Aircraft Engine Combustion Chambers

Combustion Section Overview

• It is located between the compressor diffuser and turbine section
• Includes combustion chambers, a fuel injection system, an ignition source, and a fuel drainage system

Combustion Chamber (Combustor)

• Where fuel and air mix and burn in a turbine engine
• It features an outer casing with a perforated inner liner
• Perforations size and shape affect flame propagation

Fuel Injection System

• Meters fuel through nozzles into combustors
• Nozzles are in the combustion chamber case or compressor outlet elbows
• Fuel is delivered as a finely atomized spray for thorough mixing with air
• Finer spray increases combustion speed and efficiency

Ignition Source

• A typical ignition source is a high-energy capacitor discharge system
• It includes an exciter unit, two high-tension cables, and two spark igniters
• The system produces 60 to 100 sparks per minute, creating a ball of fire at the igniter electrodes
• Some systems have sparks that shoot out several inches
• Use caution during maintenance tests to avoid lethal shock

Fuel Drainage System

• Drains unburned fuel after engine shutdown
• Reduces the risk of fire due to exceeding temperature limits
• Prevents gum deposits in fuel manifolds and combustion chambers

Combustion Chamber Efficiency Requirements

• Mixes fuel and air effectively
• Efficiently burns the mixture
• Cools combustion gases to a temperature the turbine blades can tolerate
• Distributes hot gases evenly to the turbine section

Airflow Division

• Airflow through a combustor is divided into primary and secondary paths for mixing, ignition, and cooling
• 25-35% of incoming air is primary air
• 65-75% of incoming air is secondary air

Primary (Combustion) Air

• Directed inside the liner in the front end of the combustor
• Passes through swirl vanes to create radial motion
• Axial velocity is reduced to about 5-6 feet per second
• Velocity reduction accommodates the slow flame propagation rate of kerosene-type fuels
• Excessively high airflow can cause a flameout
• A vortex is created for turbulence to properly mix fuel and air
• Combustion completes in the first third of a combustor

Secondary Airflow

• Flows at hundreds of feet per second around the combustor's periphery
• Forms a cooling air blanket on both liner sides
• Centers combustion flames to prevent liner contact
• Some is slowed and metered through liner perforations to ensure combustion of unburned fuel
• Secondary air mixes with burned gases and cool air to evenly distribute energy to the turbine nozzle

Combustion Chamber Types

• Multiple-can
• Annular (basket)
• Can-annular
• Functionally same, differing design and construction

Multiple-Can Type

• Consists of individual combustor cans acting as burner units
• Well-suited for centrifugal compressor engines due to even air distribution at the diffuser
• Each can has a perforated stainless steel liner inside the outer case
• The Inner liner is heat resistant and removable for inspection
• High curvature provides resistance to warpage
• Inefficient shape in terms of space and weight

Multiple-Can Interconnection

• Individual combustors connected by small flame propagation tubes
• Combustion starts in two cans with igniter plugs
• Flame travels through tubes to ignite fuel/air mixture in other cans
• Flame propagation tube has a small inner tube carrying flame
• The outer tube carries airflow between the cans, cooling and insulating
• A typical combustion section has 8 or 10 cans
• Cans are numbered clockwise facing the rear of the engine in American-built engines, with the number one can on top
• All combustor cans discharge exhaust gases into an open area at the turbine nozzle inlet

Annular Type

• Commonly used in small and large engines
• Most efficient in terms of thermal efficiency, weight, and physical size
• Consists of a housing and perforated inner liner (basket)
• Liner encircles the outside of the turbine shaft housing
• Shroud shaped to contain one or more concentric baskets
• An annular combustor with two baskets is a double-annular combustion chamber
• Normally, the ignition source consists of two spark igniters
• Airflow enters at front and discharges at rear similar to multiple-can design
• It must be removed as a single unit for repair where complete engine separation is usually required

