume distribution towards the smaller droplet range

when compared to the distribution obtained at 100 m/s (Figure 4-7), as expected. But

doubling the liquid flow rate still leads to an increased formation of larger droplets. More

liquid volume (65%) is in droplets less than 1 mm in diameter at a liquid flow rate of

0.1 L/s, while only 20% of the total liquid volume is taken up by droplets of the same size

at 0.2 L/s. However, the shatter jet was still effective enough to completely reduce the

liquid stream into droplets less than 3 mm in size even when the liquid flow was doubled.

Figure 4-9 presents Sauter mean diameters with respect to air velocity at the twoaforementioned liquid flow rates:


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(/)

()

Figure 4-9: Effect of liquid flow rate on SMD. l= 2.5 cP, Nls= 7.5 cm

An increase in liquid flow rate from 0.1 to 0.2 L/s leads to a significant rise in droplet

mean diameter regardless of the air velocity. At 100 m/s the average droplet diameter

increases from 1.25 to 2 mm when the liquid flow rate is doubled, while at 300 m/s the

droplet diameter increases from 0.6 to 1.2 mm.

4.3 Effect of ViscosityThis section presents the effects of liquid viscosity on droplet formation. All of the data is

for a liquid flow rate of 0.1 L/s, while the nozzle proximity to the liquid stream is 7.5 cm.

Representative snapshots of liquid breakup are presented in Figure 4-10.


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Figure 4-10:Shattering images at different liquid viscosities at uair= 100 m/s,

Ql= 0.1 L/s, and Nls= 7.5 cm

Upon visual inspection, increasing the liquid viscosity from 1 to 50 cP leads to a small

effect on droplet size. In particular, there seems to be less fine drop formation when theliquid viscosity is 50 cP, as opposed to when the liquid viscosity is 1 cP.

Figure 4-11 compares shattering results for four liquid viscosities at an air velocity of

100 m/s. Figure 4-12presents volume distribution curves for the four liquid viscosities.


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()

(%)


and Nls= 7.5 cm

()

(%

)


and Nls= 7.5 cm


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Figure 4-11shows a distinct effect of liquid viscosity on droplet size distribution. While

the droplet distribution curve remains unchanged as the liquid viscosity is increased from

1 to 2.5 cP, there are fewer smaller droplets (0.15 to 0.3 mm in diameter) as the liquid

viscosity increases to 10 and 50 cP. A slight increase in the number of droplets larger

than 0.6 mm is also present at 10 and 50 cP. The change of viscosity does not affect

droplet distribution curve maximum.

The volume distribution plot does not show a distinct impact of liquid viscosity on

droplet breakup at 100 m/s. By comparison the effect of air velocity or liquid flow rate on

shattering is much more apparent. The liquid volume in droplets less than 2 mm in

diameter increases as the liquid viscosity is lowered from 50 to 2.5 cP. However, the

volume distribution curve for 1 cP does not fit that trend.

Figure 4-12indicates that the shatter jet has difficulty atomizing a highly viscous (50 cP)

liquid stream. There is a significant amount of liquid volume in droplets greater than

3 mm in diameter for viscosities of 1, 2.5, and 50 cP. While these droplets are uncommon

for lower viscosities and could be considered outliers, large droplets appear regularly

when a highly viscous (50 cP) liquid stream is shattered.

The effect of viscosity is also assessed at a higher air velocity. Figure 4-13 compares

droplet distributions attained for four liquid viscosities at uair= 300 m/s. Viscous effects

on liquid breakup were also examined with the use of volume distribution plots, and the

results are presented in Figure 4-14.


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()

(%)

Figure 4-13: Effect of viscosity on droplet distribution at uair= 300 m/s,Ql= 0.1 L/s,

and Nls= 7.5 cm

()

(%

)

Figure 4-14: Effect of viscosity on volume distribution at uair= 300 m/s,Ql= 0.1 L/s,

and Nls= 7.5 cm


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At a higher air velocity, a smaller effect of viscosity on droplet distribution is seen. There

are 15% fewer small droplets (less than 0.15 mm in diameter) and 5% more larger

droplets (0.6-0.9 mm in diameter) as the liquid viscosity increases from 1 to 50 cP.

The increase in liquid viscosity has a small effect on shattering at 300 m/s. More liquid

volume is in small droplets (


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Overall the effect of viscosity on Sauter mean diameter is not significant at air velocities

above 200 m/s. However, the effect of viscosity on average drop size is more apparent at

lower air velocities (100 and 150 m/s). This is most likely due to the ability of

aerodynamic drag (at uair > 200 m/s) to overcome forces that resist deformation. The

Sauter mean diameter for a highly viscous liquid (50 cP) is noticeably higher when

compared to the average droplet size obtained for less viscous liquids (1, 2.5, and 10 cP).

