Compressor Inlet Duct Design PDF

Compressor Inlet Ducts Compressor Inlet Duct Design When a gas turbine engine is installed in an aircraft, it usually requires a number of accessories fitted to it and connections made to various aircraft systems. The engine, jet pipe and accessories, and in some installations a thrus...

Compressor Inlet Ducts Compressor Inlet Duct Design When a gas turbine engine is installed in an aircraft, it usually requires a number of accessories fitted to it and connections made to various aircraft systems. The engine, jet pipe and accessories, and in some installations a thrust reverser, must be suitably cowled and an air intake must be provided for the compressor, the complete installation forming the aircraft power plant. Air intakes working hard The main requirement of an air intake is that, under all operating conditions, delivery of the air to the engine is achieved with the minimum loss of energy occurring through the duct. To enable the compressor to operate satisfactorily, the air must reach the compressor at a uniform pressure distributed evenly across the whole inlet area. The air entrance or flight inlet duct is normally considered part of the airframe, not part of the engine. Nevertheless, it is usually identified as engine Station 1. The function of the inlet and its importance to engine performance are necessary to any discussion of gas turbine engine design and construction. Even a small discontinuity of airflow can cause significant efficiency loss as well as many unexplainable engine performance problems. Therefore it follows that if the inlet duct is to retain its function of delivering air with minimum turbulence, it must be maintained in as close to new condition as possible. If repairs to this inlet become necessary, expertly installed flush patches are mandatory to prevent drag. Moreover, the use of an inlet cover is recommended to promote cleanliness and to prevent corrosion and abrasion. Relevant Youtube link: Video Lesson: Aircraft Engine Intakes 2022-08-24 B1-15a Gas Turbine Engine Page 52 of 244 CASA Part 66 - Training Materials Only  Inlet Types and Locations Many air inlet ducts have been designed to accommodate new airframe/engine combinations and variations in engine mounting locations. In addition, air inlets are designed to meet certain criteria for operation at different airspeeds. Some of the most common locations where engine inlets are mounted are: In the wing On the engine On the fuselage Within the fuselage. Engine inlet 2022-08-24 B1-15a Gas Turbine Engine Page 53 of 244 CASA Part 66 - Training Materials Only  Inlet Construction in Wing Some early commercial and military aircraft had engines installed in the wings. In-wing design streamlined the turbojet engines of the era. The inlet ducts were constructed from aluminium alloy and built into the wing, forming part of the secondary structure. Since the introduction of high-bypass turbofan engines, this design has become impractical. In-wing intakes on a de Havilland Comet 2022-08-24 B1-15a Gas Turbine Engine Page 54 of 244 CASA Part 66 - Training Materials Only  Inlet Construction on Engine Most multi-turbojet and turbofan aircraft have the inlet ducts mounted directly onto the front of the engine. Some turboprops have engine-mounted inlets as part of their power plant assembly. Inlets are constructed from aluminium alloy and/or composites such as carbon fibre and Kevlar®. Typically, turbofan inlets are bolted to the forward flange of the inlet case or fan case. This allows for short, efficient inlet ducts with minimal internal skin friction. Engine-mounted inlet 2022-08-24 B1-15a Gas Turbine Engine Page 55 of 244 CASA Part 66 - Training Materials Only  Inlet Construction on Fuselage Some multi engine jet aircraft have the engines mounted on the aft fuselage. The inlet ducts may be mounted directly onto the front of the engine or form part of the fuselage, engine pylon, or stub wing structure. Inlets are constructed from aluminium alloy and/or composites such as carbon fibre and Kevlar®. Inlets forming part of the aft fuselage may have a long “S” shaped duct. Inlet Construction on Fuselage 2022-08-24 B1-15a Gas Turbine Engine Page 56 of 244 CASA Part 66 - Training Materials Only  Inlet Construction Within Fuselage Single-engine and some twin-engine military aircraft have their engines mounted within the fuselage. The inlet may form part of the fuselage structure. Inlet ducts may be mounted in the nose, under the fuselage or on both sides of the fuselage. In Fuselage intake 2022-08-24 B1-15a Gas Turbine Engine Page 57 of 244 CASA Part 66 - Training Materials Only  Effects of Inlet