Types of combustion chamber
The following types of combustors – can-type (individual type), annular type and cannular type – are known to be mainly used for turbine engines.
The design of individual or can-type combustion chambers seems to be rather simple, however, it is quite complex in operation. Its operation is characterized by the necessity of flame stabilization that is the smooth continuous ignition process. In order to stabilize the flame and secure complete combustion, a strong reverse flow of heated air and gas must be produced. The replacement of individual combustion chambers and liners is possible without complete engine disassembly.
The annular combustion chamber is a single chamber completely surrounding the engine. Fuel is fed through fuel nozzles, which distribute the flame around the circumference of the chamber. The chamber outer case (liner) is punched with uniform rows of holes, which diffuse the compressed air and help to maintain an efficient flame pattern.
The annular chamber has important advantages: minimized friction between the gas and the combustor surfaces, improved annular pressure equalization and reduced weight, all adding to the efficiency of this combustor type.
Harmful emission of jet engines
Because it is an internal combustion engine whose exhaust gases flow directly into the environment, a jet engine is a serious source of air pollution. Because of its high level of noise, it’s also causes noise pollution.
Air pollution results from the combustion process of the gas-turbine engine. Jet-engine emissions, including carbon dioxide, carbon monoxide, hydrocarbons, and nitrogen oxide gas, contribute to both the greenhouse effect and atmospheric ozone depletion. They also endanger the health of people especially near airports.
Nitrogen dioxide emissions contribute to acid-rain formation. The emission of hydrocarbons contributes to ozone formation. In terms of these emissions, the new high-bypass turbofan jet engines pollute much less than older turbofan and turbojet engines. Nitrogen oxides have a possible role in ozone depletion, and its reduction can only be effected by less air traffic in general.
The noise from a nearby jet takeoff is about 110 decibels. The main source of jet-engine noise is the propulsion system and the resultant noises generated by both internal and external processes.
Turboprop engines
If a gas generator (turbojet) turns an aircraft propeller through a system of gears, it becomes a turboprop. The gas generator for a turboprop might be either a single- or dual-compressor type, although there are no dual-axial compressor turboprops in production.
Although a turboprop is more complicated and heavier than a turbo-jet engine of equivalent size and power, it will deliver more thrust up to medium-high airspeeds. However, the advantage decreases as flight speed increases. In normal cruising-speed ranges, the propulsive efficiency of a turboprop remains more or less constant, whereas the propulsive efficiency of a turbojet increases rapidly as airspeed increases. The spectacular performance of a turboprop during take-off and climb is the result of the ability of the propeller to accelerate a large mass of air while the aircraft is moving at relatively low ground and flight speed.
Turbofan engines
Fanjets and turbofans are one and the same thing. In principle, the turbofan (or fanjet) is the same as the turboprop except that the ratio of the secondary airflow to the primary airflow through the basic engine is less.
Turbofans are rapidly becoming the most widely used of all the types of jet engines, particularly in large multi-engine aircraft. The turbofan is a compromise between the good operating efficiency and high-thrust capability of a turboprop and the high-speed, high-altitude capability of a turbojet. At cruising altitude, the engine-propeller combination of a turboprop loses efficiency rapidly at airspeeds above 400 knots. Not only does the turbofan have no such limitation but it is much simpler than a turboprop.
The fact that the fan air does not pass through the basic engine enables a turbofan to achieve relatively low specific fuel consumption. In addition, because it accelerates a large mass of air to relatively low velocity, even at very low aircraft speed, a turbofan, like a turboprop, produces much more thrust than a turbojet during take-off and the initial climb.
Another advantage of the turbofan is a lower noise level, which is an important feature at all commercial airports. The lower level of noise occurs because a turbo-fan engine has at least one additional turbine stage to drive the fan. Less velocity through the jet nozzle results in less noise.