Quantification of liquid water saturation distribution in a diffusion medium is critical to establishing a basic understanding of the two-phase flow and flooding occurrence in proton exchange membrane ͑PEM͒ fuel cells. We have used X-ray microtomography to obtain high-resolution ͑10 ϫ 10 ϫ 13.4 m͒, three-dimensional images of liquid water distribution in a gas diffusion layer ͑GDL͒ during gas purge. We report on temporally resolved liquid saturation profiles across the GDL thickness and demonstrate the feasibility of using X-ray microtomography to quantify liquid water distribution at the component level. The results show that the drying rate decreases exponentially with purge time and no significant liquid water removal takes place after 6 min of purge, at room temperature.The importance of water management in a proton exchange membrane ͑PEM͒ fuel cell has been stressed widely in the literature. 1,2 There is a great need to quantitatively measure liquid water saturation in porous layers such as gas diffusion layer ͑GDL͒, microporous layer ͑MPL͒, and catalyst layer ͑CL͒, as this is a key parameter in flooding occurrence under normal operations as well as ice formation during cold start of PEM fuel cells. The water present in these porous layers, at shutdown, may freeze under subzero temperatures and makes cold start of a PEM fuel cell difficult. In practice, a fuel cell is typically purged with inert gas such as nitrogen before shutdown to remove the liquid water from it. Quantification of liquid water inside the GDL, during the purge process, can provide significant insight into the purge mechanisms and cold start characteristics of PEM fuel cells.Unfortunately, the need to quantitatively measure liquid water saturation in the porous layers of a fuel cell has yet to be addressed. Prior measurements of liquid water distribution in PEM fuel cells have included optical visualization using transparent fuel cells, 3-5 neutron radiography, 6-9 and magnetic resonance imaging ͑MRI͒. [10][11][12] Optical visualization is focused primarily on channel two-phase flow and clogging as well as liquid water distribution on the surface of GDL. Neutron radiography provides a two-dimensional map of liquid water and hence can neither distinguish between the liquid water present in various components of a fuel cell nor resolve the liquid water saturation distribution along the thickness of the GDL. In general, neutron radiographs quantify the liquid water under the assumption that all the liquid water is present in the cathode GDL only. Such an assumption may not be accurate under various operating conditions. MRI is used to visualize the transport of liquid water across a polymer electrolyte membrane. However, the presence of magnetically inductive materials like carbon makes MRI unsuitable for quantifying liquid water in GDL. Recent developments in microscale visualization techniques offer new opportunities for imaging pore-scale flow and multiphase transport in porous media. In recent years, X-ray microtomography has been used wi...

The presence of hydrates also tends to lower the heat capacities ties of sediments, but these effects are coinparative'ysmall and not useful for exploration.

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The isothermal linear and volume compressions of the explosive pentaerythritol tetranitrate (PETN) were measured to 10 GPa using a high pressure x-ray diffraction technique. The fits to the data are a/a0 = 1−2.052×10−2P + 2.230×10−3P2 − 1.041×10−4P3, and c/c0 = 1−2.832×10−2P + 3.295×10−3P2 − 1.458×10−4P3, for P<10 GPa, and ust = 2.16 + 3.007 upt − 0.682 upt2, for upt < 0.95 km/s, and ust = 2.76 + 1.71 upt, for upt ≳ 0.95 km/s, where ust = [PV0/ (1 − V/V0)]1/2, upt = [PV0(1 − V/V0)]1/2, and ρ0 = 1/V0 = 1.774 g/cm3. The shock compression Hugoniot of PETN calculated from the isothermal compression fit is us = 2.24 + 2.95 up − 0.605 up2, for up < 1.00 km/s, and us = 2.81 + 1.75 up, for up ≳ 1.00 km/s, where us is the shock velocity and up is the particle of mass velocity behind the shock wave. The isothermal compression fit and the Hugoniot are initially quadratic because of the rapid change in the strength of repulsive forces between the PETN molecules during initial compression.

Flow‐through tests are completed on a natural fracture in novaculite at temperatures of 20°C, 80°C, 120°C, and 150°C. Measurements of fluid and dissolved mass fluxes, and concurrent X‐ray CT imaging, are used to constrain the progress of mineral dissolution and its effect on transport properties. Under constant effective stress, fracture permeability decreases monotonically with an increase in temperature. Increases in temperature cause closure of the fracture, although each increment in temperature causes a successively smaller effect. The initial differential fluid pressure‐drop across the fracture increases by two orders of magnitude through the 900 h duration of the test, consistent with a reduction of an equivalent hydraulic aperture by a factor of five. Both the magnitude and rate of aperture reduction is consistent with the dissolution of stressed asperities in contact, as confirmed by the hydraulic and mass efflux data. These observations are confirmed by CT imaging, resolved to 35 microns, and define the potentially substantial influence that benign changes in environmental conditions of stress, temperature, and chemistry may exert on transport properties.

Carbon dioxide injection into coal formations provides an opportunity to sequester carbon while simultaneously enhancing methane recovery. Although powdered coal samples provide a quick indication of the gas sorption capacity, underground storage takes place within compact coal monoliths, and therefore, it may be necessary to account for in situ conditions, specifically confining stress, for meaningful estimates. This study presents the sorption rates and sorption capacities of CO 2 and CH 4 for a bituminous coal sample in a whole sample and in pulverized form. The impact of confining stress on these sorption capacities of coal cores is evaluated with a multiple-point isotherm over a prolonged time period. The kinetics of the complex, heterogeneous processes occurring in a bituminous coal sample are quantified while under confining stress. Sorption capacities for a powdered sample are 1.17 and 0.66 mmol/g for CO 2 and CH 4 , respectively. The application of 6.9 and 13.8 MPa of confining stress contributed to 39 and 64% CO 2 sorption capacity reduction. Similarly, 85 and 91% CH 4 uptake capacity reduction is observed at those confining stresses. The time-dependent gas diffusion parameters are quantified using the volumetric method with a mathematical analysis of the pressure-decay data. Carbon dioxide diffused through the coal faster than CH 4 . Initial exposure over a few days showed a rapid reduction in diffusion presumably as the macro-and mesopores filled. With longer exposure, 10 additional days, a steady slower diffusion is observed for CO 2 . The steady-state slower diffusion is achieved within a few days for CH 4 . It was found that the overall gas movement, specifically diffusion, is hindered by confining stresses and takes place at rates significantly less than in unconfined powder coal.

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