m. stranzinger* nestec ltd. nestlé product technology ... angle of the pressure-swirl atomizer...

___________________________________________

*Corresponding author: [email protected]

ILASS Americas 27th Annual Conference on Liquid Atomization and Spray Systems, Raleigh, NC, May 2015

Film Stabilization as key step of Atomization Process for Spray-drying of highly

concentrated Emulsions or Suspensions.

M. Stranzinger*

Nestec Ltd.

Nestlé Product Technology Centre

Nestlé Strasse 3

3510 Konolfingen

Switzerland

Abstract

Food industry applies spray-drying to achieve shelf-stable and fast reconstitutable powders for a variety of products,

such as dairy, coffee, culinary and beverage products. The process of spray-drying converts emulsions, suspensions

and other type of dispersions into powder. Spray nozzles create droplets, which are dried in hot air by evaporating

water. The final powder quality, the final powder texture, the dryer process design, the drying efficiency and process

economy, the operational safety, to name only a few characteristics, are directly linked to the spray quality and thus

the atomization process. Drying efficiency and process economy are key cost drivers, which are triggered from

product side by the product water content to be evaporated. Thus during spray-drying the aim is to atomize

concentrate solids contents, as high as possible. A drawback for Dairy emulsions is, that concentrate viscosity

increases exponentially with increasing solids of the liquid to be atomized. Single-phase atomization typically used

in industry as most reliable technology by utilizing pressure-swirl atomizers has limitations in viscosity, resulting in

air-core break down. Air-core variation and break-down cause liquid film variations and thus droplet size variations.

The objective is to maintain uni-modal and polydisperse droplet size distributions. Therefore liquid film stabilization

is key before atomization of highly concentrated emulsions or suspensions starts. This paper describes the features

of single-phase pressure-swirl atomization, compares with other atomization technologies, describes the air-core

break-down process, as well as dependence on process and material properties and limitations found in powder

production processes, which are triggered by atomization. An experimentally found stability criterion based on the

spray angle of the pressure-swirl atomizer explored in this study allows to correlate with the liquid film in the swirl-

chamber of this type of atomizer.

March 2015 Stranzinger Michael 2

Table of Contents Introduction ................................................................................................................................................................... 3

Materials and Methods .................................................................................................................................................. 7

Non-Dimensional Parameter Approach ...................................................................................................................... 10

Results ......................................................................................................................................................................... 11

Conclusions ................................................................................................................................................................. 17

Acknowledgement ....................................................................................................................................................... 17

Nomenclature .............................................................................................................................................................. 17

References ................................................................................................................................................................... 18


Introduction

This work explores hollow-cone swirl-flows

generated by pressure driven swirl-flow atomizers

widely used in the manufacturing of food powders to

find an experimental stability criterion based on the

spray angle of this type of atomizers. The aim of food

atomization is to achieve shelf-stable and fast

reconstitutable powders by spray-drying of

emulsions, suspensions and other type of dispersions.

Figure 1 shows typical powder particles under the

microscope, which show different reconstitution

properties. For full-cream milk powders best

reconstitution properties are achieved with “compact

grape” like powder agglomerates, as shown with the

right picture in Figure 1. Such “compact grape” like

powder agglomerates are achieved with sprays,

which show uni-modal and polydisperse droplet size

distributions. Different types of powder agglomerates

are schematically classified by Pisecký [12], see

Figure 2.

Figure 1. Examples of spray-dried dairy powders; non-agglomerated powder (see left picture) and agglomerated

powder with a “compact grape” like structure (see right picture).

Figure 2. Schematic views of different types of powder agglomerates; Pisecký [12].

In this study Delavan SDX type spray-nozzles

(Goodrich Corporation [6]) are investigated, which

have the widest application range in food industry.

Figure 3 shows the main components of SDX spray-

nozzles, consisting of an end plate (see top part), a

swirl chamber which enables angular momentum to

the bulk fluid creating a swirl-flow (see centre part)

and an orifice disc from which the hollow-cone spray

releases (see bottom part). The right picture in Figure

3 illustrates the conversion of a liquid bulk product

flow (at product inlet) into a liquid film spray

consisting of a strong angular momentum flow

component with a hollow cone. Inside the hollow-

cone film the spray disintegrates into ligaments and

finally into spray droplets (for spray mechanism see

for instance Lefebvre [9]).


