25 September 2018
I have put answers to your questions below. I hope this helps.
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.