Research On Combustion Processes And Combustion Optimization In Magnesium-Based Water Ramjet Engines

Investigations on water ramjets are being extensively conducted for their application inthe cruise propulsion system of high-speed torpedoes currently. Based on amagnesium-based water ramjet engine, this research focuses on combustion mechanismof a magnesium particle in steam, internal combustion processes and combustionoptimizationoftheengine.Approachesoftheoreticalanalysis,numericalsimulationandexperimentalstudywereappliedfortheresearch.A high speed camera and an infrared thermometer were employed to observe thecharacteristics of ignition and combustion of a magnesium particle in steam.Mechanisms of ignition and combustion were determined in the experimental study.The results show that, a transition from surface oxidation to gas-phase combustion tookplace; a different phenomenon from combustion in air occurred, with white flocculesformedabovethe flame, andtheproduct ofmagnesium oxidealso inthe shapeof whitefloccules; the interaction was controlled bychemical kinetics prior to the ignition point,and then by diffusion process afterwards; ignition temperatures were measured withvaluesbetween1150~1200K.A high speed camera was also used to observe the variation of combustion life withdifferent diameters of magnesium particles in steam. Prolonged combustion life wasrecorded with an increasing particle diameter, and the variation rule got closer to theD~2law while the diameter was decreasing. Based on the droplet combustion model, amodified D~2law was established, taking into account the oxide cap covering theparticlesurface.Fromtheexperimentaldata,the coveringcoefficientwas determinedofthemodified D~2law.The magnesium-droplet ignition and combustion model was developed, in order todescribe the transformation of magnesium particles inside water ramjet engines. Inthismodel, the whole procedure is divided into three phases: oxidation phase, ignition andcombustion phase. An oxidation kinetics model is adopted to calculate the thickness ofthe oxide layer in oxidation phase; while in combustion phase, the particle diameter isgovernedbythemodified D~2law.Adirect-connecttestingmethodforwaterramjetengineswasdeveloped.Withwater-jetentering the chamber vertically, the influence of its impulse to axial thrust waseliminated. With a comparison of different data processing ways, a processing methodin terms ofthe stable operation period was confirmed to be more eligible. In addition,the numerical method was improved by employing the magnesium-droplet ignition andcombustionmodel,andthemethodwasverifiedbyasimulationofonetest.Influences of ignition energy, starting sequence, water/fuel ratio, water injection pressure and metal content on the ignition process were studied experimentally, fromwhich the characteristics and mechanism of ignition process were determined. Thewhole ignition process was found to follow four phases: ignition time lag,self-surporting combustion interval, primary precombustion interval and secondaryprecombustion interval. Within the self-surporting combustion interval, the ignition iscontrolled bysolid-phase ignition theory, while forprimaryprecombustion interval andsecondaryprecombustioninterval,thegas-phaseignitiontheorydominatestheignition.Characteristics of combustion processes inside the water ramjet engines were analyzedwith different approaches. A multi-zone combustion model was developed to describethe internal processes.Influences of water injection distance, injection angle, water/fuelration and its partition, and atomization characteristics were estimated on thecombustion processes, from which combustion optimization methods were concluded.A magnesium-based water ramjet engine was designed using the optimization methods,and a testof it was conducted with a Type 73 solid propellant, which has a high metalcontent. The results show that the combustion efficiencyof the engine was enhanced to91.02%aftercombustionoptimization.