【作者】 南希；

【导师】 黄伟光； 林峰；

【作者基本信息】 中国科学院研究生院（工程热物理研究所） ， 工程热物理， 2014， 博士

【摘要】 多年来,周向槽机匣处理的扩稳效果和扩稳机理一直是叶轮机械领域持久热议的题目。然而囿于压气机流动失稳的复杂过程及各异性,周向槽机匣处理的设计难以形成普适准则,现阶段周向槽的实际应用设计仍需依靠大量试验建立完备的数据库供以筛选。为缩短研制周期,本文从实际应用角度出发,立足于对动叶端区对压气机流动失稳的影响机理的认识,发展了一种作用于动叶端区的控制体分析方法。基于压气机内部流动的三维数值模拟,锁定动叶端区轴向动量为反映流场稳定性的特征量,通过对动叶端区建立控制体量化提取稳定性特征量,由此比较不同周向槽方案扩稳能力,并利用实验验证了该方法在亚音速压气机环境和跨音速压气机环境下的有效性和准确性。研究表明,绝大多数轴流压气机属于叶尖敏感型,发生失速时主流和叶顶泄漏流的交界面自转子前缘溢出,产生突尖形失速先兆,而推动这一交界面在节流过程中不断向前缘移动的机制是动叶端区的轴向动量平衡。基于这一认识,本文提出,加槽后,主流与泄漏流交界面自前缘溢出的趋势将被抑制,压气机的稳定性边界得以拓宽。这一现象可通过加槽后动叶端区的轴向动量分布反映出来。进而,不同周向槽方案的扩稳能力则可通过对比端区轴向动量的分布判断出来。因此,为捕捉这一密切反映压气机流动稳定性的特征参数,本文提出了在动叶端区建立一系列离散控制体的方法,用以量化压气机动叶端区的轴向动量分布情况。以光壁为基准,可通过比较不同周向槽边界下动叶端区轴向动量分布的改变情况,对其相应的扩稳能力进行定量比较,为周向槽初期工程设计提供初选手段。本文选择一典型的跨音速风扇转子NASA Rotor67为研究示例介绍控制体分析方法的建立。首先利用单通道定常／非定常全三维数值模拟得到压气机内部流场,然后在动叶端区合理的径向范围内设置一系列轴向离散的控制体,提供轴向动量沿转子叶顶轴向的定量分布。同时,依次不断累加控制体单元上的轴向动量,可得到一条形似钟形的动量曲线,简称为“钟形曲线”。该曲线的顶点可视为主流和泄漏流交界面在三维空间上的积分效果。对光壁结构和机匣处理结构节流过程中的钟形曲线予以考察,发现随流量不断减小,钟形曲线的顶点不断前移,这与节流过程中主流／泄漏流交界面前移机制相互吻合。加槽后,在同一流量下,钟形曲线的顶点轴向位置被后推,此时主流／泄漏流交界面在周向平均意义上的位置比光壁更靠近通道下游,这一表现反映了周向槽抑制了主流／泄漏流交界面的前溢趋势,加入处理机匣的压气机仍可保持稳定运行。基于对这一扩稳机理的认识,提出对比光壁近失速状态下不同周向槽方案的钟形曲线,可评估出不同方案的扩稳能力。在提出动叶端区轴向动量的控制体分析用于评估周向槽扩稳能力的方法后,分别在一台亚音速和一台跨音速压气机环境下利用实验进一步证实了这一判断方法的有效性。在实验室自有低速轴流压气机实验台IET-LAC上,利用动叶端区控制体分析方法比较了三组双槽的扩稳能力并通过实验验证了钟形曲线预测的结果正确。选取美国圣母大学跨音速轴流压气机ND-TAC为高速压气机转子研究示例,利用钟形曲线对其设计的七种不同周向槽方案的扩稳能力进行预判,并通过实验证实了七种不同周向槽方案的扩稳裕度提升值与钟形曲线的预估结果完全一致。进一步,借助于在亚音速和跨音速压气机环境下的周向槽实验结果发现,利用钟形曲线的几个要素可以判断周向槽扩稳的水平,并可在同一扩稳水平下进一步区分更细致的扩稳能力高低。考虑到周向槽机匣处理的设计还应兼顾压气机的效率和稳定性,在以量化的轴向动量来描述周向槽对压气机稳定裕度影响的启发下,开展周向槽对转子峰值效率影响的初步研究。根据不可逆损失的严格热力学定义,确定熵产为描述压气机流动损失的特征物理量,并利用气动热力学和流体力学基本公式,推导了用于压气机流动控制体形式的熵产表达式。研究中发现,有槽和无槽情况、不同周向槽处理情况的绝热效率分布在动叶端区体现明显差别。因此,对动叶端区和周向槽内分别建立控制体,量化了峰值效率工况下,不同周向槽方案引起的熵产变化。研究结果表明,周向槽内和动叶端区是压气机内部流场中具有较高流动损失的两个区域；不同轴向位置处的周向槽对叶顶流动损失的影响有显著不同。周向槽改善了动叶端区局部的流动,降低了这一部分的不可逆流动损失,而槽内的复杂流动结构又引起了额外的熵产。周向槽对压气机峰值效率的影响是这两方面作用的综合效果。在上述研究基础上,进一步探讨了周向槽几何对扩稳作用的影响。在若干周向槽的几何因素中,选取了三个有代表性的几何参数——周向槽的槽深、槽数和轴向位置。借助周向槽与叶片通道的唯一接触面——开口面为桥梁,分别考察了不同几何边界下,周向槽内和动叶端区控制体上的轴向动量的分布。研究发现,在三个几何因素中,周向槽的轴向位置对扩稳效果的影响最大。通过对各项轴向力的分析中发现,处理机匣的动叶端区轴向动量分布被改变的根本原因是动叶端区的负载被改变。在以上工作基础上,为跨音压气机J69优化多槽方案。利用动叶端区控制体分析方法快速的对比了多种不同周向槽结构,得到了兼顾扩稳和效率性能的周向槽方案。更多还原

