Ignition And Explosion Of Flowing Dust Clouds In The Pipe

Dust explosion is the most common accident in the powder-related industry.Once an explosion accident occurs,it can easily cause heavy casualties and property losses.Dust removal system is a high-prone area and the focus of prevention and control of dust explosion.A starch explosion in Hebei Jianmin Starch Sugar Industry caused by static sparks due to static electricity accumulation is a typical example.Therefore,preventing dust explosions originating in pipelines is very important for powder-related industries to reduce the dust explosion risk.However,the existing research mainly focuses on the ignition and explosion laws of suspended dust clouds in enclosed vessels,and there is no in-depth study on the ignition and explosion mechanism of flowing dust cloud under the action of airflow in the pipe.Therefore,this paper designed and implemented the fire explosion test device independently for the flowing dust cloud in the pipe.Based on the experimental results of the airflow characteristics in the pipe,the calculation method for the concentration of the flowing dust cloud in the pipe was determined,and further experiments were conducted to explore the fire and explosion of the flowing dust cloud.The influence of the process and various factors on the ignition sensitivity of corn starch provides new ideas and experimental basis for the prevention and control of dust explosion accidents in dust removal systems in industrial production occasions.First,a testing device for the ignition and explosion of the flowing dust cloud in the pipe was designed and built independently.Based on this device,we developed a test method for the airflow characteristics of the pipe,the dust supply rate,the flame propagation process of the flowing dust cloud,and the sensitive characteristic parameters.In order to analyze the airflow characteristics of the pipeline and further carry out dust explosion experiments under different conditions,the experiment measured the real-time curves of the airflow velocity at different radial positions of the pipe section at the ignition position and obtained the cross-sectional airflow velocity distribution.The average air velocity was obtained by the maximum air velocity method.The empirical formula of the average air velocity with respect to umax at the center of the section circle,and finally combined with the fitting formula of umax with respect to the rotation speed of centrifugal fan,the fitting formula of the average air velocity with respect to the rotation speed of centrifugal fan is obtained.The experimental results show that the velocity of the measuring point of the pipe section fluctuates with time.The larger the average velocity of the measuring point,the weaker the pulsation phenomenon;the closer to the pipe wall,the smaller the air velocity,and the velocity change is almost linear;the farther from the pipe wall,the greater the air velocity;the air velocity reaches umax at the center of the cross section.Overall,the air velocity in pipelines is distributed in an inverted U shape under the cross-sectional radial coordinate system.Secondly,using the high-power arc generator as the ignition device and the corn starch before drying as the experimental medium,the experiment explored the influence of the airflow characteristics on the ignition arc and the ignition process of the flowing dust cloud in the pipe.The results show that with the increase of the average air velocity in the pipe,the arc shape changes drastically,the brightness decreases,the discharge time is shortened,and the maximum breakdown distance is reduced,which finally leads to the loss of ignition energy and the actual ignition energy reduction.After igniting the flowing dust cloud,two completely different flame forms are obtained.When the concentration of corn starch is relatively low,a local flame will appear probabilistically.With the moving downstream of the local flame,the length of the flame gradually decreases,the brightness decreases,and finally disappear;with the increase of the average air velocity,the existence time of the local flame will gradually shorten.Therefore,the presence of airflow at low concentrations is not conducive to dust cloud ignition and explosion.When the concentration of corn starch is relatively high,a self-sustaining flame will appear.With the moving downstream of the self-sustaining flame,the length of the flame gradually increases,the brightness increases,the flame fills the pipe,and the propagation speed continues to increase.Therefore,contrary to low-concentration conditions,airflow under high-concentration conditions is conducive to flame propagation.The appearance of the two flames is highly correlated with the concentration of dust clouds.When the number of ignitions under the same conditions is constant,the frequency of self-sustained flames decreases with the decrease of dust cloud concentration;the frequency of local flames first increases and then decreases with the decrease of dust cloud concentration.Finally,the experiment determined the influence of dust cloud concentration,moisture content,and average air velocity on minimum ignition energy and minimum explosion concentration of corn starch.The results show that minimum ignition energy of corn starch before and after drying decreases first and then increases with the increase of dust cloud concentration.Minimum ignition energy of corn starch after drying is significantly lower than that before drying.In particular,when the dust cloud concentration is 1200 g/m~3,minimum ignition energy of corn starch before drying is 2J,while minimum ignition energy of corn starch after drying is 0.1J,the former is 20 times the latter in value.Minimum ignition energy increases as the average air velocity increases.When the average air velocity is 28m/s and the dust cloud concentration is 300 g/m~3,the ignition energy rises sharply,even if the output energy is adjusted to the maximum value of 30J,it fails to ignite.Limited by the probabilistic nature of the dust cloud ignition itself and the small setting gradient of ignition energy,the change trend of minimum ignition energy under some conditions is not obvious.On the whole,minimum explosion concentration of corn starch before and after drying increases with the increase of the average air velocity.When the average air velocity is 16m/s,minimum explosion concentration is slightly reduced,which may be related to the probability of ignition.Minimum explosion concentration of corn starch after drying is significantly lower than that before drying.This paper experimentally studies the ignition and explosion laws of the flowing dust cloud in the pipe,and the research results have guiding significance for the prevention and control of dust explosion accidents in the dust removal system.