|英文摘要||Aluminum powders are widely used as a metal additive in various high-energy propellants because of its high combustion enthalpy. In order to understand the combustion mechanism of high-energy propellants, the ignition and combustion characteristics of aluminum powder are two of the important factors. It has been concerned to increase fuel heat value to improve engine thrust performance by adding metal powder and other energetic materials to hydrocarbon fuels. Aluminum powder can also release a large amount of heat through secondary combustion with hydrocarbon fuel combustion products H2O and CO2. Aluminum powder produces more stable Al2O3 by secondary combustion, which also plays a catalytic recombination role for the decomposition of hydrocarbon fuel combustion products.
When aluminum powders are added to hydrocarbon fuels, the ignition and combustion properties of the mixtures are closely related to aluminum particle size, ambient temperature and pressure. At present, nano-aluminum powders have been widely applied. As the particle size of aluminum powder decreases from micron to nanometer, the combustion control process of aluminum powder will exhibit a transition from diffusion limit to reaction kinetic limit. Therefore, the study of ignition delay and burn time of aluminum powders with various particle sizes is of great significance to the establishment of combustion mechanism in engine combustor and the design and optimization of combustor, it also provides important basic data for establishing and validating the aluminum powder ignition and combustion chemical kinetic mechanisms. At the same time, the present study will provide a basis for exploring the new approach to increase engine thrust in a scramjet.
The ignition delay and burn time of aluminum powders with various particle sizes under various oxidant environments (O2, H2O, and CO2) and the combustion interaction between aluminum powder and hydrocarbon fuel were studied in this dissertation. The dependence of the ignition delay and burn time of aluminum powders with the environmental temperature, pressure, type and concentration of oxidant, as well as aluminum particle size was systematically obtained in shock tube experiments. The special attention was paid to the dependence of ignition delay and burn time of aluminum powders with various factors in the transition regime from the diffusion limit to kinetic limit in combustion. Furthermore, aluminum powders with various particle sizes were added into two typical fuels H2 and C2H4 to investigate the combustion interaction between aluminumpowder and fuel. Based on the shock tube experiments, the existing chemical kinetic mechanisms of aluminum powder were applied to kinetically simulate the burn time of aluminum powders.The main conclusions are as follow:
1) The relations of the ignition delay time and burn time of aluminum powders with particle size (50 nm, 200 nm, 1 μm, 10 μm, 20 μm, 50 μm) were obtained at the temperatures from 2320 to 2470 K and the pressure of 8atm as follow:
D is the aluminum powder size in μm
2) The relations of the ignition delay time and burn time of 200 nm aluminum powders with temperature, pressure and oxidant concentration were obtained at the temperatures from 1400 to 3200 K, the pressures from 2 to 13 atm, and oxidant concentrations from 20% to 80% as follow:
τ_ign=10.6[〖X_(O_2 )]〗^(-0.93) P^(-0.95) exp(26710/RT)
τ_ign=16.7[〖X_(〖CO〗_2 )]〗^(-0.6) P^(-0.7) exp(32158/RT)
τ_ign=94.1[〖X_(H_2 O)]〗^(-0.99) P^(-0.41) exp(56528/RT)
τ_burn=12.3[〖X_(O_2 )]〗^(-0.43) P^(-0.54) exp(21550/RT)
τ_burn=42.8[〖X_(〖CO〗_2 )]〗^(-0.71) P^(-0.44) exp(36981/RT)
τ_burn=73.1[〖X_(H_2 O)]〗^(-0.69) P^(-0.28) exp(39666/RT)
where the ignition delay time τign and the burn time τburn are in μs, T is the temperature in Kelvin, XO2、XCO2、XH2O are the mole fraction of O2、CO2、H2O in Ar, and P is the pressure in MPa, R is gas constant in J•mol-1•K-1.
3）The relations of the ignition delay and burn time of 10 μm aluminum powders with temperature, pressure and oxidant concentration were obtained at the temperatures from 2100 to 3000 K, the pressure from 2 to 13 atm, and the oxidant concentrations from 20% to 80% as follow:
τ_ign=789.5/(P^0.32 T^0.31 (X_(O_2 )+0.64X_H2O+0.51X_CO2 ) )
τ_burn=1572.3/(P^0.34 T^0.29 (X_(O_2 )+0.67X_H2O+0.47X_CO2 ) )
where the ignition delay time τign and the burn time τburn are in μs, T is the temperature in Kelvin, XO2、XCO2、XH2O are the mole fraction of O2、CO2、H2O in Ar, and P is the pressure in MPa.
4) The ignition and combustion of 200 nm aluminum powders in the O2, CO2 and H2O are kinetically controlled. The ignition and combustion of 10μm aluminum powders in the O2 are kinetically controlled. The ignition and combustion of 10 μm aluminum powders in the CO2 and H2O gradually transit from kinetically controlled to diffusion-controlled.
5) The kinetic simulation results of burn times of 200 nm aluminum powders show a good consistency with the experimental results on the change trend with temperature, pressure and oxidant concentration, but there are some numerical differences between the simulation and the experimental results.
6) 1μm aluminum powders can ignite in H2 with 70% dilution at 1276 K and 1.3atm. 10μm aluminum powders can ignite in C2H4 with 88% dilution at 1277 K and 1.2 atm. As the particle size decreases, the aluminum powders can ignite more easily in hydrogen and ethylene with the higher dilution. The aluminum powders ignition and H2 or C2H4 ignition take place simultaneously under present experimental conditions. The ignition of aluminum powder delayed to H2 or C2H4 was not observed. The addition of aluminum powders can shorten the ignition delay of hydrogen and ethylene.|