Learning Forum

[Anadi_Mondal]

Hello,

Would anyone tell me the conditions of using EWF ? Why EWF may fail? Actually, I am getting negative film velocity for the upward vertical tube(air/water annular flow in a tube). I notice that when maximum film thickness and film velocities are 0.67mm and 1.3 m/s respectively , everything is running smoothly. But, when the maximum film thickness in the simulation is 1.05mm , film velocity is negative. Although, film Re is less than 400 for both cases. So, what may be the reason for negative film velocity or film Re?

I will be highly pleased if you respond with a reference showing critical film thickness/film velocity/film Re number for the EWF model. Actually, I am struggling with this problem for a longer period but didn't able to solve the problem. From my simulation experience I notice that when the film thickness is more than 1 mm, it is showing negative film velocity.

Regards,

Anadi

[Rob]

It depends on the flow. The film model assumes the film is "thin" relative to the mesh and flow. A film could be a few cm thick in some cases (probably a little extreme) for a roof or the like.

[Anadi_Mondal]

I didn't catch you properly. Here you just told 'it depends on flow', but would you give an example with flow parameters( like air/water Re) and mesh geometry( mesh size in the axial or radial direction, near-wall mesh thickness?
Moreover, you mention 'the film is "thin" relative to the mesh and flow'. What is the comparable mathematical relation for film thickness, mesh and flow for the EWE model?
Regards Anadi

[Rob]

I don't have fixed numbers, but it's covered here https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v221/en/flu_th/flu_th_ewf_intro.html If there is sufficient mass in the film it will flow against the bulk fluid flow - I've been working on such a problem on and off for a larger project for a couple of years.