Oddities in CFX
 

Hi everyone!

My problem is as follows:
I modelate a pipe counter heat exchager. It includes a lot of pipes, so I've decided to consider one elementary cell (even its quater) - see first 2 picture. So hot water (a coolant) flows into one side within pipe (without phase change), and the cold water flows from the other side in shell side, where it starts boiling.
All parallel the pipe axis planes have symmetry BC.
Inlet BC: inlet velocity
outlet BC: outlet pressure (for both medium)

After I got a decision, I saw a really strange oddity: The tempreture of water for both medium doesn't change at the start of flowing. You can see a little plateau in the both charts. Of course it desagrees the real situation.

Maybe somebody knows what is it?

(The solver did about 1500 itarations and the residuals stopped changing after 500-700 itarations).

You only mentioned the residuals are not changing anymore.

No changing residuals do not mean the solution has converged yet. Level of the residuals matters considerably.

You have not explained how is your mesh setup. The snapshot seems coarse on the boundaries.

I tried to decriase the size of cells, but nothing change. I think the problem isn't in the mesh. Cells seem long in mesh, but aspect ratio quite good (average less than 10)
The maximum level of residual is as follows: (all other residuals a little smaller)
They are not very small. I think it because I didn't turn double precision on (my computer can't even start solving with double precision)

What about momentum/continuity/energy residuals?

Okey. They are:

I'm guessing the heat has not reached you line yet (radially). You define an inlet temperature, and it takes time (distance) for that heat to reach an area away from the wall. I would expect these plateaus, and they will get smaller if you move you lines closer to the wall.

Yes, you are right. I've created 3 more lines. One of them right between pipe's boundaries, and two between pipe wall and center. This plateau became smaller. But what should I do, to get rid of it?
If I do some more iterations, will it help?

It is recommended for CHT simulations like this you add imbalances as a convergence criteria. You can also look at the imbalances in the solver manager to see if it has reached steady state - imbalances of around 1% are the usual target.

May I ask why is the plateau incorrect?

What is your inlet velocity profile? if you start with the uniform flow, i.e. plug flow, the flow must develop and it will take a while (length along the pipe, not time) before the center line is heated.

See "entrance length" in wikipedia, or a textbook for details. Similarly, there is the thermal entrance length

For flow in a circular pipe, the thermal entrance length is approximately x/D = 0.05 * Re * Pr for laminar flow, and about 5-10 diameters for turbulent flow if I recall correctly.

There is no error here. That is the correct result.
Apparently I wasn't clear in my explanation the first time.

Wow. Thank you for your thoughts. I haven't ever knew and thought about thermal entrance length existence, though I know about velocity entrance length.
But It sounds sanely. For example, if we consider h+(v^2)/2=const (I don't know how this equation is called in English; maybe "Bernoulli integral in the heat form"), we'll see, If velocity doesn't change, than h=const, or the same, t=const. It's only my thoughts (I don't know how they far from truth).

The main thing is that the average tempreture is changed along pipe in this region.

If I change inlet BC from velocity inlet to mass flow rate inlet, then velocity profile won't be uniform, isn't it?

Velocity and mass flow rate are both uniform inlets.