Figure 4 - uploaded by Vladimir Gurau
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Contour lines of liquid volume fraction and liquid streamlines in the GDL in the vicinity of the attached droplet (detailed area in (a)) during 5 seconds of simulations; (b) time step 15 seconds; (c) time step 16 seconds; (d) time step 17 seconds; (e) time step 18 seconds; (f) time step 19 seconds
Source publication
We analyze the impact of the interfacial phenomena at the macroscopic interfaces between fuel cell components on the water management and on the two-phase transport in proton exchange membrane fuel cell (PEMFC) electrodes using multiphase multi-fluid computational fluid dynamics (CFD). We present the numerical approach used to capture multiphase ph...
Contexts in source publication
Context 1
... backpressure (capillary pressure) in the droplet decreases at a faster rate during the sequence B-C-D when the droplet radius increases spontaneously. Figure 3. Evolution of the capillary pressure in the GDL at the interface with channel and in the attached droplet Figure 4 shows the contour lines of liquid saturation and the liquid water streamlines (red lines) in the GDL in the vicinity of the attached droplet (see the detailed area in (a)) at time steps 15, 16, 17, 18 and 19 seconds of simulation. Note the timescale in Figures 2 and 3. Second 15 corresponds to the moment immediately before droplet detachment (stage A in Figures 2 and 3) when the streamlines converge towards the attached droplet. ...
Context 2
... the timescale in Figures 2 and 3. Second 15 corresponds to the moment immediately before droplet detachment (stage A in Figures 2 and 3) when the streamlines converge towards the attached droplet. Time steps 16 through 19 correspond to sequence A-B when liquid water accumulates in the GDL along streamlines without exiting the GDL. Figure 5 illustrates the liquid saturation in the GDL at points 1 through 4 shown in Figure 4. Point 1 corresponds to a location above the attached droplet. ...
Citations
... Unlike low-temperature perfluorosulfonic acid (PFSA)-based membranes such as Nafion ® , PA-doped PBI membranes do not need to be rehydrated during operation to improve their proton conductivity, therefore the hydrogen and air entering the domain are considered dry gasses. Water in the cathode flow-field is considered at equilibrium with the PA aqueous solution in the membrane, and since the electro-osmotic drag coefficient of water in PA-PBI membranes is nearly zero [6], sorption/desorption of water and electro-osmotic discharge of water [20][21][22][23][24][25] are neglected in the model. be as vapor, therefore the model is single-phase. ...
... Unlike low-temperature perfluorosulfonic acid (PFSA)-based membranes such as Nafion ® , PA-doped PBI membranes do not need to be rehydrated during operation to improve their proton conductivity, therefore the hydrogen and air entering the domain are considered dry gasses. Water in the cathode flow-field is considered at equilibrium with the PA aqueous solution in the membrane, and since the electro-osmotic drag coefficient of water in PA-PBI membranes is nearly zero [6], sorption/desorption of water and electro-osmotic discharge of water [20][21][22][23][24][25] are neglected in the model. ...
The automated process of coating catalyst layers on gas diffusion electrodes (GDEs) for high-temperature proton exchange membrane fuel cells results inherently into a number of defects. These defects consist of agglomerates in which the platinum sites cannot be accessed by phosphoric acid and which are the consequence of an inconsistent coating, uncoated regions, scratches, knots, blemishes, folds, or attached fine particles—all ranging from μm to mm size. These electrochemically inactive spots cause a reduction of the effective catalyst area per unit volume (cm2/cm3) and determine a drop in fuel cell performance. A computational fluid dynamics (CFD) model is presented that predicts performance variation caused by manufacturing tolerances and defects of the GDE and which enables the creation of a six-sigma product specification for Advent phosphoric acid (PA)-doped polybenzimidazole (PBI)-based membrane electrode assemblies (MEAs). The model was used to predict the total volume of defects that would cause a 10% drop in performance. It was found that a 10% performance drop at the nominal operating regime would be caused by uniformly distributed defects totaling 39% of the catalyst layer volume (~0.5 defects/μm2). The study provides an upper bound for the estimation of the impact of the defect location on performance drop. It was found that the impact on the local current density is higher when the defect is located closer to the interface with the membrane. The local current density decays less than 2% in the presence of an isolated defect, regardless of its location along the active area of the catalyst layer.
... A simple model of the water accumulation in the system may illustrate one possible mechanism leading to oscillations under two-phase conditions. Simplifying a model for liquid water formation and transport by Gurau et al. (2009) , it is assumed that the • water transport between bulk phase and the MEA is due to a linear driving force such that the water balance reads ...
Over the last decade, nonlinear phenomena in low-temperature fuel cells as well as high-temperature fuel cells have been reported in the open literature. Experimental and theoretical studies found multiple steady states as well as periodic oscillations. The present article gives an overview of publications on this subject. Instead of sorting the analyses according to the types of fuel cells, this work used the source of the nonlinearity for classification. In the first part of the contribution, a very simple prototype fuel cell model is introduced. The model helps to give a qualitative explanation of the majority of nonlinear effects reported in literature. It is further used to identify potential sources of nonlinear behavior in reaction kinetics, membrane properties, and mass transport mechanisms. A classification scheme that is based on types of negative differential resistance (NDR) and was originally introduced by K. Krischer in Modern Aspects of Electrochemistry (Vol. 32, p. 1, Plenum Press, 1999) for electrochemical systems is applied to fuel cells. The second part of the work classifies the findings from literature according to their NDR type. Instabilities resulting not from electrochemistry but from other mechanisms such as water formation and reactant starvation are also discussed.
A two-phase flow process model for the gas diffusion layer (GDL) of a polymer electrolyte membrane fuel cell, considering also the cathode catalyst layer (CL), is presented. For this purpose, a systematic analysis of the factors affecting flooding and drying, including the liquid accumulation in the gas channel (CH), was performed using a one-dimensional reference model for the GDL and a compact channel model. The treatment proposed for the CH–GDL interface was compared with other boundary conditions in the literature. It was concluded that the liquid accumulation in the channel is determinant for estimating the steady state and transient GDL flooding, but that predicting the saturation level in the CL can help for determining operation policies for precluding flooding in the GDL–CL composite, in the absence of an adequate channel model. Bifurcation behavior, associated with the water phase change, was identified by means of the compact model.
