[冲压发动机进气道]The effects of intake modification on a Ramjet engine

Under consideration for publication in J.Fluid Mech.1The effects of intake modification on aRamjet engineBy P.N.Beuchat†Department of Mechanical Engineering,University of Melbourne,VIC,3010,Australia(Received26November2007)A simple modification to the intake nose cone of a can-type ramjet engine was investi-gated.The original design was suspected to be sub-optimal and a biconical modification was proposed.The analysis tools indicated that the modified design provides a10% performance improvement to pressure recovery and massflow rate into the combustion chamber.The analysis performed here required crucial assumptions of theflow and shock systems involved that require either experimental validation,or a more complex theoretical model.The conclusion is that a biconical design,if implemented,will provide improved performance,and that investigation should be continued by determining the optimum deflection angle combination and then performing experimental validation.1.IntroductionThe major motivation for the development of ramjet engines is the desire to build and test advanced aircraft which can travel exceedingly faster.Ramjets are integral to achieving this goal because of where their operating conditions lie relative to other propulsion methods.They are able to provide effective thrust over a wideflight Mach range,which takes over from turbojet power at the low Mach range(M≈2)and leads into the supersonic combustion ramjet(scramjet) at high Mach range(M≈6)(Fry(2004)).The intake design of a ramjet is particulary crucial as it fully determines the inlet conditions to the combustion chamber.The major requirements on the intake are to produce a high pressure environment relative to the local atmospheric without introducing significant turbulence or distortions into theflowfield.Both†Student No217014:pbeuchat43@2P.N.Beuchatof these contribute to a more efficient combustion process allowing more thrust to de developed by the engine.The feasibility of ramjets is usually demonstrated by comparing with rocket power under similar requirements.The major limitation of ramjet is that they provide no static thrust.The ram compression of intake air requires the engine to be moving faster than Mach1.5before the engine becomes self-sustaining.Hence all ramjets must be accelerated to operating speeds by an auxiliary source,in most missiles this is integral with ramjet engine(Fry(2004)).Development of the ramjet engine wasfirst conceptualised in the early1900’s and its development throughout the20th century was driven by the military application for fast,long range missiles.The speed capability makes ramjets ideal for standoffsituations.The main advantages over rocket power is that for ramjets”the engines produce much higher engine efficiency over a large portion of the flight and a longer powered range than rockets,there is thrust modulation for efficient cruise and acceleration,they have the ability to change efficiently powered fight path and maneuverability,and they are reusable not just refurbishable”.(Fry(2004)).Two of the more important developments during the century were the introduction of powerful CFD code for modelling high speed turbulence& combustion dynamics allowing for efficient propulsion performance to be achieved, and variable intake geometry to increase the range of efficient operatingflight Mach numbers(Fry(2004)).Interestingly,as the ram compression heats the intake air it cannot be effectively used for engine cooling,hence cooling is commonly effected using an expendable coolant or the onboard fuel.This means the cooling capacity must also be con-sidered when choosing a fuel.Hydrogen gives the best thrust and cooling,while methane is the leading natural hydrocarbon(Hunczak&Low(1957)).More re-cently JP-7is a synthetic fuel created in response to the demands of these extreme applications.2.BackgroundRamjets are airbreathing engines as all of the oxidizer necessary for combustion comes from the atmosphere(Fry(2004)).The intake is intended to increase the static pressure ratio and reduce theflow speed to be subsonic at the combustionThe effects of intake modification on a Ramjet engine3chamber.At supersonicflight Mach numbers a system of normal and oblique shocks will develop around the engine dependent on intake geometry.Across a shock theflow Mach number is decreased and the static pressure in-creased,hence shocks are ideal for achieving a ram compression effect.The geometry of the intake design is intended to create a system of shocks which result in theflow satisfying the two combustion requirements discussed above. Shocks are an anisentropic phenomena and although the static pressure increases across the shock,the stagnation pressure