Research Of Skin Friction Reduction Mechanisms And Applications Caused By Boundary Layer Combustion

With the increase of flight speed,the drag of hypersonic aircraft increases dramatically,and it becomes more and more difficult to obtain the net thrust of the propulsion system.For a hypersonic aircraft,the viscous drag accounts for more than 50%of total drag of the aircraft,and the skin friction accounts for more than 60%of the total drag of the engine.High skin friction has become a severe challenge for hypersonic propulsion systems.For air-breathing propulsion,especially for scramjet engine,how to reduce skin friction as much as possible and increase net thrust has become an urgent problem to be solved.Boundary layer combustion（BLC）is considered as an effective way to reduce skin friction,which has attracted widespread attention from scholars all over the world.In this paper,the mechanism of skin friction reduction caused by BLC and BLC applications in scramjet are carried out by theoretical analysis and numerical simulations.The thesis consist of the theoretical model and the mechanism of skin friction reduction caused by BLC,the calculation of skin friction coefficient,numerical simulation of turbulent BLC,the applications of BLC in scramjet components and the effects of BLC on scramjet propulsion performance.First,according to the classical gas dynamics theory,a theoretical model of hydrogen-fueled BLC is studied,including the distribution of aerodynamic parameters,skin friction reduction mechanism and wall heat flux variation,skin friction coefficient calculation and the influenced factors.The results show that the flame is the boundary in boundary layer,and the composition,density and temperature of the mixture gas all show the characteristics of layered distribution.BLC can greatly reduce the skin friction because the mixture gas viscosity and the wall normal gradient of stream-wise velocity are all reduced dramatically.The reduction of velocity gradient is the dominated factor of skin friction reduction.By introducing Prandtl mixing length,the formula of the compressible turbulent BLC skin friction coefficient is obtained and verified based on Van Driest theory.The influences of hydrogen jet velocity,wall temperature and main-stream temperature on the skin friction coefficient are also explored.The results show that the influence of hydrogen jet velocity on skin friction is about 5%,and that of wall temperature does not exceed 10%.The influence of main-stream temperature can be ignored in engineering applications.Then,the theory of hydrogen-fueled BLC is extended to hydrocarbon-fueled BLC.Due to the different physical properties of hydrogen and hydrocarbon fuel,the magnitudes of skin friction reduction are different.The skin friction coefficient with hydrogen is only 57.35%of that with hydrocarbon fuel.Secondly,numerical studies are conducted by turbulent flat-plate BLC simulations.Although BLC generates an additional heat source near wall,the presence of hydrogen prevents the wall temperature gradient from sharply rising but sharply falling.More than 50% skin friction reduction can be obtained by BLC,and the magnitude of reduction can exceed 60% if the wall hydrogen mass fraction could maintain a high level.Density decrease caused by the heat release can weaken the turbulent transport capacity in boundary layer,but it cannot suppress the level of turbulence fluctuation.Due to changes of aerodynamic parameters,Van Driest wall velocity law is no longer suitable to describe velocity distribution of BLC,while the modified White&Chiristoph wall velocity law is accurate in turbulent boundary layer both with and without combustion.The influences of relevant parameters on skin friction and wall heat flux with BLC are also researched.It is found that there are no significant differences between the skin friction coefficient and wall heat flux obtained by different reaction mechanisms.Although high turbulence intensity leads further combustion in boundary layer,the skin friction reduction benefits cannot offset the increase in skin friction caused by the high turbulence intensity.Fuel jet conditions can also affect the level of skin friction,high jet height and low velocity is helpful to obtain lower skin friction.In addition,the effects of pressure gradient on skin friction reduction caused by BLC are studied,and it is found that further reduction can be achieved with adverse pressure gradient while little with favorable pressure gradient.Thirdly,BLC application researches are conducted in typical components of scramjet engine,such as strut combustion chamber and inlet-combustor.BLC cannot change the flow field structure in combustion chamber.However,BLC can effectively improve the combustion efficiency,and reduce skin friction of combustion chamber.The skin friction of φ_B,inj=5.90% is only 58.5% of that without BLC.As hydrogen equivalent ratio increases,further skin friction reduction can be obtained,but the reduction efficiency may decrease.Moreover,BLC can decrease wall heat flux,but it is achieved when hydrogen mass fraction of the wall is maintained at a high level.In addition,adiabatic wall are easier to obtain lower friction than isothermal wall.At the front of inlet-combustor,hydrogen fuel injection can improve combustion efficiency and reduce skin friction,without causing unacceptable total pressure loss.When the hydrogen equivalent ratio reaches 0.2,skin friction reduction can be achieved by 22.31%,which is equivalent to 9.83%reduction in total drag.Meanwhile,hydrogen injection also reduces the wall temperature gradient,it is helpful to relieve the thermal load of the wall.Finally,the effects of BLC on the propulsion performance of scramjet are explored.BLC is adopted in inlet and combustor of Hyshot scramjet engine,respectively.It is found that there is little influence on flow structure of scramjet inner path.Larger injected angle and higher injected pressure ratio in inlet is beneficial to obtain a lower skin friction.Inlet fuel injection can reduce skin friction by about 21.61%,and increase the outlet impulse by about 3.73%.Moreover,combustor boundary layer injection can obtain 20.79%skin friction reduction and increase the outlet impulse by 3.52%.Similar researches are conducted on the original-size Busemann scramjet.Inlet injection can reduce skin friction of the engine by 24.21%,and the outlet and impulse can be increased by 2.90%;while the reduction magnitude of the combustor boundary layer injection is 27.20%and the outlet impulse promotion is 2.89%.Combined the inlet injection and combustor boundary layer injection,the outlet impulse can be promoted by 5.4% at the condition of φ_I,inj=0.15 and φ_C,inj=0.15.In summary,BLC can greatly reduce skin friction and increase total impulse of the engine,so it can be widely used in scramjet.This paper has explained the mechanisms of skin friction reduction,conducted numerical simulations of BLC with flat-plate turbulent boundary layer,scramjet components and the whole engine,and obtained abundant and detailed flow-field data and aerodynamic performance data.While laying the foundation for subsequent studies of BLC,it also blazes a trail in the field in improving propulsion performance of hypersonic engine.