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The effects of inlet temperature and turbulence characteristics on the flow development inside a gas turbine exhaust diffuser.

机译：进气温度和湍流特性对燃气轮机排气扩压器内部流动发展的影响。

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摘要

The overall industrial gas turbine efficiency is known to be influenced by the pressure recovery in the exhaust system. The design and, subsequently, the performance of an industrial gas turbine exhaust diffuser largely depend on its inflow conditions dictated by the turbine last stage exit flow state and the restraints of the diffuser internal geometry. Recent advances in Computational Fluid Dynamics (CFD) tools and the availability of computer hardware at an affordable cost made the virtual tool a very attractive one for the analysis of fluid flow through devices like a diffuser. In this backdrop, CFD analyses of a typical industrial gas turbine hybrid exhaust diffuser, consisting of an annular diffuser followed by a conical portion, have been carried out with the purpose of improving the performance of these thermal devices using an open-source CFD code "OpenFOAM". The first phase in the research involved the validation of the CFD approach using OpenFOAM by comparing CFD results against published benchmark experimental data. The numerical results closely captured the flow reversal and the separated boundary layer at the shroud wall where a steep velocity gradient has been observed. The standard k --epsilon turbulence model slightly over-predicted the mean velocity profile in the casing boundary layer while slightly under-predicted it in the reversed flow region. A reliable prediction of flow characteristics in this region is very important as the presence of the annular diffuser inclined wall has the most dominant effect on the downstream flow development. The core flow region and the presence of the hub wall have only a minor influence as reported by earlier experimental studies. Additional simulations were carried out in the second phase to test the veracity of other turbulence models; these include RNG k--epsilon, the SST k--o, and the Spalart-Allmaras turbulence models. It was found that a high resolution case with 47.5 million cells using the SST k--o turbulence model produced a mean flow velocity profile at the middle of the annular diffuser portion that had the best overall match with the experiment. The RNG k --epsilon, however, better predicted the diffuser performance along the exhaust diffuser length by means of the pressure recovery coefficient. These results were obtained using uniform inflow conditions and steady-state simulations. As such, the last phase of our investigations involved varying the inflow parameters like the turbulence intensity, the inlet flow temperature, and the flow angularity, which constitute important characteristics of the turbine blade wake, to investigate their impact on the diffuser design and performance. These isothermal CFD simulations revealed that by changing the flow temperature from 15 to 427°C, the pressure recovery coefficient significantly increased. However, it has been shown that the increase of temperature had no effects on the size of the reversed flow region and the thickness of the separated casing boundary layer, although the flow appears to be more turbulent. Furthermore, it has been established that an optimum turbulence intensity of about 4% produced comparable diffuser performance as the experiment. We also found that a velocity angle of about 2.5° at the last turbine stage will ensure a better exhaust diffuser performance.
机译：已知整个工业燃气轮机效率受排气系统中压力恢复的影响。工业燃气轮机排气扩压器的设计以及其性能在很大程度上取决于其进入条件，该条件由透平末级出口流动状态和扩压器内部几何形状的约束所决定。计算流体动力学（CFD）工具的最新进展以及以可承受的成本提供的计算机硬件，使得该虚拟工具成为分析流经扩散器等设备的流体的极具吸引力的工具。在这种背景下，已经进行了对典型的工业燃气轮机混合排气扩散器的CFD分析，该扩散器由环形扩散器和圆锥形部分组成，目的是使用开源CFD代码“ OpenFOAM”。研究的第一阶段涉及通过比较CFD结果与已发布的基准实验数据，使用OpenFOAM验证CFD方法。数值结果紧密捕获了流动的逆流和在已观察到陡峭的速度梯度的护罩壁处分离的边界层。标准的kε湍流模型略微预测了套管边界层中的平均速度剖面，而略微预测了反向流动区域中的平均速度剖面。由于环形扩压器倾斜壁的存在对下游流动的发展具有最主要的影响，因此对该区域中流动特性的可靠预测非常重要。正如早期实验研究所报道的那样，岩心流动区域和轮毂壁的存在只具有很小的影响。在第二阶段进行了额外的仿真，以测试其他湍流模型的准确性。其中包括RNGk-ε，SST k-o和Spalart-Allmaras湍流模型。结果发现，使用SST k-o湍流模型在一个具有4750万个细胞的高分辨率情况下，在环形扩散器部分的中间产生了平均流速分布，该分布与实验的整体匹配最佳。然而，RNGkε通过压力恢复系数更好地预测了沿排气扩压器长度的扩压器性能。这些结果是使用均匀的流入条件和稳态模拟获得的。因此，我们研究的最后阶段涉及改变流入参数，例如湍流强度，入口流动温度和流动角度，这些参数构成了涡轮叶片尾流的重要特征，以研究它们对扩散器设计和性能的影响。这些等温CFD模拟显示，通过将流动温度从15更改为427°C，压力恢复系数显着增加。然而，已经表明，尽管流动似乎更湍流，但温度的升高对反向流动区域的尺寸和分离的壳体边界层的厚度没有影响。此外，已经确定，最佳湍流强度约为4％，可产生与实验相当的扩散器性能。我们还发现，在最后一个涡轮级，大约2.5°的速度角将确保更好的排气扩压器性能。

著录项

  • 作者

    Bomela, Christian Loangola.;

  • 作者单位

    The University of North Carolina at Charlotte.;

  • 授予单位 The University of North Carolina at Charlotte.;
  • 学科 Engineering Mechanical.;Energy.
  • 学位 Ph.D.
  • 年度 2014
  • 页码 194 p.
  • 总页数 194
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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