 ## Fluids

#### Need answer of very basic questions: What is back flow in Fluent simulation?

• rumth
Subscriber

I need to clear the concepts about some simple questions:

1. What does back flow mean in Fluent?

2. What is Back flow volume fraction for VOF model?

3. In my simulation, more mass is going out through outlet than inlet. I have set velocity inlet = 553 m/s. At pressure outlet, Gauge pressure = 0 atm. Operating pressure = 1 atm.

If I set Gauge pressure = 2atm (greater than operating pressure) then mass flow through outlet will decrease, right?

4. For two phase, I am selecting water by marking region. How can I know the volume fraction of water in the domain?

regards,

Raju.

• jmccasli
Ansys Employee

Hi Raju,

I have put answers to your questions below. I hope this helps.

Regards,

Jeremy

1. What does back flow mean in Fluent?

When you have recirculation near an outlet, flow will enter the domain from the outlet. This reverse flow into the domain from the outlet is called "back flow". Back flow must have prescribed values for the field variables (e.g., pressure and temperature), so it is necessary to specify this.

2. What is Back flow volume fraction for VOF model?

This specifies the phase volume fraction of any occurrence of back flow at an outlet. For example, let's say you have a VOF model with primary phase as air, and secondary phase as water. You will need to specify the secondary phase back flow volume fraction at an outlet. If you set it equal to 0, this will mean that all back flow is air. If you set it equal to 1, this will mean all back flow is water.

3. In my simulation, more mass is going out through outlet than inlet. I have set velocity inlet = 553 m/s. At pressure outlet, Gauge pressure = 0 atm. Operating pressure = 1 atm.

If I set Gauge pressure = 2atm (greater than operating pressure) then mass flow through outlet will decrease, right?

What drives the fluid through the outlet is the mean pressure gradient (pressure drop over length of pipe). Increasing the outlet pressure will decrease the pressure drop (in magnitude), so yes you should see the flow rate through the outlet decrease.

4. For two phase, I am selecting water by marking region. How can I know the volume fraction of water in the domain?

Perhaps you can clarify what you mean? The volume fraction for each phase is available as a post-processing quantity, so you can post-process it in any way you like. E.g., you can create contours, perform surface and volume reports, etc.

• rumth
Subscriber

Hi Jeremy,

Thank you so much for your nice explanations. It helps me a lot.

Regards,

Raju.

• Ani94
Subscriber

Hello

My question is regarding backflow total temperature that need to be set to avoid backflow. I am simulating flow through heat exchanger, where at process outlet I have given pressure outlet, hot fluid is entering shell at 493k and utility fluid at 338K through tubes. Solver is giving me the reverse flow is happening. My question here is what should be the backflow temperature when I don't know temperature at outlet.

It should be near atmospheric or it should be higher to avoid backflow.

(Physically there is no backflow at outlet) 