The large amount of dust produced in coal mining not only seriously affects the health of the miners, but the coal dust is also explosive, threatening the safety of coal production. In recent years, with the increase in coal mining intensity, the problem of dust prevention and control has become increasingly prominent. At present, the main dust prevention measures for coal mines in China are spray dust reduction, and atomization nozzles are used to clean the air. As the basic element of spray dust reduction, its atomization ability (mist flow shape and particle size) directly determines the effect of spray dust reduction.
1. Nozzle classification and its characteristics
according to the shape of the mist flow According to the shape of the mist flow formed by the nozzle, the nozzle can be divided into two categories: cone-shaped solid nozzle and cone-shaped hollow cone nozzle. The solid nozzle is mainly used for dust suppression, and the hollow nozzle is mainly used for dust suppression. The cone-shaped solid mist column sprayed by the solid nozzle has a higher mist flow velocity, and the dust collided by the mist particles can generally be lowered. However, because of the high speed of the mist flow, the air drawn around it can easily blow away the respirable dust with a small particle size, which objectively affects the dust reduction effect. The cone-shaped fog curtain sprayed by the hollow nozzle is mainly for dust prevention. In order to increase the area covered by the fog screen, there is generally a large cone angle of the fog screen, and the nozzle is relatively far away from the dust source. This also causes the fog particle velocity to be reduced to a very low level at the larger diameter end of the fog screen. In addition to not being able to capture dust particles, it also loses its dust-blocking effect. From the analysis of the shape of the fog body, the density of the solid spray fog body is higher than that of the hollow spray fog body in its full length area. In the effective range of the solid spray, it is generally difficult for coal dust to pass through the fog screen, so , The solid cone-shaped fog body is better than the hollow cone-shaped fog body.
2. Classified by atomization method
(1) Mechanical atomization
Mechanical atomization mainly relies on the high-speed jet generated by the liquid to atomize itself under the action of pressure difference, so it can be divided into direct nozzle, centrifugal nozzle and rotary nozzle. Direct atomization and centrifugal atomization can be collectively referred to as pressure atomization. Direct injection nozzles mainly rely on water jets to achieve the purpose of atomization. The water pressure requirements are relatively high, and the larger the diameter of the nozzle hole, the thicker the atomization, so the nozzle hole diameter cannot be too large, and the flow rate adjustment range is relatively small. The centrifugal nozzle uses the centrifugal force generated by the high-pressure water through the swirling device to generate a liquid film, which is broken and atomized by the air. The effect of centrifugal atomization is better than direct atomization, but it also requires a higher water supply pressure, so the application conditions are limited. Rotary nozzles are roughly divided into two categories: rotating body type and rotating nozzle type. The rotating body type is divided into the rotating cup type and the rotating disc type. Rotary cup atomization is to spray water into the front end of a conical rotor cup, and spread the water into a thin film with the help of a high-speed rotating rotor cup, and atomize the liquid by the combined action of “centrifugal force spray” and “speed spray”. In the same way, rotary-disk atomization relies on a high-speed rotating disk to atomize liquid.
(2) Media atomization
is divided into pneumatic atomization and bubble atomization according to different atomization methods. Pneumatic atomization nozzles are widely used. Pneumatic air atomization nozzles rely on a certain pressure of gas (compressed air or steam) to form a high-speed airflow, so that the air and water form a high relative velocity to achieve the purpose of atomization. The advantage is that a good atomization effect can be obtained under lower water pressure, and the working conditions can be adjusted in a larger range. But the power source is not single, and the system composition is complicated.
(3) Special nozzle atomization
Special nozzles generally adopt the principles of ultrasonic, electromagnetic field, electrostatic effect and other principles for atomization. Although this type of nozzle has a good effect in some other industrial applications, it is less used due to the harsh underground environment of coal mines.
