Learning Forum

[pratheeba.nagarajan]

Hi,

I am simulating vertical upward flow in an annular geometry in ANSYS Fluent.

• Flow direction: +z
• x and y are radial directions
• Inlet at z = 0 (annular face, inner tube is wall)
• Two outlets at z = 0.4:
  • Annular outlet
  • Tube outlet

Reported mass flow rates (Fluent → Reports → Fluxes):

• Annular inlet: +2.98E‑05 kg/s
• Annular outlet: +2.89E‑05 kg/s
• Tube outlet: −5.87E‑05 kg/s

From control‑volume mass conservation, I expect:

m˙tube,out=m˙ann,in−m˙ann,out≈9.3E ⁣− ⁣06  kg/s\dot m_{\text{tube,out}} = \dot m_{\text{ann,in}} - \dot m_{\text{ann,out}} \approx 9.3E\!-\!06 \;\text{kg/s}m˙tube,out​=m˙ann,in​−m˙ann,out​≈9.3E−06kg/s

However, Fluent reports a negative mass flow at the tube outlet.

Velocity vectors show:

• Flow entering the domain at the annular inlet
• Flow leaving at the annular outlet
• Reverse flow at the tube outlet

My understanding is that Fluent computes mass flow as:

mdot=∫ρv.ndA  

where n is the outward normal. Hence, reverse flow at the tube outlet leads to negative reported mass flow.

My questions:

1. Is the large negative value at the tube outlet simply due to backflow driven by pressure gradients?
2. In the presence of outlet backflow, what is the correct way to verify global mass conservation?
3. Would redefining the tube outlet as a pressure inlet be more appropriate?

Any clarification would be appreciated.

Thank you

Pratheeba

 

[Rob]

As I can't easily visualise what you're modelling, nor whether it's converged it's a little hard to comment. 

Backflow is typically due to the flow, and the "prevent reverse flow" button should only be used if you REALLY understand what's going on in the flow. For a steady flow I'd expect mass to balance once the model has converged.