# Question on boussinesq approximation

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 January 11, 2010, 05:58 Question on boussinesq approximation #1 Member   Panos Join Date: Nov 2009 Posts: 33 Rep Power: 9 Hello cfd users I simulate the air flow in a heater. The wall temperature is approximately 1200 oC. The height is 3 meters and the diameter of the tube is 13 cm. The air flow range from 10 to 220 lit/min. i simulated the procedure in fluent. I would like to use the density of the air in correlation to the air temperature. Can i use the boussinesq approximation? i have read that boussinesq approximation is not appropriate when the temperature differences are large. But this refers to the neighboor cells or to the Toutlet-Tinlet air temperature? Thank you in advance

January 11, 2010, 12:18
boussinessq
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CFDtoy
Join Date: Mar 2009
Location: United States
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Hi there,
Boussinessq goes : rho_modified = rho_ref*(1+beta*deltat). Now, if you have large deltat (Tcell - Treference with which fluid properties are defined)...beta*deltat starts to go close to 1...which means its no longer a small change in rho_ref in a given cell. thats why they indicate u cant use bouss at high Tdiff.

For your case, twall ~ 1200 C - pretty high..yes..but you could definitely try boussinessq approx here. If you see oscillations in the solution, probably you can gain more insight.

/CFDtoy
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Quote:
 Originally Posted by panos_metal Hello cfd users I simulate the air flow in a heater. The wall temperature is approximately 1200 oC. The height is 3 meters and the diameter of the tube is 13 cm. The air flow range from 10 to 220 lit/min. i simulated the procedure in fluent. I would like to use the density of the air in correlation to the air temperature. Can i use the boussinesq approximation? i have read that boussinesq approximation is not appropriate when the temperature differences are large. But this refers to the neighboor cells or to the Toutlet-Tinlet air temperature? Thank you in advance

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