Troels Dyhr Pedersen.indd - Solid Mechanics

10 The reaction kinetics of DME
- 33 - -
10.1 Elementary reaction model
All elementary reactions are modeled using the three parameter form of the Arrhenius
expression:
k(
T ) = A ⋅T
e
with units
T : K
Cal
E A :
mol
Cal
RU
:
mol ⋅ K
The unit of A depends on the reaction type.
−E
A
b
RU
T
The parameters needed to calculate the forward reaction rate coefficient are the frequency
factor A, the temperature exponent b and the activation energy EA. These parameters are
supplied for both forward and reverse reactions in the reaction mechanism used to model
DME combustion.
10.2 The detailed reaction scheme
The reaction kinetics of DME is described in details by a reaction scheme named DME
2000 from Lawrence Livermore National Laboratory. The reaction scheme is based on
studies on premixed DME/air flames at atmospheric pressure. Extensive documentation is
available for this scheme [21]. Units are in mol, Calories and Kelvin.
Low pressure and Troe fall-off corrections are given for some reactions. The details of
these corrections are beyond the scope of this report, but the corrections are included in
the simplified scheme for completeness.
10.3 Reducing the mechanism for lean combustion
The full scheme was developed to model both lean and rich flames. Rich flames involve
much more complicated reaction paths than lean flames. Reactions leading to the
formation of higher hydrocarbons are included, such as ethylene. Since ethylene is
believed to be an important precursor to soot, the mechanism can potentially be used to
predict soot formation in rich flames. The formation of higher hydrocarbons is however
very low at lean and premixed conditions, since the possibility of local rich combustion
zones is very limited. Hence there is no need to include these species in the description of
lean combustion.