How to control the noise of diesel generator set

2022-09-22
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How to control the noise of diesel generator set

the noise of diesel generator sets often becomes the main pollution source of ambient noise. At present, the society has higher and higher requirements for environmental protection. How to effectively control its noise pollution is a difficult work with great promotion value. This is also the main work of our environmental protection, which should be paid more attention. In order to do this work well, we must first understand and analyze the noise composition of diesel generator sets

I. cause analysis of diesel generator noise:

diesel engine noise is a complex sound source composed of many sound sources. According to the noise radiation mode, diesel engine noise can be divided into aerodynamic noise and surface radiation noise. According to the generation mechanism, the surface radiation noise of diesel engine can be divided into combustion noise and mechanical noise. Aerodynamic noise is the main noise source

(I). Aerodynamic noise:

aerodynamic noise is caused by the disturbance of gas and the interaction between gas and object, because the unstable products of gas can be increased by about 10 ℃ after annealing. Aerodynamic noise radiated directly into the atmosphere includes: intake noise, exhaust noise, and cooling fan noise

1. Intake noise:

intake noise is one of the main aerodynamic noise of diesel engine, which is formed by the pressure fluctuation caused by the periodic opening and closing of intake valve. When the intake valve is opened, a pressure pulse is generated in the intake pipe, and as the piston continues to move, it is damped; When the inlet valve is closed, a pressure pulse with a certain duration is also generated. As a result, periodic intake noise is generated. Its noise frequency components are mainly concentrated in the low-frequency range below 200 Hz. At the same time, when the air flow flows through the flow section of the inlet valve at a high speed, turbulence separation occurs, resulting in the generation of high-frequency noise. Because the flow section of the inlet valve is constantly changing, the turbulence noise has a certain frequency range, mainly concentrated in the high-frequency range above 1000 Hz. When the natural frequency of the air column of the intake pipe is consistent with the main frequency of the periodic intake noise, the resonance noise of the air column will also be more prominent in the intake noise

for turbocharged engines, the intake noise of turbochargers is significantly higher than that of non turbocharged engines because the speed of turbochargers is generally high. The noise of turbocharger is caused by the rotating noise generated by the blade periodically cutting the air and the turbulent noise formed by the high-speed air flow. It is a continuous high-frequency noise, mainly distributed in the frequency range of 500~10000 Hz. At present, most of our company adopts turbocharged engines

intake noise is related to design factors such as intake mode, intake valve structure, cylinder diameter, cam profile, etc. For the same engine, it is most affected by the speed. Doubling the speed can increase the intake noise by 10~l5db (a)

2. Exhaust noise:

exhaust noise is the main sound source of engine noise, and its noise is generally 10~15db (a) higher than that of the whole engine. The engine exhaust is a high temperature (800~1000 ℃) and high pressure (3~4 atmospheres) gas. The exhaust process is generally divided into two stages, namely, free exhaust stage and forced exhaust stage. Engine exhaust gases rush out of the exhaust valve at high speed, enter the muffler along the exhaust manifold, and finally discharge into the atmosphere from the tailpipe. In this process, broadband exhaust noise is generated

exhaust noise includes complex noise components: exhaust noise with the number of exhaust times per unit time as the fundamental frequency, gas column resonance noise in the pipeline, air blowing noise at the exhaust manifold, exhaust gas injection and impact noise, Helmholtz resonance noise of the cylinder, Carmen vortex noise, turbulence noise inside the exhaust system, etc

the main factors affecting engine exhaust noise are: cylinder pressure, exhaust valve diameter, engine displacement, exhaust valve opening characteristics, etc. For the same engine, engine speed and load are the most important factors affecting its exhaust noise

3. Cooling fan noise:

fan noise consists of rotating noise and turbulent noise. The rotating noise is caused by the pressure pulsation of the air caused by the periodic cutting of the fan blades, with the blade passing frequency as the fundamental frequency and accompanied by high-order harmonics. Turbulent noise is a wide-band noise caused by the turbulent separation of the surrounding air caused by the movement of the fan, which disturbs the air and forms the compression and thinning process of the gas

the noise of the cooling fan is most affected by the speed. Doubling the speed can increase its sound level by 10~15db (a). At low speed, the fan noise is much lower than the engine noise, and at high speed, it often becomes the main noise source. At present, the speed of diesel engine used by our company is 1500 rpm, which belongs to high-speed diesel engine

(II). Surface radiated noise:

combustion noise and mechanical noise are difficult to be strictly distinguished. Generally, the noise radiated outward through the cylinder head, piston connecting rod crankshaft body due to the pressure vibration formed by combustion in the cylinder is called combustion noise. The noise produced by the impact of the piston on the cylinder liner and the mechanical impact vibration between the moving parts such as timing gear, valve train and fuel injection system is called mechanical noise. Generally, the combustion noise of direct injection diesel engine is higher than the mechanical noise, while the mechanical noise of non direct injection diesel engine is higher than the combustion noise, but the combustion noise is higher than the mechanical noise at low speed

II. Control measures to solve noise:

(I) aerodynamic noise control:

1. Intake noise control:

generally, engines are equipped with air filters, and the intake noise can be greatly attenuated, becoming a "secondary sound source", said Dr. James Mitchell, general supervisor of Solvay engineering plastics global automotive market. When other sound sources are further controlled, the intake noise may become the main sound source. At this time, it is necessary to consider using an intake muffler with good performance. Generally, the intake muffler should be combined with the air filter for integrated design, which can not only meet the requirements of intake and filtration, but also effectively control the intake noise

2. Exhaust noise control:

the most effective way to control exhaust noise is to install an exhaust muffler. The actual situation is often that the noise reduction effect is not ideal. The main reasons are that the structural design of muffler is unreasonable and there are problems in processing technology. The latter problem can be improved by improving the process level; The former problem involves the design idea of muffler. Generally, the muffler design is mainly based on experience, and some design and calculation procedures are carried out under some ideal assumptions. In these assumptions, the actual influence is to ignore the existence of air flow, and the existence of high-pressure, high-temperature and high-speed pulsating air flow. The air flow in this state will affect the sound field distribution, sound velocity and sound propagation law inside the muffler, especially the air flow velocity. The main reason why the air flow affects the performance of the muffler is the regeneration noise of the high-speed pulsating air flow of the engine exhaust. Secondly, this air flow will impact the acoustic components of the muffler, such as the pipeline, shell, diaphragm, etc., and then excite the vibration radiation noise. When the structural parameters of the muffler are improperly selected, or the structure is unreasonable, or there are problems in the processing technology, the silencing performance of the muffler will decline. At the same time, too high air velocity will also increase the pressure loss of the muffler and cause the silencing performance to decline

(II). Control of engine surface radiated noise:

the control of engine surface radiated noise (combustion noise and mechanical noise) is subject to various restrictions on engine performance. From a technical point of view, it is very difficult and the amount of noise reduction is limited. The practice shows that taking measures on the structure can reduce the surface radiation noise of the engine to a certain extent, so as to reduce the noise of the whole machine. The basic measure of control is to increase the stiffness and damping of the structure, so as to reduce the surface response of the structure under the same excitation force. At the same time, reducing the surface area of radiated noise is also an effective measure to control radiated noise

(III). Practical application of the idea of comprehensive noise control:

usually a 500 kW imported unit, the noise in the machine room can reach 105~108 dB (a). Without treatment, the ambient noise outside the machine room is 70~80 dB (a) or higher, and the noise of domestic units with the same power parameters is greater. At present, when assessing whether the environmental noise meets the standard, China adopts the environmental noise standard for urban areas or the noise standard for industrial enterprises at the boundary, in which there are different noise limits corresponding to different areas. Generally, it is a class I area in urban areas, and the limit standard is 55 dB (a) in the daytime and 45 dB (a) at night; In the suburbs, there are mostly class II areas, and the corresponding limit standards are 60 dB (a) in the daytime and 50 dB (a) at night. It can be seen from the comparative data that the required noise reduction range is very large, and the corresponding control technology is also very difficult.

in practical work, because our company selects all configured engine machines, it is very difficult and unrealistic for the unit itself to take control measures. Considering that the noise of oil engine generally pollutes the surrounding environment during its operation, how to effectively control the external radiation of oil engine noise in the machine room is a very practical problem that must be solved. The selected scheme should not only effectively reduce the environmental noise, but also organize the air flow in the machine room to meet the air flow required for the operation of the generator unit, so as to ensure the normal operation of the unit. Simply reducing the leakage of noise and sacrificing the air flow in the oil machine room will cause uneven cooling of the oil machine surface, reduce the power generation capacity of the oil machine, and affect normal use. After years of cooperation with the environmental protection department not to point the instrument at anyone's Department, we have accumulated some treatment experience in noise elimination of the oil engine room, mainly because the corresponding control scheme must be determined according to the specific project of the engine room, which should consider the environmental standards of the area where the engine room is located, the form of enclosure structure of the engine room, the model, power, cooling air volume and other factors of the engine room. The core of the integrated control is the concept of equal sound insulation, that is, a closed enclosure is used to isolate the unit from the outside world and reduce the external sound radiation of the sound source. The so-called equal sound insulation concept means that the sound insulation of all parts of the entire enclosure structure (such as civil structure and doors, windows, etc.) should be equal. The channels reserved for connecting the machine room with the outside world (such as cooling fan outlet, engine exhaust outlet, machine room ventilation outlet, etc.) must be designed as silencing channels, and their insertion loss should also be equivalent to the sound insulation of the enclosure structure. Only in this way can the environmental noise outside the machine room meet the standard. We still take a 500 kW imported unit as an example. The indoor wall of the oil machine room is designed to be pasted with sound-absorbing board, and at the same time, the sound-absorbing board ceiling is used. After such sound-absorbing treatment, it not only increases the sound insulation of the enclosure structure, but also reduces the reverberation sound in the oil machine room, which can generally have an effect of 3~5db (a). Mario Enriquez from Johnson Controls highlighted intelligent materials (the properties of such materials can respond to environmental changes) as an important trend of interior decoration. High frequency noise in engine noise can be achieved by blocking because of its wave length. Because the low-frequency components of engine noise are more abundant, simple blocking cannot achieve satisfaction

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