Configurations Single-Entry (Pitot) Type Duct The ideal air inlet for a turbojet engine fitted to an aircraft flying at subsonic or low supersonic speeds is a single, short, pitot type circular inlet as shown below. This type of inlet makes full use of ram effect on the air due to forward speed and suffers the minimum loss of ram pressure with changes in aircraft attitude. However, as sonic speed is approached, its efficiency begins to fall due to the formation of a shock wave at the inlet lip. Single-Entry (Pitot) Type Duct Although this short, straight duct results in the minimum pressure drop, the engine tends to suffer from inlet turbulence, especially at low airspeed and/or high angles of attack (AOA). The pitot type inlet can be used for engines which are mounted in pods, wings or other flying surfaces, although the inlet sometimes require a departure from the circular cross section due to the area of the surface. Even the wing pylon-mounted engine inlets of some aircraft are squared due to their proximity to the ground when the wing is flexed. 2022-08-24 B1-15a Gas Turbine Engine Page 58 of 244 CASA Part 66 - Training Materials Only  Squared inlets Single-engine aircraft sometimes use a pitot type inlet, but this requires a long duct ahead of the compressor, with a resultant drop in pressure. However, it achieves smooth airflow into the compressor. Pitot type inlet on a single engine aircraft 2022-08-24 B1-15a Gas Turbine Engine Page 59 of 244 CASA Part 66 - Training Materials Only  Subsonic Inlet Ducts Inlet ducts such as those found on business and commercial jet aircraft are of fixed geometry and have a divergent shape. A diverging duct progressively increases in diameter from front to back. This duct is sometimes referred to as an inlet diffuser because of its effect on pressure. Air enters the aerodynamically contoured inlet at ambient pressure and starts to diffuse, arriving at the compressor at a slightly increased static pressure. Usually the air is allowed to diffuse (increase in static pressure) in the front portion of the duct and to progress at a fairly constant pressure past the engine inlet fairing, also called the inlet centre body, to the compressor. In this manner, the engine receives its air with minimal turbulence and at a more uniform pressure. Subsonic inlet duct Inlet pressure increases add significantly to the mass airflow as the aircraft reaches its desired cruising speed. It is here that the compressor reaches its aerodynamic design point and produces its optimum compression and fuel economy. At this point the flight inlet, compressor, combustor, turbine and tailpipe are designed to be in match with each other. If any one section does not match the others for whatever reason – damage, contamination or ambient conditions – engine performance will be affected. The turbofan inlet is similar in design to the turbojet except that it discharges only a portion of its air into the gas generator, with the remainder passing into the fan. 2022-08-24 B1-15a Gas Turbine Engine Page 60 of 244 CASA Part 66 - Training Materials Only  Turbofan installation 2022-08-24 B1-15a Gas Turbine Engine Page 61 of 244 CASA Part 66 - Training Materials Only  Ram Pressure Recovery When the aircraft engine is operated on the ground, there is a negative pressure in the inlet because of the high velocity of the mass airflow being drawn into the inlet by the compressor. As the aircraft begins to move forwards, air is rammed into the inlet and ram recovery takes place. The resultant increase in inlet pressure cancels the drop in pressure in the inlet, and conditions return to ambient, as shown. Ram recovery normally begins to occur at speeds between Mach 0.1 and Mach 0.2 in most aircraft. As aircraft speed continues to increase, ram compression increases. The engine can use this effect to increase the compression ratio and thus create more thrust with less fuel usage at a set altitude. Ram Pressure Recovery - Airspeed vs Thrust graph 2022-08-24 B1-15a Gas Turbine Engine Page 62 of 244 CASA Part 66 - Training Materials Only  Supersonic Inlets A Convergent-Divergent (C-D) inlet duct (fixed or variable) is required on all supersonic aircraft. A supersonic transport, for example, is configured with an inlet that slows the airflow to subsonic speed at the face of the engine, regardless of aircraft speed. Subsonic airflow into the compressor is required if the rotating aerofoils are to remain free of shock wave accumulation, which is detrimental to the compression process. In order to vary the geometry, or shape, of the inlet, a movable restrictor is often employed to form a C-D shape of variable proportion. The