During the last couple of years at Nestlé new spray-

drying concepts and increasing amounts of food

recipes were introduced into existing food powder

production lines. As a consequence different spray

characteristics based on the production mass-flow-

rate, spray pressure and the food properties of the

recipe during spray creation (such as dynamic

viscosity, surface tension and density of the liquid

bulk) are leading to increasing demands in terms of

process automation, to allow high production

flexibility changing from one recipe to another one

using the same spray-drying equipment. To match

these demands, a flexible atomization in the spray-

drying process is required, which can only be met by

process automation. This study provides means for

atomization automation based on an experimental

stability criterion which enables to maintain uni-

modal and polydisperse droplet size distribution

during a production cycle for different recipes and

material properties applied, by utilizing the same

process equipment.

Figure 3. Sketch from Kelson [8] of main components of the spray-nozzle considered (Pressure Driven Swirl-Flow

Atomizer, type Delavan SDX); end plate (top part), swirl chamber (centre part) and orifice disc (bottom part). The

swirl chamber height Hswirl is characterized with the notation SA, SB, SC, etc. until SJ (e.g. SA = 0.8mm, SC =

1.4mm, SJ = 8.7mm in height Hswirl). The height Hswirl increases in alphabetical order. The orifice diameter D is

characterized in inch = orifice number / 1000 (e.g. D = 49 = 0.049inch = 1.24mm, D = 150 = 0.15inch = 3.81mm).

Explorations of different atomization technologies

(Tratnig [3], Strixner [4]) showed, that for reliable

industrial application, the high-pressure single-phase

atomization is the preferred technology, compared to

pneumatic atomizers or rotary atomizers; technology

types are shown in Figure 4. Pneumatic atomizers

utilize high amount of air for atomization, which is

cost intensive and requires more heat during spray-

drying to compensate for the cooling effect of this air.

Rotary atomizers show limitations for highly viscous

sprays in terms of droplet creation and thus cause

significant wall fouling. Typical spray-drying

conditions are shown in Figure 5, comparing single-

phase with two-phase atomization (Strixner [4]).


Figure 4. Different types of atomization technologies used for food spray-drying applications. Atomizers

categorized according to the kind of energy supplied; Tratnig [3].

Single-Phase

Technology

Two-Phase

Technology

Principle of

droplet creation

Centrifugal forces &

oscillations

within concentrate film

Shear-stress between concentrate

(1st phase) and air (2

nd phase)

Features Spray-pressure and concen-

trate mass-flow-rate are

coupled and thus droplet size

varies with changing mass-

flow-rate. Simple design,

easy to clean.

Droplet size is decoupled from

spray-pressure and mass-flow-rate

of concentrate and thus can be kept

constant. More complex design,

needs air supply!

Concentrate

viscosity range

1mPas -150mPas 1mPas -150mPas

Droplet sizes

achieved D32<70m at P=110bar with

100mPas

D32<70m at ALR > 0.6 with

100mPas

ALR: air-to-liquid ratio [kg/h air

mass-flow-rate / kg/h liquid mass-

flow-rate]

Droplet-

particle

residence times

in spray-dryers

Egron: 25sec – 5minutes

Niro: 5 – 40 minutes

Egron: 25sec – 5minutes

Niro: 5 – 40 minutes

Figure 5. Comparison of single-phase versa two-phase atomization for food spray-drying applications; Strixner [4].


In this study, the spray quality is judged in terms of

the consistency of the droplet size spectrum (uni-

modal and polydisperse) and its corresponding

droplet size mean value (e.g. Sauter mean value D3,2).

Figure 6 shows an experimental characterisation of

water-sugar-ethanol model sprays in terms of the

Sauter diameter D32,global and the standard

deviation of the droplet size spectrum, given by the

standard deviation RMS,global(D) of the drop

diameter (Tratnig [3]). Tratnig et al. [2, 3 and 15]

found these data in Figure 6 from Phase-Doppler

anemometry. Besides model sprays, Tratnig et al. [2,

3, 15 and 16] also explored food emulsions, utilized

in the Dairy powder business.

Figure 6 shows with increasing Sauter diameter also

an increase of the droplet size standard deviation,

with a linear dependence. In other words, with

increasing Sauter diameter, the polydisperse width of

the spray increases also. For spray-drying hence best

reproducible operating conditions are found with

minimum droplet size deviation possible or smallest

RMS,global(D) value (Tratnig [3]).

Delavan-SDX-Nozzle

y = 0.3024x

R2 = 0.9394

0

10

20

30

40

50

60

0 50 100 150 200

D32,global [µm]

RM

S,g

lob

al(

D)

[µm

]

µ approx. 10 mPas

µ approx. 50 mPas

µ approx. 150 mPas

Figure 6. Droplet size characterisation of water-sugar-ethanol model sprays at 3 liquid bulk viscosities combining

the Sauter diameter D32,global with the droplet size deviation, given by the standard deviation RMS,global(D) of

the drop diameter measured along a defined cross-section of the spray (Tratnig [2,3 and 15]). The dependency is

linear.

D32,st = D32,global RMS,global(D) (1) Based on the explorations of Tratnig et al. [2,3,15

and16] optimum spray stability D32,st for Delavan

SDX type pressure-swirl nozzles is achieved, if the

droplet size deviation RMS,global(D) for a certain

Sauter diameter is minimal (see definition 1).

To quantify film and spray stability, this study has

combined experimental and numerical spray angle

investigations of water-sugar-ethanol solutions with

definition 1 to determine stabilized films and sprays

within the atomizers explored. Spray angles can be

easily determined by visual exploration. Figure 7

shows the chosen approach of this study to determine

a liquid film and spray stability criterion.


Figure 7. Step by step evolution of pressure-swirl-nozzle explorations with the objective to find an experimental

stability criterion for the nozzle film and spray. Materials and Methods

Experimental determined spray angles are

correlated with the following 3 process quantities for

given food material properties (characterized by the

dynamic viscosity and density of the liquid bulk) and

mass-flow-rates Qm through the spray-nozzle:

spray pressure P at the spray-nozzle

spray angle 2 and spray droplet size D32,global

The surface tension only plays a role, if the droplet

break-up is also considered. This is not the case for

the experimental spray angle explorations.

Figure 8 shows the pilot test set-up used for the

experimental spray angle explorations. From the

experimental explorations the spray pressure P

(online measured with a pressure transmitter Hänni

ED520 0-600bar roughly 50cm upstream of the

Delavan spray-nozzle as indicated in Figure 8 with PI

in between the high-pressure pump type Rannie HP-

IND 24.60 and the Delavan spray-nozzle) and the

spray angle 2 (as defined in Figure 9) have been

determined. Spray angles have been analysed from

digital photographic pictures; Stranzinger [13].

Furthermore the mass-flow-rate Qm (FI in Figure 8)

and the liquid bulk density (DI in Figure 8) have

been determined online via a Foxboro mass-flow

meter CFT10 (FMI in Figure 8). The liquid bulk

dynamic viscosity (VI in Figure 8) together with

the temperature T (TI in Figure 8) have been

measured online via a Hydra-Motion viscometer

XL/7-100-T10-3/4“BSPP. To avoid damage of the

Foxboro mass-flow meter and enable reproducible

viscosity measurements, a safety valve (with 5bar

peak pressure, monitored via the PI indicator in the

low-pressure line, see Figure 8) was mounted in

between the feeding pump (Mono-Pump, Socil SL

20) and the Foxboro mass-flow meter (see Figure 8).

To enable a wide range of liquid bulk product

viscosities (0.001Pas – 1.5Pas) two model fluids have

been used for the experimental explorations of this

study:

water at room temperature

water-sugar-ethanol solutions

Figure 8. Pilot test set-up; Stranzinger [13].


Definition of Spray Cone Angle 2

2c

m

2

Figure 9. Definition of the spray angle 2 along the hollow-cone spray; Stranzinger [13].

Figure 10. Axisymmetric view of a pressure-swirl spray-nozzle. Definition of the air-core size 2R at the orifice disc

outlet plane (see dashed line) of the spray-nozzle. The liquid film thickness LF is analysed at the outlet plane (with

dark blue colour).

Figure 10 shows a cross-section view of such a swirl-

nozzle geometry and the definition of the air-core

size or diameter 2R, as well as the liquid film

thickness LF at the outlet plane:

LF = ½ * (D – 2R) (2)

Figure 11 shows a numerical visualization of the air-

core break-down process and the correlation with the

experimentally determined spray angle.

