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1. INTRODUCTION

In the applications of suspension atomization there are

many atomizers, which do not form jets of liquid, but rather

flat or conical sheets e.g. fan atomizers and hollow cone

pressure nozzles [1]. It was assumed that the solid particles

in the liquid influence the break-up mechanisms of the

sheet in the manner that the solid particles achieve the

perforated-sheet disintegration. The solid particles lead the

suspension sheet to form holes. These holes grow rapidly in

size until the rims of adjacent holes coalesce to produce

ligaments of irregular shape that finally break-up into drops.

The sizes of these drops are strongly influenced by the

diameter of the ligaments, which are also affected by the

sheet thickness. Several researchers have investigated the

perforation mechanism in film formation atomization. The

source of the perforations has been attributed to a variety of

cases such as suspended solid particles, liquid droplets, air

bubbles and/or turbulence [2]. Dombrowski and Fraser [3]

studied the break-up of water and alcohol sheets containing

3 to 60µm suspended solid particles. They found that if the

particles were wetted by the liquid they had no effect on the

manner of disintegration of the sheet. On the other hand,

when suspensions with unwettable particles are used, they

have a marked effect and cause perforation of the sheet.

Butler et al. [4] studied the disintegration of dilute emulsion

sheets. They found that the perforations did not appear at

the position where the particle diameter was equal to the

sheet thickness and they assumed that the emulsion

particles interact with the local perturbations within the

flow. These perturbations grow causing a hole in the sheet.

Glaser [5] studied the break-up of suspension sheets

containing different solid particles. He found that solid

particles with a small relative density affect the sheet

stability when the sheet thickness is thinner than the solid

particle size. The acceleration of the solid particles with a

large relative density achieves the instability and the

turbulence of the suspension sheet. Similar results were

presented by Shimizu et al. [6], who investigated the

influence of abrasive particles on the jet structure. Their

results showed that small abrasive particles tend to suppress

the disintegration of the jet, whereas large abrasive particles

tend to promote jet break-up. The effect of abrasive

particles on the jet structure is clear when using abrasive of

high density. Dahl [7] analysed the suspension flow inside a

swirl nozzle and he found that the cyclone-theory can be

used to calculate the friction losses and the velocities in a

swirl nozzle. Further more his measurements showed that in

the case of a lime suspension the side friction is comparable

with that for the pure liquid for low Reynolds numbers,

while it becomes a higher value in the case of higher

Reynolds numbers.

In the following the influence of the solid particle

loading on the break-up of the suspension sheet produced

by means of a hollow cone pressure nozzle is discussed in

some cases in comparison with results from a flat jet

atomizer.

Paper ID ICLASS06-143 Break-up of Hollow Cone and Flat Suspension Lamellae of Pressure Atomizers

Basel Mulhem, Ghaias Khoja, Udo Fritsching and Günther Schulte

University of Bremen, Chemical Engineering Department, gs@iwt.uni-bremen.de

ABSTRACT The effect of the solid particle size and particle loading on the break-up of the suspension sheet was

investigated through the visualization of the disintegration of model suspension sheets formed by means of a hollow cone and a

flat pressure spray nozzle. Various model suspensions based on water and mixtures of glycerol/water with different solid particle

sizes and particle loadings were atomized. From the experiments with the dilute suspension it was found that the small solid

particles dP = 6µm did not change the break-up mechanism of the liquid sheet, while the liquid sheets with larger solid particles

(dP = 56, 94, 228µm) were dominated by perforations in the film. The position of the perforation seems to be influenced by a

number of parameters such as ρL, ρP, ηL, σ, uL and dP. However, only suspensions with a high liquid viscosity have shown

perforations at the position where the particle diameter was equal to the sheet thickness δhole = d90,3. From the experiments with

the dense suspensions it was found that increasing of the solid particle concentration stabilises the suspension sheet.

Measurements on the suspension sheets have shown that solid particles influence the break-up of the hollow cone and the flat

sheets in a similar way. The interaction between the solid particles and the disturbances in the sheet control the perforation

process.

Keywords: Lamellae break up, Suspension lamellae, Suspension atomization

2. EXPERIMENTAL WORK

2.1 Sprayrig and processfluids

In order to determine the effect of suspended solid

particles on the break-up of the hollow cone and flat

lamellae different model suspensions based on water were

used as modelfluids. Fig. 1 shows a schematic illustration

of the experimental set-up. The suspensions were

discharged from a hollow cone atomizer (SS-QUICK-

WHIRLJET QA-5; dout = dinlet = 3.6mm, dchamber = 11mm)

and a flat spray atomizer (SS 1/4P-5040; D = 3,6mm).

Ethanol/watermixtures and glycerol/watermixtures with

various suspended Clay, Glass, Polymer and

Siliciumcarbide particle fractions were atomized, while a

pressure transducer measured the liquid pressure at the

nozzle.