C-D shaped duct becomes necessary to reduce supersonic airflow to subsonic speeds. At this point, it is important to remember that at subsonic flow rates, air flowing in a duct acts as an incompressible liquid, but at supersonic flow rates, air is compressed to the point of creating the familiar shock wave phenomenon. Convergent-divergent engine inlet The supersonic diffuser type inlet provides a means of creating both a shock wave formation to reduce air velocity and a variable C-D shape to meet the various flight conditions from take-off to cruise. Air velocity drops to approximately Mach 0.8 behind the final shock wave and then to Mach 0.5 by diffusion. Moveable spike inlet 2022-08-24 B1-15a Gas Turbine Engine Page 63 of 244 CASA Part 66 - Training Materials Only  The illustration depicts a movable wedge which provides a similar function of convergence, divergence and shock wave formation. It also has a spill valve to dump unwanted ram air overboard at high speed. Many high-performance aircraft have an excess of mass flow at cruising speeds. The Concord inlets shown provide a good illustration of how complicated an inlet may have to be to take full advantage of the energy recovery that is possible. At the speed of sound, half the pressure needed by the engine for combustion may be provided by ram effect, and the other half by compression through the engine. At twice the speed of sound, pressure ratios in the vicinity of 30:1 are possible, and at 3 times the speed of sound, this may rise to 50:1. As aircraft speed increases, the compression provided by the engine becomes relatively minor and there is no need for complicated anti‑surge devices (which stop pressure fluctuations in the compressor that can lead to damage and engine failure). The modest pressure rise over each of the compressor stages is such that control of fuel flow alone provides a sufficient safeguard against surge. NOTE: In the illustration, the wedge has been lowered during supersonic flight to force a controlled sonic shock wave at the inlet. This slows air velocity to subsonic. The divergent area further reduces air velocity, and the open dump valve permits the escape of excessive pressure. In subsonic flight, the wedge is fully retracted for maximum nozzle area and the dump valve reversed to act as an air scoop. Moveable wedge inlet 2022-08-24 B1-15a Gas Turbine Engine Page 64 of 244 CASA Part 66 - Training Materials Only  Another method of varying the geometry of an inlet duct uses a movable spike, or plug, which is positioned as necessary to alter the shape of the inlet as aircraft speed changes. The shape of the spike and surrounding inlet duct combine to form a movable C-D inlet. During transonic flight (Mach 0.75 to 1.2), the movable spike is extended forwards to produce a normal shock wave, or bow wave, at the inlet. As airspeed increases, the spike is repositioned to shift the C-D duct for an optimum inlet shape at the new airspeed. As airspeed increases to supersonic, the bow wave changes to multiple oblique shock waves, extending from the tip of the spike, and a normal shock wave develops at the lip of the inlet. Moveable spike or 'mouse' Spike or 'mouse' in F-111C Intake The ‘mouse’ would expand to almost fill the intake when supersonic. 2022-08-24 B1-15a Gas Turbine Engine Page 65 of 244 CASA Part 66 - Training Materials Only  Bellmouth Inlets Bellmouth Inlets on a helicopter Bellmouth compressor inlets, shown in the diagram below, are convergent in shape and are commonly found on helicopters and engine test cells. They present a mouth considerably wider in circumference than the engine compressor inlet and smoothly converge, funnelling air down to compressor inlet circumference. You may have seen similar fittings on classic car or motorcycle carburettors. Bellmouths eliminate the ‘necking down’ effect of an airstream passing through a plain orifice, and allow the engine to draw all the air it can use. A bellmouth inlet increases aerodynamic drag on the airframe, but it is the most efficient option when there is little or no ram pressure available to force air into the compressor. This condition exists on helicopters and engine test cells. Bellmouth intake 2022-08-24 B1-15a Gas Turbine Engine Page 66 of 244 CASA Part 66 - Training Materials Only  As the duct losses are very small, bellmouth ducts are often used during ground testing and calibration, fitted with mesh screens to protect technicians from ingestion hazards while making trimming adjustments on running engines. The screens also provide FOD protection. Screens have been tried on aircraft during flight, but fatigue and maintenance trouble created as many