This paper presents the results of a parametric experimental study of free swirling flow at the exit of a flat-vane axial swirler. A total of 16 data sets were acquired by combining four swirler vane angles (22°, 29°, 50.5°, and 58.3°) and four exit nozzles of different diameters (30, 40, 52, and 76 mm). Sophisticated pressure probes consisting of precise microphones and a two-component LDV system were used to investigate the effect of these geometrical parameters on swirling flow regimes characterized by the swirl number. Particular attention was paid to the precessing vortex core (PVC) phenomenon observed at the exit of the swirler nozzle. It has been shown that by varying the vane angle and the diameter of the exit nozzle, it is possible to independently control the swirl number value and the occurrence of a PVC. A distinct correlation has been found between the PVC-induced pressure pulsations detected by acoustic probes and the tangential velocity fluctuations measured by LDV. The use of microphones provides a quick way to measure the frequency response of swirl flow in a wide range of geometries and flow configurations. The PVC effect does not occur at low subcritical values of the integral swirl number (S < 0.5) and in the case of strong swirl flow (S_g = 0.9 and 1.2) in the absence of constriction by the nozzle (D_e/D₀ = 1). The disappearance of the PVC effect for strong swirl flow without constriction is due to the extreme displacement of the flow to the nozzle walls. The absence of a PVC in the flow was inferred not only from measurements of the frequency response of the flow over a wide range of Re numbers, but also from the absence of specific markers in velocity RMS distributions. Measurement results are used to derive an empirical correlation of the integral swirl number and the Strouhal number with a modified geometric swirl number. This allows a generalization of the frequency characteristics of swirling flows with a PVC for flat-vane axial swirlers, which are widely used in engineering.

本文介绍了平叶片轴向旋流器出口处自由旋流的参数化实验研究结果。通过组合四个旋流器叶片角度（22°、29°、50.5° 和 58.3°）和四个不同直径（30、40、52 和 76 毫米）的出口喷嘴，总共获得了 16 个数据集。由精密麦克风和双组件 LDV 系统组成的精密压力探头用于研究这些几何参数对以旋流数为特征的旋流流态的影响。特别注意在旋流器喷嘴出口处观察到的进动涡核 (PVC) 现象。已经表明，通过改变叶片角度和出口喷嘴的直径，可以独立控制旋流数值和 PVC 的发生。已经发现声学探头检测到的 PVC 引起的压力脉动与 LDV 测量的切向速度波动之间存在明显的相关性。麦克风的使用提供了一种快速测量各种几何形状和流动配置中的旋流频率响应的方法。PVC 效应不会在积分涡流数 (S  < 0.5) 和在 没有喷嘴收缩的情况下 ( D _e / D ₀  = 1)的强旋流 ( S _g = 0.9 和 1.2 )。对于没有收缩的强旋流，PVC 效应的消失是由于流向喷嘴壁的极端位移。流中不存在 PVC 不仅可以从对宽范围Re的流的频率响应的测量中推断出来数字，但也来自速度 RMS 分布中缺少特定标记。测量结果用于推导出积分涡流数和 Strouhal 数与修正几何涡流数的经验相关性。这允许使用 PVC 对广泛用于工程的平叶片轴向旋流器的旋流频率特性进行概括。