Gas Turbine Engine Exhausts PDF

Gas Turbine Engine Exhausts Purpose of the Gas Turbine Engine Exhaust After the gas flow leaves the turbine, it enters the exhaust section. The design of a turbojet engine exhaust section exerts tremendous influence on an engine’s performance. Its shape and size affect: Tur...

Gas Turbine Engine Exhausts Purpose of the Gas Turbine Engine Exhaust After the gas flow leaves the turbine, it enters the exhaust section. The design of a turbojet engine exhaust section exerts tremendous influence on an engine’s performance. Its shape and size affect: Turbine inlet temperature Mass airflow through the engine Velocity and pressure of the exhaust jet. Therefore, an exhaust section determines, to some extent, the amount of thrust developed. Exhaust cone 2022-08-24 B1-15a Gas Turbine Engine Page 168 of 244 CASA Part 66 - Training Materials Only  The purpose of an exhaust section is to collect the gas flow, straighten it and increase its velocity. A typical turbojet engine exhaust section extends from the rear of the turbine section to the point where the exhaust gases leave the engine. An exhaust section is comprised of several components, including the: Exhaust cone Exhaust duct or jet pipe Exhaust nozzle. Turbojet engine exhaust 2022-08-24 B1-15a Gas Turbine Engine Page 169 of 244 CASA Part 66 - Training Materials Only  Exhaust Cone A typical exhaust cone assembly consists of an outer duct, an inner exhaust cone, three or more radial hollow struts, and a group of tie rods that assist the struts in centring the inner cone within the outer duct. The outer duct attaches to the rear flange of the turbine case. The purpose of an exhaust cone assembly is to channel and collect turbine discharge gases into a single jet. Due to the diverging passage between the outer duct and inner cone, gas velocity within the exhaust cone decreases slightly, while gas pressure rises. Radial struts between the outer shell and inner cone support the inner cone and help straighten the swirling exhaust gases that would otherwise exit the turbine at an approximate angle of 45°. The outer duct and exhaust cone are commonly made of nickel alloys or titanium. Exhaust case and cone assembly 2022-08-24 B1-15a Gas Turbine Engine Page 170 of 244 CASA Part 66 - Training Materials Only  Jet Pipe A jet pipe is an extension of the exhaust section that directs exhaust gases safely from the exhaust cone to the exhaust, or jet nozzle. The use of a jet pipe penalises an engine’s operating efficiency due to heat and duct friction losses. These losses decrease the exhaust gas velocity and, hence, the thrust. Jet pipes are used almost exclusively with engines that are installed within an aircraft’s fuselage to protect the surrounding airframe. In engines installed in a nacelle or pod that may require no jet pipe, the exhaust nozzle is mounted directly to the exhaust cone assembly. Jet pipe 2022-08-24 B1-15a Gas Turbine Engine Page 171 of 244 CASA Part 66 - Training Materials Only  Exhaust Section Nozzles Exhaust Nozzle An exhaust, or jet, nozzle provides the exhaust gases with a final boost in velocity. An exhaust nozzle mounts to the rear of a jet pipe if a jet pipe is required, or to the rear flange of the exhaust duct if no jet pipe is necessary. The size, or area, of the nozzle must be carefully matched to the mass airflow of the engine. If it is too small, it restricts gas flow and the resulting high back-pressure causes a significant rise in jet pipe temperature, a slowing of the turbine and high turbine temperatures. If the nozzle area is too large, there is insufficient exhaust gas acceleration, a considerable loss of thrust and therefore a lower temperature in the jet pipe. An excessive nozzle area also causes a lower back-pressure on the engine turbine, causing the turbine to speed up. This could result in an engine overspeed. Types of exhaust nozzle designs used on aircraft are convergent, divergent and convergent/divergent. Relevant Youtube link: Turbine Engine Exhaust System Convergent Nozzles The convergent nozzle is used on most subsonic aircraft. On a converging exhaust nozzle, the nozzle diameter decreases from front to back and the flow area cannot be altered. This convergent shape produces a venturi that accelerates the exhaust gases to Mach 1 and no faster. Converging exhaust nozzle 2022-08-24 B1-15a Gas Turbine Engine Page 172 of 244 CASA Part 66 - Training Materials Only  On fan or bypass type engines, there are two gas streams venting to the atmosphere. High- temperature and -velocity gases are discharged by the turbine, while a cool air mass is moved rearwards at a slower velocity by the fan section. Low bypass engine cold and hot exhaust flow In a low-bypass engine, the flows of cool and hot air are