Method and apparatus for determining aerosol size distributions

• US 4,211,487 A
• Filed: 01/22/1979
• Issued: 07/08/1980
• Est. Priority Date: 01/22/1979
• Status: Expired due to Term

• Current Assignee: The United States Of America As Represented By The Secretary Of The Army
• Original Assignee: The United States Of America As Represented By The Secretary Of The Army
• Inventors: Wortman, Donald E., Morrison, Clyde A., Karayianis, Nick
• Primary Examiner(s): Corbin, John K.
• Assistant Examiner(s): Arnold, Bruce Y.
• Application Number:
  US06/004,979
• Time in Patent Office: 533 Days
• Field of Search: 356/336, 356/318, 250/574
• US Class Current: 356/336
• CPC Class Codes: G01N 15/0205 by optical means

Abstract

A method and apparatus for determining aerosol size distributions, in whichhe droplets of an aerosol region are placed in an amplitude modulated electric field, which produces resonant elongation and contraction of particularly sized droplets, in the form of mechanical oscillations, depending on the modulating frequency of the modulated electric field. A light beam is directed against the oscillating droplets and scattered thereby, with the intensity of the scattered light beam varying as a result of the changing reflection surfaces presented by the oscillating droplets. Differently sized drops are sequentially resonantly oscillated by selectively varying the modulation frequency of the electric field, with the varying intensity of the oscillating droplets at each modulation frequency yielding the size distribution of a given aerosol.

Claims

1. An apparatus for determining the size distribution of aerosol droplets in an aerosol region, comprising:

drive means for generating an undirectional electric field and directing said field toward said aerosol region such that said field penetrates said aerosol region;

means for amplitude modulating said electric field such that said field is defined by the following relationship,

space="preserve" listing-type="equation">E(t)=E.sub.0 cosω

t[1=ε

.sub.M cosω

.sub.M t]

1 where

E₀ =peak amplitude

ω

=carrier frequency

ε

_M =modulation depth

ω

_M =modulating frequency;

t=time

whereby the modulation of said electric field produces resonant elongations and contractions in the form of mechanical oscillations of selectively sized droplets, where the oscillating frequency of said mechanical oscillations is defined by the following relationship, ##EQU4## where ω

_n =mechanical oscillation frequency

n=oscillation mode number

τ

=droplet surface tension

ρ

=droplet density

a=droplet radius;

whereby for a known τ and

ρ

, and an assumed oscillation mode n, only those droplets having a radius a oscillate at a preselected oscillation frequency ω

_n ;

sampling means for directing a light beam against the mechanically oscillating aerosol droplets, said sampling means arranged such that said light beam is scattered by said aerosol region with the scattered light beam exhibiting a time varying intensity produced as a result of reflection of said light beam off a time varying droplet reflection surface of each of said oscillating droplets;

detector means in the path of the scattered light beam for measuring the time varying intensity of said scattered light;

data processing means coupled to said amplitude modulating means and said detector means for sequentially selecting the oscillation frequencies ω

_M and for determining an aerosol size distribution based on the intensity of the time varying portion of the scattered light beam at each oscillating frequency ω

_n as measured by said detecting means.

2. An apparatus according to claim 1 wherein said drive means comprises:

a CO₂ cw-laser.

3. An apparatus according to claim 1 wherein said sampling means comprises:

a He-Ne cw-laser.

4. A method of determining the size distribution of aerosol droplets in an aerosol region, comprising:

generating a unidirectional electric field and directing said field toward said aerosol region such that such field penetrates said aerosol region;

amplitude modulating said electric field such that said field is defined by the following relationships,

space="preserve" listing-type="equation">E(t)=E.sub.0 cosω

t[1+ε

.sub.M cosω

.sub.M t]

where

E₀ =peak amplitude

ω

=carier frequency

ε

_M =modulation depth

ω

_M =modulating frequency

t=time

whereby the modulation of said electric field produces resonant elongations and contractions in the form of mechanical oscillations of selectively sized droplets, where the oscillating frequency of said mechanical oscillations is defined by the following relationship, ##EQU5## where ω

_n =mechanical oscillation frequency

n=oscillation mode number

τ

=droplet surface tension

ρ

=droplet density

a=droplet radius;

directing a sampling light beam against the mechanically oscillating aerosol droplets such that said light beam is scattered thereby, with the scattered light beam exhibiting a time varying intensity depending on the number of oscillating aerosol droplets;

varying the modulation frequency ω

_M to obtain data indicative of the time varying scattered light intensity of variously sized aerosol droplets with the intensity of the time varying portion of the scattered light beam at each modulation frequency indicative of the number of particularly sized aerosol droplets associated therewith; and

,

processing said data to obtain an overall size distribution of the droplets in said aerosol region.