Recirculation of flocs in settling column
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h1. Recirculation of flocs in settling column {composition-setup} cloak.toggle.exclusive=false {composition-setup} h2. |
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{toggle-cloak:id=Concerns} |
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Concerns {cloak:id=Concerns} |
The
...
Tube
...
Floc
...
team
...
observed
...
a
...
slow
...
but
...
seemingly
...
interminable
...
recirculation
...
of
...
flocs
...
inside
...
the
...
settling
...
column
...
during
...
the
...
settling
...
state
...
of
...
each
...
experimental
...
run.
...
This
...
recirculation
...
was
...
troubling
...
because
...
it
...
violated
...
our
...
assumption
...
that
...
the
...
fluid
...
inside
...
the
...
settling
...
column
...
was
...
quiescent
...
enough
...
to
...
allow
...
flocs
...
to
...
be
...
accelerated
...
only
...
by
...
gravity.
...
Since
...
the
...
experimental
...
apparatus
...
was
...
designed
...
to
...
observe
...
floc
...
sedimentation
...
inside
...
the
...
settling
...
column,
...
if
...
flocs
...
were
...
actually
...
being
...
accelerated
...
by
...
a
...
recirculating
...
flow,
...
the
...
data
...
we
...
had
...
collected
...
cannot
...
be
...
interpreted
...
as
...
information
...
about
...
floc
...
sedimentation.
...
This
...
recirculation
...
problem
...
may
...
be
...
a
...
reason
...
for
...
why
...
we
...
have
...
not
...
been
...
seeing
...
ideal
...
settling
...
curves
...
as
...
we
...
would
...
have
...
expected
...
in
...
our
...
results.
...
Changes
...
need
...
to
...
be
...
made
...
to
...
assure
...
that
...
the
...
settling
...
column
...
is
...
quiescent
...
during
...
the
...
settle
...
state,
...
or
...
a
...
new
...
approach
...
must
...
be
...
taken
...
to
...
interpret
...
the
...
data.
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{cloak} h2. |
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{toggle-cloak:id=Possible Causes} |
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Causes {cloak:id=Possible Causes} |
Upon
...
identifying
...
the
...
recirculation
...
problem
...
and
...
bringing
...
it
...
up
...
with
...
Dr.
...
Weber-Shirk,
...
the
...
Tube
...
Floc
...
team
...
attempted
...
to
...
identify
...
the
...
cause
...
of
...
the
...
recirculation
...
problem.
...
After
...
weeks
...
of
...
diagnosing
...
the
...
experimental
...
apparatus,
...
the
...
team
...
was
...
able
...
to
...
identify
...
several
...
potential
...
causes
...
for
...
the
...
recirculation:
...
- The
...
- 90
...
- degree
...
- elbow
...
- connectors
...
- at
...
- each
...
- end
...
- of
...
- the
...
- settling
...
- column
...
- was
...
- originally
...
- thought
...
- to
...
- be
...
- the
...
- cause
...
- of
...
- the
...
- circulation
...
- because
...
- such
...
- a
...
- sharp
...
- turn
...
- was
...
- bound
...
- to
...
- introduce
...
- some
...
- vorticity
...
- into
...
- the
...
- flow.
...
- Although
...
- the
...
- recirculation
...
- occurred
...
- after
...
- the
...
- mean
...
- flow
...
- has
...
- been
...
- stopped,
...
- the
...
- redirection
...
- of
...
- momentum
...
- caused
...
- by
...
- the
...
- 90
...
- degree
...
- bend
...
- may
...
- still
...
- have
...
- lingered
...
- and
...
- thus
...
- could
...
- have
...
- contributed
...
- to
...
- the
...
- recirculation.
...
- The
...
- connection
...
- between
...
- the
...
- 3/8
...
- inch
...
- diameter
...
- flocculator
...
- tube
...
- and
...
- the
...
- 1/2
...
- inch
...
- settling
...
- column
...
- has
...
- always
...
- been
...
- a
...
- source
...
- of
...
- concern.
...
- The
...
- abrupt
...
- change
...
- from
...
- small
...
- to
...
- big
...
- diameter
...
- should
...
- theoretically
...
- cause
...
- flow
...
- instabilities
...
