MAE 5360: Hypersonic Airbreathing Engines

1. MAE 5360: Hypersonic Airbreathing Engines Overview Mechanical and Aerospace Engineering Department Florida Institute of Technology D. R. Kirk

2. Physical Aspects of Hypersonic Flow • Flow regimes • Subsonic, incompressible: M∞ < 0.3 • Subsonic, compressible: M∞ < 0.7 • Sonic: M∞ = 1.0 • Transonic: 0.7 < M∞ < 1.3 • Supersonic: M∞ > 1.0, 1.0 < M∞ < 5.0 • Hypersonic flow typically defined: M∞ > 5 • Physical aspects of supersonic flow • Physical aspects of hypersonic flow • Thin Shock Layers • Entropy Layer • Viscous Interaction • High Temperature Effects • Low-Density Flow

3. Physical Aspects of Hypersonic Flow Thin Shock Layers • Flow field between the shock wave and the body surface is called the shock layer • Shock layers at hypersonic speeds are very thin • A characteristic of hypersonic flow is that the shock waves lie close to the surface, thus creating thin shock layers, which in turn can cause many physical complications • The boundary layer on the surface of the body can grow to the same thickness of the thin shock layer itself. This leads to a merging of the shock wave with the boundary layer, constituting a fully viscous shock layer. • Note that the thin shock layer does allow us to develop some simplified aerodynamic theories for the prediction of surface pressure at hypersonic speeds (called the Newtonian Law for Hypersonic Flow)

4. Physical Aspects of Hypersonic Flow Thin Shock Layers • Flow field between the shock wave and the body surface is called the shock layer • Shock layers at hypersonic speeds are very thin • A characteristic of hypersonic flow is that the shock waves lie close to the surface, thus creating thin shock layers, which in turn can cause many physical complications • The boundary layer on the surface of the body can grow to the same thickness of the thin shock layer itself. This leads to a merging of the shock wave with the boundary layer, constituting a fully viscous shock layer. • Note that the thin shock layer does allow us to develop some simplified aerodynamic theories for the prediction of surface pressure at hypersonic speeds (called the Newtonian Law for Hypersonic Flow)

5. Physical Aspects of Hypersonic Flow:Thin Shock Layers

6. Physical Aspects of Hypersonic Flow Entropy Layer • Good example is detached shock in front of a blunt nosed body • The shock waves curves downstream of the nose and at hypersonic speeds essentially wraps itself around the nose of the body • At supersonic speeds the shock wave at the nose is also curved, but the magnitude of the curvature is far less than at hypersonic speeds • In the nose region, the shock layer is very thin, and the shock wave is highly curved • This strong curvature induces very large velocity gradients in the flow behind the shock in the nose region. • These large velocity gradients are accompanied by strong thermodynamic changes in the flow. • This region of strong gradients is called the entropy layer and extends downstream close to the body surface • Downstream of the nose, the entropy layer interacts with the boundary layer growing along the surface, and this interaction increases aerodynamic heating of the surface above and beyond what would be predicted without the entropy layer • Strength of the entropy layer is related to the shock curvature • Very important phenomena at hypersonic speeds

7. Physical Aspects of Hypersonic Flow:Entropy Layer

8. Physical Aspects of Hypersonic Flow Viscous Interaction • Boundary layer thickness drivers: • The thickness of a laminar boundary layer is inversely proportional to the square root of the Reynolds number • For compressible flow, boundary layers shock that the thickness is also proportional to the Mach number squared • Boundary layers at hypersonic Mach numbers can grow very large (especially at high altitude) • The boundary layer thickness can become so large that the flow outside the inviscid flow field external to the boundary layer • This creates a viscous interaction: the thick boundary layer flow affects the outer inviscid flow, and the changes in the inviscid flow feed back and influence the boundary layer growth • Consequences of viscous interaction on hypersonic vehicles is an increase in surface pressure and skin friction, leading to increased drag and increased aerodynamic heating • Consider hypersonic flow over a flat plate: the outer inviscid flow no longer sees a flat plate, and instead sees a body with some effective thickness induced by the thick boundary layer (displacement thickness) • The actual pressures exerted on the flat plate are higher than the ambient pressure (induced pressure increment)

