Fan noise consists of rotational noise and vortex noise. Rotational noise is produced by periodic disturbances created when the rotating fan blades cut through the airflow, while vortex noise is generated when airflow separates over the section of the rotating blades and, due to gas viscosity, slips off or splits into a series of vortices, thereby radiating unstable flow noise. The exhaust duct communicates directly with the outside atmosphere, and the air velocity is high, so airflow noise, fan noise, and mechanical noise are radiated through this duct.

The main means of controlling fan and exhaust duct noise is to design an effective sound-absorbing exhaust duct. The sound-absorbing duct may consist of an air guide channel and an exhaust noise reduction box, or of an air guide channel plus one or several groups of sound-absorbing baffles. The working principle of the exhaust noise reduction box is similar to that of an absorptive silencer. The sound absorption effect can be improved by replacing sound-absorbing materials (changing the material's sound absorption coefficient) and by adjusting parameters such as the thickness of the sound-absorbing material, as well as the length and width of the exhaust duct. When designing an exhaust sound-absorbing duct, special attention must be paid to ensuring that the effective area of the exhaust outlet meets the heat dissipation requirements of the generator set, so as to avoid increased exhaust outlet resistance, which can lead to higher exhaust noise and generator set shutdown due to high coolant temperature.

4. Control of intake noise

When the generator set operates in an enclosed generator room, broadly speaking, the intake system includes both the generator set air inlet passage and the engine intake system. Like the exhaust duct, the air inlet passage communicates directly with the outside atmosphere. The air velocity is high, and both airflow noise and generator operating noise are radiated to the outside through the air inlet passage. Noise from the engine intake system is formed by pressure fluctuations generated by the periodic opening and closing of the intake valve, and its noise frequency is generally in the low-frequency range below 500Hz.

For turbocharged engines, because the turbocharger rotates at very high speed, the intake noise is significantly higher than that of naturally aspirated engines. Turbocharger compressor noise consists of rotational noise generated by the periodic impact of the blades on the air and vortex noise formed by high-speed airflow, and it is a continuous high-frequency noise whose main energy is generally distributed in the 500Hz~10kHz range.

The air filter equipped on a diesel generator set itself already has a certain silencing effect. Considering that intake noise is relatively low, the engine intake system is generally not treated separately. For the air inlet passage of the generator set, comprehensive control should be carried out from aspects such as air duct design and the selection of sound insulation materials. The basic approach is:

1) The net air inlet area should comply with design specifications to ensure that the engine intake system and the generator set cooling system have sufficient fresh air intake.

2) The air inlet passage should undergo sound absorption treatment. Generally, a combination of air inlet louvers, an air guide channel, and silencing baffles is used. If there is sufficient space, a combination of air inlet louvers and a noise reduction box may also be used.

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