Flow mechanisms, heat transfer and discharge coefficient characteristics for a representative part of a turbine casing cooling system, consisting of an array of 20 impinging jets, were numerically investigated. The study focused on the influence of the jet Mach number while maintaining the Reynolds number constant at Re = 7,500. Therefore, the orifice bore diameter or the fluid density had to be varied. The objectives of the current CFD simulations have not been adressed before in literature, not only because heat transfer characteristics and pressure drop are given for impingement jet Mach numbers up to 0.72 at a constant relatively low Reynolds number, but also because fundamental understanding of physical phenomena of the flow in the cylindrical plenum and in the small sharp-edged orifices at the bottom side of the tube is provided. Increasing the Mach number by simultaneously reducing the orifice diameters led to slightly decreasing Nusselt numbers, with average deviations of the order of 14%. However, the heat transfer coefficient increased considerably with increasing Mach number. On the contrary, the variation of the Mach number by varying the density showed only a slight influence on the heat transfer coefficient. The predicted discharge coefficients increased significantly by augmenting the Mach number.
A representative part of an active clearance control system for a low pressure turbine has been numerically investigated. The setup consisted of a cylindrical plenum with 20 inline arranged impinging jets at the bottom side discharging on a flat plate. The study focused on the influence of the nozzle geometry on the flow as well as heat transfer characteristics at the impingement plate and the discharge pressure drop. CFD (Computational Fluid Dynamics) simulations were performed for a constant Reynolds number ReD = 7,500 and different mean jet Mach numbers up to 0.7. Different length-to-diameter ratios of the jet holes (L/D) and various hole shapes (cylindrical, elliptic, convergent and divergent conical) were investigated to evaluate the performance of the impingement cooling configurations. The predictions showed a significant influence of the length-to-diameter ratio of the orifice bores on the heat transfer and the pressure losses. For L/D = 2 no suction of the ambient air in the nozzles was observed. In comparison to the configuration with L/D = 0.25 an improvement of the discharge coefficient of 9% for Ma = 0.7 and 20% for Ma = 0.17 was achieved. However, the highest heat transfer was observed for the smallest L/D-ratio of 0.25. The shape variation showed that only the elliptic jet holes with a ratio of AR = 0.5 enhanced the overall heat transfer and simultaneously reduced the pressure losses because of discharging onto the target plate. This result was found to be valid for all investigated jet Mach numbers. Additionally, for both elliptic jet aspect ratios of 0.5 and 2 the axis-switchover phenomenon of the flow was observed.
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