For the atomizer case shown in Figure 11 the air-core

break-down happens below =0.1Pas. A constant

mass-flow-rate of Qm=300kg/h is applied. This air-

core breakdown behaviour is also reflected in the

spray angle. But even with an air-core break-down,

the spray angle does not disappear completely.

The top row in Figure 11 shows spray cone pictures

for different product viscosities. Underneath of the

pictures the dimensionless spray angle number 2* (see definition (5) in the following paragraph)

indicates the loss in spray angle 2 relative to the

maximum possible spray angle 2max for the swirl-

nozzle type used. The maximum possible non-

dimensional spray angle at 2*=100% is found with

water as spray product. Before air-core break-down

the spray angle loss is 19% for 2*, from 2*=92%

to 2*=73% as shown in the example of Figure 11.

A mathematical description of the air-core formation

and break-down process can be found in the

publication of Wimmer and Brenn [5].

Film thickness LF


=0.017Pas / 300 kg/h =0.026Pas / 300 kg/h =0.049Pas / 300 kg/h =0.14Pas / 300 kg/h =1.5Pas / 300 kg/h

SC703-49

2*=92% 2*=79% 2*=73% 2*=63% 2*=49%

0

0.2

0.4

0.6

0.8

1

0.001 0.01 0.1 1 10

VISCOSITY [Pas]

AIR

-CO

RE

RA

DIU

S R

[mm

]

Figure 11. Photographs of the spray cone formation and air-core radius as a function of viscosity. The percentages

represent the size of the non-dimensional spray angle 2*. Air-core radius is compared at orifice outlet plane,

according to the definition in Figure 10. The air-core radius is found from axial symmetric 2-phase numerical

calculations (Moser [10] and Stranzinger [13]) using a commercial CFD code from Fluent [14]. Model fluids with

increasing viscosities are atomized at constant mass-flow-rate of Qm=300kg/h for a Delavan SDX SC-49 type

nozzle.


Non-Dimensional Parameter Approach To reduce the amount of parameters defined and

to compare different spray-nozzle configurations with

large operating range differences the non-dimensional

parameter approach allows a simpler interpretation of

the results in order to define an experimental stability

criterion with respect to the spray angle. For

atomization studies the non-dimensional parameter

approach is a common method, see for instance Hahl

[7].

For this study, the following seven process quantities

are considered:

Qm: the mass-flow-rate [kg/s]

P: the total spray pressure before spray-

nozzle [Pa]

: the liquid bulk density [kg/m3]

: the liquid bulk viscosity [Pas]

Hswirl: the swirl chamber height [m]

2: the spray angle [deg]

2max: the maximal possible spray angle

for a specific spray-nozzle type [deg]

The surface tension between liquid bulk and

ambient air is not used, since the break-up of the

liquid bulk into primary droplets is not part of the

experimental stability criterion for spray angles.

Applying the -theorem (Panton [11]) the 7

dimensional parameters reduce to 3 characteristic

non-dimensional numbers (Stranzinger [13]):

2

2

*

swirlH

Qm

PP

(3)

the non-dimensional pressure number,

swirlH

QmRe

(4)

the Reynolds number

max2

1002*2

(5)

and the non-dimensional spray angle.

Additionally in terms of the numerical explorations

done by Moser [10] and Stranzinger [13] the air-core

diameter 2R is made dimensionless using the orifice

diameter D:

D

RD

1002*

(6)

the non-dimensional air-core diameter,

given in percentage of the orifice diameter D.

In order to correlate the spray angle stability criterion

with the experimental droplet size data from Tratnig

[2,3] the following non-dimensional definition of the

global Sauter diameter is used:

D

globalDD

,32*32 (7)

the non-dimensional Sauter diameter,

given as the ratio between the global Sauter diameter

D32,global at a given spray cross-section (as defined

by Tratnig [3]) divided by the spray-nozzle orifice

diameter D.


Results The non-dimensional representation of the swirl-

flow parameters (as defined in the previous

paragraph) given here with P*, Re and 2* (see

Figure 12 and Figure 13) demonstrate the spray

pressure and spray angle behaviour of the swirl-

nozzle as a function of mass-flow-rate Qm and of

viscosity (represented with Re) from the

experimental explorations done.

