The Combustion Chamber - ATPL 021 — Airframe, Systems and Powerplant

Between compressor and turbine sits the only place in the engine where heat is added: a chamber in which a flame burns continuously, hour after hour, inside a machine moving air at hundreds of metres per second. This lesson covers how that trick is done: the slowed air, the divided flows, the swirl that anchors the flame, and the chamber shapes that engineering has settled on.

Purpose and requirements

The purpose of the combustion chamber is to add heat energy to the airflow by burning fuel in it, at essentially constant pressure, raising the gas temperature to the highest value the turbine downstream can accept. The requirements for combustion are the same three a match needs, engineered for permanence: fuel and air mixed in burnable proportions, an ignition source (needed only for starting: once lit, the flame sustains itself), and air moving slowly enough that the flame is not simply blown out.

The working principle

The slow-air requirement is the design's starting problem: kerosene's flame burns at only a few metres per second, while the air leaving the compressor moves at well over a hundred. That is the reason for the snout and the reduced axial velocity at the chamber's inlet: the diffusing entry slows the air sharply, and only then is a flame possible at all. The swirl vanes (swirler) around the fuel nozzle then finish the job: they spin the entering air into a vortex whose core flows backwards, a recirculation zone of nearly stationary, continuously re-ignited mixture on which the flame anchors, while the swirl also tears the fuel spray into a well-mixed cloud.

Primary and secondary airflow

Only a fraction of the compressor's air can be allowed near the flame, and the chamber divides the flow accordingly:

Primary airflow — roughly a fifth of the total, entering through snout and swirl vanes (and the first liner holes) to burn with the fuel in the combustion zone.
Secondary airflow — the remaining majority, flowing along the outside of the flame tube: it cools the liner walls (entering through the many small holes as a protective film), and its later portion is the dilution air, joining through the large downstream holes to mix with and cool the burnt gas to a temperature the turbine can accept.

The two mixture ratios follow from the split: in the primary zone, air and fuel meet at roughly 15:1 by mass, near the chemically correct ratio, because that is what burns completely and stably. Counting the total airflow through the whole chamber against the same fuel, the overall ratio is far leaner, roughly 45:1 to 130:1 across the operating range: most of the air was never there to burn, only to cool and dilute.

Drains, gas parameters and the outlet limit

The drain valves at the chamber casing's low points answer a starting hazard: fuel sprayed during a start that fails to light pools at the bottom of the chamber, and a later successful start would meet it as a sheet of flame. The drains let unburned fuel escape overboard; chamber pressure closes them automatically once the engine runs.

The gas parameters through the chamber: temperature rises enormously, the point of the exercise; static pressure stays essentially constant, falling only slightly to the friction and turbulence of pushing gas through the tube (the combustor loss the working-cycle graph showed); and velocity, kept deliberately low for the flame, rises moderately toward the exit as the heated gas expands. In the flame's core the temperature reaches around 2,000°C, far beyond what any turbine metal survives, which is why the dilution air exists: a typical maximum outlet temperature is of the order of 1,000 to 1,500°C, set not by the chamber but by what the turbine's first blades can bear.

Chamber types

Can (multiple can) type — separate flame tubes each in its own casing, connected by interconnector tubes so a start's flame can propagate between them. Simple to develop and maintain (one can at a time), but heavy, bulky and uneven in its delivery to the turbine.
Can-annular (cannular, turbo-annular) — individual flame tubes inside one shared annular casing: the maintenance convenience of cans with much of the annular design's compactness, the classic compromise of an older engine generation.
Annular — a single continuous flame ring around the engine: lightest, most compact, least liner surface to cool for its volume, and the most even temperature distribution at the turbine face, the modern standard.
Reverse-flow annular — an annular chamber in which the air doubles back: it flows rearwards outside the flame tube, turns, and burns flowing forwards before turning again into the turbine. The folding shortens the engine markedly, which suits small turboprops and helicopter engines built around centrifugal compressors.

What the exam asks

The component functions and the two ratios are recall; the slow-air reasoning, the primary/secondary division and the type comparisons carry the understanding marks. The overview, following the learning objectives:

Define the chamber's purpose and list the requirements for combustion

Describe the working principle, the reason for the snout's reduced axial velocity, and the swirl vanes' function

Define primary and secondary airflow and their purposes

Explain the primary-to-fuel and total-to-fuel mixture ratios

State the drain valves' function, the gas-parameter changes, and a typical maximum outlet temperature

Describe the can, can-annular, annular and reverse-flow annular types and their differences