Experimental flow analysis in a modern turbine rear structure with 3D polygonal shroud under realistic flow conditions

Abstract

The development of the geared and ultra-high bypass ratio engines has increased the demands and operational envelope for the low-pressure system in commercial aircraft engines. This, in turn, requires improvement of the existing design of the turbine rear structure (TRS) with further experimental investigation of the flow. This paper presents a first comprehensive study of the effect of 3D polygonal shroud design in a modern TRS including several vane types: regular vanes, vanes equipped with bump served for better structural behaviour of the TRS, and vanes with increased thickness to give more internal space for the passage of the bearing oil pipes and scavenge tubes. Improvement of structural properties of the TRS and modification of the axisymmetric flow passage due to these engine-relevant compromises lead to new challenges in terms of the aerodynamic efficiency. Therefore, detailed experimental aero studies of the TRS with 3D polygonal shroud equipped with all vanes are required. Experimental investigations were performed at Chalmers University of Technology in a modern test facility where a low-pressure turbine (LPT) is located upstream to provide engine-realistic flow field to the TRS. The three vane types were arranged in the TRS into an engine-realistic configuration and the flow around each vane type was studied at aerodynamic design point and off-design conditions with constant Reynolds number 350,000. The measurements were performed by multihole pressure probes, wall static pressure taps on the vane and shroud and by the oil-film visualization technique. The results show that the thickened vane is handling the on-design and off-design conditions with good turning and aerodynamic performance. A presence of the shroud bump significantly influences the wake formation expressed into the additional vorticity region created from the bump itself which leads to extra pressure losses. The oil-film visualization at the off-design condition with a higher swirl angle has revealed a small flow separation with further reattachment near the hub suction side corner for the three types of TRS vanes. Moreover, from the aerodynamic analysis and flow visualization, it was shown that the bump flow separates in the shroud suction side corner under on- and off-design conditions. Therefore, an overall study represents an important validation case for future CFD calculations. Further details about the loss mechanisms due to the realistic vane geometries and polygonal shroud geometry will be discussed in the full version of the paper.