Physicomathematical modeling of a pulse atomizer

Olga Kudryashova,

Natalya Korovina, Boris Vorozhtsov

IPCET SB RS

HEMS

2012 HEMs 2012

PRACTICAL USE

o Non-lethal weapon (NLW): screening smoke, stopping aerosols.

•

o Sedimentation of harmful solid aerosols with preliminary dispersion of superfine liquid aerosols.

o Firefighting on transport.

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PROBLEMSpeed of the creation, superfine dispersion, autonomy.

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Ultrasonic nebulizer:+ Superfine,- LOW speed,- Demands an electricity

Pneumatic spray:- TOO large particles,+ HIGH speed,+ Autonomous

Destruction of liquid streams: Cavitation:

DECISION

To use energy of HEMS for creation of cavitation and autonomy of the compressed gas for liquid dispersion. Expected effect:

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+ HIGH Speed of the creation,

+ Superfine dispersion,

+ Autonomy.

So, cavitation + dispersion by the compressed gasSo, cavitation + dispersion by the compressed gas

Cavitation gives high dispersion.But how to create cavitation quickly? Cavitation gives high dispersion.But how to create cavitation quickly?

PREVIOUS MODEL OF ATOMIZER

The sprayer design is a combination of a hydrodynamic explosive tube and a centrifugal atomizer (HEMs’2011).

+ HIGH dispersion (< 10 μm)

+ HIGH speed (< 1 sec)

+ Autonomy

BUT

- Dangerous to use for a large volume of liquid.

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Figure 1 – Scheme of an explosive-type centrifugal atomizer: the charge chamber 1 contains an explosive charge. Gases (reaction products) push out, by means of a piston 2, water from a container 3. Water enters into a vortex chamber 5 through n of openings 4, and then escapes from a nozzle 6.

Decision:HEMs only for cavitation + dispersion by the compressed gas

Decision:HEMs only for cavitation + dispersion by the compressed gas

STAGE 1: Shock wave Cavitation The pressure of the shock wave pm:

where

Zm – the amplitude of particle displacement in the excited wave:

ω – the vibration frequency,

Q – the explosive transformation energy,

V1 – charge chamber volume,

L – the height of the water column,

S1 – the cross-sectional area of the liquid column,

γ – the adiabatic exponent of detonation products,

c – the wave propagation velocity in the liquid (sound velocity),

Ml – liquid mass.

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Figure 2 – Scheme of an impulse atomizer1 – compressed gas 1, 2 – liquid container,3 – membranes, 4 – openings for gas,5 – nuzzle,6 – pre-nuzzle volume.

1

1,m

Qp

V

21

2,m

l

QZ

t S c

Wo1 2m

l

Z Q

L с M

Cavitation begins at Wo>0,01Cavitation begins at Wo>0,01

MOVEMENT OF CAVITATED LIQUID

• Abramovich-Klyachko theory:• Geometrical parameter Ae (1),

where S2 – the area of entrance openings, n – their number, S3 – the area of nuzzle.

• The friction factor λ (2) is defined by speed of the incoming flow (3).

• The effective cross-section coefficient ε (4).

• The spray angle φ (5).• The empirical formula for

calculation of dispersion of an aerosol (6).

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31

2 2

1A 1

SS

S S n

e1 2

AA ,

1 / 2 / AS S

0,3

21,05, _ Re

Reinur

(1 ) 8

(1 1 )tg

3

1

/ 2eA

0,10,6 0,7147,8A Re Пd

nuz

D

D

2

1П2

l

nuz lr

2 1

01

1 2

1l l

in gl g g

p pv p

S p p

(1)

(2)

(3)

(4)

(5)

(6)

Dd > 10 μm

CAVITATION BUBBLES Cavitation index:

k = Vw/Ve≈0.8, where Ve – the element volume, Vw – the liquid volume.

At the moment of outflow each bubble bursts into droplets with a diameter equal to the water layer thickness h.

The expansion is considered to occur instantaneously (adiabatic process):

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D1

D2

D3

D4

h

a b

Figure 3 – Cavitation elements:bubbles and liquid in the liquid

layer

Figure 4 – Cavitation element:a) before outflow, b) after outflow

3 3

1 (1 ) ,mD k Z

1(1 )

Wo2

L kD

1/ 1/3

1 3 -2 1- atm atm

m md

p pD kD

k p p

Dd~ 1…5 μm depending on shock wave pressure pm

(7)

1(1 )

l

L k QD

с M

CRITERIA AND DISPERSION CHARACTERISTICS

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Figure 6 – Dependence of a spray angle φ on parameters S1/S2 (1), S1/S3 (2), А (3)

Figure 6 – Dependence of a spray angle φ on parameters S1/S2 (1), S1/S3 (2), А (3)

Figure 5 – Dependence of the droplet diameter (6) on parameters S1/S2 (1), S1/S3

(2), А (3)

Figure 5 – Dependence of the droplet diameter (6) on parameters S1/S2 (1), S1/S3

(2), А (3)

S1/S2 – the relation of the area of section of the vortex chamber to the area of openings; S1/S3 – the relation of the area of section of the vortex chamber to the nozzle area; Abramovich parameter А.

AEROSOL DISPERSION AND Wo PARAMETER

To receive a high-disperse aerosol (diameter of drops about 7-8 microns), it is necessary to create a condition for cavitation (Wo>0.01) but not to allow too big pulse impact on liquid (Wo<0.09)

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Figure 7 – Dependence of the droplet diameter on Wo: calculated by (6) – aerodynamic mechanism (1) and by

(7) – cavitation mechanism (2)

Figure 7 – Dependence of the droplet diameter on Wo: calculated by (6) – aerodynamic mechanism (1) and by

(7) – cavitation mechanism (2)

CONCLUSIONS• The physicomathematical model of a centrifugal

pneumatic atomizer at pulse influence of HEMs is offered.

• Expressions for calculation of the defining parameters of atomization from input parameters of an atomizer are received.

• It is shown that for achievement higher dispersion of an aerosol it is necessary to provide pulse nature of impact on liquid; the dimensionless criterion of Wo defining the mechanism of an atomization is offered.

• The new atomizer is autonomy, gives a superfine aerosol for a shot time.

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THANK YOU FOR ATTENTION

HAPPY ATOMIZING!

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