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Supersonic Combustion

Supersonic Combustion. Theresita Buhler Sara Esparza Cesar Olmedo. Purpose & Goals Introduction to combustion Engine parameters Jet Engine Ramjet Scramjet Jet Engine vs. Scramjet Model Reference stations Analytical approach Compressible flow Shockwaves Inlet: Diffuser design

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Supersonic Combustion

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  1. NASA Grant URC NCC NNX08BA44A Supersonic Combustion Theresita Buhler Sara Esparza Cesar Olmedo

  2. NASA Grant URC NCC NNX08BA44A Purpose & Goals Introduction to combustion Engine parameters Jet Engine Ramjet Scramjet Jet Engine vs. Scramjet Model Reference stations Analytical approach Compressible flow Shockwaves Inlet: Diffuser design COSMOSWorks design Engine: Cowl design Combustion schemes & fuels Exhaust: Expansion Prototype design Materials Design Specifications Installation in the wind tunnel Location Fuel lines and ignition wires Hydrogen safety History Cost Acknowledgements Questions Supersonic Outline

  3. NASA Grant URC NCC NNX08BA44A Hypersonic Vehicle • High speed travel • Commercial flight • Reaction engines • Circumnavigation in four hours • NASA Goals • Global reach vehicle • Reduced emissions • Challenges • Shockwaves • High heat • Combustion instability • Flight direction control NASA X-43 Vehicle NASA X-51 Testing

  4. NASA Grant URC NCC NNX08BA44A Combustion • Fuel • Air • Heat • High pressure flow, at high compression • Quickly changing conditions • Temperature difficulties • Frictional heating • High forced convection • Highly turbulent • Shock

  5. NASA Grant URC NCC NNX08BA44A Engine Parameters Fit engine to aerospace system Jet Engines – Low orbit, max Mach 3 Ramjets – High altitude, supersonic flight, subsonic combustion Scramjets – High altitude, hypersonic flight, supersonic combustion

  6. NASA Grant URC NCC NNX08BA44A Jet Engines • Combustion chamber • Introduce fuel • House combustion • Turbine blades • Capture expansion of exhaust gases • Inlet design • Feed air into chamber • Compressor blades • Increase pressure of flow

  7. NASA Grant URC NCC NNX08BA44A Ramjet • Vehicle travels at supersonic speed • Simplest air-breathing engine • No moving parts • Compression of intake achieved by supersonic flow – inlet speed reduction • Shockwave system • Relatively low velocity • Combustions at subsonic speeds • Very high reduction in speed • High drag • High fuel consumption • Temperature at 3000 K (4940°F) • Diffuser • Exit plane contracts • Exhaust at supersonic speed • Travel: M = 3 • Combustion: M= 0.3

  8. NASA Grant URC NCC NNX08BA44A Scramjet • Hypersonic flight • No moving parts • Combustion at Supersonic speed • Flow ignites supersonically • Fuel injection into supersonic air stream • Steer clear of shock waves • Is Aerodynamically challenged

  9. NASA Grant URC NCC NNX08BA44A ScramjetBoeing

  10. NASA Grant URC NCC NNX08BA44A Then and Now

  11. NASA Grant URC NCC NNX08BA44A What is Supersonic Combustion Combustion maintained at supersonic speed How is it achieved? Design Shockwave Fuel Injector Detonation Combustion

  12. NASA Grant URC NCC NNX08BA44A Oblique shocks Mach number decreases Pressure, temperature, and density increase Attached to vehicle Normal shocks Mach number decreases Pressure, temperature, and density increase Creates subsonic region in front of nose Detached Shock Waves

  13. NASA Grant URC NCC NNX08BA44A Oblique shock Mach number decreases Pressure, temperature, and density increase Expansion wave Mach number increases Pressure, temperature, and density decrease Shock Waves

  14. NASA Grant URC NCC NNX08BA44A Diffuser Development • Wind tunnel specifications • Inlet speed • Mach 4.5 • Cross-sectional area • 6 x 6 in • Length of test section • 10 in

