total NOx is less than Thermal NOx alone in FLUENT
In some cases, why total NOx is lower than Thermal NOx only.
This is due to reduction reactions in prompt and fuel NO models. Therefore,
net NO formation = overall NO formation - overall NO reduction
The reduction is done by HCN or NH3 present as intermediate species.
The selectivity of reduction reactions depends on temperature and
the other species (eg., CHi, OH etc). Therefore, it may be possible
that the net NO formation rate from these two (fuel+prompt) models is -ve.