Overview

Much that has been said about compressors applies equally well to turbines, but two factors lead to major differences between turbines and compressors. First, the high gas temperature at a turbine inlet introduces material problems much more serious than those associated with a compressor, and has led to blade cooling in modern aircraft engines. (The high temperature also leads to lower tangential Mach numbers for turbine blades than for compressor blades with the same blade speed, and this eases the aerodynamic problems somewhat.) Second, the pressure falls through the turbine rather than increasing as in the compressor. This dropping pressure thins the boundary layers, reducing separation problems and rendering the aerodynamic design less critical.

The turbine efficiency is less critical to the performance of a turbojet engine than the compressor efficiency (see section 3.7), and because of this and other factors there was considerably less detailed aerodynamic development of the turbine than of the compressor in early work on aircraft engines. The situation has now changed because turbine efficiencies are critical in high-bypass turbofans, and the weight of the turbine can be a major part of the total weight of such engines. The cost of the turbine can be a major part of the total cost of any engine because of the difficult and expensive materials. So there is a great incentive to reduce the number of turbine stages while increasing efficiency.

To clarify the relationship between the turbine and compressor, consider a turbojet engine with equal compressor and...