thanks!

i tell you how to define this argument inside the UDF. *****for example****** today ,you have three gas (include air ,water vapor , hydrogen)to simulate chemical reaction in your model . (i used a Fluent's 5.5) *first , you found out the order of species react in the Defines/materials panel inside "mixtures species" edit ......"selected species" small window have order of the species .air , water vapor , hydrogen respectively .

*second , air ---0, water vapor---1 , hydrogen---2

*third , if you used UDF to simulate your physical phenomena , you want to get mass fraction of hydrogen in your code , you can write C_YI(c,t,2)

best regard hsin-ping chang