MAE 4261 AIRBREATHING ENGINES Velocity Triangles Example April

MAE 4261: AIR-BREATHING ENGINES Velocity Triangles Example April 12, 2012 Mechanical and Aerospace Engineering Department Florida Institute of Technology D. R. Kirk

EXAMPLE: SEE SECTION 8. 2 FROM H&P a b c • Draw velocity triangles assuming that wr = 2 times the axial velocity w (w = constant)

VELOCITY TRIANGLES AT b w Start by drawing the axial velocity to some scale (10 units here)

VELOCITY TRIANGLES AT b w bb=75º Draw the absolute velocity vector Vb

VELOCITY TRIANGLES AT b w Vb bb=75º vqb Draw vq in direction of rotation from the axis to absolute velocity vector

VELOCITY TRIANGLES AT b w Vb bb=75º wr vqb Add the rotational velocity (wr) and remember Vabs=Vrel+Vcs

VELOCITY TRIANGLES AT b w Vb bb=75º wr vqb Draw in the velocity to the rotor as seen from the rotating frame

VELOCITY TRIANGLES AT b relative frame inlet velocity to rotor w Vb bb=75º Stationary frame inlet velocity to rotor wr vqb

VELOCITY TRIANGLES AT c wr Either start with the fixed axial velocity or fixed rotational speed

VELOCITY TRIANGLES AT c bc’=55º w wr Add the velocity from the rotor blades in the relative frame

VELOCITY TRIANGLES AT c bc’=55º w wr vqc Add the velocity exiting the rotor in the absolute frame

VELOCITY TRIANGLES AT c stationary frame exit velocity of rotor relative frame exit velocity of rotor bc’=55º w wr vqc Again, draw vq in the direction of rotation to the absolute velocity vector

COMPOSITE TRIANGLE Fixed or ‘metal’ blade angles bc’=55º w bb=75º w wr vqb vqc To draw the composite velocity triangle, overlay the rotational velocity

QUESTIONS • Is this a compressor or a turbine? How can you tell? • On which blade row(s) is there a torque applied? Why? • Describe in words the energy exchange process in each of the two blade rows

QUESTIONS • Is this a compressor or a turbine? – This is a turbine. The stationary frame tangential velocity (vq) in the direction of rotor motion is reduced across the moving blade row • On which blade row(s) is there a torque applied? Why? – Torque is applied to both blade rows since there is a change in angular momentum across each of them. However, power is extracted only from the moving blades. • Describe in words the energy exchange process in each of the two blade rows – In the first blade row, fluid internal energy is converted to swirling kinetic energy by accelerating the flow through a nozzle. No additional energy is added or removed from the flow. – In the second blade row, swirling kinetic energy is extracted from the flow reducing the overall level of energy in the flow and transferring it to the spinning rotor blades.

ADDITIONAL QUESTION • So far, we have looked at trailing edge angles of the blades (bb and bc’) • Why do we care about exit velocities from stator in the relative frame? Why do we even draw this on velocity triangles? relative frame inlet velocity to rotor w Vb bb=75º Why draw this? Stationary frame inlet velocity to rotor wr vqb

ADDITIONAL QUESTION Information about how to shape leading edge of rotor blade Doesn’t come into ideal Euler equation but obviously important for aerodynamic Purposes (rotor relative inflow angle)