Study On The Spectral Characteristics Of Collision Between Gas Molecules And Ceramic Target In Hypersonic Flow

Aerospace aircraft,high-speed missiles,spacecraft and other types of aircraft all fly at hypersonic speeds during re-entry into the atmosphere.These aircrafts collide with the molecules in the atmospheric,generating friction and ionization,producing plasma.The surface of the aircraft is thus wrapped with a certain thickness of"plasma sheath".This process leads to a disruption to the communication of the aircraft,causing a"blackout"phenomenon.Meanwhile,high-speed friction makes the temperature at the stagnation point of the aircraft shell reach several thousand Kelvin.This process is easy to cause the structural strength of the body material to be weakened and the shape to be damaged or even disintegrated.This is the phenomenon of"thermal barrier".At present,there are still unsurmountable obstacles in the"blackout"and"thermal barrier"during the operation of hypersonic vehicles,which seriously threatens the safety of aircrafts and astronauts.The electron density,electron temperature,and electron oscillation frequency of the"plasma sheath",as well as the temperature and emissivity of the surface of the vehicle,are important basic parameters for solving the problems of"blackout"and"thermal barrier".However,the complex environment in the reentry process brings great difficulties in direct measurements of these parameters.The spectral method has the advantages of rich information,fast response speed,high sensitivity and remote detection,and it can be used to obtain target parameters in complex environments.In the thesis,spectral detection and analysis techniques were used to study the spectral characteristics of collisions between gas molecules and ceramic targets in hypersonic airflow.A high-precision small-field spectrum detection system was designed and built to measure the spectra of the hypersonic airflow in the wind tunnel in the whole process of interaction with the ceramic target.The electron temperature,electron density,and electron oscillation frequency in the plasma were calculated based on the separated plasma spectrum.Based on the thermal radiation spectrum,the change law of temperature and spectral emissivity during the interaction of the ceramic target and the hypersonic airflow is inverted.At present,the plasma generated during hypersonic flight is usually described by the equilibrium high-temperature plasma statistical model Saha equation.However,in the actual reentry process,equilibrium and non-equilibrium exist at the same time,and the plasma generation mechanism is more complicated.One part comes from high temperature ionization,and the other part comes from collision ionization between aircraft and gas molecules.Therefore,the sole use of the Saha equation is not sufficient to comprehensively describe the process of hypersonic flight.In the thesis,the kinetic energy and potential energy of the molecules in the equivalent hypersonic airflow and the interaction potential between the molecules and the ceramic target surface are considered.The Schr（?）dinger equation is established to describe the energy transfer process between the gas molecules and the ceramic target.At the same time,the selection rules are considered according to the different energy level structures of nitrogen atoms and the silicon atoms.The transition wavelengths of some atomic spectral lines are given,which provides a reference for subsequent spectral identification.According to the actual requirements of the"blackout"and"thermal barrier"spectral detection in the hypersonic re-entry process,a small field-of-view and wide-band spectral detection system was constructed.For the first time,the radiation spectrum of the whole process of hypersonic airflow and high temperature resistant quartz composite ceramics in the wind tunnel has been obtained.The hypersonic airflow velocities are 5 Ma,7 Ma,and 10Ma,respectively,and the spectral detection band is 0.2μm～15μm.The analysis results of collision spectrum characteristics show that:in the visible band,the spectrum is composed of the linear characteristic spectrum superimposed on the continuous thermal radiation spectrum.In regard to the near-infrared and mid-infrared bands,the spectrum is mainly reflected in the continuous thermal radiation spectrum characteristics.The measured spectral information can be used for the inversion of plasma parameters and material thermal parameters.In terms of a plasma spectroscopy analysis,based on the NIST spectral database,the spectrum of the incoming flow in the wind tunnel and the interaction spectrum between the incoming flow and the ceramic target at the stationary point were analyzed and identified.The results show that（1）The incoming flow plasma spectrum contains only the emission lines of air elements,mainly including nitrogen and oxygen atoms.In addition to the characteristic emission of nitrogen and oxygen atoms in the collision spectrum,characteristic spectral lines of silicon are also detected,indicating that the target material atoms have been excited and ionized.