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The performance of turbine section of a gas turbine deteriorates over operation because of working in high temperature conditions and characteristics of the entry gas. On the other hand, due to complexity of the flow field within the turbine, three-dimensional analysis is required. This paper presents a numerical study of roughness effects on turbine flow field and performance. In this paper, effects of blade surface roughness caused by operation conditions on turbine performance were numerically calculated. Numerical calculations were carried out for the fourth stage of an axial turbine which was experimentally tested in the technical university of Hannover, using ANSYS software. Calculated results were verified with the measured data and showed a good agreement. To find out the effects of blade surface roughness on turbine stage performance and flow field, Two equivalent sand-grain roughness heights of 106㎛ (transitionally rough regime) and 400㎛ (fully rough regime) in four different mass flow rates were considered. Results showed that summation of efficiency reductions of the rough stator and rough rotor approximately equals to that of the totally rough stage for each roughness height and effect of stator roughness on efficiency reduction is same as the effect of rotor roughness on stage efficiency.

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Roughness of vanes’ outer surface and that of cooling channels’ inner surface have considerable impact on temperature distribution. Using a rougher surface leads to increased turbulence in near-surface flows and increases the rate of heat transfer. In this study, vane of a C3X turbine cooled via 10 cooling channels was simulated -three-dimensionally- by ANSYS-CFX software based on SST turbulence model, and then the effects of roughness of said surfaces were examined. The results showed that increasing the roughness of the blade’s outer surface, which absorbs the heat of the hot fluid, to values below the threshold of fully rough regime ( Reks < 70 ) makes no significant impact on vane’s surface temperature distribution; but increasing the roughness to values higher than this threshold leads to 8% increase in surface temperature. This indicates that outer surface of the blade should always exhibit a transitionally rough regime. Opposite to the outer surface, increasing the roughness of cooling channels’ inner surface, which transfers the heat to the cooling fluid, found to be the very beneficial, as even a slight increase in the roughness of this surface (within the domain of transitionally rough) decreases the blade’s surface temperature by up to 8%, and improves the hydraulic-thermal performance factor by about 250%.