Flame Propagation Behaviors Of Nano PMMA Dust Explosions

The dust explosion flame propagation behaviors should be fully understood in order to predict the explosion consequences accurately and develop the effective protection measures essentially.Due to the larger particle forces,the dispersion process and explosion characteristic of combustible nano dusts are very different from those of micro dusts.However,the essential issues of nano dust explosions are not well acknowledged,especially for the microscopic phenomena and mechanisms in the flame propagation developments.In this study,the flame propagation dynamic processes in nano dust explosions were studied,the physical model of nano dust flame microstructures were established,and the flame propagation mechanisms in nano dust explosions were revealed.The main contents and conclusions are listed below:(1)The TGA and DSC characteristics of nano and micro PMMA dust particles were measured.Results showed that the thermal characteristics were significantly influenced by the particles size distributions,the monodisperse/polydisperse characteristics and the agglomeration phenomena.On the whole,the PMMA dust particles were mainly pyrolyzed in the temperature range of 250??400?.The pyrolysis/devolatilization products were mainly contained MMA,CO2 and CH3OH.And the pyrolysis/devolatilization products were reacted with oxygen severely in the range of 400?-600?.In this oxidation reaction process,O2 was rapidly consumed and amounts of heat was released.(2)Dust particle size distributions before and after dispersion were presented.The inner particle forces were larger for smaller particles,which could easily led to serious agglomeration phenomenon.A dimensionless parameter(?)was defined to evaluate the degree of agglomeration before and after dispersion.It was revealed that not only the particle size distributions,but also the forces caused by collision and friction between particles and high speed air flow in the dispersion process determined the agglomeration phenomenon.Besides,the mixed dust samples contained particles in various size distributions exhibited a complex dispersity mode and agglomeration form.(3)Effects of turbulent intensity on nano dust flame propagation behaviors were investigated.Results showed that the residual turbulent intensity of 100 nm PMMA dust cloud was decayed with increasing ignition delay time.And with the smaller residual turbulent intensity,the dominant effect of flame self instability became more significant,the flame became more unstable,the flame front became irregular,the combustion reaction intensity became weaker,the average flame propagation velocity became smaller and the velocity pulsating degree became stronger.(4)Comparative experiments were conducted to reveal the flame structure characteristics,the flame propagation behaviors,the flame chemical reaction kinetics(the thickness of preheat zone and the temperature distribution)and the flame microstructures in micro and nano dust explosions.30?m PMMA dust flame was characterized by clusters of flames,whereas 100 nm PMMA dust flame was similar to premixed gas-phase flame coupled with the luminous spot flames.Compared with the 30 ?m PMMA dust flame,the 100 nm PMMA dust flame front was more regular,the propagation was more continuous,the preheat zone was thinner,the maximum flame temperature was higher,the average flame propagation velocity was larger and the velocity pulsating degree was weaker.(5)Effects of particle size distributions,concentration conditions and agglomeration characteristics on the flame structure characteristics,the flame propagation behaviors,and the temperature distributions in nano dust explosions were systematically investigated.Results showed that smaller particles in the flame front dominated the flame propagation.The macroscopical developments of the flames were determined by the major mass proportion dust particles.The flame front was smoother,the flame structure was more continuous,the average flame propagation velocity was larger and the maximum flame temperature was higher with more proportion of smaller dust particles.This phenomenon indicated a transition of the diffusion dominant combustion mode to the kinetic controlling combustion mode.(6)The py roly sis/de volatilization and combustion mechanisms of nano agglomerated dust particles were discussed.Combustion reaction firstly occurred around agglomerated particles,where high concentration premixed combustible volatiles were available.Thus the luminous spot flames were formed.The diffusion combustion coupled with the local premixed gas phase combustion mechanism of agglomerated particles influenced the nano dust flame propagation behaviors essentially.(7)A physical model of nano dust flame propagation mechanism was established.Considering the dynamic processes that contained the particle movement around flame front,the particle pyrolysis/devolatilization process,the mixing with oxygen,ignition and combustion,the nano dust flame propagation dynamic mechanism was proposed.Based on the physical model of pyrolysis/devolatilization and combustion of single PMMA dust particle,dimensionless numbers Bi,Th,and Da were introduced to evaluate the controlling regimes of pyrolysis/devolatilization and combustion processes.The results indicated that the pyrolysis/devolatilization process of 100 nm PMMA dust particles was dominated by the external heat transfer sources(Bi<1 and Th>1),and the overall combustion and flame propagation process was controlled by the pyrolysis/devolatilization rate(Da>1).