Learning Forum

[Stefanotepo0]

Hi I am using VOF method in ansys fluent. I have a dissertation project which is evaluating the effect of surfactants in the surface tension of a mixture between water and gas (methane in this case).   Since surfactants are not available in Ansys, I decided to only use water and methane (and air) mixture (three phases), and varying the surface tensions referring to some surface tension datas from past journals.   I modelled three scenarios which each has its own surface tensions, but all showed the same results 

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(Note: gas in this case = methane)

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Scenario 1; SFT water-gas = 73 mN/m, SFT water-air = 73 mN/m, SFT gas-air = none. I also include wall adhesion where as; air-gas = 90deg, water-air = 90deg, water-gas = 90 dec

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Scenario 2; SFT water-gas = 30 mN/m, SFT water-air = 30 mN/m, SFT air-gas = none. Wall adhesion; air-gas = 90, water-gas = 150, water-air = 150 

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Scenario 3; SFT water-gas = 0.01, SFT water-air = 0.01, SFT air-gas = none. Wall adhesion; water-gas = 180deg, water-air = 180deg, and air-gas = 90deg  

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Please someone kindly help me to solve this because I have spent a week for nothing figuring all by myself.    

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Also other datas such as:

Ansys fluent CFD Method: VOF 2D modelling

Geometry: rectangular with 3 ft tall and 0.333ft width. Has two inlets ( bottom right for gas(methane), bottom left for water) and one outlet at the top.

Explicit formulation, interface modelling sharp, interfacial anti-diffusion is activated, Model: continuum surface force, wall adhesion is activated (I want to activate jump adhesion but it says “jump adhesion is only supported with cell based smoothing") 

Velocity water inlet = 200 ft/s, Velocity gas(methane) inlet = 190 ft/s.   The meshing has element size of 5e-003 m which doesn’t include refinement adaption. I attached the meshing as a picture here.  

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Once again, please someone help, I have spent a week without any progress  

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Sincerely, Stefano P  

   

[Rob]

Wall contact angle effects how the phase interaction behaves at the wall. Depending on mesh and flow speed this may have no effect on the results. What does the phase contour look like? Which way is up?

[Stefanotepo0]

Dear sir,

Thank you so much for your interest, I appreciate it so much. Yes, it goes up. I attached the contour results in this chat. I want to make all of the three results to be different so I can evaluate the effect of surface tension in a water-gas mixture. 
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In addition, the way before I run the calculation, I patched and smoothed volume fractions of both water and gas(methane) (don't know either that is necessary) then I initialized it. Also, for the method I used PISO, and the time steps is 1000 with a size of 1e-5. is there anything I missed? 

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Sincerely,
Stefano P

 

[Stefanotepo0]

Dear Sir,

I tried to reduce the flow velocity and they were different now. Thank You so much for helping me moving on from this dead end.

Nevertheless, if someone have another idea, would be appreciated to let me know.

Sincerely,
Stefano P

[Rob]

That looks about right, and with time the blue phase (air) will disappear. Remember surface tension is a fairly weak force so has little effect on high speed (ie high inertia) flows or in large domains. 

As an aside, if the methane is a gas you have two phases:  liquid and "gas". Where the gas phase is a species mixture of air & methane.