Research On Ablation Mechanism Of Insulator Under High Temperatrue Alumina Depositoin Condition

The combustion chamber of Solid Rocket Motor(SRM)is subjected to the action of high temperature and high pressure two-phase flow,where the internal insulation layer is the key to ensure the normal and reliable operation of SRM.However,the insulator is also a part of the negative mass of the engine.Therefore,the lightweight,anti-ablation insulation materials and advanced thermal protection design becomes the key technologies of SRM,and mastering these key technologies requires a profound understanding of the ablation mechanism of insulation material.In the future,in order to meet the needs of emergency space launching and seizing the commanding heights of space,high-thrust and high-performance SRM have attracted great attention of military powers.However,with the adoption of highly-aluminized propellant and the restriction on the overall length of the Solid Rocket Motor(SRM),the working environment in the combustion chamber becomes extremely harsh,especially,the high temperature alumina droplets are easy to deposit in the charge section and back wall of submerged nozzle,which will lead to the abnormal heat transfer and ablation of insulation materials.However,at present,there is still a lack of in-depth understanding and effective predictive methods for the insulator ablation mechanism.Therefore,it is of great academic significance and engineering value to study the ablation mechanism of insulator under slag deposition condition,explaining its heat transfer mechanism and establishing an accurate ablation model of thermal protection system for improving the performance and reliability of large SRMs.In this paper,a two-phase flow numerical model for predicting alumina deposition in SRM was established.Focused on the ground firing test results of a 2m-two-stage engine,numerical calculations were carried out under the droplet/wall impact models of section-area capture,Weber criteria and Sommerfeld criteria,respectively.By comparing the numerical results with the experimental deposition measurements,it is concluded that the properties of the experimental medium of Weber criteria model are quite different from high temperature alumina,which is not suitable for the prediction of slag deposition.The calculation results of the Sommerfeld criteria and the section-area capture model are close to the experimental results,however,the section-area capture model has certain experience,while the Sommerfeld criteria has better generality,so the Sommerfeld criteria is more recommended for SRM slag deposition calculation.The research results provide an effective method for predicting the slag deposition and boundary condition for ablation research as well.A multi-function deposition experimental engine was designed to simulate the deposition state of real SRM,measuring the thermal increment of alumina deposition and observing the alumina deposition phenomenon.The ground firing experiment was carried out and the macroscopic ablation characteristics of insulation material under deposition conditions were obtained.The observation of the deposition of high temperature alumina was realized,and the deposition state of alumina was summarized.The phenomenon of“alumina blowing”caused by the over flow of pyrolysis gas generated by insulation material under deposition condition was discovered for the first time.The temperature response characteristics of graphite calorimeter under deposition conditions were measured,which laid a foundation for further calculation of alumina heat increment and research on heat transfer characteristics.In order to calculate deposition heat flux of high temperature alumina,the concept and model of inverse heat conduction problem(IHCP)were described in detail.The uncertainty of the calculation solution of IHCP and its sensitivity to measurement error were discussed.Through the analysis of existing calculation methods,Beck's sequence function method was selected to compile and validate the heat flux calculation program.Based on the measured data,the parameters of heat increment and temperature of deposition interface were obtained by IHCP code.Combined with the experimental RTR image,the variation of temperature response in graphite calorimeter with the dynamic deposition process of alumina was analyzed.The heat transfer form dominated by alumina heat was determined under the deposition condition,and the heat transfer characteristics of deposition were expounded.In order to obtain the kinetic parameters of the alumina-charring layer reaction system and reveal the reaction mechanism,the main factors affecting the ablation of charring layer were analyzed.Experiments on high temperature thermochemical reaction of alumina and carbon powder with different mass ratios were carried out.The dominant reaction form of alumina and charring layer under deposition conditions was determined,and the dominant reaction equation was given.The kinetic parameters of the overall reaction of Al₂O₃-C system were given through thermodynamic analysis and kinetic experiments.It is deduced that the reaction of Al₂O₃-C system at high temperature is controlled by two-dimensional boundary phase reaction(R₂).Meanwhile,combined with the analysis of experimental differential thermal curves and the calibration of reaction components,the step-by-step reaction equation and kinetic parameters of the overall reaction process were further studied,and the reaction mechanism of Al₂O₃-C system was revealedFinally,on the basis of the original thermochemical ablation model of the previous work,the alumina heat increment and Al₂O₃-C system reaction sub-models were added,the deposition ablation model was established,and the deposition ablation program was compiled.The calculated ablation thickness of insulation material is close to the experimental results.Therefore,it is considered that the ablation model established in this paper can reflect the ablation process of insulation material under deposition conditions,which can provide theoretical guidance for thermal protection design of SRM.