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how to set the heat transfer of the wall for compressible gas case

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Old   March 6, 2019, 09:07
Default how to set the heat transfer of the wall for compressible gas case
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Hi everyone! I have a problem for the setting of the heat transfer for the wall. In my case I focused on the cooling process which hapens in the chamber, and there is only one inlet without the outlet for the gas, so I used the ideal gas namely the density is a variable. The problem is that when I use the 'mass flow rate' as the inlet boudary, because the gas is compressible so in the chamber there is a influence of the heat produced by the increasing gas. The solution what I used now is to set the heat transfer coefficient of the wall, to make sure the temperature of the gas in the chamber dosen't increase (stable), that means I have eliminate the heat influence of the compressible gas.
The only one problem is that my 'mass flow flow rate' is different in different cases, so I have to adjust the heat transfer coefficient of the wall everytimes, and that waste time. Is there a better solution to solve that problem? Thank you!
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Old   March 6, 2019, 17:50
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Yes, the better solution is to model whatever the actual device has acting at its boundary. Don't forget the golden rule of boundary conditions is that if you cannot define the conditions at the boundary then you can't use it as a boundary condition.

Likewise for your mass flow rate inlet boundary with no outlet - are you sure this is right? This is a very unusual situation if it is correct. Most systems like this would have a inlet mass flow rate which decreases over time as the pressure in the chamber increases.
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Old   March 7, 2019, 02:45
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Yes, the better solution is to model whatever the actual device has acting at its boundary. Don't forget the golden rule of boundary conditions is that if you cannot define the conditions at the boundary then you can't use it as a boundary condition.

Likewise for your mass flow rate inlet boundary with no outlet - are you sure this is right? This is a very unusual situation if it is correct. Most systems like this would have a inlet mass flow rate which decreases over time as the pressure in the chamber increases.
Yes, I just simulate the experiment simulation. Only inlet so the pressure in the chamber will keep increasing (not very high, about 0.2 bar to 0.6 bar). And there is no sensor on the inlet, actually I don't get the value (velocity or mass flow rate) directly, and in this situation I can only use the ideal gas law to caculate the mass flow rate (pressure difference is known)
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Old   March 7, 2019, 05:32
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On your inlet valve - a more common situation would be a constant pressure source with a throttling valve. This can only be approximated as a constant mass flow if the pressure source is much higher than the vessel pressure.
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