Centrifugal pumps are widely used in engineering applications, consuming a considerable amount of energy across the globe. However, in many cases they operate under off-design conditions, such as multiphase flows, implying in an even higher energy consumption. A prominent example is the mixture of water and viscous oil, where the phases may exhibit a dispersed flow pattern depending on superficial velocities. Understanding the flow dynamics within the impeller during two-phase liquid–liquid operation is crucial for grasping the mechanisms underlying energy dissipation and pump performance. In this work, we investigate experimentally oil–water flows in dispersed regime within a centrifugal pump impeller, and propose a framework for automatically identifying the dispersed phase and measuring the velocity field of the continuous phase. For that, we carried out time-resolved particle image velocimetry (TR-PIV) in a transparent pump operating under two-phase flow conditions. An image processing technique based on deep learning was developed to dynamically mask oil droplets (dispersed phase) and distinguish them from water-seeded particles (continuous phase) in the raw TR-PIV data. Additionally, a method to evaluate the phase-ensemble average velocity was designed and implemented. The results revealed that neglecting dynamic masking in the TR-PIV images caused an inversion of velocity values between the pressure and suction blades, driven by the accumulation of oil droplets in recirculation zones near the suction blade. This result highlights the importance of accurately tracking the dispersed phase. Our findings indicate higher turbulent kinetic energy (TKE) values at lower flow rates when the dispersed phase consists of larger oil droplets. These findings expand our understanding of multiphase flows in centrifugal pumps, which can be proven useful for validating numerical simulations, proposing new mathematical models, and contributing to the design of improved and energy-saving impellers.
Tag: PIV
Particle image velocimetry in the impeller of a centrifugal pump: Relationship between turbulent flow and energy loss
Turbulent flows play a dominant role in the operation of centrifugal pumps, which find widespread use in industrial settings and various aspects of human life. The dissipation rate of turbulent kinetic energy emerges as a key parameter within these devices, with its local values exerting a significant influence on centrifugal pump performance. Recent advances in particle image velocimetry (PIV) techniques have expanded the ability to analyze complex turbulent flows across a broad spectrum of scales. In this context, this paper aims to deepen our understanding of the turbulent flow field and its correlation with energy loss in centrifugal pump impellers. To achieve this, experiments were conducted using PIV on a transparent pump operating under different conditions. Statistics of the turbulent flow were then obtained from phase-ensemble averages of velocities, vorticity, turbulence production, and local dissipation of turbulent kinetic energy. To overcome the limited spatial resolution constraint of PIV, the large-eddy PIV (LES-PIV) method was employed to estimate the local dissipation rate. In this method, it is assumed that the motion of larger scales is measured by the PIV technique, while the smaller scales (unresolved scales) are modeled by a sub-grid scale model, calculated from the strain rate tensors obtained from the measured fields. Energy losses in the impeller were studied using two methodologies: (i) a conventional method based on power measurements, and (ii) an alternative approach based on the budget of turbulent kinetic energy. Our results reveal that turbulent loss caused by turbulence production is the main source of energy loss in the pump impeller, and it is particularly pronounced in low-flow operating conditions characterized by large-scale structures. On the other hand, in situations where flow rates exceed the best efficiency point (BEP) condition, the predominant flow structures are marked by small-scale features, mainly attributed to local dissipation of turbulence. Our findings clarify the characteristics of energy losses in centrifugal pump impellers and their relationship with the turbulent flow field, and, in addition, providing a methodology for calculating the local turbulent dissipation rate and its limitations when derived from PIV measurements.
Particle image velocimetry in the impeller of a centrifugal pump: A POD-based analysis
The flow field within the channels of a centrifugal pump impeller is usually complex, containing turbulent structures in a wide range of time and length scales. Identifying the different structures and their dynamics in this rotating frame is, therefore, a difficult task. However, modal decomposition can be a useful tool for detecting coherent structures. In this paper, we make use of proper orthogonal decomposition (POD) of time-resolved flow fields in order to investigate the flow in a centrifugal pump. For that, we carried out experiments using time-resolved particle image velocimetry (TR-PIV) in a pump of transparent material operating at different conditions and obtained the statistical characteristics of the turbulent flow from phase-ensemble averages of velocities and turbulent kinetic energy. The results reveal that at the pump’s best efficiency point (BEP) the flow is well-organized, with no significant flow separation. For flow rates below the BEP, flow separation and vortex structures appear in the impeller channels, making the flow unstable. At flow rates above the BEP, intense jets appear close to the suction blades, while small instabilities occur on the pressure side. The POD analysis shows that at low flow rates, the flow is dominated by large-scale structures with intense energy levels, while at the BEP and higher flow rates, the flow is dominated by small-scale structures. Our results shed light on the turbulence characteristics inside the impeller, providing relevant information for reduced-order models capable of computing the flow in turbomachinery at much lower costs when compared to traditional methods.
A coupled PIV/PTV technique for the dispersed oil-water two-phase flows within a centrifugal pump impeller
The current work presents a framework for the simultaneous characterization of different phases in a dispersed oil-water two-phase flow. The framework is based on the coupling of the PIV and PTV techniques in raw PIV acquisition images. The PIV technique computes the water velocity fields, while the PTV technique calculates the oil drop velocities. Thus, the proposed technique allows the simultaneous measurement of the water phase and dispersed oil drop velocity from the same image. In order to present the advantages of the coupled PIV/PTV technique, the flow within a centrifugal pump impeller is completely analyzed by computing the oil and water phase-ensembled velocities.