Investigation On Laser-induced Ignition And Combustion Characteristics Of Micro/Nano-sized Aluminum Particles At The Micro-scale

This paper carries out a study on laser-induced ignition and combustion characteristics of micro/nano-aluminum powders in the micro-scale combustor.During the study,a variety of technical methods are used,which include infrared laser ignition technology,micro-combustion technology,microscopic measurement technology,image processing technology.An optical diagnostic test system about ignition and combustion of solid micro-particles fuel in the micro-scale combustor are developed,what can measure particle shape,particle size,ignition delay time,particulate combustion emission signal and other parameters.The main contents of this paper are as follows:(1)Study on laser-induced ignition and combustion characteristics of single micro-aluminum particle in the micro-scale combustor.The laser ignition and combustion of single aluminum particle are successful in a stagnation air flow of the micro-scale combustor at atmospheric temperature and pressure,in this study ignition and combustion characteristic parameters were measured.Experimental results show: When the primary particle diameter of micro-aluminum particles is 2-4?m,the minimum laser ignition power is 35.1 mW.When the size range of aluminum powder particles is 5-8?m,the minimum laser ignition power is 63.2 mW.When the size range of aluminum powder particles is 9-10?m,the minimum laser ignition power is 84.1 mW.When the laser ignition power is 72.7 mW,the ignition delay time of micro-aluminum particles with the size range of 2-8?m is 0.2-0.4ms.When the laser ignition power is 111.7 mW,the ignition delay time of micro-aluminum particles with the size range of 4-8?m is 0.3-0.4ms and the ignition delay time of micro-aluminum particles with the size range of 9-12?m is 0.6-1.2ms.So the minimum laser ignition power of single micro-aluminum particle increases as the particle size and ignition delay time increases as the particle size in a stagnation air flow of the micro-scale combustor.According to the thermal ignition theory,the ignition occurs the time when the energy obtaining and energy loss of the fuel is balanced.The particle size of metal fuel greatly affects the ignition delay time.The smaller the particle size is,the greater the specific surface area is and the faster the heat gets and the more easily the particle is ignited.Single aluminum particle in the micro-scale combustor occurs firstly heterogeneous ignition,then transforms into a homogeneous ignition.In the combustion process of single aluminum particle,flame brightness severe oscillations because of large heat losses of the combustion chamber wall cause flame extinguishing.(2)Study on laser-induced ignition and combustion characteristics of nano-aluminum powders in the micro-scale combustor.The ignition and combustion mechanism of nano-aluminum powders at the micro-scale is presented,which is based on nano-aluminum oxidation theory,interacting particle sintering model and the ignition and combustion phenomena of nano-aluminum powders at the micro-scale.In a stagnation air flow of the micro-scale combustor at atmospheric temperature and pressure,the minimum laser ignition power of nano-aluminum powders is 28.7 mW,and the ignition delay time is in the magnitude of micro seconds order.As the laser ignition power is the same,the ignition delay time is influenced by the roundness,equivalent diameter and bulk density of nano-aluminum powders.The classical combustion of nano-aluminum powders includes four stages,i.e.,the startup of combustion,diffusive combustion,weak flame and extinction.The nano-aluminum powders are preheated by laser,and then heterogeneously ignited as the alumina shells rupture due to the melting of the core.At the moment of rupture,the micro-explosion might take place.After ignition,nano-aluminum powders combust diffusively in the naturally convective air flow.The fluctuation of the flame front and flame brightness shows instability of combustion.At the micro-scale,the diffusion combustion followed by the weak flame,and the flame eventually extinguishes.