Mach number and flow compressibility

Guosheng Kang

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