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.

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.

Low‐complexity de‐mapping algorithms for 64‐APSK signals

Junwei Bao,Dazhuan Xu,Xiaofei Zhang,Hao Luo 한국전자통신연구원 2019 ETRI Journal Vol.41 No.3

Due to its high spectrum efficiency, 64‐amplitude phase‐shift keying (64‐APSK) is one of the primary technologies used in deep space communications and digital video broadcasting through satellite‐second generation. However, 64‐APSK suffers from considerable computational complexity because of the de‐mapping method that it employs. In this study, a low‐complexity de‐mapping method for (4 + 12 + 20 + 28) 64‐APSK is proposed in which we take full advantage of the symmetric characteristics of each symbol mapping. Moreover, we map the detected symbol to the first quadrant and then divide the region in this first quadrant into several partitions to simplify the formula. Theoretical analysis shows that the proposed method requires no operation of exponents and logarithms and involves only multiplication, addition, subtraction, and judgment. Simulation results validate that the time consumption is dramatically decreased with limited degradation of bit error rate performance.

Design and Optimization for Distributed Compress-and-Forward System based on Multi-Relay Network

( Junwei Bao ),( Dazhuan Xu ),( Hao Luo ),( Ruidan Zhang ) 한국인터넷정보학회 2019 KSII Transactions on Internet and Information Syst Vol.13 No.6

A novel distributed compress-and-forward (CF) system based on multi-relay network is presented. In this system, as the direct link between the source and destination is invalid due to some reasons, such as the limited power, special working environment, or even economic factors, relays are employed to receive analog signals and carry on distributed compressed encoding. Subsequently, the digital signals are transmitted to the destination via wireless channel. Moreover, a theoretical analysis for the system is provided by utilizing the Chief Executive Officer (CEO) theory and Shannon channel capacity theory, and the rate-distortion function as well as the connection between the transmission rate and the channel capacity are constructed. In addition, an optimal signal-to-noise ratio (SNR) -based power allocation method is proposed to maximize the quantization SNR under the limited total power. Simulation result shows that the proposed CF system outperforms the amplify-and-forward (AF) system versus the SNR performance.

Cross-Layer Resource Allocation Scheme for WLANs with Multipacket Reception

Lei Xu,Dazhuan Xu,Xiaofei Zhang,Shufang Xu 한국전자통신연구원 2011 ETRI Journal Vol.33 No.2

Tailored for wireless local area networks, the present paper proposes a cross-layer resource allocation scheme for multiple-input multiple-output orthogonal frequency-division multiplexing systems. Our cross-layer resource allocation scheme consists of three stages. Firstly, the condition of sharing the subchannel by more than one user is studied. Secondly, the subchannel allocation policy which depends on the data packets’ lengths and the admissible combination of users per subchannel is proposed. Finally, the bits and corresponding power are allocated to users based on a greedy algorithm and the data packets’ lengths. The analysis and simulation results demonstrate that our proposed scheme not only achieves significant improvement in system throughput and average packet delay compared with conventional schemes but also has low computational complexity.

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.

Stator modification methods for diagonal flow fans to achieve noise reduction of rotor-stator interaction

Jieting Han,Yu Zhang,Shiyang Li,Bin Huang,Dazhuan Wu 대한기계학회 2022 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.36 No.2

The effect of three noise reduction methods for rotor-stator interaction in diagonal flow fans was studied. The unsteady flow field and directivity of tonal noise were obtained by numerical simulation based on computational fluid dynamics (CFD) and the Ffowcs Williams and Hawkings (FW-H) method; the baseline model was tested in a semi-anechoic chamber. The generation mechanism and location of noise were investigated. In addition, the characteristics of tonal noise at blade-passing frequency (BPF) were studied, along with its harmonics. The results show that the tonal noise at 1 BPF dominates the overall sound pressure level. Proper rotor-stator spacing, a large forward-leaned angle, and a large backward-swept angle are beneficial to reduce tonal noise. Also, the noise reduction effect is related to the low amplitude and to the phase shift caused by the pressure response of the rotor wake impinging on stator surfaces, especially near the leading edge at 1 BPF.

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.