Gas Turbine Engine Fundamentals

Fundamentals

  • After this lesson, you should be able to:
    • Distinguish between Potential & Kinetic Energy
    • Apply Newton’s Laws of Motion in GTE
    • Define the relationship between force, work, power, energy, velocity, acceleration
    • Understand the basic working principle of GTE
    • Differentiate between the four types of GTE

Energy

  • Energy is the ability to do work.
  • SI unit is Joules.
  • Forms of energy include electrical, chemical, heat, nuclear, and mechanical energies.
  • Energy cannot be destroyed; it can only be converted from one form to another.
  • Energy conversion is not 100% efficient; it will not convert completely into work.

Kinetic Energy and Potential Energy

  • Potential energies are due to gravity and the state of a body.
  • Kinetic Energy
  • Example: How to stop an aircraft traveling along a runway with 4MJ of kinetic energy?

Newton’s First Law of Motion

  • A body will remain in its state of rest, or if in motion, will continue to move at a constant velocity, unless acted upon by some external force.

Newton’s Second Law of Motion

  • The acceleration of an object is directly proportional to the force acting on it and is inversely proportional to its mass, taking place in the direction of the force.
  • F = ma

Newton’s Third Law of Motion

  • To every acting force, there will be an equal and opposite reacting force.

The Relationship Between Force, Work, Power, Energy, Velocity, and Acceleration

  • Velocity = \frac{Displacement}{Time}
  • Acceleration = \frac{(Final\ velocity – Initial\ Velocity)}{Time}
  • Force (Thrust) = mass \times acceleration
  • Work\ done = Force \times Distance
  • Power = \frac{Work}{Time}

What is Propulsion?

  • Propulsion is the action of driving or pushing forward.
  • Examples include Hero’s Aeolipile and Newton’s Steam Carriage.

Principles of Jet Propulsion (Balloon Analogy)

  • When the stem is closed, the force is balanced.
  • When the stem is open, the force is unbalanced.
  • Illustration of Newton’s 3rd law of motion.

Principles of Jet Propulsion (Balloon Analogy) with Burners

  • When burners are added, air temperature is raised, and air velocity increases
  • Thrust = F = m \frac{vj - v0}{t}

Principles of Jet Propulsion (Balloon Analogy) with Compressors

  • When a compressor is added, pressure and airflow are maintained.

Principles of Jet Propulsion (Balloon Analogy) with Turbine

  • A turbine is placed in the path of the heated air.
  • Some of the energy is used to drive the turbine, which in turn drives the compressor.
  • The remaining energy is used to expel the hot gases through the stem.

Thrust

  • To create the forward reaction, there must be an acting force, which is known as Thrust.
  • The gas turbine engine is designed to accelerate a stream of air to an exceptionally high velocity and to obtain useful thrust from the reaction.
  • The thrust obtained is proportional to the mass and the acceleration.
  • Force = mass * acceleration

Principles of Jet Propulsion - Thrust

  • The same amount of propulsive thrust can be obtained by either:
    • Accelerating a LARGE mass through a SMALL increase in velocity (turboprop).
    • Accelerating a SMALL mass through a LARGE increase in velocity (turbojet).

Types of Aircraft Turbine Engines

  • Non-Air Breathing Engines
    • Rocket
  • Air-Breathing Reaction Engines
    • Ramjet
    • Pulsejet
    • Gas Turbine Engine (GTE)
      • Turbojet, Turbofan, Turboprop, Turboshaft

Liquid-Fuel Rocket Engine

  • Metal tube filled with gunpowder or rapid-burning mixture of chemicals.
  • Fuel burns and is expelled out from the back of the tube, and the rocket is pushed forward.
  • Liquid oxygen in one tank and liquid fuel in the other tank.
  • Gas rushes out from the nozzle at the back, and thrust is produced.
  • High-velocity exhaust gases.

Ramjet Engine

  • At high supersonic speeds (M ≥ 2.5), sufficient compression of incoming air can be attained through shock formation at the inlet ram followed by the inlet diffuser.
  • Commonly used in military UAVs.
  • Contains no moving parts.
  • Must be assisted to attain a speed of more than 400km/h before it can be started.

Ramjet Engine - Operation

  • Air is compressed by means of shock formation at high aircraft speed.
  • Fuel is injected, mixed with air, and burned.
  • Heated gases are accelerated through the nozzle.

Pulse Jet Engine

  • Shutters open to allow air to enter.
  • Fuel is then mixed with the air and ignited.
  • As the heated gas expands and leaves the exhaust, pressure decrease causes the shutter to open again.

Types of GTE

  • Turbojet
  • Turbofan
  • Turboprop
  • Turboshaft

Turbojet Engine

  • Small frontal area
  • Ability to take advantage of high ram-pressure ratios
  • Derives its thrust by highly accelerating a small mass of air, all of which goes through the engine
  • Highest thrust-to-weight ratio
  • Low thrust specific fuel consumption (TSFC) at high airspeeds

Turbofan Engine

  • Two gas streams: cold bypass air and hot turbine discharged air.
  • Fan air accounts for 80% of the thrust.
  • Two types of turbofan:
    • Low bypass (Bypass ratio < 2:1)
    • High bypass (Bypass ratio > 4:1)

Turbofan Engine Components

  • Fan Blade
  • Spinner
  • Combustor
  • High-Pressure Compressor
  • Low-Pressure Compressor (Booster)
  • High-Pressure Turbine
  • Low-Pressure Turbine

Turbofan Engine Characteristics

  • Low TSFC at higher airspeeds up to Mach 1
  • Thrust-to-weight ratio falls between turbojet and turboprop
  • Ground clearance is less than turboprop but not as good as turbojet
  • Lower noise level

Turboprop Engine

  • Fitted with reduction gearbox to reduce the speed of the propeller to about 1000-2000 rpm.

Turboprop Engine Characteristics

  • Low TSFC at low speeds, but deteriorates rapidly as airspeed increases
  • Efficient reverse thrust
  • Complicated design and lower thrust-to-weight ratio than a turbojet
  • Large frontal area of propeller requires longer landing gears

Turboshaft Engine

  • Fitted with reduction gearbox to reduce the speed of the output shaft

Turboshaft Engine Characteristics

  • Similar to a turboprop but without a propeller
  • Delivers torque (shaft horsepower) through an output shaft
  • Uses almost all the exhaust energy to drive the output shaft
  • Commonly used in helicopters and auxiliary power units (APU)