ديسمبر 06, 2020
Turbines
The final part of the gas turbine is the turbine section. This is where the energy from the fuel is converted into a form of mechanical energy, the rotation of the turbine shaft generating a torque. All gas turbines except for some very small machines use axial flow turbine sections. As with the compressor, the axial flow turbine will consist of a number of stages, each stage including a set of stationary vanes, usually called nozzles and a set of rotating blades that are attached to the turbine shaft.
There are two major types of turbine/blade design that can be applied to a gas turbine, each defined by the way it extracts energy from a fluid. The two are called reaction turbines and impulse turbines. One way of understanding the difference is to observe that reaction turbines exploit the static pressure in a fluid whereas impulse tur vbines exploit the dynamic pressure. This means that as fluid passes through a reaction turbine the static pressure drops but the fluid velocity which defines its dynamic pressure remains relatively constant. In contrast when a fluid passes through an impulse turbine stage the velocity drops while the static pressure remains constant. The stages of a modern axial gas turbine will generally combine the two, extracting part of their energy from the static pressure and partly from the dynamic pressure. It is common for the first stages to be predominantly impulse type while the latter stages are more reaction type. However all stages usually exploit both.
The order of the stationary vanes and rotating blades in the turbine is the reverse of the compressor. The high pressure, high temperatures gas from the combustor meets a stage’s vanes first and then is directed to its blades. The vanes form convergent ducts which turn static pressure into dynamic pressure, increasing the speed of the air passing through them. This dynamic pressure is then used to drive round the rotating blades. As in the compressor, both vanes and blades are shaped like aerofoils in order to ensure the smooth flow of air through the complete turbine. Each stage extracts a portion of the energy contained in the air.
In a simple gas turbine the compressor and the turbine blades are all on a single shaft. However there are more complex arrangements. In some machines there are two concentric shafts. One of these carries the compressor blades and the first one or two stages of turbine blades. The later turbine stages are attached to a second shaft that drive the generator to produce electrical power. In some aeroderivative gas turbinesthis is taken further still and the compressor stages are divided too. The low pressure compressor blades are then mounted onto the same shaft as low (or medium) pressure turbine stages while the high pressure compressor stages are on the same shaft as the high pressure turbine stages.
The efficiency of a gas turbine will depend on the temperature drop across the stages. In order to achieve high efficiency the turbine stage inlet temperature must be very high. In some modern gas turbines the inlet temperature can reach 1600°C. It requires very special materials and design techniques to design turbine components that can withstand this temperature.
The efficiency of a gas turbine will depend not only on the inlet gas temperature but also on the temperature of the gas as it leaves the final stage of the gas turbine. The exhaust gas from a simple cycle gas turbine, that is one not in a combined cycle configuration, needs to be a cool as possible in order to achieve maximum efficiency. However in a combined cycle power plant a part of the energy is captured in a steam generator that exploits waste heat in the gas turbine exhaust. The temperature of the exhaust gas exiting a turbine in this type of plant will be much higher. The temperature at the exhaust of a high efficiency aeroderivative gas turbine will probably be in the 400°C to 500°C range. While this is relatively high it still enables efficiency of up to 46% for the best machines. Other small industrial turbines will have efficiencies of up to 42%. Conversely large industrial gas turbines designed for combined cycle operation may have exhaust gas temperatures above 600°C. Efficiencies may be as low as 38% but will typically be up to 42%.
Do Gas Turbines Work?
Gas turbines are comprised of three primary sections mounted on the same shaft: the compressor, the combustion chamber (or combustor) and the turbine. The compressor can be either axial flow or centrifugal flow. Axial flow compressors are more common in power generation because they have higher flow rates and efficiencies. Axial flow compressors are comprised of multiple stages of rotating and stationary blades (or stators) through which air is drawn in parallel to the axis of rotation and incrementally compressed as it passes through each stage. The acceleration of the air through the rotating blades and diffusion by the stators increases the pressure and reduces the volume of the air. Although no heat is added, the compression of the air also causes the temperature to increase.
The compressed air is mixed with fuel injected through nozzles. The fuel and compressed air can be pre-mixed or the compressed air can be introduced directly into the combustor. The fuel-air mixture ignites under constant pressure conditions and the hot combustion products (gases) are directed through the turbine where it expands rapidly and imparts rotation to the shaft. The turbine is also comprised of stages, each with a row of stationary blades (or nozzles) to direct the expanding gases followed by a row of moving blades. The rotation of the shaft drives the compressor to draw in and compress more air to sustain continuous combustion. The remaining shaft power is used to drive a generator which produces electricity. Approximately 55 to 65 percent of the power produced by the turbine is used to drive the compressor. To optimize the transfer of kinetic energy from the combustion gases to shaft rotation, gas turbines can have multiple compressor and turbine stages.
The compressed air is mixed with fuel injected through nozzles. The fuel and compressed air can be pre-mixed or the compressed air can be introduced directly into the combustor. The fuel-air mixture ignites under constant pressure conditions and the hot combustion products (gases) are directed through the turbine where it expands rapidly and imparts rotation to the shaft. The turbine is also comprised of stages, each with a row of stationary blades (or nozzles) to direct the expanding gases followed by a row of moving blades. The rotation of the shaft drives the compressor to draw in and compress more air to sustain continuous combustion. The remaining shaft power is used to drive a generator which produces electricity. Approximately 55 to 65 percent of the power produced by the turbine is used to drive the compressor. To optimize the transfer of kinetic energy from the combustion gases to shaft rotation, gas turbines can have multiple compressor and turbine stages.