As part of our continuing series on topics in the FDS 6release notes, this blog post will focus on FDS 6 pyrolysis model.
The basic 1-D heat transfer and pyrolysis model for solid
surfaces remains almost the same. However, several of the input parameters have
been changed to expand the functionality and readability of the input file. The most
significant change in functionality deals with the potential swelling or
shrinking of the surface layers. This requires a slightly different treatment
of the pyrolysis reactions' residue materials.
The purpose of the changes in the input parameters has
been to support and to make use of the major developments that have taken place
in the species/combustion solver. FDS 6 has much more versatile way of handling
gaseous species and multiple combustion reactions than the versions before - a
topic worth of a separate blog post. You can now associate the products of
the burning surfaces to any of gases that are being tracked by FDS. This is
true for both surfaces with specified burning rate and surfaces with pyrolysis
reactions. We have tried to make the logic of using keywords, such as SPEC_ID,
consistent throughout the code.
The solid products of the pyrolysis reactions are called
residue. In FDS5, they were identified using a RESIDUE keyword, now using
MATL_ID keyword. So, not much change here. But unlike before, the densities of
the residue materials can now have a great influence on the behaviour of the
pyrolysis process. Previously, the bulk density of the residue, such as a char
layer, was determined by the density of the initial material layer and the
effective yield of the material from one or more reactions producing it. Now,
the thickness of the layer will be adjusted so that the residue material's density
becomes what has been actually specified as a density of that material. Some
practical applications of this feature are listed below:
• Non-charring materials will shrink as material is
removed from the condensed phase to the gas phase.
• Porous materials like foams would shrink when the
material melts and forms a non-porous layer.
• Some charring materials swell, i.e., get thicker, when
a porous char layer is formed.
• Intumescent fire protection materials would swell
significantly, creating an insulating layer.
For example, if the original material with DENSITY = 500
is completely converted into another material with DENSITY = 1000, the thickness of
the layer will become half of the original. Or, if the original
material with DENSITY = 1000 is completely converted into a material with
DENSITY = 500, the thickness will become twice the original. You can, of
course, prevent this from taking place if you know that the material
will retain the original thickness.
See the User's guide for an explanation on how the
shrinking / swelling will be determined if there are more than one
simultaneously reacting materials. Examples of shrinking and swelling materials
are given in verification case shrink_swell.fds.
As a summary, you must pay more attention on the
specification of MATL densities for the pyrolysis model than before.
