Effect of Geometry and Fluid Viscosity on Dynamics of Fluid-Filled Cracks: Insights From Analog Experimental Observations

Fluid-filled volumes in geological systems can change the local stress field in the host rock and may induce brittle deformation as well as crack propagation. Although the mechanisms relating fluid pressure perturbations and seismicity have been widely studied, the fluid-solid interaction inside the crack of a host rock is still not well understood. An analog experimental model of fluid intrusion in cracks between planar layers has been developed to study stress conditions at the margins and tips. A combined high-speed shadowgraph and a photoelasticity imaging system is used to visualize the fluid dynamics and induced stresses on the solid matrix. Cavitation, as well as bubble growth and collapse, occurs along the sawtooth crack margins, which produces a highly localized stress concentration to initiate new subcrack systems. The presence of the bubbles at the crack tip during fluid pressure perturbation can enhance crack propagation

Capillary penetration method for measuring wetting properties of carbon ionomer films for proton exchange membrane fuel cell (PEMFC) applications

In this work, capillary rise experiments were performed to assess the wetting properties of carbon-ionomer (CI) films. The samples were attached to a micro-balance and then immersed into liquid water to (i) measure the mass gain from the liquid uptake and (ii) estimate the (external) contact angle to water (typical value around 140°). The results showed that drying the CI films under low vacuum significantly impacted the CI film wettability. The influence of the ionomer content on the CI films’ wettability was investigated with various ionomer to carbon (I/C) ratios: 0.8, 1.0, 1.2 and 1.4. No significant variation of the contact angle to water extracted from the capillary rise experiment was measured. However, water uptake increased with the I/C ratio suggesting a more hydrophilic behavior. This observation was in good agreement with the measurement from the sessile drop method showing a slight decrease of the contact angle to water: from 155° for an I/C of 0.8 to 135° for I/C = 1.4

Water transport in complex, non-wetting porous layers with applications to water management in low temperature fuel cells

An experimental setup was designed to visualize water percolation inside the porous transport layer, PTL, of proton exchange membrane, PEM, fuel cells and identify the relevant characterization parameters. In parallel with the observation of the water movement, the injection pressure (pressure required to transport water through the PTL) was measured. A new scaling for the drainage in porous media has been proposed based on the ratio between the input and the dissipated energies during percolation. A proportional dependency was obtained between the energy ratio and a non-dimensional time and this relationship is not dependent on the flow regime; stable displacement or capillary fingering. Experimental results show that for different PTL samples (from different manufacturers) the proportionality is different. The identification of this proportionality allows a unique characterization of PTLs with respect to water transport. This scaling has relevance in porous media flows ranging far beyond fuel cells. In parallel with the experimental analysis, a two-dimensional numerical model was developed in order to simulate the phenomena observed in the experiments. The stochastic nature of the pore size distribution, the role of the PTL wettability and morphology properties on the water transport were analyzed. The effect of a second porous layer placed between the porous transport layer and the catalyst layer called microporous layer, MPL, was also studied. It was found that the presence of the MPL significantly reduced the water content on the PTL by enhancing fingering formation. Moreover, the presence of small defects (cracks) within the MPL was shown to enhance water management. Finally, a corroboration of the numerical simulation was carried out. A threedimensional version of the network model was developed mimicking the experimental conditions. The morphology and wettability of the PTL are tuned to the experiment data by using the new energy scaling of drainage in porous media. Once the fit between numerical and experimental data is obtained, the computational PTL structure can be used in different types of simulations where the conditions are representative of the fuel cell operating conditions

2D Parametric study of viscous fingering

When a low viscosity fluid is forced to displace another immiscible fluid with a higher viscosity inside of a porous media a particular flow structure called viscous fingering is generated. The study of that particular flow structure has special relevance in understanding the diffusion process and the transport characteristics of a fluid inside of a porous media. This work examined the effect of two fundamental parameters like the injected volumetric flow rate and the domain aspect ratio over the viscous fingering pattern. In order to perform that parametric study, a set of numerical simulations using the 2D network simulator model are used. A large viscosity ratio between the injected and displaced fluid is used to focus the work only on the unstable behavior state. Copyright © 2007 by ASME

Experimental investigation of Krauklis wave-propagation velocity in trilayer using dynamic photoelasticity

