1.1 Introduction to Propulsion

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Gas Turbine and Jet Propulsion

Xin ZhangRoom: 2577C. Telephone: (852) 3469-2220Email: [email protected]

1.1. Introduction to propulsion

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Why propulsion Aircraft is a feasible alternative for long distance travel (>3 hrs)

Aircraft is a good option for high-speed travel (>300km/h)

What is propulsion To push forward or drive an object forward

Thrust is generated through the application of Newton's ? law of action and reaction.

In an aircraft engine a working fluid is accelerated and the reaction to this acceleration produces a thrust force on the engine

Net force on CV = Momentum flux out Momentum flux in

Thrust is produced if there is an increase in momentum of the working fluid

Momentum

Control Volume (CV)

Momentum in Momentum out

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The environmental impact of air transport

Environmental noise a local nuisance near airports*, a major constraint on the growth of air transport

NOx emissions a threat to local air quality and public health

Carbon emissions (fuel burn) a contributor to global warming .. currently 3% of global carbon emissions + contrails

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* En route noise may be an issue with new propulsion systems

WHO guidelines for community noise

Noise-induced hearing impairment

Interference with speech communication

Sleep disturbance

Cardiovascular and physiological effects

Mental health effects

effects of noise on performance

effects of noise on residential behaviour and annoyance

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Newtons laws of motion

First law

Second law

Third law

When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force.

The vector sum of the forces on an object is equal to the mass m of that object multiplied by the acceleration vector of the object

When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body

= 0 = 0= =

Lateral and longitudinal accelerations

Longitudinal acceleration:

Lateral acceleration:

sV

tVa

=

=211

RVa

22=

R

V1

V2

s

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Classification of gas turbine engines

There are four main types of aerospace propulsive devices:

Piston engine Gas turbine engines Airbreathing Ramjets Rockets

The list is given in ascending order of vehicle flight speed.

Method of thrust generation

Engine type Working fluid

Cycle Thrust generation

Piston Air Intermittent Propeller, rotor

Gas turbine Air Continuous Jet, fan, propeller, rotor

Ramjet Air Continuous Jet

Rocket Various Continuous Jet

In addition to thermodynamic cycle, the method of thrust generation can differ for each of these engines

Newtons 2nd law of motion

Force = rate of change of momentum

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Variants and hybrids

Obvious examples Turbine powered

Turbojet Turbofan Turboprop and turboshaft Propfan

Non-continuous combustion Less obvious hybrids

Turboramjet

Turborocket

Ram-rocket

Liquid air cycle engines (LACE)

Q. What dictates choice of engine types?

A1: Vehicle mission vs. engine capability

The vehicle must operate within a flight corridor which is governed by Lift limit (loss of lift at high altitude and/or low speed) Aerodynamic force limit (excessive structural loading at high speed and

low altitude) Temperature limit (excessive aerodynamic heating at high speed)

Engine capability is determined by

Ability to provide sufficient thrust Ability to operate with adequate fuel economy, as indicated by, for

example, the engines specific impulse:

= where FN is the thrust, is the total mass flowrate of stored propellant

(fuel+oxidiser), and is the Earths gravitational constant at sea level.

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Q. What dictates choice of engine types?

A2: Economics Development cost Purchase cost Operation cost

A3: Reliability and safety Mean time between overhaul (MTBO) Mean time between failure (MTBF)

A4: Technology and legislation Aerothermodynamics Materials Emissions Others (fuels, lubrication, cooling).

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Rocket propulsion

O2

Fuel

Combustionchamber

Propelling nozzle

Rocket propulsion

O2

Fuel

Combustionchamber

Propelling nozzle

Working fluid is the exhaust products (no momentum in)

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Propeller propulsion

Air

Propeller propulsion

Air

Working fluid is Air (momentum inflow from incoming air)

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Lorins concept for jet propulsion (1913)

Air Fuel Exhaust

Combustionchamber Propelling nozzle


Air Fuel Exhaust


Working fluid is Air & Exhaust gases

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Air Fuel Exhaust


No rotating parts ~ Ramjet principle

Requires high speed flight velocity

Vin

Combustionchamber

Compressor

Exhaust

Turbine Propelling nozzle

Vjet

Whittles turbo-jet 1930(Co-inventor Ohain 1936)

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Vin

Combustionchamber

Compressor

Exhaust


Vjet


The compressor and the turbine are turbomachines: Machines which transfer energy from or to working fluid through the fluid-dynamic action of rotating blade-rows

Vin

Combustionchamber

Compressor

Exhaust


Thrust ~ mass flow x (Vjet Vin)

Vjet


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Gas turbine cycle

P3

P2

2

4

5

3

Entropy

Turbine entry temperature

2 3 45

Work input to actual compressor is greater than work input for the ideal one!Actual turbine produces less work than the reversible, adiabatic one!

=

Gas turbine cycle

P3

P2

Entropy


= WorkoutHeatin = Heatin HeatoutHeatin

Heat in

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Gas turbine cycle

P3

P2

Entropy



Heat out

Gas turbine cycle

P3

P2

Entropy



Useful work+Lost work

Lost work

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Gas turbine cycle

P3

P2

Entropy




Lost work

Gas turbine cycle

Entropy




Lost work

As turbine entry temperature (TET) and pressure ratio rises useful work to heat input rises

Higher efficiency

Higher specific work ~ thrust : weight

Pressure ratio

Documents

1.1 Introduction to Propulsion