Chemically Reacting Channel Flow

Fig. 1.4 Illustration of the chemically reacting boundary-layer flow in a single channel of a catalytic-combustion monolith.

There is growing interest in a variety small micro power sources that deliver a few Watts. Such systems, which can provide direct mechanical power or serve as battery alternatives for electronic devices, often rely on the flow and reaction of fuels in small channels. In addition to fuel cells, other technologies include thermoelectrics and small-scale internal-combustion engines. These applications require attention to low-speed chemically reacting flow, often with significant surface interactions. 

This problem considers the chemically reactive flow in a long, straight channel that represents a section of an idealized porous media (Fig. 4.32). Assume that the flow is incompressible and isothermal, but that it carries a trace compound A. The compound A may react homgeneously in the flow, and it may react heterogeneously at the pore walls. Overall, the objective of the problem is to characterize the chemically reacting flow problem, including the development of an effective mass-transfer coefficient as represented by a Sherwood number. 

There are many chemically reacting flow situations in which a reactive stream flows interior to a channel or duct. Two such examples are illustrated in Figs. 1.4 and 1.6, which consider flow in a catalytic-combustion monolith [28,156,168,259,322] and in the channels of a solid-oxide fuel cell. Other examples include the catalytic converters in automobiles. Certainly there are many industrial chemical processes that involve reactive flow tubular reactors. Innovative new short-contact-time processes use flow in catalytic monoliths to convert raw hydrocarbons to higher-value chemical feedstocks [37,99,100,173,184,436, 447]. Certain types of chemical-vapor-deposition reactors use a channel to direct flow over a wafer where a thin film is grown or deposited [219]. Flow reactors used in the laboratory to study gas-phase chemical kinetics usually strive to achieve plug-flow conditions and to minimize wall-chemistry effects. Nevertheless, boundary-layer simulations can be used to verify the flow condition or to account for non-ideal behavior [147]. 

Consider a model problem that is motivated by the chemically reacting plug flow in a channel that has both homogeneous and heterogeneous chemistry (Section 16.3). Assume... 

Chapters 6 and 7 discuss many of the underlying fluid mechanical aspects of stagnation flows and channel flows. The intent here is to put those fundamentals to use in terms of practical, chemically reacting flows. There are numerous applications that could be discussed. We choose a few to illustrate salient points about the modeling. 

There are numerous applications that depend on chemically reacting flow in a channel, many of which can be represented accurately using boundary-layer approximations. One important set of applications is chemical vapor deposition in a channel reactor (e.g., Figs. 1.5, 5.1, or 5.6), where both gas-phase and surface chemistry are usually important. Fuel cells often have channels that distribute the fuel and air to the electrochemically active surfaces (e.g., Fig. 1.6). While the flow rates and channel dimensions may be sufficiently small to justify plug-flow models, large systems may require boundary-layer models to represent spatial variations across the channel width. A great variety of catalyst systems use... 

Creslaf, simulates chemically reacting, two-dimensional, boundary-layer flow in channels and ducts, including heterogeneous wall chemistry. 

CRESLAF (Chemically Reacting Shear Layer Flow) A FORTRAN Program for Modeling Laminar, Chemically Reacting, Boundary Layer Flow in Cylindrical or Planar Channels. M. E. Coltrin, H. K. Moffat, R. J. Kee, and F, M. Rupley, Sandia National Laboratories, Livermore, CA, 1991 (see also M. E. Coltrin, R. J. Kee, and J. A. Miller, J. Electrochem. Soc., 133, 1206, 1986.). CRESLAF is a... 

The disadvantages are that PDMS devices typically can only handle low pressure drops before catastrophic rupture, and untreated PDMS can become swollen or chemically react with some liquids, thus making the channels not reusable. Recently, there have been reports demonstrating that the walls of such channels can be coated with material that can improve the flow properties [11]. 

Coltrin ME, Moffatt HK, Kee RJ, Rupley FM (1996) CRESLAF (Version 4.0) a Fortran Program for Modeling Laminar, Chemically Reacting, Boundary-Layer Flow in Cylindrical or Planar Channels, SAND93-0478 Sandia National Laboratories... 

A single-channel manifold also can be used for systems in which a chemical reaction generates the species responsible for the analytical signal. In this case the carrier stream both transports the sample to the detector and reacts with the sample. Because the sample must mix with the carrier stream, flow rates are lower than when no chemical reaction is involved. One example is the determination of chloride in water, which is based on the following sequence of reactions. ... 

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