Theory

Question 1

Name the two ways a specified thrust force can be produced?

Answer 1

1. Small mass of gas and a large increase in velocity
2. Large mass of gas and a small increase in velocity

Question 2

What two assumptions can you make to simplify the thrust equation?

Answer 2

1. Mass fuel rate of fuel is approximately 0
2. Air and Exit pressure are equal

Question 3

What are advantages of thrust reversers?

Answer 3

• Reduces landing distance
• Improves ground handling in wet/icy conditions
• Reduce brake and tyre wear (especially in wet/icy conditions)
• Improves rejected take-off margins
• Military: Operate from shorter runways (increases operational flexibility)

Question 4

What are disadvantages of thrust reversers?

Answer 4

• Hot gases cannot impinge on wind
• Must be impossible to deploy in flight
• Hot gases cannot be re-ingested into the engine
• Need to minimise lift to improve braking performance

Question 5

What’s the difference between hot and cold stream thrust reversers?

Answer 5

Hot Thrust Reversers: Core/all flow reversed (turbojet/small turbofan)

Cold thrust Reversers: Bypass flow reversed (large turbofan engines)

Question 6

What’s the definition of compressibility?

Answer 6

The fractional change in volume of a fluid element per unit change in pressure

Question 7

From what Mach Number does Compressibility become important for high speed flows?

Answer 7

Greater than 0.3

Question 8

What’s the difference between heat, work and internal energy?

Answer 8

Heat is the form of energy that is transferred due
to a difference in temperature

Work is basically any form of energy transferred that isn’t due to a temperature difference

Internal energy is the energy stored in the form of
excited atoms (function of temperature only for an ideal gas)

Question 9

What’s the difference between enthalpy and entropy?

Answer 9

Enthalpy is the capacity to do work (function of temperature only for an ideal gas)

Entropy is a measure of the reversibility of a process (J/K)

Question 10

When is a flow reversible in terms of entropy?

Answer 10

When ∆s=0 when the temperature between two places are equal (Thot=Tcold)

Question 11

What’s the definition for a steady flow?

Answer 11

No properties within the control volume, at the boundaries of a control volume or heat and work interactions DON’T change with time

Question 12

What’s the definition for flow work?

Answer 12

Work that is required to push mass into or out of a control volume

Question 13

What are two reasonable assumptions for aero engines?

Answer 13

1. Adiabatic combustion chamber (Q=0)
2. No shaft power output (Wshaft =0)

Question 14

What’s the definition for Propulsive Efficiency?

Answer 14

Propulsive efficiency is a measure of how well an engine produces thrust power relative to the amount of kinetic energy it adds to the airflow.

Question 15

What’s the definition for Thermal Efficiency (cycle efficiency)?

Answer 15

Thermal efficiency is a measure of how well an engine adds kinetic energy to the airflow relative to the total energy input from the fuel.

Question 16

What’s the two main ways of improving thermal efficiency?

Answer 16

1. Increase overall pressure ratio
2. Increase turbine entry temperature

Question 17

Whats the the definition for overall efficiency?

Answer 17

Combination of propulsive and thermal efficiency

Question 18

What are the equations for thrust specific fuel consumption and specific thrust?

Answer 18

tsfc = Mf/F

specific thrust = F/Ma

Question 19

Whats true about disturbances in supersonic flows?

Answer 19

In supersonic flow disturbances cannot propagate upstream

Question 20

Explain what happens to Area, Pressure and Velocity in a nozzle at subsonic mach number.

Answer 20

dA < 0 area is decreasing
dV > 0 velocity is increasing
dp < 0 pressure is decreasing

Question 21

Explain what happens to Area, Pressure and Velocity in a Diffuser at subsonic mach number.

Answer 21

dA > 0 area is increasing
dV < 0 velocity is decreasing
dp > 0 pressure is increasing

Question 22

Explain what happens to Area, Pressure and Velocity in a nozzle at supersonic mach number.

Answer 22

dA > 0 area is increasing
dV > 0 velocity is increasing
dp < 0 pressure is decreasing

Question 23

Explain what happens to Area, Pressure and Velocity in a Diffuser at supersonic mach number.

Answer 23

dA < 0 area is decreasing
dV < 0 velocity is decreasing
dp > 0 pressure is increasing

Question 24

When can the mach number be 1 in a nozzle?

Answer 24

When dA=0 so when the duct walls are parallel.

The flow is with the minimum of supersonic, maximum of subsonic or at sonic velocity.

Question 25

How do oblique shocks come about?

Answer 25

Nozzle exit pressure is lower than for normal shock at exit.

A series of oblique shock waves occur outside the nozzle.

Question 26

What happens to under expanded flow through a con-di nozzle?

Answer 26

A series of oblique expansion waves outside the nozzle continue the expansion until the external pressure is
reached.

Question 27

Whats the definition for the degree of reaction?

Answer 27

Measure of the extent to which the rotor contributes to
the overall static enthalpy change in the stage

Question 28

What type of pressure gradient occurs in a compressor?

Answer 28

Adverse pressure gradient (p2>p1)

Question 29

Explain what happens if the degree of reaction is 0 in a compressor

Answer 29

• Pressure rise is entirely over stator (pressure drop if turbine)
• Pressure gradient through stator is high (sealing difficult)
• Boundary layer prone to separation

Question 30

Explain what happens if the degree of reaction is 0.5 in a compressor

Answer 30

• Pressure rise is split evenly between stator and rotor
• Pressure gradient is modest (sealing easy)
• Boundary layer likely to remain attached

Question 31

What happens across a stator in terms of work done and enthalpy?

Answer 31

No work done across a stator so ∆ho=0 (ho1=ho2)