Since wastewater pumps used in sewage systems have a trade-off relationship between non-clogging ability and efficiency, many internal flow and performance estimation studies so far have focused on optimizing both parameters. Here we focus on vortex pumps with a large space that allows foreign matters to pass through inside the casing that are used as sewage pumps.
In this paper, using experimental and numerical methods, we clarified clogging mechanism in the pumps using CFD (Computational Fluid Dynamics), Motion Capture and PIV (Particle Image Velocimetry) method.
Firstly, since it was difficult to observe the motion of foreign matter in the pumps, a simple experimental apparatus was built to quantify it. Five cylindrical rods were set in a rectangular water passage made of acrylic that has an 80mm*80mm cross section. The motion of strings in the passage was studied with experiments and computations. Strings were chosen as foreign matter because they are comparatively easy to compute and experiment with compared to fabric for example. In the experiments, the motion of strings could be recorded with a high-speed video camera and their motions were quantified with two-dimensional motion capture system.
In the computations, the motion of strings was simulated using DEM (Discrete Element Method) in the CFD software STAR-CCM+ by connecting particles with straight lines and coupling CFD with DEM. It is a numerical value method in which the motion and interaction of a large number of disintegrating objects is simulated. Its characteristic is that the point of contact between the particles is included in the equation of motion. Particles are connected by massless rods. These rods transmit force and momentum to each particle. Furthermore, the force between each particle is computed of the interaction between soft-particles and their bonding strength. In order to formulate the contact force between the particles, the spring-dashpot system is used. In addition, the optimal parameters of the DEM particles were obtained by conducting a parameter study.
We confirmed that the motion of the strings in the flow direction coincided with at a high precision in the passage. Furthermore, by applying the computational method with the above results, we were able to simulate the motion of strings in a vortex pump. We found that strings were pulled back into the pump by the backflow in the tongue of the pump. This backflow was observed using PIV results, providing experimental confirmation of this mechanism. Moreover, we were able to simulate the motions of fibrous materials in the passage and vortex pump.
The strong swirling flow at the exit of the runner of a Francis turbine at part load causes flow instabilities and cavitation surges in the draft tube, deteriorating the performance of the hydraulic power system. The unsteady cavitating turbulent flow in the draft tube is simplified and modeled by a diffuser with swirling flow using the Scale-adaptive simulation method. Unsteady characteristics of the vortex rope structure and the underlying mechanisms for the interactions between the cavitation and the vortices are both revealed. The generation and evolution of the vortex rope structures are demonstrated with the help of the iso-surfaces of the vapor volume fraction and the Qcriterion. Analysis based on the vorticity transport equation suggests that the vortex dilatation term is much larger along the cavity interface in the diffuser inlet and modifies the vorticity field in regions with high density and pressure gradients. The present work is validated by comparing two types of cavitation surges observed experimentally in the literature with further interpretations based on simulations.
Model tests and CFD were carried out to find out the cause of cavitation surge in hydraulic power plants. In experiments the cavitation surge was observed at flow rates higher and lower than the swirl free flow rate, both with and without a surge tank placed just upstream of the inlet volute. The surge frequency at smaller flow rate was much smaller than the swirl mode frequency caused by the whirl of vortex rope. An unsteady CFD was carried out with two boundary conditions: (1) the flow rate is fixed to be constant at the volute inlet, (2) the total pressure is kept constant at the volute inlet, corresponding to the experiments without/with the surge tank. The surge was observed with both boundary conditions at both higher and lower flow rates. Discussions as to the cause of the surge are made based on additional tests with an orifice at the diffuser exit, and with the diffuser replaced with a straight pipe.
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