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CHAPTER 18. DUO
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Reformers that are based on monolithic structures include thousands of equally sized small channels. To
resolve these structures with numerical methods, grids with high spatial resolution are needed, which leads
to large counts of cells. As the type of the basic differential equations is elliptical, the solution process is
always iterative. The catalytic reforming kinetics of practical fuels (e.g., diesel) depend on gas-phase and
heterogeneous reaction mechanisms, which may involve over a thousand elementary reactions. For each of the
hundreds of species, a transport equation in a CFD code has to be solved. This would lead to simulation times
in the time-span of weeks up to months or years, even if a large number of processors (e.g., several thousands)
were available.
Therefore, direct coupling of chemistry at this level greatly exceeds the capability of computational fluid
dynamics (CFD) models which can handle geometric complexity, but not in combination with complex chemistry. Therefore, the development and investigation of reformer systems using solely a common CFD code is
not practical. Beyond that, the licensing of a commercial CFD code is very expensive and not suitable for small
companies.
18.1.3
Simulation Tools DUO and DC4OpenFOAM
DUO stands for the coupling of the two computer codes DETCHEM Und (German for and) OpenFOAM and
is a synonym for the joint utilisation of these two programmes. OpenFOAM is an open source CFD tool
which enables the calculation of three-dimensional fluid and solid regions. It is free of cost. DETCHEM
is a purchasable package of tools specifically designed for the modeling and simulation of reacting flows,
particularly for heterogeneous systems such as catalysis, materials synthesis and fuel cells. The simulation
tool DUO is the gateway between the two codes. The basic idea is to combine the advantages of the two
programmes.
Parts of the DETCHEM software can be used in different ways additionally to OpenFOAM. The user has to
distinct between two different approaches DUO and DC4OpenFOAM, which can be applied solely or together.
In DUO single tools of DETCHEM are called as external executables from OpenFOAM (external coupling).
These tools calculate flow and the complex reactions in straight channels in one or two dimensions in space
(axial or axial/radial). This feature can be used to calculate monolithic structures with many channels or tubes.
In this case the flow regions inside the channels are not included in the CFD grid. The data exchange between
the codes is managed with ASCII data files.
In DC4OpenFOAM some features of DETCHEM are provided by calling a precompiled shared library
(internal coupling). This feature is used to calculate the fluid properties and the source terms of the chemical
reactions in arbitrary regions. This capability is needed, if gas phase reactions in flow regions outside of the
channels of a monolith (e. g. upstream of a monolith) have to be considered. Additionally, the surface reactions
at walls can be calculated. Therefore, it is also possible to calculate the full three-dimensional flow and the
chemistry inside the channels of a monolith. In this case the flow regions inside the single channels would be
included in the CFD grid.
Both tools, DUO and DC4OpenFOAM can be used independently or can be mixed up. The approach
allows, for example, the calculation of a reformer in which the combined gas phase and surface chemistry is
confined within geometrically simple tubes or channels and additional reactions take place in the gas phase
out of the monolith and at the outer surface. In figure 18.1, a schematic drawing of a monolith with nine
channels located in a surrounding flow field is shown. The flow covers the region upstream of the monolith
as well as the channels in the monolith. Flow over the external surfaces of the catalyst tubes may be used to
achieve thermal control. For example, hot exhaust products from an SOFC tail-gas combustor can be used to
support endothermic steam reforming. Heat transfer occurs inside the fluid and the solid regions as well as at
the interfaces between these regions.
18.1.4
How DUO and DC4OpenFOAM work
A: DUO (External Coupling to OpenFOAM)
As mentioned above, the task is to model a reformer based on a monolithic structure. The approach exploits the
structure of reformers in which the combined gas-phase and catalytic chemistry is confined within geometrically
simple tubes or channels. In figure 18.1, a schematic drawing of a monolith with nine channels located in a
surrounding flow field is shown. The flow covers the region upstream of the monolith as well as the channels
in the monolith. Flow over the external surfaces of the catalyst tubes may be used to achieve thermal control.
For example, hot exhaust products from an SOFC tail-gas combustor can be used to support endothermic steam
reforming. Heat transfer occurs inside the fluid and the solid regions as well as at the interfaces between these
regions.