combined in a mixer unit that ensures mixing of the two streams before they exit the engine. High-bypass engines (depending on the engine manufacturer) may have a common or integrated nozzle to partially mix the hot and cold gases prior to their ejection or may exhaust the two streams separately through two sets of nozzles arranged coaxially around the exhaust nozzle. The two exhausts are referred to as the hot and cold streams. High bypass fan integrated exhaust flow 2022-08-24 B1-15a Gas Turbine Engine Page 173 of 244 CASA Part 66 - Training Materials Only  High bypass fan two streams Divergent Nozzles Helicopter tailpipes are often divergent in shape to nullify thrust produced, which enhances hover capabilities. As the power required to drive the rotors is extracted through the turbines, the residual thrust from the tailpipe produces a thrust on the aircraft. To nullify this thrust, a divergent duct is used to decrease the velocity of the exhaust gas and thereby reducing the thrust produced. The photo shows a divergent nozzle. Divergent exhaust 2022-08-24 B1-15a Gas Turbine Engine Page 174 of 244 CASA Part 66 - Training Materials Only  Convergent/Divergent Nozzles The diameter of a converging/diverging duct decreases, then increases from front to back. The converging portion of the exhaust nozzle accelerates the turbine exhaust gases to sonic (Mach 1) speed at the narrowest part of the duct. Once the gases are moving at the speed of sound, they are accelerated further in the nozzle’s divergent portion, so the exhaust gases exit the nozzle well above the speed of sound. Converging/diverging exhaust A variable geometry nozzle (converging/diverging) is necessary on engines that utilise an afterburner. When an afterburner is being used, the area of the exhaust nozzle must be increased. If this is not done, an area of back pressure at the rear of the turbine is created which could increase the turbine temperature beyond its safe level. Variable nozzles are typically operated with pneumatic, hydraulic or electric controls. Afterburners are used to accelerate the exhaust gases, which in turn increases thrust. An afterburner is typically installed immediately aft of the exhaust cone assembly and forward of the exhaust nozzle. Variable geometry exhaust nozzles 2022-08-24 B1-15a Gas Turbine Engine Page 175 of 244 CASA Part 66 - Training Materials Only  Exhaust Insulation Engine insulation blankets are used to shield portions of an aircraft’s structure from the intense heat radiated by the exhaust duct. Although these blankets protect the fuselage from heat radiation, they are used primarily to reduce heat loss from the exhaust system. Reducing heat loss improves engine performance. Common places where insulation blankets may be used include the combustion, turbine and exhaust sections. Aluminium, glass fibre and stainless steel are among the materials used to manufacture engine insulation blankets. Several layers of fibreglass, aluminium foil and silver foil are covered with a stainless steel shroud to form a typical blanket. The fibreglass is a low-conductance material and the layers of metal foil act as radiation shields. Each blanket is manufactured with a suitable covering that prevents it from becoming oil soaked. Although insulation blankets were used extensively on early engine installations, they are typically not required with modern turbofan engine installations. Engine insulation blankets 2022-08-24 B1-15a Gas Turbine Engine Page 176 of 244 CASA Part 66 - Training Materials Only  Engine Noise Gas Turbine Noise Suppression Noise is best defined for gas turbine engine purposes as ‘unwanted sound’ because it can be both irritating and harmful. The sound level of the average business jet or airliner during take-off, as heard by persons on the airport near the end of the runway, is likely in the range of 90–100 dB. This noise level is similar to a subway train noise as heard from the boarding platform. Right next to the aircraft, the noise level of a turbojet could be as high as 160 dB and painful to the ears. Jet exhaust noise is caused by the violent and hence extremely turbulent mixing of the exhaust gases with the atmosphere and is influenced by the shearing action caused by the relative speed between the exhaust jet and the atmosphere. The small eddies created near the exhaust duct cause high- frequency noise, but downstream of the exhaust jet, the larger eddies create low-frequency noise. The noise level reduces if the mixing rate is accelerated or if the velocity of the exhaust jet relative to the atmosphere is reduced. Most fully ducted turbofan engines are designed with what is termed exhaust mixing to blend the fan and hot airstreams more effectively and lessen the sound emission from a common