Modeling and Performance of Gas Turbine Cycles with Various Means of Blade Cooling
Abstract: A method for simulating the cooled gas turbine is demonstrated. Based on equations and other knowledge found in the literature, a first-law thermodynamic, non-dimensional model is established and implemented in the equation-solving, programmable software IPSEpro. Increasing the water vapor or carbon dioxide contents of the gas turbine hot gas is found not to have an impact on the Stanton number, which is employed for the modeling of the heat transfer to the blade. The model is validated using experimental data from an industrial gas turbine and data found in the literature, and is found to give results easily in concordance with the case with which it is matched. Based on the validation, a reference gas turbine is established and employed for various cycle calculations. First, it is shown how gas turbine efficiency can increase with decreased coolant temperature and with a decrease in the ratio of specific heats between hot gas and coolant. Simple-cycle simulations are done to illustrate the impact of coolant pre-cooling, cooling with humid air and cooling with steam. Due to its high specific heat, steam is shown to have the best capacity for cycle performance enhancement. For the single-pressure combined cycle, it is shown how steam for cooling of the first gas turbine vane can be flashed from the water leaving the economiser in a split stream boiler. This is found to give a considerable increase in cycle thermal efficiency (more than three percentage points), but the concept needs to be further evaluated in order to establish whether it is technically feasible. For the studies of the HAT cycle, the role of the compressor modeling is emphasized. For a compressor with a given geometry, extracting a reduced amount of air for cooling will give an increase in the pressure ratio, which will have a positive impact on the HAT cycle thermal efficiency, but it will also, in practice, lead to the surge margin being approached. For cooling with humid air, there will be no significant increase in the HAT cycle thermal efficiency if the compressor ratio is maintained constant. Designing gas turbine cycles with a high thermal efficiency is not a goal in itself; also good economic performance is important. The impact of cooled blades for the profitability of the gas-turbine based power plant is illustrated briefly, with focus on heat resistance versus materials cost and on compressor washing intervals.
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