Experimental Investigation And Numerical Simulation Of Minimum Ignition Energy Of Dust Cloud

With the development of modern industry, flammable dust has been widely used, the potential risk of dust explosion has greatly increased. Dust explosion hazards involving flammable dust preparation, using and disposal, such as coal, food, metallurgical, chemical, textile and other industries, has been a continuing threat. In particular, with application of new technologies and expansion of production scale, the growing frequency of dust explosion caused serious casualties and property damage. Therefore, it is very necessary for the deep study of dust explosion parameters, and it has important practical significance and scientific value for prevention and control of such industrial disasters. Minimum ignition energy of dust cloud is an important characteristic parameter of dust explosion. It has important significance in assessment of dust explosion hazard and taking effective measures to avoid ignition sources.Scientists who have long been engaged in dust explosion protection work, are using various methods in testing and researching of minimum ignition energy of dust cloud. Currently, the testing of minimum ignition energy of dust cloud is mainly according to the Association of German Engineers’s VDI2263standard and the International Electrotechnical Commission’s IEC31standard. In this paper, research methods of combination of theoretical analysis, numerical simulation and experiment are used, we research and analysis the ignition characteristics, conditions and affecting factors in spark ignition of dust cloud base on both theoretical and experimental study, a complete mathematical model is established for spark ignition of dust cloud according to the spark ignition conditions and dust cloud ignition mechanism. Theory study of ignition proccess of dust cloud is realized.First, we design the spark discharge and minimum ignition energy testing system from designing auxiliary spark triggering three-electrode spark discharge system based on comprehensive analysis of the existing7spark trigger circuits, independently developed a set of minimum ignition energy testing device including high-voltage relay triggering double electrode system, electrode movement triggering two-electrode system, auxiliary spark triggering three-electrode system, different levels of energy in spark discharge can be achieved, provide different options for test requirements.The minimum ignition energy of corn starch, lycopodium and wheat flour is tested through experiment. By combining experiment with a large number of literature research, a comprehensive, systematic analysis of affecting law of sensitive conditions,such as dust concentration, ignition delay time and so on, on the minimum ignition energy is done, combined discharge process of electric spark with ignition mechanism of dust cloud,analyze the internal substantive laws detailly, provide effective guidance for quickly and accurately gain of measured conditions in experiment.Self-designed high energy spark triggering system according to the shortcomings of chemical igniter used in dust cloud explosion index measuring, dust explosion parameters testing device,which is the only supporting device for10kJ of high energy ignition is developed. Difference of high energy spark ignition and chemical ignition in measurement of dust explosion index is studied through experiment, feasibility of high energy spark ignition measurement is verified, provides an effective laboratory equipment for alternative of traditional chemical ignition to static spark ignition.Using the oscillometric method, the voltage and current across the discharge electrodes are gained respectively by using high voltage probe and shunt resistor, the spark energy is calculated by integration of discharge power. Spark discharge waveforms are analyzed under inductive load;Effect of charging energy on the discharge waveforms and spark energy is also analyzed.Energy conversion rate under different charging energy and energy error are analyzed.R-C-L discharge equation is established based on equivalent circuit of ignition system under inductive load, gap resistance and gap current changes during the process of electric spark discharge are modeled using Runge-Kutta method, discharge energy(spark energy) in electrode gap is calculated through discharge model. The calculating results of discharge model combined with the experimental results provides effective guidance for the experimental determination of parameters in ignition circuit,and provides important experimental results for the use of spark energy to measure minimum ignition energy of dust cloud.Secondly, based on detailed analysis of dust cloud ignition mechanism, combined with two-phase flow, heat transfer and chemical reaction dynamics theory, according to the process of spark ignition of dust cloud in experiment, a mathematical model for minimum ignition energy of dust cloud is established. Ignition process of homogeneous reaction, heterogeneous reaction and homogeneous-heterogeneous reaction is analyzed respectively according to different ignition mechanism,and the ignition criterion is proposed.Combustion of dust particles in model consideres the water evaporation, devolatilization, gas phase reaction and particle surface reaction. Particle surface reactions is controled by both oxygen diffusion to the particle surface and chemical reaction rate, the model also consideres radiation heat transfer effects. Turbulence effect on the spark ignition process is taked into account by eddy diffusion coefficient, overcomes calculation errors caused by ignoring turbulence in existing models. Effect of spark discharge time, spark diameter, dust cloud concentration, particle size on minimum ignition energy of dust cloud is analyzed through mathematical model. Minimum ignition energy of aluminum powder and corn starch is calculated, and model is validated by using experimental results. Theoretical analysis shows that the results are consistent with the experimental results. Under the conditions of ignoring discharge shock, electric spark discharge time has little effect on calculation results, and spark energy distribution in space have a major impact on the results.The development of testing system for minimum ignition energy of dust cloud has important practical value, the understanding of spark ignition process and mechanism of dust cloud can be further deepened through experimental analysis; The complete mathematical model for minimum ignition energy of dust cloud has important theoretical value. The experimental measurements combined with model results can make determination of minimum ignition energy of dust cloud more reasonable and accurate, and provides a reliable scientific basis for the prevention of dust explosion.