Module 15 Gas Turbine Engine flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Brayton cycle efficiency

Ideal thermal efficiency of a gas turbine cycle equals 1 minus 1 divided by pressure ratio raised to (gamma minus 1)/gamma, so higher pressure ratio and higher gamma improve efficiency.

Net thrust formula

Net thrust equals mass flow times jet velocity minus flight velocity, plus nozzle exit pressure minus ambient pressure times exit area; the second term vanishes for a choked convergent nozzle.

Station 4

Combustor exit and turbine inlet station, where gas temperature is highest; turbine entry temperature is measured or inferred here and limits blade material and cooling design.

Bypass ratio

Ratio of fan bypass mass flow to core mass flow; high-bypass turbofans (5 to 12) have better propulsive efficiency and lower specific fuel consumption than low-bypass engines.

Assumed temperature derating

A takeoff thrust reduction method that uses a fictitious higher outside air temperature in the thrust calculation, lowering thrust and EGT to extend hot-section life while increasing takeoff distance.

Engine pressure ratio

EPR is turbine exit total pressure divided by compressor inlet total pressure; it is used as the thrust-setting parameter on some engines, especially Rolls-Royce and Pratt & Whitney types.

Specific fuel consumption

Fuel flow divided by net thrust, expressed in kilograms per newton-hour or pounds per pound-force-hour; lower values indicate better fuel efficiency at a given operating condition.

Max continuous thrust

The highest thrust rating with no time limit, used for climb and cruise; unlike takeoff thrust it is not restricted to a five- or ten-minute period.

Rotating stall

A localized cell of stalled compressor blades that rotates around the annulus, causing vibration and asymmetric pressure, distinct from a full-axisymmetric surge with flow reversal.

Variable stator vanes

Compressor vanes whose angle is scheduled with corrected speed to keep the front stages from stalling at low power and to widen the stable operating range.

Compressor surge margin

The percentage difference between the stall pressure ratio and the operating pressure ratio at a given corrected flow; inlet distortion and bleed extraction reduce it.

Corrected mass flow

Mass flow multiplied by the square root of theta (T/T reference) and divided by delta (p/p reference); it lets compressor maps be compared across altitude and temperature conditions.

Combustor dilution zone

The downstream portion of the combustor liner where air is added to cool the gas to a turbine-safe temperature profile before it enters the nozzle guide vanes.

Air-blast fuel nozzle

A nozzle that atomizes fuel using high-velocity compressor air rather than pressure swirl alone; it gives good atomization at high pressure ratios and low fuel flows.

Pattern factor

A measure of the non-uniformity of combustor exit temperature; a high pattern factor means hot streaks that reduce turbine blade life and force lower thrust ratings.

Lean blowout

Flame extinction that occurs when the fuel-air ratio in the primary zone falls below the stable limit, typically during rapid deceleration or idle operation at high altitude.

Thermal barrier coating

A ceramic layer, usually yttria-stabilized zirconia, applied over a bond coat on turbine blades and vanes to reduce metal temperature and extend component life.

Active clearance control

A system that sprays cooling air on the turbine casing to shrink tip clearance at cruise, reducing leakage and specific fuel consumption; clearance opens at takeoff.

Turbine nozzle guide vanes

Stationary vanes that accelerate and turn hot gas to the correct angle for the turbine rotor blades; they are usually cooled and often cast as segments.

Creep in turbine blades

Time-dependent elongation of blades under centrifugal load and high temperature; it increases tip clearance and can lead to blade failure if limits are exceeded.

Cascade thrust reverser

A reverser with a translating sleeve and blocker doors that turn fan air forward through cascades; it is used on high-bypass engines and is inhibited in flight.

Choked convergent nozzle

A nozzle in which flow reaches sonic velocity at the throat when nozzle pressure ratio exceeds about 1.85; further pressure ratio increase does not raise thrust.

Exhaust gas temperature

Temperature measured at turbine exit (station 4.5 or 5); it is the primary indicator of hot-section health, and a rising EGT at fixed thrust signals degradation.

Squeeze-film damper

A bearing support with a thin oil film that absorbs rotor vibration and reduces loads transmitted to the engine casing, improving bearing life and reducing vibration.

Labyrinth seal

A non-contacting seal with knife-edge fins running against a land; it restricts air or oil leakage between compartments and is common at bearing and shaft locations.

Buffer air

Compressor air routed to a bearing compartment to keep oil in and contaminants out; loss of buffer pressure leads to oil loss and possible fire.

FADEC

Full-authority digital engine control; a dual-channel digital system that schedules fuel, variable vanes, bleed valves, and ignition, with hydromechanical backup and protection limits.

Fuel metering unit

The component that meters fuel to the nozzles based on throttle position and engine parameters; it is controlled by the FADEC or by hydro-mechanical governors on older engines.

Fuel/oil heat exchanger

A unit that uses fuel to cool engine oil and warms the fuel to prevent ice formation; it is part of the fuel system between the LP pump and the metering unit.

Air turbine starter

A starter driven by bleed air that cranks the engine through the accessory gearbox; it cuts out at a set N2 and has a duty cycle limit to prevent overheating.

Hot start

A start in which EGT exceeds its limit because too much fuel is introduced or ignition is delayed; the crew or FADEC must abort the start and motor the engine to purge fuel.

EGT margin

The difference between the EGT at takeoff thrust and the EGT redline; a declining margin indicates hot-section degradation and drives shop visit planning.