Propulsion

• Total temperature in a nozzle is constant
• Total temperature in an intake is constant, frictional effects manifest as a total pressure loss
• If a nozzle or intake is 100% efficient (isentropic flow), there will be no loss in total pressure
• SFC = f / F_s (1 + B)
• mdot_c = mdot_in / B + 1
• mdot_b = mdot_in B / B + 1
• A multiple shaft engine design is used because:

  • Better matching and performance for off design conditions
  • Easier to have different components operating at optimum conditions
  • Better surge margin
• Pros for 2 shaft design:

  • Less complex
  • Lighter

  Cons:

  • Not as good surge margin at lower speed
  • Variable vanes + bleed needed
• Pros for 3 shaft design:

  • Better off design matching
  • Better surge margin

  Cons:

  • Heavier and more complex
• 1. B2
  2. a2
  3. C2
  4. V2r
  5. U
  6. V3r
  7. C3
  8. B3
  9. a3
  10. Cw3
  11. Cw2

  
• 1. a1
  2. B1
  3. C1
  4. C2
  5. U
  6. V1r
  7. a2
  8. B2
  9. V2r
  10. Cw2
  11. Cw1

  
• de Haller number, DH = V2r / V1r > 0.72
• Free vortex assumptions:

  • Constant loses radially; Entropy constant with radius
  • Constant work radially; Specific work constant with radius
  • No circumferential variation of properties
  • No radial component Cr = 0
  • Axial velocity constant with radius
• Features that improve convective cooling include: Ribs, Impingement Plates, Bends, Bleeds, Pedestals and Pin Fins
• Increasing thrust and efficiency means high turbine entry temperature and pressure ratio
• Film cooling drawbacks include

  • High aerodynamic loses
  • Large amounts of bleed air used
  • holes prone to blocking
  • Boundary layer may transition
• Can Combustor

  Advantages:

  • Easy to develop
  • Fuel spray well matched to swirl air and secondary air jets
  • Easier to test due to being segmented
  • Low engine diameter

  Disadvantages:

  • Complex and heavy ducting at inlet and exit
  • Large frontal area for given mass flow
  • Poor aerodynamically
  • Light around problem
• Annular Combustor:

  Advantages:

  • Better aerodynamically with less pressure loss
  • Less complex and lighter ducting
  • Minimum size and weight
  • Light around problem solved

  Disadvantages:

  • Difficult to develop as they require full mass flow
  • High buckling load on outer flame tube. Weaker structurally
  • Poor matching of spray and air jets. Can penetrate radially inwards and ouwards instead of circumferentially
  • More difficult to obtain even fuel air ratio and hence uniform and stable exit temperature profile
• Propulsive efficiency is a measure of how much of the total engine power is actually used to power the aircraft
• Thermal efficiency is a measure of how much of the heat input to a propulsion system is converted into net power
• Overall efficiency is a measure of how much heat energy is used for actual propulsive power
• For a nozzle expanded to ambient:

  Propulsive Efficiency = 2V_i / (V_e + v_i)