crit_cond.dat
 

when should we use this option? since it is for multi species simulation, suppose for two gases like methane and air as two separate species coming separately into the combustion chamber, then in this case two gas species are coming, so should we check crit_cond.dat option and provide critical values for each species value ?

For premixed combustion, it should not matter that much. For non-premixed combustion like the combustion in a burner it will be better to use crit_cond.dat to capture the mixing process more accurately.

What if we do not provide crit_cond.dat file for non-premixed type combustion, then whether it wil affect the results the we will get? whether the error is considerable?

Yes, the error could be considerable if crit_cond.dat is not provided.