Aircraft Propulsion AENG 31102 Aircraft Gas Turbine Performance & Design Revision Lecture Lecture 7 Fundamentals ~Turbojet • The compressor raises the pressure of the air before combustion • The turbine extracts work from the hot high pressure combustion products to drive the compressor Each stage consists of a row of rotor blades followed by a row of stator blades The first stage is often preceded by an inlet guide vane Each stage consists of a row of Nozzle Guide Vanes which direct the gas onto the rotor blade Total or Stagnation Temperature The stagnation temperature is the temperature at the stagnation point in a fluid flow. At a stagnation point the speed of the fluid is zero and all of the kinetic energy has been converted to internal energy and is added to the local static enthalpy. Hence ho = h + V2/2 For Constant Cp; Enthalpy = Cp T Hence To = T + V2/2 Cp or To/T = 1 + (γ – 1)/2 M2 = 1 + 0.2 M 2 The Practical Turbojet Cycle Joule or Brayton Cycle: • Calculate Total Temperatures & pressures at inlet • Step by step calculation of Total Temperatures & Pressures through engine • Check whether nozzle is choked • Calculate Jet Velocity & static pressure at nozzle plane • Thrust rate of change of momentum 4 Variations • Remove turbo machinery ~ RAMJET • Add power turbine ~ TURBOSHAFT/TURBO-PROP • Split the flow after initial compression & add more stages ~ TURBOFAN • Add heat exchanger ~ AIR BREATHING ROCKET Different Turbofan Types Civil Turbofan~ Trent High By-pass Ratio 5 ~ 12 “Low” Specific Thrust Specific Thrust Thrust per unit mass flow Units: N/kg/sec or m/s Military Turbofan ~ EJ200 Low By-pass Ratio 0.3 ~ 1 “High” Specific Thrust Axial Compressors Rotor Speed = U Combining the vectors of C1 and U to give V1 and 1 Axial velocity = 𝑪𝒂𝟏 Whirl velocity = 𝑪𝒘𝟏 Assume Constant Axial Velocity i.e. 𝐶𝑎1 = 𝐶𝑎2 = 𝐶𝑎3 = 𝐶𝑎 Hence at Exit 3 and C3 are the same as 1 and C1 Compressors ~ diverging passages Turbines ~ converging passages Factors effecting Stage Pressure Ratio • Blade Speed • Axial velocity • High Deflection in Rotor Blades (β1 - β2 ) • Efficiency UCa p03 1 p01 p 1 tan 1 tan 2 C pT 01 de Haller Number = V2 / V1 For minimum losses V2 / V1 > 0.72 Effect of increasing deflection Typical Transient Compressor Working Lines 9 Centrifugal Compressors pO 3 isenU 2 1 1 pO1 Cp TO1 𝑪 Slip Factor: 𝝈 = 𝒘𝟐 𝑼 Power Input Factor: 𝝍 = 𝑅𝑎𝑡𝑖𝑜 𝑜𝑓 𝑎𝑐𝑡𝑢𝑎𝑙 𝑤𝑜𝑟𝑘 𝑖𝑛𝑝𝑢𝑡 𝑡𝑜 𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑤𝑜𝑟𝑘 𝑖𝑛𝑝𝑢𝑡 Tip Clearance Issues Core CF6 Thrust 50,000 Lb Fan Diameter 2.2m OPR 29 TFE 731 4000 Lb Fan Diameter 0.72 m OPR 15 11 Tip Clearances Stator Inlet Stator Outlet SOT Turbine Entry (Rotor) TET Rotor Inlet RIT Deflection of flow in rotor = β2 + β3 Axial Flow Turbines Turbine Layout & Cooling A typical uncooled turbine blade showing twisted contour Cooling Effectiveness 𝑇𝑏– 𝑇𝑐𝑟 = 𝑇𝑔 𝑟𝑒𝑙 – 𝑇𝑐𝑟 Aircraft Flight Envelope 16 Cycle Choice SPECIFIC THRUST (Thrust per unit Mass Flow) = (𝑪𝒋 − 𝑪𝒂 ) (Unit of velocity) Specific Thrust - Variation with design flight speed AIRCRAFT PROPULSION AENG 31102 • Length of Exam 2 hours (max time 2 hrs 30 minutes to cover connectivity issues. • The paper contains questions of the type: – – – – Fill in the blanks Multiple Choice True or False Essay Questions* • * Questions on specific topics covered during the lectures – short answers in bullet point form. • The maximum for this paper is 60 marks. Hints • For the calculation questions be careful to fill into the stated accuracy – there is some latitude for rounding issues. • For the essay questions – bullet points covering the main points are fine • The exam is “open book” therefore you do not need to memorise equations.
