1. MASS TRANSFER TO SPHERE AND HEMISPHERE ELCTRODES BY IMPINGING JET
Eiran Kochavi, Yoram Oren, Abraham Tamir, Tuvia Kravchik BEN-GURION UNIVERSITY OF THE NEGEV, FACULTY OF ENGINEERING SCIENCES, DEPARTMENT OF CHEMICAL ENGINEERING, BEER-SHEVA, ISRAEL

2. 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.

3. The goals of this work (con’d):
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.

4. The experimental system

5. 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.

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

7. Scheme of solution domain

8. 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

9. Theoretical model and PHOENICS settings (cont’d)
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)

10. Theoretical model and PHOENICS settings(cont’d)
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).

11. Concentration profile of impinging jet over a submerged sphere electrode

12. Flow vectors map of impinging jet over a submerged sphere electrode

13. 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

14. Comparison between Experimental and Theoretical limiting currents

15. PHOENICS characteristics
PHOENICS X11 – Version 2.1.3
Operating system – UNIX
Convergence
The number of iterations

16. 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)
C2 (mass fraction of water)linear relaxation (LINRLX)
V1(velocity vector in Y-axis) false-time-step relaxation (FALSDT) - 1*10-5
W1(velocity vector in Z-axis) false-time-step relaxation (FALSDT) - 1*10-6

17. 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 ( %) by comparison to experimental results.

18. 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.