[CZL]

Thank you all once again for your replies.

It seems that there isnt a problem with the assumption which i have made, rather i think that the problem lies with the possibility that my result may not have converged.

I have attached images of the stream lines and the plots of the residuals of 2 different grid resolution.

High resolution is about 8x that of the low res one.

https://www.dropbox.com/sh/pgdjf1ivi...WnZpguz7a?dl=0
for the high resolution one, the mass flow rate at the outlet does not match the one which i have specified in the boundary condition.

any suggestions as to how i could reduce the iteration count?

[RodriguezFatz]

Hey CZL, why the energy equation? If the temperatur changes along the pipe, there is no way this can be periodic. Or am I wrong?

[flotus1]

Some thoughts on the statements in this thread:

1) Translational periodic boundary conditions can be used even for straight pipe flows
This simply implies that the flow is fully-developed, an assumption that may be valid at least for some part of the geometry we are dealing with here.
And of course we have to assume that the fluid we are dealing with is incompressible.

2) Fluent is able to deal with temperature variations even for straight pipes with translational periodic boundary conditions applied. The temperature is rescaled at the "inlet" based on the temperature value you provide at the periodic interface. Again, this only works with constant-density fluids.
Basically the same procedure as with a pressure gradient along a translational periodic flow.
Periodic flow with heat transfer is even one of the basic tutorials for Fluent.

Since the energy equation does not seem to converge very well in your cases, try decreasing the under-relaxation factor for this equation (->Solution controls)
The other equations seem to converge perfectly at least on the coarse grid.
If convergence is too slow on the finer grid, try interpolating the solution from the coarse grid as an initial condition for the fine grid. (File -> Interpolate)

[RodriguezFatz]

Originally Posted by flotus1

2) Fluent is able to deal with temperature variations even for straight pipes with translational periodic boundary conditions applied. The temperature is rescaled at the "inlet" based on the temperature value you provide at the periodic interface. Again, this only works with constant-density fluids.
Basically the same procedure as with a pressure gradient along a translational periodic flow.
Periodic flow with heat transfer is even one of the basic tutorials for Fluent.

Thanks flotus, good to know.

[CZL]

thanks for the inputs!

will try to do as suggested.

I used energy equation to track how this passive geometry is better in terms of transferring heat rather that the straight case as the curvature kind of induces fluid mixing, enhancing heat transfer.

That means that the absolute values of the solution is not of concern, rather delta T will be taken note of.

My understanding is that if delta T is better than the conventional straight geometry, it will definitely also be better in the developing region due to the flow profile and hence, this be concluded to be overall more efficient...

does it sound logical? i mean is this method applicable?

[flotus1]

Sounds reasonable.
But keep in mind that the curved geometry not only enhances heat transfer compared to a straight channel, it also increases pressure loss and might be quite difficult to manufacture.
So if pressure loss is a relevant factor check that your method is better than simply a longer straight channel.

[hyugakojiro999]

@Flotus1

Could you please help me,

I cant get the real pressure values when I use translational periodic B.C. in my calculations. Is there a way to do it?

[emreaskin]

First of all, you need to know "what is the periodic boundary condition in CDF?"

This is from Cimbala and Cengel:

"Flow field variables along one face of a periodic boundary are numerically linked to a second face of identical shape (and in most CFD codes, also identical face mesh). Thus, flow leaving (crossing) the first periodic boundary can be imagined as entering (crossing) the second periodic boundary with identical properties (velocity, pressure, temperature, etc."