No abstract

No abstract

The influence of surface roughness on the pump performance was deeply analyzed based on computational fluid dynamics (CFD). A series of numerical calculations with different grid numbers, turbulence models, and surface roughness were made for a typical multistage centrifugal pump. Moreover, the external characteristic experiments were also conducted to verify the numerical calculations. The results show that the surface roughness has enormous influences on the pump performance. With the increase of the surface roughness, the head and the efficiency of the pump decreases continuously, but the decreasing rate slows down gradually, and the surface roughness has a greater influence on the efficiency than that on the head. Moreover, the influence of surface roughness on the disk friction loss power is much greater than that on the hydraulic power. Besides, the total efficiency of the pump reduces mainly by decreasing the hydraulic efficiency and the mechanical efficiency, due to the negative effect of surface roughness. In addition, the surface roughness of the impeller and the diffuser mainly affects the hydraulic efficiency, the surface roughness of the shroud's outer wall mainly influences the mechanical efficiency, and the surface roughness of the inner wall of the pump cavity mainly affects the volume efficiency, but the influence of surface roughness on the pump performance is interconnected. Therefore, due to that, it's very difficult to make the precision-machine inside the impeller and diffuser, polishing the impeller shroud and pump cavity is beneficial to improve the pump efficiency and reduce the pump shaft power.INDEX TERMS Mechanical engineering, pump, surface roughness.

No abstract

A computational platform for direct numerical simulation of fluid-particle two-phase flow in porous media is presented in this study. In the proposed platform, the Navier-Stokes equations are used to describe the motion of the continuous phase, while the discrete element method (DEM) is employed to evaluate particle-particle and particle-wall interactions, with a fictitious domain method being adopted to evaluate particle-fluid interactions. Particle-wall contact states are detected by the ERIGID scheme. Moreover, a new scheme, namely, base point-increment method is developed to improve the accuracy of particle tracking in porous media. In order to improve computationally efficiency, a time splitting strategy is applied to couple the fluid and DEM solvers, allowing different time steps to be used which are adaptively determined according to the stability conditions of each solver. The proposed platform is applied to particle transport in a porous medium with its pore structure being reconstructed from micro-CT scans from a real rock. By incorporating the effect of pore structure which has a comparable size to the particles, numerical results reveal a number of distinct microscopic flow mechanisms and the corresponding macroscopic characteristics. The time evolution of the inlet to outlet pressure-difference consists of large-scale spikes and small-scale fluctuations. Apart from the influence through direct contacts between particles, the motion of a particle can also be affected by particles without contact through blocking a nearby passage for fluid flow. Particle size has a profound influence on the macroscopic motion behavior of particles. Small particles are easier to move along the main stream and less dispersive in the direction perpendicular to the flow than large particles.

To improve the stability of the multistage centrifugal pump, a special test bench was built based on the Bently ADRE 408 portable vibration tester. Taking a cantilever multistage centrifugal pump as the research object, the axis orbit and the vibration spectrum of the pump were obtained at 0Q d (zero flow point), 1.0Q d (design flow point) and 1.5Q d (large flow point). Moreover, based on the Butterworth filter of the original axis orbit, we derived the axis orbit curve and the displacement time-domain diagram with fault information under different frequency multiplication, and also deeply studied the rules of vibration characteristics changing with different operating conditions and with different series at each monitor location. The results show that the axis orbit at the zero flow point is unstable and the vibration velocity is the highest which is caused by the imbalance of the mass; the axis orbit at the large flow point is stable and the amplitude of the vibration velocity is the smallest; the axis orbit of the design flow point is the most stable and the amplitude of the vibration velocity is the smallest. The maximum axis orbit displacements in the three operating conditions are all smaller than the unilateral space between the impeller and the diffuser, which avoids the radial friction between the rotor components and the static components. The axis orbit after the filter indicates that the rotor part has an unbalanced mass, and there is a fit clearance between the inner bore and the shaft diameter of the impeller, which can easily lead to the misalignment. Under different flow operating conditions, the blade frequency and its frequency multiplication are the main excitation frequencies of the multistage centrifugal pump, and to the pump casings at every stage, the main vibration frequency is one time and two times the blade frequency. This experimental study provides a new method to improve the stability and restrain the instability of the multistage centrifugal pumps.

No abstract

The operating range of axial flow pumps is often constrained by the onset of rotating stall. An improved method using a double inlet nozzle to stabilize the performance curve is presented in the current study; a single inlet nozzle and three kinds of double inlet nozzle with different rib gap widths at the inlet of axial flow pump impeller were designed. Three dimensional (3D) incompressible flow fields were simulated, and the distributions of turbulence kinetic energy and velocity at different flow rates located at the inlet section, as well as the pressure and streamline in the impeller, were obtained at the same time. The single inlet nozzle scheme and a double inlet nozzle scheme were studied; the experimental and numerical performance results show that although the cross section is partly blocked in the double inlet nozzle, the head and efficiency do not decline at stable operation flow rate. On small flow rate condition, the double inlet nozzle scheme effectively stabilized the head-flow performance, whereby the block induced by the backflow before the impeller was markedly improved by using a double inlet nozzle. It has also been found that the rib gap width impacts the efficiency curve of the axial flow pump.