The solid lines in Figure 12 and Figure 13 represent

the dimensionless pressure number P* as a function

of the Reynolds number Re. The dashed lines

represent the dimensionless spray angle 2* as a

function of Reynolds Re. Increasing Re depicts lower

product viscosity. All lines in Figure 12 and Figure

13 are found by logarithmic curve fitting. With the non-dimensional pressure number P* one

can distinguish the sensitivity of the spray-

performance as a function of the flow-state given by

the Reynolds number Re. A high line slope

corresponds to a highly sensitive spray-performance.

A flat line represents a very robust spray-

performance, since fluctuations of Re have low

impact on the pressure number P*. Thus increasing

the swirl-chamber size and keeping the orifice size

constant (see Figure 13) the pressure number P*

becomes less sensitive to mass-flow-rate or viscosity

changes (represented with Re). This can be seen with

the little slope of the swirl-nozzle examples SG, SI

and SJ, respectively (see Figure 13). Thus an increase

in nozzle size levels out the significant pressure

dependence found for the pilot-plant swirl chamber

types SA, SC or SE, respectively (see solid lines with

high slope in Figure 13). This means at higher flow

rates (typically above 1000kg/h per nozzle which is

the case for factory conditions) or very low product

viscosities the swirl-chamber geometry (due to its

increased chamber size) has reduced impact on the

spray formation and hence the spray quality. In other

words the swirl-nozzle behaves less sensitive.

Nevertheless also for factory scale conditions large

changes could occur in product material properties. In

particular at start-up and shut-down procedures it

happens, that a specified nozzle type responds

inappropriate (spray-cone breakdown or dripping

behaviour) to viscosity or mass-flow-rate fluctuations

(= Re-fluctuations), causing quality deteriorations in

spray and finally powder quality, respectively.

On the other hand small-size swirl-chambers for

instance chamber size SC can be made less sensitive

to Re-fluctuations adjusting the orifice size (see

Figure 12, SC-nozzle diagram with orifice sizes 24,

49, 90 and 133). As one can see in the diagram a

decrease in orifice size from size 133 to size 24

reduces the slope of the P*-trend-lines and thus the

swirl-nozzle behaves less sensitive to Re-fluctuations

(see in particular cases with orifice size 24 and 49).

From the dimensionless spray angle 2* one can

distinguish a lower limit before a sudden drop in

spray angle occurs, which is common for all nozzle

combinations considered. Using the example in

Figure 12 and the swirl-nozzle type SC-24, the angle

2* (see asterix symbols in the chart) remains above

2*>70% for the flow-states (Re>1000) considered,

but drops by several decades (~30%), below

Re<1000. This behaviour is also true for the other

swirl-nozzles considered, but at different critical

flow-states (Re). As a rule of thumb a minimum

dimensionless spray angle of 2*=70% can be

specified, above which the spray depicts best possible

quality, avoiding an air-core breakdown. This

minimum spray angle allows to define a swirl-flow

stability criterion.


Swirl Chamber Types: SC

Orifice Sizes: 24, 49, 90, 133

0

20

40

60

80

100

10 100 1000 10000 100000

Re=f(Qm,DNOUT,eta) [-]

tota

l p

res

su

re P

*=f(

p,r

o,D

NO

UT

,Qm

)

[-]

0

20

40

60

80

100

sp

ray

-an

gle

2a

lfa

*=f(

2a

lfa

, 2

alf

am

ax

)

[-]

SC-24: P*

SC-49: P*

SC-90: P*

SC-133: P*

SC-24: 2alfa*

SC-49: 2alfa*

SC-90: 2alfa*

SC-133: 2alfa*

Figure 12. Experimental data of non-dimensional correlations found from pilot-plant trials; Stranzinger [13]. Swirl

chamber geometry type SC is shown here, varying the orifice size. The correlations are shown for the non-

dimensional spray angle 2alfa* versa the Reynolds number Re and the non-dimensional pressure number P* versa

the Reynolds number Re, respectively. The lines represent logarithmic fits of measured data. The solid lines show

trends of P* as a function of Re. The dashed lines show trends of 2alfa* as a function of Re.

