Principle of Operation Gas Turbine

In an ideal gas turbine, gases undergo four thermodynamic processes: isentropic compression, isobaric (constant pressure) combustion, isentropic expansion, and heat rejection.Together these make up the Brayton cycle.In a real gas turbine, when gas is compressed (in a centrifugal or axial compressor), mechanical energy is irreversibly (due to internal friction and turbulence) converted into pressure and heat energy. With increased heat in the combustion chamber,the specific volume of the gas increases while the pressure decreases slightly.During expansion through the stator and rotor passages in the turbine, irreversible energy conversion occurs again. Inhale fresh air instead of expelling heat.If the engine is augmented with a power turbine to drive an industrial generator or a helicopter rotor, the outlet pressure will be as close to the inlet pressure as possible, leaving just enough energy to overcome the pressure loss in the exhaust duct and expel the exhaust.With a turboprop, there is a specific balance between propeller power and jet thrust for the most economical operation.In a turbojet engine, only enough pressure and energy can be extracted from the airflow to drive the compressor and other components.The remaining high-pressure gas is accelerated through the nozzle to provide a jet to propel the aircraft.The smaller the engine, the higher the rotational speed of the shaft must be to achieve the desired tip speed.Tip speed determines the maximum pressure ratio that can be achieved by the turbine and compressor.This in turn limits the maximum power and efficiency the engine can achieve.To keep tip speed constant, if the diameter of the rotor is reduced by half, the rotational speed must be doubled. For example, a large jet engine spins around 10,000-25,000 rpm, while a microturbine spins up to 500,000 rpm.Mechanically, a gas turbine is much more complex than an internal combustion piston engine.A simple turbine may have only one major moving part, the compressor/shaft/turbine rotor assembly, along with other moving parts in the fuel system. In turn, this can translate into prices.For example, it turned out that the Jumo 004 had a material cost of 10,000 ℛℳ, cheaper than the Junkers 213 piston engine at 35,000 ℛℳ[26],and required only 375 hours of low-skilled labor to complete (including fabrication, assembly, and shipping), while BMW 801 vs. 1,400.However, this also translates into inefficiencies and reliability.More advanced gas turbines, such as those found in modern jet engines or combined cycle power plants, may have 2 or 3 shafts (spools), hundreds of compressor and turbine blades, movable stator blades, and Extensive external ductwork for air; they use temperature resistant alloys and are fabricated to exacting specifications requiring precision fabrication.All of these generally make the construction of a simple gas turbine more complex than a piston engine.Furthermore, for optimum performance in modern gas turbine power plants, the gas needs to be prepared according to precise fuel specifications.The gas conditioning system processes the natural gas to the exact fuel specifications in terms of pressure,temperature, gas composition and associated Wobbe Index before entering the turbine.The main advantage of a gas turbine engine is its power-to-weight ratio.Gas turbines are well suited for aircraft propulsion due to the large amount of useful work that can be produced by a relatively lightweight engine.Thrust and journal bearings are a critical part of the design.They are hydrodynamic oil bearings or oil-cooled rolling bearings.Foil bearings are used in some small machines such as microturbines,and also have great application potential in small gas turbines/auxiliary power units. 

Creep

A major challenge in turbine design, especially turbine blades, is reducing creep caused by the high temperatures and stresses experienced during operation.Higher operating temperatures are constantly being sought to increase efficiency, but at the expense of higher creep rates.Consequently, several approaches have been employed to attempt to achieve optimum performance while limiting creep, the most successful of which are high performance coatings and single crystal superalloys.These techniques work by constraining deformations caused by mechanisms that can be broadly categorized as dislocation slip, dislocation climb, and diffusive flow.The protective coating provides thermal insulation to the blade and provides resistance to oxidation and corrosion.Thermal barrier coatings (TBCs) are usually stabilized zirconia-based ceramics, while oxidation/corrosion resistant coatings (bond layers) are usually composed of aluminides or MCrAlY (where M is usually iron and/or chromium) alloys.Using a TBC limits the temperature exposure of the superalloy substrate, thereby reducing the diffusion rate of active species (typically vacancies) within the alloy and reducing dislocation and vacancy creep.A coating of 1–200 μm has been found to reduce blade temperature by up to 200 °C (392 °F).The bonding layer was directly coated on the surface of the substrate by carburizing, which had a dual purpose of improving the adhesion of the TBC and the oxidation resistance of the substrate.Aluminum in the bond layer forms Al2O3 at the TBC bond layer interface, providing oxidation resistance but also causing poor interdiffusion (ID) zones between itself and the substrate.Oxidation resistance outweighs the disadvantages associated with the inner diameter zone as it increases the service life of the blade and limits the loss of efficiency from buildup on the outside of the blade.

Nickel-based superalloys have higher strength and creep resistance due to their composition and resulting microstructure. Gamma (γ) FCC nickel is alloyed with aluminum and titanium to precipitate a homogeneously dispersed coherent Ni3(Al,Ti) γ-' (γ') phase.Finely dispersed γ' precipitates impede dislocation motion and introduce threshold stress,increasing the stress required for creep initiation. Furthermore, γ' is an ordered L12 phase,which makes it harder for dislocations to pass through it.Refractory elements such as rhenium and ruthenium can also be added in solid solution to increase creep strength.The addition of these elements reduces the diffusion of the gamma phase, thereby maintaining fatigue resistance,strength and creep resistance.The development of single crystal superalloys has also led to significant improvements in creep resistance.Due to the lack of grain boundaries, single crystals eliminate Coble creep and thus deform in fewer modes-reducing the creep rate.Although single crystals have low creep at elevated temperatures,they have significantly lower yield stress at room temperature, where the strength is governed by the Hall-Petch relationship.Care needs to be taken to optimize design parameters to limit high temperature creep without reducing low temperature yield strength.