Unit 5: Combustion Chambers - Subjective Questions

Gas-turbine combustion chambers are principally classified as:

• Can or tubular chamber: Consists of several separate cylindrical flame tubes, each enclosed in its own casing. Each can usually has an individual fuel injector and igniter. It is easy to develop, test, inspect, and replace, but it has greater weight and frontal area.
• Annular chamber: Uses a single continuous annular flame tube between concentric inner and outer casings. It is compact, light, and provides a more uniform turbine-inlet temperature, but inspection and development are comparatively difficult.
• Can-annular or tubo-annular chamber: Contains several flame tubes arranged inside a common annular casing. Interconnector tubes propagate the flame between cans. It combines the mechanical strength and maintainability of can chambers with some compactness of annular chambers.

Modern aircraft engines generally use annular chambers because of their low weight, small size, and superior exit-temperature distribution.

The annular chamber offers the best overall aerodynamic performance, whereas the can chamber is simpler to develop and maintain.

An effective combustion-chamber design must satisfy the following requirements:

• High combustion efficiency: Nearly all supplied fuel should release its chemical energy.
• Low total-pressure loss: Flow resistance must be minimized because pressure loss reduces available turbine work and engine thrust.
• Stable combustion: The flame must remain stable over wide fuel-air ratios, altitudes, and engine speeds.
• Uniform outlet temperature: Hot streaks at the turbine inlet must be avoided to protect turbine blades.
• Reliable ignition and relighting: Starting on the ground and relighting at altitude must be dependable.
• Compactness and low weight: Chamber length, diameter, and structural mass must be small.
• Low emissions and smoke: Formation of , unburned hydrocarbons, soot, and should be limited.
• Adequate liner cooling: Flame-tube metal temperature must remain below the allowable limit.
• Low tendency to pulsate: Pressure oscillations, combustion noise, and thermoacoustic instability must be controlled.
• Durability and maintainability: The chamber must withstand thermal cycling, vibration, oxidation, and creep while permitting inspection and repair.

The combustion process occurs in the following stages:

1. Diffusion: High-velocity compressor air enters a diffuser, where velocity decreases and static pressure rises.
2. Air distribution: The diffuser flow is divided among the primary, secondary, and dilution zones.
3. Fuel injection and atomization: Fuel injectors convert liquid fuel into fine droplets and distribute them in the primary zone.
4. Evaporation and mixing: Droplets absorb heat, evaporate, and mix with a portion of the compressor air.
5. Ignition: An igniter initially creates a flame kernel. Once established, combustion becomes self-sustaining.
6. Primary-zone combustion: Recirculating hot gases ignite the fresh mixture and maintain a locally near-stoichiometric flame.
7. Secondary combustion: Additional air completes oxidation of partially burned fuel, carbon monoxide, and hydrocarbons.
8. Dilution: The remaining air lowers the gas temperature to the value permitted at the turbine inlet and improves temperature uniformity.
9. Discharge: The hot products enter the turbine with a controlled velocity and temperature profile.

• Primary zone: Approximately one-quarter to one-third of the total air is introduced near the fuel injector. Swirlers and liner holes form a recirculation zone that retains hot products and supports continuous ignition. The local mixture is maintained near a readily combustible value.
• Secondary zone: Additional air is admitted downstream to complete combustion. It oxidizes carbon monoxide and unburned hydrocarbons and gives the chemical reactions sufficient residence time.
• Dilution zone: Most of the remaining air enters through relatively large dilution holes. It reduces the gas temperature to the allowable turbine-inlet temperature and produces a uniform radial and circumferential temperature distribution.

Correct air distribution is essential. Excess primary air may produce a lean blowout, whereas insufficient secondary air causes incomplete combustion and smoke. Poor dilution produces turbine-damaging hot spots.

Combustion efficiency is the ratio of the actual heat released by combustion to the chemical energy supplied with the fuel.

