Weber Number
 

Hi all, it shold a very simple question, but i have found different answers to that. In the Weber number i should use the density of the droplet or the continuous density? In my case i got liquid droplets injected in gas phase. Thx in advance.

Hi Fabio,

The Weber number definition depends on what you are using it for. Are you using the Weber number to calculate another quantity, or in a model such as drop breakup?

Hi, i need to calculate this number to choose the proper breakup model.
Which of the twi density i got i need to put in the formula?

Fabio,

Without seeing the code or knowing what the breakup models are, I would say it is probably gas density. You need to verify this though. Do you have any documentation for the code you are using (what code is it)?

Sorry, i was thinking that was only a matter of definition of the Weber number.
Anyway the code is FLUENT and docementation says that there are two breakup model TAB nad WAVE, for the latest shall be used. Documentation also says that i should use continuous phase density but the result is very low We=0.4776.

Hi Fabio,

What type of spray are you modeling?

I'm modeling a solid 3d cone spray of a liquid in an hot gas. Did you mean this?

Fabio,

I was just wondering if this is a gasoline spray, Diesel spray, or some other type of spray. You mentioned that the Weber number that you calculated seems very low and so I just wanted to see if that makes sense. For example, the Weber numbers for Diesel sprays are typically higher than gasoline sprays.

Sorry, it isn't an engine injection i'm studing an SCR the fluid density is a bit more of water density 1084-1092 .

How about using both models and checking how close you go to the desired results; experimental results if available? Although it is a very non-technical answer, I couldn't help replying your question.

Unfortunately i haven't experimantal data to match, i have to study the trajectories of the injected particle with diffent duct geometries. So i need one right model to use for the different simulations to match the results. i hope to have expressed well which is the point of the situaton.

continuous phase is in this case the gas phase

So the definition in the manual is right there isn't any ambigous definition?

The spray for SCR requires high pressure injection involving higher velocities and hence higher inertial forces. Certainly it will call for the use of wave breakup model. You will have to choose the model constants carefully.

As documentation says Weber number guide the choice of the breakup model, for that reason i have asked here the right density to use, with the gas density as suggested on manual i got a result much less than We=100 that indicates TAB as correct to use. I'm getting confused, injestion pressure is 600kPa.

Hi,

I think the Weber number must be based on gas phase (continous phase) as you can find in the papers which classify mechanisms of drop breakup (e.g. Liu and Reitz, "An analysis of the distortion and breakup mechanisms of high speed liquid drops", 1997).

In these papers you can find the classification of the drop breakup mechanisms, and see that Kelvin-Helmholtz instability occurs when gas Weber number goes over 80, and Taylor Analogy Breakup is not good when this instability is the dominant mechanism (as FLUENT suggests We > 100).

This point is clear, i have used TAB model because We<100 but i would like to know i have computed this in the proper way using gas phase density.
Thanks for the help.