I am validating my CFD mini rectangular channel model. For that purpose, I need to know how to get the Nu number accurately.
For Calculating Nusselt no. from fluent post-processing as follows:
Nu = h Dh /kf
h = q’’/(Tw-Tf)
where q'’ is the heat flux at the solid-liquid interface, Tw =∫TdA/dA is the
interface temperature and Tf =Tin + Tout/2 is the bulk fluid temperature.
By following these formulations, I have computed the Nu number. But I am not sure whether it is right or wrong. I am humbly requesting you to review my process that is given below in the doc file.
Qs:
Do I need to consider three interfaces(two side face, one bottom face, considering top face insulated) to calculate Tw and q"? Could you please provide me a way to find Nu number from the mini rectangular channel?
Dear Md.Wasi Uddin
Numerically, your calculation is not correct. For bulk temperature, you need to calculate Tf locally at several cross-sections along the channels using "mass-weighted average" from "Report type" in the software. Also, local Tw can be extracted by creating lines on interfaces. Then, you calculate h_local at first, and using different methods for integrating, you obtain average h. However, this type of calculation is almost impossible in experimental studies, and investigators try to use the equations you mentioned (using inlet and outlet temperatures for Tf). On the whole, if you want to validate your simulation, you should use the calculation defined in experimental studies to strongly compare the results and errors.
Good luck
Dear Md.Wasi Uddin
Numerically, your calculation is not correct. For bulk temperature, you need to calculate Tf locally at several cross-sections along the channels using "mass-weighted average" from "Report type" in the software. Also, local Tw can be extracted by creating lines on interfaces. Then, you calculate h_local at first, and using different methods for integrating, you obtain average h. However, this type of calculation is almost impossible in experimental studies, and investigators try to use the equations you mentioned (using inlet and outlet temperatures for Tf). On the whole, if you want to validate your simulation, you should use the calculation defined in experimental studies to strongly compare the results and errors.
Good luck
Dear Mojtaba Bezaatpour,
Thanks for your informative answer.
But I can not understand, how many cross-sections and lines on the interface of solid-fluid should I take?
Would you please give any tutorial or any document that may help me in finding the Nu number?
Thanks in advance.
Hi Md.Wasi Uddin
you should go to reports, surface integrals, wall fluxes, then you can get the heat transfer coefficient and it is true way based on bellow reference and the symmetrical situation that you have:
El Maakoul, A., Laknizi, A., Saadeddine, S., Abdellah, A. B., Meziane, M., & El Metoui, M. (2017). Numerical design and investigation of heat transfer enhancement and performance for an annulus with continuous helical baffles in a double-pipe heat exchanger. Energy conversion and management, 133, 76-86.
After that, you should calculate the Nusselt number base on Nu=hD/K
However, if you want to calculate the average Nu or h in a specific area, you'd better define a line it the area you have validated data(Numerical or experimental), and using h = q’’/(Tw-Tf), that Tw is the wall temperature and Tf is the area-weighted of average for temperature in the line. For example, in the bellow reference, the heat transfer coefficient could be achieved by h = q’’/(Tw-Tf) in X/D=147.
X=lenght and D= diameter
Moraveji, M. K., Haddad, S. M. H., & Darabi, M. (2012). Modeling of forced convective heat transfer of a non-Newtonian nanofluid in the horizontal tube under constant heat flux with computational fluid dynamics. International Communications in Heat and Mass Transfer, 39(7), 995-999.
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