Fig. 1: Spray rig for analysis of lamellae

More than 30 photographs of the liquid sheet were taken

with CCD-camera immediately downstream of the nozzle

for each operating condition (s. Fig. 2). The “width” of the

liquid sheet Bhole and the length of the sheet Zhole at the

position of the occurrence of perforation were measured

from the photographs. Further more the sheet angle θ was

calculated from the following equation:

���

����

�

⋅⋅=

hole

hole

B0.5

Zarctan2� .

θ/2θ/2θ/2θ/2 Zh

ole

Zb

rea

k

10mm

Fig. 2: Hollow cone lamella: parameters studied

Table 1: Model suspensions

Suspension Cp

v.%

dp

µm

Clay / {water}

{ρL= 1,00g/cm³, ηL= 1,00mPa.s,

σL= 72mN/m}

0 - 30 10

Clay /

{20v.%Ethanol+80v.%water}

{ρL= 0.966g/cm³, ηL= 1,84mPa.s,

σL= 44,86mN/m}

0 - 30 10

Clay /

{75v.%Glycerol+25v.%water}

{ρL= 1.188g/cm³, ηL= 57,4mPa.s,

σL= 53,58mN/m}

0 - 30 10

Siliciumcarbide /

{75v.%Glycerol+25v.%water}

{ρL= 1.188g/cm³, ηL= 57,4mPa.s,

σL= 53,58mN/m}

0 - 30 10

Glass /

{75v.%Glycerol+25v.%water}

{ρL= 1.188g/cm³, ηL= 57,4mPa.s,

σL= 53,58mN/m}

5 35 - 95

Polymer / {water}

{ρL= 1,00g/cm³, ηL= 1,00mPa.s,

σL= 72mN/m}

5 53

Table 1 shows the properties of suspensions studied.

2.2 Experimental results

Fig. 3 shows photographs of the break-up process of the

hollow cone sheet of two different processfluids, water and

a high concentrated Clay/water-suspension (Cp = 30v.%),

generated by the above mentioned atomizer (injection

pressure prel = 1.2bar). The photographs show that the

hollow cone liquid sheets will disintegrate into ligaments

and drops according to aerodynamic waves. For a given

nozzle geometry the liquid flow rate through the nozzle and

the formation of the hollow cone sheet is strongly

influenced by the injection pressure and the friction losses

occurring in the nozzle. The increase of the liquid viscosity

leads to higher losses inside the nozzle, which consequently

leads to a lower tangential velocity of the liquid. Therefore

the diameter of the air core in the nozzle decreases and as a

result the liquid flow rate increases while the sheet angle

(θ) decreases as a result of the decrease of the tangential

velocity component.

Suspension

Atomizer

Pressure-Signal

Suspension

Air

Tank CCD-Camera

Lamella

Light

10mm

10mm

Water Clay/water-suspension

Cp = 30v.%

Fig. 3: Hollow cone lamella for different process fluids

By adding solid particles to the basic processfluid water its

flow behaviour changes i.e. it becomes shear-thinning

rather than newtonic as for the pure carrier liquid.

Moreover generally the value of the viscosity of the

suspension increases with increasing particle concentration.

Therefore it is also to expect that increasing of the solid

particle concentration leads to increase of the flow rate and

to decrease of the sheet angle. Fig. 4 shows that the high

solid particle concentration in the suspension based on

water (Cp = 30v.%) results in a higher flow rate than in case

of pure water and also in a smaller spray angle (Fig. 5).

One can realize a rather small difference of the flow rates

and a more significant difference of the spray cone angles.

The characteristic trait of the suspension flow rate can be

explained as a result of the shear thinning behaviour of the

suspension, because the suspension shows a high viscosity

at low shear rate (e.g. at pressure 0.2bar and the suspension

flow rate is comparable with the flow rate of

glycerol/water-mixture with 57.4mPa.s). For high shear

rates (e.g. at prel = 2bar) the suspension shows a low

viscosity and this results in a low flow rate (smaller liquid

outlet). The strong influence of the solid particle

concentration on the sheet angle is a result of the decrease

of the tangential velocity of the suspension in the swirl

chamber of the nozzle, which is, as mentioned above,

controlled by the pressure losses inside the nozzle, which

increase with increasing the solid particle concentration in

the suspension (high side friction in swirl chamber) [7]. Fig.

6 shows that the increase of the solid particle concentration

to 30v.% leads to lower break-up lengths, on the other hand

the break-up length seems to be slightly dependent on the

injection pressure. This behaviour in spraying the high

concentrated suspension by means of the hollow cone

nozzle disagrees with the results observed by flat jet nozzle

at CP = 30v.%, where the break-up length for high

concentrated suspension was higher than in case of pure

carrier liquid (water).

