Turbofan:Combustors

Turbofan combustor

Typical designs

A good summary can be found at chapter 5 of the book "Jet engines: fundamentals of theory, design, and operation". We summarize it even more here. Three main designs exist: can combustors, can-annular combustor, and annular combustor. They were invented in that order, to solve some of the flaws of previous models.

The basic principle is that the combustor takes the compressor discharge flow, which has a high velocity, a high pressure, and a high enough temperature for the combustion to occur with a reduced risk of flaming-out. The first job of the combustor is to slow down the flow so that it can be mixed to fuel and provide a stable flame. The second task of the combustor is to prevent the flame from spreading outside it, and thus preventing a too hot flow to spread on other parts of the engine. Its design ensures that the flame will be properly confined, and that the output gas temperature will be acceptable for the turbine. The hotter the gas is discharged in the turbine, the more energy will be gained from the combustion. To limit this temperature, and also to limit the combustor temperature to avoid its own melting down, diluting air is introduced in the combustor.

A can combustor features mechanisms to slow down the input air flow, fuel injection nozzle, air dilution flows, and a hot gas discharge. Several can combustors are then placed circumferentially in the engine's core. The main disadvantage is the loss of useful volume in the combustor section, and the non-uniformity of the hot gas discharge.

The can-annular combustor has a single air dilution diffuser for all combustion cans, making more efficient use of the volume and reducing the design complexity and weight.

Finally, annular combustors are designed as a single annular part taking the maximum available volume from the section. All modern engines use this design, so do we.

Combustor properties

Combustor, like other parts of the engine, operate with air flow. It thus affects its properties, like temperature, pressure and velocity. A combustor is said efficient when it minimizes the pressure drop. The air used for combustion must be slowed down, and this is generally done by both expanding the input flow and swirling a part of it. This swirl causes turbulences that imply pressure loss, and thus efficiency reduction of the combustor.

The produced hot gas will be used to provide energy to the engine, and this energy needs to be in an easily extractable form. High temperatures and high velocities are appreciable for turbine work extraction. There is probably no upper bound for velocity, but there is one for temperature, the temperature that the turbine inlet vanes can sustain, and the temperature that the turbine blades can sustain.

Our design

Flame holding, Cooling, Material, Ignition.