Studies On The Combustion Mechanism And Model Of Aluminum Particles In Multicomponent Burning Flow

Aluminum particles have high volume energy density,ideal ignition and combustion performance.It is ideal energy additives and has been widely used in space propulsion.With the development of diversified spacecraft missions,the revolution of propulsion system is changing rapidly.The advance of new propulsion type is driving the innovation of the utilization way for aluminium particles in power system.Powder motor is a new type of power propulsion.The propellant is stored in solid particles and transported as two-phase flow.The raw materials are green and can be obtained easily,the process of pretreatment and packaging are more convenient.This powder type propellant gives the system strong flow control ability and adaptability in extreme ambient temperature.It needs not to consider the compatibility problem of propellant preparation,avoids particle agglomeration combustion,and gives great potential in the exploration of outer space in the future.As an important component of powder propellant,aluminium powder has not been studied thoroughly on the basic issues such as cold injection ignition and flow combustion in approximate-real environment,and the combustion mechanism is still unclear.Based on the combustion conditions in Al/AP powder rocket motor,the research scheme of particle flow and combustion under multi-component conditions is proposed.Firstly,based on the experimental method of synchronous thermal analysis,the weight loss and heat release characteristics of particles under multi-component conditions were studied,and the quantitative analysis data of reaction process of aluminum particles with time and temperature were obtained.Then,the surface reaction kinetics data were obtained by particle dynamics analysis method.Secondly,engine environment is simulated for particle combustion according to the design parameters of Al/AP powder motor.Technical difficulties of carbon deposition on the obsvertion window and environmental pressure simulation are overcomed.The powder samples are injected by several times in diferent positions in order to seize the whole process during defferent flow stages under the designed flow velocity.Finally,the flow combustion characteristics of aluminum particles were studied on the basis of the combustion platform under the condition of approximate real combustion environment.The ignition and combustion mechanism about core and shell was studied in the multi-compotent environment.Based on the kinetic combustion data obtained from experiments and the combustion pictures in the multi-compotent environment,a combustion model for flow Al particle was established.Through this study,the main results and conclusions are as follows:(1)The thermal oxidation study is conducted in H₂O,CO₂ and O₂ environment.In mixed conditions,the oxidiation temperature range decreases becasuse of high oxidiation rate,the process trend is similar to that in H₂O environment.The particle thermal oxidation model based on thermodynamic analysis can be used to predict the and heating process before the particles get ignition.The results show that The initiation temperature of oxidation increases when heating rate increases.When the heating rate reaches 10³K/s,particle oxide layer gets thicker in ignition stage.As the heating rate increase,the growth of oxide layer slows down.When the heating rate exceeds 10⁶ K/s,the growth of oxide layer can be neglected.(2)In flow conditions,the ignition time has a strong linear relationship with the particle size.The agglomerated particles ignite a little longer than its source powder,and the heating process of agglomerated particles is obviously accelerated when oxidant(KCl O₄)is added.The experimental photographs show that the particle profile and radiation characteristics will change significantly during the ignition stage.For smaller samples,the processes of combustion in paticle local,deformation and splitting are relatively independent in time.For larger samples,these processes may occur simultaneously.The essence of particle ignition process is particle heating and core-shell reorganization on particle surface.(3)There are two different combustion patterns of aluminum particles in the actual flow environment.For particles with smaller size(<10 um),the combustion time distributes above Beackstead formula curve(corresponding mechanism A).For larger particles(>20um),the combustion time distributes below the curve(mechanism B).The combustion acceleration of mechanism is related to the fluid-particle velocity difference and the fluid disturbance.For the particles of 10-20 um,the combustion pattern is in the transition stage between combustion mechanism A and mechanism B.For agglomerated particles,the combustion time doesn't significantly shorten compared with that of Al particles with the same particle size(200 um).Particle combustion morphology consists of three parts:molten alumina shell,molten Al core and evaporating flame layer.The ratio of flame radius to particle radius varies greatly(1.12-1.64),which is related to shel-core posture of particle and the flow direction of ambient gas.The coverage of oxide shell increases gradually(15-95%)?(4)Flame color image methord is adopt to anlysis the temperature characteristics.The results show that the ignition temperature of the particles is about 1900-2000K,the surface temperature of the particles is uniform before ignition.The ignition process starts from the local temperature change and the change gradually spreads to the whole particles.The local temperature of the burning aluminum particles is related to the local component.The core temperature is 2100-2200K,and oxide shell temperature is 2300-2500K.When ambient pressure rises(0.3MPa),particle temperature increases significantly.The temperature of particle flame is 3400-3700K,the shell temperature is2600-2700K,and the core temperature is more than 2300K.(5)According to the change of particle size and combustion surface temperature,three different core-shell combustion mechanisms are given:hard shell,soft shell and whole shell.When the particle size is smaller,the particle shell remains intact,and the particle burns through mass transfer in the oxide shell;when the particle size is larger,the mass transfers through evaporation on core surface and diffusion in shell layer.Core-shell temperature is related to environmental pressure,composition and environmental temperature.When the core temperature of the particle is lower than the melting point of alumina,the oxide shell has hard shell structure.When the temperature of the core is higher,the oxide shell has soft shell structuret.(6)The results of particle ignition and combustion model is conducted.The mass change of core and shell is mainly controlled by surface heterogeneous reaction during ignition stage,the increase of particle temperature is caused by convective heat transfer.When the particle temperature approaches ignition temperature,the effect of surface heterogeneous reaction on temperature change increases gradually.At combustion stage,the evaporation and combustion of aluminium core are the dominant factors of particle reaction,and the interaction between them determined the temperature trend.The core-shell heat transfer is similar to heat release in surface reaction.Both magnitude is at low level.The heat transfer between the core and shell is an important factor to keep the temperature difference between the core and shell.