conservation of angular momentum in swirling flow

[lt1980]

Hi, I am working with CFD to analyze a swirling burner with a square section combustor. When I calculate the angular and the axial momentum inside the combustor, it decreases the more I check downstream to the burner exit. Do you know what would create a decrease in the angular and axial momentum inside a square section combustor? Should the swirl number variate inside the combustor?

Thank you very much for your help.

 

[btrueblood]

If you are modelling the walls with any kind of viscid flow, then there will be dissipation of the swirl as the flow moves thru the combustor. Or are you saying the rate of decrease is higher in the model than you measure in testing?

 

[lt1980]

Thanks for your answer. I don´t have experiments, but I was looking to know what can generate a decrease of the momentum. I was thinking that it should be conserved along the combustor, but it decreases. Should the swirl number be conserved with a decrease of angular momentum along the combustor?

Thanks

 

[btrueblood]

Angular momentum is "conserved", you just aren't looking closely enough. The theory of conservation of momentum and angular momentum is the statement "a body's momentum is conserved unless outside forces act upon the body". If you compute the shear forces at the wall, there will be a tangential component (drag). These shear forces arise from the swirl flow, but in the equivalent but opposite sense, the wall acts upon the flow, thus is an outside force that changes the flow's angular momentum.

Even in a free jet, the flow will interact with the outside atmosphere eventually (unless exhausting into vacuum), and the swirl will dissipate.

That said, in cylindrical swirling flows, the core region of the flow often persists a long distance downstream; I could imagine that a square channel would cause more rapid dissipation due to the disturbances formed in the corners (additional axial vorticity that gets (robs) its energy from the core flow).

Swirl number is defined by...? I assume some average swirl velocity or momentum divided by axial velocity or momentum? Again, no, the swirl number would be preserved only in the core region, away from the wall disturbance.

 

[lt1980]

Thank you very much for your answer. Do you know if the shear flow and the generation of turbulence by this shear flow would influence that too, or should it be only the influence of the wall or viscous flow what it decreases the momentum?
Thank you very much for your support.