Tube Wall - Roughness Coefficients
 

We currently model the exterior of tube walls (the boundary of our model) that look like this:

http://w30.indonetwork.co.id/pdimage...1_walltube.jpg

...as flat walls without a boundary layer mesh. (Can['t do it anyother way due to limited computing and time resources.)

The mesh height on the walls is 8" to 12", and the tubes are 2-1/2" OD (R1-1/4")

Does anyone have any experience on the best Roughness Height and Roughness Coefficient to use for this situation? Radiation is dominant, but Convection and Conduction are not negligible.

TIA,

R.