  15. NASA Grant URC NCC NNX08BA44A Design of Diffuser • Initial design of diffuser • Use manifold design to introduce fuel • Diffuser was designed in to two separate pieces Goal Seek 18° 28.29° 19.67°

  16. NASA Grant URC NCC NNX08BA44A Design of Diffuser • Top part of the diffuser • Has machined holes for fuel and ignition wires. • Also four holes for securing the base of the diffuser

  17. NASA Grant URC NCC NNX08BA44A Design of Diffuser

  18. NASA Grant URC NCC NNX08BA44A 2D Shockwaves

  19. NASA Grant URC NCC NNX08BA44A Inefficient Designs Bow Shock – Cowl Interference Oblique Shock – Cowl Spillage

  20. NASA Grant URC NCC NNX08BA44A Cosmo Flowork Analysis

  21. NASA Grant URC NCC NNX08BA44A Cosmos Flowork Analysis Velocity Profile Mach Speed Profile

  22. NASA Grant URC NCC NNX08BA44A Cosmos Flowork Analysis Pressure Contours Inlet Mach = 4.5

  23. NASA Grant URC NCC NNX08BA44A Cosmos Flowork Analysis Temperature Contours Inlet Mach = 4.5

  24. NASA Grant URC NCC NNX08BA44A Cosmos Flowork Analysis

  25. NASA Grant URC NCC NNX08BA44A Ramp Fuel Injections Ramped Outward Ramped Inward

  26. NASA Grant URC NCC NNX08BA44A Cosmos Flowork Analysis

  27. NASA Grant URC NCC NNX08BA44A Cosmos Flowork Analysis

  28. NASA Grant URC NCC NNX08BA44A Cosmos Flowork Analysis

  29. NASA Grant URC NCC NNX08BA44A Cosmos Flowork Analysis

  30. NASA Grant URC NCC NNX08BA44A Combustion • Combustion Stoichiometry • Ideal fuel/ air ratio • Recommended fuel for scramjets • Hydrogen • Methane • Ethane • Hexane • Octane • Only Oxidizer is Air • Maximum combustion temperature • Hydrocarbon atoms are mixed with air so • Hydrogen atoms form water • Oxygen atoms form carbon dioxide • Most common fuel for scramjets • Hydrogen • In scramjets, combustion is often incomplete due to the very short combustion period. • Equivalence ratio • Should range from .2 -2.0 for combustion to occur with a useful time scale • Lean mixture ratio below 1 • Rich mixture ratio above 1

  31. NASA Grant URC NCC NNX08BA44A CombustionParallel Mixing Fuel- Air Mixing at mach speeds Gas phase chemical reaction occurs by the exchange of atoms between molecules as a results of molecular collisions. The fuel and air must be mixed at near-stoichiometric proportions before combustion can occur Parallel Mixing of Fuel- Air U1 Mixing Layer δm U2

  32. NASA Grant URC NCC NNX08BA44A CombustionParallel Mixing • Zero shear mixing • Both air and fuel velocities are equal • Shear stress doesn’t exist between streams • Coflow occurs • Lateral transport • Occurs by molecular diffusion • At fuel – air interface • No momentum or vorticity transfer • Axial development of cross –stream profiles of air mole fraction YA in Zero shear (U1=U2) • Fuel Mole fraction Profile is YF=1-YA • Mirror Image Ya Ya U1 δm U2

  33. NASA Grant URC NCC NNX08BA44A CombustionParallel Mixing • Molecular diffusion • Fick’s Law • Air molecular transport rate into fuel • Proportional to the interfacial area times the local concentration gradient. • Proportionality constant • DFA, = molecular diffusivity • Where DFA*ρ is approximately equal to molecular viscosity μ for most gases Ya Ya U1 δm U2

