The chemistry model now solves a system of mass fractions, temperature
and pressure, rather than a system of concentrations, temperature and
pressure.
The new form now accounts for the change in reaction rate associated
with thermal expansion. Thermal expansion (or contraction) can dilute
(or concentrate) the species concentrations, thereby reducing (or
increasing) the reaction rates. Previously it was not possible to
include this term because it was not computationally feasible to
evaluate it in a system in which the state variable was concentration.
The reaction rate interface has been simplified with respect to the
generation of derivatives. Reactions are defined as k*C, where k is the
reaction rate and C is the product of concentrations (raised to their
stoichiomentric coefficients and/or specified powers). Reaction rate
classes now provide two logical functions governing the derivatives of
k; i.e., ddT (derivative w.r.t. temperature) and ddc (derivative w.r.t.
concentration). Previously the reaction rate interface was closely
related to the form of third-body reactions, which made it inconvenient
to implement rates that were not very third-body-like.
It is now possible to verify the implementations of the jacobian methods
by comparison with finite-difference based evaluations of the rate
methods. This has been done and a number of bugs have been found and
fixed in the reaction rate classes.
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