The growth of the gas turbine application in recent decades has been brought about most significantly by four factors:
1- Metallurgical advances that have made possible the employment of high temperatures in turbine components (especially turbine blades) and the combustors.
2- The cumulative background of aerodynamic and thermodynamic knowledge (especially from aircraft and spacecraft industries).
3- The utilization of advanced computer technology in the design and simulation of turbine airfoils, combustor and turbine blade cooling configurations.
4- The ability to simplify the control of this highly responsive machine through the use of modern control/digital technology including start/stop and to govern the minute to minute operation and also report on condition (diagnostics) and predict future failures (prognostics).
Combining the above has led to improvements in air-compressor design (increase in pressure ratio), combustor design (regenerators, various low NOx designs, etc), turbine design (single crystal blades, cooling, etc) and overall package performance.
Gas turbines have always been tolerant of a wide range of fuels including liquids and gases (high and low heating values) and also gasified coal, wood and bio fuels.
Air-compressors are generally axial with numerous stages (some designs use 19 stages for axial air-compressor). Centrifugal air-compressor design (one or two impellers) may be used in some small machines. The increase in compressor ratio is one of the contributors in the overall increase in simple cycle thermal efficiency to above 35% (especially for aero-derivative). Now modern aero-derivative gas turbine can offer more than 45% efficiency (in simple cycle) and more than 60% in combined cycle.
Combustor design is a complex task, often referred to a “black art”. The combustor design took two distinct configurations fairly early in the evolution of the gas turbine. These are the can-annular combustor and the annular section (including the single combustor). There are two types of can-annular combustors: 1- Straight flow combustor. 2- Reverse flow combustor. The advantage of the reverse flow combustor (as used in some modern gas turbine) is in use of a regenerator, which improves overall thermal efficiency. A further distinctive design approach is number, design and arrangement of fuel nozzles (per combustion chamber).
4 TURBINE SECTION
Present gas turbines use impulse-reaction turbine design. Aero-derivative units use high aspect ratio (long and thin) blades usually incorporating tip shrouds to dampen vibration and improve blade tip sealing characteristics. The heavy frame industrial gas turbines incorporate a low aspect ratio (short and thick) blade usually with no shroud. Traditionally where long thin airfoils have been used, lacing wire is employed to dampen vibration. Improvements in metallurgy and manufacturing techniques have allowed eliminating mid-span shrouds and lacing wires. Turbine blades are subject to considerable stresses resulting from high temperature, high centrifugal forces and thermal cycling.
5 GAS TURBINE PERFORMANCE CURVES
Minimum required performance curves: net output, net heat rate, exhaust temperature, exhaust flow versus ambient temperature for the specified fuel(s) at site conditions.
Amin Almasi is senior rotating equipment consultant in Brisbane, Queensland, Australia.