exhaust duct. On these engines, the sound from the inlet is likely to be louder than from the exhaust. Noise-absorbing materials convert acoustic energy (air pressure) into heat energy. However, one can still find what looks like the old-style noise suppressor being fitted to some newer engines to meet the new noise standards. Exhaust mixing 2022-08-24 B1-15a Gas Turbine Engine Page 177 of 244 CASA Part 66 - Training Materials Only  Noise Suppression Units Types of noise suppression units used on gas turbine engines are: Corrugated nozzle Chevron nozzle Lobe type nozzle Multiple tube nozzle. These devices all operate under the principle of increasing the mixing rate of the exhaust gases with the atmosphere, thus reducing the larger ‘eddies’ within the exhaust stream. This has the effect of dampening low-frequency sound. It is accomplished by increasing the contact area of the atmosphere with the exhaust gas stream through lobe, corrugated, multi-tube and chevron nozzles. Because low-frequency noise tends to linger at relatively high volume, noise reduction is achieved by raising the frequency. Frequency change is accomplished by increasing the perimeter of the exhaust stream, which provides more cold and hot air mixing space. This reduces the tendency of hot and cold air molecules to shear against each other and breaks up the large turbulence in the jet wake, which produces the low-frequency (loud) noise. Corrugated Nozzle Corrugated nozzle 2022-08-24 B1-15a Gas Turbine Engine Page 178 of 244 CASA Part 66 - Training Materials Only  Chevron Nozzle Chevron nozzle Lobe Type Nozzle Lobe type nozzle 2022-08-24 B1-15a Gas Turbine Engine Page 179 of 244 CASA Part 66 - Training Materials Only  Multiple Tube Nozzle Multiple tube nozzle Noise Attenuating Materials Newer aircraft have inlets and tailpipes lined with noise-attenuating materials to keep sound emission within the established effective perceived noise decibel limits (EPNdB). These noise- absorbing materials convert acoustic energy (air pressure) into heat energy. However, noise- suppression nozzles (corrugated and chevron) are still being fitted to some newer engines to meet the new reduced noise level regulations set down by different authorities. Exhaust noise suppression 2022-08-24 B1-15a Gas Turbine Engine Page 180 of 244 CASA Part 66 - Training Materials Only  Noise-attenuating linings are fitted to engine inlet, fan and exhaust ducts. This noise-absorbing lining material converts acoustic energy into heat. It normally consists of a porous skin supported by a honeycomb backing and separates the face sheet from the solid engine duct. For optimum suppression, the acoustic properties of the skin and the liner are carefully matched. The disadvantage of liners is the slight increase in weight and skin friction, which leads to a slight increase in fuel consumption. However, they provide very powerful noise suppression. Noise attenuating lining locations 2022-08-24 B1-15a Gas Turbine Engine Page 181 of 244 CASA Part 66 - Training Materials Only  Thrust Reversers Purpose of Thrust Reversers Airliners, most commuter aircraft and an increasing number of business jets are equipped with engine thrust reversers. They are used to: Aid in braking and directional control during normal landing and reduce brake wear and maintenance Provide braking and directional control during emergency landings and aborted take-offs Back some types of aircraft out of a parking spot in a ‘power back’ operation. Thrust reversers in use Thrust reversers redirect the flow of cold and/or hot exhaust to provide thrust in the opposite direction. They provide approximately 20% of the braking force under normal conditions (wheel brakes provide the other 80%). Reversers must be capable of producing 50% of rated thrust in the reverse direction. However, exhaust gas exits a typical reverser at an angle to the engine’s thrust axis. Because of this, maximum reverse thrust capability is always less than forward thrust capability. 2022-08-24 B1-15a Gas Turbine Engine Page 182 of 244 CASA Part 66 - Training Materials Only  Thrust reversers diagram Operating in reverse at low ground speeds can cause re-ingestion of hot gases and compressor stalls. It can also cause ingestion of fine sand and other runway debris. The most frequently encountered thrust reversers can be divided into two categories: The mechanical blockage type The aerodynamic blockage type. Thrust reversers can be further divided into two groups: Hot stream Cold stream. Aircraft can use a combination of mechanical and aerodynamic blockage reversers. 