Gas Turbines - Introduction — Page 206, Lesson 277

Let me walk you through the duct design inside a gas turbine engine. This is where we control the airflow's velocity and pressure as it moves through the engine. As the air passes through the engine, we demand various changes in its velocity and pressure. For example, throughout the compression stage, the air must be compressed but without any appreciable increase in its velocity. Another example is at the exhaust nozzle, where the pressure of the gas is dropped to that of ambient — that's atmospheric pressure — with a considerable increase in its velocity. These changes in pressure and velocity are accomplished by the different shaped passages or ducts through which the air must pass before it exits the engine. The design of these ducts is extremely important because the efficiency with which the changes from velocity energy — that's kinetic energy — to pressure energy — that's potential energy — and vice versa occur, are reflected in the overall efficiency of the engine. Let me show you two examples of duct shapes used within the engine. In the top example, we have a divergent duct. A divergent duct widens as the air flows through it. This duct increases the pressure of the air after it leaves the final stage of the compressor and before it enters the combustion chamber. This air is sometimes called 'compressor delivery air' — it's the highest pressure air in the engine. The advantage here is twofold. First, an increase in pressure with no expenditure of energy in driving the compressor. Second, a decrease in velocity, which will serve in making the task of the combustion chamber less difficult. In the bottom example, we have a convergent duct. This narrows as the air flows through it. It's used to accelerate the gas as it passes through the nozzle guide vanes on its way to the turbine blades. So the key principle here: a divergent duct converts velocity energy into pressure energy, and a convergent duct converts pressure energy into velocity energy. The efficiency of these conversions directly affects the overall efficiency of the engine.