15.4 compressors

Question 1

what is primary function of compressor

Answer 1

supply sufficient quantity of air to satisfy the requirements of the combustion chamber

Question 2

what is the secondary function of the compressor

Answer 2

supply air to bleed air systems

Question 3

what does the igv do

Answer 3

inlet guide vanes guide the air into the compressor - improve the airflow into the first compressor stage and they can be fixed or variable

Question 4

what is always the first part of the compressor (rotor or stator)

Answer 4

rotor

Question 5

what are the two main types of compressors used in gas turbine engines

Answer 5

centrifugal or axial

Question 6

what is the pressure rise per stage in a centrifugal compressor

Answer 6

8:1

Question 7

why are centrifugal compressors limited to two stages

Answer 7

due to efficiency concerns

Question 8

how does axial flow work

Answer 8

compresses air while the air continues in its original direction of flow

multiple stages of blades

Question 9

benefit of axial flow

Answer 9

avoids energy losses caused by turns

Question 10

where are axial flow compressors normally found

Answer 10

turbofan engines

Question 11

where are centrifugal compressors usually found

Answer 11

turboshaft and turboprop

Question 12

why is there a gradual decrease in the diameter of axial compressor blades

Answer 12

to maintain a steady axial velocity

Question 13

what is a stage referring to in a compressor

Answer 13

rotor-stator pair

Question 14

centrifugal flow compressor pros

Answer 14

pros
high pressure rise per stage
efficiency over a wide rotational speed range
simple to manufacture/cheap
low weight
low starting power requirements

Question 15

centrifugal compressor cons

Answer 15

large frontal area
losses in turns between stages

Question 16

axial flow compressor pro

Answer 16

high peak efficiencies
small frontal area
straight through flow
high ram efficiency
increased pressure rise by increasing number of stages

Question 17

axial flow cons

Answer 17

good efficiencies only at a narrow speed range
difficult to manufacture
high weight
high starting power requirements

Question 18

what does the centrifugal forces on engine depend on

Answer 18

rotor speed
blade mass
the distance of centre of gravity of the fan blade from the rotational axis

Question 19

what is the moment weight

Answer 19

product of the blade mass and the distance of the centre of gravity from the rotational axis

Question 20

how is moment weight calculated

Answer 20

special blade scale

Question 21

how does moment weight change

Answer 21

blade mass and the position of centre of gravity of the fan blade

Question 22

where can the moment weight be seen

Answer 22

blade root

Question 23

where are balance weights installed

Answer 23

on the fan disc o in the spinner cone

Question 24

what is a method for changing a fan blade

Answer 24

change a pair of blades with similar moment weights - even if only one is damaged

Question 25

what is the one thing in common between axial flow and centrifugal flow compressors

Answer 25

they are very sensitive to changes in airflow

Question 26

what is compressor stall

Answer 26

when there is airflow separation within the compressor - airflow slows down, stops or reverses direction

Question 27

how does compressor stall occur

Answer 27

when angle of attack increases when inlet air velocity decreases - causing airflow to separate from the compressor aerofoils

Question 28

what is compressor surge

Answer 28

a very severe form of compressor stall

Question 29

can compressor surge form in all compressors

Answer 29

yes

Question 30

what is the reason for stall

Answer 30

damaged engine components dirty compressor blades dents or ice on engine inlet engine below designed speed incorrect acceleration or deceleration

Question 31

how can you prevent stall

Answer 31

compressor bleed valves use of dual or triple spool rotors use variable compressor stator vanes

Question 32

how do bleed valves work to prevent stall

Answer 32

they open at low engine speeds so that the excessive airflow can escape overboard

Question 33

how do dual or triple spool rotors prevent stall

Answer 33

each compressor rotor is driven by a separate turbine

Question 34

what is the most effective way to prevent compressor stall

Answer 34

variable stator vanes

Question 35

what is the compressor ratio of an axial compressor

Answer 35

1.3:1

Question 36

(1.3:1) if incoming air is 10 psi, what is the psi of the air leaving

Answer 36

13psi

Question 37

how are fan blades numbered

Answer 37

generally in the direction of movement

Question 38

how many stages are centrifugal compressors limited to

Answer 38

two