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HOME > PAST ISSUE > July-August 2013 > Article Detail

TECHNOLOGUE

The Adaptable Gas Turbine

Whether creating electricity or moving planes, this engine continues to inspire innovation

Lee S. Langston

Why Turbines?

The gas turbine has some design advantages over other power systems. It is capable of producing large amounts of useful power for a relatively small size and weight. Because motion of all its major components involves pure rotation (there is, for instance, no reciprocating motion as in a piston engine), its mechanical life is long and the corresponding maintenance cost is relatively low. However, during its early development, the deceptive simplicity of the gas turbine caused problems, until aspects of its fluid mechanics, heat transfer and combustion were better understood. In the words of Edward Taylor, the first director of the MIT Gas Turbine Laboratory, early gas turbine compressor designs foundered on a rock, and the rock was stall. Stall is the sudden blockage and even reversal of engine flow, caused when fluid separated away from the compressor airfoil surfaces instead of flowing evenly over them. Taylor paraphrased P. T. Barnum’s words to describe two kinds of stall: You can operate a compressor so it stalls all of the blades some of the time (called surge) or some of the blades all of the time (called rotating stall). It took much early research and development to avoid such stall conditions.

Although a gas turbine must be started by some external means (a small external motor or other source, such as another gas turbine), it can be brought up to full load (peak output) conditions in minutes, in contrast with a steam turbine plant whose startup time is measured in hours.

Gas turbines can also use a variety of fuels. Natural gas is commonly used in land-based gas turbines, whereas light distillate (or kerosene-like) oils power aircraft jet engines and marine gas turbines. Diesel oil or specially treated residual oils (such as biodiesel) can also be used, as well as combustible gases (such as methane) derived from blast furnaces, refineries, landfills, sewage and gasification of solid fuels such as coal, wood chips and bagasse (the crushed stalks of sugarcane or sorghum). Some recent work in South Africa on a type of nuclear power plant called a pebble bed reactor (which uses tennis ball–sized spheres of graphite embedded with fissile material) provided helium gas to power a type of turbine that has a closed cycle, meaning it uses a gas preheated by an external source that is recirculated through the system.

An additional advantage of gas turbines is that the usual working fluid is atmospheric air, and the machine does not require liquid cooling—an important consideration in many parts of the world, where cooling water is in short supply.

In the early days of its development, one of the major disadvantages of the gas turbine was its lower efficiency (hence higher fuel usage) when compared to other engines and steam turbine power plants. However, over the past 70 years, continuous engineering development has pushed the thermal efficiency (18 percent for the 1939 Brown Boveri gas turbine) to present levels of about 45 percent for simple cycle operation. Efficiencies can reach over 60 percent for combined-cycle operations, where exhaust gases are put to additional use.





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