problems as the FOD they prevented. They may still be seen, however, on some helicopters. Bellmouth used in testing engines 2022-08-24 B1-15a Gas Turbine Engine Page 67 of 244 CASA Part 66 - Training Materials Only  Inlet Screens The use of compressor inlet screens is usually limited to rotorcraft, turboprops and ground turbine installations. This may appear peculiar to the casual observer who realises the appetite of all gas turbines for debris such as nuts, bolts, stones, etc. Screens have been tried in high subsonic flight engines in the past, but icing and screen fatigue failure caused so many maintenance problems that the use of inlet screens has for the most part been avoided. When aircraft are fitted with inlet screens for protection against foreign object ingestion, they may be located internally or externally at either the inlet duct or compressor inlet. Inlet Screen One type of separator used on some turboprop aircraft incorporates a movable vane which extends into the inlet airstream. Once extended, the vane creates a prominent venturi and a sudden turn in the engine inlet. Combustion air can follow the sharp curve, but sand and ice particles cannot because of their inertia. The movable vane is operated by a control handle in the cockpit. Another type of particle separator uses several individual filter elements that act as a swirl chamber. With this type of system, as incoming air passes through each element, a swirling motion is imparted by helical vanes. The swirling motion creates enough centrifugal force to throw the dirt particles to the outside of the chamber. 2022-08-24 B1-15a Gas Turbine Engine Page 68 of 244 CASA Part 66 - Training Materials Only  The particles then drop to the bottom of the separator, where they are blown overboard by compressor bleed air through holes on each side of the filter unit. As the foreign particles are swirled out of the intake air, clean air passes through the filter into the engine inlet. Turboprop inlet screen Divided Entry Inlets Divided entry inlets, as shown in the photo below, are used on single-engine aircraft to avoid using long inlet type ducts. Usually the twin divided inlet ducts merge into the wing leading edges on each side of the fuselage. Divided entry intakes 2022-08-24 B1-15a Gas Turbine Engine Page 69 of 244 CASA Part 66 - Training Materials Only  The airflow may remain divided until it reaches the engine compressor or merge smoothly before reaching the engine, as shown below. Divided entry intakes - exposed The disadvantage of the divided type of inlet is that when the aircraft yaws, a loss of ram pressure occurs on one side of the inlet as shown in the diagram below, causing an uneven distribution of airflow into the compressor. Ram differences due to yaw 2022-08-24 B1-15a Gas Turbine Engine Page 70 of 244 CASA Part 66 - Training Materials Only  Secondary Air Inlet Doors Some aircraft utilise a system of doors which allow extra air into the inlet duct. Secondary air inlet doors are designed to react to excess negative pressure within the inlet. If the pressure within the inlet falls below a predetermined limit, the suck-in doors are pushed open by the high external air pressure and allow extra airflow to the compressor. The tendency for the doors to open is counteracted by spring tension against the door. Therefore, the pressure required to open the door may be altered by adjusting the tension of the door spring. As with all aircraft maintenance tasks, any adjustments must be done in accordance with the manufacturer’s directions. Secondary inlet 'blow in' doors 2022-08-24 B1-15a Gas Turbine Engine Page 71 of 244 CASA Part 66 - Training Materials Only  Inlet Duct Losses Inlet duct losses can occur if the aircraft or conditions exceed expected flight attitudes, such as very high AOAs or sideslipping, in which the smooth inlet airflow into the inlet duct is disrupted. Sometimes these conditions can lead to a compressor stall. During ground running, crosswinds can disrupt the inlet airflow and increase the AOA of the air into the compressor, causing compressor stall. Some aircraft are more prone than others to this phenomenon, which is why it is important to face an aircraft into the wind before carrying out an engine ground run. B747s are very susceptible to crosswind-induced compressor stall with power settings above idle. Intake airflow distortion at excessive flight attitudes The engine inlet duct must provide a uniform supply of air to the compressor if the engine is to perform at optimum efficiency. To do this, the duct must create as little resistance as possible. To aid in the prevention of intake drag or resistance, the duct should be kept smooth and clean, and any damage in the intake area must be immediately repaired in accordance with the manufacturer’s instructions. Curves or bends must be minimal and carefully blended. The design of the intake should reduce turbulence to a minimum. This ensures that the engine receives its air at a uniform pressure across the face of the compressor. If a curve is necessary, it must be as gentle as possible. The walls of the duct must have flush rivets or fasteners if fitted. The seal or joint between engine and duct must be as accurate as possible. 