[m&s]

hi every body,
how can I calculate nusselt number in a tube?
meanwhile all values for "convective heat transfer coefficient" (h) which I attempted for that were zero, is this value (zero) logical?
thanks for your attention.

[Hamidzoka]

describe your geometry and boundary conditions.
check if you have chosen adiabatic wall boundary conditions.
h is normally fucnction of both fluid and flow type.

[m&s]

thank you Hamid
wall tempreture is constant and geometry is a rectangle and its boundry is velocity inlet,axymetric,wall and pressue outlet
how can i calculate nu number?

[Hamidzoka]

Dear Ramin;
It seems that you are about to solve an axisymetric pipe flow with constant wall temperature. for such this case nusselt number is calculated as a function of Reynolds and Prandtl numbers for both laminar and turbulent flows and can be found in heat transfer text books. then, "h'"coefficient is calculated from nusselt number. in case pipe flow calculated formula for nusselt number is not applicable to your case, firstly you have to calculate "h" manually using energy equation at solid wall control surface like this:
Q(convective;fluid to solid)= Q(conductive;through the wall)
knowing h and k of fluid, nusselt will be calculated.
does it help?

[m&s]

thank you but i don't know how can i do this job.
please help me.
why fluent has surface nusslet number but doesn't have flouid nu number?

[Hamidzoka]

Dear Ramin;
this is because of the definition of nesselt number:
"In heat transfer at a boundary (surface) within a fluid, the Nusselt number is the ratio of convective to conductive heat transfer across (normal to) the boundary."
in fact nusselt number is meaningless when there is no surface.
for more info. look at the following link:
http://en.wikipedia.org/wiki/Nusselt_number

does it help?