Unsteady conjugate heat transfer Negative initial temperature T0: -3918.142

Cagatayemre · January 10, 2018, 05:08

Hello OpenFOAMers. I am trying to simulate unsteady conjugate heat transfer from cylinder to the air and I need to draw a graph of Nusselt numbers with respect to Reynolds numbers. I will compare the results with the graph that I attached. In this case, there are 3 different velocity values 0.3 m/s, 0.43 and 0.60 m/s. 5 different cylinder wall temperature =300, 373, 473, 573, 673 K. And I also need to see the shedding vorticity in Paraview like in the attached screen shot. My files are attached. Any help will be appreciated. Thank you very much. An also I need Strouhal number vs Reynold number graph or drag coefficient vs Reynold number graph.

What solver do I need to use?
How can I get the shedding vortex shape?
How can I plot Nusselt number vs Reynolds Number?

peterhess · January 10, 2018, 11:39

Hello Cagatayemre!

Do you have a forced or natural convection?

You are talking about Reynolds number. Well in the case you uploaded there is no forced flow but free convection via gravity! All velocities for all boundaries are 0 in the case you uploaded!

If the case you try to solve is free (natural) convection, then Re number is useless!!!

In free convection the govering number is Rayleigh number. it decides as Re number if the flow laminar or turbulent.

Anyway, if you have a forced convection, then you need to change your case and define the inlet(s) and the outlet(s) of the flow.

Which solver?

well it depends on what you want to do.

If you want to simulate a transient problem, then you use chtMultiRegionFoam. If SSSF (steady state steady flow) then chtMultiRegionSimpleFoam.

As I understood you want to simulate different velocities and different temperatures. That means you should use chtMultiRegionSimpleFoam, cause you dont simulate what happens DURING changing those variabels but different SSSF cases (First velocity with first temperature, 2nd with 4th ...).

Please describe what you want to do:

- free convection or forced one?

- flow inlet and outlet if it is forced flow

Something else...

If the small cylinder in your case (heat source) have a constant wall temperature, why are you interessted simulating it at all?

All you need to do is simulating the fluid region...

Regards

Peter

Cagatayemre · January 10, 2018, 12:05

Thanks Peter, I want to study natural convection, steady case.

peterhess · January 10, 2018, 12:16

Well, in this case you need to think about if it is the right target to find a relation Nu=f(Re)!

I would make a relation Nu=f(Ra)!

https://en.wikipedia.org/wiki/Rayleigh_number

See:

Fundamentals of Heat and Mass Transfer. ISBN-10: 0470501979 - ISBN-13: 978-0470501979

Chapter 9.

You will see that Ra number is here relevant (Grashof number * Pr).

In your case (SSSF) it is sufficient to use chtMultiRegionSimpleFoam!

chtMultiRegionFoam (pimple) works also, but unnecessery.

Anyway, there are other solvers that also usable for natural convection like buoyantSimple/PimpleFoam...

I personly like to use chtMultiRegionSimpleFoam for such a problem.

The chart you uploaded is for forced flow with different velocities. It is not natural convection.

Regards

Peter

Cagatayemre · January 10, 2018, 17:20

Thanks Peter. Great explanation. I will refine the mesh in the border but now I am trying to rotate the cylinder. I explained in the link a little bit. Another question how can I measure the a when clculating Nu_overall. After solving these problems, I will read and ask you again. I will open a new thread now I will put the link in here. My lasts decision is that the case is steady conjugate the small cylinder should be a solid cylinder and there should be water flow.

Illegal dictionary entry or environment variable name "0"

January 10, 2018, 12:16		#4
peterhess Senior Member Peter Hess Join Date: Apr 2011 Location: Austria Posts: 250 Rep Power: 16	Well, in this case you need to think about if it is the right target to find a relation Nu=f(Re)! I would make a relation Nu=f(Ra)! https://en.wikipedia.org/wiki/Rayleigh_number See: Fundamentals of Heat and Mass Transfer. ISBN-10: 0470501979 - ISBN-13: 978-0470501979 Chapter 9. You will see that Ra number is here relevant (Grashof number * Pr). In your case (SSSF) it is sufficient to use chtMultiRegionSimpleFoam! chtMultiRegionFoam (pimple) works also, but unnecessery. Anyway, there are other solvers that also usable for natural convection like buoyantSimple/PimpleFoam... I personly like to use chtMultiRegionSimpleFoam for such a problem. The chart you uploaded is for forced flow with different velocities. It is not natural convection. Regards Peter Last edited by peterhess; January 10, 2018 at 19:10.

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January 10, 2018, 11:39		#2
peterhess Senior Member Peter Hess Join Date: Apr 2011 Location: Austria Posts: 250 Rep Power: 16	Hello Cagatayemre! Do you have a forced or natural convection? You are talking about Reynolds number. Well in the case you uploaded there is no forced flow but free convection via gravity! All velocities for all boundaries are 0 in the case you uploaded! If the case you try to solve is free (natural) convection, then Re number is useless!!! In free convection the govering number is Rayleigh number. it decides as Re number if the flow laminar or turbulent. Anyway, if you have a forced convection, then you need to change your case and define the inlet(s) and the outlet(s) of the flow. Which solver? well it depends on what you want to do. If you want to simulate a transient problem, then you use chtMultiRegionFoam. If SSSF (steady state steady flow) then chtMultiRegionSimpleFoam. As I understood you want to simulate different velocities and different temperatures. That means you should use chtMultiRegionSimpleFoam, cause you dont simulate what happens DURING changing those variabels but different SSSF cases (First velocity with first temperature, 2nd with 4th ...). Please describe what you want to do: - free convection or forced one? - flow inlet and outlet if it is forced flow Something else... If the small cylinder in your case (heat source) have a constant wall temperature, why are you interessted simulating it at all? All you need to do is simulating the fluid region... Regards Peter

January 10, 2018, 12:05		#3
Cagatayemre Member Çağatay Emre Ayhan Join Date: Sep 2017 Location: Istanbul, Turkey Posts: 31 Rep Power: 8	Thanks Peter, I want to study natural convection, steady case.

January 10, 2018, 17:20		#5
Cagatayemre Member Çağatay Emre Ayhan Join Date: Sep 2017 Location: Istanbul, Turkey Posts: 31 Rep Power: 8	Thanks Peter. Great explanation. I will refine the mesh in the border but now I am trying to rotate the cylinder. I explained in the link a little bit. Another question how can I measure the a when clculating Nu_overall. After solving these problems, I will read and ask you again. I will open a new thread now I will put the link in here. My lasts decision is that the case is steady conjugate the small cylinder should be a solid cylinder and there should be water flow. Illegal dictionary entry or environment variable name "0"