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首页> 外文期刊>Experimental Thermal and Fluid Science: International Journal of Experimental Heat Transfer, Thermodynamics, and Fluid Mechanics >Forced and unforced unsteadiness in an axial turbomachine
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Forced and unforced unsteadiness in an axial turbomachine

机译：轴向涡轮机中的强迫和非强迫不稳定

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摘要

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 phe nomena (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-deter ministic, 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 compo nents contributing to the total unsteadiness of the flow are also observed, in order to determine their par ticular 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 con tribute equally to the rest of the energy. In addition, it is observed that higher levels of unforced unstead iness 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 anisot ropy, 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.
机译：本文对低速轴流透平机械中的各种不稳定因素进行了识别和分类。从经典图片中，将非周期性机制（湍流）与周期性现象（不稳定）分开，概述了进一步的分解，以区分强迫（确定性周期性）和非强迫（不确定性）不稳定。将原始速度轨迹（在带有热线风速仪的典型工业风扇中针对几种测试条件进行测量）进行平均，以获得时间分辨的波动。然后，基于频率的滤波过程被用来从剩余的湍流波动中分离出不确定的，但也是非混沌的扰动，从而导致所谓的非强迫不稳定。该术语揭示了连贯的流动结构，该流动结构涉及“大规模”不稳定，其周期性特征不同于叶片旋转尺度（BPF）。首先，根据运行条件来分析与总不稳定相关的动能（以时间平均流量的动能的百分比表示）。接下来，为了确定它们在整体不稳定情况中的特殊意义，还观察到了导致整个流动不稳定的不同成分。结果表明，湍流动能达到总非稳态能量的大约50-60％，而受迫和不受迫成分均对其余能量贡献相等。另外，观察到，较高水平的非强迫不稳定状态集中于端壁边界层，在该壁中边界状态由于尾流芯结构的损失而显着减少了强迫不稳定状态。相反，在转子通道的内部区域，明显的强迫不稳定。此外，在建立了与尖端泄漏涡旋相关的大规模的尖端区域中，非强迫不稳定尤其强烈。结果表明，对于低速轴向涡轮机械，无力不稳定的估计构成了叶尖泄漏流存在的准确指示。此外，这可以通过代表典型各向异性结构的各向异性ro的程度来证实。最后，通过引入非受力组件的功率谱密度，可以确定这些大波动的典型涡流大小。

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