Ji Pei

A Comparison of One-Way and Two-Way Coupling Methods for Numerical Analysis of Fluid-Structure Interactions

The interaction between fluid and structure occurs in a wide range of engineering problems. The solution for such problems is based on the relations of continuum mechanics and is mostly solved with numerical methods. It is a computational challenge to solve such problems because of the complex geometries, intricate physics of fluids, and complicated fluid-structure interactions. The way in which the interaction between fluid and solid is described gives the largest opportunity for reducing the computational effort. One possibility for reducing the computational effort of fluid-structure simulations is the use of one-way coupled simulations. In this paper, different problems are investigated with one-way and two-way coupled methods. After an explanation of the solution strategy for both models, a closer look at the differences between these methods will be provided, and it will be shown under what conditions a one-way coupling solution gives plausible results.

Numerical prediction of 3-D periodic flow unsteadiness in a centrifugal pump under part-load condition

Numerical simulation and 3-D periodic flow unsteadiness analysis for a centrifugal pump with volute are carried out in whole flow passage, including the impeller with twisted blades, the volute and the side chamber channels under a part-load condition. The pressure fluctuation intensity coefficient (PFIC) based on the standard deviation method, the time-averaged velocity unsteadiness intensity coefficient (VUIC) and the time-averaged turbulence intensity coefficient (TIC) are defined by averaging the results at each grid node for an entire impeller revolution period. Therefore, the strength distributions of the periodic flow unsteadiness based on the unsteady Reynolds-averaged Navier-Stokes (URANS) equations can be analyzed directly and in detail. It is shown that under the 0.6Qdes. condition, the pressure fluctuation intensity is larger near the blade pressure side than near the suction side, and a high fluctuation intensity can be observed at the beginning section of the spiral of the volute. The flow velocity unsteadiness intensity is larger near the blade suction side than near the pressure side. A strong turbulence intensity can be found near the blade suction side, the impeller shroud side as well as in the side chamber. The leakage flow has a significant effect on the inflow of the impeller, and can increase both the flow velocity unsteadiness intensity and the turbulence intensity near the wall. The accumulative flow unsteadiness results of an impeller revolution can be an important aspect to be considered in the centrifugal pump optimum design for obtaining a more stable inner flow of the pump and reducing the flow-induced vibration and noise in certain components.

Application of different strategies of partitioned fluid–structure interaction simulation for a single-blade pump impeller

In this article, the one-way and two-way coupling strategies are used, respectively, for partitioned fluid–structure interaction simulation to numerically investigate the hydrodynamic force and vibration of single-blade pump impeller at design and off-design conditions, and the effect of mesh resolution on the two-way coupling results is also considered. The comparison analysis of the calculated results with different strategies, involving aspects of amplitude and phase, has been carried out. In addition, the simulated impeller deflection results are also compared with experimental results by proximity sensors. The analysis of the results show that for different mesh resolutions, the amplitude of deflection with fine mesh is bigger than that with coarse mesh for almost all the operational conditions, and the phase differences can be clearly found for part-load conditions. The amplitude of hydrodynamic force calculated with coarse mesh is smaller than that is calculated with fine mesh in almost all quadrants for each flow rate, except the first quadrant for Q = 11 L/s. Furthermore, the amplitude of deflection calculated by two-way coupling strategy is bigger than that is calculated by one-way coupling strategy in certain quadrants for each flow rate, and the phase results have a good agreement for different coupling strategies. The amplitude of hydrodynamic force calculated by one-way coupling is just slightly smaller, and the phase difference can only be observed for Q = 42 L/s and Q = 11 L/s conditions.

Experimental investigation on the flow-induced noise under variable conditions for centrifugal pumps

