In order to improve the efficiency, ultra-high bypass ratio engine attracts more and more attention because of its huge advantage, which has larger diameter low pressure turbine (LPT). This trend will lead to aggressive (high diffusion) intermediate turbine duct (ITD) design. It is necessary to guide the flow leaving high pressure turbine (HPT) to LPT at a larger diameter without any severe loss generating separation or flow disturbances. In this paper, eight ITDs with upstream swirl vanes and downstream LPT nozzle are investigated with the aid of numerical method. These models are modified from a unique ITD prototype, which comes from a real engine. Key parameters like area ratio, inlet height, and non-dimensional length of the ITDs are kept unchanged, while the rising angle (radial offset) is the only changed parameter which ranges from 8 degrees to 45 degrees. In this paper, the effects of rising angle (RA) on ITD, as well as nearby turbines, will be analyzed in detail. According to the investigation results, RA could be as large as 40 degrees in such model of this paper to escape separation; When RA increases, local inlet flow field of LPT nozzle appears to be with apparent variation; while a positive result is that outlet flow field could be kept almost unchanged through modifying blade profile. On the other hand, it seems optimistic that the overall total pressure loss could be kept nearly equivalent among different RA cases. And a valuable conclusion is that outer wall curvature is more important for pressure loss, which advises a clear direction for optimizing ITD.
Numerical method is used to investigate the effect of ITD with different RAs ranging from the original 8 degrees to 45 degrees. Even higher RA increment result is not given in the paper to make sure that separation within the ITD is avoided, which might incur numerical inaccuracy. However, this result should be a helpful advice for designers, that RA of ITD in a real engine should be smaller than 45 degrees if separation is averted in the duct.
Within the passage of ITD with RA smaller than 25 degrees, SP on outer wall is smaller than that on inner wall, which will induce the fluid near inner wall migrating towards outer wall, especially the low energy fluid in hub boundary layer. However, the trend is opposite near outlet when RA is larger than 25 degrees, which is a critical model for pressure distribution. The stream-wise pressure will affect the flow field structure, but span-wise pressure will change the strength of vortices. Near hub, vortex pairs are induced, and consumed in A30 and A40; in the domain region, positive wake vortex is more difficult to be dissipated in passage with larger RA to cause more loss as one main source of energy deficit. On the other hand, the flow field will be more uneven for downstream LPT in larger RA ducts with such flow structures, which is troublesome for downstream LPTs.
The total pressure loss analysis shows that the loss is not necessarily larger in ITD with higher outlet radius, which comes from mixing and boundary layers. So an acceptable loss level with relatively larger RA is possible through choosing a proper RA primarily, and then optimizing the curvatures of walls, especially outer casing.
The overall trends of yaw angle at outlet of LPT nozzle are almost similar. This means that the influence of ITD RA can be diminished, even can be eliminated by adjusting the LPT vanes. This is believed to be an important finding, because the ITD’s performance can be further improved by increasing RA to be more aggressive, while the LPT rotor inlet flow field can be guaranteed by only modifying LPT nozzle without extra performance penalty, and with an unchanged LPT nozzle blade number, which is very important for engine safe running.
As a conclusion, the RA’s selection plays a key role in making sure a well-designed ITD. This paper should be a start of further work that how to supply better inlet flow for LPT nozzle in larger RA ITDs, and downstream LPT rotor with the same nozzle vane number through optimizing blade profile. Further experimental work is on going by the authors to explore the underneath flow mechanism in depth.
Fig. 1 Definition of ITD model
