Abstract This study presents numerical investigations of turbulent single-phase (water) and two-phase (air-water) flows in a horizontal diverging channel (diffuser), extending our previous experimental work (Mansour et al., 2018a). The main target is to examine and discuss the prediction accuracy of available computational fluid dynamics (CFD) models under such turbulent two-phase flow conditions with separation, based on direct comparisons with detailed experimental data. After performing a mesh-independence test, the numerical results for single-phase flows have been validated against experimental data of the axial pressure change in the channel. Four different turbulence models, including the Realizable k − ϵ , the k − ω shear stress transport (SST), the Spalart-Allmaras, and the Reynolds Stress Model (RSM) were considered and compared. The results show that the Realizable k − ϵ and RSM models can predict the pressure change in single-phase flows more accurately, while only RSM could as well predict a velocity field close to the experiments. Accordingly, only Realizable k − ϵ and RSM have been used for further two-phase flow simulations, which were performed using a transient setup together with the Volume of Fluid (VOF) method to model the interaction between the two phases. It was observed that only RSM performed reasonably well concerning flow regimes and air accumulations. Finally, considering higher flow rates, even the two-phase flow regimes predicted by RSM start to deviate from the experiments. The present study underlines the limitations of existing CFD models when applied to such complex two-phase flows.

中文翻译：

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扩散器中湍流单相和两相流的数值研究

摘要 本研究对水平发散通道（扩散器）中的湍流单相（水）和两相（空气-水）流动进行了数值研究，扩展了我们之前的实验工作（Mansour 等人，2018a）。主要目标是基于与详细实验数据的直接比较，检查和讨论在这种带有分离的湍流两相流条件下可用计算流体动力学 (CFD) 模型的预测精度。在执行网格无关性测试后，单相流的数值结果已经根据通道中轴向压力变化的实验数据进行了验证。考虑并比较了四种不同的湍流模型，包括 Realizable k − ϵ 、k − ω 剪应力传输 (SST)、Spalart-Allmaras 和雷诺应力模型 (RSM)。结果表明，Realizable k − ϵ 和 RSM 模型可以更准确地预测单相流中的压力变化，而只有 RSM 才能预测接近实验的速度场。因此，只有 Realizable k − ϵ 和 RSM 已用于进一步的两相流模拟，这些模拟是使用瞬态设置和流体体积 (VOF) 方法进行的，以模拟两相之间的相互作用。据观察，只有 RSM 在流态和空气积聚方面表现得相当好。最后，考虑到更高的流速，即使是 RSM 预测的两相流态也开始偏离实验。本研究强调了现有 CFD 模型在应用于这种复杂的两相流时的局限性。