Low Specific Centrifugal Pump Research Articles

Improve of Unsteady Pressure Pulsation Based on Jet–Wake Suppression for a Low Specific Centrifugal Pump

Abstract In this study, three impellers with different blade pressure side (PS) profiles are designed and the influence on the hydraulic and dynamic performance of a low specific speed centrifugal pump is investigated by numerical simulation and experimental research. The results show that blade PS modification introduced in this study can efficiently alleviate the unsteady pressure pulsation of model pump. In order to study the effects of blade modification on the internal flow filed, the volute domain is replaced by an even outlet region for computational fluid dynamic (CFD) analysis. Relative velocity distribution is extracted to visualize the three-dimensional (3D) flow characteristics at the impeller outlet. Results show that the flow at impeller outlet presents a typical jet–wake structure, which is significantly suppressed after the blade modification. The suppression of jet–wake structure, which is attributed to the redistribution of pressure and velocity in the impeller caused by the change of blade work capacity, can directly reduce the intensity of pressure pulsation in the volute by increasing the velocity uniformity at impeller outlet. Given that the existence of jet–wake flow results in large mixing loss and velocity deviation at the impeller outlet, entropy generation rate and slip velocity calculation are introduced here to measure the extent of jet–wake configuration. Results show that both indicators introduced here can be used to quantify the extent of the jet–wake structure at impeller outlet, and thus, indirectly predict the strength of unsteady pressure pulsation in pump volute.

Numerical study of operating performance of a centrifugal pump as turbine at overload with special emphasis on unsteady blade load

Three-dimensional (3D) unsteady Reynolds-averaged Navier–Stokes (URANS) simulations are conducted to investigate the blade load and internal flow field for a low specific centrifugal pump operating as a turbine (PAT). Validation and grid independence of the simulation method are performed and ensured. A thorough inspection to flow variables in terms of pressure and radial velocity as well as circumferential velocity at strong rotor–stator interaction region is performed. Unsteady performance characteristics in terms of head and shaft power as well as transient blade loads are evaluated to assess the unsteady PAT performance. Significant decreasing of the blade load is revealed when impeller passage passes the volute tongue and associated with the strong vortex close to passage suction side caused by the sharp angle of suction side trailing edge with a discernible flow incidence angle. High negative radial velocity in the region close to suction side is originated from the vortex inducing velocity at the blade suction side and a high pressure gradient in the trailing edge region. The decrease of blade load is caused by periodical development and restriction of the vortex with an impeller passage passing tongue and a subsequent pressure variation on suction side that leads to temporally decreased blade loading. High positive radial velocity in pressure side region is originated from the blocking effect from the strong vortex at suction side, leading to the generation of a relatively weaker vortex fluctuating with impeller rotation at pressure side. That is the source of pressure variation on pressure side, contributing to the blade loading variation. These results provide a detailed insight into the complex overload flow field that might be utilized for an improved PAT design.

Investigation of flow instability characteristics in a low specific speed centrifugal pump using a modified partially averaged Navier–Stokes model

In this study, a modified partially averaged Navier–Stokes (MPANS) model is applied to investigate the flow instability characteristics in a low specific centrifugal pump. In MPANS model, the unresolved-to-total ratio of kinetic energy fk is determined according to the local grid size and turbulence length scale. The numerical results by MPANS model are compared with that simulated by SST k-ω model and the available experimental data. It is noted that MPANS model shows better performance for investigating the unstable flow in the current pump under part-load operation conditions. The time-averaged internal flow and flow incidence in the pump impeller depicts that with the decreasing flow coefficient, flow separation develops in the impeller. Owing to the strong separation flow as well as vortex evolution, incidence angle is large and varies remarkably at the entrance of blade-to-blade passage in the pump impeller. The evolution dynamics of rotating stall is further discussed in detail based on vorticity transport equation. During the evolution of rotating stall, the vortex stretching term has an important effect on vorticity transport under the part-load conditions. The analysis of the pressure fluctuations excited by periodic evolution of rotating stall shows that the rotating stall cell propagates along the rotational direction, and identifies the rotating stall frequency ( fstall), which is much lower than the rotational frequency of the impeller, fn ( fstall = 8.76% fn). Finally, two-dimensional Lagrangian coherent structure (LCS) is used to reveal the separation flow in blade-to-blade passages of the pump by monitoring the trajectory of the particles. Both LCS and vortex structure by λ2 can clearly demonstrates the passage blockage and flow separation under the part-load operation conditions, depicting that the separation flow occurs at blade suction side and develops from the leading edge to the main passage in the impeller.