We developed a photoelasticity optical apparatus to investigate the Krauklis wave propagation in a fluid-filled fracture. In our experiment, we modeled a fluid-filled fracture by transparent photoelastic sensitive polycarbonate and unsensitive acrylic plates. Using a high-speed camera and linear polarized light, we visualized propagation of the Krauklis wave. We processed and analyzed the high-speed camera data using the pixel method to estimate the group velocity of the Krauklis wave. We noted that the group velocity of the Krauklis wave is much lower than that of the fluid or fracture matrix. This observation is consistent with the theoretical expressions on the Krauklis velocity at low frequencies. In addition, we observed high attenuation of the Krauklis wave in the fracture

Physical modeling of fluid-filled fractures using the dynamic photoelasticity technique

We have developed an optical apparatus based on the dynamic photoelasticity technique to visualize and analyze the propagation of the Krauklis wave within an analog fluid-filled fracture. Although dynamic photoelasticity has been used by others to study seismic wave propagation, this study adds a quantitative analysis addressing dispersion properties. We physically modeled a fluid-filled fracture using transparent photoelastic-sensitive polycarbonate and nonsensitive acrylic plates. Then we used a pixel-based framework to analyze the dispersion of a Krauklis wave excited in the fracture. Through this pixel-based framework, we thus demonstrate that the dynamic photoelasticity technique can quantitatively describe seismic wave propagation with a quality similar to experiments using conventional transducers (receivers) while additionally visualizing the seismic stress field. We observe that an increase in the fluid viscosity results in a decrease in the velocity of the Krauklis wave. We also determine the capability of the method to analyze seismic data in the case of complex geometry by modeling a sawtooth fracture. The fracture’s geometry can strongly affect the characteristics of the Krauklis wave as we note a higher Krauklis wave velocity for the sawtooth case, as well as greater perturbation of the stress field

Dynamic photoelasticity study of the Krauklis wave: The effects of fluid viscosity and fracture geometry

The Krauklis waves are slow guided waves generated in fluid-filled fractures and are characterized by their dispersive and resonating nature. We have developed an unconventional experimental apparatus to study the effects of fluid viscosity and fracture geometry as two import factors on the velocity of Krauklis waves. Our apparatus is based on the dynamic photo elasticity technique that is used to visualize the stress field generated by the propagation of the Krauklis waves. We consider two fluids with different viscosities and two fracture geometries of flat and saw-teeth. Based on our results, with the increase of the fluid viscosity, the velocity of the Krauklis wave decreases. In addition, we note that the fracture geometry strongly affects the characteristics of the Krauklis waves

Sensitivity of thermal transport and phase-change in thin porous layers to the distribution of the solid matrix

Understanding the water transport in the Porous Transport Layer (PTL) is important to improve the operational performance of polymer electrolyte membrane fuel cells (PEMFC). High water content in the PTL and flow channel decreases the transport of the gas reactants to the polymer electrolyte membrane. Dry operating conditions result in increased ohmic resistance of the polymer electrolyte membrane. Both cases result in decreased fuel cell performance. Multi-phase flow in the PTL of the fuel cell is simulated as a network of pores surrounded by the solid material. The pore-phase and the solid-phase of the PTL are generated by varying the parameters of the Weibull distribution function. In the network model, the mass transfer takes place in the pore-phase and the bulk heat transfer takes place in the both the solid-phase and liquid phase of the PTL. Previous studies have looked at the thermal and mass transport in the porous media considering the pore size distribution. In the present study, the sensitivity of the thermal and mass transport to the different arrangements of the solid-phase is carried out and the effect of different solid-phase distributions on the thermal and liquid transport in PTL of PEM fuel cell are discussed. Copyright © 2013 by ASME

Effect of porosity and thermal conductivity of a Porous Transport Layer on saturation and thermal transport in a low-temperature fuel cell

Water transport in the Porous Transport Layer (PTL) plays an important role in the efficient operation of polymer electrolyte membrane fuel cells (PEMFC). Excessive water content as well as dry operating conditions are unfavorable for efficient and reliable operation of the fuel cell. The effect of thermal conductivity and porosity on water management are investigated by simulating two-phase flow in the PTL of the fuel cell using a network model. In the model, the PTL consists of a pore-phase and a solid-phase. Different models of the PTLs are generated using independent Weibull distributions for the pore-phase and the solid-phase. The specific arrangement of the pores and solid elements is varied to obtain different PTL realizations for the same Weibull parameters. The properties of PTL are varied by changing the porosity and thermal conductivity. The parameters affecting operating conditions include the temperature, relative humidity in the flow channel and voltage and current density. A parametric study of different solid-phase distributions of the PTL and its effect on thermal, vapor and liquid transport in the PTL under different operating conditions are discussed. Copyright © 2013 by ASME