Application of the semi-analytical cavitation model to flows in a centrifugal pump

Visualization experiments are carried out to investigate the cavitating flows in a centrifugal pump at different flow rates. The cavity lengths under different pump inlet pressure are obtained. The semi-analytical cavitation model is improved since it is not suitable for predicting large cavities full of vapor. The improved cavitation model is used to numerically study the steady and rotating cavitation in the centrifugal pump. Compared with the Zwart cavitation model, the numerical results predicted by the semi-analytical model agree much better with the experiments, especially for large cavities. The cavity lengths at the suction side are overestimated during the simulations, especially by using the Zwart model. At low flow rates, the prediction of the rotating cavitation effect is weaker and the cloud shedding frequency is smaller than those in the experiments.     

Numerical simulation of flow in centrifugal pump with complex impeller

Based on the Navier-Stokes equations and the Spalart-Allmaras turbulence model, three dimensional turbulent flow fields in centrifugal pump with long-mid-short blade complex impeller are calculated and analyzed numerically. The relative velocity and pressure distributions in the flowpart are obtained. It is found that the flow in the passage of the complex impeller is unsymmetrical due to the joint action between volute and impeller. The back-flow region is at inlet of long-blade suction side, near middle part of long-blade pressure side and outlet of short-blade suction side. The flow near volute throat is affected greatly by volute. The relative velocity is large and it is easy to bring back flow at outlet of the complex impeller near volute throat. The static and total pressure rise uniformly from inlet to outlet in the impeller. At impeller outlet, the pressure periodically decreases from pressure side to suction side, and then the static pressure sharply rise near the throat. The experimental results show that the back flow in the impeller has an important influence on the performance of pump.     

叶片厚度分布对两级离心泵空化性能的影响

针对叶片厚度分布对两级离心泵空化性能的影响,通过对第一级叶轮叶片的厚度分布进行优化,研究其对泵外特性、压力脉动以及流体激励力的影响。结果表明：采用线性厚度分布的对称叶片时,前缘厚度对空化性能的影响最大,叶片越薄,泵的抗空化能力越强;随着尾缘变厚,压力脉动幅值向叶轮进口方向偏移,且随着叶片厚度减小,压力脉动幅值相应减小。第一级叶轮流道内的压力脉动主频为干涉叶频,保留高幅低频的宽频特征,所受空间三向力的脉动主频为2倍叶频,轴向力较小,脉动幅值较低,径向力的主要信号成分是叶频和2倍叶频。     

Numerical investigation of influence of the clocking effect on the unsteady pressure fluctuations and radial forces in the centrifugal pump with vaned diffuser

The relative position between the diffuser vane and the volute tongue (clocking effect) has a great effect on the performance of the single-stage centrifugal pump, which however, is often ignored by designers. In this paper, the influence of clocking effect on the unsteady pressure fluctuation in a centrifugal pump and on the radial force of impeller is investigated. The hydrodynamic performance of the centrifugal pump with vaned diffuser was experimentally measured. Numerical simulation based on the Reynolds-averaged Navier–Stokes (RANS) combined with the SST k-w turbulence model was used to obtain hydrodynamic performance of the centrifugal pump. The numerical results of the hydrodynamic performance were in agreement with the experimental data. The results show that the clocking effect has great influence on the pressure fluctuation and on the unsteady radial force imposed on the impeller. When the diffuser vane approaches the volute tongue, the pressure fluctuation intensity in volute is relatively lower. Meanwhile, relatively larger radial force on the impeller and the lower efficiency are obtained when the diffuser vane is near the volute tongue. Thus, it is suggested that the volute tongue should be located near the middle of two diffuser vanes to obtain better performance.     

Numerical investigation of pressure distribution in a low specific speed centrifugal pump

To study the pressure distribution of the volute casing, front casing and back casing in a prototype centrifugal pump, the pressure experiments and numerical simulations are carried out at six working conditions in this paper. The experimental results shows that the asymmetry of static pressure distribution on volute casing and front cavity is caused by the tongue of the volute and it may result in high radial and axial resultant force which can cause vibration and noise in the centrifugal pump. With the increasing of flow rate, the asymmetry of static pressure distribution and the magnitude of static pressure values reduce. The numerical results indicate that the pressure fluctuation near the tongue is strongest and it becomes slighter at point away from the tongue. With the increasing of flow rate, the local high-pressure region in impeller passage reduces and the flow becomes smoother accordingly, whereas the fluid speed becomes much higher which may cause further flow losses. The results predicted by numerical simulation are in coincident with the experimental ones. It shows that the turbulence model for simulating the flow field in centrifugal pumps is feasible.     

