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 Guosheng Kang October 23, 1999 15:56

Mach number and flow compressibility

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

 clifford bradford October 24, 1999 17:18

Re: Mach number and flow compressibility

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.

 Demselles Hollowajiwoak October 25, 1999 14:50

Re: Mach number and flow compressibility

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

 Adrin Gharakhani October 25, 1999 15:00

Re: Mach number and flow compressibility

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)

 Guosheng Kang October 26, 1999 12:51

Re: Mach number and flow compressibility

Thank you all!

 Rainer Kurz October 26, 1999 15:56

Re: Mach number and flow compressibility

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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