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Problem in Natual Convection

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Old   June 11, 2018, 05:42
Default Problem in Natual Convection
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Hi all
I'm simulating a natural convection problem in fluent.this case hase been simulated in comsol but my homework is to do it in fluent.the problem is simulation of convection & conduction in a vacuum flask.flask contains of 90c coffee that's insulated by foam and covered by steel.flask is in ambient temp(25c).purpose is to model natural convecion in 10hours.i calculated rayleigh number and that's about 10^7(laminar).
i created my geometry in GAMBIT.meshed it fineand created zones for fluids & solids.then I imporeted mesh in fluent with this settings:

Model Settings
---------------------------------------------------------
Space 2D
Time Unsteady, 1st-Order Implicit
Viscous Laminar
Heat Transfer Enabled
Gravity Enabled
Solidification and Melting Disabled
Radiation None
Species Disabled
Coupled Dispersed Phase Disabled
---------------------------------------------------

now I have some problems.i wanted to set the initial coffee zone temp as 90c and air zone pressure as 1atm so i patched those faces,but when i ran calculation and see temp countor i see every where is 90c.i've tried to adapt regoins but my geomtry isn't simple(maybe I sould make smaller simple regions).I'm realy stucked!
5.png
my second problem is in boundary conditions.I've set all of them as "wall".when i import it in fluent i can just coupled thermal condition and I cant assign H.T.C and temp.

Last edited by Cyrus69; June 13, 2018 at 11:04.
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Old   June 11, 2018, 07:36
Smile Problem in Natural Convection
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Sankarasubramanian R
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Hello Cyrus69,

The simulation seems interesting. I have good experience working on Conjugate Heat Transfer simulations. If interested, please drop me a mail at rsankar1069@gmail.com
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Old   June 11, 2018, 10:02
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Quote:
Originally Posted by rsankar1069 View Post
Hello Cyrus69,

The simulation seems interesting. I have good experience working on Conjugate Heat Transfer simulations. If interested, please drop me a mail at rsankar1069@gmail.com
i sent u geometry and case file.
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Old   June 11, 2018, 16:33
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i encounter another problem.i've used boussinesq for density but after calculation i don't see any density diffrence in it!
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Old   June 12, 2018, 02:51
Post Problem in Natural Convection
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Sankarasubramanian R
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Hello Cyrus ,

1) Looking at the case files, I see that you have not defined an interface connection between various domains. You will have to define interface connections between two different domains in contact in order for the heat dissipated from one body to transfer to the other body.

2) It would also be better to specify the operating density under the operating conditions tab close to the density value of air at ambient conditions. Buoyant forces are driven through the difference in densities calculated with operating density as reference.

Please let me know if the case runs fine with these changes made.
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Old   June 13, 2018, 05:46
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thanks dear sankara for ur tips.
i've made some changes.i've a created a fluid zone with water properties for coffee zone and i created interface bc as wall,so fluent made coupled Bc automaticaly.
i made boussinesq density for air with 298k operating temp and 1.225 operating density.i set interface between air and coffee as wall!(i know its not correct but it gives me reasonable result.attached results).
ididnt find any answer for density problem.velocity countor is good but shows 0.5 m/s speed at the top corner of flask(i think that's high).
i'm still workin on it.i'll be glad to know ur ideas.
velocity(300sec).jpg

static temp(300sec).jpg

density.jpg
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