Today, nearly 90% of electricity consumed in the United States is generated using gas turbines. Seeing how a vast majority of the population depends on such turbines to meet its electricity requirements, designers undertake extensive research to come up with the most efficient turbine designs. Moreover, aircraft also utilize different variations of gas turbines, such as turbofans, turbojets, turboshafts, and turboprops. Therefore, designing turbines with less weight and more efficient power generation is of utmost importance to various industries. To better understand the necessity and function behind systems like gas turbines, this blog will discuss modern three-stage gas turbines and what makes them valuable.
Since their patent in 1791 by John Barber, gas turbines have come a long way in terms of efficiency, increasing by nearly 60% compared to their initial days. Turbines are bladed machines that function on the principle of the Brayton cycle, where the working fluid's potential energy is converted into the rotating shaft's mechanical energy. The generator later harnesses this rotational energy provided by the shaft to produce electricity. Moreover, designing turbine blades is a highly complicated process involving multiple factors, such as fluid dynamics, the structural strength of the blades, heat transfer, and specific manufacturing requirements. Additionally, turbine staging is another factor that significantly influences turbine efficiency.
Several OEMs (Original Equipment Manufacturers), whether for aircraft engines or Heavy Frame Gas Turbines (HFGT), show significant variations in their gas turbines' “hot section,” even at the same megawatt capacity. For instance, General Electric has three staging turbine systems, while Siemens has four, and Alstom has five. In this case, selecting the optimum turbine staging system becomes paramount.
Gas turbines use combustion products of clean air or clean gasses (such as a mixture of inert gasses like helium) and organic fuels (gaseous or liquid, combined with steam or water) to drive their turbine stages. Staging in a turbine can be defined as driving present rows of stationary vanes or rotating blades to produce mechanical energy. The source of this mechanical energy is often provided by the expansion of heated, high-pressure gasses like helium at low pressures and temperatures. To achieve this, most modern turbines typically use three-staging systems, namely impulse (or pressure) staging, velocity-compound staging, and reaction staging. Each of these stages has been described below:
Impulse or pressure staging alters the flow of incoming high-velocity air, thereby diminishing its kinetic energy while still spinning the turbine. The operating principle of pressure staging depends on Newton's second law of motion, which states that the force acting on a moving object is the product of its weight and acceleration. Impulse staging is ideal when inlet pressure is high, but flow must be less, those of which are the exact working requirements of de Laval turbines and Pelton wheels. The only pressure drop in this stage occurs at the stationary blades, a.k.a the nozzle vanes, and produces a fully optimized output of 1.3 kilowatts.
This staging type involves a set of stationary nozzles that incorporate stationary impulse blades for redirecting flow. This type of staging enables a significant pressure drop and the extraction of twice as much power as a single pressure stage. Velocity compounding can be used as the first stage in large turbines to set up control, apart from being well-suited for small turbines.
Due to the working fluid's mass or pressure, torque is developed in this type of staging. As the gas passes through the turbine's rotor blades, its pressure gradually lessens. Although similar to the basic design of impulse staging, reaction turbines may require more stages than impulse staging to achieve a more significant reduction in steam enthalpy.
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