Encyclopedia of Environmental Science and Engineering, Volume I and II

26 AEROSOLS

a particle and air at the outlet of a converging nozzle is

detected (Wilson and Liu, 1980).

Sedimentation Method

By observing the terminal settling velocities of particles it

is possible to infer their size. This method is useful if a TV

camera and He–Ne gas laser for illumination are used for the

observation of particle movement. A method of this type has

been developed where a very shallow cell and a TV system

are used (Yoshida et al., 1975).

Centrifuging Method

Particle size can be determined by collecting particles in a

centrifugal flow field. Several different types of centrifugal

IMPACTION SURFACE

SMALL

PARTICLE S

T

LARGE

PARTICLE

STREAMLINE

OF GAS

MEAN GAS FLOW

U 0

W

Re =

25000

3000

500

10

S/ W = 0.5, T/ W = 1

Re = 3000, T/ W = 2

0.3 0.4 0.5 0.6 0.7 0.8 0.9

St k

0

20

40

60

80

100

S/W=

0.25

0.5

5.0

COLLECTION EFFICIENCY (%)

FIGURE 8 Principle of operation of an impactor. Collection effi-

ciency of one stage of an impactor as a function of Stokes number,

Stk, Reynolds number, Re, and geometric ratios.

BEAM

SPLITTER

He–Ne

LASER

CLEAN

AIR

AEROSOL

NOZZLE

PHOTOMULTIPLIER

SIGNAL PROCESSING

CHAMBER PRESSURE

GAUGE

PUMP

FIGURE 9 Measurement of aerosol particle size by laser-

doppler velocimetry.

chambers, of conical, spiral and cylindrical shapes, have been

developed for aerosol size measurement. One such system is

illustrated in Figure 10 (Stöber, 1976). Particle shape and

chemical composition as a function of size can be analyzed

in such devices.

Electrical Mobility Analyzers

The velocity of a charged spherical particle in an electric field,

v e , is given by Eq. (20). The velocity of a particle having unit

charge ( n p  1) in an electric field of 1 V/cm is illustrated

in Figure 3. The principle of electrical mobility analyzers is

based upon the relation expressed by Eq. (20). Particles of

different sizes are separated due to their different electrical

mobilities.

FIGURE 10 Spiral centrifuge for particle size measurements.

AEROSOL

DISTRIBUTOR

CLEAN AIR

AEROSOL

CLEAN AIR

ROTATION

PLASTIC FILM

EXHAUST

DISTRIBUTOR

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