Jeffrey S. Allen

The Effects of Morphological and Wetting Properties of Porous Transport Layers on Water Movement in PEM Fuel Cells

A parametric study of the effect of wetting and in proton exchange membrane (PFM) fuel cells morphological properties of the cathode porous transport layer (PTL), also known as the gas diffusion layer (GDL), on water transport in proton exchange membrane (PEM) fuel cells has been conducted using a two-dimensional network model that captures the two-phase capillary flow behavior. The effect of PTL wetting properties is explored by considering four contact angle values. The effect of the morphology is analyzed using six pore size distributions. These six pore size distributions are generated by varying the scale and shape parameters in Weibull probability distribution functions. Also, the effect of a microporous layer (MPL) on water transport in the PTL is considered. Inclusion of the MPL resulted in a significant increase in the percolation pressure and a reduction in the PTL water content due to the formation of a few localized fingers which serve as water conduits. When defects in the MPL were considered, the PTL water content remained low with percolation pressures reduced to values similar to a PTL without an MPL.

Microscale Heat and Mass Transport of Evaporating Thin Film of Binary Mixture

Analytical and computational studies are presented to examine the effect of binary mixture (pentane/decane) on the microscale heat and mass transport of an evaporating meniscus formed inside a two-dimensional slotted pore. Mass conservation in the liquid film is combined with the momentum equations, energy balance, and normal stress balance and then scaled yielding two constitutive equations: 1) a fourth-order, nonlinear, ordinary differential equation for thin-film profile [Eq. (27)] and 2) a first-order, linear, ordinary differential equation for concentration profile [Eq. (30)]. The numerical results showed that the magnitude of distillation-driven capillary stress due to the composition gradient of a binary mixture can be larger than the thermocapillary stress due to temperature gradient while they are acting in opposite direction. Henceforth, the proof-of-concept has been established in that the binary mixture could facilitate improvement of the evaporating thin-film stability. It was also shown that the resulting stress elongated the length of the evaporating thin-film region without degradation of heat transport effectiveness.

Experimental investigation of convective structure evolution and heat transfer in quasi-steady evaporating liquid films

The stability, convective structure, and heat transfer characteristics of upward-facing, evaporating, thin liquid films were studied experimentally. Dichloromethane, chloroform, methanol, and acetone films with initial thicknesses of 2–5 mm were subjected to constant levels of superheating until film rupture occurred (typically at a thickness of around 50 μm). The films resided on a temperature controlled, polished copper plate incorporated into a closed pressure chamber free of non-condensable gasses. The dynamic film thickness was measured at multiple points using a non-intrusive ultrasound ranging system. Instability wavelength and convective structure information was obtained using double-pass schlieren imaging. The sequence of the convective structures as the film thins due to evaporation is observed to be as follows: (1) large, highly variable cells, (2) concentric rings and spirals, and (3) apparent end of convection. The transition from large, variable cells to concentric rings and spirals occurs ...

Scaling percolation in thin porous layers

Percolation in porous media is a complex process that depends on the flow rate, material, and fluids properties as well as the boundary conditions. Traditional methods of characterizing percolation rely upon visual observation of a flow pattern or a pressure-saturation relation valid only in the limit of no flow. In this paper, the dynamics of fluid percolation in thin porous media is approached through a new scaling. This new scaling in conjunction with the capillary number and the viscosity ratio has resulted in a linear non-dimensional correlation of the percolation pressure and wetted area in time unique to each porous media. The effect of different percolation flow patterns on the dynamic pressure-saturation relation can be condensed into a linear correlation using this scaling. The general trend and implications of the scaling have been analyzed using an analytical model of a fluid percolating between two parallel plates and by experimental testing on thin porous media. Cathode porous transport laye...

Visualization and Predictive Modeling of Two-Phase Flow Regime Transition With Application Towards Water Management in the Gas-Flow Channels of PEM Fuel Cells

Understanding the behavior of gas and water vapor flow through the microchannel gas flow passages of a proton-exchange membrane (PEM) fuel cells is critical to reliable fuel cell operation. Recent research efforts have illustrated the importance of capillarity on the behavior of two-phase flow (gas-liquid) in low Bond number systems; that is, systems where capillary forces are important relative to gravitational forces. Such systems include capillary tubes and microchannels as well as the gas flow channels of a PEM fuel cell. The key characteristic scaling factors for two-phase flow in capillaries have been determined. The choice of length scales and velocity scales in dimensionless groups used to characterize two-phase flow is critical to correctly delineating phase distribution. Traditional scaling for these types of flows have considered the interaction between gas and liquid phases to be primarily inertial in nature. The role of liquid film stability where the phase interaction is a combination of viscous and capillary effects is shown to be a more appropriate scaling for low-Bond number, low-Suratman number two-phase flows. Microscopic visualization at high frame rates has been used to identify the flow regime under various gas-liquid mass ratios, channel geometries and surface energies. The observations collected via high speed microscopy and corresponding pressure measurements are reported for square and circular cross-sectional microchannels with contact angles of 20 degrees (hydrophilic) and 70 degrees (hydrophobic). The effect of geometry and contact angle on the phase distribution and the pressure drop are dramatic.Copyright

A novel miniature dynamic microfluidic cell culture platform using electro-osmosis diode pumping.