  34. NASA Grant URC NCC NNX08BA44A CombustionParallel Mixing • Fick’s law for diffusion

  35. NASA Grant URC NCC NNX08BA44A Combustion Parallel Mixing Ya Ya U1 δm U2

  36. NASA Grant URC NCC NNX08BA44A y B1 Ya Ya U1 x δm U2 -B2 B1+B2 X=Lm X=0 Combustion Parallel Mixing • Steepest concentration gradient at x = 0 • Mixing layer reaches the wall at x=Lm the air mole fraction still varies from 1.0 at y=B1 and 0 at y= -B2 • More mixing is needed • 2Lm is recommended by experiment • enables complete micro-mixing

  37. NASA Grant URC NCC NNX08BA44A y B1 Ya Ya U1 x δm U2 -B2 B1+B2 X=Lm X=0 Combustion Parallel Mixing Mixing layer thickness equation Estimate injector height, B1+B2=B to reduce mixing length, Lm

  38. NASA Grant URC NCC NNX08BA44A Combustion Parallel Mixing Manifolding idea Multiple inlets Reduce mixing length Tradeoff: Inefficient design Adds bulk and volume Air δm B Fuel δm Air Fuel

  39. NASA Grant URC NCC NNX08BA44A Combustion Laminar Shear Mixing • Molecular diffusion alone cannot meet the requirements of rapid lateral mixing in supersonic flow • Solution shear layer between both layers • U1>U2 , Uc=0.5(U1+U2 ) • Velocity ratio r =(U1/U2 ) • Velocity Difference Δ U= (U1-U2 ) μ: dynamic viscosity ν: kinematic viscosity

  40. NASA Grant URC NCC NNX08BA44A Combustion Turbulent shear mixing • As we further increase the velocity difference delta U • Shear stress causes the periodic formation of large vortices • The vortex sheet between the two streams rolls up and engulfs fluid from both streams and stretches the mixant interface. • Stretching of the mixant interface increases the interfacial area and steepens the concentration gradients • Shear mixing increases molecular diffusion

  41. NASA Grant URC NCC NNX08BA44A Combustion Turbulent shear mixing Fuel wave Fuel vortex Micro-mixing

  42. NASA Grant URC NCC NNX08BA44A CombustionTurbulent Shear Mixing • Mean velocity profile combines • Prandtl’s number • Turbulent kinematic viscosity • Time average characteristics of turbulent shear Micro-mixing Fuel wave Fuel vortex

  43. NASA Grant URC NCC NNX08BA44A CombustionTurbulent Shear Mixing Shear layer width – Two methods Local shear layer width for turbulent shear mixing Recent research Cδ is a experimental constant

  44. NASA Grant URC NCC NNX08BA44A CombustionTurbulent Shear Mixing • Density effects on shear layer growth – compressible flow • Based on constant but different densities • A density ratio, s, is derived • s can be calculated once stagnation pressure and stream velocities are known

  45. NASA Grant URC NCC NNX08BA44A CombustionTurbulent Shear Mixing • Convective velocity for the vortex structures • With compressible flow using isentropic stagnation density equation changes to

  46. NASA Grant URC NCC NNX08BA44A CombustionTurbulent Shear Mixing • Density correct expression for shear layer growth including compressibility effects

  47. NASA Grant URC NCC NNX08BA44A CombustionTurbulent Shear Mixing Only applies to box cowl

  48. NASA Grant URC NCC NNX08BA44A Fuel CombustionTurbulent Shear Mixing Based on what we know the angle of our hydrogen injection should be To produce a hydrogen rich mixture Lm, F air Lm, A

  49. NASA Grant URC NCC NNX08BA44A Mixing Controlled Combustion High mixture temperature High reaction rates Limiting feature: mixing Reaction Rate Controlled Low mixture temperature Adequate mixing Limiting feature: reaction rates Rate of heat release Diffusion Combustion

  50. NASA Grant URC NCC NNX08BA44A Diffusion Combustion • Symmetric flame • Stoichiometric ratio • Varies across flame • Flame center • Highest temperature, fuel • Air lost around edges

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