Experimental study on a centrifugal pump with an open impeller during startup period

Transient performance of fluid machinery during transient operating periods,such as startup and stopping,has been drawn more attentions recently due to the growing engineering needs.In this paper,the transient behavior of a prototype centrifugal pump with an open impeller during rapid startup period is studied experimentally.The variations of the rotational speed,flow rate,head,and shaft power during rapid startup period are recorded in experiments at different discharge valve openings.In addition,the non-dimensional flow rate and head are also used to analyze the transient behavior.The research result shows that the rise characteristic of the rotational speed is not basically changed by working points,while mainly depends on the startup characteristics of the driving motor.Compared with the rapid rise of the rotational speed,the flow rate rises slowly in the initial stage of startup.Moreover,the flow rate lags behind the rotational speed to rise to final stable value,and the delay becomes more severe with the increase of the discharge valve opening.The shaft power impact phenomenon generally exists in the process of startup.The non-dimensional analysis shows that the non-dimensional head is very high at the very beginning of startup,and quickly falls to the minimum,then gradually rises to final stable value,while the non-dimensional flow rate always shows the rise tendency during whole startup period.In conclusion,it is found from the non-dimensional results that the quasi-steady analysis is unable to accurately assess the transient flow during startup period.     

Numerical analysis of flow in ultra micro centrifugal compressor -influence of meridional configuration

A single stage ultra micro centrifugal compressor constituting ultra micro gas turbine is required to operate at high rotational speed in order to achieve the pressure ratio which establishes the gas turbine cycle. As a consequence, the aerodynamic losses can be increased by the interaction of a shock wave with the boundary layer on the blade surface. Moreover, the centrifugal force which exceeds the allowable stress of the impeller material can act on the root of blades. On the other hand, the restrictions of processing technology for the downsizing of impeller not only relatively enlarge the size of tip clearance but also make it difficult to shape the impeller with the three-dimensional blade. Therefore, it is important to establish the design technology for the impeller with the two-dimensional blade which possesses the sufficient aerodynamic performance and enough strength to bear the centrifugal force caused by the high rotational speed. In this study, the flow in two types of impeller with the two-dimensional blade which have different meridional configuration was analyzed numerically. The computed results clarified the influence of the meridional configuration on the loss generations in the impeller passage.     

Unsteady pressure measurements in a high-speed centrifugal compressor

This paper presents the unsteady data acquisition system used to measure the pressure field in high speed compressors. Details and electronic sketches are given for the conditioners developed in-house that have been used to amplify and to filter the pressure signal with the aim of acquiring data up to 150 kHz. A discussion of the experimental results carried out in a centrifugal compressor is proposed. Through different processing of the pressure signals and a comparison with URANS simulations, the excitation of the pressure transducers by the pressure waves generated by shock waves that occur between the impeller and the diffuser is highlighted. The levels of pressure fluctuations measured when entering into surge are also presented and reveal very repetitive behaviour of the flow instabilities.     

Numerical investigation on influence of diffuser vane height of centrifugal pump

Vaned diffusers are extensively used in centrifugal pumps, but the influence of vane height on internal flow field and overall performance is not explicit. This paper mainly presents numerical investigation on influence mechanism of diffuser vane height in a single-stage centrifugal pump. The head values were carried out on a low specific speed centrifugal pump equipped with different diffuser vane height by numerical simulation and experimental method. And the deviation between numerical results and experimental results were < 5%. The diffuser vane height h/b ratio is changed as 0, 0.3, 0.4, 0.5, 0.6, 0.8, and 1 in this study. The numerical analysis shows that reducing diffuser vane height could eliminate the vortex which appears at tongue region. Meanwhile, the influence of rotor-stator interaction was reduced by reducing the vane height. Consequently, the energy loss in the volute and the diffuser could both be decreased at design flow point and over flow point. In the other hand, the circumferential velocity at partial flow point gets larger which could lead to large frictional loss. In general, reducing the diffuser vane height at design and over flow point could improve the output work of impeller.     