【Abstract】 Among several casing treatment methods, circumferential grooves (CG) are well recognized as a means for stall margin extension with the best structure integrity and the minimum negative impact on the thermal efficiency for axial compressors. However, their design rule remains undecided due to the controversies in associate with the underlining physical mechanisms. A long lasting topic in this area is to accomplish a proper circumferential groove configuration for real compressors. The most common approach is to establish a database of casing grooves through extensive experimentation arid elaborate measurements in the tip region. A new control volume analysis is developed in this paper aiming at assessing the circumferential grooves effectiveness on stability improvement. The underlying mechanism for this approach is based on the hypothesis that the spike stall precursors can be triggered by the forward spillage of the rotor tip leakage flow and the onset condition of such a spillage is determined by the axial momentum balance within the rotor tip region. Control volumes are defined to quantify the axial momentum balance of the whole region where the grooves influence the flow at the rotor tip. Through comparing the axial momentum distribution of the blade tip region of circumferential groove configurations, its stall margin extension effeteness will be identified quantitatively, which can be very helpful during the design processing. Experiments are carried out to validate this approach both in low speed compressors and transonic compressors.The control volume method provides a brief yet quantitative approach for such a prescreening purpose because it is capable of providing an insight in the flow physics related to fluid forces, momentum, energy and entropy. NASA Rotor67is first taken as an example to develop control volumes in this paper. The compressor field with and without grooves are firstly simulated by using the steady/unsteady single passage simulation. A series of control volumes are set on the rotor tip region. Each of those control volumes extends one blade pitch in the tangential direction, aiming at catching all of the flow physics of the whole pitch in the tip region. Periodic boundary conditions apply to both control surfaces in the tangential direction. The cumulative axial momentum distribution are formed to describe the flow status in the rotor tip region, whose distribution resemble bell curves. It is an integral result to reflect the final balance between the main flow and TLF and it is extremely important information on stability. Each point on the bell curves represents the value of the cumulative axial momentum from the inlet up to this local position, which is equal to the net axial force acting on the flow by this part of the blade. The value and the axial location of the peak are crucial parameters to weight the potential of flow instability. While the peak value indicates how strong the positive axial force can reach, the axial location of the peak indicates how such a force is distributed along the blade chord. Therefore, if such a force becomes increasingly stronger and concentrated towards the blade leading edge further, the flow should become less stable.Two examples are conducted to validate this control volume approach experimentally. For the low speed compressor case, three different double-groove schemes are studied. The effectiveness of these groove-schemes are compared by using the bell curves, and then validated with experiments, which shows a good agreement. Another example is the series of circumferential groove VCT3of transonic rotor ND-TAC in university of Notre Dame are studied. Their stall margin extensions are analyzed by the control volume method. It has then been proven that the bell curves get the correct stall margin extension tendency via experiments.Another application for control volume method is the analysis of the influence of grooves on compressor efficiency. Entropy generation is indentified as the critical parameters of the irreversible flow loss. The entropy generation changed by three different grooved casings are studied. Control volumes are settled down at the rotor tip region as well as the inside of each groove. Comparing with the quantitative entropy value in and out of the groove region, it is found that the groove improved the local tip flow structure and ameliorated the flow loss, besides, the inner groove produce extra flow loss. Hence, the peak efficiency changed by grooves are the final effect of these two factors.Groove depth, groove number as well as groove axial locations are studied as they are the most important groove geometries that influence the stall margin extension. By using the control volume method, the effectiveness of the grooves with different geometries are compared for a transonic compressor rotor J69. The groove axial location show a significant effect on stall margin improvement. With the control volume approach proposed in this paper, an optimized groove scheme is finally select for this rotor, which has a considerable stall margin improvement as well as a slighter negative influence on efficiency.更多还原