Purpose

Turboshaft engines usually include one centrifugal compressor due to its high-pressure ratio, stability and compactness. Many designers rely on positive raking to decrease tip gap flow and therefore losses. However recent optimization studies revealed geometries contradicting this canonic view. Hence, this paper aims to investigate how the rake angle alone can influence performance and to which extent.

Design/methodology/approach

A turboshaft representative impeller was chosen and altered for null and +/−30° rake angles. Menter's shear stress transport model is used for steady computational fluid dynamics simulations, sweeping the nominal speedline at various tip clearances. Backsweep distribution is identical in all cases, isolating rake influence.

Findings

Pressure ratio was lowered for the both positively and negatively raked blades, but through distinct aerodynamic mechanisms. Although the flow through the tip gap was lower for the positive rake, this is due to lower blade loading. Splitter comparison reveal that these effects are more pronounced in the radial regions.

Practical implications

Some of the findings may extend beyond turboshaft engines, into turbochargers, home appliances or industrial blowers. However, all extrapolations must consider specific differences between these applications. Turboshaft compressors designers can benefit from this study when setting up their free parameters and penalty functions in the early concept stages.

Originality/value

Only few similar studies can be found in the literature to date, none similar to turboshaft applications. Also, this impeller is designed to eliminate leading edge shocks and suction side boundary layer separation, which makes it easier to isolate the tip gap flow effects. The authors also provide a framework on which semi-empirical design equations can be further developed to incorporate rake into 1D design tools.

中文翻译：

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前角对涡轮轴压缩机的影响，数值研究

目的

涡轮轴发动机由于其高压比，稳定性和紧凑性而通常包括一台离心压缩机。许多设计人员依靠积极的倾斜来减少尖端间隙流动，从而减少损失。但是，最近的优化研究表明，几何形状与这种经典观点相矛盾。因此，本文旨在研究前角单独如何影响性能以及在多大程度上起作用。

设计/方法/方法

选择了涡轮轴代表叶轮，并针对零前角和+/- 30°的前角进行了更改。Menter的剪切应力传输模型用于稳定的计算流体动力学模拟，在各种尖端间隙扫过标称速度线。后掠分布在所有情况下都是相同的，从而隔离了前刀的影响。

发现

正斜刮板和负斜刮板的压力比均降低，但通过独特的空气动力学机制。尽管对于正前角而言，通过叶尖间隙的流量较小，这是由于叶片载荷较低。分离器的比较表明，这些影响在径向区域更为明显。

实际影响

一些发现可能会超出涡轮轴发动机，扩展到涡轮增压器，家用电器或工业鼓风机。但是，所有推断都必须考虑这些应用程序之间的特定差异。涡轮轴压缩机的设计者可以在概念早期阶段设置其自由参数和补偿函数时从这项研究中受益。

创意/价值

迄今为止，在文献中仅能找到很少的类似研究，而没有类似于涡轮轴应用的研究。此外，该叶轮的设计还消除了前缘冲击和吸力侧边界层的分离，从而更容易隔离叶尖间隙流动效应。作者还提供了一个框架，在该框架上可以进一步开发半经验设计方程，以将rake纳入一维设计工具中。