Forced and unforced unsteadiness in an axial turbomachine
Forced and unforced unsteadiness in an axial turbomachine
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DOI：
10.1016/j.expthermflusci.2008.10.012
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发表时间：
2009-03
影响因子：
3.2
通讯作者：
J. M. F. Oro;E. B. Marigorta;K. M. A. Díaz;R. Ballesteros-Tajadura
中科院分区：
文献类型：
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作者：
J. M. F. Oro;E. B. Marigorta;K. M. A. Díaz;R. Ballesteros-Tajadura
本文对低速轴流式涡轮机械中不同的非定常来源进行了识别和分类。从将非周期性机制（湍流）与周期性现象（不稳定）分离的经典图像出发，概述了进一步的分解，以区分受迫（确定性周期性）和非受迫（非确定性）不稳定。使用热线风速计在典型工业风扇的多种测试条件下测量的原始速度迹线经过整体平均以获得时间分辨的波动。然后，采用基于频率的滤波程序来隔离来自剩余湍流波动的非确定性但非混沌的干扰，从而产生所谓的非受迫不稳定。该术语揭示了相干流结构，涉及“大规模”不稳定性以及与叶片旋转尺度（BPF）不同的其他周期性特征。作为起点，分析与总不稳定相关的动能（以时间平均流动的动能的百分比表示）作为操作条件的函数。接下来，还观察了导致流动总体不稳定的不同组成部分，以确定它们在全局不稳定场景中的特殊意义。结果表明，湍流动能约占总非定常能量的 50-60%，而受迫分量和非受迫分量对其余能量的贡献相等。此外，据观察，较高水平的非受迫不稳定集中在端壁边界层，由于尾流核心结构的损失，受迫不稳定明显减少。相反，受迫不稳定在转子通道的内部区域更加明显。此外，非受迫不稳定在尖端区域尤其强烈，在该区域建立了与尖端泄漏涡流相关的大尺度。结果表明，非受迫不稳定性的估计构成了低速轴流涡轮机械叶尖泄漏流存在的准确指标。此外，这通过各向异性程度的表示得到证实，其中揭示了典型的各向异性结构。最后，通过引入非受力分量的功率谱密度，可以识别这些大波动的典型涡流尺寸。
Different sources of unsteadiness in low-speed axial turbomachinery are identified and classified in this paper. From the classical picture segregating non-periodic mechanisms (turbulence) from periodic phenomena (unsteadiness), a further decomposition is outlined to distinguish between forced (deterministic periodicities) and unforced (non-deterministic) unsteadiness. Raw velocity traces, measured for several test conditions in a typical industrial fan with hot-wire anemometry, are ensemble-averaged to obtain time-resolved fluctuations. Then, a frequency-based filtering procedure is employed to isolate non-deterministic, but also non-chaotic, disturbances from the remaining turbulent fluctuations, resulting in the so-called unforced unsteadiness. This term reveals coherent flow structures that involve “large-scale” unsteadiness with other periodic features different to the blade rotation scales (BPF). As a starting point, the kinetic energy associated to the total unsteadiness (in terms of a percentage of the kinetic energy of the time-averaged flow) is analyzed as a function of the operating conditions. Next, the different components contributing to the total unsteadiness of the flow are also observed, in order to determine their particular significance on the global unsteady scenario. It is shown that the turbulent kinetic energy reaches up to approximately a 50–60% of the total unsteady energy, while both forced and unforced components contribute equally to the rest of the energy. In addition, it is observed that higher levels of unforced unsteadiness are concentrated towards the endwall boundary layers where forced unsteadiness is notably reduced due to the loss of the wake–core structure. Conversely, forced unsteadiness is more evident at inner regions of the rotor passage. Furthermore, unforced unsteadiness is especially intense in the tip regions where large-scales associated to the tip leakage vortex are established. It is demonstrated that the estimation of the unforced unsteadiness constitutes an accurate indicator of the presence of tip leakage flows for low-speed axial turbomachinery. Moreover, this is confirmed through the representation of the degree of anisotropy, where typical anisotropic structures are revealed. Finally, with the introduction of power spectrum densities for the unforced components, it is possible to identify typical eddy sizes of these large fluctuations.