In this article aerodynamic effects of tip clearance on a heavy duty axial turbine are studied. Three different tip clearances are considered for each rotor. For simplicity, a simple tip profile is assumed and cooling air is not modeled. Aerodynamic behavior of all stages is studied in terms of polytropic efficiency, leakage mass flow, secondary and total losses, penetration length, and total mass flow rate for different pressure ratios. Also three well established correlations of tip clearance loss are compared with CFD results to obtain the best model for performance calculation of such a large-scale turbine. The steady states, viscous and compressible flow governing equations representing the flow field with k-epsilon turbulence model are solved using commercial code ANSYS CFX.12. Useful data are presented to predict the variation of efficiency of each individual rotor, as well as entire turbine, as a function of relative tip gap (k/h). This information may be useable in design and troubleshooting. According to the results, even though pressure drop in rear stages across tip gap is lower than pressure drop in front stages, leakage mass flow rate is considerably high for this LP stages. Consequently, tip clearance losses of rear stages have significant effect on the entire turbine efficiency.
In the current study, it is focused on blade optimization of compressor to achieve improved performance characteristics. Due to dependency of mass flow rate on the inlet temperature of the gas turbine, temperature changes influence on compressor performance and efficiency. In order to enhance the working conditions at design and off-design operation, an automated design process is applied. The process has three main steps including parametrization of the geometry, numerical simulation of flow and optimization design approach. Stochastic design approach is utilized for optimization. The objective of this improvement will push the airfoil geometry in a way that minimum loss value, extended acceptable off-design operation in constant exit flow angle can be achieved with being focused on hot day's operation. The considered case in the present study is a compressor of MGT-70 heavy-duty gas turbine and the optimization focuses on the first four stages. Based on numerical simulation of optimized compressor, 1% enhancement in efficiency in all operating conditions is achievable. Moreover, the mass flow rate can be enhanced roughly up to 0.8% and 1% for design and off-design conditions, respectively. After assembling the new developed parts, the first upgraded prototype of the gas turbine has been tested in sixth Unit of Parand power station. More than 600 signals of pressure and temperature in circumferential and radial directions were extracted from compressor section. The results show good agreement predicted in range inlet flow angle between measurements and theoretical targets.
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