Russian scientists Yu.V. Kondratyuk and F.A. Zander was the first to express the idea of the possibility and necessity of using light metals and their compounds for fuels in liquid-propellant rocket engines. The use of powdered metal fuel (PMF) essentially opened up a new direction in jet engine building. Possessing a high calorific value and high density, PMFs can significantly increase such important characteristics of propulsion systems as specific thrust impulses.
With regard to intra-chamber processes in power plants, experimental and theoretical studies should be aimed at studying those characteristics that determine the ignition and completeness of combustion of powdered metal fuel . One of these characteristics is the flame propagation speed. In this regard, experimental data are needed that will make it possible to establish regularities and reveal the features of combustion and flame propagation in a turbulent flow of airborne particles of metal particles. Aluminum is one of the most accessible metals as a powdered metal fuel.
This paper presents the results of studies of the influence of the initial parameters of the air suspension flow: aluminum particle size, velocity, turbulence, and air temperature on the flame propagation velocity. This work is a continuation of the research, the results of which were presented in .
The most important characteristic of powdered metal combustibles is the particle size. In recent years, quite a lot of experimental results have been obtained to determine the law of combustion of aluminum particles, allowing us to state that in the size range 10m, there is a change in the combustion mode. In the specified range of sizes, the combustion mode of aluminum particles changes from diffusion-controlled to kinetic, i.e., a transitional combustion mode is realized.
It was found that in the air suspension flow with ASD-4 with an average particle size 7.4, the combustion of which proceeds in the kinetic mode, the dependence corresponds to theoretical concepts. According to which, at values of 0.2 and in air suspensions of aluminum particles, heat release, temperature and flame front propagation velocity take maximum values [2,3,4].
In the air suspension flow with ASD-1 with an average particle size , it was found that the flame propagation speed increases with a decrease in the excess air coefficient in the range of , and there is only one maximum on the dependence curve, which corresponds to the value of the excess air coefficient . This is explained by the fact that ASD-1 aluminum particles with an average size burn in air at atmospheric pressure in the diffusion mode. In this case, with an increase in the mass concentration, the total surface area of the particles increases, in connection with which an intensification of the processes of radiative heat transfer between the particles is observed, leading to a corresponding increase in the speed of propagation of the flame front.
It has been established that for an air suspension of ASD-1 powder with an average particle size of aluminum , a dependence was obtained indicating that an increase in the initial air temperature leads to an increase in the flame propagation velocity . The maximum values of the flame propagation speed correspond to the values of the excess air coefficient . In this case, the boundary of the rich limit of flame propagation shifts towards stoichiometry (), remaining in the region of the over-enriched composition of the air suspension.
It was found that with an increase in the flame propagation velocity , respectively, and with an increase in the air suspension flow velocity , the range of mixture compositions within which flame propagation is possible narrows. It was found that in the model of a combustion chamber with a diameter of 0.04 m at a speed of less than 40 m/s, there was no steady propagation of the flame front. This fact confirms the existence of a critical value of the Reynolds number for the process of unsteady flame propagation in reacting air suspensions of aluminum particles.
It has been established that in the air suspension flow with ASD-1 aluminum particles in the range of , an increase in the initial turbulence from 12 to 22% and a turbulence scale from 0.01 to 0.07 mm leads to an expansion of the flame propagation limits and increase the speed of flame propagation. It was also found that the presence of a grate in the inlet channel at 0.07 m in the combustion chamber leads to an increase in the flame propagation velocity in the air suspension flow from ASD-4, and to a decrease in the air suspension flow from ASD-1.
The revealed regularities of the influence of the initial parameters of the air suspension flow on the speed of flame propagation can find practical application in the organization of the combustion process in the chambers of propulsion systems on powdered metal combustible fuel.