The gas turbine engines, use in various applications, operate under off-design conditions for considerable time duration. The performance and efficiency of the gas turbine engines deteriorates under such off-design conditions. The variable area nozzle turbine (VANT) is variable geometry concept which can be used to improve the performance of the gas turbine engine in part-load condition. However, mechanical aspects to implement VANT demands for part clearances to be provided near hub and tip endwalls. The provision of part clearances, induces leakage flow under the pressure gradient from pressure side to suction side of the vane.This leakage flow mixes with the primary incoming flow and boundary layer flow from the walls. This has detrimental effects on the overall performance of the gas turbine engine.

The vane geometry,for present study was taken from the 2^nd^ stagelow pressure turbine of energy efficient engine (EEE) proposed by Pratt and Whitney. The available vane profiles and pressure distribution data at five different span wise locations were used to construct the vane geometry. The annular cascade of vane geometry was scaled by factor 1.2 to experimentally investigate the flow field using sector annular cascade tunnel condition. In order to implement VANT, the endwalls and both ends of the vane need to be of concentric spherical shape with its centre on engine axis. Hence, the casing and hub endwalls demand for modifications. The modifications such as to accommodate the high casing endwall angle, rear step was provided at the upstream of vane leading edge near tip. The part clearances were provided for ease of vane movement near hub and tip. The leakage flows near hub and tip interact with the main passage flow eventually degrades the performance of the nozzle. The vaneis free to rotate to accommodate the change in stagger angle in the annular passage in order to change the flow area passage. The vane is held in the hub and casing endwalls by the circular pivot passing through it. However, the pivot in the part clearances near hub and tip need to be selected in such a way that the flow leakage through the pressure side to suction side can be minimised and hence the losses can also be minimised through the flow passage.

For the present study, various pivot shapes i.e. circular, elliptical and airfoil shape in the 2 mm part clearance near hub and tip regions were chosen. The present work also incorporates the effect of the pivot shape on vane passage flow field at five different vane turning angles from +10º to -10º in 5º intervals. This vane turning helps to change the throat area of the nozzle which governs the air mass flow rate through the engine turbine section.For numerical analysis, the grid was generated in the ICEM CFD^®^for nozzle flow passage with pivot shape and numerical study was performed using commercially available CFD tool ANSYS CFX^®^.

The effect of different pivot shapes on the leakage flow, formation of the leakage vortex and its interaction with the main passage flow were analysed using entropy, vorticity and turbulent kinetic energy. Also, the structure of the leakage flow and hence leakage vortex changes with the change in vane turning angle. The paper focusses more about detailed flow field study within the part clearances near hub and tip region and flow three dimensionality within the nozzle passage and exit flow field.


Numerical Investigations on the Effect of Pivot Shapes in Part Clearance Flow Field of Variable Area LP Turbine Nozzle Vane

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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