Research On Al/AP Powder Propellant Ignition Combustion And Laminar Flame Spread Modeling

The powder rocket engine uses powder energetic material as propellants.This type of engine not only has functions like multiple starts and thrust adjustment,but is simple in structure,convenient in use as well as the solid rocket motor.Pre-packaged powder propellants have no problems like propellant aging and cracking in high and low temperature storage circumstances.So powder propellants have better storage performance and enironmental adaptation.In current state,efficient optimization design of combustor,ignitor and flame holder cannot be carried out for limited understanding of powder propellants combustion mechanism.The study on ignition combustion characteristics and laminar flame propagation is the basis of powder propellants combustion theory.In this paper,parameters such as ignition delay,combustion intensity,flame temperature,ignition temperature were measured and calculation by using CO₂ laser ignitor combined with fiber optic spectrometer.The influence of particle size,environmental pressure on ignition combustion performance was analyzed.Powder fuel improved methods like Mg/Al alloy,glomerate aluminum particles and graded aluminum particles are also investigated.A Bunsen type burner was builded up to observe laminar dust flame propagation.The influence such as width of shear flow slot,flow velocity,particle size,dust concentration and grading size on Al/air laminar flame propagation was studied.Four kinds of carrier gas like N₂,air,He,N₂O was considered in energy aspect.And the influence of carrier gas on powder propellant was studied with experimental methods.Ignition combustion characteristic experiment was conducted on Metal/AP powder mixtures and influence of particle size,mass ratio and pressure was also analyzed.Then,a laser ignition model on Al/AP powder mixtures was obtained.At last,laminar flame spread characteristics of Al/AP powder propellant is requied,so we tried to get a new type Al/AP/N₂Bunsen flame by using the designed burner.And we successfully obtained the stable Al/AP/N₂ Bunsen flame under certain mass ratio and dust concentration.The geometry of flame was theoretically analyzed,and one-dimensional flame spread model was established based on heat-transfer mechanism.The main conclusion and results of this paper is as follows:?1?In the experiment,the ignition delay of aluminum particle increases with particle initial size increasing.R_AlO,which is the intensity ratio of 486nm/568nm signals,varies during buring process within 0.2,and increase with initial particle size increasing.This indicates that the vaporation rate of Al droplet can affect its homogeneous combustion intensity.Dual-wavelength method and grey-estimate method were carried out to calculate flame temperature and ignition temperature,respectively.We found that flame temperature decreases with particle size increasing,but ignition temperature increases.This is the result of oxidizer diffusion.Ignition kinetics process of smaller aluminum particles is shorten,as a result of melting and boiling point of aluminum and alumina shell decreasing due to bigger surface energy of smaller particles.It indicates that ignition delay was determined by both external heating and own property.Heterogeneous Surface Reaction?HSR?can shorten ignition delay from the calculation result of builded one-demensional ignition model.And HSR rate increases with oxidant concentration increasing.The model is well conformity with experimental data.?2?The Al/air Bunsen flame with sole particle size can be obtained only with particle size of 5,10 and 20 microns.Particle size measurement range can be broadened with using grading particles.The experimental result shows that laminar flame speed increases with particle average size decreasing.The maximum laminar flame speed is at equivalence ratio of1.2,due to dust heat conductive coefficient increases with Al particles concentration increasing.?3?Powder propellant improvement method such as Mg/Al alloy,glomerate aluminum particles and graded aluminum particles was investigated.Experimental results show that Mg can promote heating and mixing of Al with gas in Mg/Al alloy.Covered agent can also promote mixing of Al in glomerate aluminum particles and increase packing density of smaller aluminum particles.Graded aluminum particles have better combustion and packing performance than sole size aluminum particles.?4?Alternative carrier gases were comprehensively compared.Al/AP powder propellant has the maximum theoretical specific impulse with using helium,but poor combustion performance.Air and nitrogen cannot increase theoretical specific impulse,but air can improve combustion performance of aluminum particles.N₂O can dramatically decrease particle ignition delay than known kinetics mechanism with air,indicating unknown ignition mechanism with N₂O existence.Laminar flame speed,flame zone thickness and colour temperature of Al/N₂O dust are all exceeded Al/air dust.N₂O is better than air in combustion promotion.?5?Laser absorption efficiency of AP particles was measured through laser ignition experiment.Al/AP ignition model is established,considering Al particle ignition and AP particle decomposition kenitics.The calculated result is in good agreement with experiments.These achievements setted the foundation for ignitor design and study on laminar flame spread mechanism of Al/AP powder propellant.?6?Al/AP/N₂ Bunsen flame was successfully achieved by using 20?m Al and 95.86?m AP particles under 3:1 AP/Al mass ratio and 3223 g/m³ dust concentration.An inverted cone flame front was obtained.The calculated laminar flame speed is 72cm/s.One-dimensional flame spread model was established based on heat-transfer mechanism.Separation-overlap flame structure was proposed.The predicted laminar flame speed is about 37.3%smaller than experimental result.Calculation error may be caused by ignoring the change of Nu number.