Abstract
Recent advances in gas turbine development have led to wider usage of combined power plant for electrical power generation, and made it possible to reach a thermal efficiency of 55–60%. This was a result of introducing higher turbine inlet temperature (TIT) and other factors. However, this temperature is restricted by the metallurgical limit of turbine blades of about 800 °C. Thus, need arises to design efficient cooling systems to cool the turbine components subjected to such high temperatures. The performance of a combined system with different cooling techniques in the high temperature section of the turbine is evaluated. A general model of the combined system is developed and used to compare the performance relevant to the three main schemes of blade cooling, namely air-cooling, open-circuit steam cooling (OCSC) and closed-loop steam cooling (CLSC). The performance results of the combined system are expressed in terms of overall efficiency and specific power as functions of three primary variables and some other secondary variables, which depend on the considered type of cooling. The primary variables are the TIT, compressor pressure ratio ( R c), and the cooling mass ratio ( Φ c). The secondary variables are related to the geometry, aerothermodynamics, and heat transfer parameters of the gas turbine blades. The specific power and efficiency of the gas turbine are found to be sensitive to the type of cooling technique used. The combined system with CLSC is found to outperforms the OCSC system in specific power and overall efficiency. Thus, it is clear that more power is created when the cooling steam in the closed-loop is not thrown away. Under the given conditions the power of the lower steam cycle with CLSC is increased by 6%, accompanied by 19% rise in cycle efficiency relative to OCSC at similar conditions. The CLSC results in 11% enhancement in power and 3.2% in efficiency relative to air-cooling. The CLSC is less sensitive to variations of operating variables at part load.
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