（2）According to the Boltzmann oblique line method,three common spectral lines of N I 746.8 nm,N I 821.6 nm and N I 904.5 nm were selected for the plasma spectrum generated by the collision of hypersonic airflow（5 Ma）with two targets（quartz composite ceramics and silicon carbide ceramics）.Calculate the electron temperature of the collision between the airflow and the two target materials（quartz composite ceramic 0 ms～400 ms,silicon carbide ceramic 0 s～980 s）.For the quartz composite ceramic target,the minimum value of the electron temperature is 11264 K which appears at 160 ms and the maximum is 19323 K which appears at 320 ms.For the silicon carbide ceramic target,the electron temperature reaches the minimum value of 13289 K at 100 s and the maximum value of 15282 K at 700 s.It can be seen that,compared with the quartz composite ceramic,the electron temperature range of the silicon carbide ceramic target is about 6000 K smaller,which is relatively stable.（3）According to the Stark broadening method,the electron density of the plasma generated by the collision of the hypersonic airflow（5 Ma）with the two targets at the stagnation point is calculated.When the target is a quartz composite ceramic,the Si I577.2 nm from the target and the OI 777.2 nm from the incoming flow in the spectrum were selected to calculate the electron density.The electron density at the stagnation point is1.31×10¹⁷ cm^-3～4.19×10¹⁷ cm^-3 and 4.26×10¹⁷ cm^-3～4.99×10¹⁷ cm^-3 respectively.When silicon carbide ceramic is used as the target material,based on the Si I 794.2 nm and OI 777.2nm spectral lines,the calculated electron densities are 4.21×10¹⁶ cm^-3～5.97×10¹⁶ cm^-3and1.66×10¹⁷ cm^-3～1.81×10¹⁷ cm^-3.（4）According to the Langmuir oscillation,the electron oscillation frequency at the stagnation point of two thermal protection materials is given.For the thermal radiation spectrum,firstly,according to Planck’s law,the thermal radiation spectra of the interaction between the hypersonic airflow and the two targets in the wind tunnel were fitted respectively,The temperatures of the boundary layer and the target surface are given based on the similar wavelength method.It is found that when a high-speed airflow with a velocity of 5 Ma acts on a quartz composite ceramic target,the temperature of the boundary layer is 7361 K in the initial stage,and the temperature is 9881 K when the target is silicon carbide ceramic.Secondly,according to the actual application environment and reentry experience of the two target materials,the target surface temperature of the two target materials at different time stages is calculated respectively.For the quartz composite ceramic target,five different airflow velocities of 5 Ma,7 Ma and 10 Ma.In the ascending stage and 7 Ma,5 Ma in the descending stage are set,and each airflow velocity lasts for 10s.Based on the spectral calculation of these five stages,the airflow velocity is ascending,the target surface temperature is 2063 K at 5 Ma,the target surface temperature is 2362 K at 7Ma,the target surface temperature reaches 2821 K at 10 Ma;in the descending section of air velocity the target surface temperature is 2472 K at 7 Ma,and the target surface temperature at 5 Ma is 2033 K.When the target material is silicon carbide ceramic,continuously collide with ceramic target using 5 Ma hypersonic airflow and the whole process.Spectra of four moments were selected at equal intervals of 200 s starting from 300 s,and the calculated temperature of the target surface fluctuated little,which was about 1284 K±5 K.At the same time,the spectral emissivity of visible,near-infrared and mid-infrared bands is given by the similar wavelength algorithm based on the slow variation characteristic of the emissivity,and the uncertainty and relative error of the emissivity measurement are calculated.This research provides a new technical means for obtaining the parameters of"blackout"and"thermal barrier"in the extremely complex environment of the hypersonic target reentry process.It has a certain reference significance to the solution of"blackout"and"thermal barrier".