2022-08-24 B1-15a Gas Turbine Engine Page 72 of 244 CASA Part 66 - Training Materials Only  The inlet duct leading edge is susceptible to damage by bird strikes or hail. Damage to internal acoustic lining may be caused by bird strike, stones and mishandling, e.g. dropped tools, careless handling of fan blades or failure to use a protective mat when entering the intake. 2022-08-24 B1-15a Gas Turbine Engine Page 73 of 244 CASA Part 66 - Training Materials Only  Inlet Duct Anti-ice Systems The Effect of Icing Conditions on Engine Operation Icing of the engine and the leading edges of the intake duct can occur during flight through clouds containing supercooled water droplets or during ground operation in freezing fog. Icing conditions, however, are most prevalent when operating the engine at high speeds on the ground. Ice can form in the inlet at up to 4.4 °C ambient temperature in relatively dry air and up to 7.2 °C in visibly moist air, due to the cooling effect of high inlet airflow velocities. Protection against ice formation may be required since icing of these regions can considerably restrict the airflow through the engine, causing a loss in performance and possible malfunction of the engine. Additionally, damage may result from ice breaking away and being ingested into the engine or hitting the acoustic material lining the intake duct. The ambient temperature is well below -15 °C at all cruise altitudes for a gas turbine-powered aircraft, and ram pressure does not raise inlet temperature sufficiently above freezing. However, most of the flight time is above cloud level, and anti-icing is not required. When required, the usual method of initiating anti-icing is to select one engine, then watch the engine parameters stabilise, after which the remaining engine(s) are selected in a similar manner. On take-off, climb-out, descent and landing, the pilot must carefully assess the need for anti-icing according to the prevailing weather conditions. To prevent engine malfunction or damage, the operator must make the same assessment when running the engine on the ground. Ice chunks, when dislodged, could damage the compressor or fan blades and the inlet duct itself. Ice ingestion is a consideration in engine design. Inlet Anti-ice (Turbofan) Turbofan aircraft in icing conditions 2022-08-24 B1-15a Gas Turbine Engine Page 74 of 244 CASA Part 66 - Training Materials Only  Anti-ice air is directed radially inwards at the engine inlet case to heat all surfaces on which ice might form. Unlike certain de-icing systems on wing leading edges and propellers, this system does not allow ice to form. If the anti-ice system is inadvertently used to de-ice the inlet area by being turned on after compressor stalls occur from ice formation, the impact forces of ice on compressor blades and vanes can severely damage the engine or even cause the engine to fail completely. During flight, the anti-icing system is turned on before entering the icing condition. Anti-icing heat is required when visible moisture is present in the form of clouds. Inlet anti-ice - turbofan 2022-08-24 B1-15a Gas Turbine Engine Page 75 of 244 CASA Part 66 - Training Materials Only  Inlet Anti-ice (Turboprop) Some smaller turboprop and turboshaft engines use electric heat strip systems classed as electro- thermal anti-icing systems. They are constructed of electrical resistance wire embedded in layers of reinforced neoprene materials and located primarily at the lip of the nacelle flight inlet. Other possible locations are the engine inlet case and the engine inlet struts. Like the hot air anti-ice systems, the electro-thermal systems are cycled on and off as required by ambient conditions. They are designed to operate only when the engine is running since operating the strip without air passing over it tends to overheat the strip and the part of the engine it is attached to. One system uses hot engine or reduction gearbox oil to anti-ice the engine and inlet. Turboprop aircraft in icing conditions 2022-08-24 B1-15a Gas Turbine Engine Page 76 of 244 CASA Part 66 - Training Materials Only