Diffusion layer flow field interaction

Kramer et al. [272] used this same technique to compare two different flow field designs— inferdigifafed and serpentine— and their interactions with the cathode diffusion layer. If was shown thaf the bottom of the interdigitated channels got plugged with liquid water that was not removed properly. On the other hand, the serpentine FF could transport the water inside the channels more effectively, but inside the cathode DL accumulation of wafer was still evident. 

In conclusion, most of the visualization techniques that have been developed can be used to study in detail the interaction betw een diffusion layers and flow fields. However, it is important to note that there are still a number of issues with these techniques that may limit their precision. 

H. Dohle, R. Jung, N. Kimiaie, J. Mergel, and M. Muller. Interaction between the diffusion layer and the flow field of polymer electrolyte fuel cells—Experiments and simulation studies. Journal of Power Sources 124 (2003) 371-384. 

An anode configuration closely related to the AB approach is the so-called reconfigured anode, in which a thin layer of metal (such as Pt/C) or metal oxide (such as FeOx) is added to the outside of the anode GDL facing the flow field. - Unlike a normal anode electrode layer that is impregnated with ionomers for facile proton transport, this ionomer-free CO oxidation layer is hydrophobic for improved gas diffusion to help maximize the interaction between CO and O2. 

Electrokinetic effects are not usually of importance in the type of electrochemical experiments considered in this monograph, because the electric fields at the walls of the glass cells are small and significant convection is not induced. Although electrohydrodynamic flow can be induced by the interaction of an electric field with the diffuse layer near an electrode surface, the fields in the diffuse layer near the electrode surface are not sufficiently large in most electrochemical experiments to produce measurable fluid flow. However experiments in which very large fields are intentionally applied can produce... 

Electroosmotic flow is the bulk liquid motion that results when an externally applied electric field interacts with the net surplus of charged ions in the diffuse part of an electrical double layer. The term electroosmotic flow and electroosmosis are generally used interchangeably in the context of micro- and nanofluidics. Alternating current electroosmosis in which bulk flow is generated using... 

Electrokinetic phenomenon arises when the mobile portion of the diffuse double layer and an external electric field interact in the viscous shear layer near the charged surface. If an electric field is applied tangentially along a charged surface, then the electric field exerts a force on the charge in the diffuse layer. This layer is part of the electrolyte solution, and migration of the mobile ions will carry the solvent with them and cause it to flow. On the other hand, an electric field is created if the charged surface and diffuse part of the double layer are made to move relative to each other. The four electrokinetic phenomena broadly classified are... 

Since the electro-osinotic flow is induced by the interaction of the externally applied electric field with the space charge of the diffuse electric double layers at the channel walls, we shall concentrate in our further analysis on one of these 0 1 2) thick boundary layers, say, for definiteness, at... 

The principle of FFF can be explained best with the aid of Fig. 7.1. A lateral field acting across a narrow channel, composed usually of two planparallel walls, interacts with molecules or particles of a solute and compresses them to one of the channel walls in the direction of x-axis perpendicular to this wall. Hence a concentration gradient is established in the direction of the x-axis. This concentration gradient induces a diffusion flow in the reverse direction. After a certain time a steady state has been reached and the distribution of the solute across the channel can be characterized by a mean layer thickness /. At a laminar isothermal flow of a Newtonian fluid along a narrow channel, usually a parabolic velocity profile is formed inside the channel. It means that the molecules or the particles of the solute are transported in the direction of the longitudinal axis of the channel at varying... 

It is discussed in more detail below. The most basic chromatographic technique is adsorption chromatography [82,83], in which separation arises from variation in the retention of the chain units or functional groups, due to their interaction with a stationary surface. Other techniques rely on rates of sedimentation [84,85], and diffusion-adsorption phenomena (thin layer chromatography [TT-C]) [86, 87]. Thermal diffusion is the basis for thermal field flow fractionation (TFFF) [88-91], discussed later. 

The flow in the gas channels and in the porous gas diffusion electrodes is described by the equations for the conservation of momentum and conservation of mass in the gas phase. The solution of these equations results in the velocity and pressure fields in the cell. The Navier-Stokes equations are mostly used for the gas channels while Darcy s law may be used for the gas flow in the GDL, the microporous layer (MPL), and the catalyst layer [147]. Darcy s law describes the flow where the pressure gradient is the major driving force and where it is mostly influenced by the frictional resistance within the pores [145]. Alternatively, the Brinkman equations can be used to compute the fluid velocity and pressure field in porous media. It extends the Darcy law to describe the momentum transport by viscous shear, similar to the Navier-Stokes equations. The velocity and pressure fields are continuous across the interface of the channels and the porous domains. In the presence of a liquid phase in the pore electrolyte, two-phase flow models may be used to account for the interaction between the gas phase and the liquid phase in the pores. When calculating the fluid flow through the inlet and outlet feeders of a large fuel cell stack, the Reynolds-averaged Navier-Stokes (RANS), k-o), or k-e turbulence model equations should be used due to the presence of turbulence. 

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