Mechanism Analysis Of Magnetohydrodynamic Heat Shield System Including High Temperature Real Gas Effect

The design of thermal protection systems is of great importance to hypersonic vehicles because of the severe aerothermal heating incident upon their leading edges.During hypersonic flight,the weakly-ionized plasma layer post shock can be utilized for flow control by externally applying a magnetic field.The induced Lorentz force decelerates the flow in the shock layer,and consequently the shock layer thickness is increased and the convective heat flux can be mitigated.This so-called Maneto Hydro Dynamics Heat Shield System(MHDHSS)is a novel application of MHD flow control in the thermal protection field.This system has so many advantages that it is highly reliable,reusable,easy to be switched on-off while needed and does no harm to the aerodynamic configuration of the aircraft.All of these advantages have warranted popular interest in this subject area in recent years.Focusing on the MHDHSS,combined methods of theoretical analysis,numerical simulation and ground experiment are utilized in this paper.In order to explore the interaction mechanisms between the high temperature flow field and the electromagnetic field,physical models and CFD methods of the hypersonic thermochemical nonequilibrium flow field coupled with electromagnetic field are developed.By applying the numerical codes written in this paper,hypersonic flow structures,flow principles as well as the thermal protection mechnisms after coupling the electromagnetic field are detailedly analyzed and clearly revealed.Based these researches,theoretical experiment of MHD thermal protection is conducted,and experimental techniques are developed during wind tunnel tests on the high temperature real gas condition coupled with electromagnetic field,which provide theoretical and experimental support for further mechanism analysis of MHDHSS.First,a normal columned solenoid-based MHD thermal protection system model is built and fast calculation method of solenoid magnetic field is derived.By applying the low magneto-Reynolds MHD model,a series of axisymmetric simulations over the flow field of Orbital Reentry Experimental(OREX)Capsule are performed to analyze the influence of magnetic induction strengths,magnetic field types(dipole,solenoid,uniform)and geometric parameters of solenoid magnet of externally applied magnetic fields on the efficiency of MHD thermal protection.Then,the engineering feasibility of solenoid magnetic MHDHSS is analyzed and the feasible range of magnetic induction intensity is obtained to meet the limit of coil current density.After analyzing the defects of normal columned solenoid-based MHDHSS,a novel MHDHSS based on shape-following solenoid is proposed.Results indicate that,saturation effect exists in the process of MHD heat flux mitigation but not in the MHD shock control.There exists a best distance between the solenoid center and the stagnation point in the axial installation of normal solenoid-based MHDHSS,which cannot be too far to meet the limit of coil current density and too close to worsen the aerothermal environment in the shoulder area.Compared with the normal columned solenoid magnet,the shape-following solenoid-based MHDHSS works better under the same exciting coil current density with a merely one-sixth long coil.A parallel computation model is constructed for solving the thermochemical nonequilibrium flow on multiblock structured mesh.Three significant aspects are discussed which may affect the simulated thermochemical nonequilibrium heat flux,namely grid density,wall temperature and surface catalytic condition.By this research,a suggested grid convergent criterion for the nonequilibrium heat flux calculation is provided.Based on the defect analysis of the low-temperature and high-termperature surfact catalytic models,an eight-reaction Combined Surface Catalytic(CSC)model is proposed to describe the catalysis between O gas and silica surface.Then,the catalytic mechanisms are analyzed and the model uncertainty caused by the variation of parameter magnitude is discussed.Results indicate that,with the increase of the wall temperature,the dominant reaction contributing to catalytic coefficient varies from LH recombination to ER replacement,then to O2 desorption.The characteristics of the TPS material surface,namely the concentration and radii of active sites,strongly affect the magnitude of recombination coefficient.Detailed analysis of the silica-based TPS material surface characteristics should be made to properly valuate the model parameters.Numerical calculation model is constructed for solving thermochemical nonequilibrium flow coupled with electromagnetic fields based on the low magneto-Reynolds assumption.By comparing the simulated results based on perfect gas and thermochemical nonequilibrium models,the real gas effect on the thermal protection performance of MHDHSS is analyzed.The influences of electric conductivity model,the vibrational partition