Gas Turbine Combustor | Jet Engine Combustor Types & description

Combustors

A gas turbine engine's combustor is necessary for burning a mixture of fuel and air and delivering the resulting gas mixture to the turbine inlet at a temperature as uniform as feasible. A good combustion engine will satisfy the following criteria:

High efficiency:-fuel is burned almost entirely to ensure maximum energy release. Long range requires this.
Stable operation:-must be blowout-free at mass flow values ranging from idle to maximum power and at pressures spanning the aircraft's whole altitude range.
Low pressure loss:-Gases should be delivered to the turbine at their highest possible pressure to minimise pressure loss. When any cooling air travelling through the turbine re-enters the main air flow, some pressure loss is necessary to ensure the right direction of circulation.
Uniform temperature distribution:-The gases from the combustor should enter the turbine as close to the limiting metal temperature as feasible to achieve maximum engine performance. To safeguard the engine, hot areas in the flow will lower the acceptable turbine inlet temperature. Additionally, this will lower engine performance.
A well-designed combustor will be able to restart in flight over a wide range of altitudes and flight speeds.
Simple starting (primarily air starts).
Small size:- Engine capacity and diameter are directly influenced by combustor size. A high energy release rate in a tiny size is desired.
Low smoke:- a smoky combustion chamber is only a minor nuisance. A smoke-producing burner could help locate military aircraft in flight.
Low carbon formation:- Carbon buildup in the burner can obstruct vital air passages, raising metal temperatures above desired levels and shortening engine life.

Types of Gas Turbine Combustor

Combustors for aircraft engines have developed into three basic types in order to satisfy these requirements.

Can combustor
Annular combustor
Can-annular combustor

The Can Type Combustor

Can Type Combustor

The photo in the image depicts the Whittle W.1 turbojet's can combustor setup from the 1930s. 7 to 14 of these cans normally make up a complete engine combustor. In order to provide uniform combustion characteristics in each tube and to allow flame travel between cans for ignition since, in the typical installation, only two cans will be outfitted with igniters, these individual cans are interconnected by tubes located between the cans. the sequence of separate burner cans made up of an inner liner and an outer shell that make up the Can type combustor found in gas turbine engines. A centrifugal compressor outside rim is surrounded by the individual cans. Direct hot gas flow into the turbine occurs from the cans.

Can combustors have the drawback of not maximizing available space, which results in a huge engine diameter. They have relatively substantial pressure losses and are very hefty. The individual cans are simple to remove for inspection, and they allow for easier control of the fuel-air patterns than annular designs do.

Annular Combustor

The compressor and turbine are connected by a single annulus that is positioned around the gas generating shaft in the annular combustor. These combustors are particularly well suited for axial flow compressor engines and make the maximum use of the available space in the engine. They are the smallest and lightest type of combustion engine. The annular combustor also offers the lowest pressure loss and the greatest efficiency.

There are drawbacks to the annular combustor as well. Because of the wide diameter and thin walls, there can be structural issues. The complete combustor must be taken out of the engine for inspection or significant repairs from a maintenance perspective. Additionally vulnerable to combustion instability is the annular combustor.

Can-annular combustor

Can-annular combustor is a particular kind of combustion chamber seen in several large turbojet and turbofan engines. It is made up of small cans into which fuel is sprayed before being lit. The hot gases are collected and uniformly sent into the turbine by these cans, which mount on an annular duct. Some of the advantageous characteristics of the other two combustor types are combined in the can-annular type. It utilises the available space well while also having individual, replaceable inner liners. Additionally, compared to the annular combustor, it has better structural stability and a smaller pressure drop.

The universal traits present in all types of combustion processes serve as the basis for combustor design. Whether contemplating combustion in a fireplace or a jet engine, the needs resulting from these features are fundamental. them being:

(1) proper mixture ratio

(2) temperature of the reactants

(3) turbulence for good mixing and

(4) time for burning.

Additionally, the combustion process for aviation turbine engines must be completed with a respectable pressure loss.

Any combustion process must have a certain mixture ratio because there are lean and rich flammability limits above which burning is impossible. In terms of fuel-air ratio, the lean limit for JP-type fuels is approximately 0.04, and the rich limit is approximately 0.20.

The stoichiometric fuel-air ratio for the majority of hydrocarbon fuels is approximately 0.0667. As a result, it is clear that any combustor must maintain a mixture ratio that is within permissible limits if burning is to occur at all, and within much stricter limits if good burning is to occur. It is important to operate the combustor with a significant amount of extra air to provide appropriate cooling in order to lower the temperature of the gases leaving the combustor to an admissible value. The significant amount of extra air needed lowers the overall fuel-air ratio to a value that is often less than 0.02. This fuel-to-air ratio is obviously too low for combustion, thus the burner design must include a way to remove between 60 and 75 percent of the air surrounding the actual combustion zone. The air that is bypassed is referred to as secondary air because it does not participate in combustion, whereas the remaining air, which does, is referred to as primary air. The amount of primary air is dictated by the fuel-air ratio in the actual combustion zone.

Reverse-flow combustor

In Reverse-flow combustor air from the compressor enters the outer casing before flowing backward into the inner liner. As it enters the inner lining, it changes direction once more. Before passing past the turbine, it turns around once more. Where engine length is important, reverse-flow combustors are used.