Modeling diffusion limitation in solid-oxide fuel cells

Effect of surface diffusion on the performance of solid-oxide fuel cell is investigated. A methodical approach for the evaluation of surface diffusion coefficients of various adsorbed species based on bond-order conservation Morse potential (BOC-MP) method is presented. The surface diffusion fluxes are used for the evaluation of temporal changes in surface coverages. Our analysis shows that surface diffusion does not lead to the concentration losses in solid oxide fuel cells. Further analysis is carried out and results are presented to substantiate the significance of interface diffusion on the behavior of voltage at limiting current

Computational Fluid Dynamics of Catalytic Reactors

Catalytic reactors are generally characterized by the complex interaction of various physical and chemical processes. Monolithic reactors can serve as example, in which particle oxidation and reforming of hydrocarbons, combustion of natural gas, and reduction of pollutant emissions from automobiles are frequently carried out

Advances and challenges of ammonia delivery by urea-water sprays in SCR systems

Over the past decades, selective catalytic reduction (SCR) using aqueous urea sprays as ammonia precursor has become the prevalent technique for NO$_{X}$ emission control in mobile applications. Preparation of ammonia from urea water sprays still represents a challenge in aftertreatment engineering as complex interactions of multi-phase physics and chemical reactions have to be handled. Increasingly stringent emission legislations and the ongoing development of fuel-efficient engines and close-coupled aftertreatment systems raise high demands to SCR systems. Due to highly transient conditions and short mixing lengths, incomplete spray evaporation can result in liquid/wall contact and formation of solid urea deposits lowering ammonia selectivity and homogeneity. This article reviews the ongoing development of SCR systems with focus on the efficient evaporation and decomposition of the injected spray for a homogeneous ammonia distribution in front of the SCR catalyst. Critical aspects of spray evaporation and impingement, liquid film and deposit formation are pointed out and potentials for system optimization are discussed

Inflight estimation of gyro noise

A method is described and demonstrated for estimating single-axis gyro noise levels in terms of the Farrenkopf model parameters. This is accomplished for the Cosmic Background Explorer (COBE) by comparing gyro-propagated attitudes with less accurate single-frame solutions and fitting the squared differences to a third-order polynomial in time. Initial results are consistent with the gyro specifications, and these results are used to determine limits on the duration of batches used to determine attitude. Sources of error are discussed, and guidelines for a more elegant implementation, as part of a batch estimator or filter, are included for future work

Modeling of Solid-Oxide Electrolyser Cells: From H2, CO Electrolysis to Co-Electrolysis

In this analysis, we report an in-house model to describe the complex fundamental and functional interactions between various internal physico-chemical phenomena of a SOEC. Electrochemistry at the three-phase boundary is modeled using a modified Butler-Volmer approach that considers H2 and CO, individually, as electrochemically active species. Also, a multi-step elementary heterogeneous reaction mechanism for the thermo-catalytic H2/CO2 electrode chemistry, along with the dusty gas model (DGM) to account for multi-component diffusion of ideal gases through porous media, are used. The model is geometry independent. Results pertaining to detailed chemical processes within the cathode, electrochemical behavior and irreversible losses during SOEC operation are demonstrated