Learning Forum

[Mahi97]

Could someone help with the procedure of how to simulate the film boiling on a heated sphere at 1063K and the surrounding water at saturation temperature.There is initially a thin vapour film over the sphere and with time it enlarges and gets dispersed as a bubble. please someone help if possible.

[Rob]

Multiphase model is needed and phase change.

[Mahi97]

Can we do it by the VOF model and is any UDF required or just the evaporation-condensation mass transfer enough ?
Can you please describe the steps to be followed using the VOF initializing with a thin vapour film around the sphere ?

[Rob]

Check the evaporation/condensation model to ensure it'll work. The evaporation side should be OK, but the condensation may get messy as the bubbles shrink. You may want the multi-fluid VOF. To get the vapour film you'll need to patch the vapour into an adaption register or cell zone.

[Mahi97]

Yeah, I have done that...the patching part ..and also included the multi phase vof ...taken two phases namely water and vapor , but my heat transfer coefficient is becoming very large in the order of some 10,000 but it should be in the range of 250 or something like that.Also my simulation is correct,but it is achieved slow compared to a similar research paper with this problem.

[Mahi97]

Sir, i have attached the case of the problem that Iam trying to simulate.Please suggest if i could do anything to get my results correct.

[Rob]

Check the HTC is calculated using the same reference values: it's a common oversight. As for the rest, assuming the mesh is good, what else is different compared to the paper. If you recall from the rules, staff are not permitted to open attachments.

[Mahi97]

Iam using the same reference values as that in the paper.Could I somehow send that paper to you, by mail or something because my problem would be clearer to you, if you'd see that paper.

[Rob]

No, we're not permitted to: please read the rules to understand what we can and can't do.

[Mahi97]

Sir, I have a small query.In my problem IAM intializing a vapour film around the sphere ,but after certain time steps the vapour film is getting detached completely and is getting formed again and that's the reason for the sudden jump in heat transfer coefficient.Is there a way to maintais a constant vapour film around the sphere and still be able to produce bubbles around it.

[Rob]

You'll need to set boundary conditions that support the film. The film is detaching as that's what the solver thinks is the correct answer based on your boundary conditions. Note, if the mesh and other settings aren't suitable that may not be physically what we'd expect.

[Mahi97]

Thanks a lot for the reply, but what are the boundary conditions to be set for the film and how can we set them, since I'm patching a certain region to be a vapor film and intializing it.So, how can we setup appropriate B.C , so that the vapor film doesn't detach completely.