The Modulation Of Energy Transport In The Hypersonic Gas-Solid Interface And Its Application In Thermal Protection System

Severe aerodynamic heating has become a bottleneck for developing a hypersonic flying vehicle in troposphere with long navigation time and high reliability.Reducing aerodynamic heating or designing a reliable thermal protection system is the key technology of realizing long-time navigation for a hypersonic flying vehicle.In order to propose efficient solutions to the problems of aerodynamic heating and thermal protection,molecular dynamic(MD)simulation and molecular collision theory are used to study energy transport mechanism in a hypersonic gas-solid interface.In experiments,thermal conductivity for graphite films with different thickness is measured.The following research results have been obtained.A MD model of a nanoscale cuboid flying in an argon environment is established to mimic a hypersonic vehicle flying in troposphere with different flying speeds.The simulated temperature rise of the flying cuboid from the MD model agrees well with the theoretically predicted value based on the molecular collision theory,which verifies the accuracy of the MD model.Based on the MD model,the stagnation-point heat flux can be calculated by a statistical formula,and the change of the heat flux on a constant-temperature cuboid with its cross-section size and surface geometry is discussed.It is demonstrated that the peak amplitude of the hypersonic flow in the velocity distribution function of the gas atoms at the stagnation point increases with the decrease of the cuboid size,which means that more kinetic energy is carried by the gas flow,leading to a larger stagnation-point heat flux.Therefore,decreasing the peak amplitude of the hypersonic flow can effectively reduce the stagnation-point heat flux.Our MD simulation results confirm that the heat flux can be decreased 16.8%for the spherical cavity surface geometry compared with the flat surface due to the reduced hypersonic peak.In practice,the velocity distribution function at the stagnation point is dominated by the interactions between the gas atoms and the solid surface.A method of studying the atomic collisions between the gas atoms and surface in a hypersonic flight vechicle is proposed to reveal the energy transport mechanism in the gas-solid interface,in which the stagnation-point heat flux and energy accommodation coefficient(EAC)are discussed systematically.It is found that the EAC has a significant size effect,and the EAC decreases with the increasing size until it converges at the size larger than 4?4nm~2.Moreover,results of energy transport in the interface under different atmospheric pressures illustrate that the stagnation-point heat flux increases logarithmically with the atmospheric pressure,while the EAC hardly changes with the pressure.EAC at different flying speeds is also investigated,and it is demonstrated that the average interaction time of gas atoms colliding with the surface decays exponentially and the average interaction depth increases linearly when the flying speed increases,which induces that the EAC first declines and then slowly increases with the flying speeds.Based on the theory of energy transport in the gas-solid interface,the modulation of energy transport is explored by the methods of changing the relative atomic mass,cuboid stiffness,and LJ potential of atomic interactions.It is illustrated that the phonon spectrum of solid atoms shifts to the higher frequency with the decrease of atomic mass and the increase of cuboid stiffness,which leads to the spectrum deviating away from the corresponding spectrum of gas atoms and noticeably reduces the stagnation-point heat flux across the gas-solid interface.For example,the heat flux can be decreased by14.19%after reducing the solid atomic mass by half or can be reduced by 25%via doubling the cuboid stiffness.Also,the energy transport in the interface can be dropped a lot by increasing the average reflected kinetic energy of gas atoms,which is accomplished through decreasing the depth of potential well or increasing the equilibrium distance in the LJ potential.Analogously,decreasing the depth of potential well by three quarters can reduce the heat flux by 20%,and an increase in the equilibrium distance by 0.5 times can reduce it by 31.47%.In view of the wide applications of graphite-based composite in the thermal protection system of a flight vehicle,the time-domain thermoreflectance(TDTR)technique is utilized to measure the thermal conductivity for pristine graphite films and graphite films treated with focusing ion beam(FIB)or oxygen plasma.The cross-plane thermal conductivity of the pristine graphite and the graphite treated with oxygen plasma increases with the increase of the thickness of graphite films,which agrees well with the results reported in the previous literatures.Compared with the pristine graphite films,the cross-plane thermal conductivity of the FIB treated graphite goes down to a value of about 0.45 W/m K with different thickness.Moreover,the stagnation-point heat flux between a graphite cuboid and the gas atoms is increased by more than 30%compared with the silicon cuboid due to the low equivalent stiffness between graphite layers.