Please use this identifier to cite or link to this item: http://scholarbank.nus.edu.sg/handle/10635/79261
Title: Computationally - efficient modeling & simulation of transport phenomena in fuel cell stacks
Authors: ASHWINI KUMAR SHARMA
Keywords: Fuel cell stacks, Mathematical modeling, Transport phenomena, Model reduction, Decoupling of the cells
Issue Date: 8-Jan-2013
Source: ASHWINI KUMAR SHARMA (2013-01-08). Computationally - efficient modeling & simulation of transport phenomena in fuel cell stacks. ScholarBank@NUS Repository.
Abstract: In the last two decades, mathematical modeling and simulations have come to play an important role in the research and development of fuel cells. In order to capture the wide array of physicochemical processes that occur inside the cell, the models need to consider transport of mass, momentum, species, energy, and charge in multiple length scales and result in a highly coupled system of non-linear partial differential equations. As such, applying these models to stacks, comprising tens or even hundreds of single cells, will come at a hefty computational cost, both in terms of memory usage and execution time. It is therefore of interest to derive computationally-efficient strategies that can solve for and predict the local behavior of each cell in a stack at sufficiently low cost, whilst preserving all the essential physics. To reduce the overall complexity and associated computational cost for detailed mechanistic stack models, this thesis aims to investigate and exploit the underlying mathematical nature of the transport equations. First, a hybrid modeling strategy is proposed for fuel cell stacks, in which the steady-state transport equations are classified based on their regions of influence: conservation of mass, momentum and species are local to cells and their governing equations can be reduced mathematically by exploiting the slenderness at the single cell level; whereas the conservation of heat and charge are global to the stack and thus retain the original elliptic nature. The thesis further investigates the charge transport phenomena taking place across the cells. In this regard, steady-state conservation of charge in a bipolar plate between two cells is analyzed, and a dimensionless number is identified that quantifies the degree of local current density coupling across the cells. The same number is found to govern the interchangeability of potentiostatic and galvanostatic boundary conditions for fuel cells. Further, a decoupled stack simulation strategy is proposed arguing that the electrically and thermally decoupled units can be found in a fuel cell stack. The decoupled units are not influenced by their neighboring units and thus can be simulated one by one repeatedly; simulation of all the units provides a solution of a complete stack model. The thesis, thus far, demonstrates various concepts with PEMFC stack equipped with porous flow fields that allow a reduction in dimensionality as well as the linear Darcy law instead of the nonlinear Navier Stokes equations; the latter is more challenging to solve. For fuel cells equipped with straight rectangular flow channels, one needs to resolve the three-dimensional (3D) Navier Stokes equations which add to the required computational resources. In this context, a velocity-vorticity formulation is implemented to tackle the weakly compressible parabolized steady 3D Navier Stokes equations in a channel with a permeable wall - a situation that occurs in fuel cells. In summary, this thesis proposes and investigates computational-efficient strategies for modeling and simulation of the transport phenomena in fuel cell stacks. The scalability and associated low computational cost of such strategies open up the possibilities for wide-ranging parametric studies and optimization of stacks.
URI: http://scholarbank.nus.edu.sg/handle/10635/79261
Appears in Collections:Ph.D Theses (Open)

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