Scholarly Work - Chemical & Biological Engineering

Permanent URI for this collectionhttps://scholarworks.montana.edu/handle/1/8718

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Development of two-phase flow regime specific pressure drop models for proton exchange membrane fuel cells
(2015-01) Anderson, Ryan; Eggleton, Erica; Zhang, Lifeng
Water is an inevitable byproduct in proton exchange membrane fuel cells that can lead to complex two-phase flow throughout the cell's components, including the flow field channels utilized for gas delivery. A modified Lockhart–Martinelli (LM) approach based on unique water introduction through the gas diffusion layer is used here to predict the gas–liquid pressure drop in these channels by modifying the Chisholm parameter C. This paper exclusively uses experimental data of two-phase flow multipliers from four sources in the literature, all of which are obtained from active fuel cell operation. C does not appear to change strongly as a function of temperature, relative humidity, or air stoichiometry, but does vary significantly with the current density. This is especially true at low current densities (<500 mA cm−2). To capture this behavior, C is defined as a flow regime dependent parameter based on a flow regime map from the active fuel cell data. In addition to the traditionally used slug, film, and single-phase regimes, an ‘accumulating’ flow regime is proposed to capture the behavior of C and two-phase flow multipliers at low current densities. The proposed accumulating flow regime is consistent with visual observation reported in the literature. In addition, the developed LM approach can be employed to optimize fuel cell flow field design and operation.
Packed Bed Thermal Energy Storage: A Simplified Experimentally Validated Model
(2015-12) Anderson, Ryan; Bates, Liana; Johnson, Erick; Morris, Jeffrey F.
Thermal energy storage in packed beds is receiving increased attention as a necessary component for efficient implementation of concentrated solar power plants. A simplified, one-equation thermal model for the behavior of a packed bed is presented for α-alumina as solid storage material and air as the heat transfer fluid. The model successfully predicts the thermocline behavior over time. Two flow rates during storage are presented for alumina in a cylindrical packed bed. Temperature-dependent thermophysical properties are utilized to accurately model the systems. An additional study of air and alumina at high temperature (700 °C) is presented to further highlight the importance of variable thermophysical properties in real models. Explicit consideration is given to explain situations where the modeling approach is valid based on a Biot number analysis and the thermal capacities of the solid and fluid.
Quantifying Cathode Water Transport via Anode Relative Humidity Measurements in a Polymer Electrolyte Membrane Fuel Cell
(2017-08) Battrell, Logan; Trunkle, Aubree; Eggleton, Erica; Zhang, Lifeng; Anderson, Ryan
A relative humidity (RH) measurement based on pressure drop analysis is presented as a diagnostic tool to experimentally quantify the amount of excess water on the cathode side of a polymer electrolyte membrane fuel cell (PEMFC). Ex-situ pressure drop calibration curves collected at fixed RH values, used with a set of well-defined equations for the anode pressure drop, allows for an estimate of in-situ relative humidity values. During the in-situ test, a dry anode inlet stream at increasing flow rates is used to create an evaporative gradient to drive water from the cathode to the anode. This combination of techniques thus quantitatively determines the changing net cell water flux. Knowing the cathodic water production rate, the net water flux to the anode can explain the influence of liquid and vapor transport as a function of GDL selection. Experimentally obtained quantified values for the water removal rate for a variety of cathode gas diffusion layer (GDL) setups are presented, which were chosen to experimentally vary a range of water management abilities, from high to low performance. The results show that more water is transported to the anode when a GDL with poor water management capabilities is used, due to the higher levels of initial saturation occurring on the cathode. At sufficiently high concentration gradients, the anode removes more water than is produced by the reaction, allowing for the quantification of excess water saturating the cathode. The protocol is broadly accessible and applicable as a quantitative diagnostic tool of water management in PEMFCs.