粒径表示方法

粒徑大小的測定

(

原文

)

How methods and instruments have kept pace with changing technology

There are many methods and instruments available for drop size data

collection. Since repeatable test results are essential in comparing

nozzle performance data, it is essential to use testing procedures that

take

into

account

all

of

the

potential

variables

in

the

sampling

technique

for both methods and instrumentation.

By

spraying

water

into

a

pan

of

oil

and

shuttering

the

spray,

it

is

possible

to

count

and

size

individual

drops

using

a

microscope.

This

technique

is

still used by some researchers. Problems with this method involve drop

coalescence, inadequate sample size and the fact that very small drops

will be deflected away from the oil by air currents at the surface due

to the spray velocity. Also, larger drops can and do break-up from

impacting the surface.

The

same

type

of

method

is

used

when

spraying

a

dye

onto

a

stationary

card,

or water onto liquid sensitive paper. Again, the small drops might be

deflected away from the

target

and the

large drops can

break-up from the

impact. Data collected by these

of uncontrolled variables making such test results generally

non- repeatable.

While

drop

size

data

was

being

collected

in

the

early

1950's

using

methods

such as flash photography, probably the first real breakthrough in drop

sizing technology was the development in 1961 of an automated imaging

analyzer (Figure 5).1

1 The Electronic Imaging Analyzer was developed at Spraying Systems Co.

by

Dr.

Verne

Dietrich

and

built

by

the

Dage

Division

of

TRW,

Michigan

City,

Indiana.

The

design

was

awarded

U.S.

Patent

3275733

in

September

of

1966,

and is currently in its second generation.

Basically, the Electronic Imaging Analyzer incorporates the spatial

measurement technique using a strobe light to illuminate the spray and

record

the

image

with

a

vidicon

tube.

The

image

is

scanned,

and

the

drops

are sized and separated into different classes. Resulting data can be

mathematically

corrected

using

velocity

data

to

give

a

flux

distribution.

Sources

of

error

early

in

the

development

of

this

device

included

blurring,

depth

of

field

variations

and

vidicon

tube

saturation.

These

sources

were

recognized and corrected.

The imaging type analyzer is still actively promoted by some nozzle

manufacturers. The limited availability of this type of instrument,

however, prevents independent researchers and other interested members

of the drop size analyzer community from verifying data arrived at from

a particular test or comparing performance from similar designs.

More recently the development of commercially available drop size

analyzers makes it feasible to verify drop size results by independent

sources.

This

new

breed

of

analyzers

incorporates

lasers,

special

optics

and

digital

circuitry

to

minimize

imaging

error.

Some

of

the

more

commonly

recognized manufacturers of laser measurement instruments include

Malvern,

Particle

Measuring

Systems

(PMS),

and

Aerometrics.

The

following

is an analysis of three of their instruments.

Malvern Particle Analyzer

The Malvern Analyzer, which is considered a spatial sampling device,

utilizes the fact that a spray drop will cause laser light to scatter

through an angle dependent on the diameter of the drop (see Figure 6).

The

scattered

light

intensity

is

measured

using

a

series

of

semicircular

photo

diodes.

Theoretically,

the

distance

of

the

individual

photo

diodes

from

the

centerline

of

the

laser

and

the

intensity

functions

are

all

that

are needed to calculate the drop size distribution. A curve-fitting

program is used to convert the light intensity distribution into any of

several

empirical

drop

size

distribution

functions.

Since

the

Malvern

has

some

self- diagnostics,

potential

sources

of

error

are

easier

to

identify.

The

instrument

must

be

aligned

and

calibrated

periodically

using

reticle

slides with known etched drop distributions.

Perhaps the biggest source for error with this type of instrument is

multiple light scattering. If the spray is too dense, there is a

possibility that the scattered light from one drop might be scattered

again by other drops further

down

the beam

axis. The Malvern

is equipped

with an

the spray is too dense, but such a determination is often difficult. To

circumvent this in the lab, the technician typically moves the nozzle

farther away or uses special shielding to permit only a portion of the

spray to enter the sample area.

Particle Measuring Systems

Particle

Measuring

Systems,

also

know

as

PMS,

produces

instruments

known

as Optical Array Probes. The PMS Optical Array Probe is a flux sampling

instrument

(see

Figure

7).

As

the

drops

pass

through

the

sampling

plane,

the drops are sized and counted providing information which can be used