Part load flow in radial centrifugal pumps
Part load flow in radial centrifugal pumps
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DOI：
10.5075/epfl-thesis-4422
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发表时间：
2009
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影响因子：
--
通讯作者：
O. Braun
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文献类型：
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作者：
O. Braun
离心泵需要在所有运行条件下维持其所支持的系统的稳定运行。对定义泵水路的表面进行微小的修改可能会显著影响系统运行不稳定的倾向。这种修正对流动的作用尚不完全清楚，导致在解决不稳定问题时采用代价高昂的试验和错误方法。由于旋转叶轮与静止扩散器的相互作用(转子-定子相互作用，RSI)，离心泵内的部分负荷流动具有固有的时变性。此外，泵扩散器中的不利压力梯度可能会导致流动分离，潜在地导致空间上静止或旋转、稳定或间歇的对称性破坏的不均匀。在热力和水力透平机械中，已经观察到旋转失速，其特征是在圆周上存在明显的流动分离单元，以叶轮转速的一小部分旋转。径向离心泵的部分负荷流动由于其复杂性，是数值模拟方法面临的一大挑战。本文采用实验和数值相结合的方法，对径向离心泵和水泵水轮机内的部分负荷流动进行了研究，后者采用有限体积离散的雷诺平均N-S方程。通过三个实例验证了部分负荷流动的物理现象，并对数值模拟方法定性和定量再现径向离心泵部分负荷流动的能力进行了评估。采用定常方法对高比转数径向水泵水轮机内部的流动进行了数值研究，并假设相邻叶片流道之间存在角周期，证明了流动拓扑突变与粘性损失增加的关系，从而影响了能量排放特性，从而增加了不稳定运行的风险。当流量降到临界阈值以下时，带有导叶分离区的直通流动变为不对称流动。能量从平均流中抽出，并在一个大的涡状结构中消散。除了一些扩散器通道内的流动分离外，对双吸泵内流动的随时间变化的数值模拟表明，在扩散器内存在叶轮两侧的流量不平衡和不对称的流动分离。随着这种不平衡的发生，粘性损失大大增加，能量-流量特性中的正斜率的结果段被发现与测量的流量明显不同。通过对低比转数水泵-水轮机的瞬时压力测量，确定了不同的旋转失速模式，显示出3-5个分离流区，根据流量的不同，以0.016-0.028倍的叶轮转速旋转。对于有4个单元的失速最明显的操作条件，在感兴趣的位置用激光多普勒法测量速度。通过定义由同步瞬变压力测量获得的失速相来重建相对于失速单元通过的速度场。与时间相关的数值模拟预测了四单元旋转失速，速度场与测量的速度场符合较好，但出现的流速比实验结果高得多。考虑到数值模拟在数量上的缺陷，提出了一种新的建模方法：用一维模型代替昂贵的叶轮主要部分的三维模拟，可以显著节省计算资源，允许在不变计算成本的情况下改进对其余领域的建模。在旋转失速和叶轮侧流量不平衡等具有挑战性的情况下，对模型进行了验证。模拟结果与包括整个叶轮流道在内的模拟结果具有令人满意的一致性，这使得该方法适合于许多透平机械的应用。它还可以以合理的计算代价为尾水管涡绳模拟提供改进的、随时间变化的边界条件。参数研究可以在该方法的框架内实现对模型边界面上的一些量的平均流动和湍流的有意修改。
Centrifugal pumps are required to sustain a stable operation of the system they support under all operating conditions. Minor modifications of the surfaces defining the pump's water passage can influence the tendency to unstable system operation significantly. The action of such modifications on the flow are yet not fully understood, leading to costly trial and error approaches in the solution of instability problems. The part-load flow in centrifugal pumps is inherently time-dependent due to the interaction of the rotating impeller with the stationary diffuser (Rotor-Stator Interaction, RSI). Furthermore, adverse pressure gradients in the pump diffuser may cause flow separation, potentially inducing symmetry-breaking non-uniformities, either spatially stationary or rotating and either steady or intermittent. Rotating stall, characterized by the presence of distinct cells of flow separation on the circumference, rotating at a fraction of the impeller revolution rate, has been observed in thermal and hydraulic turbomachines. Due to its complexity, the part-load flow in radial centrifugal pumps is a major challenge for numerical flow simulation methods. The present study investigates the part-load flow in radial centrifugal pumps and pump-turbines by experimental and numerical methods, the latter using a finite volume discretization of the Reynolds-averaged Navier-Stokes (RANS) equation. Physical phenomena of part load flow are evidenced based on three case studies, and the ability of numerical simulation methods to reproduce part-load flow in radial centrifugal pumps qualitatively and quantitatively is assessed. A numerical study of the flow in a high specific speed radial pump-turbine using steady approaches and the hypothesis of angular periodicity between neighboring blade channels evidences the relation of sudden flow topology changes with an increase of viscous losses, impacting on the energy-discharge characteristic, and thus increasing the risk of unstable operation. When the flow rate drops below a critical threshold, the straight through-flow with flow separation zones attached to the guide vanes changes to an asymmetrical flow. Energy is drawn off the mean flow and dissipated in a large vortex-like structure. Besides flow separation in some diffuser channels, time-dependent numerical simulations of the flow in a double suction pump evidence a flow rate imbalance between both impeller sides interacting with asymmetric flow separation in the diffuser. Viscous losses increase substantially as this imbalance occurs, the resulting segment of positive slope in the energy-discharge characteristic is found for a flow rate sensibly different from measurements. Different modes of rotating stall are identified by transient pressure measurements in a low-specific-speed pump-turbine, showing 3 to 5 zones of separated flow, rotating at 0.016 to 0.028 times impeller rotation rate, depending on discharge. For operating conditions where stall with 4 cells is most pronounced, velocity is measured by Laser-Doppler methods at locations of interest. The velocity field is reconstructed with respect to the passage of stall cells by definition of a stall phase obtained from simultaneous transient pressure measurements. Time-dependent numerical simulation predicting rotating stall with 4 cells shows velocity fields that are in reasonable agreement with the measured velocity fields, but occurring at a sensibly higher flow rate than found from experiments. In consideration of the quantitative shortcomings of the numerical simulation, a novel modelling approach is proposed: Replacing the costly 3-dimensional simulation of the major part of the impeller channels by a 1-dimensional model allows a significant economy in computational resources, allowing an improved modeling for the remainder of the domain at constant computational cost. The model is validated with the challenging cases of rotating stall and impeller side flow rate imbalance. The satisfying coherence of the results with the simulation including the entire impeller channels qualifies this approach for numerous turbomachinery applications. It could also provide improved, time-dependent boundary conditions for draft tube vortex rope simulations at reasonable computational cost. Parameter studies modifying deliberately some quantities of mean flow and turbulence at the modeled boundary surfaces can be implemented in the framework of the method.