Computational Study of Geometric Parameter Influence on Aggressive Inter-Turbine Duct Performance

Abstract

Modern direct-drive turbofan engines typically have the fan turbine designed at significantly higher diameter than the gas producer turbine. Furthermore, the gas turbine industry is being pushed to shorten engine length with the goal of reducing weight. This results in a need to design very aggressive inter-turbine-ducts (ITD’s) that have high endwall slopes. The gas turbine design cycle typically begins with conceptual design where many engine configuration iterations are made. During conceptual design, there usually is little firm geometric definition or time for detailed CFD studies on aggressive ITD’s. This can cause a large amount of risk to the engine development schedule and cost if the space allocated for the ITD during conceptual design is found to be insufficient later in the design cycle. Therefore, simple analytical tools for accurately assessing the risk of an ITD in conceptual design are important. The gas turbine industry is familiar with the Sovran and Klomp annular diffuser performance chart [1] as a conceptual design tool for assessing ITD’s. However, its applicability to modern gas turbine ducts with high endwall slope is limited. The location of the maximum pressure recovery for a given length, the Cp* line, considers only two geometric parameters: area ratio and normalized length. The chart makes no distinction of risk of flow separation regarding the level of slope or the pitch-wise turning in the duct. However, intuition would suggest that a high slope duct would have more risk of separation than an equivalent area ratio duct with low slope. Similarly, a duct that turns the flow from axial to radial would be expected to be riskier than a pure axial duct. To help assess the interaction of duct slope and pitch-wise turning with area ratio and length, an analytical Design of Experiments (DOE) was run using approximately sixty different duct configurations. The DOE was carried out using 3D, steady CFD analysis. The results of the DOE are presented with insights provided into how the Cp* line may shift as a function of duct slope. Of particular interest is that slope by itself does not work particularly well as a risk indicator. However, a combination of new area ratio-length and slope-length parameters was found to segregate ducts between separated and non-separated cases.

Effects of Geometric Parameters and Inflow Conditions on Flow in Aggressive Inter-Turbine Ducts

“…They carried out detailed experimental research on three types of linear diffusers, and uncovered the relationship between the inlet-to-outlet area ratio and length to height ratio in order to maximum the static pressure lift. However, (Couey et al, 2010) pointed out that the design scope of ITD today is quite different from the experimental data of (Sovran and Klomp, 1967), and supplemented the influence of mean rise angle on aggressive ITD performance.…”

Numerical Investigation on the Effect of Height-to-Radius Ratio on Flow Separation Features in S-Shaped Diffuser with Boundary Layer Ingestion

“…This adverse pressure gradient is caused by a combination of flow diffusion and flow turning [15,16]. Based on this understanding, researchers have summarized two global parameters, area ratio (AR) and length-to-offset ratio (LOR), which are the key contributing factors that affect the flow separation features in the S-shaped diffuser [17]. The AR, which is the ratio of the outlet to inlet areas, is the contributing factor of flow diffusion.…”

Investigation on the flow-control strategy for an aggressive turbine transition ducts

“…After being investigated for years, the flow mechanisms of the aggressive intermediate turbine duct (AITD) have been described systematically. 1–3 Complicated pressure gradients along stream-wise and span-wise, as well as upstream flow characteristics, for example, geometry, 4,5 swirl angle, 6–8 produce complicated flow field in the duct. In addition to that, an important feature is the rotor-blade tip-leakage flow.…”