9. Physical Aspects of Hypersonic Flow:Viscous Interaction

10. Physical Aspects of Hypersonic Flow High-Temperature Effects • High Mach number flows are high energy flows • The ratio of kinetic energy to gas internal energy increases as the Mach number squared • When this flow enters a boundary layer it is slowed by the effects of friction, and the kinetic energy rapidly decreases and is converted to internal energy and the gas temperature, which is proportional to the internal energy, increases very rapidly • Hypersonic boundary layers are high-temperature regions of the flow, due to viscous dissipation of the flow kinetic energy • Another region of high temperature flow is the shock layer behind the strong bow shock wave of a blunt body. In this case thee flow velocity discontinuously decreases as it passes through the shock wave • High temperature cases chemical reactions to occur in the flow • Above 2,000 K, oxygen dissociates • Above 4,000 K, nitrogen dissociates and nitric oxide will form and can begin to ionize • Above 9,000 K, atoms will ionize • Hypersonic flows are also chemically reacting flows, and these chemical reactions change the flow field properties and affect aerodynamic surface heating

11. Physical Aspects of Hypersonic Flow:High Temperature Effects

12. Physical Aspects of Hypersonic Flow:High Temperature Effects

13. Physical Aspects of Hypersonic Flow Low-Density Flows (not inherent to hypersonic flows, but for hypersonic vehicles which typically operate at high altitudes) • Air at lower altitudes (typical ABE flight) is very well modeled as a continuous medium • However, at 300,000 ft, the continuum hypothesis is no longer valid. The influence of individual molecular impacts on the surface become important – the air must be treated as distinct particles (molecules, atoms, ions) which can be widely separated from one another • At standard atmospheric conditions, the mean free path is about l~2.176 x 10-7ft • At 342,000 ft of altitude, l~1 ft • All of our aerodynamic concepts, equations, and results that rely on the continuum hypothesis break down • Aerodynamics now based on kinetic theory (low-density flows) • Common to examine Knudson Number, KN = l/l • Continuum: KN << 1 • Free molecule flow (low density flow): KN > 10 • In free molecule flow, the body surface feels only a small number of distinct molecular impacts • Shock waves become very thick “lose their identity” • Aerodynamic force coefficients and surface heat transfer coefficients become strong functions of KN (in addition to Mach number and Reynolds number)

14. Physical Aspects of Hypersonic Flow:High Temperature Effects

15. Air-breathing Hypersonic Vehicle Overview • Air-breathing, hypersonic vehicles are highly integrated systems • Performance depends, largely, on complex physics and interactions between all components • Consider a vehicle with a hypersonic propulsion system flying at Mach numbers ~8: • In these circumstances, engine operates under supersonic conditions - scramjet mode • A key performance metric is amount of heat released in combustion chamber, as it is directly connected to the generation of thrust

16. Hypersonic Propulsion Subsystems • Presence of a supersonic flow stream over entire vehicle introduces a streamwise splitting of vehicle into several subsystems: • Forebody is dominated by presence of strong bow shock and associated complex thermo-chemistry effects • Extreme heating occurring at stagnation point determines choice of materials and cooling strategies • As flow decelerates towards engine, boundary layers develop on vehicle surface • Turbulence trips are designed to force transition to turbulence • Increased surface heating and friction • Turbulent boundary layers entering engine inlet lead to increased stability and mixing • Inlet/Isolator System conditions flow towards pressure and temperature conditions that are most favorable to combustion • In addition, shock train present in isolator reduces distortion of incoming flow due to angle of attack and yaw variability • Combustor is most critical component of vehicle • Injection system carefully designed to mix fuel and incoming supersonic air stream and to produce auto-ignition of mixture • Nozzle/Afterbody further accelerate flow stream producing thrust

17. Hypersonic Propulsion Challenges • One of main challenges in designing a scramjet engine is extremely short residence time of fuel within combustor • Ability to inject fuel, mix it in supersonic incoming air stream and ignite resulting mixture is critical design objective for propulsion system • Minimum combustion efficiency (heat release) is performance threshold for vehicle design • However, there is an imprecise limit to heat release that can be deposited in a supersonic stream ground tests • Excessive heating leads to thermal choking: a normal shock and a consequent subsonic flow region is established in combustor • In addition to a reduction in performance, this can lead to increased structural and thermal loads and eventually lead to failure • Normal shock can also propagate upstream in combustion chamber, and eventually interact with isolator, creating extensive regions of boundary layer separation • Shock motion can lead to engine unstart with entire isolator shock train moving upstream • Under these conditions vehicle performance is compromised and extreme actions have to be taken to restart engine. • Unstart conditions can also be reached for different reasons, not directly connected to combustion process. Indications of unstart events connected to perturbations of flow at inlet and to thermal deformations of structure also observed