[Shabi1992]

I want to calculate Nusselt number in a canal with constant wall temperature. I've tried many times, and the results do not match previous works.*
*I need to find Nu number on the wall ,so I draw a line on the top of the canal wall and calculate wall heat transfer coefficient, but based on hD/k it is*unreasonably high. I write this expression Nu=(q*D)/((Tw-Ti)*k), as well which
q= heat flux
Tw=wall temperature
Ti=inlet temperature
D=hydraulic diameter
k=thermal conductivity

but the results are difference again.

Does anyone have any experience calculating Nu number*in CFX?
I would be thankful if anyone can help.

[-Maxim-]

have you used the search here in this forum and tried all the suggestions in the other threads? Here is one example...

[evcelica]

How are you getting wall temperature? If you have only modeled the fluid and you are using an expression, it will use the conservative (adjacent node wall temperature) of the fluid, not the hybrid wall temperature.

The link maxim provided has a lot of good info in it of course.

[Shabi1992]

Originally Posted by evcelica

How are you getting wall temperature? If you have only modeled the fluid and you are using an expression, it will use the conservative (adjacent node wall temperature) of the fluid, not the hybrid wall temperature.

The link maxim provided has a lot of good info in it of course.

Thanks for your answers, The canal is constant Temperature so I use the temperature of canal for wall temperature.
I read that link and as maxim said it was good . Now my question is how calculate q? I mean q=-k*(dT/dy) at wall and how can I write this expression?

[Opaque]

You can compute it as:

MyNuNumber = areaInt(Wall Heat Flux)@MyWall * MyCharacteristicLength / (Thermal Conductivity * (MyWallTemp - MyRefTemp)

Note the above is the average Nu number (based on the total wall heat flow), and not the local number.

I have seen all kind of definitions for Nusselt numbers, it is the first time I have seen the inlet temperature used for the Reference Temperature. For flow within ducts, the reference temperature is usually the bulk mean temperature.

Hope the above helps,

[Shabi1992]

Originally Posted by Opaque

You can compute it as:

MyNuNumber = areaInt(Wall Heat Flux)@MyWall * MyCharacteristicLength / (Thermal Conductivity * (MyWallTemp - MyRefTemp)

Note the above is the average Nu number (based on the total wall heat flow), and not the local number.

I have seen all kind of definitions for Nusselt numbers, it is the first time I have seen the inlet temperature used for the Reference Temperature. For flow within ducts, the reference temperature is usually the bulk mean temperature.

Hope the above helps,

Thank you for your answer dear Opaque.
I forgot to use bulk mean temperature.This might help.

[Shabi1992]

Originally Posted by Opaque

You can compute it as:

MyNuNumber = areaInt(Wall Heat Flux)@MyWall * MyCharacteristicLength / (Thermal Conductivity * (MyWallTemp - MyRefTemp)

Note the above is the average Nu number (based on the total wall heat flow), and not the local number.

I have seen all kind of definitions for Nusselt numbers, it is the first time I have seen the inlet temperature used for the Reference Temperature. For flow within ducts, the reference temperature is usually the bulk mean temperature.

Hope the above helps,

I want to know how can I calculate the local Nusselt? If I want to calculate q for getting h, how can I calculate q?

[Opaque]

q = Wall Heat Flux