Experimental And Theoretical Studies On The Combustion Characteristics Of HAN-based Liquid Propellants At High Pressures

HAN-based liquid propellant is a new type of green high-energy liquid propellant, whose combustion behaviors will directly affect the performance of the engine, as an ideal fuel for the liquid rocket engine. The experimental and theoretical studies have been conducted, focusing on the high-pressure combustion characteristics of HAN-based liquid propellants. The main research contents and results are as follows:(1) By use of the theoretical and empirical formulas for estimating the thermal physical property parameters of liquid polar substances, the thermal physical properties of HAN-based liquid propellant under different pressures and temperatures were calculated, which can provide basic data for the theoretical research on the evaporation and combustion of HAN-based liquid propellants.(2) A simplified model for the HAN-based liquid propellant single droplet evaporation under atmospheric pressure was established, and the influences of ambient temperature, droplet size, droplet initial temperature, convection velocity on the LP1846droplet evaporation characteristics were researched. The results show that the evaporation process of the LP1846droplet can be divided into the transient heating stage and the equilibrium evaporation stage. As the ambient temperature grows, the periods of both the transient heating stage and the equilibrium evaporation stage are shortened, but the equilibrium temperature increases. The two stages are all extended with increasing droplet size, but there is no change in the equilibrium temperature. As the droplet initial temperature increases, the transient heating duration gets shorter, but the equilibrium evaporation stage is unaffected, and the equilibrium temperature remains the same. The larger the convection velocity is, the shorter the time of the transient heating stage is, and the bigger the average evaporation constant becomes, but the equilibrium temperature remains constant.(3) A high-pressure evaporation model for the HAN-based liquid propellant droplet was built, and the droplet temperature and the radius versus time curves were calculated when a LP1845droplet evaporates in a high-temperature, high-pressure nitrogen environment. The results indicate that the droplet radius increases firstly and reduces afterward as the evaporation goes on, and the reduction rate of the radius satisfies the D2law. The droplet temperature rises rapidly at the beginning, then maintains stable gradually at an equilibrium value. When the ambient temperature remains constant, with increasing ambient pressure, the time to reach the equilibrium temperature lengthens, but the droplet lifetime shortens. (4) Considering the liquid-gas interphase mass transfer, surface tension and gas component transport, the droplet evaporation mass transfer source term was derived. A theoretical model for the HAN-based liquid propellant droplet evaporation in a convection nitrogen environment was set up based on the theory of VOF two phase flow. The variation regularity of the flow field around the evaporating LP1846droplet was studied, and the Stefan flow around the droplet surface and the swirl at the droplet tail were also captured successfully.(5) Considering the gas-phase chemical reaction at the droplet surface, a simplified combustion model applicable to the HAN-based liquid propellant droplet was established. Based on the model, the mass burning rate and the droplet lifetime under different ambient temperatures were calculated when a LP1846single droplet combusts in a stationary atmospheric environment. The results show that the mass burning rate accelerates with increasing ambient temperature, and the higher the temperature, the faster the rate increases. The droplet lifetime gets longer as the ambient temperature increases. The model is valid by comparing the calculation results with the experimental data.(6) A device for measuring the linear burning rate of HAN-based liquid propellants at high pressures was designed. The high-pressure environments were generated by the combustion of solid propellants. The coated propellants which burn progressively were introduced to maintain the approximate constant-pressure environments. The spread velocity of the combustion flame surface was measured by virtue of the measuring velocity technology of ion probe. Based on the experimental device, the apparent linear burning rates of AF-315at29.4-55.0MPa and LP1846at6～28MPa was measured, respectively.(7) Referencing the steady-state combustion theory of the solid propellant, the concept of the liquid propellant one-dimensional combustion wave structure was put forward. A one-dimensional simplified model of the steady-state combustion of HAN-based liquid propellant column was developed, and the relational expression between the burning rate and the ambient pressure was also obtained. Based on the model, the burning rates of LP1846at6～28MPa was calculated. The maximum error is3.2%between the simulation results and the experimental data, which shows that the simplified model is valid.(8) A two-dimensional high-pressure steady-state combustion model for the HAN-based liquid propellant was built by use of the VOF two-phase numerical simulation method. The combustion process of AF-315in a high pressure strand burner was simulated, and the variation regularities of the temperature, pressure and velocity were researched. The liquid surface movement throughout the combustion process was captured successfully, and the burning rates of AF-315under different ambient pressures were obtained. This method is feasible by comparing the simulation results with the experimental data, so it can be used to simulate the high-pressure combustion process of HAN-based liquid propellants.