Reverse-Flow Annular Combustors

• Airflow can reverse direction in some designs
• Functions like conventional flow types, but air flows around chamber and enters from rear
• Combustion gases flow opposite to normal airflow
• First used by Whittle in his early designs

Can-Annular Type

• Combines features of both multiple-can and annular combustors
• Invented by Pratt & Whitney
• A removable steel shroud that encircles the combustion section
• Multiple burner cans assembled radially around engine axis with bullet-shaped perforated liners
• Fuel nozzle cluster is attached to forward end of each burner can
• Pre-swirl vanes enhance the combustion process and slow air velocity
• Flame propagation tubes connect individual liners
• Two igniter plugs are used for initiating combustion
• An individual can and liner is removed and installed as one unit

Flameout

• Combustion flame can be extinguished by high airflow rates
• Excessively slow airflow rates can also contribute
• Although flameout is uncommon, combustion instability can still occur
• Turbulent weather, high altitude, slow acceleration, and high-speed maneuvers can induce a flameout

Types of Flameout

• Lean die-out: Usually occurs at high altitude with low engine speeds and low fuel pressure, forms a weak flame
• Rich blow-out: Occurs during rapid acceleration, overly-rich mixture drops fuel temperature

Turbine Section

• It extracts energy after the fuel/air mixture is burned
• Transforms kinetic energy in hot exhaust gases into mechanical energy to drive the compressor and accessories
• Absorbs approximately 60 to 80% of the total pressure energy
• It is located downstream of the combustion section
• Basic elements include a case, stator, shroud, and rotor

Turbine Stator

• Commonly referred to as the turbine nozzle, turbine guide vanes, or nozzle diaphragm
• Located directly aft of the combustion section and forward of the turbine wheel
• Exposed to the highest temperatures in a gas turbine engine

Function of Turbine Nozzle

• Collect high-energy airflow from combustors
• Direct flow to strike the turbine rotor at the appropriate angle
• Vanes are contoured and set at an angle
• Angle forms converging nozzles to convert pressure energy to velocity energy
• Stator vanes are set in the direction of turbine wheel rotation

Turbine Casing

• Encloses turbine rotor and stator assembly
• Provides direct or indirect support to the stator elements
• Typical case has flanges on both ends for attaching to combustion section and exhaust assembly

Shroud

• Turbine nozzle assembly comprised of inner and outer shroud that retains and surrounds the nozzle vanes
• Vanes are assembled between the outer and inner shrouds or rings
• Vane number varies
• Nozzle vanes constructed to allow for thermal expansion
• There would be distortion or warping if not

Thermal Expansion Solutions

• Vanes loosely assembled in inner and outer shrouds with contoured slots slightly larger than the vanes for loose fit
• Inner and outer shrouds are encased in inner and outer support ring for strength and rigidity
• Vanes rigidly welded or riveted into segmented shrouds
• The inner or outer shroud ring is cut into segments to allow for expansion
• Expansion closes gaps between shroud segments, preventing stress and warping

Turbine Rotor

• Rotating elements consist of a shaft and turbine rotor (wheel)
• Turbine wheel: Dynamically balanced unit with blades attached to a rotating disk
• Turbine disk: Anchoring component for turbine blades bolted or welded to main shaft
• Shaft rotates in bearings lubricated by oil to reduce vibration and misalignment

Turbine Blades

• Airfoil-shaped components designed to extract maximum energy from hot gases
• Forged or cast depending on alloy composition
• Primarily cast nickel-based alloys
• They must be finish-ground to desired shape after forging or casting
• The development of reinforced ceramic material holds promise
• Initial use is likely to be in small, high-speed turbines that operate at very high temperatures

Turbine Blade Fit and Attachment

• Blades fit loosely into turbine disk when cold, expand to fit tightly at operating temperatures
• Common attachment method: Fir tree slots cut into turbine disk rim and matching bases cast or machined into blade base
• Once installed, a turbine blade may be retained in its groove by peening, welding, rivets, or locktabs
• Peening is frequently in various ways
• Application requires a small notch to be ground in the edge of the blade being installed
• The blade is inserted into the disk metal, which "flowed" into the notch
• Use a small punch mark, making the tool similar to a center punch