An electro-osmosis (EOS) diode pumping platform capable of culturing cells in fluidic cellular micro-environments particularly at low volume flow rates has been developed. Diode pumps have been shown to be a viable alternative to mechanically driven pumps. Typically electrokinetic micro-pumps were limited to low-concentration solutions (≤10 mM). In our approach, surface mount diodes were embedded along the sidewalls of a microchannel to rectify externally applied alternating current into pulsed direct current power across the diodes in order to generate EOS flows. This approach has for the first time generated flows at ultra-low flow rates (from 2.0 nl/s to 12.3 nl/s) in aqueous solutions with concentrations greater than 100 mM. The range of flow was generated by changing the electric field strength applied to the diodes from 0.5 Vpp/cm to 10 Vpp/cm. Embedding an additional diode on the upper surface of the enclosed microchannel increased flow rates further. We characterized the diode pump-driven fluidics in terms of intensities and frequencies of electric inputs, pH values of solutions, and solution types. As part of this study, we found that the growth of A549 human lung cancer cells was positively affected in the microfluidic diode pumping system. Though the chemical reaction compromised the fluidic control overtime, the system could be maintained fully functional over a long time if the solution was changed every hour. In conclusion, the advantage of miniature size and ability to accurately control fluids at ultra-low volume flow rates can make this diode pumping system attractive to lab-on-a-chip applications and biomedical engineering in vitro studies.

Micro- and nanofluidics for energy conversion

The application of micro- and nanotechnology in energy conversion is discussed. Fundamental understanding of micro- and nanoscale transport has significantly improved the operation reliability, durability, and system simplicity of fuel cell systems. On the other hand, energy harvesting from streaming potential holds the hope to be practical if the efficiency can be further improved to the level predicted by current theory. The development of micro- and nanofluidics has been consistently fast in pace and radical in progress. It can be expected to open many doors for the exploration of new energy conversion technologies.

Visualization of an Evaporating Thin Layer during the Evaporation of a Nanofluid Droplet

During the evaporation of a droplet, there exists an evaporating thin layer that is difficult to visualize because of optical restrictions. The present study visualized this thin layer by using a reflective-mode, confocal microscope that can provide improved signal-to-noise focal plane imaging over traditional optical microscopy while simultaneously serving as an interferometer when imaging thin liquid films. The spatial distribution of the evaporating thin layer thickness was determined from interferometric fringe analysis. Three distinct fringe patterns, or regions, were observed depending on the nanoparticle concentration. These regions are referred to as uniform, slow extension, and rapid extension. The formation of the three regions is closely associated with the variation of the evaporating thin layer thickness of a nanofluid droplet. The nanoparticle bank formed near the contact line region substantially affects the rate of change in the evaporating thin layer thickness that increases with the nanoparticle concentration.

A new experiment for investigating evaporation and condensation of cryogenic propellants

Passive and active technologies have been used to control propellant boil-off, but the current state of understanding of cryogenic evaporation and condensation in microgravity is insufficient for designing large cryogenic depots critical to the long-term space exploration missions. One of the key factors limiting the ability to design such systems is the uncertainty in the accommodation coefficients (evaporation and condensation), which are inputs for kinetic modeling of phase change. A novel, combined experimental and computational approach is being used to determine the accommodation coefficients for liquid hydrogen and liquid methane. The experimental effort utilizes the Neutron Imaging Facility located at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland to image evaporation and condensation of hydrogenated propellants inside of metallic containers. The computational effort includes numerical solution of a model for phase change in the contact line and thin film regions as well as an CFD effort for determining the appropriate thermal boundary conditions for the numerical solution of the evaporating and condensing liquid. Using all three methods, there is the possibility of extracting the accommodation coefficients from the experimental observations. The experiments are the first known observation of a liquid hydrogen menisci condensing and evaporating inside aluminum and stainless steel cylinders. The experimental technique, complimentary computational thermal model and meniscus shape determination are reported. The computational thermal model has been shown to accurately track the transient thermal response of the test cells. The meniscus shape determination suggests the presence of a finite contact angle, albeit very small, between liquid hydrogen and aluminum oxide.

Modeling and Diagnostics of Fuel Cell Porous Media for Improving Water Transport

When a fuel cell is operating at high current density, water accumulation is a significant cause of performance and component degradation. Investigating the water transport inside the fuel cell is a challenging task due to opacity of the components, the randomness of the porous materials, and the difficulty in gain access to the interior for measurement due to the small dimensions of components. Numerical simulation can provide a good insight of the evolution of the water transport under different working condition. However, the validation of those simulations is remains an issue due the same experimental obstacles associated with in-situ measurements. The discussion herein will focus on pore-network modeling of the water transport on the PTL and the insights gained from simulations as well as in the validation technique. The implications of a recently published criterion to characterize PTL, based on percolation theory, and validate numerical simulation are discussed.