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-   -   Bench Mark - Vaporizing Drop Trajectory (http://www.cfd-online.com/Forums/main/1616-bench-mark-vaporizing-drop-trajectory.html)

 R. Sureshkumar December 6, 1999 14:17

Bench Mark - Vaporizing Drop Trajectory

May i know if any published "Bench Mark" exists for evaporating droplet trajectory.

Thanx

 R. Sureshkumar December 6, 1999 14:36

OOP's - An Addition to Above

While simulating a vaporizing droplet in air; as the drop size reduces, say below 10microns the time step for the simulation (RKutta - IV) had to be reduced, otherwise numerical error creeps in (possibly due to negative mass occuring and so on). Can i say that below this size the simulation can be stopped. Is there any literature on this front. Thank you

 Andrzej Matuszkiewicz December 8, 1999 00:57

Re: OOP's - An Addition to Above

Calculate Knudsen number for your problem, i.e. ratio of molecular mean free path and droplet radius. If it is greater than 0.1 you need to include a correction to the continuum mechanics formula for the drag force. If it is on the order of 1 or greater you should use the Boltzmann equation rather than the continuum mechanics. If it is still smaller (less than critical radius) the droplet will become unstable and will disintegrate. Then, the time step is just irrelevant.

Andrzej

 R.Sureshkumar December 8, 1999 04:51

Re: OOP's - An Addition to Above

Thank you for the Insight Within the the continuum frame work (I do not want to enter into Boltzmann, atleast for now). Is there any recognised criteria say, for water drops less than 1 micron at so much temperature we can say that the drop evaporates fully ( or stop calculating further change ).

I saw a paper where for a fuel drop it was assumed that the trajectory ends when it hits the boundaries or when the size is less than 10 microns ( where it was assumed that the droplet evaporates fully).

Thank you

 Andrzej Matuszkiewicz December 9, 1999 00:20

Re: OOP's - An Addition to Above

Yes it is. it is called Knudsen number. To calculate it you do not need to know anything about Boltzmann equation. Just take a simple formula for mean free path from any book on the statistical mechanics or kinetic theory of gases and divide it by droplet diameter. If it is on the ordetr of 1, you should stop calculation because it does not make sense. There are too many assumption that are violated. Drag force, evaporation rate, etc.

Andrzej

 R. Sureshkumar December 9, 1999 11:00

Thank you Very Much - No message inside

Thank you

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