Okay so to clarify. You have a closed domain, so you must define a pressure somewhere. This is what you are doing in the fvSolution file. for the cell with id 0, you define a pressure with 2.5 pascal.
In the p_rgh file what you are setting in your case are only initial values. Also for p. Since you set the pressure to 2.5 pascal in that cell in your domain, you can assume that the pressure will be ~2.5 pascal in the whole domain. That's why it is suggested (not a must, but strongly suggested) to initialize p_rgh and p with that 2.5 pascal to help the convergence of the solver. |
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Pr is not directly, but it is calculated from the conductivity, viscosity, and specific heat. And they can depend. I'm not 100% if the Pr of air in that really small pressure is the same than on atmospheric pressure. You have to check it before you get unreal results because of that really simple mistake (wrong material props). |
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If I understood right, this is what I did: fvSolutions: Code:
solvers and changeDictionaryDict: Code:
p_rgh I hope this is correct now? |
Yep, seems fine.
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Hi..
Hope you will be fine. I have question related to fvOptions. If I am using fvOptions to put power in one volume, then what exactly this power is? Is this a power dissipation of that object? And how can I calculate that with this power, what would be the temperature of my object? Thank you |
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Hello, I have one question here: How can I find the referenceCell to put in the fvOptions? for example, in my case, I want to have 1e-5 pascal pressure on the complete AIR domain. so how can I put that? I shall be very thankful if you can help. Thank you |
Hi!
If you solve the flow equations, you cant. But as i mentioned earlier you can use frozenFlow in your fvSolution of air and you won't solve any flow equations. Just initialize the domain with that 1e-5 pascal, and zero velocity, and that's all. It won't change since you won't solve any flow equations, only the energy equation. |
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Thank you so much for your reply. Yes. I did that frozen flow, and Now the solver is not solving any flow equation. Good. Now, I reduced the pressure to a very low value for Air, to generate a vacuum. But the results after this remain exactly the same. There is no difference in the temperature gradient as with the high temperature (1 bar). I checked for the prandtl number also, and It doesn't actually change so much with the pressure (its range is approximately 0.7 to 0.8). But I don't know, how the behavior of temperature could be same even with so less pressure? Thank you |
Of course they remain the same with lower pressure. Check the energy equation what you are solving. It is not affected by the pressure.
BUT since you decrease the pressure your material props must change I think, since you have much less air particles. The thermal conductivity of the air will decreasing really fast as you decrease the pressure. In a really high vacuum you will have nearly 0 thermal conductivity. Maybe it is not significant from 1bar to 10. But when you go down from 1 bar to 1e-5 Pa, it should be. |
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Thank you so much for your response. It is helpful. One more question here, Is it possible to use P1 model for Air in my case? I tried P1 model and it is showing that the Air gets hot faster as compared to the case if I don't use radiation. I think this behaviour is correct? or NOT? Thank you |
Not really. P1 is good for optically thick media. For air it is not.
It became hot faster because i think you had wrong coefficients for your absorption emission model and the air is absorbed a lot of heat which is not correct since air is "transparent". The radiation should just go through the air without absorption/emission... That's why viewFactor model is the best choice for you I think. |
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Because I don’t know what I should see to make it sure that my radiation is working fine.? Thank you. |
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I have read in forum that If I want to check that my radiation model is working or NOT, then I must use wallHeatFlux utility on each boundary. but I don't know about that. Can you please help me in this? Thank you |
Yes, this is a good idea :D
run this (I hope i have no typo, and replace the regionName and numberOfProcessors to your needs): chtMultiRegionSimpleFoam -postProcess -func wallHeatFlux -region regionName -latestTime or in parallel: mpirun -np numberOfProcessors chtMultiRegionSimpleFoam -parallel -postProcess -func wallHeatFlux -region regionName -latestTime But you have to run it in on the case with and without radiation so you can see the difference. |
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DO I need to install anything or configure any library etc before doing this? OR I can directly do this? Quote:
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because some people are saying that something should be added in controlDict in functions etc etc. but I didn't use it, so I don't have any idea. |
I RAN it and it gave me the following error:
Code:
Create time |
What version of OF are you using?
You don't need anything for that. It should works: You can also do it in the controlDict file like this (but this is the same function call): https://openfoam.com/documentation/g...x.html#details |
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I did the following: In the controlDict I put: Code:
functions Code:
wallHeatFlux -region air -latestTime And it Works :) But on the air_to_heater interface, it is also showing some convection also. even After I am using frozenFlow=Yes Thank you |
I think you see some conduction through the air.
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