| The ramjet engine is widely used in supersonic vehicles due to its wide operating range,simple structure and good economy,and increasing the operating limit of the ramjet engine is one of the important directions in the development of ramjet engine technology.However,for sub/supercombustion ramjet engines entering the space domain at altitudes greater than 25 km,as the altitude increases,the atmospheric density decreases rapidly and the working pressure of the combustion chamber decreases at the same Mach number,which deteriorates the ignition performance and flame stability of the combustion chamber and cannot meet the power requirements of the vehicle.As RP-3 aviation kerosene is a liquid fuel widely used by domestic aircraft,it is important to reveal its ignition and combustion characteristics under sub-atmospheric pressure environment to improve the ignition performance and stability of the combustion chamber under extreme operating conditions and to expand the operating lift of the ramjet engine.In real engines,spray combustion is the main application of liquid fuels,therefore the ignition and combustion characteristics of droplets and droplet groups in the spray have a direct impact on the combustion and flame stability of the fuel,therefore droplet combustion has been widely recognised as a means of studying the combustion mechanism of liquid fuels by researchers at national and international level for many years.In this paper,the low-pressure ignition and combustion characteristics of single and double droplets of RP-3 aviation kerosene(0.2 bar-1 bar)have been investigated using optical diagnostics such as high-speed photography and self-emitting flame radiation.Since aluminium nanoparticles can increase the energy density of the fuel and thus the propulsion of the engine,the low-pressure combustion characteristics and microstructure of combustion products of Al/RP-3 two-phase fuel droplets are investigated using a combination of experimental and molecular dynamics simulations,and the effect of the addition of aluminium nanoparticles on the nucleation characteristics of n-decane liquids is also analysed.Firstly,a free-dropping experimental system with a self-designed high-temperature pressure-controlled dropping furnace was built to investigate the low-pressure ignition characteristics of RP-3 aviation kerosene under free dropping conditions.The experimental results show that as the ambient pressure decreases,the ignition delay time of the droplet increases exponentially,while an increase in ambient temperature expands the flammable pressure limit of the RP-3 kerosene droplet and shortens the ignition delay time of the droplet,and then the equations for the relationship between the ignition delay time of the droplet and the pressure and temperature are fitted.After droplet ignition,the flame growth rate decreases as the ambient pressure decreases and the flame morphology changes from a semi-enveloped flame to a trailing flame.By comparing the ignition processes of RP-3 and the binary surrogate droplets at 1100 K,it can be seen that the ignition delay times of the two droplets develop in a similar trend with pressure,but the surrogate is more likely to ignite due to the lack of large molecular components,so this binary surrogate cannot accurately predict the ignition process of RP-3 aviation kerosene.Secondly,based on the self-designed low-pressure hanging droplet method combustion optical measurement system,the low-pressure hanging combustion sequences of single and double droplets of RP-3 aviation kerosene are studied.It is found that as the ambient pressure decreases,the slow oxidation time during droplet ignition increases,while the natural convection intensity decreases,the flame shape gradually approaches a spherical flame,the droplet boiling point interval decreases,and the carbon soot generation and agglomeration effects are reduced.The inertia-controlled bubble growth rate increases due to the low pressure,resulting in more intense micro-explosions during droplet combustion.For sub-atmospheric combustion of two droplets,the results show that the time for the ignited droplet to ignite the unburned droplet is proportional to the distance between the droplets and the ambient pressure,while the reduction in pressure expands the maximum ignition distance between the two droplets,and due to the oxygen competition effect during the combustion of two droplets,the combustion rate of two droplets is less than that of a single droplet in almost all operating conditions.At the same time,the micro-explosion phenomenon of multi-component fuel droplet combustion promotes the secondary atomization of the droplet population and has an impact on the ignition and extinction of the two droplets.The combustion characteristics of Al/RP-3 two-phase fuel droplets and the microstructure of combustion products were investigated using a hanging droplet method combustion system and scanning electron microscopy.The results showed that the addition of aluminium nanoparticles significantly contributed to the micro-explosion during droplet combustion,while the combustion of two-phase fuel droplets was divided into the following stages:ignition,liquid-phase combustion,surfactant combustion,aluminium aggregate ignition,aluminium aggregate combustion and extinction.The combustion intensity of the aluminium aggregate at atmospheric pressure is significantly higher than at low pressure,while the aluminium aggregate at 0.2 bar is unable to ignite and burn.The microstructure of the combustion products also shows that the aluminium aggregates undergo solid phase combustion at atmospheric pressure with a porous structure resulting from fragmentation and melting,while the aluminium aggregates at low pressure without burning show a dense aluminium cohesive aggregation structure.Finally,molecular dynamics simulations were used to investigate the nucleation process of n-decane blended with aluminium nanoparticles to investigate the phenomenon of droplet micro-explosion in combustion promoted by aluminium nanoparticles.In order to balance the accuracy and economy of the calculations,TraPPE-UA force fields were used to simulate n-decane.It was found that the addition of aluminium nanoparticles enhanced the nucleation rate of n-decane,and that the nucleation of n-decane was promoted by increasing the number and diameter of aluminium nanoparticles.At the same time,the van der Waals forces of aluminium atoms on n-decane molecules are greater than those between n-decane molecules,and the local density of the liquid phase around the aluminium particles increases,resulting in nucleation sites with reduced local density at locations far from the surface of the aluminium particles. |
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