19-07-2014, 03:44 PM
A GAS TURBINE
A GAS TURBINE.pptx (Size: 639.54 KB / Downloads: 31)
Abstract
The influence of various levels of mathematical modelling on gas turbine performance is systematically analysed. It is shown that real gas effects give rise to an optimum turbine entry temperature which does not arise in a perfect gas analysis and has not been described previously in the literature. At any pressure ratio, the maximum possible efficiency with real gas effects is significantly lower (15-20%) than the maximum possible value predicted by a perfect gas analysis. An explicit equation has been derived for determining the optimum pressure ratio as a function of turbine entry temperature and component efficiencies. It is shown that the optimum design depends very strongly on turbine and compressor efficiencies. It is demonstrated that the optimum relation between pressure ratio and turbine entry temperature depends strongly on whether the optimisation is carried out at fixed pressure ratios or at fixed temperatures. All previous references seem to have considered only the latter method.
INTRODUCTION
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between.
GAS TURBINES IN SURFACE VEHICLES
Gas turbines are often used on ships, locomotives, helicopters, tanks, and to a lesser extent, on cars, buses, and motorcycles.
A key advantage of jets and turboprops for aeroplane propulsion - their superior performance at high altitude compared to piston engines, particularly naturally aspirated ones - is irrelevant in automobile applications. Their power-to-weight advantage, though less critical than for aircraft, is still important.
TURBOFAN
Large jetliners use what are known as turbofan engines, which are nothing more than gas turbines combined with a large fan at the front of the engine.
The turbofan is basically the combination of two engines, the turbo portion which is a conventional gas turbine engine.
The low specific thrust/high bypass ratio turbofans used in today's civil jetliners (and some military transport aircraft) evolved from the high specific thrust/low bypass ratio turbofans used in such [production] aircraft back in the 1960s.
DISADVANTAGES OF GAS TURBINE ENGINES
Cost is very high
Less efficient than reciprocating engines at idle speed
Longer startup than reciprocating engines
Less responsive to changes in power demand compared to reciprocating engines