Investigation of surge behavior in a micro centrifugal compressor

This paper reports the experimental and theoretical study of the surge occurred in prototyping an ultra micro centrifugal compressor. As the first step, the 10 times size model of an ultra micro centrifugal compressor having the 40 mm outer diameter was designed and manufactured. The detailed experimental investigations for the transient behavior of surge with several different values of B parameter were carried out. The experimental results during the surge were compared with those obtained by the non-linear lumped parameter theory in order to validate the effectiveness of the theoretical surge model for the micro centrifugal compressor. As a result, the quite different behavior of the surge appeared for the different values of B both in the experiment and in the analysis.     

Numerical and experimental study of the self‐priming process of a multistage self‐priming centrifugal pump

Pumps are a type of general machine with many varieties and extensive application. To simulate really the self‐priming process of multistage self‐priming centrifugal pump, the numerical calculation of gas‐water two‐phase flow on a four‐stage self‐priming pump was performed based on ANSYS CFX software. Moreover, a transparent plastic tube was installed at the pump outlet, and the photographic technology was used to observe the appearance of gas‐water escape during the self‐priming process of multistage self‐priming centrifugal pump. The experimental results were compared with the numerical results. It is found that the whole self‐priming process of self‐priming pump can be divided into three stages: the initial self‐priming stage, the middle self‐priming stage, and the final self‐priming stage. Moreover, the self‐priming time of the initial and final self‐priming stages accounts for a small percentage of the whole self‐priming process, while the middle self‐priming stage is the main stage in the self‐priming process, which determines the length of self‐priming time. The experimental results are very close to the numerical results in the initial and middle self‐priming stages.     

Computational cavitation flows at inception and light stages on an axial-flow pump blade and in a cage-guided control valve

Cavitation flows induced around an axial-flow pump blade and inside a high pressure cage-type valve are simu-lated by a two-dimensional unsteady Navier-Stokes analysis with the simplest treatment of bubble dynamics.Thefluid is assumed as a continuum of homogeneous dispersed mixture of water and vapor nuclei.The analysis isaimed to capture transient stages with high amplitude pressure change during the birth and collapse of the bubbleespecially at the stage of cavitation inception.By the pump blade analysis,in which the field pressure is moderate,cavitation number of the inception and locations of developed cavitation are found to agree with experimental re-suits in a wide flow range between high incidence and negative incidence.In the valve flow analysis,in which thewater pressure of 5MPa is reduced to 2MPa,pressure change responding to the bubble collapse between the vaporpressure lower than 1 KPa and the extreme pressure of higher than 10~4 KPa is captured through a stable computa-tion.Location of the inception bubble and pressure force to the valve plug is found agree well with the respectiveexperimental features.     

离心泵叶轮内流场数值模拟及分析

基于计算流体力学,对离心泵叶轮内部流场进行了数值模拟。采用有限体积中心格式、时间推进解法以及N-S方程计算。得到了叶轮压力分布等值线、叶轮相对速度矢量以及二次流分布,并进行详尽的分析,揭示了叶轮内部流动情况,为优化叶轮设计提供了理论依据和技术保障。     

Cavitation in semi-open centrifugal impellers for a miniature pump

Cavitation in miniature pumps was investigated experimentally for two semi-open centrifugal impellers. Although both impellers had the same blade cross-section, one impeller had a two-dimensional blade, while the other had a leaned blade. The flows were also analyzed using a numerical model of the three-dimensional turbulent flow in the pumps near the peak efficiency point using the k-ɛ turbulence model and the VOF cavitation model. The average cavitation performance of each impeller was satisfactorily predicted by the numerical simulations. The results show that the miniature pumps have similar cavitation performances as an ordinary-size pump, with the cavitation performance of the semi-open impeller reduced by increased axial tip clearances. Also, both the hydraulic and cavitation performance of the semi-open impeller were improved by the leaned blade. The results also show that uniform flow upstream of the impeller inlet will improve the cavitation performance of a miniature pump. __________ Translated from J Tsinghua Univ (Sci & Tech), 2006, 46(8): 1451–1454 [译自: 清华大学学报(自然科学版)]     