0

5

10

15

20

25

10 100 1000 10000 100000

Re=f(Qm,DNOUT,eta) [-]

tota

l p

res

su

re P

*=f(

p,r

o,D

NO

UT

,Qm

) [-

]

0

20

40

60

80

100

sp

ray

-an

gle

2a

lfa

*=f(

2a

lfa

, 2

alf

am

ax

) [-

]

SA-49: P*

SC-49: P*

SE-49: P*

SG-49: P*

SI-49: P*

SJ-49: P*

SA-49: 2alfa*

SC-49: 2alfa*

SE-49: 2alfa*

SG-49: 2alfa*

SI-49: 2alfa*

SJ-49: 2alfa*

Swirl Chamber Types: SA, SC, SE, SG, SI, SJ

Orifice Sizes: 49

Figure 13. Experimental data of non-dimensional correlations found from pilot-plant trials; Stranzinger [13]. Swirl

chamber types are increased in size from SA-SJ with one orifice size 49. The correlations are shown for the non-

dimensional spray angle 2alfa* versa the Reynolds number Re and the non-dimensional pressure number P* versa

the Reynolds number Re, respectively. The lines represent logarithmic fits of measured data. The solid lines show

trends of P* as a function of Re. The dashed lines show trends of 2alfa* as a function of Re.


To fit the experimental data as shown in Figure 12

and Figure 13 throughout the flow range considered,

an exponential fitting function (Aeschlimann [1]) of

type

)/exp( xcbay (8)

has been applied, which is more suitable than the

logarithmic trend-lines used for the discussion in

Figures 12 and 13.

Figure 14 shows example plots of trial results from

Stranzinger [13] using fitting constants a,b and c for

all nozzle geometries explored. The variable x refers

to the abscissa axis, here the Reynolds number Re.

Figure 14 shows fitting curves based on function (8)

for the swirl-chamber geometry Delavan SC, both for

the spray angle 2alfa* to Re correlation and the

pressure number P* to Re correlation, respectively.

This fitting function (8) has been applied to all nozzle

geometries experimentally explored; Stranzinger

[13].

As stated before, a lower dimensionless spray angle

of 2*=70% has been found from the explorations of Stranzinger [13] to suit as stability criterion.

Considering the uncertainty of the experimental spray

angle data with %9.6*2 calculated by

Stranzinger [13], a robust mean stability criterion can

be defined as:

2* = 80% (9)

with an uncertainty of

%9.6*2 .

This mean stability criterion is included into the

diagrams of Figures 15 and 16, respectively.

2alfa*-Reynolds Correlation

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

110.0

10 100 1000 10000 100000

Reynolds Number Re

Sp

ray

An

gle

2a

lfa* SC-40-Model

SC-50-Model

SC-60-Model

SC-70-Model

P*-Reynolds Correlation

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

10 100 1000 10000 100000

Reynolds Number Re

Pre

ssu

re N

um

ber

P* SC-40-Model

SC-50-Model

SC-60-Model

SC-70-Model

Figure 14. Model fits of experimental data with non-dimensional correlations applicable to the full set of pressure-

swirl atomizer geometries explored. The correlations in the left diagram are shown for the non-dimensional spray

angle 2alfa* versa the Reynolds number Re and in the right diagram for the non-dimensional pressure number P* as

a function of the Reynolds number Re. Swirl chamber geometry type SC is shown here, varying the orifice size.


2alfa*-Reynolds Correlation

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

110.0

10 100 1000 10000 100000

Reynolds Number Re

Sp

ray A

ng

le 2

alf

a* SC-40-Model

SC-50-Model

SC-60-Model

SC-70-Model

Figure 15. Definition of the experimental stability criterion based on the non-dimensional spray angle 2alfa* shown

with the red zone 2alfa*=80% with an uncertainty of %9.6*2 ; Stranzinger [13].

Spray angles above the stability criterion according to

definition (9) show stabilized air-cores and thus

stabilized liquid films and sprays, see Figure 15 and

Figure 16.

Figure 16 shows numerical calculations of the

dimensionless spray angle 2alfa* as a function of the

non-dimensional air-core diameter D* to compare

with the experimental results found by Stranzinger

[13]. These numerical calculations from Moser [10]

confirm the experimentally found spray angle to air-

core correlation shown in Figure 11. As highlighted

with zone A in Figure 16, even with a complete air-

core break-down at D*=0%, the spray angle does not

disappear.

This confirms the experimental results shown in

Figure 11. Zone B shows the experimentally defined

stability criterion for the spray angle 2alfa*=80%,

with an uncertainty of %9.6*2 . As a

consequence stabilized spray angles without air-core

break-down are found in zone C, which is the

recommended operational area for these type of

atomizers. In this zone the atomizers are operated

with stabilized liquid films.


