Q: We are still a bit unclear on how to define hollow cone sprays in CFX..
I would like to inject 4 cones of fuel, a solid cone in annulus 1, and hollow cones in annuli 2 to 4. We want to inject from the indicated injection plane.
In the definition for the particle injection region, should the "Injection Center" be point A or point B.
Does the "Injection Center" define the actual origin of the spray, or does it indicate the axial position of the injection plane/point?
If the injection center locates the plane of injection, then how is the spray angle defined?
Where is the spray angle referenced to for a particular injection annulus?
Also, In the particle injection definition box, there is single input for velocity magnitude. How do I assign the axial and theta velocity components for each annulus? - JCOOPER
The injection center locates the plane of the injection, so the injection point will be at B in the diagram
There are two angles that can be specified for particle sprays:
1. Cone angle/Spray angle ` angle inside/formed by cone - for hollow cone this is angle inside mean trajectory
2. Dispersion angle `angle by which spray deviates from the mean in either direction
(I have attached a version of your original diagram to illustrate these --see Hollow Cone Specification)
Particles can be injected on an inlet or an injection region:
1. CFX 11.0 INLET PARTICLE INJECTION
In CFX 11.0, a dispersion angle for the particles can only be set if you set the particle injection on an inlet.
To get the correct injection, you define an annular plane/disc equivalent to the cross section of the cone at B.
Then, create an expression for how the injection cylindrical velocity components from that plane/disc will vary with R.
There will be 2 expressions, one for the axial component and one for the theta component. If there is no swirl, only spread, us the Radial component instead of the theta component.
Alternatively, you could just break the annular injection surface up into discrete rings and assign a constant axial/theta couple for each ring.
I have attached a jpeg of the inputs for the CFX 11.0 inlet, showing where a dispersion angle could be created using expressions for axial and tangential velocities.
2. CFX 11.0 PARTICLE INJECTION REGION
Hollow cone particle injection control for CFX 11.0 injection regions is limited to only the cone angle (no dispersion)
For the injection region, however, only a single velocity can be input as you saw.
Because of this, the cone angle is the same for every injection point on the annulus (dispersion angle of 0).
For the 4 different annuli, you will have to set up 4 injection regions, each with a different velocity magnitude (this is the magnitude of the resultant vector), but the effect of dispersion cannot be considered.
(Note that this also results in parallel particle trajectories for different radii on the annular injection surface, i.e.: trajectories would not reference back to the same point.)
3. CFX 12.0 PARTICLE INJECTION REGION
For CFX 12, the region injection has been amended to allow the specification of a dispersion angle (in addition to cone angle) for each injection annulus ` see attached.
This will be a much easier format to use and will avoid having to inject using an inlet (not always possible)or neglect dispersion.
A good approximation of the 4 bands in CFX 12 would be:
1. Cone angle = 30, Dispersion angle = 15
2. Cone angle = 70, Dispersion angle = 15
3. Cone angle = 90, Dispersion angle = 15
4. Cone angle = 110, Dispersion angle = 15