[eng_yasser_2020]

Dear, all

I am using FLUET to model a two concentric cylinders (Steady-state & 2-D). The gap between cylinders is filled with gas. I want to add a boundary condition (convection boundary) in the wall between the gas, inner and outer cylinder. I have a convection coefficient as a function of "Rayleigh No. = f(Th-Tc)" not a constant value. My problem is:

- How to write a UDF for the convection coefficient which is a function in gas properties (thermal conductivity, visco, Cp, .....) and it is also a function in temperature difference (Th-Tc)??

- I mean how to write a UDF uses the material properties like (k, Cp, meu,....)???. and uses temperature diff. of two material (Th[solid]-Tc[gas])??

Thanks all