- that
...
- could
...
- lead
...
- to
...
- the
...
- formation
...
- of
...
- turbulent
...
- eddies.
...
- Moreover,
...
- the
...
- jaggedness
...
- of
...
- the
...
- inner
...
- walls
...
- of
...
- the
...
- pipe
...
- connectors
...
- will
...
- also
...
- interrupt
...
- the
...
- stability
...
- of
...
- the
...
- flow
...
- and
...
- compound
...
- the
...
- problem.
...
- A
...
- solution
...
- to
...
- this
...
- problem
...
- would
...
- be
...
- to
...
- fabricate
...
- a
...
- cone
...
- shaped
...
- connector
...
- that
...
- could
...
- provide
...
- a
...
- smooth
...
- and
...
- very
...
- gradual
...
- transition
...
- from
...
- small
...
- to
...
- large
...
- diameter.
...
- This
...
- connector
...
- would
...
- need
...
- to
...
- have
...
- a
...
- small
...
- enough
...
- angle
...
- to
...
- the
...
- axis
...
- of
...
- the
...
- column
...
- to
...
- ensure
...
- that
...
- the
...
- adverse
...
- pressure
...
- gradient
...
- created
...
- by
...
- the
...
- increase
...
- in
...
- diameter
...
- is
...
- weak
...
- enough
...
- that
...
- it
...
- will
...
- not
...
- cause
...
- flow
...
- separation
...
- at
...
- the
...
- boundary
...
- of
...
- the
...
- pipe.
...
- We
...
- noticed
...
- that
...
- the
...
- recirculation
...
- began
...
- the
...
- moment
...
- the
...
- valves
...
- were
...
- shut
...
- off.
...
- Since
...
- the
...
- solenoid
...
- valves
...
- close
...
- very
...
- abruptly,
...
- a
...
- small
...
- volume
...
- of
...
- fluid
...
- is
...
- displaced
...
- in
...
- an
...
- extremely
...
- short
...
- period
...
- of
...
- time
...
- each
...
- time
...
- a
...
- valve
...
- closes.
...
- This
...
- pulse
...
- of
...
- momentum
...
- may
...
- be
...
- strong
...
- enough
...
- that
...
- it
...
- traveled
...
- through
...
- the
...
- tubes
...
- and
...
- into
...
- the
...
- settling
...
- column
...
- where
...
- it
...
- may
...
- create
...
- instabilities
...
- leading
...
- to
...
- vorticity.
...
- The
...
- tungsten
...
- lamp
...
- that
...
- generates
...
- the
...
- white
...
- light
...
- inside
...
- the
...
- turbidimeter
...
- radiates
...
- a
...
- significant
...
- amount
...
- of
...
- heat
...
- to
...
- the
...
- settling
...
- column.
...
- Some
...
- summer
...
- students
...
- had
...
- performed
...
- a
...
- heat
...
- test
...
- which
...
- concluded
...
- that
...
- the
...
- heat
...
- generated
...
- by
...
- the
...
- lamp
...
- did
...
- not
...
- affect
...
- the
...
- turbidimeter's
...
- ability
...
- to
...
- measure
...
- turbidity
...
- accurately;
...
- however,
...
- because
...
- the
...
- heat
...
- tests
...
- had
...
- been
...
- performed
...
- using
...
- mixtures
...
- containing
...
- very
...
- fine
...
- clay
...
- particles,
...
- the
...
- effects
...
- of
...
- density
...
- driven
...
- convection
...
- was
...
- not
...
- apparent
...
- in
...
- the
...
- turbidity
...
- readings.
...
- If
...
- the
...
- heat
...
- transfer
...
- between
...
- the
...
- lamp
...
- and
...
- water
...
- was
...
- large
...
- enough,
...
- the
...
- change
...
- in
...
- the
...
- local
...
- density
...
- of
...
- fluid
...
- would
...
- cause
...
- convection
...
- currents
...
- that
...
- could
...
- potentially
...
- create
...
- the
...
- type
...
- of
...
- circulation
...
- we
...
- have
...
- been
...
- observing.
