Troels Dyhr Pedersen.indd - Solid Mechanics

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The reduced scheme however results in minor deviations in the heat release. The LTR
heat release is of shorter duration and with a higher peak in the reduced scheme, but as
can be seen from the accumulated heat release the total heat released in the LTR reactions
is not very different.
The HTR reactions are slightly delayed and with slightly lower peaks in the reduced
scheme. The shape of the two stage HTR combustion is however well preserved, and the
final accumulated heat release is identical for the two schemes.
It is found that the reduced scheme is adequately accurate for use in simulations. There is
however some potential for improving the scheme in order to match observations better.
The most notable deviation between simulations and experiments is in the delay from
LTR to HTR reactions, which is often significantly shorter in experiments.
10.5 Reaction paths
10.5.1 Initiation and low temperature reaction paths
Figure 11 shows the temperature and heat release plotted together. The low temperature
reactions starts approx. 750 K and are terminated at around 900 K.
Heat release rate [J/CAD]
Temperature [K]
1800
1500
1200
900
600
300
0
Heat release rate dependance on temperature
Full mechanism heat release [J/CAD]
Temperature [K]
-15 -10 -5 0 5
CAD
Figure 11: The heat release dependence on temperature
The LTR is self-terminating due to chain-terminating processes becoming dominating as
the temperature increases. This gives the characteristic blue flames, which in case of
DME releases a substantial part of the total energy in the fuel.
In experiments, the LTR appears in a very consistent manner. There is very little cycle to
cycle variation in both CAD position and magnitude. The development of the LTR heat
release is always observed to occur with the same magnitude. This is because the heat
release from the LTR is not dependant on initial fuel concentration. The concentration of
fuel molecules is not rate limiting; it is the concentration of OH radicals that determines
the heat release in the LTR combustion stage.
LTR are of high importance to the subsequent release of the remaining chemical energy.
The temperature increase of course means that the reactivity of the gas is increased, but
the LTR also creates a few stabile radicals such as hydrogen peroxide (H2O2),