For steady flow, neglecting casing heat loss and changes in kinetic energy, the useful increase in gas enthalpy is

where . The fuel-energy input is , where is the fuel calorific value. Therefore,

If constant specific heats are assumed,

An alternative chemical definition is

A well-designed chamber normally has very high efficiency near its design condition, while efficiency can decline during low-pressure, low-temperature, or very lean operation.

Important performance parameters include:

• Combustion efficiency : Measures completeness of fuel-energy release.
• Total-pressure loss: Commonly represented by
  
  A small value is desirable because pressure loss decreases net engine output.
• Outlet-temperature distribution: Indicates radial and circumferential temperature non-uniformity at the turbine inlet.
• Pattern factor: Quantifies the severity of the maximum outlet hot spot relative to the mean temperature rise.
• Stability limits: Define the rich- and lean-mixture boundaries within which a stable flame can be sustained.
• Ignition and relight limits: Show the combinations of pressure, temperature, velocity, and fuel flow at which ignition is possible.
• Emissions: Include , unburned hydrocarbons, smoke, particulate matter, and .
• Liner temperature and durability: Determine cooling effectiveness and component life.
• Combustion intensity: Relates heat release to chamber volume and operating pressure, indicating how compactly combustion is achieved.

The fractional stagnation-pressure loss is commonly defined as

where and are the combustor-inlet and combustor-outlet stagnation pressures.

Its principal causes are:

• Diffusion and wall-friction losses
• Turbulent mixing and flow separation
• Losses through swirlers, liner holes, and fuel-injector passages
• Sudden expansion and contraction of flow
• Momentum exchange between primary, secondary, and dilution air jets
• Fundamental pressure loss associated with heat addition to a flowing gas

A high pressure loss reduces the pressure available for expansion through the turbine and nozzle. It can therefore reduce turbine work, specific thrust, and overall engine efficiency. Designers must balance low pressure loss against the strong turbulence and mixing required for stable, efficient combustion.

The pattern factor measures the non-uniformity of the combustor outlet temperature. A commonly used definition is

where is the maximum turbine-entry temperature, is the average combustor outlet temperature, and is the average combustor inlet temperature.

A low pattern factor is desirable because:

• Local hot streaks can overheat turbine nozzle guide vanes and rotor blades.
• Temperature non-uniformity increases thermal stress and reduces component life.
• The maximum permissible mean turbine-entry temperature may have to be reduced when the pattern factor is high.
• Uneven flow can reduce turbine aerodynamic efficiency.

Pattern factor is controlled through proper fuel distribution, liner-hole sizing, dilution-jet penetration, mixing length, and circumferential matching between the combustor and turbine.

Effect of inlet pressure:

• Increasing pressure generally increases reaction rates and improves flame stability.
• Fuel evaporation and combustion efficiency usually improve at higher pressure.
• Low pressure, especially at high altitude, slows chemical reactions and narrows the stable operating range.
• Ignition and relighting become more difficult as pressure falls.
• Soot formation and emissions chemistry can also change with pressure.

Effect of inlet temperature:

• Higher inlet temperature promotes fuel evaporation and reduces ignition delay.
• It improves combustion efficiency and extends the lean stability limit.
• Low inlet temperature can cause poor atomization and evaporation, increasing and unburned hydrocarbons.
• Excessively high temperature increases liner thermal loading and can encourage formation because of higher flame temperatures.

Thus, the combustor is generally easier to operate at high pressure and moderate-to-high inlet temperature, but cooling and emissions become more demanding.

The average gas residence time may be represented approximately by

where is the effective combustion volume.

• An increase in air mass flow at fixed chamber geometry generally raises velocity and decreases residence time.
• High reference velocity may weaken the recirculation zone and convect reactants out before combustion is complete.
• If the local flow velocity exceeds the effective flame-propagation capability, blowout can occur.
• Very low velocity gives longer residence time but may increase chamber size requirements, wall heating, pressure pulsation, or flashback tendency in premixed systems.
• Sufficient residence time is needed for atomization, evaporation, mixing, ignition, and completion of chemical reactions.