4.4 Effect of Nozzle ProximityThis section presents the effect of nozzle proximity to the liquid stream on droplet

formation. The effect of jet diameter on shattering is also examined. All of the data

presented is for a liquid flow rate of 0.1 L/s and liquid viscosity of 2.5 cP. Representative

snapshots of break-up are presented in Figure 4-16:

Figure 4-16:Shattering images at different nozzle distances. Ql= 0.1 L/s, l= 2.5 cP

Placing the nozzle further from the liquid stream results in poorer shattering, as expected.

The droplets are much larger, and some are out of focus, when the nozzle is further away.

Figure 4-17 shows droplet size distributions for nozzle to liquid proximities of 7.5 and

15 cm. Volume distribution curves are plotted in Figure 4-18.


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()

(%)

Figure 4-17: Effect of nozzle proximity on droplet distribution at uair= 100 m/s,

Ql= 0.1 L/s, and l= 2.5 cP

()

(%

)

Figure 4-18: Effect of nozzle proximity on volume distribution at uair= 100 m/s,

Ql= 0.1 L/s, and l= 2.5 cP


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Placing the shatter jet nozzle closer to the liquid stream increases the percentage of

smaller droplets (up to 0.6 mm in diameter) and decreases the number of droplets larger

than 0.6 mm in diameter. Close to 85% of droplets are smaller than 0.6 mm when the

nozzle is 7.5 cm from the liquid stream, while only about 50% of the droplets are smaller

than 0.6 mm when the nozzle is 15 cm away. The droplet curve maxima remain in the

0.15 to 0.3 mm range.

As expected, more liquid volume is in smaller droplets when the shatter jet nozzle is

closer to the liquid stream. More than 60% of the liquid volume is in droplets smaller

than 2 mm in diameter when the nozzle is 7.5 cm away from the liquid stream, while only

25% of the liquid volume is in that size range when the nozzle proximity is 15 cm.

Furthermore, around 40% of the volume is in very large droplets (>4 mm in diameter)when the nozzle proximity is 15 cm. Conversely, placing the nozzle 7.5 cm from the

liquid stream eliminates droplets of that size.

Figure 4-19presents droplet SMD versus nozzle proximity.

(/)

()

Figure 4-19: Effect of nozzle proximity on SMD. Ql= 0.1 L/s, l= 2.5 cP


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Sauter mean diameter curves for 7.5 and 15 cm exhibit similar decreasing trends as the

air velocity increases. Droplet mean diameter decreases as the shatter jet nozzle is

positioned closer to the liquid stream. At uair = 100 m/s, the Sauter mean diameter is

halved when the nozzle proximity is decreased from 15 to 7.5 cm. A similar decrease of

the mean diameter is evident at all other air velocities.

Rather than compare the effect of nozzle distance on droplet distribution at a higher air

velocity, a comparison between two shatter jets with similar centerline velocities was

made in order to examine the effect of shatter jet span. The velocity profiles of the two

jets are presented in Figure 3-12. The air velocities of the two jets are 48 and 54 m/s,

while the jet diameters are 8 and 5 cm respectively. The corresponding shatter images are

shown inFigure 4-20

.


Visual inspection shows that the increased span of the shatter jet does not have a clear

effect on shattering. The droplet distribution plot in Figure 4-21compares the effect of

the span of the jet on shattering. Figure 4-22presents the effect of jet span on volume

distribution.


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()

(%)

Figure 4-21: Effect of shatter jet span on droplet diameter. Ql= 0.1 L/s, l= 2.5 cP

()

(%)

Figure 4-22: Effect of shatter jet span on volume distribution. Ql= 0.1 L/s, l= 2.5 cP


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The shatter jet with the greater span but a slightly lower centerline velocity produces

fewer small droplets when compared to the shatter jet with the greater centerline velocity.

Around 60% of the droplets are less than 0.45 mm in diameter when the span of the air

jet is 8 cm, while close to 80% of droplets are in that range when the jet span is 5 cm. As

a result, larger droplets form in greater numbers when the centerline air velocity is

slightly decreased. Increasing the span of the shatter jet has little effect on droplet

distribution.

Finally, while 20% of the liquid volume for the 5 cm jet span is in droplets >3 mm in

diameter, that data represents only one droplet. If that droplet is considered to be an

outlier and removed, then the volume distributions and Sauter mean diameters produced

by the two shatter jets become very similar.


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Chapter 5

5 Practical ImplicationsVarious effects on shattering effectiveness were presented in the previous section. It is

difficult, however, to make them directly applicable to shattering practices at pulp mills

since the experimental set-up was a scaled down version of the shattering process.

