The use of swirling flows in cyclone cooling systems is a promising method for internal cooling applications for instance in the leading edge of gas turbine blades. However, vortex breakdown can occur in such flows, which is associated with an axial flow reversal.

Within the scope of this investigation a numerical parameter study was conducted in order to explore the impact of convergent tube geometries on the flow pattern. For this purpose, Delayed Detached Eddy Simulations (DDES) were conducted for a Reynolds number of 10,000 and a swirl number of 5.3. First of all, the numerically obtained velocity field was compared to experimental data for a reference tube with a constant cross-section. This validation showed overall good agreement. Subsequently, the same reference tube was compared to four convergent tubes. The latter ones comprised three geometries with linearly decreasing diameters reaching convergence angles of $0.42\mathrm{deg}$., $0.61\mathrm{deg}$. and $0.72\mathrm{deg}$., respectively. Furthermore, an additional tube featuring a hyperbolic diameter decrease was considered.

The current investigation demonstrates that convergent vortex tubes impose a flow acceleration that suppresses the vortex breakdown phenomenon. Moreover, the axial velocity can be associated to different flow regimes with supercritical and subcritical character. These regimes indicate that the flow field within convergent tubes is less affected by the outlet conditions. Moreover, a physical reason is given that explains the formation of different flow regimes and their associated backflow as pressure-driven phenomenon. Furthermore, a modified Q-criterion illustrates two double helix vortex systems that can both be stabilized by choosing an appropriate geometry.

在旋风冷却系统中使用旋流是一种很有前途的内部冷却方法，例如在燃气轮机叶片的前缘。然而，在这种流动中会发生涡流破坏，这与轴向流动逆转有关。

在本次调查的范围内，进行了数值参数研究，以探索会聚管几何形状对流动模式的影响。为此，对 10,000 的雷诺数和 5.3 的涡流数进行了延迟分离涡流模拟 (DDES)。首先，将数值获得的速度场与具有恒定横截面的参考管的实验数据进行比较。该验证显示总体上良好的一致性。随后，将相同的参考管与四个会聚管进行比较。后者包括三个几何形状，直径线性递减，会聚角为$0.42\mathrm{度数}$., $0.61\mathrm{度数}$. 和$0.72\mathrm{度数}$。， 分别。此外，还考虑了具有双曲线直径减小的附加管。

目前的研究表明，会聚涡流管会施加流动加速，从而抑制涡流击穿现象。此外，轴向速度可以与具有超临界和亚临界特性的不同流态相关联。这些状态表明会聚管内的流场受出口条件的影响较小。此外，给出了一个物理原因来解释不同流态的形成及其作为压力驱动现象的相关回流。此外，修改后的 Q 标准说明了两个双螺旋涡流系统，它们都可以通过选择合适的几何形状来稳定。