Effect of a net on the limiting current distribution in a parallel plate electrochemical reactor: part I. Individual effects

Abstract

In some aqueous-metal batteries or electrochemical parallel plate reactors, the spacing between the electrodes is controlled by a porous net. This net affects the limiting current distribution because it disrupts the parabolic laminar flow velocity distribution. Here, computational fluid dynamics (CFD) is used to solve the Navier-Stokes equations surrounding the inert net and the effect of the net geometry on the limiting current density is studied. The location, spacing, and number of the longitudinal and transverse ribs of the net are shown to affect the local and average current density distributions on each of the two electrodes. The effect of transverse ribs on the current distribution was found to be much higher than the longitudinal ribs. The results show that the longitudinal ribs decrease the local current density at the electrode which is not in contact and increase the current density the space between two adjacent longitudinal ribs at the electrode in contact. The transverse ribs on the other hand, increase the local current density to very high values at the electrode that is not in contact. The current density, however, falls along the flow direction as it exits the transverse-ribs region. These effects were observed to be mainly due to the changes in flow field distribution. A deviation of -40% was observed for a system of 4 longitudinal ribs and no transverse ribs at the non-dimensional axial position 0.06. For 2 transverse ribs, the deviation at the same axial position was approximately 250% of the local current density. All the results are for a net with a spacing of 0.94 9 10 -3 m.Keywords Enhancement factor Á Limiting current Á Longitudinal and transverse ribs Á Nusselt number distribution Á Obstructed channel flowThickness of transverse rib in z direction (m) h 2Thickness of longitudinal rib in z direction (m) iLocal current density (A m -2 ) i avg Plate-average current density (A m -2 ) i avg-noribs Plate-average current density for no-ribs case(A m -2 ) i * Normalized local current density (dimensionless) i local current density averaged over g direction (A m -2 ) i à Normalized local current density averaged over g direction (dimensionless) k mMass transfer coefficient (m s -1 ) L Electrode length in y direction (m) l Equivalent diameter (m) l 1Live cell spacing in x direction (m) l 2Live cell spacing in y direction (m) n l Number of longitudinal ribs n t Number of transverse ribs NuLocal Nusselt number (dimensionless)

Effect of a net on the limiting current distribution in a parallel plate electrochemical reactor. Part II: combined effects

“…As in Part I [1], all the longitudinal ribs are taken to be of the same size. The transverse ribs are the same size, although they differ from the dimensions of the longitudinal ribs (Table 1, Part I [1]).…”

“…The transverse ribs are the same size, although they differ from the dimensions of the longitudinal ribs (Table 1, Part I [1]). The full net is formed by 29 transverse and 14 longitudinal ribs.…”

“…A varies significantly with the number of ribs present. (Equations (6) and (7), in Part I [1]). Hence, a more consistent definition for enhancement factor would be based on the geometrical plate area A noribs as given in Eq.…”

“…In Part I [1], our CFD study predicted the individual effects of the longitudinal and transverse ribs as a function of the number, position and spacing of the ribs. A non-dimensional current density (i*) and a deviation factor (D) were introduced to quantify the effect of the ribs on the limiting current distribution of the two electrodes (refer Equations (11) and (15), Part I [1]). It was shown that the transverse ribs affected the limiting current density distribution more significant than the longitudinal ribs.…”

“…The electrolyte fluid enters through a diffuser (not shown) at a constant velocity 0.01 m s -1 , and the free stream velocity is well established prior to contacting the inert net. The current density (i) is calculated based on a centerline concentration and is nondimensionalized (i*) by dividing it with the corresponding average plate current density (i avg ) which is calculated based on the available plate area (Equation (11), Part I[1]). …”