Study on numerical methods for transient flow induced by speed-changing impeller of fluid machinery

Dazhuan Wu,Tao Chen,Youbo Sun,Wentao Cheng,Leqin Wang 대한기계학회 2013 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.27 No.6

In order to establish a reliable numerical method for solving the transient rotating flow induced by a speed-changing impeller, two numerical methods based on finite volume method (FVM) were presented and analyzed in this study. Two-dimensional numerical simulations of incompressible transient unsteady flow induced by an impeller during starting process were carried out respectively by using DM and DSR methods. The accuracy and adaptability of the two methods were evaluated by comprehensively comparing the calculation results. Moreover, an intensive study on the application of DSR method was conducted subsequently. The results showed that transient flow structure evolution and transient characteristics of the starting impeller are obviously affected by the starting process. The transient flow can be captured by both two methods, and the DSR method shows a higher computational efficiency. As an application example, the starting process of a mixed-flow pump was simulated by using DSR method. The calculation results were analyzed by comparing with the experiment data.

A new transient CFD method for determining the dynamic coefficients of liquid annular seals

Dazhuan Wu,Xinkuo Jiang,Shiyang Li,Leqin Wang 대한기계학회 2016 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.30 No.8

Currently, the dynamic characteristics of annular seals are numerically investigated mainly by solving the bulk flow equations using perturbation method, or by simulating the perturbed flow field of annular seal using CFD method. The adopted disturbance way is generally the circular whirling motion of rotor around seal centre with constant speed. Relative to the transient CFD simulation, the quasisteady CFD simulation introducing Moving reference frame (MRF) has been widely used by researchers. Both the dynamic mesh problem and the time-consuming problem suppress the use of transient CFD simulation in annular seal research. In the paper, a new transient CFD method based on rotor’s variable-speed whirl is presented to improve the time-consuming problem and all the (total 20) dynamic coefficients of concentric liquid seal can be obtained by only two transient CFD simulations, one for the variable-speed cylindrical whirl and the other for the variable-speed conical whirl. The results are compared with those from the experiment, the quasi-steady CFD method and the traditional transient CFD method based on constant-speed whirl. The comparisons show that the new transient method can keep the good accuracy of traditional transient method and meantime largely save the computational time.

Experimental study on hydrodynamic performance of a cavitating centrifugal pump during transient operation

Dazhuan Wu,Leqin Wang,Zongrui Hao,Zhifeng Li,Zhiren Bao 대한기계학회 2010 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.24 No.2

An experimental study has been carried out in order to analyze the cavitation of a centrifugal pump and its effect on transient hydrodynamic performance during transient operation. The transient characteristics of the centrifugal pump were tested under various suction pressure and starting conditions. In transient operation of continuous starting and stopping process, instantaneous rotational speed, head,flow rate and suction pressure of the pump were measured. The effect of cavitation on transient performance of the centrifugal pump during transient operation was analyzed, and then the effects of starting acceleration rate and suction pressure of pump on cavitation were presented. Results showed that the cavitation would be delayed during rapid starting period. However, in the condition of low suction pressure and high rotational speed, pump cavitation is inescapable even if the starting period is less than a second. After the serious transient cavitation occurred, the transient performance of centrifugal pump would decline obviously, and the instantaneous head of pump would fluctuate.

Experimental and numerical study of transient flow in a centrifugal pump during startup

Zhifeng Li,Peng Wu,Dazhuan Wu,Leqin Wang 대한기계학회 2011 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.25 No.3

Transient characteristics and flows in a centrifugal pump during its starting period were experimentally and numerically investigated. The two-dimensional particle image velocimetry technique was used to capture transient flow evolutions in the pump’s diffuser. A new dynamic slip region method that combines the dynamic mesh method with the non-conformal grid boundaries is proposed to resolve the transient flows caused by the started impeller. Numerical self-coupling was realized by establishing a circulation pipe system along with the pump model equivalent to the experimental pump system. Numerical and experimental results agree well in both explicit characteristics and internal transient flow structures, confirming the validity of the proposed method. Analysis of the instantaneous flow in the impellers indicates that for the early stage of the startup, the transient vortex evolution between blades is the main reason for the transient head coefficient being lower than the steady state value. The reversed flow at the blade inlet is a less important reason for this effect. In later stages, the weakening of the intensity of the spatial vortex visible on S_2m and the main flow stream are the main reasons for the explicit performance slowly rebounding to the steady value.