decreases.Efficient ram compression design reduces the loss of stagnation pressure,this is also referred to as pressure recovery.The stagnation pressure drop is an indicator of lost potential for max-imum possible combustion pressure,so intake design is also intended to reduce this loss.The system of shocks used is a very simplified model for the aerodynamics of a supersonic intake.It is useful for indicating possible trends in design mod-ifications for further investigation but direct application of the results is not advisable.Even with more complex theoretical methods it is difficult to predict the aerodynamics characteristics,thus practical development of ramjets requires a sound knowledge base of experimental data(Satyanarayana et al.(2005)).The experimental investigations performed by Hirschen et al.(2007)make an indepth investigation of intakeflow andflow phenomena for the shock systems,boundary layer separation and buzzing.This investigation was carried out on a2-D model with dual nose angles and the possibility of bleeding some of the intake air.The high speed photography images of theflow highlights the complex nature of the flowfield and possible shock systems which can occur inside the cowl.They specifically investigated improving pressure recovery by slight alterations to in-take duct geometry to reduce boundary layers,our investigations are to intended to produced pressure recovery by simple alteration of the shock system.The study by Hartfield et al.(2007)developed a genetic algorithm for intake design based on a very similar shock model to our analysis.They also modeled a single normal shock at the cowl entrance and ignored possible reflected shock systems,and they’re analysis requires similar assumptions.As well as maximising pressure recovery,intake design may need to also consider reducing the system weight.For example design to reduce drag,temperature and structural loads allows for construction of lighter frames and casing.As usual in engineering simultaneous optimisation of the above requirements will lead to compromises.4P.N.BeuchatFigure1.The original simplified ramjet intake with R c=2R i.3.Intake analysis3.1.Shock predictionThe original design of the simplified can-type ramjet intake is shown infigure1, with all key features labelled.Thefirst part of the analysis was to predict possible shocks systems that would develop based on research and the presented geometry. The numerical analysis was then performed using simple2-D compressibleflow theory,the key equations are all contained in the notes from Monty et al.(2005).Figure1shows the shocks system for design point orientation,defined as when the oblique shockfirst intersects the cowl lip.From the geometry of the given schematic the oblique shock infigure1makes an angle ofβcrit=32.85◦to the central axis.Given the cone deflection angle ofθ=15◦we used the oblique shock relation to show that design-point operation corresponds to aflight Mach number of M∞,crit=2.92.Figure2(a)highlights that forflight Mach numbers1.0<M inf<1.62the oblique shock it detached and theflow downstream is subsonic in a region surrounding the nose point and results in non-uniform downstream conditions.At subcritical operation(figure2(b))theflow is predicable and this is the operating range of the ramjet,with the lower limit being M∞required for sustainable ram compression.The effects of intake modification on a Ramjet engine5(a)At1<M∞<1.62(b)At1.62<M∞<2.92Figure2.The ramjet at subcritical operation(a)At M∞=2.92(b)At2.92<M∞Figure3.The ramjet at design point and supercritical operationFigure4.Proposed biconical intake design,shocks system shown is for subcritical operation.Figure3(b)shows that supercritical operation introduces turbulence into the combustion chamber.This results in reduced thrust and the ramjet engine will not be able to sustain these speeds,decelerating back to design-point operation. Hence numerical analysis was only performed for the subcritical range with at-tached shocks.Our new design replaces the current nose cone with a biconical nose,which has two deflection angles,seefigure4.Again the shock systems were predicted and then the theory applied.The two deflection angles produce two oblique shocks which focus and become a single shock(called a lambda foot).A vortex sheet is produced from the focus point due to a velocity difference on either side,thus it is imperative for this design to ensure that the locus of these intersection points remains above the cowl lip so that the turbulent vortex sheet does not enter the combustion chamber.6P.N.BeuchatFigure5.Locus of Lambda Foot intersection over the subcritical Mach Range.Left end is just after attached shocks occur at both apertures,Right end is at design point operation.3.2.Numerical analysisThe analysis data was produced using the numerical software develop specifically for this investigation by Beuchat&Midgley(2007).All results are summarised graphically infigure6&7andfigure5proves that our condition on the locus of the lambda foot intersections is satisfied.The analysis is based on the shock systems discussed above,and required the following assumptions.