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Most modern passenger and military aircraft are powered by gas turbine engines, which are also called jet engines. There are several different types of gas turbine engines, but all turbine engines have some parts in common. All gas turbine engines have a power turbine located downstream of the burner to extract energy from the hot flow and turn the compressor. Work is done on the power turbine by the hot exhaust flow from the burner.

Description of Images

The bottom of the figure shows:

• computer drawings of a turbojet with the location of the turbine relative to the other engine components, on the right
• the turbine section alone with the central shaft attached to the turbine, on the left.

In both drawings, the turbine is magenta in color and the shaft is colored blue. The left end of the shaft would be attached to the compressor, which is colored cyan in the drawing on the right. Here is an animated version of the turbine section:

The upper left of the figure shows an actual power turbine. The turbine, like the compressor, is composed of several rows of airfoil cascades. Some of the rows, called rotors, are connected to the central shaft and rotate at high speed. Other rows, called stators, are fixed and do not rotate. The job of the stators is to keep the flow from spiraling around the axis by bringing the flow back parallel to the axis.

Depending on the engine type, there may be multiple turbine stages present in the engine. Turbofan and turboprop engines usually employ a separate turbine and shaft to power the fan and gear box respectively. Such an arrangement is termed a two spool engine. For some high performance engines, an additional turbine and shaft is present to power separate parts of the compressor. This arrangement produces a three spool engine. The power turbine shown on the upper left of the figure is for a two spool, turbofan engine.

Design Details

There are several interesting turbine design details present on this slide. Since the turbine extracts energy from the flow, the pressure decreases across the turbine. The pressure gradient helps keep the boundary layer flow attached to the surface of the turbine blades. Since the boundary layer is less likely to separate on a turbine blade than on a compressor blade, the pressure drop across a single turbine stage can be much greater than the pressure increase across a corresponding compressor stage. A single turbine stage can be used to drive multiple compressor stages. Because of the high pressure change across the turbine, the flow tends to leak around the tips of the blades. The tips of turbine blades are often connected by a thin metal band to keep the flow from leaking, as shown in the picture at the upper left.

Turbine blades exist in a much more hostile environment than compressor blades. Sitting just downstream of the burner, the blades experience flow temperatures of more than a thousand degrees Fahrenheit. Turbine blades must be made of special materials that can withstand the heat, or they must be actively cooled. At the upper right of the figure, we show a picture of a single, actively cooled turbine blade. The blade is hollow and cool air, which is bled off the compressor, is pumped through the blade and out through the small holes on the surface to keep the surface cool.

• Parts of a Jet Engine:
• Power Turbine:
• Turbojets:
• Afterburning Turbojets:
• Turbofans:

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