高载荷低压涡轮内尾迹-动力学粗糙度耦合流动控制机理研究

From takeoff to cruise, the operating Reynolds number for the high-loaded low-pressure turbine(LPT) in an aircraft gas turbine engine decreased significantly. At low Reynolds numbers, the boundary layer on the LPT blades are largely laminar, making them susceptible to flow separation near the aft portion of the blade suction surface, with associated loss increase and performance drop. Many active flow control techniques, which can be shut off when not required for flow control, have been developed to decrease the separation in an attempt to reduce the total pressure losses at different Reynolds numbers. Dynamic roughness is new active flow control method, which suppressed separation over the entire leading edge of the airfoil by unsteady surface roughness and firstly proposed by Huebsch in recent years. It has the advantages of low energy consumption, blade structure integrity and separation control potential etc. In this project, this method is introduced to control the separation flow of low Reynolds number in high-loaded LPT. The effect of dynamic roughness and the influence of control parameters on LPT blade loss is studied using large eddy simulation. Then the analysis method based Hilbert-Huang transformation is developed for unsteady flow control mechanism exploration. Using this analysis method, the response mechanism of boundary layer transition and separation to dynamic roughness is investigated in LPT. Accounting for the unsteady nature of the flow in an actual LPT, combined effect of the unsteady wake produced by the upstream blade row and dynamic roughness, that including continuous excitation and pulse excitation, on blade boundary development are studied in detail. And the flow control mechanism of dynamic roughness used in LPT is obtained finally. The research results will provide a new flow control method for LPT.

从起飞到巡航状态，航空发动机中高载荷低压涡轮工作雷诺数大幅降低，叶片后半部分的逆压力梯度容易诱发层流分离。主动流动控制可以实现低压涡轮不同雷诺数工况下高效运行的需求。动力学粗糙度是一种外流低雷诺数流动的主动控制方法，利用表面粗糙元小幅值非定常振动来完全抑制翼型层流分离，具有保持叶片结构完整性、控制耗能少、控制分离潜力大等优点。本项目将此方法引入到高载荷低压涡轮流动控制，采用大涡模拟数值计算方法，研究动力学粗糙度及流动控制参数对叶栅损失的影响，采用发展的基于希尔伯特-黄变换的流动分析方法，分析动力学粗糙度启动过程涡轮叶片表面边界层转捩和分离的响应机制，以及涡轮内非定常尾迹与动力学粗糙度连续/间歇式激励相互作用过程、两者共同影响叶片边界层发展的内在规律，获得涡轮内动力学粗糙度流动控制机理。研究成果将为低压涡轮提供一种新的流动控制方法。

从起飞到巡航，航空发动机低压涡轮中雷诺数急剧降低，导致叶片后半部分边界层极易分离。本项目讨论了局部振动壁面主动抑制低雷诺数低压涡轮叶栅中大尺度层流分离的可行性以及流动机理。.首先，吸力面上局部振动壁面以二维半正弦凸包垂直于壁面正弦振动，数值计算了凸包位置、频率、幅值对均匀进气叶栅分离流动控制的效果，结果表明最优凸包位于峰值速度点上游附近，当凸包振动频率为100-200Hz、幅值为1.0-1.5mm时，控制叶栅损失最小。最优的振动凸包幅值对应当地边界层厚度。振动凸包频率的选择是保证相邻凸包脱落涡之间不会出现间歇性的自然分离涡以及脱落涡之间不会因为间隔太短而出现掺混。流动控制机理来源于附着于吸力面的连续涡团加强了边界层与主流之间的能量交换。同时还研究了进口湍流和雷诺数对振动凸包控制叶栅损失的影响，控制叶栅损失与来流湍流度无关；随着来流雷诺数增加，当地边界层厚度降低，进口来流速度增加，雷诺数对可控叶栅损失的影响来源于凸包振动折合频率和幅值的综合作用。.然后，研究了三维点振凸包对叶栅损失的影响。凸包分别按单列间隔排布或双列交错排布。凸包位置、幅值及频率基于二维振动凸包计算获得的最佳值给定。结果表明凸包脱落涡与凸包间距内诱导分离涡之间的相互作用使叶栅流场相当复杂，但吸力面仍被大尺度涡团覆盖，使得三维振动凸包控制叶栅损失与二维凸包控制叶栅损失相当。当相邻三维振动凸包之间的间距为凸包宽度5倍时，流动控制所需能量约为二维振动凸包流动控制所需能量的20%。.最后，研究了非定常尾迹与振动凸包的耦合作用，单独尾迹作用下叶栅损失降低，而尾迹与振动凸包的耦合作用稍微降低了叶栅损失，上游尾迹掺混损失使得非定常来流控制叶栅损失仍大于定常来流的控制叶栅。. 因此，局部壁面振动能有效减少低雷诺数低压涡轮叶栅内的大尺度分离损失。由于驱动壁面振动的元件可以为柔性膜或电聚合物，其低成本、低重量和功耗、对叶片表面无破坏的特征使得其应用前景较好。

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