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[2D Axi] Hot airflow through a tube with three solid material layers
Posted 1 giu 2011, 08:40 GMT-4 Fluid & Heat, Heat Transfer & Phase Change, Studies & Solvers Version 4.1 7 Replies
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Hello everyone,
I have a problem that I would like to have solved. It is a relatively simple model in which a stream of air (non-isothermal flow, 873 K) flows through a tube of which the inner layer is made of ceramic material, the next is made of mineral wool and the most outer (thin) layer is made of structural steel (length: 1m).
I have done a stationary simulation which gives believable results. The temperature gradient is plotted in the attached 'SteadyState_2D_Axi.jpg'. Now I wanted to make an instationary model as well, to see how the temperature in the solid material changes in time. My idea was that for longer simulation times, the instationary solution would converge to the stationary solution. However, I have been unable to find a good instat. solution at all.
MODEL (stationary version also attached)
Inflow velocity (v_in) : 0.1 m/s
Inflow temperature (defined at inlet boundary) : 873 K
Material properties well defined and green checks for all.
Mesh: Extra Fine with 12 boundary layers on the fluid/solid boundary
Inlet BC : 'Velocity' (normal inflow) : v_in
Outlet BC: 'pressure, no viscous stress', P=0
Solid+fluid pressure P: user defined: 1 atm
Convective cooling at the outside of the model (h=5 W/m2K)
IC: u2:r=0, u2:z=v_in, P2=0, T0=293 K
Ouflow defined at outlet boundary
If the exact same settings are used for the instationary calculation, it seems there is no heat flow to the solid matter at all. (see attached picture 'Instationary Wrong Result 1 (time range 0 1 300).jpg') .
Also, for some settings there is only an extremely small temperature difference at the air-inlet and no difference in temperature in the rest of the model at all, as if there is no flow. This would seem like a pressure problem, partly also because I find the way COMSOL describes pressures a bit difficult to understand. You need a pressure difference to be able to have flow, but I don't know the delta P and I can't define both an inlet pressure AND velocity.
And sometimes it is unclear whether P is P_absolute or P_gauge.
My main questions are: How do I get the instationary model to work? And: Why is it not possible to define an inlet velocity AND pressure? There is also a fluid pressure, but where does this apply, everywhere, including at the inlet/outlet boundaries?
Please forgive me if I missed something very obvious, but I just couldn't find the problem (I'm quite new to COMSOL by the way). Thanks in advance for your trouble.
I have a problem that I would like to have solved. It is a relatively simple model in which a stream of air (non-isothermal flow, 873 K) flows through a tube of which the inner layer is made of ceramic material, the next is made of mineral wool and the most outer (thin) layer is made of structural steel (length: 1m).
I have done a stationary simulation which gives believable results. The temperature gradient is plotted in the attached 'SteadyState_2D_Axi.jpg'. Now I wanted to make an instationary model as well, to see how the temperature in the solid material changes in time. My idea was that for longer simulation times, the instationary solution would converge to the stationary solution. However, I have been unable to find a good instat. solution at all.
MODEL (stationary version also attached)
Inflow velocity (v_in) : 0.1 m/s
Inflow temperature (defined at inlet boundary) : 873 K
Material properties well defined and green checks for all.
Mesh: Extra Fine with 12 boundary layers on the fluid/solid boundary
Inlet BC : 'Velocity' (normal inflow) : v_in
Outlet BC: 'pressure, no viscous stress', P=0
Solid+fluid pressure P: user defined: 1 atm
Convective cooling at the outside of the model (h=5 W/m2K)
IC: u2:r=0, u2:z=v_in, P2=0, T0=293 K
Ouflow defined at outlet boundary
If the exact same settings are used for the instationary calculation, it seems there is no heat flow to the solid matter at all. (see attached picture 'Instationary Wrong Result 1 (time range 0 1 300).jpg') .
Also, for some settings there is only an extremely small temperature difference at the air-inlet and no difference in temperature in the rest of the model at all, as if there is no flow. This would seem like a pressure problem, partly also because I find the way COMSOL describes pressures a bit difficult to understand. You need a pressure difference to be able to have flow, but I don't know the delta P and I can't define both an inlet pressure AND velocity.
And sometimes it is unclear whether P is P_absolute or P_gauge.
My main questions are: How do I get the instationary model to work? And: Why is it not possible to define an inlet velocity AND pressure? There is also a fluid pressure, but where does this apply, everywhere, including at the inlet/outlet boundaries?
Please forgive me if I missed something very obvious, but I just couldn't find the problem (I'm quite new to COMSOL by the way). Thanks in advance for your trouble.
Attachments:
7 Replies Last Post 15 giu 2011, 03:58 GMT-4
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Posted:
1 decade ago
I found out that if I put the turbulance option from 'none' to 'RANS', then calculate (which failes due to wrong initial values apparently), then put it back to 'none', my (still wrong) solution is completely different from the first solution I posted and indeed shows the behaviour that I described at the end of my first post. I have included a picture of it.