Fluid-structure interaction analysis of annular seals and rotor systems in multi-stage pumps

Qinglei Jiang,Lulu Zhai,Leqin Wang,Dazhuan Wu 대한기계학회 2013 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.27 No.7

Annular seals play an important role in determining the vibrational behavior of rotors in multi-stage pumps. To determine the critical speeds and unbalanced responses of rotor systems which consider annular seals, a fluid-structure interaction (FSI) method was developed,and the numerical method was verified by experiments conducted on a model rotor. In a typical FSI process, rotor systems are modeled based on a node-element method, and the motion equations are expressed in a type of matrix. To consider the influence of annular seals,dynamic coefficients of annular seals were introduced into the motion equations through matrix transformation. The test results of the model rotor showed good agreement with the calculated results. Based on the FSI method proposed here, the governing equations of annular seals were solved in two different ways. The results showed that the Childs method is more accurate in predicting a rotor’s critical speed. The critical speeds of the model rotor were calculated at different clearance sizes and length/diameter ratios. Tilting coefficients of long seals were added to the dynamic coefficients to consider the influence of tilting. The critical speeds reached their maximum value when the L/D ratio was around 1.25, and tilting enhanced the rotor’s stability when long annular seals were located in either end of the shaft.

Investigation of the impact of splitter blades on a low specific speed pump for fluid-induced vibration

Guitao Zeng,Qianqian Li,Peng Wu,Bo Qian,Bin Huang,Shiyang Li,Dazhuan Wu 대한기계학회 2020 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.34 No.7

Low specific speed centrifugal pumps are widely applied in the chemical industry and agricultural irrigation fields. The vibration characteristics are of great importance for centrifugal pumps. Apart from rotor installation, fluid-induced vibrations caused by internal flow also play an important role in the vibration performance, and special attention should be paid to low specific speed centrifugal pumps for long and narrow meridian flow channels where secondary flows are easy to generate. In this paper, splitter blades were studied for decreasing the pressure and radial force fluctuations of a low specific speed centrifugal pump at the design point, both numerically and experimentally. Numerical simulations based on the Reynolds averaged Navier–Stokes (RANS) turbulent model were performed for fluid-field analysis. The steady state and unsteady simulations were computed based on ANSYS Fluent. Here, the circumferential location, leading edge location, and the deflection angle of the splitter blades were considered to suppress the secondary flow and to reduce vibration. The experiments show the effectiveness of splitter blades for decreasing fluid-induced vibrations.

Effect of meridional shape on performance of axial-flow fan

Xin Chen,Linlin Cao,Peng Yan,Peng Wu,Dazhuan Wu 대한기계학회 2017 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.31 No.11

In this study, the effect of impeller meridional shape on the performance of axial-flow fan is investigated by CFD method. Three axialflow fan impellers with different meridional shapes are designed. The blade angle, blade stacking condition and other structure factors of the impellers are all remained consistent. The performance curves of the three impellers are calculated and compared. In almost all the interested flowrate range, the impeller W3 with an inverted-isosceles-trapezoid meridional shape and the longer blade camber achieves both the higher pressure rise and the higher efficiency than the other two impellers. A two-stage axial-flow fan designed on basis of W3 is manufactured and tested. Test results show good agreement with the calculated performance curves. Further, analyses of the CFD results are conducted to reveal the reasons for the different performance. A newly-defined Local Euler head (LEH) is introduced to represent the distribution of the major Euler work in the axial-flow fan. And the LEH distributions in the three impellers are obtained. W3 achieves the highest LEH at blade Trailing edge (TE), because it could perform the most Euler work to the fluid with the longest blade camber. Then losses in the impellers are analyzed by means of the entropy generation. Among the losses in impeller, the tip leakage loss and endwall friction loss are dominated at design flowrate. The generation condition of the tip leakage loss shows significant differences among the three impellers. And the whole power loss in impeller of W3 is slightly higher than those of the other two models. However, the power loss difference among the three impellers is negligible. And due to the highest shaft power, the efficiency loss of W3 is the lowest of all.

