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Movement of air due to thermal expansion
Posted Dec 16, 2012, 12:13 p.m. EST Fluid & Heat, Heat Transfer & Phase Change, Computational Fluid Dynamics (CFD), Heat Transfer 3 Replies
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Hi everybody!
I'm trying to simulate the movement of the air in 2D and I only get errors. I use two physics: laminar flow with heat transfer. I've created a square full of air and I put easy boundary conditions:
-Left edge: 120 ºK
-Right edge: 300 ºK
-Top side and down side have thermal insulation and don't have any loss of heat.
The square is closed and it hasn't got inflows or outflows.
I have used a lot of different volume forces: Boussinesq approximation, a reference density...However I think the best idea is to put F=g_const*spf.rho.
I saw Buoyancy Flow in Free Fluids in Model Gallery and followed every step though it uses adimensional parameters and I want to draw on the material library using air properties. I've noticed that when I use a liquid instead of the air all works properly, I think it is due to the viscosity. I use a parametric sweep starting on a value of viscosity which doesn't give any error and decreasing it until the real air's viscosity.
I get a solution with this parametric sweep but I'm sure it's erroneous because I get the values of total heat flux according to each value of viscosity and they aren't the same on the right edge than on the left edge and they must be. However when the viscosity have high values, the total heat flux are the same.
The mesh I use is finer and the study is stationary.
I'm so frustrated because it should be really simple.
Sorry for my English level and thank you for your time.
I'm trying to simulate the movement of the air in 2D and I only get errors. I use two physics: laminar flow with heat transfer. I've created a square full of air and I put easy boundary conditions:
-Left edge: 120 ºK
-Right edge: 300 ºK
-Top side and down side have thermal insulation and don't have any loss of heat.
The square is closed and it hasn't got inflows or outflows.
I have used a lot of different volume forces: Boussinesq approximation, a reference density...However I think the best idea is to put F=g_const*spf.rho.
I saw Buoyancy Flow in Free Fluids in Model Gallery and followed every step though it uses adimensional parameters and I want to draw on the material library using air properties. I've noticed that when I use a liquid instead of the air all works properly, I think it is due to the viscosity. I use a parametric sweep starting on a value of viscosity which doesn't give any error and decreasing it until the real air's viscosity.
I get a solution with this parametric sweep but I'm sure it's erroneous because I get the values of total heat flux according to each value of viscosity and they aren't the same on the right edge than on the left edge and they must be. However when the viscosity have high values, the total heat flux are the same.
The mesh I use is finer and the study is stationary.
I'm so frustrated because it should be really simple.
Sorry for my English level and thank you for your time.
3 Replies Last Post Mar 3, 2013, 8:02 a.m. EST
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Posted:
1 decade ago
Hi
the natural convection model is far from "simple", probably your mesh is still too coarse, and are you using "boundary mesh" on the no-slip wall boundaries ?
--
Good luck
Ivar
the natural convection model is far from "simple", probably your mesh is still too coarse, and are you using "boundary mesh" on the no-slip wall boundaries ?
--
Good luck
Ivar
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Posted:
1 decade ago
Hi All,
i hope this theme is still "hot" enough so i've made some simple model to make sure the best boundary condition for this simulation.
it's in 2D, a pipe filled with air and surrounded by air, free convection. the inner pipe is heated with 40 degC constant.
Non Isothermal flow is used so that air convection due to different density (because of temperature) can be evaluated. in attached modell (*comm.mph) you can see that hot air moves up, coorect. but nitf.U (fluid velocity) shows that air surround it moves also, i dont think that makes any sense. and there is some area above the circle that doesnt move, it makes no sense. this modell was made based on this tutorial (gravity_tutorial.mph some where you can find it's pdf)
i'm still questioning: for BC Outlet or Inlet which boundary condition is the best velocity, preasure, no viscous stress, normal stress or laminar flow.
that the outer circle should little bit cooler than inner circle, it's correct.
i hope for interesting discussion.
best regard
akmal
i hope this theme is still "hot" enough so i've made some simple model to make sure the best boundary condition for this simulation.
it's in 2D, a pipe filled with air and surrounded by air, free convection. the inner pipe is heated with 40 degC constant.
Non Isothermal flow is used so that air convection due to different density (because of temperature) can be evaluated. in attached modell (*comm.mph) you can see that hot air moves up, coorect. but nitf.U (fluid velocity) shows that air surround it moves also, i dont think that makes any sense. and there is some area above the circle that doesnt move, it makes no sense. this modell was made based on this tutorial (gravity_tutorial.mph some where you can find it's pdf)
i'm still questioning: for BC Outlet or Inlet which boundary condition is the best velocity, preasure, no viscous stress, normal stress or laminar flow.
that the outer circle should little bit cooler than inner circle, it's correct.
i hope for interesting discussion.
best regard
akmal
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Posted:
1 decade ago
Hi
I believe your models are starting too far from some realistic steady state conditions, the temperature gradient is rather high for a not too dense mesh around the "hot source", by ramping up a stationary case you might get better results. Also your steady state get stuck in vortex calculations.
CFD and in particular when you ad heat convection is very tricky and need some careful thinking and model set up
--
Good luck
Ivar
I believe your models are starting too far from some realistic steady state conditions, the temperature gradient is rather high for a not too dense mesh around the "hot source", by ramping up a stationary case you might get better results. Also your steady state get stuck in vortex calculations.
CFD and in particular when you ad heat convection is very tricky and need some careful thinking and model set up
--
Good luck
Ivar