J. M. Zalc

Fuel processing for PEM fuel cells: transport and kinetic issues of system design

Abstract In light of the distribution and storage issues associated with hydrogen, efficient on-board fuel processing will be a significant factor in the implementation of PEM fuel cells for automotive applications. Here, we apply basic chemical engineering principles to gain insight into the factors that limit performance in each component of a fuel processor. A system consisting of a plate reactor steam reformer, water–gas shift unit, and preferential oxidation reactor is used as a case study. It is found that for a steam reformer based on catalyst-coated foils, mass transfer from the bulk gas to the catalyst surface is the limiting process. The water–gas shift reactor is expected to be the largest component of the fuel processor and is limited by intrinsic catalyst activity, while a successful preferential oxidation unit depends on strict temperature control in order to minimize parasitic hydrogen oxidation. This stepwise approach of sequentially eliminating rate-limiting processes can be used to identify possible means of performance enhancement in a broad range of applications.

Practical chaotic mixing

We present experimental and computational analyses of three-dimensional chaotic laminar mixing in one of the most widely utilized mixing devices, the stirred tank. We 7nd that 98% of the power expended in mechanical stirring produces no detectable mixing at all, and in fact convective mixing is generated solely by tiny perturbations to a dominant non-chaotic :ow. By analyzing the role of these perturbations, dramatic improvements in performance are easily achieved: for example, minor judicious, changes in agitator design produce order of magnitude improvements in mixing e>ectiveness. ? 2002 Elsevier Science Ltd. All rights reserved.

Parallel-competitive reactions in a two-dimensional chaotic flow

Abstract A parallel-competitive reaction system, A+B 1 →2P and A+B 2 →2W, where A and B 1 are the reactants, B 2 is a reactive impurity, P is the desired product, and W is a byproduct, is simulated in a chaotic flow by solving the differential convection–diffusion–reaction equations. Three different flow conditions are investigated: a predominantly regular system, a globally chaotic flow, and an intermediate case that displays predominantly chaotic behavior but which has four small islands of regular flow. The time evolution and spatial distribution of species concentration depend strongly both on the nature of the flow and on the relative rates of the two reactions. Even in the globally chaotic flow, significant spatial heterogeneity exists throughout the duration of the reactive mixing simulation. Product and waste accumulate in different spatial regions, depending on the relative characteristic times of the two reactions. When the primary reaction is the faster one, waste tends to accumulate in local A-rich regions and the selectivity of product to waste is strongly affected by the degree of chaos in the system. On the other hand, when the side reaction is faster, waste accumulates in segregated regions which have local excesses of B 2 , but mixing has minimal effect on the quantity of waste produced.

The evolution of material lines curvature in deterministic chaotic flows

Abstract It is by now well established that curvature plays a fundamental role in the description of the topology emerging from the partially mixed structures advected by chaotic flows. This article focuses on the dynamics of curvature in volume-preserving time-periodic flows. Previous work on the subject dealt with the evolution of curvature in the time-continuous framework. Here we derive the dynamical equations for the time-discrete dynamical system associated with the Poincare return map of the flow. We show that this approach allows one to gain more insight into understanding the mechanisms of folding of material lines as they are passively stirred by the mixing process. By exploiting the incompressibility assumption, we analyze dependence on initial conditions (i.e. on the initial curvature and tangent vectors), and discuss under which circumstances the dependence on the initial curvature vector becomes immaterial as time increases. This analysis is closely connected with the properties of an invariant geometric structure referred to as the global unstable manifold associated with the flow system. Direct numerical simulations for physically realizable systems are used to provide concrete examples of the results that arise from theoretical considerations. The impact of this information on the prediction of the behavior of diffusing-reacting mixing processes (e.g. pattern formation and generation of lamellar structures) is also addressed.