•Theflow upstream of the oblique shock is considered to be supersonic and uniform,true to1→2%(Hartfield et al.(2007)).•Viscous boundary layers which may develop in the duct after the normal shocks are ignored.•Subsonicflow from the duct into the combustion chamber was isentropic.The effects of intake modification on a Ramjet engine7(a)(b)parison between current and proposed inlet,the data was obtained fromnumerical analysis(a)(b)Figure7.Adjusted comparison to match the intake ares for both designsparisonThe biconical design was was specified to maintain the majority of variables as constant except the introduction of a second angle.Hence the two angle summed to beθ1+θ2=15◦(we chooseθ1=θ2=7.5◦),the length of the nose was generated so the subcritical operating range was similar.The numerical software generated the length so that the two oblique shocks intersected with the cowl lip at M∞=3.0.The third set of results found infigures6&7labeled’extended’is the identical to the original design only with the nose cone moved out of the cowling slightly to match the intake area with that of the proposed design.8P.N.BeuchatComparisons between ramjet intakes are primarily based on pressure ratio and massflow rate(˙m intake).Both of these indices must be considered because an increase in either allows more thrust to be developed in combustion,but increas-ing one significantly is often compromised by decreasing the other.The results show that the proposed design is an improvement in all aspects presented.This can be split into two conclusions.Figures6show the proposed design when incorporated into the current cowling gives improved results,approximately10%improvement.If we presume the cowling to be mounted further back and simply consider the pressure ratio across the lambda foot versus across the single oblique(and then the normal shock), we see infigure7(b)that the biconical nose continues to give improved pressure recovery at higher Mach numbers.NOTE:The analysis used actually modeled the engine as the cross-section shown extended into the page and not as an axisymmetric object because this is the2-Dflow theory we were using.It is expected that the results using axisymmetric theory would qualitatively show the same trends.Interestingly the concept Earth-orbit vehicles shown infigure8has an engine intake of the type modeled for this analysis.4.ConclusionsWe proposed a modified intake design and used simple2-D compressibleflow theory to validate the improvements of the new design.We convincingly proved that for a general intake design a biconical nose would outperform a single gra-dient nose.We achieved a10%increase in the static pressure ratio using the design specified here.The analysis tools developed as part of this investigation are adaptable for use in an optimisation algorithm to design a more effective biconical nose cone which could maximize the static pressure ratio.The effects of intake modification on a Ramjet engine9Figure8.France LEA conceptflight test vehicle(Fry(2004)).REFERENCESFry,R.S.,A century of ramjet propulsion technology evolution.Journal of Propulsion and Power,2004.p27–58.Hirschen,C.,Herrmann,D.and G¨u lhan,Experimental investigations of the performance and unsteady behaviour of a supersonic intake.Journal of Propulsion and Power,2007.p566–574.Knight,D.,Design Optimization of Air Vehicle Intakes.6th Annual modeling and optimization: Theory and applications conference,University of Waerloo,2006.Hunczak,H.R.and Low,G.M.,III.Preliminary analysis of hypersonic ramjet cooling problems.NASA Glenn Research Centre,Document ID19710070022,1957.p.37–51. Henneberry,H.M.,Zimmerman,A.V.,Dugan,J.F.,Schramm,W.B.,Breitwieser,R.and Povolny,J.H.,3.Engines.NASA Glenn Research Centre,Document ID19710070023, 1957.p.53–76.Satyanarayana,A.,Santhakumar,S.,Panneerselvam,S.and Ahmed,S.,Effect of intake ge-ometry on longitudinal aerodynamics of airbreathing vehicles.Journal of Spacecraft and Rockets,2005.p1011.Hartfiel,R.J.,Jenkins,R.M.and Burkhalter,J.E.,Ramjet powered missile design using a genetic algorithm.Journal of Computing and Information Science in Engineering,2007.p167–173.Beuchat,P.N.and Midgley,W.J.,.au/wmidgley/Undergraduate stu-dents of the Dept.of Mech.&Manuf.Engineering,University of Mebourne,2007. Monty,J.P.,Jones,M.B.and Ooi,A.,436-352Thermofluids3—Compressible Flow.Lecture series distributed by the Dept.of Mech.&Manuf.Engineering,University of Mebourne, 2005.。