Distribution, current density and

In the electrochemical cell the workpiece is the anode and the tool is the cathode. The electrolyte is fed through the tool at a rapid flow rate, 9—60 m s in such a way that the supply of electrolyte is uniform over the surface being machined. In practice the electrolyte flow pattern is as important as the arrangement of conducting surfaces on the tool in determining the current density distribution and both factors must be considered in the design of the tool. 

It is the design of the tool that determines the current density distribution and hence the shape of the feature being machined although, in general, the tool will not be exactly the same shape as the feature. 

Beming et al. [55] used the imified approach to develop a three-dimensional non-isothermal fuel cell model. The model studied reactant concentrations, current density distributions and temperature gradients within the cell as well as water flux and species... 

During operation, temperature distribution can be determined by the current density distribution and related heat release from the surface. This implies that the thermal stress can be properly treated only by the combined analyses on electrochemical performance, gaseous concentration, and thermal conduction. When stacks will be cooled down, temperature distribution can be determined by cooling mechanism so that the thermal stress distribution may change from those under operation. 

The problem of current density distribution and the need to control electrode potential distribution, in comparison to chemical reactors, is an additional problem tn electrochemical reactors. This aspect and some additional factors relevant to electrolytic processes will be reviewed in section 2.S.6 as a prelude to the discussion of cell design. 

A thorough analysis of these effects has been given by Chang et al. (2006). It shows that not only the fluid dynamics of the reactants and coolant in the manifold can have a distinct influence on the cell homogeneity, but also the resistivity of the bus plates can lead to systematic current density distribution and cell voltage differences of individual cells. 

Following this, it is possible to evaluate the balance between the damage related to the galvanic contribution estimated from the Faraday s law applied to the calculated surface current density distribution and the damage related to the dissolution related to the pH change relation. 

FIGURE 5-11. Water profiles in fuel cell a) assuming uniform current density distribution and isothermal conditions b) assuming realistic current density distribution and air temperature increase from inlet to outlet. 

The fluid flow, heat, and mass transfer in fnel ceU are commonly expressed by Navier-Stokes equation, energy equation, mass conservation equation, the associated chemical reaction equations, the charge conservation equation for the current density distribution, and their derivatives [132]. The theoretical maximum electrochemical work that a fuel cell can do may be determined by the Nemst equation as follows ... 

---