Liquid Water Transport in the Reactant Channels of Proton Exchange Membrane Fuel Cells

Water management has been identified as a critical issue in the development of PEM fuel cells for automotive applications. Water is present inside the PEM fuel cell in three phases, i.e. liquid phase, vapor phase and mist phase. Liquid water in the reactant channels causes flooding of the cell and blocks the transport of reactants to the reaction sites at the catalyst layer. Understanding the behavior of liquid water in the reactant channels would allow us to devise improved strategies for removing liquid water from the reactant channels. In situ fuel cell tests have been performed to identify and diagnose operating conditions which result in the flooding of the fuel cell. A relationship has been identified between the liquid water present in the reactant channels and the cell performance. A novel diagnostic technique has been established which utilizes the pressure drop multiplier in the reactant channels to predict the flooding of the cell or the drying-out of the membrane. An ex-situ study has been undertaken to quantify the liquid water present in the reactant channels. A new parameter, the Area Coverage Ratio (ACR), has been defined to identify the interfacial area of the reactant channel which is blocked for reactant transport by the presence of liquid water. A parametric study has been conducted to study the effect of changing temperature and the inlet relative humidity on the ACR. The ACR decreases with increase in current density as the gas flow rates increase, removing water more efficiently. With increase in temperature, the ACR decreases rapidly, such that by 60°C, there is no significant ACR to be reported. Inlet relative humidity of the gases does change the saturation of the gases in the channel, but did not show any significant effect on the ACR. Automotive powertrains, which is the target for this work, are continuously faced with transient changes. Water management under transient operating conditions is significantly more challenging and has not been investigated in detail. This study begins to investigate the effects of changing operating conditions on liquid water transport through the reactant channels. It has been identified that rapidly increasing temperature leads to the dry-out of the membrane and rapidly cooling the cell below 55°C results in the start of cell flooding. In changing the operating load of the PEMFC, overshoot in the pressure drop in the reactant channel has been identified for the first time as part of this investigation. A parametric study has been conducted to identify the factors which influence this overshoot behavior

Effect of temperature on the in-plane permeability of the gas diffusion layer of a PEM fuel cell

Proton Exchange Membrane (PEM) Fuel Cells convert hydrogen into water by causing electrochemical reaction with oxygen, producing an electric current which can be used to power electric motors. This is seen as a viable alternative to the Internal Combustion Engine which operates on fossil fuels and is often blamed for contribution to the global climate change. Due to the low temperature operation, compared to other forms of fuel cells, it is possible to adapt the PEM Fuel Cell for automotive application. By running on hydrogen, the PEM Fuel Cell promises to enable a clean mode of transport. Water vapor transport inside the fuel cell takes place by two primary mechanisms: diffusion and permeability. Diffusion is important in the through-plane direction, whereas permeability is most important in the in-plane direction. Some work has been done to measure the permeability; it has been correlated with the porosity. However, the work has focused on the permeability at room temperature for ease of measurement. The PEM fuel cell works most efficiently between 60⁰C and 95⁰C. In this work, we direct our efforts at verifying whether there is any change in permeability in the in-plane direction with change of temperature. The in-plane permeability has been measured at 25⁰C, 40⁰C, 60⁰C and 80⁰C. The work has also obtained qualitative changes in permeability with the introduction of a Microporous Layer and impregnation of PTFE onto the Gas Diffusion Layer

Ion-acoustic solitary waves in a partially degenerate plasma

The propagation of arbitrary amplitude ion-acoustic (IA) solitary waves (SWs) is studied in unmagnetized, collisionless, homogeneous electron-positron-ion (e-p-i) plasmas with finite temperature degeneracy of both electrons and positrons. Starting from a set of fluid equations for classical ions and Fermi-Dirac distribution for degenerate electrons and positrons, a linear dispersion relation for IA waves is derived. It is seen that the wave dispersion is significantly modified due to the presence of positron species and the effects of finite temperature degeneracy of electrons and positrons. In the nonlinear regime, the Sagdeev's pseudopotential approach is employed to study the existence domain and the evolution of nonlinear IA-SWs in terms of the parameters that are associated with the finite temperature degeneracy, the background number densities, and the thermal energies of electrons and positrons. It is found that in contrast to classical electron-ion plasmas both the subsonic and supersonic IA-SWs can exist in a partially degenerate e-p-i plasma.Comment: 9 pages, 5 figures; Revised version to appear in IEEE Transactions on Plasma Scienc

Micro-Farming Situation Observed through Manual Discriminant Analysis

Micro-farming situation is a relatively homogeneous farming situation conceptualised by the farmers themselves on the basis of certain agro-ecological and socio-economic criteria. These criteria are more holistic in its coverage than those used during identifying conventional recommendation domains. The article outlines the rationale of observing a micro-farming situation, proposes a method to carry on such exercises and points to certain areas of further research. Not only technology positioning, but also a methodological insight can be developed from such exercises. With some modifications this can be used as a participatory research tool during the study of farming systems, especially in the resource-poor CRD agriculture

Effect of guidewire on contribution of loss due to momentum change and viscous loss to the translesional pressure drop across coronary artery stenosis: An analytical approach