0

20

40

60

80

100

0 20 40 60 80 100

AIR-CORE DIAMETER D* [%]

SP

RA

Y A

NG

LE

2

* [%

]

SPRAY-NOZZLE SA-69

SPRAY-NOZZLE SC-49

Figure 16. Non-dimensional relations between the air-core diameter D* and the spray angle 2* of 2 spray-nozzle

geometries based on numerical results (Moser [10]) throughout the Reynolds number range considered. 2 examples

are shown here; SA-69 and SC-49 type geometries.

Figure 17 shows film stability zones (zone B in the

diagrams) for the example spray-nozzle size Delavan

SC-70. These zones are found by the 80% spray

angle stability criterion (see zone A), which defines

the left border of zones B. Within the stability zone B

the film is stable whereas no air-core break-down

occurs and thus consistent uni-modal and

polydisperse droplet size distributions can be

maintained. These zones are operating windows for

consistent spray-drying manufacturing. Nevertheless

also within the stability zone the Sauter diameter has

a range shown from D32*,st-min to D32*,st-max,

based on definition (1) and the correlation shown in

Figure 6.

B

C

A


0%

20%

40%

60%

80%

100%

0

2

4

6

8

10

12

14

16

1 100 10000

Sp

ray a

ng

le 2

alf

a*

Sa

ute

r d

iam

ete

r D

32

,glo

bal

/ O

rifi

ce

dia

me

ter D

= D

32

* [%

]

Reynolds number Re [-]

Nozzle type Delavan SC-70

D32*,st-max

D32*

D32*,st-min

2alfa*

A

B

stabilized filmand spray zone

0%

20%

40%

60%

80%

100%

0

2

4

6

8

10

12

14

16

0.0 2.0 4.0 6.0 8.0

Sp

ray a

ng

le 2

alf

a*

Sa

ute

r d

iam

ete

r D

32

,glo

bal

/ O

rifi

ce

dia

me

ter D

= D

32

* [%

]

Pressure number P* [-]

Nozzle type Delavan SC-70

D32*,st-max

D32*

D32*,st-min

2alfa*

A

B

stabilized film and spray zone

Figure 17. Zones with stabilized liquid film and spray conditions for a Delavan SC-70 atomizer. Zone A shows the

80% spray angle stability criterion, which defines the left border of zone B. Within zone B no air-core break-down

occurs and thus the operating window with liquid film and spray stabilization conditions is defined. The upper

diagram shows the non-dimensional spray angle 2alfa* and the non-dimensional Sauter diameter D32* with a

deviation range given by D32*,st-min to D32*,st-max as a function of the Reynolds number. The lower diagram

shows the spray performance for the same non-dimensional numbers as in the upper diagram as a function of the

non-dimensional pressure number P*.


Conclusions Numerical and experimental explorations of

swirl-flows with single-phase pressure-swirl

atomizers of type Delavan SDX have been

established which in both approaches show a

significant dependency of spray-pressure,

air-core size, spray angle and droplet size on

product viscosity.

A systematic approach by variation of the

flow-state (given in terms of the mass-flow-

rate, the spray pressure and the material

properties of the spraying liquid) comprising

numerical explorations and pilot-plant trials

provided a non-dimensional spray angle

stability criterion – defined as 2*=80%.

Application of the spray angle stability

criterion gives operating windows

(stabilized liquid film and spray zone)

without air-core break-down within the swirl

chamber of the atomizer.

The operational windows enable stabilized

atomization, whereas the droplet Sauter

diameter (representing a droplet size

spectrum with uni-modal and polydisperse

distribution) still varies within a range,

which becomes smaller with decreasing

Sauter diameter.

Due to the deviation sensitivity of the

droplet Sauter diameter it requires process

automation of the atomization process to fix

a droplet size spectrum throughout the

manufacturing time of spray-dried food

powders even within a defined operational

window.

Acknowledgement

Thanks are expressed to the whole team at Nestlé

PTC Konolfingen for their excellent support

establishing the experimental trials and everyone’s

personal effort. In particular the good collaboration

with Ritchie Parker at Nestlé PTC Orbe as well as

with Lukas Moser from the ProcEng Moser GmbH

are very much acknowledged. Quantitative

explorations of droplet size spectra were established

as part of a cooperation project between Dr. Günter

Brenn and Dr. Andreas Tratnig from Technical

University Graz and Nestlé PTC Konolfingen. The

outstanding collaboration with Dr. Brenn and

Dr. Tratnig is very much acknowledged. The author

thanks Nestlé PTC Konolfingen for the financial

support to establish this study.

