Moving up in complexity from plug-flow assumptions, accounting for the boundary-layer interactions in a channel-flow can be important for many applications. In a boundary-layer approximation, we include the effects of radial diffusion and dispersion for species, mass, and energy. However, convection is assumed to dominate in the axial direction (that is, along the channel), such that axial diffusion is neglected. These flow conditions can represent laminar flow in cylindrical or planar channels for a variety of applications, including thin film processing in channel-flow reactors, catalytic conversion in a representative pore of a catalyst monolith, or flow within a microchannel reactor, for example. This section describes the governing equations for boundary-layer approximations applied to channel flows. The reactor models addressed here include:

The Shear-layer Flow Reactor Models simulate the coupled hydrodynamics, gas-phase chemistry and surface chemistry in laminar-flow channels. Detailed mathematical formulation of this model and a demonstration of its application to chemistry in the chemical vapor deposition (CVD) of silicon from silane have been reported previously in the literature [85], [86]. The model is general, in that it can be applied to any channel-flow system for which gas-phase and surface kinetic mechanisms are known. The model predicts gas-phase temperature and velocity fields, concentration fields for any number of chemical species, deposition or etching rates and surface-species coverage. Results will depend on user-specified flow conditions or boundary conditions, such as surface temperature, flow rate, inlet partial pressure of the reactants, total pressure, and reactor dimensions. The models are restricted to two-dimensional geometric representations, using either planar or radial coordinates for Cartesian or axisymmetric flows, respectively.

The Shear-layer Flow Reactor Models require gas transport properties. These are determined using the Transport Pre-processor and subroutine libraries for calculating thermal diffusion coefficients and for the rigorous calculation of ordinary multicomponent transport properties. The effects of thermal diffusion, which is the separation of species of differing size in a temperature gradient may be included when requested by the user. Thermal diffusion can have an important effect on predicted concentration profiles.[86] The boundary conditions describing chemical reactions at the surface are formulated using the Surface Kinetics Pre-processor, while the gas-phase kinetics calculations employ the Gas-Phase Kinetics Pre-processor.