Recirculation of flocs in settling column
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h1. Combating the Recirculation problem in the 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} It was recently noticed by our team that after the loading state had switched to the settling state, a slow but seemingly never-ending recirculation of particles was occurring in the settling column. This recirculation is affecting our settling data, and maybe the important clue into why we haven't been seeing "great" settling curves as we would have expected in our results. To rid our setup of this recirculation problem will give us more accurate results. {cloak} h2. |
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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{toggle-cloak:id=Possible Causes} |
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Causes {cloak:id=Possible Causes} It was hypothesized that this could have been a result of the elbow connections, the change in diameter tubing from the flocculator to the settling column, the valve shutoff, upflow versus downflow through the column, heat induced currents from the turbidimeter lamp, or the abrupt stopping of the pumps in the transition of states. All of these ideas were put to the test in simple visual experiments. {cloak} h2. |
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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{toggle-cloak:id=Visual Experimentation} |
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Experimentation {cloak:id=Visual Experimentation} |
Several
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visual
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experiments
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were
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conducted
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to
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determine
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what
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the
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actual
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cause
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of
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the
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reciruclation
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was
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from.
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First,
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we
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conducted
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experiments
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changing
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the
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pump
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control
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shutoff,
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then
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we
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experimented
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with
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the
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changing
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diameter
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of
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the
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elbow
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to
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the
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column,
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and
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lastly,
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we
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experimented
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with
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the
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heat
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factor.
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{cloak} h3. |
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{toggle-cloak:id=Pump Control Shutoff} |
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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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{toggle-cloak:id=Elbow Connections} |
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Connections {cloak:id=Elbow Connections} |
Another
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possible
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source
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of
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recirculation
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in
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the
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settling
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column
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is
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the
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elbow
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connections.
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The
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settling
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column
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is
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bounded
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by
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both
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an
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elbow
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connection
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coming
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into
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the
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column
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and
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going
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out
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of
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the
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column.
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Not
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only
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do
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these
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connections
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cause
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a
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sharp
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turn
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in
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flow,
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but
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there
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is
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a
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diameter
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change
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between
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the
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elbow
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and
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the
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column.
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The
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change
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in
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diameter
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could
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be
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causing
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preferential
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flow
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in
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the
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column,
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therefore,
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a
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recirculation
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current.
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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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{toggle-cloak:id=Heat Induced Currents} |
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Induced Currents
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Currents {cloak:id=Heat Induced Currents} |
Next,
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it
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was
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hypothesized
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that
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the
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heat
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produced
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from
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the
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tungsten lightbulb
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in
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the
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turbidimeter
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may
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be
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causing
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the
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recirculation.
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Comparison
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experiments
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were
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conducted
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with
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the
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settling
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column
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outside
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the
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turbidimeter,
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where
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heat
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from
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the
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lamp
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would
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not
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be
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an
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issue,
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and
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inside
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the
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turbidimeter,
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where
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the
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heat
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could
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have
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an
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affect
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on
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the
...
column.
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When
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the
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column
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was
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placed
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outside
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the
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turbidimeter,
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little to no
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recirculation
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was
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observed.
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However,
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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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{toggle-cloak:id=Conclusions} |
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Conclusions {cloak:id=Conclusions} |
While
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the
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recirculation
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problem
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in
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the
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settling
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column
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was
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finally
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treated
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by
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removing
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the
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heat
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source
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from
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the
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column,
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the
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team
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believes
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that
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the
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problem
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could
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have
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been
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caused
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by
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several
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different
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sources
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and
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the
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culmination
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of
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all
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of
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these
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experiments
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probably
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helped
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in
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solving
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the
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recirculation
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currents.
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To
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remove
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the
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heat
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source,
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the
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team
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is
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working
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with
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HF
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Scientific
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to
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replace
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the
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IR
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lamp
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with
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a
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non-heat
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producing
...
...
...
.
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Additionally,
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the
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team
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has
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replaced
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the
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elbow
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connections
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with
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straight
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connections.
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In
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order
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to
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be
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able
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to
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use
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these
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straight
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connections,
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the
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team
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needs
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to
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figure
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out
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a
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way
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to
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stop
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the
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flow
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traveling
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into
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the
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settling
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column
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after
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the
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settle
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state
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has
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begun,
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in
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order
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to
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prevent
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watching
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the
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settling
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of
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a
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long
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vertical
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tube
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besides
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just
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the
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sample
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in
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the
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settling
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column.
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By
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introducing
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a
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slanted
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tube
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with
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a
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simple
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trap
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before
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the
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tube
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inlet,
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the
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team
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has
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managed
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to
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collect
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settling
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flocs
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in
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the
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tube
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from
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the
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top
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of
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the
...
board
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during
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no-flow
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state.
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Also,
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the
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team
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will
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determine
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whether
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or
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not
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the
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straws
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are
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helping
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to
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reduce
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the
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recirculation
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problem
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even
...
after
...
the
...
straight
...
connections
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have
...
been
...
added
...
and
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the
...
heat
...
source
...
is
...
removed,
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or
...
if
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they
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are
...
unnecessary.
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