What are 3 design conditions for the velocity vector diagram?
- incoming flow tangential to the camber line
in the relative coordinate system - relative system rotates with u=f(r) in radial direction
- outcoming flow tangential to the camber line in the relative coordinate system
Which coordinate systems need to be considered?
- Fixed coordinate system for absolute flow
- Rotating coordinate system for relative flow
What is the goal of having a meanline?
The goal is to have a general coordinate system which can be used for all kinds of flow channel geometries
- the m-coordinate represents the main flow direction of the local mass flow
- the u-coordinate represents the rotational movement or the swirl of the flow
- the n-coordinate only describes secondary and tertiary flow phenomena
Why do we need a stator in a turbine stage?
- Flow acceleration results in a
pressure drop due to nozzle principle
gz+∫dp/ρ(p)+c^2/2 = const - Increase of swirl to increase potential
work extraction in the rotor stage - Preparation of the inlet flow for the
rotor stage
Why do we need a stator in a compressor stage?
- Flow deceleration results in a pressure rise
due to diffusor principle
gz+∫dp/ρ(p)+c^2/2 = const - Reduction of angular flow momentum to
reduce loses due to shear movement in
boundary layers - Preparation of the inlet flow for the next stage
How are forces on the airfoil generated?
- Pressure and shear forces result from the flow around the airfoil
- These surface forces result in a resultant force which can be
divided in axis parallel and rotational parallel forces or camber normal and camber parallel forces - Forces from nS airfoils result in a
torque on the driveshaft
What is the relation between twist and distance to propeller core?
The closer to the propeller core, the more twisted the rotor blades are
Why is an airfoil generally twisted?
- Rotational speed increases with increasing radius: u = ω ∗ r
- Relative incoming flow less steep
towards tip - Camber line is twisted towards
machine axis closer to the hub - Magnitude of the relative speed
increases towards the tip of the airfoil
What are the differences between compressor and turbine stage?
- Stator after rotor (C) vs. Rotor after stator (T)
- Indexing 1,2,3 (C) vs. Indexing 0,1,2 (T)
- Pressure side is leading (C) vs. Suction side is leading (T)
- Resulting force vector in negative Φ-direction (C) vs. Resulting force vector in positive Φ-direction (T)
