SHORTEN THE INTERMEDIATE TURBINE DUCT LENGTH BY APPLYING AN INTEGRATED CONCEPT

摘要

The demand of further increased bypass ratio of aero engines will lead to low pressure turbines with larger diameters which rotate at lower speed. Therefore, it is necessary to guide the flow leaving the high pressure turbine to the low pressure turbine at larger diameters minimising the losses and providing an adequate flow at the LP-turbine inlet. Due to costs and weight this intermediate turbine duct has to be as short as possible. This would lead to an aggressive (high diffusion) s-shaped duct geometry.It is possible to shorten the duct simply by reducing the length but the risk of separation is rising and losses increase. Another approach to shorten the duct and thus the engine length is to apply a so called integrated concept. These are novel concepts where the struts, mounted in the transition duct, replace the usually following LP-vane row. This configuration should replace the first LP-vane row from a front bearing engine architecture where the vane needs a big area to hold bearing services. That means the rotor is located directly downstream of the strut. This means that the struts have to provide the downstream blade row with undisturbed inflow with suitable flow angle and Mach number. Therefore, the (lifting) strut has a distinct three dimensional design in the more downstream part while in the more upstream part it has to be cylindrical to be able to lead through supply lines. In spite of the longer chord compared with the base design this struts have a thickness to chord ratio of 18%. To apply this concept a compromise must be found between the number of struts (weight), vibration, noise and occurring flow disturbances due to secondary flows and losses.The struts and the outer duct wall have been designed by ITP. The inner duct was kept the same as for the base line configuration (designed by MTU). The aim of the design was to have similar duct outflow conditions (exit flow angle and radialmass flow distribution) as the base design with which it is compared in this paper. This base design consists of a single transonic HP-turbine stage, an aggressive s-shaped intermediate turbine duct and an LP-vane row. Both designs used the same HP-turbine and were run in the continuously operating Transonic Test Turbine Facility (TTTF) at Graz University of Technology under the same engine representative inlet conditions. The flow field upstream and downstream the LP-vane and the strut, respectively has been investigated by means of five hole probes. A rough estimation of the overall duct loss is given as well as the upper and lower weight reduction limit for the integrated concept.This work is part of the EU-project AIDA (Aggressive Intermediate Duct Aerodynamics, Contract: AST3-CT-2003-502836).