Learning Forum

[krany3]

How to calculate swirl number in Fluent using custom field functions?

I calculated swirl number as,

Angular Momentum = abs(swirl velocity) * abs(axial velocity) * radial coordinate

Axial Momentum = axial velocity * axial velocity * radius of the cross-sectional plane

Then, I calculated the integral of the respective momentum, to get the flux. I did not get the correct swirl number.

Should I use the FLOWRATE for calculating the flux?

[Rob]

If you define the cell zone centre of rotation to align with wherever you're trying this calculation then I think you just need to divide axial velocity by angular velocity (or the inverse!). 

[krany3]

I did not get you. Can you please explain it in detail? 

[Rob]

In the cell zone you can set the axis: default is +z.  If you alter this to tie up with your domain you'll get sensible values for axial, tangential and circumferential & angular velocity.  Check section 6.2.1.1. Inputs for Fluid Zones  of the Fluent User's Guide and specifically 6.2.1.1.6. Specifying the Rotation Axis. 

 

 

[krany3]

Okay. But, don't we need to find the momentum? How to justify that velocity ratio to get the swirl number?

Thanks

[Amine Ben Hadj Ali]

To add: The swirl number is defined in Fluent under Turbulence Modeling for Swirling Flow. Just try to setup the custom field function or expression to get it:

You need area-weighted average of radius across the section and the flowrate of radius times tangential velocity, and flowrate of axial velocity.

 

[krany3]

Why do we have to take the area-weighted average of the radial coordinate? Instead, we can get the flowrate of radial coordinate times tangential velocity. 

I followed the way you suggested. But, I didn't get to the expected value. 

Why do I get negative flow rates?

Thanks

[Amine Ben Hadj Ali]

Just check the formula in ANSYS Fluent. 

[krany3]

I did not find anything like an average value in the swirl number formula. I've couple of questions.

1. Where is the swirl number of the entire system is defined? Is it at the exit of the swirler or in the combustion chamber?

2. How to calculate swirl number from a velocity profile?

Thanks

[Amine Ben Hadj Ali]

How it is formulated can be found in the manual. In some references there no averaged radius in the denominator. Where to define it: you will need to find the most appropriate location: inlet outer or any other important locations.  If you have swirl already at inlet you have axial Flux of angular momentum.

[Amine Ben Hadj Ali]

Just to add,.. there are a lot of definitions for swirl number. Ask your profs and colleagues and check literature.

[krany3]

You have earlier mentioned to take the area-weighted average of the radius across the cross section. Should I use this averaged radius, to find the flow rate?

Also, the radius in the denominator, a lot definitions were present in the literature. Some defined it as the hydraulic radius, some defined as the average radius, and some defined it as the radius of the cross-sectional plane. What should I consider for the swirl number calculation?

Thanks

[Rob]

Pretty much you can choose which radius you want: it comes back to the definition. The definition I've seen used most effectively to compare swirl burner design was angular velocity / axial velocity: the units are a bit weird rad/m but it worked well for what we needed. 

[krany3]

When I calculate the swirl number as a ratio of (angular velocity/axial velocity), I'm no way near to the expected value. But, when I used the radius of the cross-section in the denominator, I got an approximate value of the expected one.

I'm calculating the swirl number variation in the axial direction. When I follow the above procedure, I observed that the swirl number was decreasing from the swirler exit to the combustion chamber in cold flow. 

But, in reacting flow, the swirl number was increasing along the axial direction. 

Thanks

[Rob]

Plot both the axial and radial (or angular) velocities. If the swirl number is changing one or other of the factors is the cause. 

Re the difference between my definition of angular velocity and yours and comparing that with your expected value: how is that defined. As we've said there are several definitions so we need to check the definition. 

[krany3]

Thanks! I'll look into that.