Liquid water flux

Finally, the liquid water flux found in the DMFC cathode exhaust is... 

In the case of hot springs, the steam and gas flux is subordinate to the liquid water flux. Geysers are... 

The role of the porous structure and partial liquid-water saturation in the catalyst layer in performance and fuel cell water balance has been studied in Ref. 241. As demonstrated, the cathode catalyst layer fulfills key functions in vaporizing liquid water and in directing liquid-water fluxes in the cell toward the membrane and cathode outlet. At relevant current densities, the accumulation of water in the cathode catalyst layer could lead to the failure of the complete cell. The porous structure controls these functions. 

The parameter is the maximum current density for which complete conversion of liquid water into vapor is possible. It is an important characteristic for evaluating successful CCL design. High rates of evaporation and vapor diffusion facilitate liquid-to-vapor conversion. The minimum liquid water flux relative to the total water flux at z — L is given by... 

Figure 2.11. Liquid water fluxes at the CCL GDL boundary, calculated in the model of water management for the CCL with different fractions of primary and secondary pores as depicted in Figure 2.2 [25].

Local proton, water vapor and liquid water flux densities in CCL Liquid water and proton flux density at PEM I CCL boundary Critical current density for complete liquid-to-vapor conversion in the CCL Critical current density for flooding within the CCL... 

Cathode transfer coefficient Relative liquid water flux at the CCL GDL interface... 

The fluxes out of the catalyst layer are described by three interrelated parameters the current density, I, the heat flux out of the catalyst layer, i, and the water (liquid plus vapor) flux out of the catalyst layer, These last two quantities are thought of as the heat and water production of the catalyst layers, which are clearly related to the current density /, adding or subtracting heat and water flux between the membrane and catalyst layers. The heat and water fluxes between the membrane and catalyst layer depend upon the state of the membrane and the anode channel, and as such we take and as given quantities. In the flxed boundary layer at the catalyst layer, the phase change resolves the catalyst water flux into a vapor flux and a liquid water flux... 

The vapor flux will be determined by the temperature profile in the vapor-saturated two-phase region. In the event that the vapor flux is toward the membrane, the liquid water flux into the GDL can exceed the water flux out of the catalyst layer. New The phase change within the catalyst-GDL boundary layer also impacts the heat flux, N, into the GDL,... 

The jump in liquid water flux across the evaporative layer is given by (86), while the flux is constant to leading order across the two-phase region (z+, 1). 

Wa,Wc . Liquid water flux in anode and cathode channels per unit orthogonal width (z) in moles/m s. ap. Cathode transfer factor, taken to be 1[4). 

The cathode liquid water flux Wc can also be determined... 

This section presented a three-dimensional, two-phase model of the cathode and anode of a PEM Fuel Cell. The mathematical model accounts for the liquid water flux inside the gas diffusion layers by viscous and capillaiy forces and hence is capable of predicting the amount of liquid water inside the gas diffusion layers. The physics of phase change are included in this model by prescribing the local evaporation term as a function of the amount of liquid water present and the level of undersaturation, whereas the condensation has been simplified to be a function of the level of oversaturation only. 

Transient effects in permeability measurements. Liquid water flux of PFSA membrane as a function of time. Membrane thickness 4 mils and temperature 29 C. 

Resistive limiting current density (A cm ). Equation 4.221 Methanol-limiting current density (A cm ). Equation 5.222 Limiting current density due to oxygen transport in the GDL at the channel inlet (mol cm ). Equation 4.210 Liquid water flux density (Acm )... 

At the membrane-catalyst layer interface, z = 6m, flux of liquid water, flux of dissolved oxygen and current in the electrolyte phase are continuous, and therefore the following boundary conditions apply ... 

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