Unit 5: Combustion Chambers

ASE202 — Propulsion-I 10 min read

I. Orientation — Purpose and Governing Principles

A gas-turbine combustion chamber, or combustor, converts the chemical energy of fuel into thermal energy by continuous combustion between the compressor and turbine. It must release heat efficiently while delivering a stable, uniform, low-loss gas stream at a turbine-compatible temperature.

Primary function: Fuel is atomized, vaporized, mixed with compressed air, ignited, and burned; dilution air then reduces the combustion products to the required turbine-entry temperature.
Steady-flow operation: Unlike a reciprocating-engine cylinder, the combustor burns continuously at approximately constant pressure while air and products flow through it.
Overall energy balance: Neglecting kinetic-energy change and casing heat loss,
TEXT
```
ṁa cp T3 + ṁf QLHV = (ṁa + ṁf) cp T4
```
- ṁa = air mass-flow rate, kg/s.
- ṁf = fuel mass-flow rate, kg/s.
- cp = mean specific heat at constant pressure, J/(kg·K).
- T3, T4 = combustor-inlet and outlet temperatures, K.
- QLHV = fuel lower heating value, J/kg.
Operating requirement: The chamber must retain a flame despite inlet velocities commonly exceeding the turbulent flame speed; recirculation zones provide the necessary low-velocity region.
Design characteristics: High combustion efficiency, low pressure loss, stable ignition, wide operating range, low emissions, compactness, durability, and a uniform outlet-temperature profile are required simultaneously.

II. Combustor Configuration and Design — Architecture for Controlled Heat Release

A. Classification of combustion chambers

Combustion chambers are classified mainly by the arrangement of their flame tubes and the manner in which compressor air is distributed around them.

Can or tubular chamber:
- Construction: Several separate cylindrical flame tubes are arranged around the engine axis, each with its own fuel injector; interconnector tubes spread ignition between cans.
- Advantages: Individual cans are easy to develop, inspect, remove, and test; structurally simple liners tolerate thermal expansion.
- Limitations: A large frontal area, greater mass, and uneven circumferential outlet temperature make the arrangement less suitable for compact modern engines.
Annular chamber:
- Construction: A single continuous annular flame tube surrounds the engine shaft, with injectors distributed around its circumference.
- Advantages: It offers the shortest chamber, low surface-area-to-volume ratio, reduced weight, lower cooling-air demand, and good circumferential temperature uniformity.
- Limitations: Development and rig testing are more complex; liner replacement may require extensive engine disassembly.
Can-annular or tubo-annular chamber:
- Construction: Separate flame tubes are placed inside a common annular casing and connected by cross-fire tubes.
- Advantages: It combines modular cans with a common pressure shell and was widely used in early axial-flow engines.
- Limitations: It is heavier and longer than a comparable annular combustor.
Flow-based variants:
- Straight-through combustor: Compressor air and combustion gases move mainly in the same axial direction.
- Reverse-flow combustor: Flow reverses direction within the chamber, reducing engine length but increasing turning losses; it is common in compact turboshafts.

B. Important factors affecting combustion chamber design

Combustor design balances aerodynamic, thermal, chemical, structural, and environmental requirements that often conflict with one another.

Combustion intensity: High heat release per unit volume reduces chamber size, but excessive loading shortens residence time and may cause incomplete combustion.
Pressure loss: The total-pressure drop must be sufficient to drive air through liner holes yet remain small enough to preserve cycle efficiency; practical combustors generally use only a small percentage of compressor-delivery pressure.
TEXT
```
Pressure-loss fraction = (Pt3 − Pt4) / Pt3
```
- Pt3 = combustor-inlet total pressure.
- Pt4 = combustor-outlet total pressure.
Air distribution: Approximately stoichiometric conditions are created locally in the primary zone, while substantial remaining air supports secondary combustion, dilution, and liner cooling.
Residence time: Combustion requires enough time for atomization, evaporation, mixing, and reaction:
TEXT
```
tr = Vz / Q
```
- tr = residence time, s.
- Vz = effective zone volume, m³.
- Q = volumetric flow rate through that zone, m³/s.
Outlet-temperature pattern: Hot streaks must be limited because turbine blades exposed above their design temperature experience accelerated oxidation, creep, and thermal fatigue.
Mechanical constraints: Liners must withstand vibration, pressure loading, thermal gradients, repeated starts, and differential expansion relative to the outer casing.
Fuel and emissions: Injector design must accommodate fuel properties while controlling smoke, unburned hydrocarbons, carbon monoxide, and nitrogen oxides.

