Power Consumption and Mixing

Efficiency in Agitation


SPECIFICATIONS:   Agitation and mixing experiment, showing terminal box with ribbon cable to A/D board in computer, Servodyne stirrer motor controller on shelf, Cole-Parmer conductivity meter connected to conductivity probe in tank at bottom, variable speed DC stirrer motor on ball-bearing slide, fiberglass shaft with downward-driving impeller, 20 liter polycarbonate tank with tracer injection funnel, lamp and optical bead sensor.


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  DETAILED OVERVIEW:   The apparatus involves of a variable speed stirrer.  The stirrer controller allows students to set the RPM in a range of 60 to 2000 RPM, and also to read the torque.  Three polycarbonate tanks with removable baffles are supplied, as are a number of turbines and propellers of various sizes. The tanks can be mounted on a roller bearing based torque table equipped with a load cell connected to an Omega panel meter.  Thus torque can be measured in two ways.  The 20 liter tank is equipped with a conductivity cell mounted near the bottom and connected to a Cole-Parmer conductivity meter driving an RTD A/D board mounted in a 486 or Pentium computer (computer supplied as an option).  The tank is also equipped with an optical sensor and lamp assembly designed to detect the approach of small plastic beads suspended in the tank.  Finally we supply a resistance heated aluminum cylinder equipped with a temperature sensor and driven by a variable transformer.  This is used to determine the effect of stirred speed and impeller and baffle design on the heat transfer coefficient.

The experiment operates in four modes, namely:

  • The students select a tank, impeller, baffle presence, and liquid (either water, Karo corn syrup, or catsup).  Then the stirrer RPM is varied over a range, and the torque vs. RPM data are collected and plotted and compared to correlations in the literature.  The data for a single torque vs. RPM run are obtained quite rapidly, but the many combinations of tank size, baffle presence, impeller design, and fluid type allow for extensive studies.

  • Using water in the 20 liter tank, the students set an RPM and inject about 30 ml of salt solution using a funnel mounted at the top of the tank.  As the tracer is dispersed, the transient conductivity at the bottom of the tank is digitized and recorded.  Then a nonlinear regression program is used to determine two parameters of a six-pool model of the flow pattern in the tank.  One of the parameters is the circulation rate in the tank, from which the mixing time can be calculated.  Ten or more tracer injection runs can be made before refilling the tank.

  • Several hundred plastic beads are added to the 20 liter tank, and the optical sensor is mounted on the side of the tank under the halogen lamp.  A BASIC program acquires the sensor data, identifies bead entries into the illuminated region, and computes the entry interval distribution.  This is typically a Poisson distribution, with characteristics that depend on stirrer speed.

  • The aluminum heat transfer probe is mounted in the 20 liter tank, the probe power is set, and the temperature of the probe is determined as a function of stirrer speed.  From these data the heat transfer coefficient can be calculated and compared to the literature.

Please contact us at spencer@columbia.edu for more details on the experiment, and for price and delivery.



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