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{cloak} h2. |
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{toggle-cloak:id=Visual Experimentation} |
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Experimentation {cloak:id=Visual Experimentation} |
Several
...
visual
...
experiments
...
were
...
conducted
...
to
...
determine
...
what
...
the
...
actual
...
cause
...
of
...
the
...
reciruclation
...
was
...
from.
...
First,
...
we
...
conducted
...
experiments
...
changing
...
the
...
pump
...
control
...
shutoff,
...
then
...
we
...
experimented
...
with
...
the
...
changing
...
diameter
...
of
...
the
...
elbow
...
to
...
the
...
column,
...
and
...
lastly,
...
we
...
experimented
...
with
...
the
...
heat
...
factor.
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{cloak} h3. |
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{toggle-cloak:id=Pump Control Shutoff} |
...
...
Control Shutoff
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Shutoff {cloak:id=Pump Control Shutoff} First, the pump control shutoff was removed from the beginning of the settle state to a new state before settle state. This allowed for the pumps to be shutoff seconds before the valves were shutoff. Different times between the two shutoff points were experimented with, but each time, at the time of the valve shutoff, a jerk in motion would seem to cause a recirculation in flow. Perhaps we need to not shutoff the valve in order to prevent this recirculation. Monroe programmed a new variable in Process Controller to ramp down the pump controls to zero instead of abruptly switching from a high pump control value to zero (shutoff) automatically. We installed this ramp down function in Process Controller and tested it on our apparatus. The ramp down would bring the flow down to an easier flow. We tried this new function with both a downward flowing and upward flowing column to detect any differences. After careful observation, it was concluded that the recirculation was occurring even without the valve shutoff (about 20-30 seconds into settling state) and it maybe from either the pump shutoff causing a "push/pull" in the water (even though the pump is being ramped down, there is still a slight pressure change when the pump is completely shutoff) or from the diameter change from the flocculator tube to the settling column causing eddies of turbulence in the top of the column. {cloak} h3. |
We ran some tests to gauge what contribution the sudden stoppage of flow had on the recirculation problem. First, we introduced a new Process Controller state between the Loading state and the Settle state that allowed the pumps to ramp down before shutting off. We hypothesized that a more gradual reduction in flow rate would provide an improvement over abrupt shut-off. We also wanted to see the effects of shutting the valves verses not shutting the valves, so we ran some test without shutting the valves.
After careful observation, we concluded that the recirculation was occurring even without the valve shut-off (about 20-30 seconds into settling state.) We did observe a slight reduction in the intensity of the circulation with the increase in ramp down time, but circulation was still observed after a ramp down time of 60 seconds. The instabilities maybe a result of either the pump shutoff causing a "push/pull" in the water (even though the pump is being ramped down, there is still a slight pressure change when the pump is completely shutoff) or from the diameter change from the flocculator tube to the settling column causing eddies of turbulence in the top of the column.
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{cloak} |
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{toggle-cloak:id=Elbow Connections} |
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Connections {cloak:id=Elbow Connections} |
Another
...
possible
...
source
...
of
...
recirculation
...
in
...
the
...
settling
...
column
...
is
...
the
...
elbow
...
connections.
...
The
...
settling
...
column
...
is
...
bounded
...
by
...
both
...
an
...
elbow
...
connection
...
coming
...
into
...
the
...
column
...
and
...
going
...
out
...
of
...
the
...
column.
...
Not
...
only
...
do
...
these
...
connections
...
cause
...
a
...
sharp
...
turn
...
in
...
flow,
...
but
...
there
...
is
...
a
...
diameter
...
change
...
between
...
the
...
elbow
...
and
...
the
...
column.
...
The
...
change
...
in
...
diameter
...
could
...
be
...
causing
...
preferential
...
flow
...
in
...
the
...
column,
...
therefore,
...
a
...
recirculation
...
current.
...
The recirculation occurs over the entire diameter length scale of the column; therefore, if these large eddies could be made smaller, either viscosity will dampen the smaller eddies out or the recirculation would be confined to small scales. We tried to implement a technique similar to the use of honeycomb screens in wind-tunnels to homogenize flow by placing a bundle of plastic drinking straws that had been cut to 2.5 inches in the upper portion of the settling column where the column attaches to the elbow connection. This, the team thought, would help to reduce the preferential flow pattern that could be caused by the change in diameter. After placing the straws into the settling column, we observed that the recirculation currents were reduced. It is important to note for the next experiment that the majority of these elbow experiments were conducted with the settling column outside the turbidimeter.