<p>Abstract</p> <p>Background</p> <p>Guidewire (GW) size and stenosis dimensions are the two major factors affecting the translesional pressure drop. Studying the combined effect of these parameters on the mean pressure drop (Δ<it>p</it>) across the stenosis is of high practical importance.</p> <p>Methods</p> <p>In this study, time averaged mass and momentum conservation equations are solved analytically to obtain pressure drop-flow, Δ<it>p</it>-<it>Q</it>, curves for three different percentage area blockages corresponding to moderate (64%), intermediate (80%), and severe (90%) stenoses. Stenosis is considered to be axisymmetric consisting of three different sections namely converging, throat, and diverging regions. Analytical expressions for pressure drop are obtained for each of these regions separately. Using this approach, effects of lesion length and GW insertion on the mean translesional pressure drop and its component (loss due to momentum change and viscous loss) are analyzed.</p> <p>Results and Conclusion</p> <p>It is observed that for a given percent area stenosis (AS), increase in the throat length only increases the viscous loss. However, increase in the severity of stenosis and GW insertion increase both loss due to momentum change and viscous loss. GW insertion has greater contribution to the rise in viscous loss (increase by 2.14 and 2.72 times for 64% and 90% AS, respectively) than loss due to momentum change (1.34% increase for 64% AS and 25% decrease for 90% AS). It also alters the hyperemic pressure drop in moderate (48% increase) to intermediate (30% increase) stenoses significantly. However, in severe stenoses GW insertion has a negligible effect (0.5% increase) on hyperemic translesional pressure drop. It is also observed that pressure drop in a severe stenosis is less sensitive to lesion length variation (4% and 14% increase in Δ<it>p </it>for without and with GW, respectively) as compared to intermediate (10% and 30% increase in Δ<it>p </it>for without and with GW, respectively) and moderate stenoses (22% and 48% increase in Δ<it>p </it>for without and with GW, respectively). Based on the contribution of pressure drop components to the total translesional pressure drop, it is found that viscous losses are dominant in moderate stenoses, while in severe stenoses losses due to momentum changes are significant. It is also shown that this simple analytical solution can provide valuable information regarding interpretation of coronary diagnostic parameters such as fractional flow reserve (FFR).</p

Carbon felt electrodes for redox flow battery: Impact of compression on transport properties

In a flow battery setup, carbon felt materials are compressed to obtain higher performance from the battery. In this work, a commercially available carbon felt material, commonly used as electrodes in Vanadium Redox Flow Battery setups was evaluated for the transport properties (diffusivity, permeability, pressure drop required for maintaining flow, among others) while under seven set levels of compression, using an image analysis coupled with pore network modeling approach. X-ray computed tomography has been used to obtain the microstructure of a commercially available electrode under compressed conditions. An open-source pore network modeling tool, OpenPNM has been used to investigate the transport properties of the porous felt material at each of the set compression levels. The results from the modeling are compared against experimentally obtained electrolyte transport patterns visualized using synchrotron X-ray radiography. The electrical resistance of the carbon felt electrode was measured experimentally using a four-probe method. The compression resulted in a 58% reduction in permeability, and a 25% reduction in single-phase diffusion. This combination of ex-situ characterization of the electrical and fluid transport through the electrodes provides valuable data for modeling flow battery systems, and validating hypothesis from in situ testing

Understanding biophysical and socio-economic determinants of maize (Zea mays L.) yield variability in eastern India

AbstractThe aim of this paper was to investigate the key factors limiting maize (Zea mays L.) productivity in eastern India to develop effective crop and nutrient management strategies to reduce yield gap. A series of farm surveys was conducted in two distinct agro-ecological zones of eastern India to evaluate the importance of crop management and structural constraints for maize productivity in a range of socio-economic settings prevalent in smallholder farms. Surveys revealed yield gap and yield variations among farms across growing seasons. Lower yields of farmers were mainly associated with farmer's ethnic origin, availability of family labor, land ownership, legumes in cropping sequence, irrigation constraints, seed type, optimal plant population, labor and capital investment, and use of organic manure. These constraints varied strongly between sites as well as growing seasons. Stochastic Frontier Analysis suggested intensification of farm input use and removal of socio-economic and structural constraints for increasing efficiency in maize production. The use of multivariate classification and regression tree analysis revealed that maize yield was affected by multiple and interacting production constraints, differentiating the surveyed farms in six distinct resource groups. These farm types lend scope for introducing typology-specific crop management practices through appropriate participatory on-farm evaluation/trials. Summarily, this research indicated that interacting production constraints should be addressed simultaneously, considering the need of different farm types, if significant productivity improvements are to be achieved. This will be, however, more challenging for less endowed farms due to lack of social and financial capital to improve management intensity.A typology-specific farm support strategy may be formulated to offset this lack of entitlement among resource-poor farmers

In vitro Quantification of Guidewire Flow-Obstruction Effect in MODEL CORONARY STENOSES FOR INTERVENTIONAL DIAGNOSTIC PROCEDURE

The objective is to quantify the guidewire ( and preangioplasty). The diagnostic parameters measured before guidewire insertion (CFR and FFR) and during guidewire insertion (gCFR and gFFR) were validated numerically and correlated with the new diagnostic parameter &quot;lesion flow coefficient (LFC).&quot; There was significant flow reduction with increased trans-stenotic pressure drop due to guidewire insertion. The FFR-gFFR and CFR-gCFR correlations wer