Gas Turbine Off Design

Question 1

Describe stages of a gas turbine

Answer 1

a-1: intake nozzle (isentropic compression)
1-2: compressor (isentropic compression)
2-3: combustion (nearly constant pressure)
3-4: turbine (isentropic expansion)
4-5: nozzle (isentropic expansion)

Question 2

What is the difference between aircraft has turbine and shaft power cycles

Answer 2

• aircraft has turbine useful power output is produced fully/in part due to expansion in nozzle
• also need to consider effect of forward speed and altutude

Question 3

Requirements for gas turbine intake

Answer 3

• compress airflow

- ensure uniform pressure and velocity at compressor inlet to avoid surge and flame out and blade vibrations and damage

Question 4

How does intake differ for cruise and takeoff/low speeds

Answer 4

Cruise:
Intake acts as diffuser (decreases airflow velocity)

Takeoff:
Intake acts as nozzle (increases airflow velocity)

Question 5

What is constant in adiabatic system

Answer 5

To = constant

Question 6

How do you know if nozzle is choked or not

Answer 6

Po4/Pa > Po4/Pc : nozzle choked

Question 7

What are mechanical losses

Answer 7

Occur during power transmission from turbine to the compressor due to bearing friction and windage

Question 8

Requirements for gas turbine combustion chamber

Answer 8

• supply turbine with smooth heated has flow
• minimum pressure loss, max heat release
• burn large fuel quantities
• coping with large volume of air
• coping with high speed air
• coping with high temperatures

Question 9

Describe difference between gas turbine and IC (internal combustion) engine

Answer 9

• for gas turbine, processes of compression, combustion and expansion do not occur in a single component as they do in a IC engine

Question 10

Why is off design important

Answer 10

• turbine and compressors rotate at same speed and a great condition for one might be a terrible condition for the other
• when components are linked, the operating range of each component will be considerably reduced
• because of this, gas turbine flexibility is very poor compared to IC engine

Question 11

What is a constant-speed propeller

Answer 11

• variable pitch aircraft propeller that automatically changes blade pitch in order to maintain chosen rotational speed
• this allows gas turbine to function at optimum operating conditions

Question 12

Describe mechanics behind a constant-speed propeller

Answer 12

1. High-pressure oil enters through central shaft
2. Hydraulic piston connected to each blade by piston rod
3. Oil pressure moves piston
4. The pistons cause blades to twist towards low-pitch position
5. Nitrogen pressure or springs oppose oil pressure
6. Counterweights also help move blade towards high-pitch position

Question 13

When are cycles often combined

Answer 13

Maritime ships

- as gas turbines cannot operate in some conditions, need to be paired with diesel engines sometimes

Question 14

Draw combined gas or gas system
and
Draw combined diesel and gas system

Answer 14

pic1

pic2

Question 15

What are the 2 ways to avoid surge during start-up

Answer 15

1. Bleed off air from the compressor

2. Increase the nozzle area

Question 16

How to improve power on hot days

Answer 16

• water sprays injected into the gas turbine inlet to cool down intake air
• surplus electricity can be used to produce ice, which is then applied to cool inlet air
• this method is better than having an always functioning system as not always required

Question 17

What conditions are used in non-dimensional numbers

Answer 17

stagnation conditions

Question 18

What is an equilibrium running line

Answer 18

• the equilibrium running points for a series of speeds can be plotted on the compressor characteristics
• joined up, this makes an equilibrium running line

Question 19

What is the solution to equilibrium point matching

i.e. finding operating pointing on the characteristics of each component when engine is running at steady speed or in equilibrium

Answer 19

1. Establish turbine inlet temp for each point on compressor characteristics (using flow compatibility eqn., compressor temp rise eqn. and turbine temp drop eqn.)
2. Then obtain power output from net power output eqn.
3. Then plot turbine inlet temp vs. net power output to find required temp.

Question 20

What are the steps for gas generator calculation

Answer 20

1. Select point on compressor characteristics and use to find ΔT012/T01
2. Guess value for P03/P04 and obtain mSqrt(T03)/P03
3. Use value T03/T01 to calculate N/Sqrt(T03)
4. Use turbine characteristics to get ηt
5. Use ΔT034/T03 eqn. to find T03/T01
6. See if T03/T01 values agree
7. Guess new value for P03/P04 an repeat until T03/T01 values are the same

Question 21

How does equilibrium line for nozzle relate to flight speed

Answer 21

Independent when nozzle is choked

• when nozzle is choked the non-dimensional flow will be independent of pressure ratio and hence forward speed
• turbine will be unchanged due to flow compatibility
• hence when choked, equilibrium line is independent of flight speed

Question 22

What is the variation of thrust with engine rotational speed

Answer 22

• thrust is a function of flight speed; direct dependence as momentum drag increases, indirect dependence ram pressure increase
• at low speeds drag dominates thrust reduces as speed increases
• at high speeds ram pressure dominates

Question 23

What happens when ambient temperature increases

Answer 23

1. Turbine stresses is f(N)
2. T03 increases with N, turbine creep limited
3. Ambient temp, increases for max N move down the running line, reduced m and pressure ratio (hence reduced thrust)
4. T03/T01 also reduced so does T01
5. Therefore need to reduce N to keep within allowable T03 and hence thrust further reduced