Critical cavitation coefficient analysis of a space low specific centrifugal pump with micro gravity

Centrifugal pump was used in the loop as a baselined unit. The flow rate of the pump was very small, while the head was high. This space pump must work stable for a long time (more than a year), so the performance of the pump attracted public attention. The rotational speed of the impeller was limited for stability, so the pump belonged to low specific centrifugal pump. In this paper, a single-phase centrifugal pump, which was designed for single-phase fluid loops in satellites, was modeled for numerical simulation. The hydraulic region of the pump was discretized by structured mesh. Three dimensional (3-D) flow in the pump was studied by the use of computational fluid dynamics. Partially-Averaged Navier- Stokes (PANS) model based on RNG k-ε turbulence model was developed for the simulation of the unsteady flow. Velocity inlet and pressure outlet was used as the boundary conditions. Interface was used between the impeller and the casing, as well as the impeller and inlet pipe. Performances and pressure fluctuation of the pump were investigated. The dominant frequency of the pressure fluctuation is blade passing frequency at the region close to the tongue of the casing, while it is twice of blade passing frequency at the other region.

Experimental research on internal flow in impeller of a low specific speed centrifugal pump by PIV

For the purpose of investigating the influence of two different impellers, one is with splitter blades and the other one is without splitter blades, on a low-specific centrifugal pump. The experimental investigation in impellers was conducted at different conditions and phases by means of PIV (Particle Image Velocimetry) to study the internal flow. Meanwhile, the absolute and relative velocity distributions in impellers were obtained. Experimental results show that the head value is higher in the impeller with splitter blades and both two head curves appear hump phenomena at small flow rate. The absolute velocity value increases with radius and from pressure side to suction side at the same radius gradually. The splitter blades can scour the wake, making outlet velocity distribution more uniform and improving the internal flow. The velocity distribution becomes less even in the process of closing to tongue due to reinforced interference of tongue on internal flow.

A nonlinear Partially-Averaged Navier–Stokes model for turbulence

Most of the RANS turbulence models solved the Reynolds stress by linear hypothesis and they were isotropic models. At present, Partially-Averaged Navier–Stokes (PANS) has been found to be better than that of the LES with less mesh in the near-wall region. In this paper, a new nonlinear PANS turbulence model was proposed, which was modified from RNG k–ε turbulence model and the Reynolds stresses were solved by nonlinear methods. The new nonlinear PANS turbulence model was used to simulate the flow fields around a NACA0015 hydrofoil, in a curved rectangular duct and a low specific centrifugal pump. The results based on the new nonlinear PANS model agreed well with the experimental results. The calculation results proved that the nonlinear PANS model is available for high pressure gradient flows, large curvature flows, and it can be used to capture complex vortexes in a turbomachinary.

A new nonlinear turbulence model based on Partially-Averaged Navier-Stokes Equations

Partially-averaged Navier-Stokes (PANS) Model was recognized as a Reynolds-averaged Navier-Stokes (RANS) to direct numerical simulation (DNS) bridging method. PANS model was purported for any filter width-from RANS to DNS. PANS method also shared some similarities with the currently popular URANS (unsteady RANS) method. In this paper, a new PANS model was proposed, which was based on RNG k-ε turbulence model. The Standard and RNG k-ε turbulence model were both isotropic models, as well as PANS models. The sheer stress in those PANS models was solved by linear equation. The linear hypothesis was not accurate in the simulation of complex flow, such as stall phenomenon. The sheer stress here was solved by nonlinear method proposed by Ehrhard. Then, the nonlinear PANS model was set up. The pressure coefficient of the suction side of the NACA0015 hydrofoil was predicted. The result of pressure coefficient agrees well with experimental result, which proves that the nonlinear PANS model can capture the high pressure gradient flow. A low specific centrifugal pump was used to verify the capacity of the nonlinear PANS model. The comparison between the simulation results of the centrifugal pump and Particle Image Velocimetry (PIV) results proves that the nonlinear PANS model can be used in the prediction of complex flow field.

511 遠心ポンプのキャビテーション流れ解析

The cavitation flow in a low specific centrifugal pump is analyzed by using two types of cavitation CFD codes. A locally homogeneous model of compressible air-vapor-liquid two-phase medium is applied to calculate the objective flow. Constant Enthalpy Vaporization model is also applied. Numerical results are compared to measured results. At the cavitation breakdown of the BEP flow rate, the throat is choked by cavities on both suction surface and pressure surface and head drops steeply. The steep head drop is well captured by both CFD codes. The NPSH value at the breakdown point can be predicted reasonably. At the high flow rate further modification is necessary for both codes to achieve stable and accurate results.