Improved clearance designs to minimize aerodynamic losses in a variable geometry turbine vane cascade

Abstract

Variable geometry turbine exists in small mobile gas turbines or some marine gas turbines to enhance the part-load performance. However, there are efficiency penalties associated with the vane partial gap, which is needed for the movement of variable vanes. This paper investigates the vane-end clearance leakage flow for a flat tip, a cavity tip, a winglet tip, a tip with passive injection, and a cavity-winglet tip to assess the possibility of minimizing vane-end clearance losses in a variable geometry turbine cascade. First, calculations were done at the test rig conditions for comparison with measured data, and they were used for validation of computational fluid dynamics model. Then, numerical calculations were done for turbine typical conditions. Specific flow structures of the various clearance designs of variable vanes are described, and then the effects of vane turning, including exit Mach numbers of 0.34, 0.44, and 0.54 as well as turning angles of –6°, 0°, and 6° on total pressure losses and outflow yaw angle for different vane tips are shown. In addition, the sensitivity of aerodynamic losses to vane tip gap height is evaluated. Results show that the strong interactions near the tip endwall region change the near-tip loading distribution significantly. With winglet and cavity-winglet tip designs, the loading distribution becomes very similar to the typical fixed vane, and the total loading is reduced, thus reducing the vane-end losses. Among the different vane tips presented, the cavity-winglet tip achieves the best aerodynamic performance, and the cavity tip has the lowest sensitivity to vane tip gap height. Overall, the cavity-winglet tip is found to be the best choice for variable vanes. The research results can provide useful reference for the vane design in a real high endwall-angle variable geometry turbine.

Advances in aerodynamic, structural design and test technology of variable geometry turbines

“…Therefore, the tip cavity structure should also be used to control the clearance gap leakage loss of variable vanes, 43–46 which has been confirmed by the calculation and test results, that is, the vane-end cavity structure can effectively reduce the leakage losses under all working conditions (Figure 21). Based on the same principle, the tip winglet technology can also be used in the variable vane, 16,47 which can significantly reduce the load near the vane-end, and effectively restrain the tip clearance leakage flowrate and leakage loss (Figure 22). It should be noted that the extended winglet structure parameter is a small value relative to the cascade pitch, and does not affect the actual installation.…”

“…Aerodynamic performance of variable-vane with winglet tip structure. 16,47 (a) Winglet tip structure and near-tip loading distribution (b) Entropy-increase contours in a variable vane cascade with flat and winglet tips (c) Cavity-winglet tip structure and spanwise total pressure loss distribution.…”

“…Schematic of flow streamlines over the flat tip and entropy-increase contours in a variable vane cascade at the design vane turning angle. 16 (a) Flow streamlines over the tip region overlay with casing dimensionless static pressure distribution (b) Entropy-increase contours for flat tips.…”

Experimental study on the aerodynamic performance of variable geometry low-pressure turbine adjustable guide vanes

“…Gao et al 20,21 used numerical and experimental methods to verify the effect of different leaf top structures on the aerodynamic performance of variable geometry turbine adjustable guide vane. Based on the above study, Gao et al 22 also carried out a clearance structure design for reducing the gap leakage flow in variable geometry turbines and achieved better results.…”

Investigation of Unsteady Aerodynamic Excitation on Rotor Blade of Variable Geometry Turbine

“…In turbine stages the upstream wake segments, upstream and downstream potential filed, and trailing edge shock impingement and reflection are the deterministic flow distortions which is seen by the blade [13,14]. The periodic disturbance can significantly affect turbine performance and heat transfer, and it may lead to blade failure finally [13,15]. And the unsteady aerodynamic excitations on the blade have received growing attention.…”