extracted. Moreover, an improvement would also mean that the machine would be able to show some level of efficiency. The efficiency ofa gas turbine, or more correctly the overall thermal efficiency, is the ratio of mechanical work done to the heat supplied. in the case of the Elling machine, the mechanical work was zero, and the efficiency was therefore also zero. Developments in gas turbine design and technology since then have led to ever increasing efficiencies of the machines. How far can efficiency go? This delicate question is set by the laws of thermodynamics. The person to first outline this was the Frenchman Carnot who, in 1824, described the heat engine working cycle and its efficiency. The Carnot efficiency is defined as: where: W Tmax -Tmin T\carnot = - = �=.c..----'� Q Tmax w = mechanical work Q = heat supplied T max = maximum temperature T = minimum temperature min With the Carnot formula, efficiency can be expressed as temperatures. For gas turbines, Tmax is the temperature of the hot gases leaving the combustion chamber gases and Tmin is the ambient temperature. Assume we bum fuel at stochiometric conditions and get a flame temperature of 2500 K. Assume also that our turbine is designed to withstand that temperature. Further assume that our engine is working at a pressure ratio of 100, which is very high but not unreasonable. Fi nally, assume !hat internal losses are negligible. üur highly hypothetical gas turbine results in an efficiency of approximately 65%. in comparison, where are we today? The best-performing, simple, open-cycle, single-shaft machines show an efficiency of approximately 40%. lmproving The Working Cycle Few things could be more exciti ng to gas turbine developers than finding improvements in efficiency that have not been invented before. Many ideas have seen the light over the years but only a few have resulted in commercial products. üne issue that has been on engineers' agenda for a long time is the development of modified working cycles. However, efficiency improvement measures often go hand-in-hand with increased complexity and cost. MAKALE / ARTICLE For example, the simple, open-cycle, single-shaft gas turbine comprises a compressor and a turbine on one shaft. it is simple in its design but also has a limited efficiency capability. This machine can be modified i nto a two-shaft machine with a high-pressure rotor coaxial to the low-pressure rotor. The modification will improve efficiency but will also increase engine complexity and cost. However, this two-shaft engine is available on the market today and is an example of what can be done to improve efficiency. There are many other successful working-cycle modifications such as compressor intercooling, recuperating exhaust gases, sequential combustion, ete., but these will not be discussed here. Some efficiency improvement efforts have made significant progress in recent years. From the Carnot efficiency formula above, we find that by increasing the temperature span between the heat source and heat sink we will i ncrease efficiency. According to the second law of thermodynamics, heat can only go from a higher temperature to a lower. The heat sink cannot be made to go to a lower temperature, according to the second law, but the heat-source temperature can be increased, for example by firing more fuel. Firing more fuel raises the temperature and increases the temperature span. High-pressure compressor wash system nozzle interconnection (blue hose) The challenge in increasing the firing temperature is to find durable materials for use in combustor linings and turbine blades. When new materials become difficult to fi nd, then other ways have to be found to get around the problem of overheating. üne recent accomplishment is to protect the turbine blades with a thin film of steam injected from the leading edge of the turbine blade. The film not only protects the blade from coming into direct contact with the hot combustion gases, but also the blade interior is cooled by the same steam flow. ENERJİ & KOJENERASYON DÜNYASI 89
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