I'm working on a 2-phase model with boiling phenomenon involved. the working fluid is liquid nitrogen (the process is Cryogenic); I need help with two question here:
1- Why can't one get a mixture temperature (as in VOF Model) in boiling models? I need to get a mass-weighted temp average at outlet, but no such option is available. Anything I can do instead?
2- There are a lot of models/modification (specially in force setup section) for boiling models, which I have no acquaintance and experience in. can anyone kindly suggest a reference that can help me find out which options are suitable or even effective for my simulation? also any experience and recommendation is highly appreciated.
Further info about my case:
- axisymmetric pipe
- k-e RNG or standard
- scalable wall function (y+ around 15)
- velocity inlet: 20m/s
- pressure outlet about 3.9 barg
- temp inlet: 77 K
- vof gas inlet: 0.3
- wall temp: constant 293 K
The questions which need to be answered first are:
* What is the physical reason why you think that the Nitrogen flow starts to boil/evaporate?
- depressurization?
- a heat source - flow internal or at walls?
* What is the flow regime of the flow - free surface, bubbly, anything else?
The answer on how the interfacial area of a developing multiphase flow looks like, will finally determine on how such a flow is boiling/evaporating. It will determine the interphase mass and heat transfer --> therefore the different models.
* Is it subcooled boiling (liquid temperature lower than evaporation temperature) and a heat source makes it nevertheless boil? Or is the liquid at saturation temperature and it evaporates to the vapor phase at a phasic interface due to either general temperature increase or pressure decrease?
Without answers to the above questions, no recommendation can be given.
A sketch of the flow configuration would be helpful as well.
Regards,
Dr. Th. Frank.
Thomas Frank
Thanks for your answer Dr. Frank.
1- It's actually an ambient vaporizer. so ambient air temp (as a wall heat source) is used to vaporize LN2 through an internal flow (pipe). It's clear that also a bit of pressure drop also occurs along the stream since we have an internal flow.
2- The flow regime is bubbly and that's why I'm using Boiling Model. Before that, I tried using VOF evaporation/condensation model in Fluent, to validate my simulation; but after so many attempts and trying various settings, it seemed impossible and therefore implied the importance of bubbly regime in the ambient vaporizer cases.
3- It's a subcooled boiling since the N2 Diagram shows sat. temp. equal to about 90 K @ 4 bar pressure, whereas inlet temp is 77 K. However it seems (according to literature and research results) that due to a very high temp difference between the fluid and the wall temperature, a vapor film develops rapidly (in a bubbly regime) near the pipe shell, so that VOF of liquid drops (from .7 @ inlet) to about .66 at outlet, in a 0.4 m long pipe. The average temp of the fluid reaches about 77.4 K at this point.
A sketch of the case is also attached to the answer. Outlet conditions are according to the research I try to validate my results with.
Regards,
O. Shekari
Dear Mr. Shekari,
due to the high velocity, we can leave out here buoyancy / gravity affects and then the Fluent boiling model for subcooled nucleate boiling should be fairly well be applicable. The only limitation might be, that the flow does not stay in the regime of nucleate subcooled boiling. The heat flux might be that high, that you exceed the critical heat flux and in this case you need to account for a flow regime transition from a nucleate bubbly boiling regime into film boiling and finally through churn turbulent flow into a droplet flow. Attempts have been made to predict this for nuclear reactor conditions for water/vapor under high pressure. But it requires quite some amount of experience and customization of the solver to achieve this. I personally have only experience with ANSYS CFX in that regard, not with Fluent. And the submodels in Fluents boiling model are most likely validated for water/water vapor flows as well, but uncertain to be accurately applicable for boiling liquid nitrogen. This would be necessary to get investigated. So with your case and this rather high heat fluxes you are obviously on the forefront of scientific research here.
Best regards,
Dr. Th. Frank.
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