Cavitation in semi-open centrifugal impellers for a miniature pump

Cavitation in miniature pumps was investigated experimentally for two semi-open centrifugal impellers. Although both impellers had the same blade cross-section, one impeller had a two-dimensional blade, while the other had a leaned blade. The flows were also analyzed using a numerical model of the three-dimensional turbulent flow in the pumps near the peak efficiency point using the k-? turbulence model and the VOF cavitation model. The average cavitation performance of each impeller was satisfactorily predicted by the numerical simulations. The results show that the miniature pumps have similar cavitation performances as an ordinary-size pump, with the cavitation performance of the semi-open impeller reduced by increased axial tip clearances. Also, both the hydraulic and cavitation performance of the semi-open impeller were improved by the leaned blade. The results also show that uniform flow upstream of the impeller inlet will improve the cavitation performance of a miniature pump.     

Dynamical System Analysis of Unsteady Phenomena inCentrifugal Compressor

INTRODUCTI0NUnsteadyphenomenaappearedinacentrifugalcompressorsystematlowflowratearecharacterizedbysurgeandrotatingstall.Thesurgeisknowntocauseoscillationswithlargeamplitudeofthefiowrateandthepressurerisethrough0utacompressorsys-tem.Ontheotherhand,ther0tatingstallcauseslocalflowoscillationsandappearsinsomepartsofacompressor,suchastheinducer,theimpellerandthediffuser-Sincetheydeterioratethecompressorperfor-manceands0metimesleadtothesystembreakd0wn,itisimportanttoknowthephysicalmechanismsofth…     

Conjugated heat transfer in unsteady channel flows

The exact analytical solution of the unsteady impulsive Thermo-Fluid Dynamic field arising in a two-dimensional channel with thick solid walls is presented when the thermal field in the fluid is coupled with the thermal field in the solid (conjugated heat transfer). The cases studied in this paper depend on the boundary conditions imposed on the unwetted sides of the channel walls: assigned temperature and adiabatic condition. Moreover the case of a given heat loss at the unwetted wall is also considered in an appendix. The temperature and heat flux at the solid–fluid interface are analyzed as function of time and of the nondimensional parameters governing the problem.     

Numerical study on the cavitating flow in liquid hydrogen through elbow pipes with a simplified cavitation model

Cavitation is usually caused by the pressure difference between the static pressure and the saturated vapor pressure under the local temperature and may result in huge damage to the pipelines. This paper developed a simplified cavitation model based on Rayleigh–Plesset bubble equation and Zwart cavitation model, and conducted a series of numerical simulations with the process of phase change and latent heat added to the solver by UDFs (User Defined Functions). The aim of the paper is to study the affecting factors on the cavitation process of liquid hydrogen in elbow pipes. The results show that the thermal effect can suppress the occurrence and development of cavitation. As the process of cavitation goes on, the suppression of thermal effect is more remarkable. Before the cavitating flow reaches its steady state, cavitation process is very sensitive to the changes of inlet velocity and outlet pressure. Increasing the inlet velocity or decreasing the outlet pressure can both strengthen the cavitation process. The turbulent viscosity ratio has little effects on cavitation process of liquid hydrogen, but the increase of turbulent viscosity ratio can enhance the thermal effect and lower the temperature gradient in the cavity. In addition, the structure of the cavity is found to be related to the bend angles. The cavitation process is enhanced with the decrease of the angles since the duration of centrifugal force is longer.     

Modeling centrifugal,multiphase, turbulent flows with a mixture-averaged drift-flux algorithm

Abstract

A two-dimensional, axisymmetric, mixture-averaged multiphase flow model, developed to predict solid phase stratification arising from flow acceleration, has been generalized for three dimensions. The model is exercised, with results compared to a multiphase model having separate conservation equations for the gas phase and the solid phase, and momentum exchange between phases handled by source terms. Both models are exercised with a constant particle-gas drag coefficient, characteristic of turbulent flow, and a variable, Reynolds-based coefficient. Modeling results are compared against video data acquired at the Applied Research Laboratory at Penn State University. Qualitative comparisons data show comparable results using both models.