Combustor design therefore uses diffusion, swirl, and recirculation to provide low local velocities in the flame region while allowing a high overall engine mass flow.

• At a very lean fuel-air ratio, the heat release may be too small to compensate for heat losses and dilution. Reaction rates fall, combustion efficiency decreases, and lean blowout can occur.
• At a moderately lean overall ratio, local primary-zone recirculation can still maintain a near-stoichiometric flame while the total airflow limits outlet temperature.
• Near the appropriate primary-zone ratio, combustion is rapid and stable, and efficiency is high.
• At a rich ratio, oxygen becomes insufficient for complete combustion. Carbon monoxide, unburned hydrocarbons, smoke, and soot may increase.
• Excessively rich operation can produce rich blowout, although aircraft combustors usually encounter lean blowout more readily.
• Increasing fuel flow generally raises turbine-entry temperature until limited by available oxygen, cooling requirements, and turbine material temperature.
• tends to increase where local flame temperature and oxygen availability are high, whereas and hydrocarbon emissions become important at low temperature or incomplete-combustion conditions.

The flame tube or liner is protected by several cooling methods:

• Film cooling: Air enters through slots or rows of small holes and forms a relatively cool insulating layer along the hot-gas-side surface.
• Effusion cooling: A large number of closely spaced, inclined holes distribute cooling air uniformly and create overlapping protective films.
• Impingement cooling: Jets strike the cold side of the liner at high velocity, producing a high local heat-transfer coefficient.
• Convection cooling: Compressor air flows along liner walls or through channels and removes heat before entering the combustion zone.
• Transpiration cooling: Air passes through a porous wall and forms a continuous protective layer; it is highly effective but difficult and costly to manufacture.
• Thermal barrier coatings: Ceramic coatings reduce the heat conducted into the metal and supplement air cooling.

Practical chambers commonly combine impingement, convection, film, and effusion cooling.

In film cooling, relatively cool compressor air is discharged tangentially or at a shallow angle through liner slots or small holes. The air attaches to the liner surface and forms a protective film between the metal and hot combustion gases.

Cooling effectiveness may be expressed as

where is hot-gas temperature, is wall temperature, and is cooling-air temperature.

Its effectiveness depends on:

• Cooling-air mass flow and temperature
• Hole or slot geometry, spacing, and injection angle
• Momentum ratio between cooling jets and the main flow
• Density ratio of coolant to hot gas
• Surface curvature and pressure gradients
• Turbulence and mixing downstream of injection
• Uniformity of coolant distribution

Too much jet momentum can lift the film away from the wall, while excessive cooling-air use leaves less air available for combustion and dilution.

Flame stabilization is the process of maintaining a continuous flame at a fixed region despite the high through-flow velocity in a combustion chamber.

A stable recirculation zone works as follows:

• Swirlers, bluff bodies, or sudden expansions create a region of reversed and low-velocity flow.
• Hot combustion products are carried upstream toward the incoming fuel-air mixture.
• These products supply heat and chemically active species to ignite fresh reactants continuously.
• The low local velocity provides adequate residence time for fuel evaporation, mixing, and reaction.
• The recirculation boundary acts as the principal flame-anchoring region.

For stable operation, the local rate of flame propagation and continuous re-ignition must balance the transport of reactants. If the main flow becomes too fast or the mixture becomes too lean, the recirculation zone may no longer supply sufficient heat, causing flame blowout.

An air swirler consists of fixed vanes arranged around the fuel injector. The vanes impart a strong tangential velocity component to primary air entering the combustion zone.

Its operation involves:

• The rotating flow develops a radial pressure gradient, with lower pressure near the axis.
• At sufficiently high swirl, the adverse axial pressure gradient causes vortex breakdown and creates a central toroidal recirculation zone.
• Hot products circulate upstream and ignite freshly injected fuel-air mixture.
• Strong shear layers improve fuel-air mixing and increase flame speed.
• The swirler also helps spread atomized fuel and shapes the primary-zone flame.