It seems there is no flow at all, even though I changed nothing (in absolute terms anyway).
PS.
I just discovered that it returns to 'normal' when the entire solver is deleted first before recalculation.
But the simulation still doesn't give the right results and the instationary solution never starts to look like the steady state solution (for longer simulation times).
If anyone has a good suggestion I would love to hear it.
Thanks and good luck to everyone,
Ray
It seems there is no flow at all, even though I changed nothing (in absolute terms anyway).
PS.
I just discovered that it returns to 'normal' when the entire solver is deleted first before recalculation.
But the simulation still doesn't give the right results and the instationary solution never starts to look like the steady state solution (for longer simulation times).
If anyone has a good suggestion I would love to hear it.
Thanks and good luck to everyone,
Ray
Attachments:
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Posted:
1 decade ago
I have tried everything I can possibly think of and it still won't give the right solution. I have tried using conjugate heat transfer and that didn't work, so I tried splitting it up using laminar flow and solid heat transfer, but that didn't work either. So I tried only using heat transfer with a fluid flow item in it and although all mentioned models did calculate and did converge, they all give the same wrong answer.
Then I tried using a heat influx instead of a inflow with a certain temperature, again same wrong result.
It seems like there is completely no heat transfer from the hot flow to the solid materials. WHY?
The steady state solution looks fine and I would think the time dependent solution should at least look a bit like it after a while, but I get totally wrong temp. gradients. (or actually NO gradients). I have 16 boundary layers as well, so that can't be it.
Could anyone PLEASE help, because this is taking so much time and I really don't see what is going wrong.
Included are an overview of the model, the steady state T distribution, the velocity field (which seems ok) and the wrong output for the time dependent T distribution that I keep getting all the time.
The model itself is also included.
Please, if you have a few minutes, I would be very grateful if this gets solved.
Thanks and kind regards,
Ray
Then I tried using a heat influx instead of a inflow with a certain temperature, again same wrong result.
It seems like there is completely no heat transfer from the hot flow to the solid materials. WHY?
The steady state solution looks fine and I would think the time dependent solution should at least look a bit like it after a while, but I get totally wrong temp. gradients. (or actually NO gradients). I have 16 boundary layers as well, so that can't be it.
Could anyone PLEASE help, because this is taking so much time and I really don't see what is going wrong.
Included are an overview of the model, the steady state T distribution, the velocity field (which seems ok) and the wrong output for the time dependent T distribution that I keep getting all the time.
The model itself is also included.
Please, if you have a few minutes, I would be very grateful if this gets solved.
Thanks and kind regards,
Ray
Attachments:
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Posted:
1 decade ago
Hi Ray,
It's difficult to heat thick layer of insulation by hot air. Therefore try your simulation to, say, 20000s instead of 200s.
BR,
Andrew K.
It's difficult to heat thick layer of insulation by hot air. Therefore try your simulation to, say, 20000s instead of 200s.
BR,
Andrew K.
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Posted:
1 decade ago
Hi,
In your time-dependent model several BCs are wrong, just compare it with the steady-state one.
At any rate, if you use the steady-state model and add a time dependent study and add time dependent functions to have a time dependent inlet, it seems that you still get the 'wrong' solutions. I didn't look at it very carefully but I would recommend to send it to support, also because I have been having serious issues with heat transfer in the past and I am hoping 4.2 will fix them.
Cheers
In your time-dependent model several BCs are wrong, just compare it with the steady-state one.
At any rate, if you use the steady-state model and add a time dependent study and add time dependent functions to have a time dependent inlet, it seems that you still get the 'wrong' solutions. I didn't look at it very carefully but I would recommend to send it to support, also because I have been having serious issues with heat transfer in the past and I am hoping 4.2 will fix them.
Cheers
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Posted:
1 decade ago
Dear Amir and Lechoslaw,
many thanks for your input. Just setting a much longer simulation time did indeed do the trick. I must say I am a bit surprised because I did try longer times before and it didn't seem to matter.
I'm just very glad the models were OK after all.
Thanks again!!
many thanks for your input. Just setting a much longer simulation time did indeed do the trick. I must say I am a bit surprised because I did try longer times before and it didn't seem to matter.
I'm just very glad the models were OK after all.
Thanks again!!
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Posted:
1 decade ago
Hi,
Excellent, out of curiosity how long did it take to go to steady-state? Did you do an energy balance to check the amount of heat transferred to the solid?
Cheers
Excellent, out of curiosity how long did it take to go to steady-state? Did you do an energy balance to check the amount of heat transferred to the solid?
Cheers
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Posted:
1 decade ago
Well, it took about 26000 seconds for it to reach steady state. I haven't done a check yet because I don't really know how to do that, but I'm working on it. If I have new results I will post them.
Thanks again guys.
Ray
Thanks again guys.
Ray