An experimental examination of the effect of trailing edge thickness on the aerodynamic performance of gas turbine blades
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This thesis documents the experimental research conducted on a transonic turbine cascade. The cascade was a two-dimensional model of a jet-engine turbine with an, approximately, 1.2 design, exit Mach number, and was tested in a blow-down type wind-tunnel. The primary goal of the research was to examine the effect of trailing edge thickness on aerodynamic losses. The original cascade was tested and, then, the blades were cut-back at the trailing edge to make the trailing edge thicker. The ratios of the trailing edge thickness to axial chord length for the two cascades were 1.27 and 2.00 percent; therefore, the ratio of the two trailing edge thicknesses was 1.57. To simulate the blade cooling method that involves trailing edge coolant ejection, and to examine the effect of that on aerodynamic losses, CO₂ was ejected from slots near the trailing edge in the direction of the flow. Two different blowing rates were used, in addition to tests without CO₂. A coefficient, L̅, was used to quantify aerodynamic losses, and this was the mass-averaged total pressure drop, normalized by dividing with the total pressure upstream of the cascade. The traversing, downstream total pressure probe was stationed at one of three different locations, in order to investigate the loss development downstream of the cascade. The two cascades were tested for an exit Mach number ranging from 0.60 to 1.36. The research suggested that the main influence of the trailing edge thickness on losses is through affecting the strength of the trailing edge shock system, since L̅ was almost the same for the two cascades in the subsonic Mach number region. The losses mainly differed (larger for the cut-back cascade) in the Mach number region of 1.0 to 1.2. In this region, the difference in loss maximized, showing a loss for the cut-back cascade 20 to 30 percent more than the original cascade. The CO₂ was found to have no significant effect for high Mach numbers; for low Mach numbers, the high blowing rate slightly decreased the loss. Finally, the loss, nearly, stopped to increase after one axial chord length downstream of the cascade.
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