just using fluent we identify only power law type reaction equation... but I have to enter a rate equation which consists of numeratior, denominator and summation... so do I have to use UDF? but I don't know C language... and it's hard to understand the tutorial

pleas help me....

What kind of help are you looking for? In my experience with implementing a customized reaction rate with a UDF is that it can easily take several weeks. The example in the UDF manual is very straight forward.

If you are looking for advice, why don't you try putting your rate into the standard structure used by Fluent? Use something like Excel to plot your reaction rate versus temperature for your equation. Then look if you can approximate the curve with what is in Fluent. You may have to use different equations for different temperatures. You may have to fool with the temperature exponent or the stoichiometric coefficients.

Will there be other factors that control the reaction rate? Turbulence?

-r=k1K1PCH4[sqrt(K2PO2)]/(1+K1PCH4+sqrt(K2PO2))^2

this type of rxn rate eqn. which is identified for reaction. and I think that your advice is not applied to this eqn.. so can you help me with a different way?

I guess not.