III. Combustion and Performance — From Fuel Injection to Turbine Entry

A. Combustion process

The combustion process is organized into distinct aerodynamic zones so that a stable flame can coexist with a high-velocity continuous airflow.

Fuel preparation: A pressure-swirl, air-blast, or vaporizing injector divides liquid fuel into droplets; smaller droplets have a higher surface-area-to-volume ratio and evaporate faster.
Primary zone: Swirl vanes and primary holes admit roughly enough air to establish an ignitable mixture around the spray. A toroidal recirculation zone returns hot products and active radicals toward the injector.
Stoichiometric requirement: For a hydrocarbon represented by CxHy, complete combustion is:
TEXT
```
CxHy + (x + y/4)O2 → xCO2 + (y/2)H2O
```
- x, y = numbers of carbon and hydrogen atoms in one fuel molecule.
Secondary zone: Additional air completes oxidation of carbon monoxide, unburned hydrocarbons, soot precursors, and remaining fuel fragments.
Dilution zone: Large liner holes introduce relatively cool air, lowering the mean gas temperature and shaping the radial and circumferential profiles before the turbine nozzle.
Actual mixture strength: The equivalence ratio compares the actual and stoichiometric fuel–air ratios:
TEXT
```
φ = (F/A)actual / (F/A)stoichiometric
```
- φ = 1 is stoichiometric, φ < 1 is lean, and φ > 1 is rich.
Ignition sequence: An igniter lights one local mixture during starting; cross-fire openings or circumferential propagation ignite the remaining region, after which continuous ignition is normally unnecessary.

B. Combustion chamber performance

Combustion chamber performance is judged by how completely and stably heat is released with minimum aerodynamic and thermal penalties.

Combustion efficiency: Efficiency compares the actual stagnation-temperature rise with that available from complete fuel combustion:
TEXT
```
ηb = actual heat added to gas / (ṁf QLHV)
```
- ηb = combustion efficiency.
- ṁf QLHV = chemical-energy input rate, W.
- Values near unity are expected at normal design operation; efficiency decreases during weak ignition or poor atomization.
Total-pressure loss: Loss arises from liner-hole jets, mixing, wall friction, flow turning, and heat addition. Greater loss lowers turbine work and engine thrust.
Stability limits: The stable range lies between lean blowout, where heat generation cannot sustain ignition, and rich extinction or smoking operation, where oxygen and mixing are inadequate.
Pattern factor: Outlet-temperature non-uniformity may be expressed as:
TEXT
```
PF = (T4,max − T4,avg) / (T4,avg − T3)
```
- T4,max = maximum combustor-outlet temperature.
- T4,avg = average outlet temperature.
- A smaller PF indicates better turbine protection.
Relight capability: The chamber must reignite under specified altitude, pressure, temperature, and airflow conditions following flame extinction.
Emission performance: Carbon monoxide and unburned hydrocarbons indicate incomplete low-temperature combustion; smoke indicates rich local regions, while nitrogen oxides rise strongly with high flame temperature and residence time.

C. Effect of operating variables on performance

Combustor behavior changes with pressure, temperature, airflow, fuel flow, and engine transient conditions.