from Joule heating and wall catalytic condition are also investigated.After that,the effective working condition during the hypersonic reentry flight of OREX capsule is obtained and the performance enhancement is evaluated by seeding the inflow with potassium particles.In order to cover the shortage of dipole magnetic field in the MHDHSS,the concept of MHDHSS based on multipolar magnetic field with central and peripheral solenoids is proposed.Results indicate that,after taking the real gas effect into account,there still exists a saturation effect and the influence of magnetic field distribution on the MHD flow control performance remains the same,but the magnetic induction strenghth is about 4 times larger than the perfect gas model to provide the same MHD flow control performance.As for the OREX capsule,the MHD heat flux mitigation is not obvious until the magnetic interaction parameter is above 1.0.The performance of the system can be significantly improved by seeding the inflow with potassium particles,especially under the cases of low flight speed,but there also exists a saturation effect in the seeding amount,and an adequate seeding mass fraction is suggested to be 0.01.Compared with the dipole-magnet system,the five-magnet system,whose central polar orientation is the same with the peripheral ones,have stronger work capability and better shock control and thermal protection performance.A physical model is constructed for revealing the thermal protection mechanism of MHDHSS.By analyzing the effect of the Lorentz force components in the counter and normal directions,the dominating Lorentz force components are found for the MHD heat flux mitigation in different regions of OREX.Then,a novel magnetic field with variable included angle between magnetic induction line and streamline is designed.Results indicate that the thermal protection and shock control mechanisms are not the same.The MHD thermal protection is mainly determined by the Lorentz force’s effect on the boundary layer.From the stagnation to the shoulder region,the flow deceleration effect of the counter-flow component is weakened while the flow deflection effect of the normal component is enhanced.However,the MHD shock control depends mainly on the counter-flow Lorentz force right after shock.Nevertheless,once a good Lorentz force’s effect on the boundary layer is guaranteed,the thermal protection performance can be further improved by strengthening the counter-flow Lorentz force right after shock.In order to analyze the Influence of Hall effect on the performance of MHDHSS,numerical methods are constructed to solve the Hall electric field equations in the thermochemical nonequilibrium flow field with Approximate Factor(AF)and Alternating Direction Implicit(ADI)method.Relations between the convergence property and the virtual stepping factor are revealed by theoretical analysis and numerical simulations,and then a local variable stepping factor method is proposed to accelerate the iterating process.Then,coupling numerical model are constructed and validated to solve the thermochemical nonequilibrium flow field and the electro-magnetic field.Based on the nonequilibrium model and constant model of Hall parameter,numerical simulations of the MHDHSS are conducted under different magnetic induction strengths and different wall conductivity.Results indicate that,there exists a best stepping factor for particular mesh as well as a particular Hall parameter,and the local stepping factor method yields better convergence property than the regular constant one while employing locally refined mesh.Wall conductivity has remarkable influence on the performance of MHD heat shield system.Considering that this system becomes invalid under the conductive wall condition,the surface material of the hypersonic vehicles equipped with such system is suggested to be highly insulating.However,under the high magnetic induction strength,the performance of such system still has a big discount even if the insulating material is employed after taking the Hall effect into account.Theoretical experiment of MHD thermal protection is conducted in the plasma wind tunnel of CARDC.After a series of preparation work,namely a elaborate survey,numerical simulations of the flow field within the wind tunnel as well as the nonequilibrium flow around the test article,fluid-solid coupling simulations of thermal field,simulations of magnetic field and a prior thermal test of high temperature ceramic shell,the flow condition of wind tunnel,the amount of seeding particles,the choice of shell material,the magnet scheme,water cooling scheme as well as the thermal insulator scheme are confirmed,and then the whole experiment design is finished.Experimental results shows that the stagnation and shoulder wall temperature is reduced by 90 K and 252 K respectively after applying the magnetic field,and the cold wall temperature is reduced by 100 K,which qualitively proves the feasibility of heat flux mitigation by applying the MHDHSS.