ARTICLE
Bangladesh Energy Sector: Anatomy & Roadma
Khondkar Abdus Saleque

Introduction

The gas turbine engine as a prime mover is used in many different applications to provide motive power at sea, on land and air. The main principals of the engine however remain the same in each type of use. Air is taken into the engine and it is accelerated and compressed. It is then burnt with fuel (usually kerosene for aircraft engines) and passed into a turbine where its kinetic energy is either converted completely to power the turbine shaft for ground or sea based units or it is only partially used to drive the compressors in aircraft engine. The remaining energy is used to impart thrust on the engine when the gas is expelled through the rear jets. This process provides high levels of engine efficiency and with no reciprocating parts the engine has a high degree of reliability. The engines are complicated and expensive to manufacture, hence expensive. A dissected gas turbine aircraft engine is shown below; showing its relevant components.

Typical Gas Turbine Aircraft Engine

These engines are highly efficient in converting fuel energy to safe motive power. It is the popular power source for various types of aircrafts, both for the air force (military) and civilian air transport. Now-a-days, propeller driven aircrafts are practically outdated; except as light aircrafts used in flying as a pastime or hobby. Large propeller aircrafts are turbo-props, where the propeller is powered by a gas turbine engine. The jet engine is based on Newton 's third law and works by taking in and then accelerating a small mass of air. The forces used to accelerate this air through the jet engine produces an equal and opposite reaction on the engine, and the resultant thrust exerted on the engine. The thrust is proportional to the mass of air expelled by the engine and to the velocity change imparted to it. The main advantage of the gas turbine engine is its ability with a turbine driven compressor to produce this thrust at low velocities.

The turbo jet engine works by drawing air from the atmosphere, compressing it and heating it by burning with fuel and then using its momentum to force it out of the propelling nozzle at high speed. During the work cycle, the working combustion gases and air give up some of its energy to drive the turbine, which powers the compressor part of the engine.

Because of aerodynamic considerations, a pure jet engine is less efficient at aircraft speeds below 350m.p.h than a propeller type aircraft engine. The turbo-prop engine efficiency drops down sharply as the air speed increases beyond 400m.p.h. In contrast, the jet engine efficiency rises steadily with increase in forward velocity without dropping off, thus making the jet engine the logical and popular choice for all high speed long range aircrafts.

Working Principals

The jet engine works in a continuous cycle at more or less a fixed pressure. The temperature and volume of the air passing through the engine is considerably increased while the pressure remains nearly constant. This has a great advantage as the combustion chambers can be made of relatively light weight heat resistant materials. The work cycle in its simplest form can be represented by figure 1.1 below. Point 1 represents the air as it is taken into the engine. The air is compressed from 1-2. From 2-3 heat is added to the air by the burning of fuel at constant pressure. This considerably increases the volume of air. Pressure losses in the combustion chambers are represented by the drop from 2-3. From 3-4 the hot gasses after combustion, expand through the turbine and out of the nozzle back to the atmosphere. During this part of the cycle some of the energy is converted into mechanical power (for driving the compressor) and the rest of the energy is discharged into the atmosphere, providing the thrust.  

The three main types of aircraft engines work as follows:

1. The Turbo-prop type uses almost all of the energy from the turbine to drive the propeller and very little thrust is generated from the exhaust flow. Most of the available energy is converted in the turbine stages in the engine and is used to power the propeller, which drives the aircraft.

1. The ducted fan engine may have the fan mounted, either at the front or rear of the engine, and it is enclosed in a duct. The fan draws in and accelerates the cold air which gives a low velocity jet effect thus, providing higher efficiency for the engine.
2. The by-pass engine is a ducted fan engine where energy is taken from the working gas stream by a turbine which drives an axial flow type compressor. This low pressure compressor then passes the gases on to a high pressure compressor and a duct surrounding the engine. This gives a low velocity with a high propulsive efficiency.

Engine Compressors:

There are two main types of compressors; the centrifugal flow compressor and the axial flow compressor. The centrifugal compressor is more robust than the axial compressor, and it can be more easily manufactured, and hence it is cheaper. The axial compressor however consumes substantially more air than the centrifugal compressor for the same air intake area. The axial compressor can also be adapted for high pressure ratios more easily. The air flow is important in determining the amount of thrust that can be generated by an engine. The axial compressor engine can also generate more thrust than the centrifugal compressor engine. The high pressure ratio also leads to higher engine efficiencies due to the improved specific thrust and better specific fuel consumptions.

The centrifugal impeller is usually built from forged aluminium alloys. The inlet section is sometimes made of steel. The rotor and stator blades of an axial compressor can be manufactured from an aluminium alloy, alloy steels or from titanium alloys, depending on the engine location in the aircraft and the engine working temperatures. The rotor shaft and discs are usually made from alloy steel forgings with the compressor casings being made either of aluminium or magnesium alloys at the front and steel alloys at the rear and hotter end of the engine.

The combustion chamber brings together fuel with large amounts of air at high temperatures, maintaining more or less uniform pressure. The heat of combustion expands the gases generated and increases its velocity, producing a fairly uniform stream of hot combustion gases. The gas temperature to the turbine depends on engine speed and the combustion chamber design. This is the most important part of the engine. It must be capable to ensure stable and uniform combustion condition over a wide range of operating conditions for extended periods. It may not be an exaggeration to call it the heart of the engine!

