[nrram]

I am trying to model a heat transfer flow in a pipe. I have two cases. The Reynolds number is in transitional region for both the case. For case 1, the Re is about 2400 and case 2, Re is between 3000-3800. What is the flow model in fluent that I can use to model these flow with higher accuracy?

[jsm]

If you ask best turbulence model with higher accuracy for transitional Re flow, then I recommend to use LES or DES turbulence model.

If your geometry is simple, then you can use these models with highly refined mesh. Otherwise, you have to compromise the results accuracy by using transition turbulence models.