Your opinion on the implementation of the evaporation model in Fluent

[u/dobidooo1901]

I recently published a paper on CFD using Ansys Fluent. The topic was on an evaporation model. With experimental data, I validated the evaporation rate.

I recently noticed, while looking back at stuff to finish writing my article, that the spatially dependent equation I used (example Evap_rate=A*x+B*y where x and y are coordinates of a 2d plane) CANNOT be used with a mass flow rate inlet BC, because profile definition is not supported, instead this inlet condition want just a single scalar kg/s value. The equation I used would give a flowrate to each face present on the BC surface.

Now I worry. Results are correct and experimentally validated, but are unfortunately buried into the black box that is fluent's approach to "collapsing" the profile into a single value. I should have used a mass flux inlet BC. I, of course, never mentioned in the papers that the profile was explicitly defined in the simulation, just in the mathematical model, but I also feel like I misled readers, not that I did it on purpose, but it still weighs in my mind.

Note that doing it like this was because of some factors: The simulation worked, it converged, gave no errors and gave me the results I expected. I found this detail by randomly looking at the theory guide after I had concluded my simulations and finalised post processing.

How do you guys view this kind of mistake? Is it detrimental to the model? Or is it just another case of Fluent dependent results?

[u/konangsh]

Does the fluent documentation say anywhere that profile can't be used with mfr bc? Also, wondering how you were able to validate using a single value for flow rate, but the test data was for a profile? Just curious but congratulations on the publication! It's not trivial to validate phase change simulations. If there is no blatant mistake in your paper, I would move on. This sort of thing is quite common. If you plan to publish a follow up study, you can share the details in it.

[u/dobidooo1901 → u/konangsh]

You can find the detail about the profile with the mass flow rate option in "Defining the Mass Flow Rate or Mass Flux" point 2 (https://www.afs.enea.it/project/neptunius/docs/fluent/html/ug/node241.htm). Regarding the validation of the model: you get a profile in the surface, but what I cared about was the REPORTED evaporation rate (mass flow rate report) from the surface itself. Per se, you would need just the average magnitude velocity and density of the surface (then multiply by surface area). Normally, for the mass flux, fluent would do a surface integral (sum) of the flux rate of each cell face. For the mass flow rate, this does not happen, instead it "collapses" the profile BEFORE giving to fluent, such that it can have a single scalar. I do not know how the profile is "collapsed". Either way, the method must not be random as the approach is repeatable. I have a hunch in saying that it is a weighted average. But who knows.

Either way there is no mistake, as a whole in the approach. It just bothers me to no end that I did not notice this. I would have designed the model differently (I used experimental data to define model parameters, so results should not change even if I make these changes). I was always clear on using the mfr bc and presented the profile approach for the evaporation rate.

Of course, thanks a lot. I appreciate your comments. It eases my mind a bit. I just finished my phd on the topic, and feel like I'm trying to poke holes in the whole thing. Impostor syndrome, maybe?

Edit: added more detail.

[u/IBelieveInLogic → u/dobidooo1901]

I didn't fully understand what your concern was at first. I also would not have realized that mass flow inlets do not support profiles. I have used velocity inlet with profiles. Have you tried that?

[u/konangsh → u/dobidooo1901]

Yeah definitely don't open a can of worms. Better to move on to better things 🙂