Convection in a refrigerator

CaglarCoskun · March 1, 2013, 11:28

Hello

I am trying to simulate the mixed convection of the air inside of a household refrigerator cabin for my thesis. Geometry is somewhat complex (shelves, drawers, compartments etc.)

My case is transient. It includes the stops and starts of compressor of the refrigerator, so boundary conditions are time-dependent. The temperatures at certain points inside the refrigerator cabin are fluctuating with respect to the boundary conditions.

My problem is, the temperatures of numeric results are not as responsive as the experimental data. An example comparison is given in an Excel file in the link below. As you can see the plot in the file, the initial temperatures of experimental and numeric data are almost the same, but as the time passes the error between them increases. What can you suggest about this?

My settings are as follows:
Turbulence Model: Spalart - Allmaras
Boussinesq approximation is used for density changes
Pressure, Momentum, Energy and Turbulent viscosity schemes: 2. Order Upwind.
Pressure - Velocity Scheme: SIMPLE
Under - relaxation factors are default.
Time step: 10 sec.

The file is in the following link:
http://dl.dropbox.com/u/53986989/dd7...nuc_forum.xlsx

Thanks in advance...

CaglarCoskun · March 2, 2013, 14:24

Hey!

Any ideas? =)

macfly · March 3, 2013, 13:26

Why did you choose de S-A turbulence model? Maybe you should try a 2-equations model ( $\kappa$ - $\varepsilon$ or $\kappa$ - $\omega$ ).

From the Fluent Theory Guide:
"The Spalart-Allmaras model was developed for aerodynamic flows. It is not calibrated for general industrial flows, and does produce relatively larger errors for some free shear flows, especially plane and round jet flows. In addition, it cannot be relied on to predict the decay of homogeneous, isotropic turbulence."

CaglarCoskun · March 3, 2013, 13:51

Thanks for your reply.
I have read the part about turbulence models in Theory Guide. In my problem, I did some steady simulations to validate my mesh at first. In that steady simulations, I tried Spalart - Allmaras, K - e, and K - w and their variants. However, I got the most accurate results by the Spalart - Allmaras turbulence model, so I am still going on with Spalart - Allmaras.

March 1, 2013, 11:28	Convection in a refrigerator	#1
CaglarCoskun New Member Çağlar Coşkun Join Date: Feb 2012 Posts: 20 Rep Power: 14	Hello I am trying to simulate the mixed convection of the air inside of a household refrigerator cabin for my thesis. Geometry is somewhat complex (shelves, drawers, compartments etc.) My case is transient. It includes the stops and starts of compressor of the refrigerator, so boundary conditions are time-dependent. The temperatures at certain points inside the refrigerator cabin are fluctuating with respect to the boundary conditions. My problem is, the temperatures of numeric results are not as responsive as the experimental data. An example comparison is given in an Excel file in the link below. As you can see the plot in the file, the initial temperatures of experimental and numeric data are almost the same, but as the time passes the error between them increases. What can you suggest about this? My settings are as follows: Turbulence Model: Spalart - Allmaras Boussinesq approximation is used for density changes Pressure, Momentum, Energy and Turbulent viscosity schemes: 2. Order Upwind. Pressure - Velocity Scheme: SIMPLE Under - relaxation factors are default. Time step: 10 sec. The file is in the following link: http://dl.dropbox.com/u/53986989/dd7...nuc_forum.xlsx Thanks in advance... Last edited by CaglarCoskun; March 2, 2013 at 05:08.

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March 2, 2013, 14:24		#2
CaglarCoskun New Member Çağlar Coşkun Join Date: Feb 2012 Posts: 20 Rep Power: 14	Hey! Any ideas? =)

March 3, 2013, 13:26		#3
macfly Senior Member François Grégoire Join Date: Jan 2010 Location: Canada Posts: 392 Rep Power: 17	Why did you choose de S-A turbulence model? Maybe you should try a 2-equations model ( $\kappa$ - $\varepsilon$ or $\kappa$ - $\omega$ ). From the Fluent Theory Guide: "The Spalart-Allmaras model was developed for aerodynamic flows. It is not calibrated for general industrial flows, and does produce relatively larger errors for some free shear flows, especially plane and round jet flows. In addition, it cannot be relied on to predict the decay of homogeneous, isotropic turbulence."

March 3, 2013, 13:51		#4
CaglarCoskun New Member Çağlar Coşkun Join Date: Feb 2012 Posts: 20 Rep Power: 14	Thanks for your reply. I have read the part about turbulence models in Theory Guide. In my problem, I did some steady simulations to validate my mesh at first. In that steady simulations, I tried Spalart - Allmaras, K - e, and K - w and their variants. However, I got the most accurate results by the Spalart - Allmaras turbulence model, so I am still going on with Spalart - Allmaras.