Turbomachinery stage performance parameters Flashcards
What is the work coefficient/stage loading coefficient? When is it positive and when is it negative?
The ratio between the total enthalpy rise per stage to the square of the rotational speed.
Positive for turbines.
Negative for pumps/compressors/fans.
What is the range of the work coefficient for an aero-engine?
Between 0.8 to 2.8
What is the range of the work coefficient for a stationary gas turbine?
Between 0.9 to 2.1
Why does the work coefficient have to be used together with the flow coefficient?
Because the work coefficient cannot specify aerodynamic or BL loads on a blade row or inner/outer annulus walls.
For the same work coefficient and high or low Cx, the required flow deflection through any blade row could be high or low!
Because of which parameters do the flow coefficient vary?
It varies
- Between inlet and outlet of the rotor
- With the radius
What does a high or low flow coefficient indicate?
High:
- High flow throuch a stage relative to the blade velocity
Low:
- Low mass flow
- Increased density
- Low in first stage = even lower in last stage
- May cause stall in last stage(s)
What are the consequences of a too high or too low flow coefficient, compared to a flow coefficient that is equal the design conditions?
Equal design conditions: Angle beta is correct
Too high or too low:
- Beta will also be too high or too low
- Causes steep approach of the flow on the blad
- Can causes stall on top or bottom of blade
- Minimizes or stops the TM from doing work on/by the fluid
What are typical values of the flow coefficient for
- Compressor
- Jet engine turbines
- Industrial turbines
Compressor: 0.4-1.0
Jet turbine: 0.8-1.0
Industrial turbines: 0.4-0.9
What is the definition of the degree of reaction?
The ratio between the static enthalpy change to the stagnation enthalpy change
What is the definition of the degree of reaction for a simple velocity diagram where U1=U2?
The ratio of the mean tangential-velocity to the rotational velocity, deducted from 1.
What does it mean if the degree of reaction is 50%?
50% of the static enthalpy rise/drop occurs in the stator, and 50% in the rotor.
What does it mean for a turbine if the degree of reaction is 0%?
All of the enthalpy rise/drop occur in the stator. The energy stored in the fluid is transferred in the rotor by an impulse.
What does it mean for a turbine if the degree of reaction is 100%?
All enthalpy rise/drop occur in the rotor. The energy transfer is a reaction force cause by very high air velocity.
How are pure impulse blades characterized?
They have strong curvature
How are pure reaction blades characterized?
They are thick and have highly loaded BL with a tendency to stall.