2022-08-24 B1-15a Gas Turbine Engine Page 183 of 244 CASA Part 66 - Training Materials Only  Thrust reversers Relevant Youtube link: Thrust Reverser Video 2022-08-24 B1-15a Gas Turbine Engine Page 184 of 244 CASA Part 66 - Training Materials Only  Mechanical Blockage Mechanical blockage is accomplished by placing a movable obstruction in the exhaust gas stream either before (pre-exit) or after the exhaust exits the duct (post exit). The engine exhaust gases are mechanically blocked and diverted to a forward direction by an inverted cone, half sphere or other device. The mechanical blockage system is also known as the ‘clamshell’ thrust reverser because of its shape. On the selection of reverse thrust, the doors rotate to uncover the ducts and close the normal gas stream exit. Cascade vanes then direct the gas stream forwards so that the jet thrust opposes the aircraft motion. Pre-exit clam shell with cascade vanes The post exit system is commonly known as a bucket type. It is hydraulically actuated and uses bucket type doors to reverse the hot gas stream. The bucket system is used where reversal of the hot jet nozzle gas interferes with the aerodynamics of the fan section or causes hot gas re-ingestion into the engine inlet and prevents hot gases from coming into contact with the fuselage. 2022-08-24 B1-15a Gas Turbine Engine Page 185 of 244 CASA Part 66 - Training Materials Only  Post exit bucket type diagram Post exit bucket type example The cold stream mechanical blockage type places a movable obstruction in the pre-exit fan airstream. The blocker doors open to redirect the fan airflow forwards. As they open, they mechanically block the normal gas stream exit. They are typically hydraulically operated. 2022-08-24 B1-15a Gas Turbine Engine Page 186 of 244 CASA Part 66 - Training Materials Only  Cold stream mechanical blockage type 2022-08-24 B1-15a Gas Turbine Engine Page 187 of 244 CASA Part 66 - Training Materials Only  Aerodynamic Blockage A modern aerodynamic thrust reverser system consists of a translating cowl, blocker doors and cascade vanes that redirect the fan airflow to slow the aircraft. Some aircraft may use a combination of the aerodynamic blockage and the mechanical blockage type reversers. Mixed low-bypass exhaust turbofans are configured with one reverser, while unmixed or bypass exhaust turbofans may have both cold stream and hot stream reversers. High-bypass turbofans have only cold stream reversing because most of the thrust (up to 90%) is present in the fan discharge and a hot stream reverser would be of minimum value and become a weight penalty. Cold Stream Thrust Reverse Cold stream mechanical blocker doors with cascade vanes of the pre-exit type are used on high- bypass engine thrust reverser systems. As the thrust reverser sleeve moves rearwards, the blocker doors close and the cascade vanes are exposed. The blocker doors mechanically block the normal airstream exit. Cascade vanes then direct the airstream in a forward direction. Cold stream blocker door type normal flow diagram 2022-08-24 B1-15a Gas Turbine Engine Page 188 of 244 CASA Part 66 - Training Materials Only  Cold stream blocker door type normal flow example Cold stream blocker door type reversed flow diagram 2022-08-24 B1-15a Gas Turbine Engine Page 189 of 244 CASA Part 66 - Training Materials Only  Cold stream blocker door type reversed flow example 2022-08-24 B1-15a Gas Turbine Engine Page 190 of 244 CASA Part 66 - Training Materials Only  Thrust Reverser Operation The thrust reversal lever is locked until the main engine power lever is in the idle position and the undercarriage squat switch is closed. Initial movement of the reverser control lever locks the main throttle lever in the idle position and allows the reversers to unlock and deploy. Full deployment and locking in position of the reversers releases a solenoid in the throttle quadrant, allowing the reverser control lever authority over the engine power and thus controlling the amount of reverse thrust. Then, as the aircraft slows to 60–80 kt, power is reduced back to reverser idle and then to forward thrust as soon as practical. Thrust reverse throttle Boeing 737 thrust reverse throttle 2022-08-24 B1-15a Gas Turbine Engine Page 191 of 244 CASA Part 66 - Training Materials Only  While some thrust reversers are electrically powered, most large transport category aircraft use hydraulically actuated reversers powered by main system hydraulic power, or pneumatic actuators powered by engine bleed air. Cockpit indicators show thrust reverser status – unlocked and deployed. Safety devices prevent deployment while the aircraft is in flight. Warning lights and tones alert the crew to an unlocked thrust reverser in flight. Thrust reverser indication 2022-08-24 B1-15a Gas Turbine Engine Page 192 of 244 CASA Part 66 - Training Materials Only  Thrust reverse in use 2022-08-24 B1-15a Gas Turbine Engine Page 193 of 244 CASA Part 66 - Training Materials Only