Flow Mechanism And Flow Control Investigation Within Inter-Turbine Duct

The inter-turbine duct (ITD) of a gas turbine,is located between the high-pressure (HP) and low-pressure (LP) turbines. The design of these ITDs, is likely to become more aggressive with the demands for lighter, more efficient and environment-friendly aircraft engines. Such Aggressive Inter-Turbine Ducts (AITD) could have larger HP-to-LP radial offset and/or shorter axial length, has the potential for significant length reduction and therefore engine weight reduction and/or aerodynamic performance improvement. This potential arises because very little is understood of the flow behavior in the duct in relation to the hub and casing shapes, and the flow entering the duct (e.g., swirl angle, turbulence intensity, periodic unsteadiness and blade tip vortices from upstream HP turbine blade rows).An improved understanding and systematical discussion to the flow performance within AITD is desirablebefore AITDs can be designed with confidence.In the present work, four different ITD configurations are examined experimentally and numerically to discuss the detail flow field within ITD and AITDs.(1) The Build A baseline is an existing ITD, which is representative of a modern design, is investigated first.The investigation was conducted in a large scale, low-speed, axial turbine facility at the National Research Council Canada.Detailed measurements were made by using7-hole pressure probe, surface static pressure taps and flow visualizations. Results show that the flow field within ITD is complicated and dominated by casing and hub counter-rotating vortices and boundary layer separation. On the casing, the boundarylayer develops first and then generates casing counter-rotating vortices or produces boundary layer separation. While on the hub, the hub counter-rotating vortices is generated first, which is significantlyinfluent the hub boundary layer development. The inlet flow condition is very important to the ITD flow field. With increasing casing swirl angle, the casing boundary layer separation is delayed as well as the casing counter-rotating vortices. However, the loss in ITD is higher with larger inlet swirl angle. It is because the loss source in ITD is the flow fiction loss. The separation here is very weak. With increasing hub swirl angle, the hub boundary layer is not change much. The hub counter-rotating vortices is affected by the inlet swirl gradient.(2) The geometry of the AITD Build B, C and D are optimizedby Numeca/Design3D based on the validated numerical method. Build B was designed to have a25%increase in the mean rising angle. The Build C has an AR of20%larger than that of the Builds A. And Build D has an 25%higher mean rising angle and20%larger AR than Build A. All the geometry is keeping the same axial length as Build A.Based on the geometric characteristic analysis of the ITD Builds A and B, a generic design rule for a S-shaped ITD with mild parameters has been developed. To minimize the total pressure loss, the diffusion should increase rapidly at the first bend, where the boundary layer is thin and not easy to separate even under the strong adverse pressure gradient, then decrease to accelerate the flow to avoid boundary layer separation, and then increase again to the outlet to achieve the total diffusion. The local pressure gradient can also be modified by changing the value and location of the maximum endwall curvature.(3) After the geometry optimization, the detail flow field within AITD Build B, C and D are investigated in NRC's large scale, low-speed, axial turbine facility. Results shows that the flow field within AITD contains high risk of massive flow separation, which induces stronger secondary flow. With increasing the high rising angle, the onset of casing separation is forward as well as the casing counter-rotating vortices. By rising the outlet to inlet area ratio, the whole flow field are changed. At the first bend of the AITD, wake induced casing boundary layer3D separation and then followed by2D characteristics massive boundary layer separation.Comapring with each other, the loss incrasing in case AITD-B is lower than in case AITD-C, which means that it is better to increasing high rising angle to get the higher load. Comapring the different inlet flow anlge, results show that the lower inler flow angle induces higher loss. The boundary layer separation loss is the dominated loss within AITD.(4) After the detail investigation of the flow performance within AITD, the flow control by using vortex generator is also discussed in this paper.Results show that the casing boundary layer separation is significantly reduced by using low-profile vortex generators (LPVG)'swith different configurations installed on casing.The total pressure loss in the AITD was reducedup to4.1%.These LPVGs induced streamwise vortices increase the turbulence in casing boundary layer, which leads to delay the casing separation.