GAS TURBINE ENGINE DESIGN


 


            [i]A gas turbine is also called a combustion turbine. It is a rotating engine that removes energy from the combustion gas.  Non-complex turbines might only have the shaft, the compressor, the turbine, and the alternative-rotor assembly. It only has one moving part outside of the fuel system. Yet, precision manufacturing is required for components and temperature resistant alloys to bring about maximum efficiency making the production of a simple turbine complex.


More complicated turbines have movable stator blades, an immense structure of complex piping, combustors, and heat exchanges, numerous shafts or spools, and countless numbers of turbine blades.


An essential part of the design are the thrust bearings and journal bearings. These are the oil-cooled ball bearings or the hydrodynamic oil bearings.


 In the combustor, fuel is mixed with air and ignited. Fuel combustion increases temperature and from there, combustion products are driven into the turbine section. The volume and high velocity of the gas flow goes through a nozzle where it  spins in the turbine  and as a result, the compressor gets its power. In some turbines, it powers their mechanical output. Temperature drop and exhaust gas pressure gives power to the turbine. Shaft power, thrust, or compressed air, or a mixture of the three,  extracts energy. It powers generators, trains, ships, aircraft, and in some cases, tanks.


            The Brayton cycle is composed of three thermodynamic processes: the isentropic compression, the isentropic expansion, and the isobaric constant pressure combustion.  A centrifugal or radial compressor first accelerates gases. Then, a diffuser slows down these gases. A diffuser is a diverging nozzle. Due to turbulence and resistance, energy is lost. From the diffuser, heat is added as gases pass to a combustion chamber occurring at constant pressure. The specific volume of gases increases because the pressure is always constant although this process is oftentimes accompanied with an insignificant loss in pressure due to friction. Finally, the nozzle guide vanes spreads out and accelerates the volume of gases before a turbine extracts energy. Since gases are spread out isentropic, original pressure is attained upon leaving the turbine.


                     The capacity of the materials used to create the engine are limited to endure intense stress and high temperature. To oppose this, most turbines have an ample  intricate blade cooling systems.


            The utmost pressure ratios that can be attained by the turbine and the compressor  is determined by the blade tip speed. The higher the rotation rate of the shaft needed to maintain speed, the smaller the engine. The blade tip speed limits the utmost energy and effectiveness that can be attained by the engine. The rotational speed must increase twofold if the rotor’s diameter is cut in half so that tip speed will remain constant.


            An external combustion gas turbine can also be produced. This is a turbine adaptation of a hot air engine. Thinly grounded biomass and grinded coal are used as fuel. In an external combustion gas turbine, what is used is a heat exchanger where only pure air goes through the power turbine. There are no combustion products present. In this type of combustion, thermal efficiency is lesser.


            Compared to other engines, gas turbine engines  have higher operation speed but lower operating process, lower oil consumption bringing about lower lubricating cost, have fewer moving parts that mostly move in one direction only but with lesser vibrations produced, are smaller in size but with the same power efficiency, very high power to weight ratio, and can run on a extensive selection of fuels.


            However, certain drawback designs still need to be improved on, changed or modified. Among these are extended startups, a lesser amount of responsive to changes in power, reduced efficiency when idle, and above all, the cost incurred in the production of a gas turbine engine.


            Information technology coupled with technological advances in the construction of material parts contributed vastly to greater efficiency in combustion processes,  a more enhanced cooling system of engine parts, and an elevated compression ratios and temperatures.


 



 


[i] En.wikipedia.org



Credit:ivythesis.typepad.com


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