# calculation of Nu NO.

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 March 16, 2010, 10:14 calculation of Nu NO. #1 New Member   raminj Join Date: Mar 2010 Posts: 11 Rep Power: 7 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.

 March 17, 2010, 00:35 #2 Senior Member   Hamid Zoka Join Date: Nov 2009 Posts: 162 Rep Power: 8 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.

 March 17, 2010, 02:55 #3 New Member   raminj Join Date: Mar 2010 Posts: 11 Rep Power: 7 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?

 March 17, 2010, 08:09 #4 Senior Member   Hamid Zoka Join Date: Nov 2009 Posts: 162 Rep Power: 8 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?

 March 18, 2010, 02:44 #5 New Member   raminj Join Date: Mar 2010 Posts: 11 Rep Power: 7 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?

 March 18, 2010, 04:40 #6 Senior Member   Hamid Zoka Join Date: Nov 2009 Posts: 162 Rep Power: 8 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?

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