he construction of the entire exhaust system: the exhaust case, the exhaust cone (or tail cone), and their geometry, is governed by the principles of gas dynamics and by the need to meet regulations, particularly, noise regulations.
Energy remaining in the exhaust gases after they leave the turbine can be utilized to produce thrust. In fact, in the early jet engines, this was the ONLY way to produce thrust.
To do this; components in the exhaust system must straighten and accelerate the stream of gases.
Having spent all that money, time, and effort in designing the rest of the engine, the exhaust is the culmination: will the engine produce the absolute maximum thrust possible?
Only a nozzle designed with the known principles of hot gas dynamics will give you the maximum possible thrust.
After the gases leave the turbine, they flow through a duct formed between the exhaust cone and the exhaust, or tail, pipe. Depending on the aircraft design the exhaust can be: divergent, convergent or convergent-divergent.
▲Convergent Exhaust Duct. A convergent exhaust duct is formed between the exhaust cone and the exhaust pipe. The exhaust cone is the fixed conical faking centred in the exhaust stream immediately aft of the last-stage turbine wheel.
▲A convergent-divergent Exhaust Duct, showing the “choked” nozzle, causes an increase in thrust by increased acceleration of the exhaust gases.The gases leave the turbine section and enter the convergent portion of the nozzle at a subsonic speed. Their speed increases as the duct gets smaller until they reach the speed of sound at the narrowest point where a shock wave forms and prevents further acceleration. The gases leave the narrowest point at the speed of sound, and as the duct area increases, they accelerate to a higher supersonic speed. The benefits of a CD nozzle increase as the flight Mach number of the aircraft increases.
Most engines have an exhaust collector with struts between the forward end of the exhaust cone and the tail pipe to support the rear turbine bearing and straighten the gas flow. As gases flow through the convergent duct between the exhaust cone and the exhaust pipe, they are accelerated and leave the exhaust nozzle at the highest practical velocity.
▲Construction details of the exhaust collector and discharge system
And that velocity was responsible for the terrific noise of the early jet engines.
The opening at the end of the tail pipe is called the exhaust nozzle, or jet nozzle. Its outlet area is critical because it determines the velocity of the gases as they leave the engine.
The nozzles on most turbojet and early low-bypass turbofan engines have an area that usually causes them to operate in a choked condition. By the time the gases reach the end of the tail pipe, they have accelerated to the speed of sound and can accelerate no further. The remaining energy that would otherwise be converted into velocity is now converted into a pressure differential across the nozzle. This differential produces a small increase in thrust. This condition soon turns uneconomical in terms of fuel consumption. Also temperatures within the engine would elevate significantly.
Modern high bypass turbofan engine exhaust nozzles work in unchoked condition only.
▲The complex tail cone shape of the first of the big, high-bypass engines: the Pratt and Whitney JT9D on the first Boeing 747. This “coke-bottle” shape was dictated by aerodynamic performance and noise considerations.
Higher hot exhaust airflow speed is not desirable, considering that nowadays environment pollution and noise reduction have become very important aspects in engine design. Gas parameters at turbine section outlet and flow diameter are tuned in a way that at outlet diameter core exhaust gas path pressure is reduced to ambient and maximum speed of the hot gases is never higher than the speed of sound. Sound speed of hot exhaust gases is higher than that of ambient air sound speed because of temperature difference. This phenomenon allows aircraft with unchoked exhausts to fly at speeds up to mach 1.5 (transonic).
Turbofan engines extract much more energy from the exhaust gases to drive the fan, and their exhaust gas velocities are lower than those of a turbojet engine of comparable power. For these reasons, turbofan engines do not produce enough noise to require noise suppressors at the hot gas exhaust end. (Some engines are now beginning to have noise suppressors at the fan exit.)
▲ Engineering achievement: noise suppression by design! (EPNdB is a measure of noise.)
Author – Krishna Kumar Subramanian
