Study On Primary Combustion Characteristics Of Boron-Based Fuel-Rich Propellant

Because of its high volumetric and mass heat of combustion, boron powder has been considered as the best filler candidate for the solid high-energy fuel-rich propellant of the ducted rocket. To improve the processibility and combustion properties of boron powder in the propellant, surface treatment of boron particle is necessary. To meet these requirements, the methods of surface treatment, such as, purification, coating and agglomeration on boron powder have been established. The characteristics of the different kinds of boron powders, such as thermal oxidation behavior, energy release, ignition property and primary combustion properties of the born-based fuel-rich propellant, were systematically investigated. Furthermore, the combustion mechanism of the boron-based fuel-rich propellant was studied. A novel combustion model of the boron-based fuel-rich propellant has been developed. The combustion properties of the propellant were numerically analyzed.The main results of this dissertation are as follows:To meet the requirements of boron-based fuel-rich propellant application, the surface treatment methods of boron particles, such as, purified boron powder, coated boron powder, and agglomerated boron powder, have been developed. Thermal oxidation behavior and energy release of the three types of boron powder were also investigated. The results showed that boron powder coated with LiF can reduce the temperature of boron's oxidation in oxygen environment. With higher LiF coating content, boron's oxidation degree and heat of combustion were promoted. Boron powder coated with AP or LiF can release more heat of oxidation, and the action of LiF is better. The agglomerated boron powder can release more oxidation heat than the coated boron powder.The action of mole fraction of oxygen, oxygen flux, pressure, temperature, coating and agglomeration of boron powder on the ignition characteristics of boron powder were systematically investigated. The experimental results showed that the greater the mole fraction of oxygen or pressure, the lower the ignition temperature of purified boron particle. The coating with LiF on the boron powder can reduce the ignition temperature. The ignition time of purified boron particle can be reduced by increasing the environment temperature. Based on King's ignition model of single boron particle, a theoretical model for the ignition of agglomerated boron particle has been established. The numerical results showed that agglutination of boron powder can reduce the ignition temperature of boron particle, however the ignition time is enhanced.The action of various factors, such as, the content of fine AP and the agglomerated boron powder, the types and contents of coating materials on the primary combustion characteristics of boron-based fuel-rich propellants with catalyst, were experimentally investigated. The experimental results showed that propellants with fine AP and agglomerated boron give higher burning rate and burning-rate pressure exponent. When the coating content of AP is low, the burning rate of the propellant with coated boron powder at low pressure is high and burning-rate pressure exponent is low. However, when the coating content of AP is high, the propellant with coated boron powder gives high pressure exponent and low burning rate at low pressure. The boron powder coated with LiF can increase the burning rate of the propellant at low pressure, while the high coating content of LiF has negative effect on pressure exponent. The effect of agglomerated boron powder with different particles size on the combustion properties of the propellants without catalyst was systematically investigated. The results indicated that propellant with the greater particle size of agglomerated boron powder can be ignited at lower pressure and has higher burning rate. Propellants with higher burning rate give lower combustion pressure limit.The thermal decomposition characteristics of the boron-based fuel-rich propellant and its main ingredients were investigated. The phenomenon that there is exothermal interreaction between AP and GFP catalyst has been found. The results showed that the thermal decomposition reaction of the propellant is consisted of the thermal decomposition of AP, HTPB, the reaction between the thermal decomposition products of AP and HTPB, and the oxidation of boron. The thermal decomposition of AP plays an important role on the thermal decomposition process of the propellant. There is a correlation between the peak temperature of AP thermal decomposition and the burning rate of the boron-based fuel-rich propellant with purified boron or agglomerated boron powder, which means that lower the decomposition peak temperature of AP is, the higher the burning rate is.The primary combustion mechanism of the boron-based fuel-rich propellant was studied. The flame of propellant with purified boron is brighter than that propellant with agglomerated boron. The temperature structure of the condensed reaction phase and the gas reaction phase of the propellants were measured by thermocouple. It was found that the gas reaction of the boron-based fuel-rich propellants was quite complicated, it means that there are zones where temperature was lowered. Some steady combustion parameters of the propellants, such as, the burn surface temperature, the temperature gradient in the gas phase, the final flame temperature, were obtained. The results showed that the burn surface temperature and temperature gradient in the gas phase increase with pressure enhancement. The temperature gradient in the gas phase has the greatest effect on the burning rate of the propellant with purified boron. While the burn surface temperature shows the greatest effect on the burning rate of the propellant with agglomerated boron.On the consideration of the abovementioned thermal decomposition and combustion characteristics of the boron-based fuel-rich propellant and its main ingredients, a new steady combustion model of the boron-based fuel-rich propellant based on the PEM combustion model has been developed. The main factors, such as, the action of the particle size of boron powder on the surface area ratio of AP and the propellant, the partial oxidation reaction of the oxidative products of AP thermal decomposition with binder and boron at the same time, and the feedback heat of the ignition and combustion of boron particle from gas zone on the burn surface of the propellant, were considered in the model. The results by the model showed that the numerical results are in good consistency with the experimental datas, which means that the model can reflect the combustion characteristics of the boron-based fuel-rich propellants. Some factors on the primary combustion properties of the propellant have been analyzed numerically.