Mechanical Design Flashcards
1
Q
What are 3 challenges of mechanical design?
A
- Dynamics of multiple rotor systems with large gyroscopic couples
- Cold parts are subjected and based on dynamic problems
- Hot parts are limited by fatigue and creep as well as damage tolerance philosophy
2
Q
What does mechanical design require?
A
- Determination of physical parameters (temperatures, stress, strain)
- Deep understanding of Material properties and failure conditions (very low safety factor
margins are required) - Interaction and collaboration with other disciplines
3
Q
What is an example of 2 colliding goals of mechanical design?
A
- Coaxial Rotors require the bore of the HP discs to allow the passing of the LP shaft – Stress increases with the bore radius = LP (N1) shaft diameter as low as possible
- Critical speeds should be above the range of operational speeds = LP shaft diameter should
be as high as possible
4
Q
What are the mechanical loads acting on rotor blades?
A
- Rotating Mass at high speeds -> Centrifugal Loads
- Gas loads
- Vibration phenomena are the main cause of failure (in compressors)
- Natural frequencies (i.e., Critical speeds)
- Mode-shapes and response levels
- Damping levels
- Stability (i.e., vulnerability to flutter phenomena)
5
Q
What are the 5 adverse conditions for rotor blades?
A
- Erosion prone
- Damage from foreign objects
- High Temperatures
- Working over melting temperatures
- Lifetime limited Components (Creep, fatigue)
6
Q
What are the 5 blade cooling methods?
A
- Film and convection cooling
- Transpiration cooling
- Multiple small hole transpiration cooling
- Water cooling
- Strut insert blade
7
Q
What does “shrouded” / “shroudless” mean?
A
- Shrouded: “Eingerahmt”, rotor blades have a frame around them
- Shroudless: No frame
8
Q
How did rotor blades improve over time?
A
- Sweep was used for aerodynamic improvements (“geschwungen”)
- On rotors, hardly any dihedral can be used due to structural reasons
- No need for supports between blades, as materials now stronger
9
Q
What are 7 properties of fixed stator vanes?
A
- Featuring either a free inner tip or shroud
- Single aerofoils or segments of aerofoils
- Mostly featuring T-shape root for fixing in
the outer casing - Fitted in the circumferential direction
- Dedicated vanes for nominal size, under-size,
locking and boroscope inspection - Can be rotatable about the aerofoil axis
- In most design solutions
supported at the hub and the casing
10
Q
What defines an outlet guide vane?
A
- high flow turning, many aerofoils, potentially 3D stacking,
tightly spaced, conventional platform design difficult,
often designed as an integral part with a diffuser - integral ring, cast in one piece, all aerofoils,
some post-casting machining, high roughness, low precision
(high manufacturing and build tolerances) … usually acceptable on this aerofoil
11
Q
What is the difference between HPT vanes and LPT vanes?
A
HPT needs cooling (usually film cooling)
12
Q
What are 2 axial roots?
A
- Extended root, with full formation of a platform, Blades with extended roots are held in place using retaining plates
- Non-Extended root, featuring no or a partial platform, the resulting gap is filled with segments of the
disc rim to give complete contour
13
Q
When is an axial root used and what needs to be considered?
A
- When single-blade weight is high
- Need to be combined with stators
having inner shrouds - Fitting is in the axial direction into the
the rim of the disc
14
Q
What are circumferential roots and when are they used?
A
- When single-blade weight is low
- When blade count is high and some
sealing is required at the root to
prevent reverse leakage - Can be combined with stators having
either a free inner tip or an inner
shroud - Fitting in a circumferential direction
into the rim of the disc - Dedicated blades for nominal size,
under-size and locking - Platforms are primarily rectangular,
or sheared or angled if required
15
Q
What are the 6 load sources in a rotor disc?
A
- Centrifugal body force of disc material
- Centrifugal load produced by the blades and their attachments to the disc
- Thermo-mechanical stresses produced by temperature gradients between bore and rim
- Shear stresses produced by torque transmission from the turbine to the compressor
- Bending stresses produced by aerodynamic loads on the blades
- Dynamical stresses of vibratory origin