2. Factors affecting the atomization ability of the nozzle
1. When pressure-type nozzles
adopt pressure-type atomizing nozzles (direct and centrifugal) for spray dust reduction, the dust reduction efficiency mainly depends on the water supply pressure for certain use occasions. The water pressure required for different particle sizes is different, the finer the dust The higher the pressure required for the dust. The high water supply pressure can not only obtain fine water mist, but also make the water mist particles move at a high speed and have a large water content in the space. This is extremely advantageous for dust reduction methods based on collision mechanisms. According to the actual dust particle dispersion and dust reduction efficiency requirements, referring to the corresponding graph to select the appropriate water pressure can achieve good results and good economic benefits. This conclusion is applicable to any workplace where pressure-type atomizing nozzles are used to spray and settle coal mine dust.
2. Two-phase nozzle
For two-phase nozzles, its atomization ability is affected by the following factors:
(1) The effect of the diameter and length of the
mixing tube The inner diameter of the mixing tube becomes smaller, which can increase the relative velocity of the gas-liquid two-phase, which is beneficial to atomization, but this will affect the re-agglomeration of atomized particles. Because the mixing tube is too long and the gas energy is consumed too much, it will make the liquid flow atomization worse; if the mixing tube is too short, the gas energy cannot be fully utilized, resulting in insufficient liquid flow atomization.
(2) The influence of the nozzle. The
reduction of the outlet area will increase the outlet pressure drop, so the acceleration of the gas-liquid mixture is obviously enhanced, and the relative velocity between the gas and liquid phases increases, which also promotes the liquid phase to be broken down more finely. However, the increase in outlet pressure drop will inevitably increase the pressure in the mixing tube, which will result in a decrease in the relative velocity of the gas-liquid two phases in the mixing tube, which will make the atomization worse.
(3) The relationship between gas-liquid ratio and atomized particle size
As the gas-liquid ratio increases, the atomized particle size tends to decrease. Therefore, increasing the gas-liquid ratio can increase the relative velocity of the gas-liquid two phases and make the liquid film more finely broken. However, when the gas-liquid ratio increases to a certain extent, the change in particle size is not obvious.
(4) The droplet concentration changes with the gas-liquid ratio.
As the gas-liquid ratio increases, the concentration of water particles decreases, which is caused by the decrease in the mass fraction of water in the air.
3. Ways to improve nozzle atomization
According to the atomization mechanism and experimental research, combined with the field experience of coal mines, the main ways to improve nozzle atomization are as follows:
(1) Increase the relative velocity difference between the gas and liquid phases to increase the aerodynamic force, so that the droplets can be broken into finer particles under the action of greater aerodynamic force.
(2) Increase the exit velocity of the liquid phase nozzle to enhance the collision, so that the relatively ejected droplets can be further broken during the collision. Experiments have found that if the outlet velocity of the droplets is small, large droplets will be aggregated, and if the outlet velocity of the droplets is high, the degree of atomization can be improved. However, this will reduce the relative velocity of the gas and liquid and make the aerodynamically atomized droplets worse.
(3) The experimental results show that the geometry and size of the nozzle and mixing tube have a great influence on the atomization performance. Therefore, when designing the model, in addition to considering their effects on atomization, the overall performance changes after they are combined should also be considered.
(4) Study the relationship between the flow rate and pressure of the spray water supply system and the geometric size and structure of the nozzle to improve the atomization effect. In particular, the water pressure of the water supply system has a greater impact on the atomization effect. The higher the water pressure, the finer the water mist particles. However, the problems caused by higher water pressure are: ① high energy consumption; ② all parts of the water supply system are under pressure, are prone to failure, and have a short life span, especially the internal spray system on mining equipment. This brings us to another research topic: how to improve the pressure atomization nozzle structure to obtain fine water mist particles under the limited water supply pressure.
Through the analysis of the classification and characteristics, the applicable scope of various nozzles is pointed out, and on the basis of analyzing the factors that affect the atomization ability of the nozzle, ways to improve the atomization effect of the nozzle are proposed. In order to improve the atomization effect of the nozzle, it is necessary to match the characteristics of the nozzle and the water supply system, while improving the water quality and increasing the filtration accuracy of the spray water . Our company’s nozzle is the best quality nozzle in the world, the first in China, and other companies The nozzles are low in price, low in quality, and they will rot after a few minutes of use. Don’t sell them. Those are companies without technology, and the products never sample or self-inspect.