With extensively using of centrifugal pumps, noise generation in these pumps is increasingly receiving research attention in recent years. The noise sources in centrifugal pumps are mainly composed of mechanical noise and flow-induced noise. And the study of flow-induced noise has become a hotspot and important domain in the field. The flow-induced noise closely related to the inner pressure pulses and vibration of volute in pumps, therefore, it is necessary to research the interaction and mechanism among them. To investigate the relationships, a test system is designed which includes a test loop and a measurement system. The hydrophones and pressure sensors are installed on the outlet of the pump and vibration acceleration sensors are disposed on the pump body. Via these instruments, the signals of noise, pressure pulses and vibration are collected and analyzed. The results show that the level of flow-induced noise becomes smaller as the flow increment during low flow rate operations, and it is steadily close to the design point, then it increases with the growing of flow rate in high flow rate conditions. Furthermore, there are some similar peak points in the power spectrum charts of noise, pressure pulses and vibration. The broadband noise at low flow rate is mostly focused on the region of 0–40 times shaft frequency, which is mostly made by rotating stall and vortex; while the noise at high flow rate conditions is focused on the region of 60–100 times shaft frequency, which may be mostly made by cavitations. The proposed research is of practical and academic significance to the study of noise reduction for centrifugal pumps.

Cavitation optimization for a centrifugal pump impeller by using orthogonal design of experiment

Cavitation is one of the most important performance of centrifugal pumps. However, the current optimization works of centrifugal pump are mostly focusing on hydraulic efficiency only, which may result in poor cavitation performance. Therefore, it is necessary to find an appropriate solution to improve cavitation performance with acceptable efficiency. In this paper, to improve the cavitation performance of a centrifugal pump with a vaned diffuser, the influence of impeller geometric parameters on the cavitation of the pump is investigated using the orthogonal design of experiment (DOE) based on computational fluid dynamics. The impeller inlet diameter D1, inlet incidence angle Δβ, and blade wrap angle φ are selected as the main impeller geometric parameters and the orthogonal experiment of L9(3*3) is performed. Three-dimensional steady simulations for cavitation are conducted by using constant gas mass fraction model with second-order upwind, and the predicated cavitation performance is validated by laboratory experiment. The optimization results are obtained by the range analysis method to improve cavitation performance without obvious decreasing the efficiency of the centrifugal pump. The internal flow of the pump is analyzed in order to identify the flow behavior that can affect cavitation performance. The results show that D1 has the greatest influence on the pump cavitation and the final optimized impeller provides better flow distribution at blade leading edge. The final optimized impeller accomplishes better cavitation and hydraulic performance and the NPSHR decreases by 0.63m compared with the original one. The presented work supplies a feasible route in engineering practice to optimize a centrifugal pump impeller for better cavitation performance.

Optimization on the impeller of a low-specific-speed centrifugal pump for hydraulic performance improvement

In order to widen the high-efficiency operating range of a low-specific-speed centrifugal pump, an optimization process for considering efficiencies under 1.0Qd and 1.4Qd is proposed. Three parameters, namely, the blade outlet width b2, blade outlet angle β2, and blade wrap angle φ, are selected as design variables. Impellers are generated using the optimal Latin hypercube sampling method. The pump efficiencies are calculated using the software CFX 14.5 at two operating points selected as objectives. Surrogate models are also constructed to analyze the relationship between the objectives and the design variables. Finally, the particle swarm optimization algorithm is applied to calculate the surrogate model to determine the best combination of the impeller parameters. The results show that the performance curve predicted by numerical simulation has a good agreement with the experimental results. Compared with the efficiencies of the original impeller, the hydraulic efficiencies of the optimized impeller are increased by 4.18% and 0.62% under 1.0Qd and 1.4Qd, respectively. The comparison of inner flow between the original pump and optimized one illustrates the improvement of performance. The optimization process can provide a useful reference on performance improvement of other pumps, even on reduction of pressure fluctuations.

Dynamic stress analysis of sewage centrifugal pump impeller based on two-way coupling method

Current research on the operational reliability of centrifugal pumps has mainly focused on hydrodynamic instability. However, the interaction between the fluid and structure has not been sufficiently considered; this interaction can cause vibration and dynamic stress, which can affect the reliability. In this study, the dynamic stresses in a single-blade centrifugal pump impeller are analysed under different operating conditions; the two-way coupling method is used to calculate the fluid-structure interaction. Three-dimensional unsteady Reynolds-averaged Navier-Stokes equations are solved with the SST k-ω turbulence model for the fluid in the whole flow passage, while transient structure dynamic analysis is used with the finite element method for the structure side. The dynamic stresses in the rotor system are computed according to the fourth strength theory. The stress results show that the highest stress is near the loose bearing and that the equivalent stress increases with the flow rate because the dynamic stresses are closely related to the pressure load. The stress distributions on the blade pressure side, suction side, leading edge, and trailing edge are each analysed for different flow rates; the highest stress distribution is found on the pressure side. On the blade pressure side, a relatively large stress is found near the trailing edge and hub side. Based on these results, a stress distribution prediction method is proposed for centrifugal pumps, which considers the interaction between the fluid and structure. The method can be used to check the dynamic stress at different flow rates when optimising the pump design to increase the pump reliability.

