15.04 - Compressors. Flashcards
(22 cards)
What is meant by a stage in a compressor stage
A rotor and then a stator.
On a rotor and a stator in the compression stage where does static pressure increase.
On both.
On a compressor stage when does dynamic pressure and velocity increase/decrease.
Velocity and dynamic pressure both increase on the rotor and decrease on a stator.
Centrifugal compressors.
Consist of an impeller (rotor) and a diffuser (stator), and a manifold.
Has a high pressure ride per stage of 8:1.
Limited to 2 stages due to efficiency concerns.
Normally used in turboprop and turboshaft.
Axial compressors
Normally found on turbofan engines.
Compresses air in its original direction of flow so avoids energy loss caused by turns.
Converging the casing helps maintain steady axial velocity.
Compression ratio is between 1:1 and 1:2
Pros and cons of axial compressors
Pros
- high peak efficiencies.
- small frontal area
- straight through flow allows ram air.
- increase pressure by increasing number of stages with neglible losses.
Cons
- high weight.
- high cost
- hard to manufacture.
- high starting power requirements
- good efficiencies only over a narrow rotational speed.
Pros and cons of centrifugal compressors.
Pros
- high reassure rise per stage
- efficient over a wide rotational speed range.
- the simplicity of manufacture and low cost.
- low weight.
- low starting power requirements.
Cons
Large frontal are for given airflow.
Losses in turns between stages.
Which rotating assembly is the only one that does not need to be balanced by the manufacturer only.
Fan blades.
What 3 parameters do centrifugal forces depend on
The rotor speed.
The blade mass
The distance of the centre of gravity from the rotational axis.
What is meant by moment weight.
The product of the blade mass and the distance of the centre of gravity from the rotational axis.
It is worked out by a special blade scale.
Units for fan blade moment weights.
Gram inches.
Cm.g
Ounceinches.
Where will you find the moment weight of a fan blade
On the root.
Computerised fan blade distribution.
Computer works out the best position for each blade on a fan disc
Fan blade number 1 is normally marked by an index or offset hole. They are often numbered in the direction of rotation.
Changing a a blade.
Best way to= replace blade with a new one with similar moment weight.
Other way is to replace the pair with similar moment weights.
What two factors change angle of attack.
Rotor speed and inlet speed
If rotor speed increases, AOA increases.
If inlet speed increases AOA decreases.
Vice versa.
Compressor surge.
A severe form of stall
>less airflow creates a stall-> stall causes a blockage in airflow, and a stronger stall in subsequent stages. ->air flows in reverse.
Operational reasons for a stall.
Engine is too far below design speed.
Incorrect acceleration or deceleration.
Environmental factors that can cause compressor stall
Dents or ice
Dirt
Damaged or broken combusted
Damaged or Brocken turbine.
3 ways of avoiding stall and surge.
Use compressor bleed valves
Dual or triple spool
Variable compressor stator vanes.
How do bleed valves help prevent stall and surge.
Open at low engine speeds so that excessive airflow from fwd stages can escape overboard.
Variable inlet Guide vanes.
Very complicated control mechanism
Most effective method of preventing stall and surge.
Forward stages of the high pressure compressor
Optimum AOA maintained for every airspeed.
What is the compression ratio for each stage of an axial compressor
1.3:1
