spray angle and particles diameter

[Nadezhda]

Hello!

Please help me. 

I made tests with sprinklers. I set spray angle between10 and 90.

But spray has very strong connection with particles diameter. If I set diameter 500 um, then spray distribute as I expected (between 10 and 90 degrees):

 

But if I set diameter 150 um, then particles just drop down:

 

I can’t finde connection between diameter and spray pattern in FDS User Guide. Please help me. Is it right behevior? Or there are some error in input data, or in programm?

[dr_jfloyd]

You have a 20 cm grid and the default sprinkler OFFSET is 0.05; all your drops are being inserted into a single grid cell. 

Net drag is proportional to the projected surface area over all the drops. The number of drops and their mass are proportional to volume.  Going from 500 to 150 um increases the net drag by a factor of 3 and decreases drop mass by a factor of 37.

When you inject water spray, the drops impose drag on the air which results in air being entrained into the spray. More drag means more entrainment. The velocity of that entrained air will act on the particles and bend their path. With large drops, the combination of less air entrainment due to less drag and less change in the path as a result of lower force and higher mass means the particles do not deviate much from their initial direction. With small drops this is not the case. With all the drops in a small number of cells, the large amount of drag, and the small particle mass; the entrained air collapses the spray. Set a larger OFFSET and/or have a smaller grid.

[Nadezhda]

Thank you!

среда, 13 сентября 2023 г. в 16:26:19 UTC+5, dr_jfloyd: