Gas Turbine Engines – Module 15 Comprehensive Notes

15.1 FUNDAMENTALS

• Jet propulsion = reaction force generated opposite to the flow of a pressurized gas/liquid escaping a nozzle. This principle is based on Newton’s Third Law of Motion, where the rearward expulsion of mass creates an equal and opposite forward thrust.

• Historical milestones:

  • 2nd century BC – Hero’s Aeolipile: An early demonstration of jet propulsion, this device consisted of a spherical vessel mounted on bearings, with two L-shaped tubes that directed steam tangentially from the sphere, causing it to rotate.

  • 1200 AD – Chinese solid-fuel rockets (black powder): Developed for warfare, these rockets used black powder as propellant, demonstrating the practical application of jet propulsion for thrust generation.

  • 1930 – Sir Frank Whittle patents first turbojet (pure reaction): Whittle's design was a groundbreaking internal combustion turbine engine that compressed air, mixed it with fuel and ignited it, with the hot exhaust gases directly creating thrust, revolutionizing aviation.

• Energy concepts:

  • Potential Energy (PE): Stored energy due to position (e.g., gravitational PE), pressure (e.g., compressed air within a diffuser), or elasticity. In an engine, pressure energy is crucial for guiding airflow and converting it into kinetic energy.

  • Kinetic Energy (KE): Energy of motion, possessed by the moving air, fuel, and exhaust gases within the engine.

  • Conservation of Energy: States that energy can change form (e.g., chemical to heat, heat to kinetic) but cannot be created or destroyed, forming the basis of all energy transformations within a jet engine.

  • Brayton Cycle (constant-pressure cycle) vs. Otto Cycle (constant-volume):

    • Brayton Cycle: The thermodynamic cycle that gas turbine engines operate on, characterized by heat addition and rejection occurring at constant pressure. This cycle is continuous, making it suitable for steady-state power generation.

    • Otto Cycle: The thermodynamic cycle for piston engines, where heat addition occurs at approximately constant volume during combustion. This cycle is intermittent, involving distinct intake, compression, combustion, and exhaust strokes.

  • Phases (for Brayton Cycle):

    • 1–2 Compression: Air drawn into the engine is compressed, increasing its pressure and temperature.

    • 2–3 Heat addition (p=\text{const}): Fuel is introduced and burned, adding heat to the air at nearly constant pressure, significantly raising its temperature and volume.

    • 3–4 Expansion: The high-energy, hot gases expand through the turbine and nozzle, doing work to drive the compressor and producing thrust.

    • 4–1 Heat rejection: In an open cycle (like a jet engine), heat is rejected to the atmosphere as exhaust gases are expelled.

  • Continuous in gas turbine; intermittent in piston engine: Highlights the fundamental difference in operation between jet engines and reciprocating engines.

• Newton’s Laws applied to propulsion:

  • 1st Law (inertia) – mass remains at rest or constant velocity unless acted on: An engine must overcome inertia to initiate movement and maintain flight speed.

  • 2nd Law – F=Ma; thrust proportional to mass flow × acceleration: This fundamental law governs thrust generation in a jet engine. Thrust (F) is the product of the mass of air expelled by the engine (m) and the acceleration (a) imparted to it (change in velocity over time). For a continuous flow, it's expressed as the mass flow rate multiplied by the change in velocity of the exhaust gases. Example: A larger mass of air or a greater increase in its velocity at the exhaust will result in more thrust.

  • 3rd Law – action / reaction (balloon, garden sprinkler $\Rightarrow$ jet thrust): For every action, there is an equal and opposite reaction. The engine expels hot gases rearward (action), and the resulting reaction force pushes the engine and aircraft forward (reaction).

• Force, work, power, energy, velocity, acceleration:

  • Force F=P\times A (psi $\times$ in²): The push or pull exerted by the engine. Pressure acting over an area generates force.

  • Work W=F\times D: Energy transferred when a force causes displacement. In an engine, work is done by the expanding gases on the turbine blades.

  • Power P=\dfrac{W}{t}: The rate at which work is done; units hp, W. Engine power output is crucial for performance and determining thrust capabilities.

  • Acceleration a=\dfrac{V2-V1}{t}: The rate of change of velocity, critical for generating thrust by accelerating the air mass.

• Thermodynamics:

  • 1st Law – heat $\leftrightarrow$ work; engine converts chemical $\rightarrow$ heat $\rightarrow$ mechanical $\rightarrow$ kinetic: Describes the energy conversion process where the chemical energy of fuel is converted into heat during combustion, which then drives the turbine (mechanical energy) and accelerates the exhaust gases (kinetic energy).

  • 2nd Law – heat flows hot$\rightarrow$cold; losses inevitable: This law states that processes occur spontaneously in the direction of increasing entropy. It implies that energy conversion is never 100% efficient, and some energy is always lost as unusable heat to the surroundings.

  • Conversion chain: inlet diffuser (KE$\rightarrow$PE), compressor (PE$\leftrightarrow$KE), combustor (heat +PE), turbine (KE$\rightarrow$mechanical), nozzle (PE$\rightarrow$KE): This sequence details how energy is transformed at each major section of the engine to produce thrust.

• Gas-turbine configurations: turbojet, low-bypass turbofan, high-bypass turbofan, turboshaft, turboprop. Each configuration is optimized for different operational envelopes: turbojets for high-speed flight, turbofans for fuel efficiency and reduced noise, turboshafts for helicopters and power generation, and turboprops for lower-speed, short-field operations.

• Basic sections: inlet, compressor, combustor, turbine, exhaust, accessories. These components work in sequence to ingest air, compress it, add heat, extract energy, and expel high-velocity gases.

• Engine station numbering (Pt & Tt symbols) standardises positions (e.g., Pt2, Tt7): A system of designating specific locations within the engine using numbers (e.g., 2 for compressor inlet, 7 for turbine exhaust) to provide precise measurement points for pressure (Pt for total pressure) and temperature (Tt for total temperature).

15.2 ENGINE PERFORMANCE

• Thrust definitions:

  • Gross (static) thrust Fg=\dfrac{Wa(V2-V1)}{g}: The total thrust produced by the engine at a standstill (static conditions), without accounting for the aircraft's forward motion. Wa is the mass flow rate of air, V2 is exhaust velocity, V_1 is inlet velocity (zero for static), and g is gravitational acceleration.

  • Net thrust Fn=Fg - \dfrac{Wa V1}{g} (subtract aircraft momentum drag): The actual thrust available to propel the aircraft, which is the gross thrust minus the momentum drag caused by the air entering the engine due to the aircraft's forward motion. This is the more relevant thrust for airborne performance.

  • Choked-nozzle/pressure thrust Fp=Aj(Pj-P{amb}): The thrust component generated by the pressure difference between the nozzle exit (Pj) and ambient air (P{amb}) when the nozzle is choked (flow reaches sonic speed). A_j is the nozzle exit area.

  • Thrust distribution: forward forces (compressor, diffuser, combustor, exhaust duct) vs. rearward (turbine, nozzle). Forward forces are generated by the acceleration of gases within the engine prior to the turbine, while rearward forces are associated with energy extraction by the turbine and pressure losses in the nozzle.

  • Resultant thrust = forward – rearward forces. This is the net force available to propel the aircraft.

  • Fan thrust = additional cold-stream momentum: For turbofan engines, significant thrust is generated by the fan's bypass air, which is accelerated rearward without passing through the hot section, contributing to efficiency and noise reduction.

• Engine power terms:

  • Thrust horsepower \text{THP}=\dfrac{\text{Thrust(lb)}\times Va(ft/s)}{550}: A measure of the propulsive power delivered by the engine, converting thrust into an equivalent horsepower value based on flight velocity (Va).

  • Equivalent SHP \text{ESHP}=\text{SHP}+\dfrac{Fg}{2.5}: Used for turboprop engines, this metric combines the shaft horsepower (SHP) delivered to the propeller with an equivalent power from any residual jet thrust (Fg), providing a total power output.

  • Specific Fuel Consumption (SFC): A measure of fuel efficiency. For jets, it's \dfrac{\text{lb fuel}}{\text{hr}\cdot\text{lb thrust}}, indicating how much fuel is consumed per unit of thrust per hour (lower is better). For turboprops, it's lb/hr/ESHP.

• Efficiencies:

  • Thermal (internal) – heat $\rightarrow$ KE (\approx45 %): Measures how effectively the heat energy from burning fuel is converted into useful kinetic energy of the exhaust gases within the engine.

  • Propulsive (external) \etap=\dfrac{2V}{Vj+V}: Measures how efficiently the engine converts the kinetic energy of the exhaust jet into propulsive work to move the aircraft. It is high when the jet velocity (V_j) is close to the aircraft's flight velocity (V), meaning less energy is wasted in accelerating the exhaust to a much higher velocity than needed.

  • Overall \etao=\eta{\text{t}}\times\eta_p: The product of thermal efficiency and propulsive efficiency, representing the total efficiency of the engine in converting fuel chemical energy into propulsive power.

• By-pass ratio classifications: low $\le$1 : 1, medium 1.5–3.5 : 1, high >3.5 : 1. The bypass ratio is the ratio of the mass flow rate of air that bypasses the core engine to the mass flow rate of air that flows through the core. High bypass ratios are typical for commercial airliners for fuel efficiency and reduced noise, while low bypass ratios are found in military aircraft for high speed.

• Engine Pressure Ratio ext{EPR}=\dfrac{P{t7}}{P{t2}}; IEPR integrates hot & cold streams: EPR is the ratio of total pressure at the turbine discharge (P{t7}) to the total pressure at the compressor inlet (P{t2}). It's a key indicator of engine thrust and performance. Integrated Engine Pressure Ratio (IEPR) accounts for both the hot (core) and cold (bypass) streams in turbofans.

• Gas-law relationships; Bernoulli; subsonic vs. sonic duct behaviour: These fundamental principles govern airflow within the engine. Gas laws relate pressure, temperature, and volume. Bernoulli's principle describes the inverse relationship between fluid speed and pressure. Subsonic and sonic duct behavior dictate how air accelerates or decelerates through varying duct areas, affecting pressure recovery and thrust.

• Influences on thrust:

  • Speed: As aircraft speed increases, ram recovery (the compression of incoming air due to forward motion) improves, increasing pressure at the compressor inlet and boosting thrust.

  • Altitude: At higher altitudes, both pressure and temperature decrease. The dominant effect is the significant drop in air density (due to lower pressure), which leads to a substantial decrease in mass flow rate through the engine, causing thrust to fall even though thermal efficiency might slightly improve due to lower ambient temperatures.

  • Temperature: An increase in ambient temperature leads to a decrease in air density. This results in a lower mass flow rate into the engine, which directly reduces the thrust produced if other factors remain constant. Hotter air also means less temperature rise is achievable in the combustor for a given fuel flow.

• Flat-rating: engine limited to constant rated thrust up to a specified OAT (e.g., 30 °C) for longevity. Flat-rating ensures that the engine can deliver its maximum certified thrust up to a certain ambient air temperature (OAT) or altitude. Beyond this limit, thrust will decrease. This practice protects the engine from operating at excessive internal temperatures or pressures, extending its lifespan and reducing maintenance.

15.3 INLET

• Functions: The inlet's primary functions are to supply a smooth, uniform, and high-pressure airflow to the compressor while minimizing pressure losses. It converts the kinetic energy of the free stream airflow into pressure energy for efficient compression.

• Subsonic pitot-type divergent ducts; ram effect begins $\approx$160 mph: For subsonic aircraft, inlets typically use a divergent duct shape. As air enters the duct, its velocity decreases, and according to Bernoulli's principle, its static pressure increases. This "ram effect" becomes significant at speeds above approximately 160 mph, aiding compressor efficiency.

• Supersonic intakes: convergent-divergent (variable geometry spikes, wedges, ramps) create shock-controlled compression: For supersonic flight, inlets use convergent-divergent designs. These designs, often with movable spikes, wedges, or ramps, are engineered to create a series of oblique and normal shock waves. These shocks are carefully managed to slow the supersonic airflow to subsonic speeds before it reaches the compressor, converting kinetic energy into pressure with minimal losses.

• Special forms:

  • Bellmouth (ground/rotorcraft): A smooth, rounded inlet used primarily for static engine testing or on helicopters, designed for maximum airflow efficiency at low or zero forward speeds.

  • Split side intakes: Used on some aircraft where the engine is mounted within the fuselage (e.g., in the wing root or aft fuselage), splitting the intake duct to provide air from either side of the aircraft.

  • Plenum for double-entry centrifugal: A chamber surrounding the compressor entrance on double-sided centrifugal compressors, designed to distribute airflow evenly before it enters the impeller.

• Icing protection:

  • Hot-air anti-ice (HP bleed): Prevents ice formation by directing hot high-pressure (HP) bleed air from the compressor to critical areas like the inlet cowl lip. The heat raises the temperature above freezing.

  • Electrical heater mats (turboprop spinners, intakes): Heating elements embedded in mats that provide localized heat to prevent ice accumulation on surfaces such as turboprop spinners and smaller intake areas.

  • Hot-oil circulation: Less common for inlets, but hot oil from the engine lubrication system can be circulated through some components to prevent icing.

15.4 COMPRESSORS

Types & construction

• Centrifugal: Consists of an impeller (rotating disc with radial blades), a diffuser (stationary vanes that convert velocity to pressure), and exit elbows. Typically 1–2 stages, offering a pressure ratio of approximately 4 : 1 per stage. They are robust, less susceptible to foreign object damage (FOD), and have a wider operating range, but are large in diameter for their thrust output.

• Axial: Features alternating rows of rotor blades (rotating) and stator vanes (stationary). Can incorporate multi-spool designs (N1, N2, N3) where different sections of the compressor rotate independently at varying speeds, improving efficiency across a wide range of operating conditions. Each stage provides a pressure ratio of approximately 1.2 : 1, but overall ratios can exceed 30 : 1 due to the large number of stages. They offer high efficiency and a small frontal area, ideal for high-speed flight.

• Centri-axial (compound): A hybrid design combining an axial flow compressor at the front with a centrifugal stage at the rear. This configuration offers the small frontal area advantage of axial compressors with the robust characteristics and high pressure ratio per stage of centrifugal compressors, leading to a more compact engine design.

• Fan: The ducted first-stage of the low-pressure (LP) compressor in turbofan engines. It is designed with wide-chord, low-aspect ratio blades to efficiently move a large volume of air, a significant portion of which bypasses the core engine. Bypass ratios can be up to 5–8 : 1 (or even higher), contributing significantly to thrust, fuel efficiency, and noise reduction.

Blade details

• Rotor blades: Aerodynamically twisted along their length to optimize airflow pitch as it passes through the varying radii of the blade. Squealer tips are designed with a groove or cut-out at the tip to minimize tip leakage and reduce aerodynamic losses. Materials selection depends on the operating temperature: Stainless Steel (SS) for front stages (cooler), Titanium (Ti) for mid-stages, and Nickel (Ni)-based alloys for rear stages (hotter).

• Stators: Stationary vanes positioned between rotor stages. Their primary function is to diffuse the airflow from the preceding rotor stage (converting velocity into pressure) and to guide it at the correct angle into the next rotor stage. Inlet Guide Vanes (IGVs) are stators at the compressor inlet, and some may be variable (VSVs - Variable Stator Vanes).

• Tip clearance: The small gap between the rotating blade tip and the stationary casing. Maintaining optimal tip clearance is critical for compressor efficiency. Wear fit and abradable liners (a soft sacrificial coating on the casing) are used to allow the blade tips to lightly abrade into the liner during initial engine runs or under high G-loads, creating a minimal and efficient clearance.

• Balancing: Critical for smooth and safe engine operation. Blades are moment-weighed to ensure they have the same weight distribution relative to their root. Trim balance is performed on the assembled rotor to correct any remaining rotational unbalance, typically by adding small weights to the rotor disc.

Operation & control

• Characteristic map: A graphical representation of the compressor's performance, plotting pressure ratio against corrected airflow for various corrected rotor speeds. It shows the operating limits and efficiency islands.

• Surge line vs. operating line: The surge line on the characteristic map represents the absolute limit of stable compressor operation. Beyond this line, airflow breaks down, leading to surge (a sudden, violent flow reversal). The operating line is the actual path the compressor follows during engine operation, designed to remain at a safe margin below the surge line.

• Stall types: Conditions where airflow separation occurs from the compressor blades, leading to a loss of pressure rise. Types include:

  • Low-speed stall (front): Occurs at low engine speeds, typically in the front stages, due to excessive angle of attack.

  • High-speed stall (rear): Occurs at high engine speeds, typically in the rear stages, due to airflow choking.

  • Acceleration stall: Occurs during rapid acceleration when the fuel flow increases too quickly for the compressor to handle the increased airflow.

  • Rotating stall: A localized region of stalled air that rotates around the compressor annulus, reducing efficiency and potentially leading to full surge.

• Bleed valves, Variable IGV/VSV, ACC to maintain surge margin: Control systems are implemented to prevent compressor stall and surge:

  • Bleed valves: Vents air from intermediate compressor stages, particularly during start-up or low-speed operation, to reduce the workload on downstream stages and shift the operating line away from the surge line.

  • Variable IGV/VSV: Adjust the angle of the inlet guide vanes and/or stator vanes to optimize airflow angles into the rotor blades across different engine speeds and maintain an adequate surge margin.

  • ACC (Active Clearance Control): A system that controls the temperature of the compressor casing to precisely manage the tip clearance between the rotor blades and the casing, improving efficiency and surge margin.

• Compression ratio calculations; cycle pressure ratio: Compression ratio refers to the pressure increase across a single stage or the entire compressor. Cycle pressure ratio is the total pressure ratio of the entire engine cycle, which is a key determinant of engine thermal efficiency.

15.5 COMBUSTION SECTION

• Tasks: The primary tasks of the combustion section are to efficiently burn fuel at nearly constant pressure (p=\text{const}), add a substantial amount of heat (raising temperature by 700–1200 °C), achieve this with minimal pressure loss, and ensure stable combustion over a wide range of overall air/fuel (A/F) ratios (typically 45–130 : 1, much leaner than stoichiometric in the overall engine).

• Airflow distribution (annular can example):

  • 20 % primary via swirl, flame tube snout: This portion of air enters the primary zone through swirl vanes and the flame tube snout. Swirl promotes rapid mixing of fuel and air and creates a stable recirculation zone for continuous combustion.

  • 20 % through primary holes (toroidal recirculation): Air entering through primary holes supports the main combustion process and helps establish a stable toroidal (doughnut-shaped) recirculation pattern that continuously brings hot gases back to the ignition point, ensuring flame stability.

  • 40 % dilution/secondary for cooling/exit Tt: After combustion, a large portion of air is used for dilution, reducing the gas temperature to an acceptable level for the turbine section (maintaining the turbine entry temperature, Tt). This air also acts as secondary cooling for the flame tube walls.

  • 20 % liner cooling/NGV cooling: This air forms a cooling film along the inner surface of the combustor liner to protect it from the extremely high flame temperatures and is also channeled to cool turbine Nozzle Guide Vanes (NGVs).

• Combustor types: multiple can, can-annular, annular, annular reverse-flow. Each type offers different advantages regarding manufacturing complexity, maintenance, and combustor volume. Annular combustors are most common in modern engines due to their compact size, uniform heat distribution, and lower pressure losses.

• Ignition: high-energy capacitor discharge, 60–100 sparks/min. Ignition is typically achieved using high-energy igniter plugs (similar to spark plugs but more powerful) powered by a capacitor discharge unit. These plugs deliver a high-voltage, high-current spark with sufficient energy to ignite the fuel-air mixture, especially at altitude or during wet starts.

• Fuel nozzles:

  • Simplex (single-flow): A simpler design that sprays fuel through a single orifice, common in smaller engines or older designs.

  • Duplex (primary/main): Features two concentric orifices (or passages) to provide efficient fuel atomization over a wider range of fuel flows. A primary flow for starting and low power, and a main flow that comes on at higher power settings.

  • Spill-return: A system where excess fuel not required for combustion is returned to the fuel tank, allowing for precise control of fuel flow to the nozzles and better atomization at lower flow rates.

  • Air-blast: Fuel is mixed with air before exiting the nozzle, promoting superior atomization and cleaner combustion, reducing emissions.

  • Vaporising: Fuel is pre-heated and vaporized before being introduced into the combustion zone, improving combustion efficiency.

  • Fuel/air ratios: stoichiometric 15 : 1, engine overall 45–130 : 1. Stoichiometric ratio is the ideal chemical ratio for complete combustion. However, jet engines operate at much leaner overall A/F ratios to lower turbine entry temperatures and facilitate cooling, even though the primary combustion zone is closer to stoichiometric.

• Flameout (rich / lean); relight envelope: Flameout occurs when the flame in the combustor is extinguished. This can happen due to an excessively rich (too much fuel) or excessively lean (too little fuel or too much air) mixture, or disrupted airflow. The relight envelope defines the flight conditions (altitude, airspeed) within which the engine can be successfully restarted in flight.

• Drain valves & ecology systems: Drain valves are incorporated in the combustor lower casing to evacuate unburnt fuel after engine shutdown or during an unsuccessful start, preventing fuel pooling and potential fire hazards. Ecology systems are designed to minimize unburnt hydrocarbon emissions discharged into the atmosphere during start-up and shutdown.

15.6 TURBINE SECTION

• Purpose: The primary purpose of the turbine section is to extract energy from the hot, high-pressure gases exiting the combustor. Approximately 60% of this energy is used to drive the compressor and engine accessories (such as fuel pumps, oil pumps, and generators). In turboshaft and turboprop engines, nearly 100% of the available energy is extracted as shaft power.

• Blade aerodynamics: Turbine blades use aerodynamic principles to efficiently extract energy:

  • Impulse: Relies on changing the direction of the gas flow to induce a force on the blade, much like water hitting a paddle wheel.

  • Reaction: Relies on accelerating the gas as it passes over the blade, similar to a nozzle, creating a reaction force.

  • Impulse-reaction (common): Most modern turbine blades utilize a combination of both impulse and reaction principles for optimal efficiency across a range of operating conditions.

• Components: The turbine section typically includes Nozzle Guide Vanes (NGVs) (stationary vanes that direct the gas flow onto the rotor blades at the optimal angle), rotor discs (which hold the turbine blades), blades themselves, and the exhaust cone (which directs the flow to the exhaust nozzle).

• Cooling:

  • HP NGV & blades: The turbine operates in extremely high-temperature environments. High-pressure (HP) turbine Nozzle Guide Vanes (NGVs) and rotor blades are intricately cooled using sophisticated methods, including internal impingement (jets of cooling air hitting the inner surface), convection (heat transfer to cooling air flowing within passages), and film cooling (a thin layer of cooling air forming a protective barrier on the external surface). Bleed air from the compressor is routed through hollow airfoils and internal passages to achieve this cooling.

  • Active Clearance Control (ACC) cools casing for tip clearance: ACC systems precisely control the thermal expansion and contraction of the turbine casing by routing cooling air around it. This active control maintains an optimal clearance between the rotating blade tips and the stationary casing, which is crucial for maximizing turbine efficiency and preventing blade rub.

• Materials & manufacture:

  • NGVs/rotors: Constructed from advanced Nickel-based super-alloys (e.g., Inconel, Nimonic) due to their exceptional strength, creep resistance, and high-temperature capability.

  • Blades: Manufactured using advanced casting techniques to withstand extreme temperatures and stresses. These include conventionally cast (polycrystalline), directionally solidified (grains are aligned for improved strength), and single-crystal (no grain boundaries, offering superior high-temperature performance). Blades often feature intricate internal cooling passages and may be ceramic-coated for thermal barrier protection.

  • Disc/blisk; fir-tree or dovetail roots; shrouded vs. open tips: Turbine blades are typically attached to the rotor disc via precise fir-tree or dovetail roots, which provide mechanical locking. A blisk (bladed disk) is a single, integral unit of blades and disk. Blades can be shrouded (having a band at the tip that forms a continuous ring for improved sealing) or open-tipped.

• Stresses: Turbine components are subjected to immense stresses:

  • Centrifugal: From the high rotational speed of the disc and blades, pulling them outwards.

  • Gas bending: Due to the aerodynamic forces exerted by the hot gas flow on the blades.

  • Thermal: Caused by extreme temperature gradients across components, leading to differential expansion.

  • Creep, fatigue: Creep is the slow, time-dependent deformation of a material under constant stress at high temperatures. Fatigue is the weakening of a material caused by repeatedly applied loads. Both are critical life-limiting factors for turbine components.

  • Creep – time-temperature-stress; monitored by measured blade growth: Creep damage is a function of time, temperature, and applied stress. It is often monitored through precise measurements of turbine blade growth, as even minor elongation can indicate significant creep and reduce component life.

