Study On The Mechanism Of Ignition,Combustion And Agglomeration On Composite Solid Propellant Burning Surface

Solid propellants is the energy source of solid rocket motor,and its performance directly affects the operation stability,flight speed,flying range and other important indicators of the aircraft.Metal fuel is an important component and main energetic material of solid propellants,which can greatly improve the energy level of propellants.However,the agglomeration of metal fuel on the burning surface of propellants will bring many adverse effects on motor performance.Based on the above background,combined with experimental tests and theoretical calculations,the ignition and combustion characteristics of conventional and new solid propellants containing aluminium（Al）,aluminium trihydroxide（Al H₃）and boron（B）respectively,as well as the combustion energy release characteristics and burning surface agglomeration mechanism of the corresponding solid propellants were studied.The research work is expected to provide theoretical guidance for reliable ignition,efficient combustion and full energy release of solid propellants.Firstly,the conventional Al-containing solid propellants were taken as the research object.On the one hand,the microstructure and multi-component synergistic thermal decomposition process of Al-containing solid propellants were investigated in detail by combining scanning electron microscopy,energy spectrum detection and thermogravimetric analysis.On the other hand,the ignition and combustion characteristics of Al-containing solid propellants and the agglomeration mechanism of aluminum particles on the burning surface were systematically studied by online diagnosis of combustion process,MATLAB digital image processing technique and offline characterization method of condensed combustion products.The results show that the thermal decomposition process of Al-containing solid propellants includes three stages:slow weight loss at low temperature（100–259℃,weight loss 6.61%）,rapid weight loss at medium temperature（259–396℃,weight loss 69.3%）and slow weight loss at high temperature（396–700℃,weight loss 2.02%）.The condensed-phase combustion products can be divided into three categories:alumina smoke particles（<0.5μm）,medium-sized particles（0.5–70μm）and agglomerates（>70μm）.The corresponding micro-morphological characteristics indicate that both melting dispersion and diffusion reactions occur during the combustion of aluminium particles.The formation of aluminum agglomerates is closely related to the structure of the"pockets"in the propellants.The agglomeration process（"accumulation–aggregation–agglomeration"）of the aluminum particles on the burning surface is the same at different pressures,but the increasing of pressure will shorten the time required for agglomeration and reduce the size and number of agglomerates.The velocity of agglomerates leaving the burning surface is maximum at atmospheric pressure,and there is no significant difference under different pressures.Secondly,the microscopic morphology,compositional composition,thermal oxidation properties and ignition and combustion characteristics of Al and Al H₃ that is potential substitute of Al were comparatively studied at macro and micro levels.The results show that the thermal oxidation process of Al and Al H₃ both include one weight loss stage and two weight gain stages.The ignition delay times for Al and Al H₃ with similar particle size（about 25μm）are 19.5 ms and 1.4 ms,respectively.There is a short period of explosive combustion in the early stages of Al combustion,whereas the entire combustion process of Al H₃ is explosive.The flame development of Al H₃ is characterised by a"double-layer flame surface"in the early stage,is closely related to the dehydrogenation characteristics.The combustion behavior of single Al H3 is similar to that of Al,and the accumulation of H₂ inside the particles will lead to"microexplosion"of Al H₃ particles.In general,Al H₃ has significantly better thermal oxidation and ignition combustion properties.Based on the above comparative study,Al/Al H₃-containing solid propellants were prepared by using Al H₃ to partially/fully replace Al in Al-containing solid propellants.The effects of Al H₃ on the characteristics of energy performance,ignition combustion and metal particle agglomeration were systematically investigated through experimental tests and theoretical calculations by NASA CEA software.The results of theoretical calculations show that the replacement of Al with Al H₃ will lead to an increase in the theoretical mass and volumetric combustion heat of the propellants,an increase in the theoretical specific impulse,and a decrease in temperature of the motor combustion chamber.The results of the experimental study show that as the Al H₃content increases,the combustion efficiency of the propellants gradually increases,the burning rate gradually increases,and the brightness of the combustion flame and the maximum combustion temperature gradually decrease.The agglomeration process of Al H₃ on the burning surface of the propellants and the final morphology of the agglomerates are similar to that of Al.The agglomeration of metal particles will be suppressed to a certain extent by adding Al H₃ with appropriate particle size into solid propellants.Next,according to the staged combustion characteristics of boron-rich solid propellants in practical application,a micro-tube burner was designed and produced to simulate the primary and secondary combustion processes.The combustion characteristics of B-containing solid propellants and the agglomeration characteristics of boron particles in propellants were studied in depth through online optical measurements and offline analysis of the combustion products.The results show that the primary combustion intensity and temperature of B-containing solid propellants are significantly lower than that of Al-containing solid propellants.There is obvious agglomeration of boron particles on the burning surface of propellants.Irregular boron aggregates near the burning surface do not turn into spherical droplets like aluminum aggregates,but after entering the secondary combustion flame zone,they will further develop into spherical agglomerates due to boron melting.The composition of the solid phase products of primary combustion of B-containing solid propellants is much more complex than that of the condensed phase products of Al-containing propellants,containing about 17 components such as B,BN,B₁₃C₂.The pressure increase basically does not change species of solid product components in primary combustion of B-containing solid propellants but only the relative content of each component.Subsequently,the numerical simulation of premixed combustion process of ammonium perchlorate（AP）/hydroxyl-terminated polybutadiene（HTPB）solid propellant was carried out based on Chemkin software.The rationality of the detailed gas-phase chemical reaction kinetic mechanism（including 37 components,127 radical reactions）was validated by comparison with experimental data on the combustion of AP and AP/HTPB propellants from the classical literature.On this basis,the premixed combustion flame structure and the corresponding microscopic chemical reaction processes in the near-burning surface region of AP/HTPB solid propellants of different formulations were investigated in depth in combination with temperature sensitivity analysis.The spatial evolution mechanism of the temperature distribution and key components in the gas phase region of propellants is obtained,and the results help to deepen the understanding of the macroscopic combustion characteristics of solid propellants.Finally,according to the random distribution characteristics of Al particles and AP particles,a geometric topological model of the fine structure of Al-containing solid propellants was constructed.The three-dimensional numerical simulation of the precipitation,movement and agglomeration of aluminum particles near the solid propellant burning surface and the prediction of the particle size distribution of aluminum agglomerates were realized by the particle direct numerical simulation technique based on the discrete element method.The accuracy of the established numerical model was verified qualitatively and quantitatively using experimental data on the morphological structure and particle size distribution of agglomerates in the region near burning surface.Subsequently,the effects of aluminium particle content in the solid propellants and environmental pressure on the agglomeration process and the particle size of the agglomerates were further investigated by changing the model parameters.The results obtained from the simulations were consistent with the actual situation,indicating that the model can effectively predict the agglomeration behaviour of aluminium particles in the solid propellants.