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EXAMPLE: SEE SECTION 8.2 FROM H&P
b c Draw velocity triangles assuming that wr = 2 times the axial velocity w (w = constant)
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VELOCITY TRIANGLES AT b
w Start by drawing the axial velocity to some scale (10 units here)
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VELOCITY TRIANGLES AT b
Vb w bb=75º Draw the absolute velocity vector
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VELOCITY TRIANGLES AT b
Vb w bb=75º vqb Draw vq in direction of rotation from the axis to absolute velocity vector
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VELOCITY TRIANGLES AT b
Vb w bb=75º wr vqb Add the rotational velocity (wr) and remember Vabs=Vrel+Vcs
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VELOCITY TRIANGLES AT b
Vb w bb=75º wr vqb Draw in the velocity to the rotor as seen from the rotating frame
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VELOCITY TRIANGLES AT b
relative frame inlet velocity to rotor Stationary frame inlet velocity to rotor Vb w bb=75º wr vqb
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VELOCITY TRIANGLES AT c
wr Either start with the fixed axial velocity or fixed rotational speed
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VELOCITY TRIANGLES AT c
bc’=55º w wr Add the velocity from the rotor blades in the relative frame
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VELOCITY TRIANGLES AT c
bc’=55º w wr vqc Add the velocity exiting the rotor in the absolute frame
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VELOCITY TRIANGLES AT c
stationary frame exit velocity of rotor bc’=55º relative frame exit velocity of rotor w wr vqc Again, draw vq in the direction of rotation to the absolute velocity vector
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bc’=55º w bb=75º w wr vqb vqc 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
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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
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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.
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Vb w bb=75º wr vqb ADDITIONAL QUESTION relative frame inlet
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 Stationary frame inlet velocity to rotor Vb w bb=75º Why draw this? wr vqb
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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)
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