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Ehsan July 15, 2008 18:01

Mixed convection_HELP

I'm interested in the Mixed-CHTC (Mixed Convective Heat coefficient)! Below you can read the problem description and other important information.


c d -----------------------------' ' HOT_CEILING <--'e g h ' (318K) '-------' ' f ^ ' ' ' ' ' ' ' 'COLD_WALL Adiabatic ' ' (273K) ' ' ' ' ' ' ' ' Adiabatic ' -------------------------------------' a b

@Dimesions(in meters) with relative coordinates(X,Y):

a=(0,0) b=(2.78;0) c=(0;2.3) d=(2.28;2.3) e=(2.28;2.255) f=(2.28;2.2) g=(2.735;2.2) h=(2.78;2.2)

@ 1- T_exhaust = T_inlet (approximates the recirculation idea) It is a steady approach(in theory after certain of time) 2- HOT_CEILING had a uniform temperature distribution; this applies also for the COLD_WALL 3- a uniform velocity_inlet of 0,4 m/s into the cavity; so with a normal [x,y] of [-1,0] 4- a pressure_outlet with a Gauge Pressure = 0 and directional [x,y] = [0,1] 5- the velocity_inlet height=0,045 mm --> D(hydraulic)=0.084 6- Almost no turbulence at the inlet or the 5% turbulence intensity at the velocity_inlet. 7- AIR --> Compressible!!!? because the Brinkman-number>1 --> Br=2,25 --> (17,5*0,4^2)/(0,0276*45) 8- Boussinesq approximation for density is used;

@Solution Strategy:

FLUENT Version: 2d, dp, pbns, ske (2d, double precision, pressure-based, standard k-epsilon) Release: 6.3.26

Model Settings ---------------------------------------------------------------- Space:2d Time Steady Viscous Standard k-epsilon turbulence model Wall Treatment Enhanced Wall Treatment Heat Transfer Enabled Radiation None

Discretization Scheme:

Variable Scheme


Pressure PRESTO!

Momentum Second Order Upwind

Turbulent Kinetic Energy Second Order Upwind

Turbulent Dissipation Rate Second Order Upwind

Energy Second Order Upwind


No Converged results...very high instability of local temperature which i do monitor by looking at the bahaviour of the temperature at a certain point in the interior..For instance at (0,2;0,2).

I tried also:

1- decoupled solution: First get converged solution by setting Energy 'Off' in SOlution Control window...and indeed it did converged but after setting energy back 'on'.it begins te oscillate too much and so i get diverged results. Even by using UnderRelaxation Factor for Energy of 0,4 (Which is according to FluentGuide a problem to do so). The lowest factor might use is not lower than 0,8!

2- Step by step solution:

-I also tried to decrease the Rayleigh-number by power of 10. It seems also to converged. But after setting the gravity acceleration back to -9,81 m/s^ begint again to oscillate...and instabillity as result! - Also i tried a nother Solver --> The coupled implicit (or the Density Based Solver)...No convergence! - I tried also another turbulence models(KNG and k-omega), and other density approximations like incompressible ideal-gas...still no success. - I also changed the mesh..but it is still the same problem.

@QUESTION ARE THERE ANY OTHER IDEAS...What should be the problem? We have here a Forced convection at the ceiling and a natural convection at the left wall (COLD_WALL). Is there any other solution strategy, which should help me to getconverged results.

If you want to see the mesh, please contact me.

Thank you all,


Ehsan July 15, 2008 18:04

Re: Mixed convection_HELP
Oh i see that it is not really working. i tried to draw the situation!

If you are interested i can e-mail it to you.

thank you Ehsan

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