Modeling Of Powdered B/Mg Fuels Ignition And Combustion

The powder fuel ramjet is a new type of ramjet with high energy metal or boron powder as fuel,and ram air as oxidant and working substance.Compared with liquid fuels,powder fuel does not dissociate at high temperature,and is more conducive to maintenance and long-term storage;compared to solid fuels,powder fuels have greater resistance to sensitivity and improve the safety of fuel in the manufacturing,storage and use process.The powder fuel flows under the action of fluidization gas,which has a strong flow rate regulation function,so the powder fuel Ramjet has multiple start-up and thrust regulation functions.In many powder fuels,boron-based powder fuel is considered as a powder fuel with great application potential due to its high energy characteristics and the low loss of two-phase flow and other excellent properties.As the precondition of the high energy performance powder fuel ramjet,adding the ignition and combustion promotion material as well as agglomeration of the small particle size powder fuel,the storage,transport and ignition-combustion performance is improved.The basis is to study the ignition combustion mechanism of powder fuel in depth.Based on the research background of powder fuel ramjet.This paper firstly determined that magnesium is an ideal material of boron-based powder fuel ignition combustion,and then determined the content of magnesium in powder fuel according to the engine specific impulse performance,and obtained B/Mg powder fuel by reasonable process and method.Surface morphology,density,thermal oxidation properties of the fuel were analyzed and tested.Then,a high-temperature ignition combustion experiment system was set up to study the dynamic ignition combustion characteristics of B/Mg powder fuel,and the influence of parameters such as particle size,magnesium content,magnesium particle size and ambient temperature on the flame morphology,ignition delay time,combustion time and combustion gain and decay time of B/Mg powder fuel ignition-combustion was analyzed.On these basis,experiments were carried out to collect the combustion?oxidation?products at different stages in dynamic ignition combustion process,and the mechanism of ignition and combustion of B/Mg powder fuel was revealed through TEM analysis of the internal structure,morphology and evolution of the combustion?oxidation?products at different Stages.Finally,the ignition combustion model of B/Mg powder fuel was established,the accuracy of the model was verified and the cause of the model error was analyzed.Through this study,the main conclusions are as follows:?1?B/Mg powder fuel ramjet is superior to the traditional ramjet in the Engine's specific impulse performance and the working stability of the combustion chamber.When the magnesium content in the B/Mg powder fuel is less than 0.3?mass fraction?,the powder ramjet will show a more prominent advantage in the specific impulse and density specific impulse.Agglomeration of B/Mg powder fuel shows a rules hape,a certain degree of spherical and the caking phenomenon obviously disappeared.However,due to the irregular shape of boron particles,small particle size caused by interference and interference between particles,there are some small voids between particles.The packing ratio of B/Mg powder is increased significantly compared with the original boron powder,with a single particle size packing ratio of 0.4,which is nearly 50%higher than the original boron powder.?2?The evolution of the flame morphology of different boron-based powder fuel samples in ignition combustion process can be divided into two categories,one is the cluster flame form with the state of the cluster,and the other is the micro-explosive combustion flame form which shows the micro-explosion phenomenon in the combustion.The complete combustion of the powder fuel in the form of a cluster takes a long time and is not conducive to the rapid release of energy in boron-based powder fuel.While micro-explosion combustion can be completed in a short period of time to complete the combustion,is conducive to the performance of powder stamping engine improvement.The change of combustion flame morphology can be achieved by changing the particle size,magnesium content and magnesium particle size of boron-based powder fuel.?3?Addition of magnesium can greatly reduce the ignition temperature of B/Mg powder fuel,which can be ignited when the ambient temperature is 1279k-1346k,while the Magnesium-free sample is 1649K.For different samples,the influence of particle size on the ignition delay time varies greatly,for sample B_1.0/Mg_0.0 and B_0.9/Mg_0.1,the ignition delay time increases obviously with the increase of particle size.For samples B_0.8/Mg_0.2 and B_0.7/Mg_0.3,the ignition delay time is not much affected by the sample Size.Because of the phenomenon of micro-explosion,the burning time of magnesium-free samples is significantly larger than that of magnesium-containing samples.For sample B_1.0/Mg_0.0 and B_0.9/Mg_0.1,the burning time increases obviously with the increase of particle size.For small-diameter B/Mg Powder fuel,the combustion gain time is not significantly shortened with the increase of magnesium content,but the attenuation time will be shortened;for the particle size 98-125?m and150-200?m of B/Mg powder fuel,when the magnesium mass fraction reaches 30%,the combustion process will show a peak duration,the combustion gain time is correspondingly shortened.?4?During heating of B/Mg powder fuel,the magnesium particles in the powder fuel will melt to liquid film and cover the surface of boron particles.With further heating,the contour of particles gradually blurred,and the melting boron oxide and magnesium will fill the gap between particles under the action of viscous flow.The liquid magnesium will form a liquid film stacked on the surface of the boron particles,the entire sample presents a multilayer structure.Under two necessary conditions the micro-explosion phenomenon will occur,the first is magnesium particles in the aggregates reach the superheat limit,and the second is the agglomeration of boron oxide/boron to form a closed liquid film completely wrapping the magnesium particles.?5?The combustion process of B/Mg powder fuel can be divided into three stages,the first stage is the stage before the temperature of the aggregation reaches the magnesium melting point,and the oxygen in the environment can be diffused to the surface of boron and magnesium particles through the gap between the particles;the second stage is the melting of the magnesium particles inside the aggregates,which spreads in the form of liquid film on the surface of boron particles and fill the gap between the particles,preventing the diffusion of oxygen into the aggregates;the third stage is the combustion of boron particles after the consumption of the oxide layer,the boron particles are gathered and fused under the action of viscous flow.