Mechanism Of Thermal-mechanical Shock And Investigation Of Protection For Laser Propulsion

Laser propulsion, which works on the counterforce generated during the interaction between high-power laser and propellant, is a new concept technique for propulsion. The plasma temperature of working fluid caused by laser can reach above 10000K which is much higher than the working temperature of tranditional rockets and the melting point of all materials, so, the resistance of thermal-mechanical shock and damage will be the great problem for reseach of laser propulsion.The problem of thermal-mechanical shock and damage for laser propulsion involve: light radiation, interaction between high-temperature plasma flow fluid and structure of the thruster, thermal-mechanical coupling, constitutive model and failure criterion, transient pulse effect and long-time accumulation etc. The breakthrough of this problem can provide basis for the design and technical tackling of the resistance of thermal-mechanical shock, which have important value.This dissertation is to study mechanism of thermal-mechanical shock and investigation of protection for laser propulsion: (l)research the evolution law of the unsteady-laser plasma flow fluid, analyse the factor for propulsive performance, provide the thermal-mechanical loads for the mechanism research of thermal-mechanical shock;(2) study the problem of thermal-mechanical shock and damage through themechanism analysis, experimental study and numerical simulation;(3) Develop the preliminary study for protection. Obtains the main result and the new understanding include:Established the numerical simulation platform of propulsion performance and the structure dynamic response, this platform may use in the propulsion performance computation and thermal-mechanical shock and damage computation.Numerical simulation for plasma flow fluid indicates that there are three stages: the main propell stage, anti-thrust stage and breathing- oscillation stage.Discussed the effects on propulsion performance: (1) the C_m (momentum coupling coefficien) increases along with the energy increases ; (2)the short pulse width is advantageous in improves propulsion performance;(3)the C_m increases along with the pulse number reduces, the C_m increases along with the frequency reduces. There are four interface-thermal loads in the wall of laser thruster :( 1) the wall-absorption of incident laser;( 2) the wall-absorption of thermal radiation caused by the high-temperature plasma;(3) the wall-absorption of transmission of laser which transmitted the plasma region;(4) convection heat transfer between the wall and the high-temperature gas.Numerical simulation for thermal-mechanical shock according to self-focusing model and annular-focusing model considering both single-pulse and multi-pulses was carried out to analyse the main controlling factors. The results indicate: (1) the main controlling factor of the temperature-rise is absorption of incident laser for self-focusing model, and the next is thermal radiation, the prediction of thermal-mechanical damage fits the experiment well; (2) the main controlling factor of the temperature-rise is absorption of transmission of laser for annular-focusing model, and the next is thermal radiation; convection heat transfer between the wall and the high-temperature gas can be ignored. The simulation of 100 pulses indicates that in the 65^th pulse, partial unit occure tension failure because of the material strength reduces, and in the 91^th pulse, partial unit reach the threshold of thermal damage. The mechanical damage appears earlier than thermal damage, the thruster will damage with the fracture and disintegration, partial region melt seriously, the thermal and mechanical damage are coupling, the computation explains the experiment's phenomena well.It's the first time to have clear understanding of the main factor of thermal-mechanical shock and damage, damage process,the rule and the way.Design the parabola thruster considering self-focusing and external-focusing, the thermal radiation can be ignored through designs, then, the main temperature rise of the two models can be control by the single factor: absorption of incident laser for self-focusing model and the absorption of transmission of laser for external-focusing model.Research for thermal-protection method is developed for the two models: (1) establishes the estimation method for average temperature rise and Compute the balance temperature: 1700K for self-focusing model and 2800K for external-focusing model;(2) propose the protection plans for the two model: Mass and heat transfer thermal protection method for self-focusing model; coupled the Thermal sinking method and the Mass and heat transfer thermal protection method for external-focusing model. The plan is feasible through simulation. Propose the protection plans for mechanical protection: using the high temperature structural material. The plan is feasible through simulation.