Effect of stacking conditions on performance of a centrifugal pump

Peng Yan,Shiyang Li,Shuai Yang,Peng Wu,Dazhuan Wu 대한기계학회 2017 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.31 No.2

In this study, the effect of stacking condition of centrifugal pump impeller was investigated by Computational fluid dynamics (CFD). The performance curves of impellers with three different stacking conditions were compared. The Local Euler head distributions (LEH distribution) at blade Leading edge (LE) and Trailing edge (TE) were obtained to evaluate the contribution of different spanwise location to the total work. The positive stacking characterised by the hub leading the shroud was considered inappropriate in centrifugal pump impellers as it deteriorated the performance of the pump in whole flow rate range. The negative stacking characterised by the shroud leading the hub could improve flow stability and efficiency in low flow rates, it was manufactured and tested. The test results showed good agreement with CFD calculations. A hump zone was observed in zero stacking impeller between 0.9Q d and Q d . It was closely related to the drastic change in pattern of LEH distribution at the two flow rates. The internal flow was analyzed to reveal the reasons responsible for the hump in head curve. It is found that the tremendous extension of low streamwise velocity region and its instant movement from the corner of shroud and blade suction surface to the midspan near blade suction surface when the flow rate decreases from Q dto 0.9Q d is the main reason for flow instability in zero stacking impeller.

Numerical study on transient hydraulic excitation force characteristics of a water jet mixed-flow pump during rapid startup period

Guitao Zeng,Weibin Chen,Juanhong Li,Peng Wu,Bin Huang,Dazhuan Wu 대한기계학회 2023 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.37 No.10

Mixed-flow pumps have been widely used in water jet propulsion field. To ensure reliability and operation efficiency, it is necessary to investigate the characteristics of hydraulic excitation force during transient operations. In this paper, the radial force of a water jet mixed-flow pump during rapid startup period was investigated numerically based on improved delayed detached eddy simulation (IDDES) and siding mesh method, the external characteristics were verified by test results. The spatial-temporal evolution of radial force was analyzed. The radial force during rapid startup presents transient and delay phenomenon, which shows disparity compared with quasi-steady state. The flow analysis and blade strip analysis show inlet recirculation significantly affects the radial force’s evolution, the radial force is quite different from blade root to tip and the blade tip contributes most to the radial force delay phenomenon. By applying boundary vorticity flux (BVF) theory and separation vortex formation diagnosis, separation regions caused by the dissipation of inlet recirculation were effectively identified and the change trend is in line with the change of radial force, which explains how inlet recirculation results the delay of radial force on blade tip strip and the whole impeller blade.

Effect of obstacle position on attached cavitation control through response surface methodology

Bangxiang Che,Linlin Cao,Ning Chu,Dmitriy Likhachev,Dazhuan Wu 대한기계학회 2019 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.33 No.9

The span-wise obstacle on the suction surface of a hydrofoil has been verified to be an effective passive control method for cloud cavitation. The position of obstacle significantly influences the performance of cavitation control. In this research, we investigated the effect of obstacle position on attached cavitation control on the suction surface of a NACA0015 hydrofoil through response surface methodology. The cavitation types covered from sheet cavitation to partial and transitional cavity oscillations. We derived regression equations and built response surfaces to illustrate the quantitative relationship between individual factors (obstacle position, cavitation number, and angle of attack) and cavitation dynamic response parameters (cavity length, acoustic intensity, and energy flux). Sheet cavitation was effectively suppressed because the obstacle increased the pressure at the near-wall region. However, the obstacle would induce a shear cavitation when its position was too close to the leading edge of the hydrofoil. Under partial cavity oscillation conditions, the obstacle was consistently performed well in cloud cavitation control. The cavitation dynamic response parameters significantly decreased. Under transitional cavity oscillation conditions, the obstacle cannot suppress the cavitation because the transitional cavity oscillation was likely a system-inherent instability. This research is beneficial for a comprehensive understanding of cavitation control mechanism using an obstacle and for further industrial application of obstacle in hydraulic machinery to control cavitation.