The influence of the flow rate on periodic flow unsteadiness behaviors in a sewage centrifugal pump

To design a single-blade pump with a good performance in a wide operational range and to increase the pump reliability in the multi-conditional hydraulic design process, an understanding of the unsteady flow behaviors as related with the flow rate is very important. However, the traditional design often considers only a single design condition, and the unsteady flow behaviors have not been well studied for single-blade pumps under different conditions. A comparison analysis of the flow unsteadiness behaviors at different flow rates within the whole flow passage of the pump is carried out in this paper by solving the three-dimensional unsteady Reynolds-averaged Navier-Stokes equations with the Shear Stress Transport (SST) turbulence model. A definition of the unsteadiness in the pump is made and applied to analyze the unsteady intensity distributions, and the flow rate effect on the complex unsteady flow in the pump is studied quantitatively while the flow mechanism is also analyzed. The CFD results are validated by experimental data collected at the laboratory. It is shown that a significant flow rate effect on the time-averaged unsteadiness and the turbulence intensity distribution can be observed in both rotor and stator domains including the side chamber. The findings would be useful to reduce the flow unsteadiness and to increase the pump reliability under multi-conditions.

Transient flow characteristics during variable operating conditions of the centrifugal charging pump in 1000 MW nuclear power plant

In order to study transient flow characteristics of the centrifugal charging pump during the process from the charging operating condition (Q = 34 m3/h) to the highest efficiency operating condition (Q = 110 m3/h), commercial software CFX was used to study the three-dimensional, unsteady, incompressible viscous flows. In this paper, the transient flow rate during the variable operating conditions was obtained from experimental data and immersed into CFX as the transient control function. The transient pressure and velocity in impellers, guide vanes, and double channels volute of the charging pump are analyzed through the simulation. The results show that the pressures in impellers increase gradually to the maximum and then decrease to the minimum in a rotational circle and looks like a sinusoidal curve. As the flow rate increasing, the fluctuant amplitude of the transient pressure tends to decrease. All transient velocities fluctuation becomes weaker and weaker and reduces to a minimum value in the 12th-stage impeller as time went on. The flow is untidy in the chamber and it causes large vortex at the small flow rate in the beginning of the variable operating conditions. The hydraulic performance of the pump is influenced both by the fluid acceleration and instantaneous evolutions of the vortex structure during the transient process. In addition, the fluctuation near the interface between impellers and guide vanes is bigger than others and the streamlines of flow get smooth under the rectification of each impeller and guide vane, which leads to the amplitude of pressure fluctuation unchanged basically in the last stage.

Optimum Hydraulic Design for a Radial Diffuser Pump Using Orthogonal Experimental Method Based on CFD

To improve the performance of the centrifugal pump with a vaned diffuser, the influence of impeller geometric parameters on external characteristics of the pump was investigated by Orthogonal Experimental Method (OEM) based on CFD. Blade outlet width b2, blade wrap angle φ, blade outlet angle β2, and blade number Z were selected as the main impeller geometric parameters and the orthogonal experiment of L9 (33*21), which contained 3 levels of the 3 factors and 2 levels of one factor, was done in this study. Three-dimensional steady simulations were conducted by solving the RANS equations in the design procedure with SST k-ω turbulence model, and about 5.3 million structured grids for the whole calculation domains were used. The experimental results were justified by the variance analysis method. The inner flow of the pump was also analyzed in order to obtain the flow behaviors that can affect the pump performance. The results showed that the blade outlet angle β2 had the greatest influence on the efficiency and power. The high efficiency area of the optimal impeller is wider. The final optimized impeller accomplished better pump performance, which meet the design requirements. The velocity distribution in the optimized impeller is more regular and the area of the high turbulence kinetic energy is smaller in the optimal impeller.Copyright