Using Synchrotron Based X-Ray Imaging Technique to Understand Liquid Water Transport Phenomena in Proton Exchange Membrane (PEM) Fuel Cells
Rahimian Esfahani, Paria 1990-
Proton exchange membrane fuel cells (PEMFCs) are considered to be one of the most promising alternative power source for the automotive industry and stationary applications. However, proper water management in PEMFCs remains a challenge, hindering broader commercialization of this technology. Water as a by-product of this technology plays a complex role in the performance of a fuel cell. Too much water will flood the cell while too little water will dry the membrane. Both cases are not desired when operating fuel cells. Due to saturation and/or condensation, liquid water inevitably forms and the uniqueness is emergence of liquid water from the interconnected water pathways of gas diffusion layer (GDL) to gas flow channels. In flow channels, liquid water emerges in the form of droplets, growing to a critical size and becoming unstable. After detachment of unstable droplets, various gas-liquid flow patterns might occur alone or simultaneously along the channel depending on the operating conditions. In this work, based on a force balance analysis an analytical approach was developed to predict which flow pattern is likely to occur under different operating conditions relevant to the fuel cell. It is found that the critical droplet size depends on the superficial gas velocity, contact angle and contact angle hysteresis of growing droplets. Also, the flow pattern assignment is influenced by the contact angle and contact angle hysteresis of droplets showing that increasing contact angle hysteresis leads to the slug flow regime. The opaque nature of fuel cell components, including GDL materials, poses significant challenges to understand liquid transport in PEM fuel cells. In recent years, much attention has been placed on development of visualization techniques to detect liquid water and diagnose flooding problems in PEM fuel cells. In this study, an advanced synchrotron X-ray imaging technique available at the Canadian Light Source (CLS) was employed for the first time to visualize liquid water transport in a PEM fuel cell under different operating parameters of relevance to fuel cell operation. Due to high spatial and temporal resolution coupled with high energy photons of the X-ray beam, synchrotron radiation can obtain high-resolution images of liquid water behaviour inside the GDL and gas flow channels of fuel cells. Using the X-ray images, evolution of droplets including emergence, growth, and detachment under different superficial gas velocities, was visualized and a cyclic pattern was obtained. In addition, the evolution of the height and chord of growing droplets as well as their dynamic contact angles were analyzed quantitatively. The critical droplet sizes were also observed from X-ray images. Increasing superficial gas velocity results in a decrease in the critical detachment diameter of the droplets. Also, the critical droplet size at detachment was compared to a simplified analytical model and there was good agreement between theoretical predictions and the experimental data.
DegreeMaster of Science (M.Sc.)
DepartmentChemical and Biological Engineering
CommitteeWang, Hui; Zhu, Ning; Sumner, David
Copyright DateSeptember 2017
droplet dynamics, X-ray radiographic imaging, PEM fuel cell, water management, contact angle hysteresis