[gustavosimiema]

Hi,

I have seen in lot´s of Posts the Suggestion to extent the outlet. But actually, I don´t know what does it means and neither how to do it. Could anyone give me a hint about this?
I´m working with a laminar model. Mixture of O2 and N2. Species Transport on and Energy Equation on. The Problem consist of a gasfeed inside a chamber. So I have an inlet (5mm) to fill in gas in the gasfeed, than I have an outlet (2mm) from this gasfeed to fill in the chamber, and then I have the outlet (260mm) in the middle-top of the chamber.
I´m using mass flow inlet (1e-5 kg/s) and pressure outlet in both outlets (1Pa).

I don´t know what is causing this reverse flow. Anyone has a hint?

thanks in advance

[pakk]

"extent the outlet" = make the outlet longer.
So add some volume behind the outlet.

[gustavosimiema]

Thank you so much for your reply. But I´m still facing reverse flow

[pakk]

Wait a minute... Do you mean you have reversed flow during the simulation, or do you have reversed flow in the final converged solution?

[gustavosimiema]

I have reversed flow durind the Simulation and also when the solution is converged...

[pakk]

OK, ignore the reversed flow during the simulation, but only focus on the one in the converged solution.

If you want to know where the reversed flow comes from: just look at the result! Look at how the flow lines go, and try to understand why you have reversed flow.

That is the best hint I can give: look at you solution... It is not much, but I can do no better...

[gustavosimiema]

I will do that.
I don´t know the relationship, but when I set the operating conditions to 0Pa in the pressure I´m not able to run the Simulation because I have Problems with mass diffusivity. When I Keep the Default pressure I can run the Simulation but reversed flow appears. Do you know something about that?


once more thank you for your time

[pakk]

But those are two physically different problems that you are simulating.

If you set the operating pressure to atmospheric pressure, your outlet pressure is 101325 Pa + 1 Pa = 101326 Pa.
If you set the operating pressure to 0 Pa, your outlet pressure is 0 Pa + 1 Pa = 1 Pa.

So if you change operating pressure, you change your simulation physically!

[gustavosimiema]

My objective is to Keep the pressure inside the chamber around 1Pa.
Making the operating pressure as 101325 Pa I could see in the countours of pressure that I achieved this objective. The pressure inside the chamber after results converged was 1Pa. So in my understanding, based on this Point of view I´m in a good way. I just Need to understand why I´m having reversed flow.


I think my geometry is not properly defined. I have a pipe with 30mm Diameter and 1060 mm lenght inside a cubic chamber (1060x1060x1060mm). I created an inlet to provide gas for the pipe. Than I create an outlet in this pipe to provide gas inside the chamber. Than I have the outlet to remove gas from the chamber. Is this construction ok? Or do you think I Need to Change something?
I´m afraid this is the origin of my Problems.



thank you so much

[pakk]

Originally Posted by gustavosimiema

My objective is to Keep the pressure inside the chamber around 1Pa.

Absolute pressure of 1 Pa or gauge pressure of 1 Pa? That is a very big difference, but you don't make clear which one you mean.

I think my geometry is not properly defined. I have a pipe with 30mm Diameter and 1060 mm lenght inside a cubic chamber (1060x1060x1060mm). I created an inlet to provide gas for the pipe. Than I create an outlet in this pipe to provide gas inside the chamber. Than I have the outlet to remove gas from the chamber. Is this construction ok? Or do you think I Need to Change something?
I´m afraid this is the origin of my Problems.

I don't understand your question. If that is your geometry, then that is your geometry. You can change your geometry, but then you are solving a different problem. What is it that you want to learn from the simulation?

[gustavosimiema]

I mean the gauge pressure 1 Pa. (This is a low-pressure Simulation)

With this Simulation I would like to see the pressure difference between the pipe and the chamber. And also the behaviour of the gas from the inlet until the outlet of the chamber.

[pakk]

Originally Posted by gustavosimiema

I mean the gauge pressure 1 Pa. (This is a low-pressure Simulation)

This is confusing.

Total pressure = 1 Pa: low-pressure simulation, close to vacuum.
Gauge pressure = 1 Pa: a normal-pressure simulation, close to atmospheric.

[gustavosimiema]

Sorry... I´m not so familiar with this concepts. I´m new in fluent. So, I mean Total Pressure = 1Pa (to have a low pressure condition)

[pakk]

What you can do is set operating pressure at 1 Pa, and set your boundary condition to 0 Pa.

(BTW: this is all probably unrelated to your reversed flow. If you want to know why you have reversed flow, look at the solution!)

[gustavosimiema]

Thank you so much...

In my result, I didn´t got a streamline from the inlet of the pipe until the outlet of the chamber. I don´t know if it is related to the reversed flow...

[pakk]

Then where do the streamlines from the inlet end?

[gustavosimiema]

The end is on the top of the pipe.
The inlet is in z direction and the top of the pipe is in the plane xy. So the inlet is perpendicular to the face of the pipe.

[pakk]

So you have streamlines that start at the inlet and end at a wall?

[gustavosimiema]

Exactly...

[pakk]

Then it is hard to believe that you have a converged solution. Check your mass flow balance. (Reports->Fluxes, Mass Flow Rate and select all boundaries)