CO and NOx formation in a DLN combustor
 

I have simulated a DLN combustor (which have a diagonal premixer and a central pilot)with a "partyally premixed" model (based on conserved scalar and a progress variable, "flamelet" chemistry model employed). In the case at load 100%, NOx CFD level is 2 ppm (7 ppm from experiments) and CO CFD level is 11 ppm (<<1 from experiments). In the case at load 40%, NOx CFD level is 60 ppm (30 ppm from experiments) and CO CFD level is 13 ppm (150 from experiments). Why does Fluent correclty determine the NOx trend emission but not the one of CO? Thanks