Investigation On Combustion Mechanism And Model Of Boron Particle Clouds

The powder fuel ramjet is a new type of air-breathing engine.The high energy metal（Al,Mg）or boron powder is used as fuel and the ram air is used as oxidant and working substance.Different with ducted rocket,the powder fuel ramjet has higher specific impulse performance,strong storage stability,and excellent ability of wide range thrust regulation due to the powder fuel has similarity flow characteristics with liquid fuel,high energy density and stored in the fuel tank independently.In the various kinds of powder fuel,boron is regarded as one of the most ideal fuel for the powder fuel ramjet for its higher energy performance and low loss of two-phase flow.Due to the higher viscosity of boron and the concentration of particles in the powder engine,the combustion of boron particles is always under the cluster shape.The reaction mechanism of boron is complex due to the various intermediate products during the combustion process of boron particle.Based on the in-depth study of the combustion mechanism of boron particle cluster,the guidance for the design of boron based powder fuel ramjet can be provided.According to the working performance of boron based powder fuel ramjet,the molecular structure and thermodynamic data of reactants in B/O system are determined by quantum chemical calculation.The transition state theory is used to solve the reaction path and kinetic parameters of B/O system,a detailed B/O reaction kinetic model is established.The model is simplified based on the reaction path of continuous reaction.The combustion characteristics of single and multiple boron particles were studied by numerical method,and the effects of environmental parameters and particle size on the combustion mechanism of single boron particle and the interaction mechanism of multiple particles under the condition of overlapping flame were obtained.According to the combustion characteristics of single boron particle,the physical model of single boron particle combustion was established.The combustion model of boron particle under cluster shape was established by the basis of combustion model of single boron particle and the effect of the interaction mechanism between particles.Finally,the working performance of a boron based powder ramjet was analyzed in detail through the combustion model of boron particle under cluster shape established in this paper.Through the study in this paper,the main conclusions were given as follows:1)The chemical kinetics of B/O system was obtained by quantum chemistry calculation.The stagnation point of molecular structure was searched at B3LYP/aug-cc-p VTZ level.15 stable molecular structures（including 8 intermediates）and 11 transition structures were obtained.The thermodynamic data of every reactant and the change of enthalpy（ΔH）,entropy（ΔS）and Gibbs free energy（ΔG）were calculated by statistical thermodynamics methods.Based on the theory of variational transition state,the rate constants of each elementary reaction were calculated,and the expression of reaction rate of is obtained by three parameter fitting method.The reaction rates of 12 reduced global reactions were obtained by steady-state approximation method.2)The combustion of single boron particle was calculated by numerical method based on the kinetics of B/O system established in this paper and the condensation of B₂O₃ was also considered.The effects of ambient pressure,temperature,mole fraction of O₂ and particle size on single boron particle combustion were studied.The results indicate that the surface reaction of boron particles with small particle size（1μm）is mainly controlled by the heterogeneous reaction 2B（s）+O₂=B₂O₂ at particle surface,and the B₂O₂ has the highest concentration at particle surface in all heterogeneous reaction products.The two main homogeneous reactions in the particle flame are BO+O₂=BO₂+O and B₂O₂+O=BO+BO₂,respectively.The flame temperature is mainly controlled by heterogeneous reaction,and the highest temperature appears at the particle surface.The ambient temperature and oxygen have no obvious effect on the reaction mechanism of boron particle.The flame radius and temperature increase with the decreasing of temperature and increasing of mole fraction of O₂.With the increasing of ambient pressure,the highest flame temperature still appears at the particle surface,but the concentration peak of some products appears gradually far from the particle surface.The total combustion rate of particles basically keeps a linear relationship with ambient pressure.With the increasing of particle radius,the mechanism of heterogeneous reaction on particle surface changed obviously.The mainly heterogeneous reaction changes from the reaction between boron and O₂ into the reaction with O and B₂O₃gradually.3)The combustion characteristics of multiple boron particle under stationary state was studied by the same numerical method for the combustion of single boron particle.The effects of particle distance,size ratio and packing structure were studied.At the same particle diameter,the reaction mechanism of each particle in the system is the same as that of single boron particle,and the particle flame is always under spherical structure.To a double particle system,the average combustion rate of the particle increases with the decreasing of the particle distance.when the initial particle distance is less than 0.1μm.Under different packing structures,the overall combustion characteristics of particles are basically the same as that of the double particle system.4)The combustion model of single boron particle was built based on the simulation results of the combustion characteristics of single particle.The ratio of oxidation and reduction reaction on the surface reaction of boron particle is determined by the concentration of B₂O₃ at both particle surface and flame sheet.The influence of evaporation on the combustion process of molten boron particle was also considered when the particle temperature higher than the melting temperature.The predicted results show a good agreement with experimental data.Based on the single particle combustion model and the combustion characteristics of multiple particle,the combustion model of boron particle under cluster shape was established by the effective reaction surface of the particle cluster.The influence of particle distance on its combustion rate under different porosity was adopted,and the predicted results by this model is in good agreement with the experimental data.The working performance of boron based powder fuel ramjet was analyzed by the model,and the phenomenon of obvious relay ignition was founded in the ramjet combustor.