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

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.

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 
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 NS 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 
Re: Mach number and flow compressibility
Thank you all!

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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