Study On Ablation Characteristics Of Dual Pulse Motor Soft Compartment Under Thermal Loadings

The dual pulse motor(DPM)solves the inherent problems of solid rocket engine that the thrust can not be adjusted and interrupted.DPM adopts the pulse isolation device with the functions of flame-retardant and heat insulation,which divides the combustion chamber into two combustion chambers with independent ignition systems.The ignition interval is controlled and accurately distributed.It has the advantages such as excellent maneuverability,long effective range,high speed at the end of flight,and good stealth effect.In this dissertation,the application of axial-diameter hybrid DPM is the research background.The ablation characteristics of the soft compartment wrapping a Ⅱ pulse grain under thermomechanical loadings were studied.The experimental analysis and numerical simulation are combined to establish a more accurate thermal insulation predictive model of material during heat transfer and ablation process to deeply study of the pyrolysis and ablation process of EPDM soft compartment material under Ⅰpulse working environment,which contribute a lot for the design of thermal protection structure of DPM.The main contents of this dissertation are as follows:(1)The internal flow field of the two-phase flow of the axial-diameter hybrid DPM duringⅠ pulse operation was simulated and the parameters of the flow field and the working environment of the soft compartment are obtained.The results reveal that the particle phase has a large thermal inertia,a small temperature drop,and a temperature and velocity lag between the gas-solid phase,resulting in the nozzle axis temperature being higher than other locations on the same section,while Mach number is lower.The particles follow the flow very well,and are carried by the airflow towards the nozzle,without entering the upstream of the combustion chamber.Part of the particles collided with the convergence section and bounced out.As the burning surface moves backward,the farther the particle’s starting point is from the axis,the closer the collision position is to the upper end of the convergence section,and the trajectory becomes more concentrated.During the whole process,few particles moved upstream,and there was no significant ablation and erosion effect on the soft compartment.(2)Experimental studies on the thermal weight loss and laser ablation carbonization were used to study the EPDM soft barrier material.By analyzing the thermal weight loss data,the pyrolysis kinetic parameters of the soft barrier material were obtained.By observing the laser ablation and carbonization process and analyzing the morphology before and after the ablation carbonization,the changes in the structure and element content of the material were obtained.Thermal weightlessness and mass loss are divided into three stages: the sample dehydration period;the main thermal weightlessness interval,with rapid loss rate,and large mass loss;the pyrolysis ends.After laser ablation and carbonization,the three-layer ablation structure of the original material layer,the pyrolysis layer and the carbonization layer was observed.The carbonized layer has a fully pyrolyzed and carbonized matrix,and randomly arranged cavities,which confirms the correctness of the three-layer pyrolysis layer model.(3)In view of the heat transfer and ablation process of pyrolytic carbonized thermal insulation model materials,a mathematical model of the specific heat capacity and thermal conductivity of the materials with time and temperature is proposed.The variable thermal properties of pyrolysis gas,material density model,material specific heat capacity and thermal conductivity model are established.A finite volume method based on the lattice center is adopted to discretize the governing equations.The temperature partial derivative is calculated by the Jacobian transformation.The time advancement is performed by the third-order Runge-Kutta method.Compared with the results of classical ablation experiments and the calculation results of normal thermal properties,it shows that the prediction of the pyrolysis process of EPDM insulation materials has higher accuracy,which fully verifies the reliability and credibility of the proposed model.(4)Combined with the simulated Ⅰ pulse working two-phase flow field characteristic parameters and the established variable thermal properties model,the energy diffusion process,pyrolysis carbonization process and thermal weight loss process of aramid / EPDM insulation material under a given heat flow were studied.The internal temperature,density distribution,thermal conductivity,specific heat capacity change history,thickness-time curve of pyrolysis layer and carbonized layer,and other thermal response characteristic parameters were obtained.In the initial stage of heat flow,the upper surface of the material heats up rapidly,and the energy continuously diffuses inward.Due to the heat of thermal desorption,the escape of the pyrolysis gas and the energy of radiation,the diffusion rate decreases and the temperature advancement rate decreases.The heat flow continues,and the thickness of the carbonized layer increases rapidly at the early stage,and then decreases almost linearly.The effect of temperature on the pyrolysis reaction rate changes exponentially,directly leading to the extremely fast pyrolysis reaction rate near the upper surface,and the slower the distance from the upper surface,and the trend is more obvious,the local pyrolysis gas generation rate decreases with distance rapidly.