Studies Of Multi-phase Transport Phenomena In Fuel Cells Using The Lattice Boltzmann Method

A fuel cell is the energy device which directly converts the chemical energy into electrical energy. The fuel cell is considered to be one of the most promising power sources for future stationary and automobile applications because of its high energy-conversion efficiency and environment-friendly operation. The proton exchange membrane fuel cell (PEMFC) has attracted much attention due to its unique advantages in transportation applications. Although many studies have been conducted and significant improvements have been achieved, a number of key issues, which are the major barriers for the commercialization of PEMFCs, still remain. The present thesis focuses on the water management problems in a PEMFC. Given the existing difficulties and limitations in experimentally detecting and observing liquid water transport and distribution in the PEMFC, the lattice Boltzmann method (LBM) is used for direct numerical simulation of the water transport phenomena. A computational program was developed for simulating the multi-phase and multi-component fluid flows, and the detail studies and conclusions are summarized as follows.First, liquid water transport inside the gas diffusion layer (GDL) and micro-porous layer (MPL) plays a crucial role in water management in a PEMFC. In this thesis, based on the reconstructed microstructures of the GDL and MPL, a two-phase LBM model is used to study the liquid water transport mechanisms in the GDL and MPL. The effects of microstructures and a large perforated pore or crack through the two porous layers on liquid water transport are examined in detail. Results indicate that liquid water transport in the porous media is mainly dictated by the capillary force. The MPL plays a significant role in controlling liquid water transport. A large perforated pore or crack through the two porous layers significantly affects liquid water transport and distribution in a PEMFC.Second, after the liquid water, which is produced inside a PEMFC, moves through the GDL, it then emerges on the interface between the GDL and gas channel (GC), in the form of liquid droplets. It is a key issue in the water management of a PEMFC to study the gas and liquid two-phase transport phenomena on the GDL surface. In this thesis, the two-phase LBM model is used to examine the transport phenomena, focusing mainly on parametric effects, including the distance between different pores, gas flow velocity, different pore size, and surface wetting property, on liquid droplets interactions. Results show that the increased pore distance, gas velocity, and the hydrophobicity of the GDL surface can prevent liquid droplets interactions and enhance liquid water removal.Third, the liquid water droplets move into the gas channel (GC) after it is detached from the GDL surface. Although many studies have been carried out for investigating the transport processes inside the GC of a PEMFC, a number of fundamental issues still remain, particularly concerning the detailed liquid water transport phenomena. In this thesis, the two-phase LBM model is further applied to study the liquid droplet transport phenomena in different configurations of the gas channel turning region. Results show that the liquid droplets tend to accumulate in a right-angled turning region and then become difficult to be removed. On the contrary, the U-shaped turning region is beneficial to liquid water removal. Increasing the gas flow velocity and surface contact angle can significantly improve liquid droplet movement.Fourth, since the transport process in the catalyst layer (CL) of a PEMFC is very complex, few pore-level studies in a CL have been reported in the open literature. In this thesis, a detailed catalyst layer is numerically reconstructed in three dimensions, with slight simplifications of the realistic microstructures. The two-dimensional LBM model used in the previous studies are further extended into a three-dimensional one. The multi-phase multi-component LBM model is then used for a preliminary study of the transport phenomena in the CL of a PEMFC. Results clearly illustrate the significant effect of the detailed porous structures on the multi-phase multi-component transport processes.In summary, the present thesis focuses on the fundamental issues of water management in the PEMFC. Various two-phase transport processes, including those in CL, MPL, GDL, and GC, are studied in detail to elucidate the pore-level liquid water transport mechanisms. Results provide useful information for improving water management in the PEMFC and lay a solid foundation for future PEMFC model development and optimization.