Air entering the combustion chamber has higher velocities than the velocity of burning kerosene (called Aviation Turbine Fuel in airline terminology). The air stream is slowed down to allow efficient and complete burning at almost all engine operating conditions. Part of the air flow is diverted to the outer casing, and about 20 percent goes through vanes causing air recirculation in the combustion chamber. Further about 15 percent of the air enters through holes in the combustion chamber wall and reacts with the re-circulating air producing a low velocity area which allows the kerosene to burn effectively. This turbulence enables better mixing of the fuel and air. Combustion zone gases reach temperatures of around 1900 degree C which is too hot for the turbines. Balance of the air initially diverted is introduced to cool the combustion gases as well as the combustion chamber.

Typical Axial Flow Compressor Engine

Combustion Chambers

The combustion chamber burns large quantities of fuel and air at high temperatures while maintaining a near static pressure. The heat from the combustion must be released in a manner that the air is expanded and accelerated to give a uniform stream of heated fluid. The temperature rise in the combustion chamber is between 500 to 800 o C. The gas temperature required in the turbine varies according to engine speed so the combustion chamber must be capable of a stable and efficient combustion over a wide range of operating conditions that can be maintained for extended periods.

The fluid entering the chamber has a velocity far in excess of the speed of the burning kerosene so must be slowed considerably to allow a flame to burn during all engine conditions. As the air enters the chamber, some of it is diverted in to the outer casing while about 18% passes through “swirl” vanes that induce recirculation of the air in the chamber. Subsequently, approximately 10-15% of the air taken into the outer casing is reintroduced through holes in the combustion chambers inner wall. This interacts with the recirculation air and produces an area of low velocity allowing the kerosene to burn. This causes turbulence in the primary combustion zone that mixes the fuel and air. The air leaving this primary combustion zone is between 1800 and 2000 o C which is too hot to introduce into the turbine. The remaining 60-75% of the air that was originally diverted into the outer casing is then progressively reintroduced into the chamber to cool the exhaust gasses to a manageable temperature. The combustion chamber is cooled by the gasses that are diverted into its outer casing: shown in the sketch below.

A Can-annular Combustion Chamber

Turbines

The turbine converts the energy of the hot gases expelled from the combustion chambers to power the compressor and accessories in the engine. It also powers the propeller shaft in turbo-jet aircraft engines or main drive shafts in ground based turbines. The turbine is put under extremely high stresses with rotational speeds exceeding 230 m/s for efficient operation. The torque is produced by several stages in the turbine, each consisting of one row of stationary nozzle guide vanes and one row of moving blades. In resent development of this design the number of stages has been progressively increased to reduce the load on each individual stage. Most modern engines have a two shaft system driving high and low pressure compressors. The average blade speed in a stage is very important to the stage efficiency. The gas velocity through the nozzle guide vanes and turbine blades can be reduced as the blade speed increases, and the loss of pressure is proportional to the square of the gas speed. However the stress on the turbine disk also increases with the square of the gas speed and thus the final design must make a compromise between efficiency and stress on the turbine.

Energy is transferred from the gas flow to the turbine with a very high efficiency of approximately 90%. The expanded gas forces its way through the nozzles of the turbine and due to the convergent shape accelerated to near the speed of sound. Simultaneously it is given a spin in the direction of rotation of the blades. The impact of the gas on the blades turns the turbine and transfers the energy to power the compressor and turbine shaft. The torque generated is governed by the gas flow rate and the energy change of the gas between the inlet and outlet of the turbine blades. The spin imparted on the gas on entry is almost completely removed while passing through the turbine. Excessive residual spin causes reduced efficiency and vibration in the jet pipe. The losses in the turbine are mainly because of aerodynamic losses in the turbine blades and nozzle guide vanes. Further there are small losses in the exhaust system and also due gas leakage over the rotor blades.

Exhaust System

The exhaust system passes the discharge gases from the turbines to the atmosphere at high velocity which provides the desired engine thrust. The design of the exhaust system is of paramount importance to the performance of the engine as the velocity and pressure of the exhaust gases govern the thrust produced. The velocity of the gases entering the exhaust system ranges between 120 and 220 m/s. At these velocities the gases produce high frictional losses and thus the gas flow is decelerated by diffusion. This is achieved by increasing the jet pipe area between the exhaust cone and the outer wall. The usual velocity of the gas in this phase of the engine is approximately 170 m/s. Additional losses also occur due to residual spin in the gas stream. This is reduced by the exhaust unit supports that help to straighten out the flow turbulence. The exhaust gases finally leave the engine through the propelling nozzle which is convergent and increases the flow velocity to the speed of sound under most operating conditions. The pressure on exit is above that of surrounding atmospheric pressure and this pressure difference across the nozzle gives a “pressure thrust” that is effective over the nozzle exit area. This additional thrust is obtained from the momentum change of the gas stream. Some nozzles have a varying aperture that maximises the thrust available at both low and high r.p.m and temperatures without compromising the engine performance. The variation in nozzle area also allows low specific fuel consumption to be achieved in one part of the engine operating range.

Dissected View of a Basic Exhaust System

The gas turbine engines for aircrafts have come a long way from Sir Frank Whittle's invention of the basic jet engine for aircraft propulsion developed towards the end of World War II. However the Germans were the first to put a jet aircraft in the air. It was a fighter aircraft used against the Allied armed forces; but it was too little and too late, to change the course of the conflict. Modern day high flying comfortable large passenger jet aircrafts flying at high speeds and at high altitudes all across the continents is the result of this wartime development, that heralded the jet age of aircrafts.

Literature References:

The Jet Engine (Second edition) – Rolls-Royce Ltd.

Principals of Jet Engine Operations

Gas Turbine Overview

Hybrid Electric Vehicles: HEV Components - Gas Turbine Engine