0

0,5

1

1,5

2

2,5

3

3,5

4

0 0,5 1 1,5 2 2,5Pressure [bar]

Flo

w r

ate

[l/

min

]

water Clay+water

Fig. 4: Flow rate as a function of the pressure

40

50

60

70

80

90

100

0 0,5 1 1,5 2 2,5

Pressure [bar]S

pra

y a

ng

le

water Clay+water

Fig. 5: Spray cone angle as a function of the pressure

0

20

40

60

80

0 0,5 1 1,5 2 2,5

Pressure [bar]

Bre

ak u

p l

en

gth

Z [

mm

]

water Clay+water

Fig. 6: Break-up length as a function of the pressure

Photographs of hollow cone sheet and flat sheet are shown

in Fig. 7. By comparing the suspension sheets (CP = 30v.%)

at identical operating pressure it can be seen that:

1) aerodynamic waves already have been formed on the

surface of the suspension sheet produced by the hollow

cone,

2) the sheet angle of the hollow cone sheet is bigger than

that of the flat sheet. Therefore, similar to the flat sheets

with less particles concentration (e.g. CP = 23v.%) the lower

break-up length can be explained as a result of the

destabilizing effect of the strong viscoelasiticity of the high

concentrated suspension [8].

The second important factor promoting the break-up of

the suspension sheet with wide sheet angle is the dewetting

of solid particles [3].

Z = 20mm; θ = 55°

hollow cone sheet

Z = 13cm; θ = 40°

flat sheet

Fig. 7: Break-up of the dense suspension sheet based on

water and with Clay dP = 10µm and CP = 30v.%

produced by the hollow cone nozzle and the flat nozzle

at Prel = 0.8bar

In the case of suspension based on water only a very light

increase of the break-up length was observed for a solid

particle concentration of Cp = 4v.% but a further increase of

the concentration leads to a significant lower break-up

length (Fig. 8). It seems that there is a critical concentration

at which the break-up length of the sheet becomes a

maximum value. This value depends on the carrier liquid.

0

20

40

60

80

0 0,4 0,8 1,2 1,6 2Pressure prel [bar]

Bre

ak-u

p len

gth

Z [

mm

]

pure water water + Clay: 4v.% water + Clay: 30v.%

Fig. 8: The break-up length Zbreak as a function of a

pressure for water as carrier fluid and different solid

particle concentrations

10mm

(Gly.-water)-mixture:

ρ = 1.18 g/cm³

η = 57 mPa.s

σ = 54 mN/m

10mm

(Gly.-water) + Glass:

d50,3 = 35µm

Cp = 5 v.%

(Gly.-water)+Glass:

d50,3 = 95µm

Cp = 5 v.%

0

10

20

30

40

50

60

70

0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2 2,2

pressure prel [bar]

Zh

ole [

mm

](Gly.-Water)-mixture

(Gly.-Water) + Glass: 35µm

(Gly.-Water) + Glass: 95µm

Fig. 9: Sheet length at perforation distance for a dilute

suspension (Cp=5v.%) versus operating pressure

The influence of the solid particle size on the break-up of

the hollow cone suspension sheet was studied for two

different dilute suspensions based on a

glycerol/water-mixture as carrier fluid with different glass

particle fractions (35µm and 95µm) (Fig. 9). For low

particle loading (Cp = 5v.%) no significant influence of the

solid particle on the suspension flow rate, on the sheet angle

and on the break-up mechanism was observed. The only

influence of the solid particle size was recognizable on the

perforation distance Zhole. The distance Zhole becomes lower

with increasing the glass particle size. These results confirm

the results obtained for dilute suspension flat sheets.The

comparison of photographs in Fig. 10 shows that the

aerodynamic waves in the case of the hollow cone sheet are

more pronounced than in the case of a flat sheet. Therefore

it can be assumed that the aerodynamic waves in the case of

the hollow cone dilute suspension sheet improve the

influence of the solid particles on the perforated-sheet

disintegration. Comparison between the ratio of the

thickness of the suspension sheet (at position of

perforation) and that of pure carrier liquid

(δhole)susp/(δhole)carrier liquid for the sheets produced by the

hollow cone nozzle does not confirm that the relationship

between the solid diameter and the sheet thickness may take

a linear behaviour.

