Turbomachinery Lecture 5 Airfoil, Cascade Nomenclature Frames of Reference Velocity Triangles Euler’s Equation
Airfoil Nomenclature Chord: c or b = xTE-xLE; straight line connecting leading edge and trailing edge Camber line: locus of points halfway between upper and lower surface, as measured perpendicular to mean camber line itself Camber: maximum distance between mean camber line and chord line Angle of attack: , angle between freestream velocity and chord line Thickness t(x), tmax
Frame of Reference Definitions
Frame of Reference Definitions
Cascade Geometry Nomenclature s pitch, spacing laterally from blade to blade solidity, c/s = b/s stagger angle; angle between chord line and axial 1 inlet flow angle to axial (absolute) 2 exit flow angle to axial (absolute) ’1 inlet metal angle to axial (absolute) ’2 exit metal angle to axial (absolute) camber angle ’1 - ’2 turning 1 - 2 Concave Side -high V, low p - suction surface Convex Side -high p, low V - pressure surface b bx Note: flow exit angle does not equal exit metal angle Note: PW angles referenced to normal not axial
Compressor Airfoil/Cascade Design Compressor Cascade Nomenclature: Camber - "metal" turning Incidence +i more turning Deviation + less turning Spacing or Solidity
Velocity Diagrams Apply mass conservation across stage UxA = constant, but in 2D sense Area change can be accomplished only through change in radius, not solidity. In real machine, as temperature rises to rear, so does density, therefore normally keep Cx constant and then trade increase with A decrease same component in absolute or relative frame Rotational speed is added to rotor and then subtracted If stage airfoils are identical in geometry, then turning is the same and C1 = C3
Velocity Diagrams Velocity Scales For axial machines Cx = u >> Cr For radial machines Cx << Cr at outer radius but Cx may be << or >> Cr at inner radius Velocity Diagrams Velocity Diagram Convention Objectives: One set of equations Clear relation to the math Conclusion: Angles measured from +X Axis U defines +Y direction Cx defines +X direction
Velocity Diagrams: Compressor and turbine mounted on same shaft Spinning speed magnitude and direction same on both sides of combustor Suction [convex] side of turbine rotor leads in direction of rotation Pressure [concave] side of compressor rotor leads in direction of rotation
Frames of Reference
Velocity Diagrams: Another commonly seen view
Axial Compressor Velocity Diagram: 3 N 2 1
Flip from previous page, i.e. rotor going DOWN
Relative = Absolute - Wheel Speed 1 Rotor (Blade) 3 Stator (Vane) 2
Turbine Stage Geometry Nomenclature
Flip from previous page, i. e Flip from previous page, i.e. rotor going DOWN and rotor second airfoil row
Analysis of Plane Cascade Forces Sit on frame of airfoil Fy Fx
Analysis of Cascade Forces Conservation mass, momentum
Analysis of Cascade Forces
Analysis of Cascade Forces L, D are forces exerted by blade on fluid: Fy Fx L D
Another View of Turbine Stage
Relative = Absolute - Wheel Speed 1 Rotor (Blade) 3 Stator (Vane) 2
Combined Velocity Diagram of Turbine Stage Work across turbine rotor Across turbine rotor
Effect on increased m
Reason for including IGVs
Euler’s Compressor / Turbine Equation Work = Torque X Angular Velocity Angular Velocity of Rotor Torque About the Axis of Rotor Periodicity @ B & D, integer # of blades pitches apart Identical flow conditions along B & D
Euler’s Equation Only tangential force produces on rotor. By momentum equation: Since flow is periodic on B & D, pressure integral vanishes :
Euler’s Equation Moment of rate of Tangential Momentum is Torque []: rate of work = F x dU = F x rd = [angular momentum][] torque vector along axis of rotation Work rate or energy transfer rate or power: Power / unit mass = H = head 1st Law:
Euler’s Equation Euler's Equation Valid for: Steady Flow Periodic Flow Adiabatic Flow Rotor produces all tangential forces Euler's Equation applies to pitch-wise averaged flow conditions, either along streamline or integrated from hub to tip.
Euler’s Equation
Euler’s Equation Euler Equation applies directly for incompressible flow, just omit “J” to use work instead of enthalpy:
Compressor Stage Thermodynamic and Kinematic View
Compressor Stage Thermodynamic and Kinematic View Variable behavior - P0, T0, K.E.
Axial Compressor Velocity Diagram: 3 N 2 1
Compressor Stage Thermodynamic and Kinematic View Across rotor, power input is Across stator, power input is From mass conservation,
Compressor Stage Thermodynamic and Kinematic View Euler’s equation Geometry = velocity triangles Flow = isentropic relations [CD] Thermodynamics =Euler eqn., etc. All static properties independent of frame of reference All stagnation properties not constant in relative frame
Turbine Stage Thermodynamic and Kinematic View Euler’s equation
Compressor Stage Thermodynamic and Kinematic View Stage pressure ratio is
Work Coefficient Define Work Coefficient: Applying Euler's Equation to E
Work Coefficient
Work Coefficient This equation relates 2 terms to velocity diagrams and applies to both compressors & turbines. The physics, represented by Euler’s Equation, matches the implications of Dimensional Analysis.
Work and Flow Coefficients Example: Solution:
Work and Flow Coefficients Solution continued: W1 C1 U Cx1 1 1
Work and Flow Coefficients Note: Similar velocity triangles at different operating conditions will give the same values of E (work) and (flow) coefficient Since angles stay the same and Cx/U ratio stays the same, E is the same W1A 1 1 C1A Cx1 UA UB
Work and Flow Coefficients Pr Flow, Wc A E A,B B B1 Pr Flow, Wc E B1 B2 A1 B2 A1 Nc1 A2 A2 Nc2
Work and Flow Coefficients Effect on velocity triangles Low E High E W1A C1A Cx1 1 1 W1A C1A Cx1 1 UA 1 UA
Work and Flow Coefficients Effect on velocity triangles of varying E = (cu2 - cu1)/U is design low E results in low airfoil cambers high E results in higher cambers Effect of varying = cx/U in design low results in flat velocity triangles, low airfoil staggers, and low airfoil cambers high results in steep velocity triangles, higher airfoil staggers, and higher airfoil cambers Prove these statements by sketching compressor stage and sketching corresponding 3 sets of velocity triangles
Nondimensional Parameters
Dimensional Analysis of Turbomachines
Returning to Head Coefficient Also "Head" is P/ (Previously shown), P2 can be a pressure coefficient. Incompressible form: Compressible form: Remembering compressor efficiency definitions, for incompressible flow: