Gas turbine fundamentels

ATA GAS TURBINE FUNDAMENTELS For training purposes only JET ENGINES FOR AIRCRAFT INTRODUCTION TO AIRCRAFT ENGINES For Training Purposes O n ly Controlled flight over long distances is only possible with a suitable aircraft engine From the aerodynamics lesson, you remember that the lift force which keeps an aircraft in the air, is only created when the aircraft moves through the atmosphere fast enough It is clear, that the main function of an aircraft engine is to create the necessary movement of the aircraft In addition, the aircraft engine also supplies hydraulic power, electric power and bleed air for the pneumatic system Page For Training Purposes O n ly introduction to aircraft engines cont There are two different types of aircraft engines, the piston type engines and gas turbine engines The first airplanes were powered by piston type engines that turned a propeller These engines are simple in design and more fuel efficient than gas turbine engines but, piston type engines have some big disadvantages compared with gas turbine engines The performance of piston engines decreases at higher altitudes Therefore, piston type engines are only used on very small aircraft Figure Advantages/Disadvantages of Piston Type Engines Page For Training Purposes O n ly GAS TURBINE ENGINES FOR AIRCRAFT Gas turbine engines however, can operate at very high altitudes They easily provide thrust, torque and bleed air and they let aircraft fly at high speeds There are different types of gas turbine engines on aircraft These are: S turbojet engines S turbofan engines S turboprop engines S and auxiliary power units The turbofan engine is usually used on modern aircraft This engine is better because it makes high aircraft speeds possible with good engine efficiency The turbofan engine was developed from the turbojet engine Turbojet engines were the first type of gas turbine engines used on aircraft These engines gives very high aircraft speeds but these engines are very loud because of the extremely high exhaust gas speeds They also need too much fuel Another type of engine which was developed from the turbojet is the turboprop engine Turboprop engines are specially designed to produce shaft horsepower only, which is used to drive a propeller This engine type is usually installed on small commuter aircraft It is a good compromise between achievable aircraft speed and fuel efficiency Another type of gas turbine engine that you will find on modern aircraft is the ’auxiliary power unit’ This small gas turbine engine is usually called ’APU It is used to supply the aircraft with electric and pneumatic power if engines are not available With the APU, the aircraft is independent of airport equipment Page For Training Purposes O n ly Turbo Jet Turbo Fan Auxiliary Power Unit Turbo Prop Figure Types of Gas Turbine Engines Page PRINCIPLES OF JET PROPULSION For Training Purposes O n ly All aircraft engines work in the same way They create a propulsion force which moves the aircraft If you hold a water hose which is spraying water, you can feel that the hose pushes in the opposite direction of the water which is spraying out of it Jet propulsion, is the propelling force which is generated in the opposite direction to the flow of mass through the jet nozzle An engine which uses jet propulsion, is called a reaction engine These engines use Newton’s laws of motion which state that for every force which acts on a body there is an opposite and equal reaction Page For Training Purposes O n ly Figure Jet Propulsion Principle Page THE THRUST FORCE For Training Purposes O n ly A force is always created, when a body of mass is accelerated This body can be any kind of matter such as fluids and gasses or, it can be a solid mass Figure Force Equation Page For Training Purposes O n ly the thrust force cont To accelerate air, the air pressure must be increased This can be done mechanically with a compressor or thermally by increasing the volume of air when a fuel / air mixture is burned or heated However, the combination of both methods guarantees acceptable thrust for aircraft engines In 1913 the French engineer Rene Lorin patented a jet propulsion engine This engine uses compressed air coming from a piston engine and burns it in a combustion chamber and then accelerates it through a jet nozzle You see that the idea of using jet propulsion to move an aircraft is simple but the application was difficult Until the late 1930s, there was no compressor which could supply a continuous and large enough airflow to produce a suitable thrust Figure Air Acceleration Methods Page For Training Purposes O n ly the thrust force cont Centrifugal flow compressors driven by turbines finally made propulsion engines for aircraft possible In 1937, Hans Joachim von Ohain built a gas turbine engine with a centrifugal flow compressor driven by a radial turbine and in 1941, Frank Whittle built his first engine which had a centrifugal flow compressor driven by an axial turbine Whittles’s and von Ohain’s engines became the base for all gas turbine engines Note that these engines were only possible after the development of materials that were heat resistant enough for continuous combustion CENTRIFUGAL FLOW COMPRESSOR Figure The first Gas Turbine Engines Page Page 120 For Training Purposes O n ly For Training Purposes O n ly turbine clearance control cont You know that all materials expand if they become warmer The amount that a material expands depends mainly on the temperature differences by which it is heated and it depends on the size of the material This is because the expansion is always a percentage of the original size The time taken for the material to expand depends mainly on the thickness of the material Thin materials are heated up much faster This means that thin materials expand much faster than thick materials At engine start high gas temperatures act on the turbine materials The turbine casing expands faster than the turbine rotor because of three reasons It is thinner than the rotor It is in contact with higher temperatures and it is also bigger in diameter than the rotor Page 121 For Training Purposes O n ly turbine clearance control cont However, when rotor speeds get faster the centrifugal forces on the rotor increase These centrifugal forces reduce the tip clearance because the rotor disk and blades expand further Note that the expansion of a material by centrifugal force is much bigger than expansion caused by heat This means that the tip clearance is much bigger on low engine speeds than high engine speeds The same kind of expansion also happens when the engine accelerates from low speeds to high speeds If the engine is shut down the diameters of the turbine rotor and of the casing decreases At engine deceleration or shut down the tip clearance changes as follows First the rotor shrinks faster than the turbine casing because of the decreasing centrifugal forces Afterwards the turbine casing shrinks faster because of the thinner casing materials Page 122 Page 123 For Training Purposes O n ly turbine clearance control cont Modern gas turbine engines have so called active clearance control systems These systems keep the tip clearance as small as possible in all operating conditions Cold fan air is normally used to cool the turbine casings On other engines compressor bleed air is used instead of fan air On some engines like the CFM56 5A the compressor rotor also has an active clearance control system to improve its efficiency FAN AIR For Training Purposes O n ly COMPRESSOR BLEED AIR Page 124 EXHAUST INTRODUCTION For Training Purposes O n ly In this segment you will look at the function of a gas turbine engine exhaust system The exhaust system guides the exhaust gases from the rear of the turbine into the atmosphere but the main task of the exhaust system depends on the type of gas turbine engine On a jet engine the main task of the exhaust system is to accelerate the airflow which comes from the turbine to efficiently make thrust In the turboprop engine the main task of the exhaust system is to safely guide the exhaust gases overboard The exhaust system does not make thrust, because most of the gas energy has been absorbed by the turbine for driving the propeller On the APU the exhaust system guides the exhaust gases overboard because all the gas energy will be absorbed by the turbine The APU exhaust system also reduces the noise of the exhaust gases by use of mufflers in the exhaust duct So in a jet engine, the exhaust system releases the gases to the atmosphere The exhaust gas flow leaves the engine in the necessary direction and with the optimum velocity to make efficient thrust Page 125 Page 126 For Training Purposes O n ly ENGINE EXHAUST COMPONENTS For Training Purposes O n ly On some jet engines the exhaust system has a tube named the exhaust duct or tail pipe This is a conical tube which is supported at its forward end by the turbine exhaust case The exhaust nozzle at the aft end of the exhaust duct accelerates the exhaust gases which leave the engine The exhaust cone guides the discharge flow and prevents excessive turbulence of the gases The cone also prevents reverse flow of the exhaust gases into the hub of the turbine rear stage Page 127 engine exhaust components cont On modern high bypass turbofan engines the two exhaust streams are usually exhausted separately The hot gas flow exhausts via the primary nozzle and the cold fan airflow exhausts via the secondary nozzle On modern long range aircraft the high bypass engines sometimes have a combined exhaust nozzle This system mixes hot and cold gas flow to reduce the velocity of the exhaust gases The common exhaust nozzle’s advantage is that it reduces the very high exhaust gas velocities of the hot gas flow by mixing of the two gas flows This gives a higher propulsion efficiency but the disadvantage of the common exhaust nozzle is that it increases the weight of the engine For Training Purposes O n ly Secondary Nozzle Primary Exhaust Nozzle Page 128 For Training Purposes O n ly engine exhaust components cont Some engines have an additional special exhaust gas mixer This mixer improves the mixing of the hot and cold gas flow to get higher propulsion efficiency But, as you would expect, these mixers also add weight to the engine Page 129 AIRFLOW IN THE EXHAUST NOZZLE For Training Purposes O n ly To achieve optimum thrust from a given mass the exhaust gases must expand completely in the exhaust nozzle This guarantees a laminar vortex free and axially orientated exhaust gas flow The exhaust airflow is caused by the pressure ratio between the turbine discharge and the ambient pressure The pressure ratio changes with the engine speed, so, increasing the engine speed increases the pressure ratio This results in a higher airflow through the exhaust nozzle Unfortunately, there is a limitation on this airflow behavior Increasing the pressure ratio eventually leads to a point where it is impossible to get more air through the smallest area of the nozzle Convergent exhaust nozzles reach maximum airflow at pressure ratios of approximately 2:1 With higher pressure ratios you not get higher airflow because the nozzle will become choked This choked condition means