Swirl must be selected carefully. Insufficient swirl gives weak recirculation and poor stability, while excessive swirl may cause large pressure loss, excessive liner heating, or undesirable combustion oscillations.

A flame holder is an obstruction or aerodynamic device placed in a high-speed combustible flow to create a sheltered wake in which a flame can remain anchored. Typical forms include V-gutters, bluff bodies, perforated plates, and struts.

Operation:

• Flow separates at the flame-holder edges.
• A low-velocity recirculating wake develops downstream.
• Hot products retained in this wake continuously ignite the fresh mixture passing around it.

Advantages:

• Extends the stable operating range
• Permits combustion in a flow whose main velocity exceeds normal flame speed
• Provides a definite flame location
• Promotes mixing and reliable ignition

Limitations:

• Causes stagnation-pressure loss and aerodynamic drag
• Experiences severe thermal loading
• Can generate wakes, vibration, and combustion instability
• May create non-uniform outlet temperature

Flame holders are especially important in afterburners, whereas main combustors commonly rely on swirl-generated recirculation.

Flame blowout:

• Occurs when the flame is swept downstream and extinguished.
• It is promoted by excessive airflow velocity, very lean or very rich mixtures, low pressure, low inlet temperature, poor atomization, and inadequate recirculation.
• Lean blowout is particularly important in aircraft combustors during rapid deceleration or high-altitude operation.

Flashback:

• Occurs when a flame propagates upstream into the premixing or fuel-delivery region.
• It is possible when local flame speed exceeds the opposing flow velocity.
• It is promoted by low flow velocity, high reactant temperature, highly reactive mixtures, boundary-layer separation, or upstream recirculation.
• Flashback is mainly a concern in premixed combustion systems; conventional diffusion-flame aircraft combustors are less susceptible.

Thus, blowout results from excessive convection or inadequate reaction, while flashback results from upstream flame propagation.

A combustion chamber must provide dependable ground starting, in-flight ignition, and altitude relighting.

• During starting, compressor delivery pressure and airflow are low, so the fuel must be finely atomized and directed near the igniter.
• The igniter creates an initial high-energy kernel that must reach a combustible local mixture.
• In can or can-annular chambers, interconnector tubes transmit flame from ignited cans to adjacent cans.
• After successful light-up, recirculating hot products sustain combustion and the igniter may be switched off.
• At altitude, low pressure and temperature reduce reaction rate, impair fuel evaporation, and increase ignition delay.
• High flight speed can increase combustor airflow and shorten residence time, further restricting the relight envelope.
• Relighting may require reduced engine speed, optimized fuel scheduling, continuous ignition, or high-energy igniters.

The relight envelope specifies combinations of altitude, flight Mach number, engine speed, and fuel flow for which reliable ignition is possible.

Combustor design involves several competing requirements:

• Compactness versus residence time: A small chamber reduces weight and drag but provides less time for evaporation, mixing, and reaction.
• Mixing versus pressure loss: Strong swirl, jets, and turbulence improve combustion and temperature uniformity but increase stagnation-pressure loss.
• Flame stability versus emissions: A high-temperature near-stoichiometric primary zone stabilizes the flame but can increase and liner thermal loading.
• Cooling versus combustion air: More cooling air protects the liner, but less air remains for primary combustion and outlet-temperature control.
• Low emissions versus stability: Very lean combustion can reduce , but it approaches lean blowout and may cause thermoacoustic instability.
• Fine atomization versus injector complexity: Small droplets improve evaporation and efficiency, but sophisticated injectors increase cost and may be sensitive to blockage.
• Uniform outlet temperature versus chamber length: Greater mixing length improves pattern factor but increases size and weight.
• Durability versus mass: Thicker, stronger liners last longer but increase weight and thermal stress.

A successful design balances these factors over the complete engine operating envelope rather than optimizing only one design condition.