Eiran Kochavi, Yoram Oren, Abraham Tamir, Tuvia Kravchik BENGURION UNIVERSITY OF THE NEGEV, FACULTY

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Eiran Kochavi, Yoram Oren, Abraham Tamir, Tuvia
KravchikBEN-GURION UNIVERSITY OF THE NEGEV,
FACULTY OF ENGINEERING SCIENCES, DEPARTMENT OF
CHEMICAL ENGINEERING, BEER-SHEVA, ISRAEL

• MASS TRANSFER TO SPHERE AND HEMISPHERE ELCTRODES
  BY IMPINGING JET

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The goals of this work

• Characterization of mass transfer rates to
  sphere and hemisphere electrodes, under impinging
  jet of solution.
• Characterization of mass transfer with both
  electrodes in two systems submerged in the
  electrolyte solution and unsubmerged.
• Development of theoretical model that
  characterizes the system of impinging jet to
  electrodes.

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The goals of this work (cond)

• Comparison between experimental and theoretical
  limiting currents
• Prediction of flow and concentration profiles at
  complex geometry system by using numerical
  simulation program (PHOENICS) with the proper
  boundary conditions.

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The experimental system
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Results

• Mass transfer coefficients increased as we
  increased the linear velocity of the electrolyte
  jet at the nozzle exit
• Mass transfer coefficients for the submerged
  hemisphere electrode, are higher than the
  coefficients for a sphere electrode.

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Results

• Mass transfer coefficients decrease by increasing
  the nozzle-electrode distance. However, the
  influence is more pronounced at smaller distances

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Scheme of solution domain

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Theoretical model and PHOENICS settings

• The grid distribution was divided into two
  sections (in Y-axis)
• 1- Near the electrode surface (20 cells) the
  distribution was according to a power-law
  expanding grid
• 2- In the electrolyte bulk (20 cells) the
  distribution was according to a uniform grid

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Theoretical model and PHOENICS settings
(contd)

• The model employed a 41x37 grid cells (BFC) in
  the y-z plane.
• The cells region was divided into two frames
• 1- zone where the solution flows out of the
  nozzle. The method of the interpolation of
  internal points was transfinite (TRANS)

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Theoretical model and PHOENICS
settings(contd)

• 2- zone where the solution impinging the
  electrode and flows over it to the outlet. The
  method of the interpolation of internal points
  was Laplace-like equation (LAP).

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Concentration profile of impinging jet
over a submerged sphere electrode

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Flow vectors map of impinging jet over a
submerged sphere electrode

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Theoretical results for impinging jet on a
submerged sphere electrode

• At the impingement zone we observed a sharp
  concentration profile that is influenced by jet
  impingement on the electrode surface.
• Because of the vortex (at the flow vectors map)
  on the electrode sides we observed the
  concentration profile to become wider along Z-axis

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Comparison between Experimental and Theoretical
limiting currents
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PHOENICS characteristics

• PHOENICS X11 Version 2.1.3
• Operating system UNIX
• Convergence
• The number of iterations - 2000

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PHOENICS characteristics

• Relaxation
• The Relaxation factors for the variables
• P1(pressure) linear relaxation (LINRLX)- 0.01
• C1(mass fraction of ferricyanid ion) linear
  relaxation (LINRLX) - 0.01
• C2 (mass fraction of water)linear relaxation
  (LINRLX) - 0.01
• V1(velocity vector in Y-axis) false-time-step
  relaxation (FALSDT) - 110-5
• W1(velocity vector in Z-axis) false-time-step
  relaxation (FALSDT) - 110-6

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CONCLUSIONS

• 1. Using a numerical simulation code (PHOENICS),
  enables us to develop a method to predict the
  theoretical limiting current values
• 2. Calculated limiting currents from the
  concentration field distribution, were in a good
  agreement (0.5-10 ) by comparison to
  experimental results.

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RECOMMENDATIONS

• 1. Development of theoretical model for submerged
  hemisphere electrode and compare the results with
  the experimental one
• 2. Developments of theoretical model for
  unsubmerged sphere and hemisphere electrode, and
  compare the results with the experimental one.