Study On Ignition And Combustion Characteristics And Combustion Promotion Mechanism Of Boron Based High-energy Nanofluid Fuel

The performance of propulsion is directly related to the fuel calorific value.Boron has a very high energy density.By adding boron nanoparticles to liquid hydrocarbon fuel through surface modification to make a stable suspension of boron-based nanofluid fuel,the energy density of the fuel can be significantly increased,and it has considerable development potential in the field of synthetic high-energy fluid fuel.To deepen the understanding of this new high-energy solid and liquid bi-phase fluid fuel,the evaporation characteristics,ignition combustion characteristics and combustion promotion mechanism of B/JP-10（exo-tetrahydrodicyclopentadiene）nanofluid fuel are systematically studied in this paper by combining experimental research and theoretical calculations.It is expected to provide reference and technical support for the basic characterization and engineering practical application of this new high-energy fuel.First,the evaporation characteristics of B/JP-10 nanofluid fuel were investigated using a hot plate test system.Boron nanoparticles were found to increase the Leidenfrost point of JP-10.When the temperature is 400～650°C,the evaporation process of 10wt%B/JP-10 fuel droplet includes three stages:Leidenfrost droplet evaporation,immersion evaporation and agglomerate penetration evaporation.Among them,Leidenfrost droplet evaporation and agglomerate penetration evaporation have an inhibitory effect on droplet evaporation,while immersion evaporation can promote the evaporation of fuel droplets.The ignition temperature of the fuel decreased with the increase of the solid content,and the ignition temperature of the fuel decreased from 706°C to 686°C when the solid content was 30wt%.The ignition temperature of the fuel decreased with the increase of the solid content,and the ignition temperature of the fuel decreased from 706°C to 686°C when the solid content was 30wt%.The ignition temperature of fuel droplets with different solid contents was calculated by building a Leidenfrost droplet heat transfer model for B/JP-10 nanofluid fuel,and the calculated results were in good agreement with the experimental results,which indicated that the model could effectively predict the ignition temperature of B/JP-10nanofluid fuel.Secondly,the single droplet ignition and combustion process of B/JP-10nanofluid fuel was investigated by using CO₂ laser ignition and combustion test system.The ignition and combustion processes of fuel droplet under air and 50%oxygen content are different.In air,only JP-10 single-phase combustion of fuel droplet can occur due to the higher oxygen competition;while at 50%oxygen content,there are two-phase coupling combustion stages and agglomerate combustion stages of fuel droplet.The ignition delay of fuel droplet decreased with increasing solid content and oxygen content,but the oxygen content contributed significantly more to the ignition delay than the solid content.The ignition delay of fuel in air decreased from 136.6 ms to 107.2 ms when the solid content was increased from 5wt%to30wt%,while the ignition delay of fuel at 50%oxygen content was around 45 ms.The formation of agglomerates during fuel droplet combustion is related to both the solid content and the oxygen content of the fuel droplet.When the solid content is greater than 10wt%,agglomerates will be formed during the combustion of fuel droplets in an air environment,while an increase in oxygen content will increase the critical solid content for agglomerate formation.Next,the combustion promoting mechanisms of B/JP-10 nanofluid fuel by two methods including Et OH（Ethanol absolute）doping in the liquid phase and boron nanoparticle modification in the solid phase were investigated.In terms of liquid-phase doping,the promotion effect of Et OH doping on the combustion of B/JP-10 nanofluidic fuel is mainly in two aspects.On the one hand,Et OH enhances the"microexplosion"of the fuel droplet itself in the early stage of combustion,which transports the hydrocarbon fuel vapor and active boron to the oxygen-rich zone and improves the utilization of oxygen around the droplet;on the other hand,after the formation of agglomerates,Et OH will gradually break the agglomerates and promote the contact between active boron and oxygen.Eventually,the overall energy release efficiency of the fuel is improved.In terms of solid-phase boron nanoparticle modification,both Li F and AP modifications can break the agglomerates formed during the combustion of fuel droplets to promote the release of boron energy from the fuel,but their destruction mechanisms are different.Li F mainly reacts with B₂O₃on the surface of boron agglomerates to produce easily sublimated material boroxyl fluoride compounds and lamellar crystals of lithium boroxyl compounds,thus making it difficult to close the shell layer.AP is mainly generated by decomposition of strong oxidizing gases,creating a local oxygen-rich environment around the agglomerates to enhance the"microexplosion"effect to break the shell structure,thus forming a porous configuration on the surface of the agglomerates.Subsequently,the atomized combustion characteristics of B/JP-10 fuel were investigated using a self-designed and constructed nanofluid fuel atomization combustion system.The center of B/JP-10 nanofluid fuel atomization flame is divided into four combustion regions along the axial direction of the nozzle:B/JP-10 coupled combustion region（exit section）,JP-10 single-phase combustion region（steady combustion section）,B/JP-10 coupled combustion region（tail flame section）,and boron single-phase combustion region.The addition of 10wt%boron nanoparticles significantly increases the combustion temperature of the exit and tail flame sections of the atomized flame,with the average temperature of the exit section increasing from 536°C to 981°C;the average temperature of the tail flame increases from1104°C to 1352°C.However,the volatilization of boron oxide fumes generated by boron combustion decreases the temperature of the fuel atomization flame steady combustion section,reducing the average temperature of this section from 1607°C to1573°C.The addition of boron nanoparticles can advance the cracking position of JP-10,reduce the generation of CO and promote the full combustion of JP-10.Finally,a two-phase coupling ignition and combustion model of B/JP-10nanofluid fuel was developed based on the experimental study coupled with the evaporative combustion of hydrocarbon fuel and the combustion mechanism of boron agglomerates.The dynamic energy release characteristics during the ignition and combustion of this fuel droplet were obtained by solving the model in Matlab.The calculated results are in good agreement with the experimental results,indicating that the model can achieve the effective prediction of the ignition and combustion time of B/JP-10 nanofluid fuel.