Furthermore, smelt and steam were not used in any of the experiments. However, the use

of non-dimensional parameters and their correlations can help make the experimental

shattering data applicable to real shattering practice.

5.1 Effect of Momentum RatioSimilar to the relation described in (2-1), the Buckingham theorem was used to find the

dependency of droplet mean diameter (Ddrop) on non-dimensional parameters that

describe liquid shattering:

= lg

g

l

l

dropWe

DDqf

DD Re,,, 5-1

DlandDgare the hydraulic diameters of the shattered smelt or water-glycerine solution,

and of the impinging steam or air shatter jet, respectively. The subscript ldenotes liquid

smelt or water-glycerine solution, while g denotes steam or air. Expression (5-1) is

identical to (2-1) with the exception of the liquid-to-gas hydraulic diameter ratio, which is

unique to the liquid shattering process. Once again, the symbol q denotes the liquid-to-

gas momentum ratios:

2

2

gg

ll

u

uq

= 2-2


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Since only q and Dl/Dg can be controlled at a pulp mill, normalized droplet mean

diameter (Ddrop/Dl) should be plotted as a function of the two non-dimensional variables.

However, due to the lack of experimental data of liquid-to-gas diameters, only the effect

of liquid-to-gas momentum ratio can be illustrated.

Figure 5-1is a plot ofwater-glycerine to air momentum ratio with respect to normalized

droplet mean diameter:

/

Figure 5-1: Plot of liquid-to-gas momentum ratio with respect to normalized droplet

mean diameter

Two liquid flow rates 0.1 and 0.2 L/s are presented in Figure 5.1. As expected, the

droplet mean diameter decreases as the liquid-to-gas momentum ratio decreases. The

slope of both curves becomes steeper as the momentum ratio decreases. It is also

apparent that increasing liquid flow rate (and liquid stream diameter as a result) leads to a

slight increase of mean droplet size.


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If the air velocity and the actual diameter of the shattered smelt stream are known, the

mean diameter of the droplets can be approximated by multiplying the normalized droplet

diameter by the smelt stream diameter. One should also take into account the effect of

viscosity, surface tension, and nozzle design, but the plot in Figure 5-1is a good start for

connecting the experimental results to actual practice.

5.2 ImplicationsTwo practical implications may be drawn from this experimental study.

1. For low viscosity liquids, the droplet distribution is unaffected by air jet velocity.This implies that recovery boiler smelt can be easily shattered with a minimum

amount of steam. Therefore, it is beneficial to reduce the steam used for shattering.

2. Although the results showed that viscosity has only a small effect on shattering evenat the maximum water-glycerine viscosity of 50 cP used in this study, it does not

mean that smelt viscosity does not have an effect on shattering in practice. This is

because as the smelt starts to freeze, its viscosity increases drastically, well above

50 cP. This could make shattering more difficult.


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Chapter 6

6 Conclusions and RecommendationsThis section summarizes the findings of this study, and draws conclusions on the

practical implications. Future work suggestions are also presented.

6.1 ConclusionsSmelt shattering research is vital if the likelihood of serious dissolving tank explosions is

to be reduced. To date, the industry has not dedicated much effort to study and improve

smelt shattering. An experimental set-up and methodology were developed to study the

shattering process with the use of water-glycerine solutions instead of smelt and air in

place of steam. Shattering experiments examined the effect of air jet velocity, liquid flow

rate, liquid viscosity, and shatter nozzle placement on liquid droplet size. A summary of

the findings is presented below.

Effect of Air Velocity

While the droplet diameter distribution shows little effect of air velocity, the volume

distribution plot confirms that more liquid volume is in smaller droplets as the air

velocity increases, as expected. The Sauter mean diameter decreases linearly with

increasing air velocity. Liquid is completely disintegrated into droplets regardless of the

air velocity.

Effect of Liquid Flow Rate

Doubling the liquid flow rate from 0.1 to 0.2 L/s leads to an increase in the number of

large droplets. Unbroken lumps appear frequently at lower air velocities. The volume

distribution plots indicate that more liquid volume is in small droplets at a lower liquid

flow rate. The Sauter mean diameter increases as the liquid flow rate increases.


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Effect of Liquid Viscosity

The effect of viscosity on droplet size is not as clear as that of air velocity or liquid flow

rate. However, at a low air velocity, it is clear that the number of smaller droplets

decreases as the liquid viscosity increases. Droplet size is affected by liquid viscosity to a

much smaller extent at a greater air velocity. Sauter mean diameter is independent of

viscosity except when a highly viscous liquid is shattered with a weak air jet.

Effect of Nozzle Proximity

Placing the nozzle closer to the liquid stream increases the number of smaller droplets, as

expected. Sauter mean diameter decreases significantly as the nozzle proximity to the

liquid stream

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