Learning Forum

[Finn Blake]

Hi,

I am injecting a solid cone of evaporating droplets. I have set the rosin rammler distribution with 1.5e-5m, 6e-5m, 7e-5m for minimum, mean and maximum. At the near injection zone the SMD follows this but the diameter doesn't. However further downstream the the SMD and the diameter both grow to significantly larger. They shouldn't be doing this and I'm not sure why/how to stop it. 

Any advice appreciated.

Thanks

[Rob]

Have you got any other DPM models switched on? 

[Finn Blake]

Hi Rob, I have the one solid cone injection and within the DPM window I have pressure dependant boiling and breakup turned on (breakup with KHRT). Also just to add I'm using 2021R1

[Rob]

Any coalesence? 

[Finn Blake]

That appears to be what is happening just by looking at the graphs but I don’t have stochoastic collision turned on (which is where coalesence would occur) so I don’t think that should be the case.

UPDATE: If I turn off breakup this problem doesn’t seem to occur and the droplet diameters are as predicted. However the velocity profile means the spray doesn't hold its cone shape as only the particles heading in the z direction have high velocity and the rest of the cone is significantly lower, it is like this:

 

Is there a setting so that the velocity is more evenly distributed in the cone so it holds its shape instead of the central core dominating it like that?

Thanks

 

 

[Rob]

The droplet trajectories are also influenced by the flow. So, for a cone I'd expect the angle to collapse in a co-current flow, or one assisted by gravity. 

Replot the above by age and velocity component (left-right coordinate) and the image may be clearer. 

[Finn Blake]

Yep thanks Rob, that makes sense. Do you have any idea why including the breakup model is causing the droplet sizing to be wrong? 

[Rob]

Can you open the DPM Numerics up and check the panel(s)? I remember that activating break up automatically turned coalesence on which you could then switch off. 

[Finn Blake]

I can’t see anything for coalesence in here. (the droplet heat-mass solution and vaporization limiting factors have been altered just to check their impact but they have been since reset to default)

 

[Rob]

Oops, Physical Models. Sorry! 

[Finn Blake]

I didn’t have coalesence in there either, it only shows up as an option when collision is turned on (which I did do just to turn coalesence off and than collision off again). I actually have narrowed it down to the KHRT breakup being unsuitable for the spray, If I use the TAB model than the diameters are correct so I appear to have solved it. I was using the jet weber number instead of droplet weber to decide which breakup was suitable but I think given my spray in a real scenario begins as a disperesed 2 phase flow I needed to use droplet weber number. Thanks for your help