Inlet pressure: Higher pressure generally accelerates chemical reactions and improves ignition and combustion efficiency; low pressure at altitude increases relight difficulty and lean-blowout tendency.
Inlet temperature: Warmer compressor-delivery air promotes evaporation and reduces ignition delay, but elevated flame temperatures can increase nitrogen-oxide formation.
Air mass flow and velocity: Increasing flow shortens residence time and raises aerodynamic loading; if recirculation becomes inadequate, the flame may blow out.
Fuel–air ratio: Increasing fuel flow from a very lean condition strengthens combustion, but excessive enrichment produces local oxygen deficiency, smoke, liner heating, and high outlet temperature.
Fuel atomization: Greater injector pressure differential or stronger air-blast action usually reduces droplet diameter. Poor atomization causes wall wetting, delayed evaporation, carbon deposits, and unburned hydrocarbons.
Altitude operation: Reduced ambient pressure lowers compressor-delivery density and reaction rates, narrowing the ignition and stability range even when flight velocity is high.
Acceleration and deceleration: Rapid acceleration temporarily enriches the mixture and raises turbine temperature; rapid deceleration may make the primary zone too lean and cause flameout.
Hardware condition: Blocked injector passages, eroded liner holes, carbon deposits, or damaged seals alter local air–fuel distribution and create hot streaks or unstable burning.

IV. Thermal Protection and Flame Control — Maintaining Liner Life and Stable Burning

A. Flame tube cooling

Flame tube cooling prevents the metal liner from approaching the much higher temperature of the combustion gases.

Cooling-air source: Compressor-delivery air flows through the annulus between the casing and liner before entering through designed holes, slots, or louvers.
Film cooling: Rows of slots introduce a thin layer of relatively cool air along the inner wall, insulating the liner from convection and direct flame radiation.
Effusion cooling: Numerous small inclined holes form an overlapping protective air film while cooling the metal within each passage.
Impingement cooling: Jets strike the liner’s outer surface, producing high local heat-transfer coefficients; it is often combined with film or effusion cooling.
Radiation control: Soot luminousity and flame proximity increase radiative heat flux, so primary-zone aerodynamics must keep the reaction core away from the wall.
Design compromise: Cooling air protects the liner but is unavailable for primary combustion or dilution; excessive cooling can quench reactions and increase carbon monoxide or unburned hydrocarbons.
Material protection: Nickel-based alloys, thermal-barrier coatings, compliant joints, and controlled expansion gaps improve resistance to oxidation and thermal fatigue.

B. Flame stabilization

Flame stabilization maintains continuous ignition by matching local flow conditions to chemical reaction and flame-propagation rates.

Velocity requirement: A flame remains stationary where the local gas velocity normal to the flame approximately equals the turbulent flame speed; otherwise it moves upstream or downstream.
Recirculation principle: Reverse-flowing hot products continuously supply heat and reactive species to incoming fuel–air mixture, acting as an aerodynamic pilot.
Swirl stabilization: Strong angular momentum creates a central low-pressure region and vortex breakdown, producing an internal recirculation zone near the injector.
Stability boundary: Lean blowout occurs when heat loss and reactant convection exceed heat release; rich extinction occurs when insufficient oxygen or poor mixing suppresses reaction.
Control measures: Proper primary-hole penetration, injector spray angle, dome geometry, and air split retain an ignitable mixture over starting, idling, acceleration, and full-power conditions.

C. Use of flame holders

Flame holders create sheltered low-velocity wakes in which combustion can remain anchored despite a high mainstream velocity.

Operating principle: A bluff body separates the flow, forming shear layers and a recirculating wake that traps hot combustion products and ignites fresh mixture continuously.
Typical forms: V-gutters, perforated gutters, rings, steps, struts, and swirl-generated aerodynamic holders are selected according to chamber geometry.
Application: Physical flame holders are especially important in ramjets and afterburners, where approach velocity is too high for an unsupported flame.
Placement: Fuel is introduced sufficiently upstream for mixing, while ignition and sustained burning occur in the holder’s wake.
Advantages: Flame holders widen stability limits, shorten the required combustion length, and improve light-up reliability.
Penalties: Their blockage creates total-pressure loss, drag, local hot spots, vibration, and thermal stress; excessive blockage can also produce non-uniform turbine-entry flow.