Nomenclature D

D32,

global

D32, st

Hswirl

LF

P

Qm

orifice diameter of the spray-nozzle

[m]

global Sauter diameter at a spray cross-

section [m]

Sauter diameter applied to the

experimental stability criterion [m]

swirl chamber height [m]

liquid film thickness at

the outlet plane [m]

total spray pressure before spray-

nozzle [Pa]

mass-flow-rate [kg/s]

R

2R

RMS, global (D)

air-core radius [m]

air-core diameter [m]

standard deviation of the global Sauter

drop diameter [m]

liquid bulk density [kg/m3]

liquid bulk viscosity [Pas]

2alfa, 2 spray angle [deg]

2max maximal possible spray angle for a

specific spray-nozzle type [deg]

D32*

D32*,st

D*

P*

non-dimensional Sauter diameter,

given in percentage of D [%]

D32* applied to the stability criterion,

given in percentage of D [%]

non-dimensional air-core diameter [-]

non-dimensional pressure number [-]

Re Reynolds number [-]

2alfa*, 2* non-dimensional spray angle [-]


References

1. Aeschlimann, J.M., Fitting Functions for

Usual Cases of Univariate Modeling, Nestec

Ltd., Switzerland, 2002.

2. Tratnig, A., Brenn, G., Strixner Th.,

Characterization of sprays of opaque liquids

from pressure-swirl atomizers using phase-

Doppler anemometry, 14th Int. Symp. on

Applications of Laser Techniques to Fluid

Mechanics, Lisbon, Portugal, Paper number

1465, 07-10 July, 2008.

3. Tratnig, A., Characteristics of sprays

produced by pressure-swirl atomizers, PhD-

thesis, Technische Universität Graz, August

2009.

4. Strixner, Th., Characterisation of highly

viscous sprays and high-TS atomization

measurements, cooperation report,

Technische Universität Graz, February

2008.

5. Wimmer, E. and Brenn, G., Viscous flow

through the swirl chamber of a pressure-

swirl atomizer, International Journal of

Multiphase Flow, 53: 100-113 (2013).

6. Goodrich Corporation, Delavan SDX,

Charlotte, NC, USA.

7. Hahl, D., Theoretische und experimentelle

Untersuchungen zur

Flüssigkeitszerstäubung mit

Hohlkegeldüsen, VDI Fortschrittberichte,

Reihe 3: Verfahrenstechnik Nr. 302, VDI

Verlag, 1992.

8. Kelson, J., Nestlé Spray-Drying, Nestec

Ltd., Switzerland, 2000.

9. Lefebvre, A. H., Atomization and Sprays,

Hemisphere Publishing Corporation, 1989.

10. Moser, L., CFD pressure swirl flow analysis

– influence on product quality, Phases 1 and

2, ProcEng Moser GmbH, Siselen,

Switzerland, 2002-2003.

11. Panton, R., Incompressible Flow, 2nd

Edition, John Wiley & Sons Inc., 1996.

12. Pisecký, J., Handbook of Milk Powder

Manufacture, Niro A/S, Copenhagen,

Denmark, 1997.

13. Stranzinger, M., Pressure-swirl-nozzle

explorations for viscous products, R&D

report RDKO-RD040020, Nestlé PTC

Konolfingen, December 2004.

14. Fluent Inc., Computational Fluid Dynamics

Software, Versions Gambit 1 and Fluent

6.1.22, D-64295 Darmstadt, Germany.

15. Tratnig, A., Brenn, G., Drop size spectra in

sprays from pressure-swirl atomizers,

International Journal of Multiphase Flow 36

(2010) 349 – 363.

16. Tratnig, A., Brenn, G., Strixner, Th.,

Fankhauser, P., Laubacher, N., Stranzinger,

M., Characterization of spray formation

from emulsions by pressure-swirl atomizers

for spray drying, Journal of Food

Engineering 95 (2009) 126-134.

m. stranzinger* nestec ltd. nestlé product technology ... angle of the pressure-swirl atomizer...

Documents