Fig. 10: break-up of the dilute suspension sheet at 1bar

1

1.5

2

2.5

3

0 50 100 150 200 250

glass particles (d50,3)P [µm]

( δ h

ole

)su

sp /

( δ h

ole

)car

rier

liq

uid

Hollow cone sheet: 1.4bar

Hollow cone sheet: 1.2bar

Hollow cone sheet: 1bar

Flat sheet: 1bar

Flat sheet: 0.8bar

Flat sheet: 1.4bar

Fig. 11: (δδδδhole)susp/(δδδδhole)carrier liquid for sheets of the hollow

cone nozzle and of the flat jet nozzle versus solid particle

size d50,3

Fig. 11 shows that the influence of the solid particles on the

perforation break-up mode in the hollow cone sheet is more

intensive than that on the flat sheet. In the case of the flat

nozzle it was found that the influence of the solid particle

size on the perforated-sheet disintegration of dilute

suspension depends upon the Reynolds number. For small

values of the Reynolds number the sheet thickness at the

hole position increases linearly with increasing the solid

particle size in the sheet and the influence of the operating

pressure on the performation mechanism becomes smaller.

Fig. 12 shows the perforation distance as a function of the

pressure for two dilute suspensions (Cp = 5v.%) with two

different solid particles (Polymer: 1.1g/cm, dp = 53µm and

Siliciumcarbide: 3.2 g/cm³, dp = 53µm) and water as carrier

liquid. It is seen that the increase of the solid particle size

and the increase of the solid particle density leads to

decrease the perforation distance Zhole. Anyway there is no

clear relationship between the thickness of the hollow cone

sheet and the solid particle size.

20 mm

water:

ρ = 1 g/cm³

η = 1 mPa.s

σ = 72 mN/m

20 m m

water + polymer:

ρP = 1.1 g/cm³

d50,3 = 53µm

Cp = 5 v.%

20 mm

Water + Silic.:

ρP = 3.21 g/cm³

d50,3 = 53µm

Cp = 5 v.%

0

10

20

30

40

50

60

70

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2 2,2

pressure prel [bar]

Zh

ole [

mm

]water

water + polymer particles

water + silic. particles

Fig. 12: Sheet length at perforation distance for a dilute

suspension (Cp = 5v.%) based on water and with

polymer and siliciumcarbid particles as a function of

the pressure

Shimizu and et.al [6] investigated the accelerations of the

spheres abrasive particles suspended in water and having

different densities and diameters. The results showed that

the velocity difference between the water and the particles

with the highest density and relatively large diameter is

much higher than the velocity difference between the other

particles (lower density and smaller diameter) and water.

Accordingly, the turbulent motion of the water phase is

enlarged and jet break-up is promoted by the particles with

the highest density and large diameter. This effect of the

abrasive particle on the water jet break-up seems to be the

similar effect of the solid particles on the perforation

mechanism in the case of suspension sheet.

3. SUMMARY

The influence of the solid particle size, concentration

and density as well as the effect of the carrier liquid on the

break-up of a hollow cone and a flat suspension sheet was

studied experimentally. The photographs of the sheet

break-up have shown that the solid particles have an

influence on the sheet break-up and its parameters. The

25mm 20mm

increase of the solid particle concentration leads at first to

increase the break-up length, a further increasing of the

concentration leads to decrease the break-up length. The

effect of the concentration seems to be influenced by the

carrier liquid. The solid particle size and density affect the

perforation mechanism in the dilute suspensions. In this

case solid particles with high relaxation time (large

diameter or high density) can not follow the turbulent

fluctuations in the liquid sheet that leads to enlarge the

velocity difference between the particles and the liquid.

Accordingly, the perforation mechanism is promoted by the

solid particles. The experimental results presented in this

work find a good agreement with the theoretical study

presented in [8] and also the results of break-up of abrasive

water suspension jets presented in [6].

4. REFERENCES 1. Masters, K.: Spray drying in parctice,

SprayDryConsulte Intl. APS, Danmark, 2002

2. Thomas M. Spielbauer and Cyrus K. Aidun. 1994

Atomization & Sprays 4 405-436

3. Dombrowski, N.; Fraser, R. P.: A photographic

Investigation into the Disintegration of liquid sheet,

1954 Phil. Trans. R. Soc. London A 247 1001-130,

1954.

4. Butler Ellis, M.C; Tuck, C.R and Miller, P.C.H 1999

Atomization & Sprays 9 385-397

5. Glaser, H. W.: Das Zerstäuben von Suspensionen mit

Ein- und Zweistoffdüsen, VDI VERLAG, Düsseldorf,

1989.

6. Shimizu, S.; Hiraoka, Y.: Instantaneous Photographic

Observation of Abrasive Water Suspension Jets,

Influence of Abrasive Particle on Jet Structure). JSME

International Journal, vol.45, No. 4, pp. 830-835, 2002

7. Dahl, H. D.: Theoretische and experimentelle

Untersuchungen zur Flüssigkeitszerstäubung mit

Hohlkegekdüse, Ph.D. thesis, Institut für Mechanische

Verfahrenstechnik, Uni. Stuttgart, 1992.

8. Parthasarathy, R. N.: Linear spatial analysis of the

slurry sheets subjected to gas flow. Atomization and

Sprays, vol. 9, pp. 519-540,1999.