Table of Contents:

  • What Is Combustion Chamber?
  • Combustion Chamber Process
  • Factors Influencing Combustion Chamber Design

What Is Combustion Chamber?

The combustion chamber of a gas turbine is where the energy that drives the whole system is added. The combustion chamber of a modern turbine typically consists of a cylinder with a second smaller cylinder called the liner inside it. A fuel air mixture passes into the mouth of the liner and additional air may pass around the outside of it, between the liner and the outer cylinder to keep the liner cool. This air is then introduced through holes and slots along the liner.

Schematic Diagram Of An Open Cycle Gas Turbine


Combustion Chamber Process:

Air from the engine compressor enters the combustion chamber at a velocity up to 500 feet per second, but because at this velocity the air speed is far too high for combustion. The first thing that the chamber must do is to diffuse it, i.e. decelerate it and raise its static pressure. Since, the speed of burning kerosene at normal mixture ratios is only a few feet per second, any fuel lit even in the diffused air stream, which now has a velocity of about 80 feet per second, would be blown away.
A region of low axial velocity has therefore to be created in the chamber, so that the flame will remain alight throughout the range of engine operating conditions. In normal operation, the overall air/fuel ratio of a combustion chamber can vary between 45:1 and 130:1. However, kerosene will only burn efficiently at, or close to, a ratio of 15:1, so the fuel must be burned with only part of the air entering the chamber, in what is called a primary combustion zone. This is achieved by means of a flame tube (combustion liner) that has various devices for metering the airflow distribution along the chamber.

Combustion Chamber Processes – Air Separation Percentage Diagram

Approximately 20% of the air mass flow is taken in by the snout or entry section. Immediately downstream of the snout are swirl vanes and a perforated flare, through which air passes into the primary combustion zone. The swirling air induces a flow upstream of the center of the flame tube and promotes the desired re-circulation. The air not picked up by the snout flows into the annular space between the flame tube and the air casing. Through the wall of the flame tube body, adjacent to the combustion zone, are a selected number of secondary holes through which a further 20% of the main flow of air passes into the primary zone.

The air from the swirl vanes and that from the secondary air holes interacts and creates a region of low velocity recirculation. This takes the form of a vortex, which has the effect of stabilizing and anchoring the flame. It is arranged that the conical fuel spray from the nozzle intersect the recirculation vortex at its center. This action, together with the general turbulence in the primary zone, greatly assists in breaking up the fuel and mixing it with the incoming air. The temperature of the gases released by the combustion is about 1,800 – 2,000 0 C, which is far too hot for entry to the nozzle guide vanes of the turbine.

The air not used for combustion, which amounts to about 60% of the total airflow, is therefore introduced progressively into the flame tube. Approximately a third of this is used to lower the gas temperature in the dilution zone before it enters the turbine and the remainder is used for cooling the walls of the flame tube. This is achieved by a film of cooling air flowing along the inside surface of the flame tube wall, insulating it from the hot combustion gases.

Combustion should be completed before the dilution air enters the flame tube, otherwise the incoming air will cool the flame and incomplete combustion will result. An electric spark from an igniter plug initiates combustion and the flame is then self-sustained. The design of a combustion chamber and the method of adding the fuel may vary considerably, but the airflow distribution used to effect and maintain combustion is always very similar to that described.

Combustion Chamber Process Animation

combustion chamber process

Factors Influencing Design (Combustion Chamber Process):

  • Low turbine inlet temperature.
  • Uniform temperature distribution at turbine inlet (i.e., to avoid local overheating of turbine blades).
  • Stable operation even when factors like air velocity, A/F ratio & chamber pressure varies greatly, especially for aircraft engines (the limit is the ‘flameout’ of the combustion chamber) & at the event of a flame-out the combustor should be able to relight quickly.
  • The formation of carbon deposits (‘coking’) must be avoided. Can damage the turbine if breaks free.
  • Aircraft engines should avoid visible smoke as it hinders visibility in airports.
  • Finally, pollutants like NOx, CO, unburned Hydrocarbons(UHCs) etc. should be limited.

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