Research Of Al/Mg Fuel-rich Propellant Combustion Characteristics And Its Influences On The Working Performance Of Ramjet

Solid Rocket Ramjet(SRRJ)is an advanced air-breathing propulsion device,with outstanding advantages like high specific impulse,long range,simple structure and good reliability.Therefore,it is ideal for the use of supersonic cruising missiles.In the 1950 s,the successful development of high-energy composite propellant lays a technical foundation for promoting the application of ramjet.To further improve the specific impulse of solid propellant,energy density,ignitability,toxicity,environmental capacity and many other effects were taken into the comprehensive consideration.Thus,magnesium and aluminum were chosen to be the ideal metal additives for composite propellant ingredients.SRRJ includes the typical structures of primary gas generator,inlet and backward-facing step,which lead to internal flow with the obvious three-dimensional turbulent effect.Such characteristic is even strong especially for the scale-span multiphase reacting issues with the motion and combustion of micron-scale metal particles.At present,experimental observation and measuring method are mainly employed in the related investigations.This paper focused on this complicated problem and a scale-span Coupled Multiphase Reacting Phenomena Solver(CMRPS)was developed for numerical simulation.Moreover,deep study was performed to investigate the influences of Al/Mg fuel-rich propellant combustion characteristics on the working performances of ramjet with advanced experimental techniques and on-ground directconnected ramjet experiment.The main work of this paper includes:(1)The 2D/3D coupled multiphase reacting solver CMRPS was developed with three independent modules for gas,solid and discrete phases.CMRPS has the ability of simulating the turbulent dense/dilute particle-laden flows with multiphase reacting and two-way coupling effects.Gas module was based on Finite Volume Method(FVM),considering turbulence,multiphase,chemical reaction kinetics and thermodynamics.The coupling approaches with solid and discrete modules were realized by the interphase source terms and unsteady iterative computation.Solid module was designed for simulating the temperature distribution inside composite propellant using the conjugated heat transfer algorithm based on the same heat flux and boundary temperature at burning surface.The method of studying the motion and burning processes of injected micron metal particles was determined by the real local volume fraction of discrete phases,including the Eulerian model based Two Fluid Model(TFM)for the dense particle-laden flow as well as the Lagrangian model based Discrete Element Model(DEM)for tracking the dilute particle trajectories.Finally,various classical benchmarks were adopted to validate and verify the reliability and accuracy in the numerical results by CMRPS.(2)The microscopic qualitative flame mode transformation of an isolated micron-scale magnesium particle was studied considering the forced-convection effects and the complicated environmental influences inside SRRJ.Thus,various factors were analyzed and fitted in defining the quantitative correlation formulas for the instantaneous burning rate and total burning time.On one hand,particle diameter and environmental parameters(pressure,temperature and oxygen concentration)can all affect the temperature distribution of combustion wave for an isolated magnesium particle.Meanwhile,the initial isotropic flame is strongly influenced by the forced-convection effects.With the increase of relative velocity,the flame mode varies from envelope to transition,then wake until the final flameout.Eventually,correlation formulas for the instantaneous burning rate and total burning time of an isolated magnesium particle were drawn using the least-square algorithm with 138 numerical results under different working conditions.(3)A laser ignition platform was built including high-speed camera,infrared thermometer,tungsten/rhenium micro-thermocouple and high frequency testing system for investigating the initial decomposition and combustion characteristics of Al/Mg fuel-rich propellant in the near burning surface area.The conjugated heat transfer between gas and solid domain was numerical investigated by CMRPS with Two Fluid Model and a simplified chemical reaction model.It was found that Vielle and Summerfield semi-empirical formulas can both well fit the burning rate versus pressure for this typical solid fuel.The simulated burning surface temperature was 1044 K for the working condition of 0.60 MPa in air,which had a relative error of 4.4% with experimental results.Moreover,flame structure and the distribution of main reacting components were analyzed under various working conditions with different pressures and oxygen concentrations.There existed the obvious “two-platform” phenomenon in the temperature distribution of combustion wave.It was influenced by the chemical reaction pathway and violent degrees of multiphase mixture,which comprehensively resulting in the high-temperature components diffusion and thermal feedback on the composite propellant in the near burning surface region.(4)A Eulerian-Lagrangian model based two-way coupling CFD-DEM algorithm was established to track the whole moving trajectory of aluminum particles.The macroscopical characteristics of the primary high temperature gas with injected micron aluminum particles through a cylindrical afterburner and JPL nozzle were studied.Then flow distribution and burning characteristics of the dilute gas-solid two-phase flow was statistically analyzed.It was found there existed obvious effects of velocity lag and temperature advance in the multiphase flow with burning aluminum particles comparing to the primary single-phase case,which has positive influences on increasing temperature and thrust.However,due to the interphase drag force,local gaseous velocity may be reduced in some partial regions.Furthermore,the effects of injecting area,initial temperature,diameter and massflow rate of aluminum particles were statistically calculated and quantitatively analyzed.Better particle dispersion meant higher gas-contacting chance and larger mass and heat transfer space,which can improve combustion efficiency.The higher the initial particle temperature and the smaller the diameter,the faster the ignition and the easier the combustion,which means more heat was released inside the length-limited flow field.Physical reactions of aluminum particles and burning products also had significant influences with combustion for the multiphase reacting problem.It needed special attention in the distribution and actual ratio for the local components as well as its mixing state.(5)The working characteristics of a full-scale SRRJ with Al/Mg fuel-rich propellant was studied by the on-ground direct-connect facility.The Ducted Rocket Engine(DRE)had dual air inlets and employed a variety of detection sensors to observe and measure the pressure field,temperature field,thrust and other working performance parameters for the air-fuel ratio of 15.Meanwhile,the ablation characteristics of thermal protective materials were analyzed.The high temperature zone inside the flow field existed symmetrically because of the dual inlet structure of DRE with strong turbulent effects.The induced ram-air resulted in recirculation zones at the upwind areas around the inlet-ports,while lead to strong collision at downwind for better mixing.Therefore,there existed higher local oxygen concentration and secondary combustion characteristics were enhanced.The motion and distribution phenomenon of discrete particles inside this DRE were successfully simulated by the two-way coupling CFD-DEM algorithm of CMRPS.The combustion efficiency of particles and more other details of this multiphase reacting coupled flow field were captured and analyzed.The ablation phenomenon was found in close relationship with high-temperature gas distribution and particle erosion including thermochemical ablation and mechanical erosion.