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{cloak} |
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{toggle-cloak:id=Heat Induced Currents} |
...
...
Induced Currents
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Currents {cloak:id=Heat Induced Currents} |
Next,
...
it
...
was
...
hypothesized
...
that
...
the
...
heat
...
produced
...
from
...
the
...
tungsten lightbulb
...
in
...
the
...
turbidimeter
...
may
...
be
...
causing
...
the
...
recirculation.
...
Comparison
...
experiments
...
were
...
conducted
...
with
...
the
...
settling
...
column
...
outside
...
the
...
turbidimeter,
...
where
...
heat
...
from
...
the
...
lamp
...
would
...
not
...
be
...
an
...
issue,
...
and
...
inside
...
the
...
turbidimeter,
...
where
...
the
...
heat
...
could
...
have
...
an
...
affect
...
on
...
the
...
column.
...
When
...
the
...
column
...
was
...
placed
...
outside
...
the
...
turbidimeter,
...
little to no
...
recirculation
...
was
...
observed.
...
However,
...
recirculation currents were clearly visible when the column was inside the turbidimeter,
Figure. Flocs in the tube showing the ideal condition when there is no heat is applied (left) and when heat is applied to the settling tube (right).
In order to ensure that heat may be the determining factor, the team wanted to test whether another type of heat source would cause recirculation. The settling column was placed outside the turbidimeter but when the experiment came to the settling state, a team member wrapped their hand around one part of the column to provide a heat source. Once again, recirculation was visible and the team determined that heat was definitely the major contributing source to recirculation in the settling column.
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{cloak} |
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{toggle-cloak:id=Conclusions} |
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Conclusions {cloak:id=Conclusions} |
While
...
the
...
recirculation
...
problem
...
in
...
the
...
settling
...
column
...
was
...
finally
...
treated
...
by
...
removing
...
the
...
heat
...
source
...
from
...
the
...
column,
...
the
...
team
...
believes
...
that
...
the
...
problem
...
could
...
have
...
been
...
caused
...
by
...
several
...
different
...
sources
...
and
...
the
...
culmination
...
of
...
all
...
of
...
these
...
experiments
...
probably
...
helped
...
in
...
solving
...
the
...
recirculation
...
currents.
...
To
...
remove
...
the
...
heat
...
source,
...
the
...
team
...
is
...
working
...
with
...
HF
...
Scientific
...
to
...
replace
...
the
...
IR
...
lamp
...
with
...
a
...
non-heat
...
producing
...
...
...
.
...
Additionally,
...
the
...
team
...
has
...
replaced
...
the
...
elbow
...
connections
...
with
...
straight
...
connections.
...
In
...
order
...
to
...
be
...
able
...
to
...
use
...
these
...
straight
...
connections,
...
the
...
team
...
needs
...
to
...
figure
...
out
...
a
...
way
...
to
...
stop
...
the
...
flow
...
traveling
...
into
...
the
...
settling
...
column
...
after
...
the
...
settle
...
state
...
has
...
begun,
...
in
...
order
...
to
...
prevent
...
watching
...
the
...
settling
...
of
...
a
...
long
...
vertical
...
tube
...
besides
...
just
...
the
...
sample
...
in
...
the
...
settling
...
column.
...
By
...
introducing
...
a
...
slanted
...
tube
...
with
...
a
...
simple
...
trap
...
before
...
the
...
tube
...
inlet,
...
the
...
team
...
has
...
managed
...
to
...
collect
...
settling
...
flocs
...
in
...
the
...
tube
...
from
...
the
...
top
...
of
...
the
...
board
...
during
...
no-flow
...
state.
...
Also,
...
the
...
team
...
will
...
determine
...
whether
...
or
...
not
...
the
...
straws
...
are
...
helping
...
to
...
reduce
...
the
...
recirculation
...
problem
...
even
...
after
...
the
...
straight
...
connections
...
have
...
been
...
added
...
and
...
the
...
heat
...
source
...
is
...
removed,
...
or
...
if
...
they
...
are
...
unnecessary.
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{cloak} |