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Aircraft Propulsion - Level 3

Gas Turbine Operation and Design Requirements

Gas Turbine Components

A greater understanding of the gas turbine and its operation can be gained by considering its three major components (Figs. 1, 2 and 3 found in the three previous sections): the compressor, the combustor and the turbine. The features and characteristics will be touched on here only briefly.

Compressors and Turbines: The compressor components are connected to the turbine by a shaft in order to allow the turbine to turn the compressor. A single shaft gas turbine (Fig. la and 1b) has only one shaft connecting the compressor and turbine components. A twin spool gas turbine, which is found in land- and marine-based applications, has two concentric shafts, a longer one connecting a low pressure compressor to a low pressure turbine (the low spool) which rotates inside a shorter larger diameter shaft. The shorter, larger diameter shaft connects the high pressure turbine with the higher pressure compressor (the high spool) which rotates at higher speeds than the low spool. A triple spool engine would have a third, intermediate pressure compressor-turbine spool.

Gas turbine compressors are either centrifugal or axial, or can be a combination of both. Centrifugal compressors (with compressed air output around the outer perimeter of the machine) are robust, generally cost less and are limited to pressure ratios of 6 or 7 to 1. They are found in early gas turbines or in modern, smaller gas turbines.
The more efficient, higher capacity axial flow compressors (with compressed air output directed along the center line of the machine) are used in most gas turbines (e.g. Figs. 2 and 3). An axial compressor is made up of a relatively large number of stages, each stage, consisting of a row of rotating blades (airfoils) and a row of stationary blades (stators), arranged so that the air is compressed as it passes through each stage.

Turbines are generally easier to design and operate than compressors, since the hot air flow is expanding rather than being compressed. Axial flow turbines (e.g. Figs. 2 and 3) will require fewer stages than an axial compressor. There are some smaller gas turbines that utilize centrifugal turbines (radial inflow), but most utilize axial turbines.

Turbine design and manufacture is complicated by the need to extend turbine component life in the hot air flow. The problem of ensuring durability is especially critical in the first turbine stage where temperatures are highest. Special materials and elaborate cooling schemes must be used to allow turbine airfoils that melt at 1800-1900°F to survive in air flows with temperatures as high as 3000°F.

Combustors: A successful combustor design must satisfy many requirements and has been a challenge from the earliest gas turbines of Whittle and von Ohain. The relative importance of each requirement varies with the application of the gas turbine, and of course, some requirements are conflicting, requiring design compromises to be made. Most design requirements reflect concerns over engine costs, efficiency, and the environment. The basic design requirements can be classified as follows:

  1. High combustion efficiency at all operating conditions.
  2. Low levels of unburned hydrocarbons and carbon monoxide, low oxides of nitrogen at high power and no visible smoke for land-based systems. (Minimized pollutants and emissions.)
  3. Low pressure drop. Three to four percent is common.
  4. Combustion must be stable under all operating conditions.
  5. Consistently reliable ignition must be attained at very low temperatures, and at high altitudes (for aircraft).
  6. Smooth combustion, with no pulsations or rough burning.
  7. A low temperature variation for good turbine life requirements.
  8. Useful life (thousands of hours), particularly for industrial use.
  9. Multi-fuel use. Characteristically natural gas and diesel fuel are used for industrial applications and kerosene for aircraft.
  10. Length and diameter compatible with engine envelope (outside dimensions).
  11. Designed for minimum cost, repair and maintenance.
  12. Minimum weight (for aircraft applications).

A combustor consists of at least three basic parts: a casing, a flame tube and a fuel injection system. The casing must withstand the cycle pressures and may be a part of the structure of the gas turbine. It encloses a relatively thin-walled flame tube within which combustion takes place, and a fuel injection system.

Compared to other prime movers (such as Diesel and reciprocating automobile engines), gas turbines are considered to produce very low levels of combustion pollution. The gas turbine emissions of major concern are unburned hydrocarbons, carbon monoxide, oxides of nitrogen (NOx) and smoke. While the contribution of jet aircraft to atmospheric pollution is less than 1%, jet aircraft emissions injected directly into the upper troposphere have doubled between the latitudes of 40 to 60 degrees north, increasing ozone by about 20%. In the stratosphere, where supersonic aircraft fly, NOx will deplete ozone. Both effects are harmful, so further NOx reduction in gas turbine operation is a challenge for the 21st century.

The original article from which this section is extracted, Introduction to Gas Turbines for Non-Engineers, by Lee S.Langston, University of Connecticut and George Opdyke, Jr., Dykewood Enterprises, can be found in the ASME International Gas Turbine Institute's "Global Gas Turbine News", Volume 37, No.2, 1997, and has been used with permission.

 

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Updated: February 23, 1999