Week # 1

lameubiquityMechanics

Feb 21, 2014 (3 years and 6 months ago)

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Week # 1

MR Chapter 1



Tutorial #1


MR #1.1, 1.4, 1.7.



To be discussed on Jan. 22
2014.


By either volunteer or
class list.



MARTIN RHODES (2008)
Introduction to Particle
Technology
, 2nd Edition.
Publisher John Wiley & Son,
Chichester, West Sussex,
England.


Describing the size of a
single particle. Some
terminolgy about
diameters used in
microscopy.



Equivalent circle
diameter.



Martin’s diameter.



Feret’s diameter.



Shear diameter.

Describing the size of a single particle


Regular
-
shaped particles



The orientation of the particle on the
microscope slide will affect the
projected image and consequently the
measured equivalent sphere diameter.


Sieve measurement: Diameter of a
sphere passing through the same sieve
aperture.


Sedimentation measurement:
Diameter of a sphere having the same
sedimentation velocity under the
same conditions.


Comparison of equivalent sphere diameters.


Comparison of equivalent diameters


The volume equivalent sphere diameter is a commonly used equivalent
sphere diameter.


Example: Coulter counter size measurement. The diameter of a sphere
having the same volume as the particle.


Surface
-
volume diameter is the diameter of a sphere having the same
surface to volume ratio as the particle.





Shape


Cuboid

Cylinder

(Example)

Cuboid: side lengths of 1, 3, 5.

Cylinder: diameter 3 and length 1.

Description of populations of particles


Typical differential
frequency distribution

( )
dF
f x
dx

F: Cumulative distribution,

integral of the frequency distribution.


Typical cumulative frequency distribution


Comparison between distributions



For a given population of particles,

the distributions by mass, number

and surface can differ dramatically.




All are smooth continuous curves.




Size measurement methods often

divide the size spectrum into size

ranges, and size distribution becomes

a histogram.

Conversion between distributions


Mass and number distributions for man
-
made objects orbiting the earth


Total number of particles, N and total surface area S
are constant.


Particle shape is

independent of size,

a
s

is constant.

V is the total volume of the
particle population and
a
v

is
the factor relating the linear
dimension of particle to its
volume.


dx
f
s
V
dx
x
Nf
x
dx
f
N
v
v
)
(
)
(
3
a

Assumptions for conversions among
different distribution functions


It is necessary to make assumptions about the
constancy of shape and density with size.


Calculation errors are introduced into the
conversions.


Example: 2% error in FN results in 6% error in
FM. (Recalling the relationship between mass and
diameter).


If possible, direct measurements be made with the
required distribution.

Describing the population by a single number


Definitions of means


Plot of cumulative frequency against weighting function
g(x). Shaded area is


Number
-
length mean: Arithmetic
mean of the number distribution
conserves the number and length
of population.



Comparison between measures of
central tendency. Adapted from
Rhodes (1990).



Surface
-
volume mean, Sauter
mean: Arithmetic mean of surface
distribution conserves the surface
and volume of population.



The values of the different
expressions of central tendency
can vary significantly.



Two quite different distributions
could have the same arithmetic
mean or median.

Equivalence of means

K
s

and K
v

do not vary with size

2
s s N
dF x k dF

Same Expression

Common methods of displaying size
distributions

Arithmetic
-
normal Distribution

Log
-
normal Distribution

log
z x

Arithmetic
-
normal distribution with an arithmetic mean of 45 and standard deviation of 12.


z: Arithmetic mean of z,
s
z
: standard deviation of log x


Log
-
normal distribution plotted on linear coordinates


Log
-
normal distribution plotted on logarithmic coordinates

Methods of particle size measurements:
Sieving


Sieving: Dry sieving using woven wire sieves is
appropriate for particle size greater than 45
m
m.
The length of the particle does not hinder it
passage through the sieve aperture.


Most common modern sieves are in sizes such that
the
ratio of adjacent sieve sizes is the fourth
root of two

(e.g. 45, 53, 63, 75, 90, 107
m
m).


Methods of particle size measurements:
Microscopy


The optical microscope may be used to measure particle
size down to 5
m
m.


The electron microscope may be used for size analysis
below 5
m
m.


Coupled with an image analysis system, the optical and
electron microscopy can give
number

distribution of size
and shape.


For irregular
-
shaped particles, the projected area offered to
the viewer can vary significantly. Technique (e.g. applying
adhesive to the microscope slide) may be used to ensure

random orientation
”.

Methods of particle size measurement


Sedimentation


Size analysis by sedimentation


Re
p

<0.3. Motion of the
particle obeys Stoke’s law.



The suspension is
sufficiently dilute (No
hindered settling).



Particles are assumed to
accelerate rapidly to their
terminal free fall velocity,
time for acceleration is
negligible.


Permeametry

C
o
: original uniform suspension density
.

Sampling point: C at time t after the start of settling.


At time t all particles traveling faster than h/t will

have fallen below the sampling point.


C represents the suspension density for all particles

which travel at a velocity <= h/t.


See Example 1.3

The diameter calculated from the Carman
-
Kozeny equation is the

arithmetic mean of the surface distribution.


Electrozone sensing


Schematic of electrozone sensing apparatus

As particle flow through
the orifice,

a voltage pulse is recorded.


The amplitude of the pulse
can be related to the
volume of particle the
orifice.


Particle range:

0.3
-
1000
m
m.