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Mach number and flow compressibility

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Old   October 23, 1999, 15:56
Default Mach number and flow compressibility
  #1
Guosheng Kang
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I was told that for combustion in industrial furnace at regular pressure (P is around 1 atm), the gaseous fuel and oxygen or air is usually introduced into the combustion chamber at a speed less than 120 m/s, so that the Mach number is less than 0.3, thus the noise will meet the regulation. I have no experience of operating a burner at high speeds, so I need advise from you all on two questions. (1). Does anyone know that the speed described above exceeds 120 m/s in practice? (2). Assume that u=140 m/s causes the Mach number is exactly 0.30, then for u=139 m/s the imcompressible flow model is used, but for u= 141 m/s a compressible flow model should be used, is there a big jump across this point? Thanks. Guosheng Kang
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Old   October 24, 1999, 17:18
Default Re: Mach number and flow compressibility
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clifford bradford
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for combustion you should probably use compressible flow to take into acc't bouyancy effect unless you're using say Boussinesq approximation. i can't answer (1) but the answer to (2) is the jump shouldn't be too big.
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Old   October 25, 1999, 14:50
Default Re: Mach number and flow compressibility
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Demselles Hollowajiwoak
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Instead of asking if the difference between M=.299 and M=.301 is siginificant, you should consider if assuming M=.299<0.3 is incompressible just because it is less than 0.3. M=0.3 is not a singularity about which compressibility effects change dramatically. It is a "rule of thumb" that must be used carefully when M is very near 0.3. I would also agree that buoyancy effects should be considered for this process.

Regards, Demselles Hollowajiwoak
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Old   October 25, 1999, 15:00
Default Re: Mach number and flow compressibility
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Adrin Gharakhani
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You can obviously not use the "incompressible" flow assumption in the case of combustion - as metnioned earlier.

What you really mean to say is that you are using a low Mach number approximation of compressible flow - which looks like the incompressible N-S equation (but you can still allow for density variation).

The order of accuracy of this "model" is Mach**2 - so for most practical problems you're safe even at as high as Ma=0.4 (though the norm is Ma=0.3)

Adrin Gharakhani
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Old   October 26, 1999, 12:51
Default Re: Mach number and flow compressibility
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Guosheng Kang
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Thank you all!
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Old   October 26, 1999, 15:56
Default Re: Mach number and flow compressibility
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Rainer Kurz
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Item 2: No. Gas flow is compressible at any Mach number. However, below Mach 0.3, the error by using the assumption of incompressible flow is typically tolerable.
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