hello! please is there a way to understand when should we change the values of the default underelaxation factors in fluent, especially, concerning "species factors???"

This is quite tricky and if you go through this forum and check earlier posts, you will definitely find some good pointers.

Do you want to solve fpr species transport with or without reaction/s? -usually under-relax only when solution is unstable in initial iterations. -never under-relax heavily. under-relaxation of species should be at the max 0.8. -if you have to under-relax heavily, try: ---------check set-up of problem (most important). ---------try step-wise solution. ---------try coupled solver in that order. use coupled solver in case you have a high speed flow. default URFs usually need not be changed.

Regards,

Swarup.