that in the smallest area of the nozzle the exhaust gas velocity equals the speed of sound Page 130 Page 131 For Training Purposes O n ly For Training Purposes O n ly airflow in the exhaust nozzle cont If we further increase the pressure ratio, the airflow would discharge at the exhaust nozzle with a static pressure which is greater than the ambient pressure This causes the exhaust jet to rapidly burst in all directions This burst of the exhaust jet is very inefficient because the remaining exhaust gas pressure cannot be converted to thrust Convergent divergent nozzles are used for pressure ratios of more than 2:1 These nozzles are also named C D nozzles They permit a further increase in the exhaust gas velocity and therefore result in a higher thrust C D nozzles are only necessary for aircraft that fly faster than the speed of sound They are very sensitive to RPM changes of the engine therefore they must be of a variable design Page 132 Page 133 For Training Purposes O n ly For Training Purposes O n ly airflow in the exhaust nozzle cont On civil aircraft high exhaust gas velocities which reached the speed of sound were only found on turbojet engines or on old turbofan engines with low bypass ratios High exhaust gas velocities must be avoided because they decrease the propulsion efficiency, they cause great turbulences with the ambient airflow and therefore make loud exhaust noise So modern turbofan engines have convergent exhaust nozzles with sufficiently wide exhaust areas that will not choke Page 134 [...]... of the chemical composition of a gas In other words, air does the same as any other gas like nitrogen or even exhaust gases Page 29 PRESSURE AND FLOW IN DUCTS For Training Purposes O n ly The pressure in a gas turbine engine changes, if heat is added or removed The pressure also changes if the quantity or the volume of the air is changed The flow velocity in a gas turbine changes if the flow area is... principle describes pressure and velocity changes in moving liquids or gases In a constant air flow, the velocity changes with the shape of the flow area Page 30 GAS TURBINE COMPONENTS For Training Purposes O n ly The main components of a typical gas turbine engine are: S - the engine air inlet S the compressor S the combustion chamber S the turbine S the exhaust duct S the jet nozzle The engine air inlet... these turbines are specially designed to drive the propeller When the hot gas is leaves the turbine, the exhaust system moves it into the atmosphere Exhaust ducts are used on some aircraft to make sure that the hot gas does not come out from the engine directly under the wing The jet nozzle which is also named the propelling nozzle, increases the velocity of the exhaust gas It then sends the exhaust gas. .. ly For Training Purposes O n ly gas turbine components cont The primary function of the turbine is to supply the torque which is necessary to drive the compressor via the connected shaft It also drives the engine accessory gearbox via bevel gears and shafts So it powers the engine fuel pump, the oil pump, the hydraulic pumps, the generators or other accessories Separate turbines are used on high bypass... air with a propeller The propeller can be driven directly from the compressor shaft or by a free turbine and a long center drive shaft Note, that a reduction gear is always required to reduce the high gas turbine engine rotation to speeds that can be managed by the propeller Turboprop engines convert all the gas energy into torque They are very efficient, but the propeller does not permit high aircraft... increases the velocity of the exhaust gas It then sends the exhaust gas in the correct direction Page 33 Page 34 For Training Purposes O n ly For Training Purposes O n ly gas turbine components cont When the air passes through an operating gas turbine engine, the air pressure, the temperature and the volume change but after the air has passed through the engine, the air pressure, the temperature and the volume... modern jet aircraft Figure 20 Propfan Page 28 OPERATION OF A GAS TURBINE ENGINE THERMODYNAMIC PRINCIPLES For Training Purposes O n ly All changes of air in an engine from the inlet to the exhaust are in accordance with thermodynamic laws These laws describe how energy is converted from heat to mechanical energy and vice versa The condition of a gas like air is shown by its pressure, its temperature and... Turbofan engines are usually twin or triple spool engines The fan is always driven by a turbine via a drive shaft Normally these engines do not have a reduction gear to reduce the speed of the fan Turbofan engines convert a large part of the gas energy into torque to drive the fan and the engine compressors The remaining hot gas energy from the airflow which discharges from the so called core engine is directly... Exhaust Nozzles on Turbofan Engines Page 27 For Training Purposes O n ly thrust on typical aircraft engines cont Latest engine research activities are about open rotor engines, or so called propfans The gas turbine engine powers either a single fan or a set of counter rotating fans via a gearbox Because of the number and shape of the fan blades, these engines have good speed and altitude capabilities They... here produce thrust by accelerating ambient air However, the difference between them is how they do it A turbojet engine is designed for one purpose and this is to produce high velocity gases On these engines, all the gas energy which is not used to drive the compressor and the accessories is converted into thrust The high outlet velocity of these engines gives high aircraft speeds, but turbojet engines