Modeling of droplet dynamics in a proton exchange fuel cell electrode channel
Fuel cells are promising alternatives to conventional energy conversion devices. Cells fueled with hydrogen are environmentally friendly and their effciency is up to 3 times higher than that of high-temperature combustion devices. However, they are still expensive and their durability is limited. On...
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| Tipo de recurso: | tesis doctoral |
| Fecha de publicación: | 2016 |
| País: | España |
| Institución: | Universitat Politècnica de Catalunya (UPC) |
| Repositorio: | UPCommons. Portal del coneixement obert de la UPC |
| Idioma: | inglés |
| OAI Identifier: | oai:upcommons.upc.edu:2117/96312 |
| Acceso en línea: | https://hdl.handle.net/2117/96312 https://dx.doi.org/10.5821/dissertation-2117-96312 |
| Access Level: | acceso abierto |
| Palabra clave: | Piles de combustible Assaigs de materials Àrees temàtiques de la UPC::Enginyeria civil |
| Sumario: | Fuel cells are promising alternatives to conventional energy conversion devices. Cells fueled with hydrogen are environmentally friendly and their effciency is up to 3 times higher than that of high-temperature combustion devices. However, they are still expensive and their durability is limited. One of the key factors in fuel cell performance is the so-called water management. Water produced within the fuel cell is evacuated through the gas channels, but at high current densities water can block the channel, limiting the current density generated in the fuel cell and thus reducing its effciency. Novel numerical analysis methods with feasible computational cost and high accuracy could help characterizing droplet transport in gas microchannels. In this work we focus on modeling and simulation of droplet emergence, deformation and detachment in fuel cell gas channels as this defines the most common mode of liquid transport in the problem at hand. However, methods presented could be applied to other problems involving a gas-liquid system, where liquid is found as small droplets or films. A semi-analytical model of a water droplet emerging from a pore of the gas diffusion layer surface in a Polymer Electrolyte fuel cell channel is developed. The geometry of the static and deformed shape is characterized and the main geometric variables (i.e. radius, height, perimeter) are assumed to depend on the contact angles only. The forces acting on the droplet are the drag force of the air and the surface tension force, which acts as adhesion force. The analytical study solves the problem of a growing droplet in a gas ow channel to see the effects of: i) air velocity and liquid mass ow in droplet deformation and oscillation; and, ii) droplet height in frequency of oscillation. The predicted values for both drag and surface tension force are higher than the results found in literature. Higher air velocity values lead to more deformation of the droplet and oscillation with lower frequency but higher amplitude. Similar effects have been identified when the liquid mass ow is increased, leading to faster detachment of the droplet. A continuum Lagrangian formulation for the simulation of droplet dynamics is proposed next. This model is developed in two and three dimensions. Using the Lagrangian framework, liquid surface can be accurately identified. The surface tension force is computed using the curvature defined by the boundary of the Lagrangian mesh. Special emphasis is given to the treatment of the surface tension term in the linearized version of governing equations. The corresponding tangent matrix allows for alleviating the severe time step size restrictions associated to the capillary wave scale. A dynamic contact angle condition is developed in order to include effects of rough surfaces in contact line evolution. Numerical examples of sessile drop in a horizontal surface and sessile drop in an inclined plane are compared to experimental results. Results show excellent agreement with experimental data. Numerical results are also compared the semi-analytical model previously developed by the authors in order to discuss the limitations of the semi-analytical approach. An embedded formulation for the simulation of immiscible coupled gas-liquid problems is then presented. Previous model considered only the liquid domain, and air ow effects were not included at the continuum level. The embedded method is particularly designed for handling gas-liquid systems where liquid represents a small fraction of the total domain. Gas and liquid are modeled using the Eulerian and the Lagrangian formulation, respectively. The Lagrangian domain (liquid) moves on top of the fixed Eulerian mesh. The location of the material interface is accurately defined by the position of the boundary mesh of the Lagrangian domain. The individual fluid problems are solved in a partitioned fashion and are coupled using a Dirichlet-Neumann algorithm. Neumann part of the coupling includes the entire stress tensor (normal and tangential components). Representation of the pressure discontinuity across the interface does not require any additional techniques being an intrinsic feature of the method. The proposed formulation is validated with several numerical examples and a convergence analysis is included as well. Finally, the embedded formulation is used to model the problem of interest, which is the dynamics of a droplet in a PEFC electrode channel. Numerical examples include a time detachment analysis, where the droplet pins and detachment occurs when a threshold value of contact angle hysteresis is reached. Results show good agreement with experimental data available, and results using the semi-analytical method again show the limitations of this model. An extension to the previous example includes water injection into the gas channel in order to compare results with previous studies in literature. |
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