• Turbine blade attachment: Specific methods are used to securely attach blades to the disc, including fir-tree (multiple serrations), dovetail (wedge-shaped), bulb (simple bulbous end), and clamping plates.

15.7 EXHAUST

• Sections: The exhaust system consists of key components: an inner cone (plug) and outer cone that shape the flow, struts that support the inner cone and may house accessories, a tailpipe that guides the hot gases, and the nozzle which is the final component determining thrust.

• Divergent annulus diffuses gases (M 0.8$\rightarrow$0.5) before nozzle: After exiting the turbine, the gases pass through a divergent annular duct. This diffusion process slows the gases down (e.g., from Mach 0.8 to Mach 0.5), which increases their static pressure, preparing them for efficient acceleration through the nozzle.

• Nozzle types:

  • Fixed convergent (subsonic) – may choke; pressure thrust Fp=A(Pj-P{amb}): The most common type for subsonic aircraft. It has a gradually narrowing passage. If the pressure ratio across the nozzle is sufficiently high, the flow at the throat (narrowest point) will reach sonic speed (Mach 1), a condition known as choking. Pressure thrust is generated by the difference between the jet pressure (Pj) at the nozzle exit and the ambient pressure (P_{amb}) acting over the exit area (A).

  • Fixed divergent (helicopter) – reduces residual thrust: Used in turboshaft engines for helicopters or power generation where residual jet thrust is undesirable. The divergent section helps to recover more energy as shaft power by further expanding the gases, thus minimizing the thrust produced by the exhaust.

  • Convergent-divergent (CD) – supersonic; variable geometry on military/reheat engines: Essential for efficient supersonic operation. The convergent section accelerates the flow to sonic speed at the throat, and the subsequent divergent section further accelerates it to supersonic speeds, especially when reheat (afterburning) is employed. Variable geometry allows the area of the throat and exit to be adjusted for optimal performance across a wide range of flight conditions and power settings.

  • Variable area (fan engines) – jet-pipe trimming: In some high-bypass turbofan engines, the exhaust nozzle area can be varied (trimmed) to optimize the engine's performance, balance the bypass and core streams, and control operating lines for best efficiency and specific thrust.

• Noise suppression:

  • Corrugated / lobed mixers, cascade mixers: These designs actively mix the hot core exhaust with the cooler bypass air, reducing the shear between the two streams and lowering overall jet noise.

  • Acoustic linings (perforated honeycomb): Sound-absorbing materials, often in the form of perforated panels backed by honeycomb structures, fitted within the engine intakes and exhaust ducts. These linings dissipate sound energy, reducing emitted noise levels.

  • High bypass mixing: Fundamentally, high bypass ratio engines are inherently quieter because a larger proportion of their thrust comes from the slower-moving bypass air, and modern designs often incorporate extensive mixing between hot and cold streams before exhaust.

• Thrust reversers:

  • Functions: Primarily used to reduce landing roll distance, especially on wet or contaminated runways, by redirecting exhaust gases forward. They also reduce brake wear and save time by allowing faster exit from the runway.

  • Categories:

    • Hot-stream: Redirects the hot core engine exhaust. Types include clamshell (or bucket-type, deploys rearward of the nozzle exit) and target (pivots to block the exhaust flow directly at the nozzle exit).

    • Cold-stream cascade (translating cowl) with blocker doors; hot-stream spoilers: Common on turbofan engines. A translating cowl slides rearward, revealing cascade vanes that redirect the bypass air forward, while blocker doors simultaneously obstruct the cold stream. Some systems also incorporate hot-stream spoilers (small vanes that deploy to disrupt hot exhaust flow for additional reversal).

  • Requirements: Thrust reversers must provide >50% of the rated forward thrust in reverse, ensure no ingestion of foreign objects into the engine, be fail-safe (not deploy inadvertently), and contribute minimal drag and weight to the aircraft.

  • Actuation: Typically hydraulic or pneumatic systems deploy the reversers. They incorporate isolation valves for safety, mechanical locks to prevent inadvertent deployment, and position sensors that provide feedback to the cockpit (e.g., REV/UNLK/FULL Rev lights).

  • Safety: Several safety features are integrated: ground/flight squat switch (prevents deployment in the air), throttle idle interlock (ensures engine is at idle before reverser activation), and maintenance lock-out pins (for securing during ground operations).