Turbine Blade Classification

• Classified as impulse, reaction, or a combination of both
• that use impulse blades change the direction of airflow, causing no change in gas pressure or velocity
• Turbine wheel absorbs force to change the direction of airflow and converts it to rotary motion
• Reaction turbine blades produce a turning force based on aerodynamic action
• Ducts increase gas velocity and reduce pressure, similar to an airfoil creating lift

Impulse Turbine System

• Nozzle vanes form converging ducts
• Increases velocity of exhaust gases
• Blades extract energy by redirecting the flow of gas

Reaction Turbine

• Nozzle guide vanes direct exhaust gas flow to strike the blades at a positive angle of attack
• Convergent shape increases gas velocity and decreases resulting pressure to create lift that rotates the wheel

Impulse-Reaction Turbine blades

• Modern engines incorporate impulse-reaction blades to distribute workload evenly
• The Blade base is impulse shaped
• The blade tip is reaction shaped
• Design achieves uniform velocity and pressure drop

Shrouded Blades

• Shrouded blades are connected at their ends
• Blades can be open or shrouded
• Open-ended blades are used on high-speed turbines
• Shrouded blades are used on turbines with slower speeds
• Shrouds of blades contact each other, providing support and reducing vibration
• Prevent air from escaping over the blade tips, increasing efficiency
• Added weight makes shrouded blades more susceptible to blade growth
• A knife-edge seal is machined around the outside of the shroud
• Minimizes air losses at the blade tip

Turbine Cooling

• Common way to cool components is using engine bleed air
• Turbine disks absorb heat from hot gases near their rim and blades through conduction
• Cooling air is directed over each side of the disk to limit temperature variations
• Compressor bleed air is directed through hollow blades and out holes in the tip/edges
• Cooling is known as convection or film cooling
• Some vanes are constructed of porous, high-temperature material
• High-temperature material uses transpiration cooling
• Air is ducted into the vanes and exits through the porous material
• Engine designs use a combination of internal and surface cooling methods
• Turbine vane shrouds may also be perforated with cooling holes

Counter-Rotating Turbines

• Some smaller turboshaft engines feature counter-rotating turbine wheels
• Chosen for dampening gyroscopic effects and reducing engine vibration

Turbine Design: Temperature

• Temperature is the most limiting factor in running a gas turbine engine
• The higher the temperature of the incoming air, the more power/thrust an engine can produce
• Effective cooling systems are important as a result
• Cooling systems allow the turbine vane and blade components to operate between 600 to 800 degrees Fahrenheit above metal alloy temperature limits

Exhaust Section

• Design exerts tremendous influence on engine performance
• Shape and size of an exhaust section affect turbine inlet temperature
• Mass airflow through the engine and velocity/pressure of exhaust jet affect engine performance
• Determines amount of thrust developed

Exhaust Section Components

• Extends from the rear of turbine section to the exhaust gases
• The exhaust section has an exhaust cone, exhaust duct, and exhaust nozzle

Exhaust Cone

• Consists of an outer duct/shell, an inner/tail cone, radial hollow struts, and tie rods

Purpose of Exhaust Cone

• Channels and collects turbine discharge gases into a single jet
• Gas velocity decreases while pressure rises
• Supports the inner cone and straightens swirling exhaust gases
• Straighten gases from turbine

Tailpipe

• Extends from exhaust cone to the exhaust or jet nozzle
• Results in a penalty on engine's operating efficiency because of heat and duct friction losses, which reduces exhaust gas velocity and therefore thrust
• Used for engines installed within an aircraft's fuselage to protect the surrounding airframe
• Engines installed in a nacelle or pod, do not usually require one

