Direct and large-eddy simulation of buoyancy-induced flows in rotating cavities
Direct and large-eddy simulation of buoyancy-induced flows in rotating cavities
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旋转腔中浮力诱发流动的直接大涡模拟
DOI：
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发表时间：
2018
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通讯作者：
D. Pitz
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文献类型：
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作者：
D. Pitz
本文采用谱单元法对旋转空腔内离心力诱导的流动进行了直接数值模拟(DNS)和隐式大涡模拟(LES)。例如，这些流动发生在燃气轮机内部空气系统的压缩机腔中，在该系统中，冷却空气被用来从压缩机盘中提取热量。由于基于雷诺平均Navier-Stokes(RANS)方程的湍流模型不能准确描述浮力和旋转相互作用引起的流动现象，用计算流体动力学(CFD)方法研究浮力诱导流动具有一定的挑战性。因此，无模型方法是可取的，因为它们可以提供流动物理的准确描述。首先，将该方法应用于层流区、过渡区和完全湍流区共存的动静叶布局，并将计算结果与文献中的实验数据进行了比较。在此基础上，采用线性稳定性分析、数值模拟和大涡模拟等方法研究了旋转密封环空中离心力诱导的流动。结果表明，旋转空腔的对流起始时间与Rayleigh-Benard对流问题的起始时间相似。对不同瑞利数值的流动统计分析表明，无论是厚度还是速度分布，圆盘边界层都表现为层流Ekman层。即使考虑了瞬时剖面，也能观察到这一点，尽管解是不稳定的。结果还表明，在重力作用下，盖热边界层尺度与自然对流尺度是一致的。引入轴向贯通的冷却空气，保持了在密封腔中观察到的一些特征，但却显著降低了核心温度，相应地增加了护罩的换热。轴向通流也促进了腔内观察到的频率范围的显著增加。
In this research a spectral element method is used to perform direct numerical simulation (DNS) and implicit large-eddy simulation (LES) of flows induced by centrifugal buoyancy in rotating cavities. These flows occur, for instance, in the compressor cavities of gas turbines internal air systems, in which cooling air is used to extract heat from compressor disks. Buoyancy-induced flows are inherently challenging to study using computational fluid dynamics (CFD), since turbulence models based on the Reynolds-averaged Navier-Stokes (RANS) equations are not able to provide an accurate description of the phenomena induced by the interplay between buoyancy and rotation. For this reason, model-free approaches are desirable, since they can provide an accurate description of the flow physics. First, the method is applied to a rotor/stator configuration, in which regions of laminar, transitional and fully turbulent flow coexist, and the results are compared with experimental data from the literature. Subsequently, flow induced by centrifugal buoyancy in a sealed rotating annulus is investigated using linear stability analysis, DNS and LES. It is shown that the onset of convection for a rotating cavity is similar to that for the problem of Rayleigh-Benard convection. Analysing flow statistics for different values of the Rayleigh number, it is shown that the disk boundary layer behaves as a laminar Ekman layer, both in terms of its thickness and of its velocity profiles. This is observed even when instantaneous profiles are considered, despite the unsteadiness of the solution. The results also show that the shroud thermal boundary layer scaling is consistent with that of natural convection under gravity. Introducing an axial throughflow of cooling air, some features observed in the sealed cavity are maintained, however a strong reduction in the core temperature and a corresponding increase in the shroud heat transfer occur. The axial throughflow also promotes a significant increase in the range of frequencies observed inside the cavity.