Sound is produced by the vibration of objects (solids, liquids and gases). When sound interferes with people's rest, study and work, it becomes noise. Noise has seriously damaged the human living environment, harmed human health, and affected people's daily work and production activities. It is listed as one of the three major international public hazards (air pollution, water pollution, and noise pollution). Currently, countries around the world are taking various measures to strictly control noise pollution.
In the combustion system, noise mainly comes from fans, air flow and flames. Combustion system noise can be divided into mechanical noise, aerodynamic noise, combustion noise and oscillating combustion noise based on different sources.
Mechanical noise mainly refers to the noise caused by mechanical vibrations from combustion and auxiliary equipment.
On high-power combustion devices, in order to obtain the air required for combustion, a blower is often used, or an induced draft fan is installed in the flue to exhaust air in order to maintain the negative pressure in the furnace. When the fan is running, it will produce strong noise, including friction noise caused by bearing rotation, mechanical transmission and unbalance during unit operation, noise caused by the vibration of the fan and the air duct itself, as well as motor cooling fan noise, electromagnetic noise etc., has become a very important noise source in the combustion system.
When the air flow in the combustion system forms turbulence, velocity and pressure pulsations occur, causing noise. Because this fluctuation is random, airflow noise is broadband noise. According to different generation mechanisms, it can be divided into jet noise, eddy current noise and boundary layer noise. These three types of noise contain noise of various frequencies. When one of the noises is the same as a certain frequency generated by the combustion noise, it will cause resonance, increase the amplitude, and emit a lot of noise.
Among aerodynamic noise, jet noise is the most common noise source. The jet flow of gas or air into the furnace and the exhaust venting of the combustion device all have jet noise problems. Its formation mechanism and suppression methods have become important research topics in contemporary environmental engineering.
Combustion noise is caused by changes in gas flow velocity and pressure in local areas caused by fluctuations in the combustion reaction. Evenly mixed laminar flames are silent. Combustion noise originates from the turbulence of air flow and the uneven composition of local areas. The size of combustion noise is proportional to the combustion intensity. Combustion intensity represents the heat release rate per unit volume of the flame. Therefore, when the heat release rate of the flame remains unchanged, the flame volume increases, the combustion intensity decreases, and therefore the combustion noise also decreases. Experimental research shows that when the flame volume is doubled while the combustion speed remains unchanged, the combustion noise can be reduced by 3dB. By changing the structure and arrangement of the burner nozzle, such as replacing one large nozzle with multiple small nozzles so that the fuel is injected in thin streams, the noise can be reduced due to the increase in flame volume.
The so-called oscillatory combustion is an unstable combustion phenomenon caused by periodic changes in combustion speed and heat release rate caused by the pulsation of the combustion exothermic reaction.
The noise system is composed of noise sources, sound transmission paths, and receivers, so noise control must be considered from these three links.
Controlling noise sources is the most fundamental and effective way to control noise. Commonly used methods include:
1) Improve the accuracy of fan assembly and eliminate imbalance. Use low-noise transmission devices to avoid direct connection of motors without acoustic treatment. Rotational noise can be reduced by using a suitable fan impeller shape and reducing the impeller speed. For fixed fans, they should be installed accurately and paid attention to maintenance to reduce mechanical noise.
2) Change the shape of the nozzle to reduce noise production. Flower-shaped nozzles and multi-hole nozzles with the same outlet cross-sectional area produce less noise than single-hole nozzles. This is due to the mutual interference of the jets causing changes in the characteristics of the jet initial section. However, it is difficult to process flower-shaped nozzles, and multi-hole nozzles are often used in engineering, especially for medium-pressure jet burners. In addition, reducing the gas pressure and the nozzle outlet flow rate can not only reduce jet noise, but also reduce combustion noise.
3) Reduce the heat load of the burner and reduce the noise. Increasing the number of burners can reduce noise power. In addition, rational selection of burner design parameters and attention to the adjustment of operating conditions to ensure stable operation of the burner are also powerful measures to reduce noise.
If it is difficult to eradicate noise from the source due to limited conditions, measures need to be taken to control the noise propagation path. It is also very effective to take measures such as sound absorption, silencing, sound insulation and damping to reduce and control the spread of noise. Commonly used noise reduction devices include:
1) Sound-absorbing materials The structural characteristics of porous sound-absorbing materials are many tiny gaps and continuous holes, which have good ventilation properties. When sound waves are incident on the surface, they will enter the material along these pores and cause the air in the pores and the tiny fibers of the material to vibrate. Because of the effects of friction and viscous resistance, a considerable part of the sound energy is converted into heat energy and is consumed. This is the principle of sound absorption in porous materials. Commonly used sound-absorbing materials include glass wool, slag wool, cotton wool, felt, wood wool boards and sound-absorbing bricks.
2) Sound insulation cover: Completely enclose the noise-emitting machines (such as fans) in a sound insulation cover to prevent the noise from spreading outward. The sound insulation cover must be lined with porous materials. When sound waves pass through the micro-pores, friction is used to consume the sound energy. Or the inner wall of the soundproof enclosure can be covered with a material with viscous damping to prevent the accumulation of sound intensity inside the enclosure. In order to prevent machine noise from being brought out of the cover through the connecting pipes, the pipes can be connected with flexible connections.
3) Muffler (acoustic filter) The muffler used in the pipeline relies on changes in acoustic impedance to prevent sound waves from passing freely and partially reflect back to the sound source to reduce noise. Commonly used methods are to change the catheter cross-section and provide side branches.
A final means of controlling noise is protection at the point of reception. When other measures cannot be achieved, or when only a few people are working in a noisy environment, personal protection is an economical and effective measure. Commonly used prevention and control devices include earplugs, earmuffs, helmets, etc.
Generation and elimination control of noise during combustion process
>Mechanical noise: mechanical vibration noise of combustion and auxiliary equipment;
>Aerodynamic noise: airflow turbulence, occurrence of velocity and pressure pulsations
>Combustion noise: air flow and pressure changes caused by combustion reaction;
>Oscillating combustion noise: Noise produced by unstable combustion.
Ways to eliminate and control noise
>Control noise sources
>Control noise transmission pathways
>Take protective measures at noise receiving points