Exhaust Nozzle

• Provides exhaust gases with a final velocity boost
• Mounts to rear of tailpipe for an existing tailpipe, to rear flange of exhaust duct with need for a tailpipe

Exhaust Nozzle Types

• Converging design: Nozzle diameter decreases from front to back, accelerates exhaust gases and increases engine thrust
• Converging-diverging design: Diameter decreases, then increases with supersonic speed
• Turbine exhaust gases accelerate to supersonic speed at narrowest duct part
• Gases are accelerated further in diverging portion

Fan or Bypass Engines

• High-temperature gases discharged by turbine, cool airmass moved rearward by fan section
• Low bypass: hot cool air combined in a mixer unit
• High bypass: Flows discharge separately through two sets of coaxially exhausts
• Some high bypass have a common part

Nozzle Opening

• Fixed or variable area
• Engines that utilize an afterburner need a variable geometry
• Operated with pneumatic, hydraulic, or electric controls

Afterburners

• Used to accelerate exhaust gases, which in turn, increases thrust
• Installed aft of last stage turbine/forward of exhaust nozzle
• Contain fuel manifold, an ignition source, and a flame holder

Afterburner Function

• Operated because gases in tailpipe contain oxygen
• 25% 75% discharge used for cooling support combustion
• A fuel manifold fits the tailpipe entrance, fuel injects fuel into the tailpipe
• The fuel and air mix, ignite and burn in the afterburner
• Additional heat accelerates the exhaust gases

Flame Holder

• Tubular grid/spoke-shaped obstruction
• It is placed downstream of the fuel nozzles for fuel and air mixing
• Causes turbulence, gases swirl

Afterburner Influence

• Increases temperature/thrust
• Exhaust nozzle area increased to the decreased back pressure
• Increasing exhaust size controls exhaust gas temperature

Afterburning

• Used to increase thrust on for military aircraft
• It can provide as much as a 100% increase in thrust with fuel flows three to five times higher

Thrust Reversers

• Assist in braking
• Redirect exhaust gases to provide thrust in opposite direction

Accessory Section

• Powers engine and aircraft accessories (electric generators, hydraulic pumps, fuel pumps, and oil pumps)
• Acting as an oil reservoir or sump
• Housing accessory drive gears/reduction gears
• Accessory drive location selected to minimize engine profile for streamlining

Power for Accessories

• From the engine's main power shaft
• Beveled gears drive an accessory shaft, then, an accessory gearbox
• The gearbox then distributes power to each accessory drive pad
• Turbine engines require the use of rpm reduction
• Some installations require an intermediate or transfer gearbox for appropriate reduction gearing

Engine Stations Numbering

• Locate certain engine areas when doing maintenance
• Numbers in the same area take in temperature and pressure
• Identifies EPR reading because engines have different ducts

Station Numbering for instruments

• P refers to pressure
• T refers to temperature

Noise Suppression

• Some energy released from burning fuel turns into noise pollution
• Noise pollution is a big issue that impacts both designers and operators
• Noise reduced by mixing fast gasses with slow gas
• Use of turbofans helps lower both inside and on-ground noise
• Turbofans do not suppress noise because exhaust cancels it internally

Turbine noise

• In older planes require devices that disrupt airflow behind the tailcone with soundproof material
• The sound intensity is measured in decibels (db)
• A decibel is the ratio of one sound to another.
• 60 db is good 130db is physically dangerous

Airport Concerns

• Federal Aviation Administration has established noise limits based on an aircraft weight for airport
• All older aircraft were granted the period of time for adjustments

Engine Mounts

• Design and construction is simple
• Turbine engines produce little torque, only need to support the weight of the aircraft and protect from stress
• A wing-mounted engine is mounted to two or four brackets
• Turboprops produce larger torque and propeller weights
• Turboshafts use stronger locations for many mounts

Bearings

• Support engines
• Located lengthwise
• Number depends on the rotor weight
• Split-spool usually more than 1