Learning Forum

[javat33489]

Hi all.

 I made a calculation for a centrifugal pump using water from a fluent base.

Now I want to add methane 10% of the total volume.

I create a mixture of two phases, the first phase is water, the second is methane (take methane from the fluent base). Methane with density = constant, incompressible.

Screenshots of the mixture settings, I think everything is correct:

At the inlet I set the pressure to 2 atm. The output is consumption. Now I have two inputs and 2 outputs (water, methane and mixture). At the mixture inlet I set the pressure to 2 atm. For methane, volume fraction = 0.1 (10% of the volume of the entire mixture).

At the output, I set the costs for water and methane. Now I have reduced the water consumption by 10%, and set this 10% for methane. Thus, we indicated to the program that 10% of methane from the entire mixture came in and came out.

Screenshots of input and output settings, I think everything is correct: 

I also turned on the power so I could measure the temperature of the environment later. My goal is to increase the amount of gas and measure the temperature.

But when I start the calculation, the k and continuity indicators increase sharply and then all the others, I get an error and the calculation ends. Why?

I also tried:

reduce relaxation coefficients

change calculation methods (coupled, simple)

change the viscosity model (SST, k-e)

I also reduced the minimum step to 1e-8

nothing helps.

Help me please.

[SRP]

Hi,

The sudden increase in turbulence kinetic energy (k) and continuity residuals, leading to an error and termination of the calculation, can be indicative of numerical instability or convergence problems.

Can you please also mentioned how did you initialize the problem?

Thank you.

[javat33489]

Sir! I found the reason why the problem is not being solved. It seems there is a bug in fluent. It is not possible to set the outlet flow rate for the methane phase. I made a simpler setup from a pipe and described everything here:

/forum/forums/topic/volume-fraction-problem-when-using-fluent-mixture/

Look here please! Thank you!

[javat33489]

P.S.

I looked at the fluent tutorial and there is no setting for the mixture so that there is pressure at the inlet and flow at the outlet:

 

But for a centrifugal pump exactly these settings are needed.

[javat33489]

PLEASE HELP IS REALLY NEEDED

[javat33489]

Help please 

[SRP]

Hi,

You can try with the pressure outlet boundary condition.

Outflow boundary conditions are used to model flow exits where the details of the flow velocity and pressure are not known prior to solution of the flow problem. They are appropriate where the exit flow is close to a fully developed condition, as the outflow bc assumes a zero streamwise gradient for all flow variables except pressure.

You can check ansys user’s guide: 7.4. Boundary Conditions (ansys.com)

[javat33489]

Yes, of course I checked this tutorial. But what should I do if I am calculating a centrifugal pump? There you need to set the flow rate at the outlet and the pressure at the inlet! How should I set the mixture in this case?

[javat33489]

Can I do it like this:

1.The first calculation is to carry out PRESSURE AT INPUT, FLOW AT OUTLET, WITHOUT A MIXTURE ONLY WATER.

2.Second calculation. Take the exit speed from the first calculation. Set INPUT PRESSURE, OUTLET SPEED, WATER + METHANE MIXTURE?

[javat33489]

When adding gas to a liquid (creating a mixture), the overall density (relatively speaking) of the mixture will decrease due to the gas. Can I rely on the pump outlet velocity from the first calculation (water only) when calculating the mixture in the second calculation?

I plan to change the volumetric content of gas in the liquid (the total volume will not change in both the first and second calculations, only the volume of gas fraction will change). Can the fluid flow rate in a centrifugal pump change when the density of the mixture changes? Can I get an incorrect result in the second mixture calculation using the data from the first calculation (water only)?

[javat33489]

PLEASE HELP