In order to clarify the relationship between different blade numbers and radial exciting force of the rotary lobe pump rotor, five rotary lobe pumps with different blade numbers were researched based on the dynamic mesh techniques using RNG k-ε turbulence model and PISO algorithm. The internal flow characteristics in the pump are compared and the influence mechanism of the different blade numbers on the transient flow structure and the exciting force inside the rotor cavity are revealed. Meanwhile, through numerical simulation and experimental verification, the comparative analysis of numerical prediction shows that the relative error is less than 5.6%, and the numerical simulation has higher accuracy. The results show that the number of blade has a significant effect on the flow characteristics and radial exciting force distribution of rotor cavity of the rotary lobe pump. With the increase of the number of blades, the average flow rate and the flow pulsation amplitude of the flow rate at the outlet of the pump are obviously decreased, which effectively inhibits the secondary flow, vortex and the velocity abrupt change between the rotor gaps region of the rotor cavity. The maximum radial exciting force at 6 blade numbers is less 36% than that at 4 blade numbers. A multistage series transition cavity is formed between the high-pressure cavity and the low-pressure cavity when the blades number of 5 and 6 which can effectively reduce the pressure difference between the two sides of the radial gaps. It can reduce the radial leakage, and effectively suppress the instantaneous pressure relief effect caused by the instantaneous opening of the gaps.

View full abstract

This study numerically investigates the flow characteristics of a multi-blade centrifugal fan in order to identify the process of low-frequency aerodynamic noise generation. Computational analysis has been carried out using the large eddy simulation (LES) model. In the preliminary study, it was confirmed that the present computational fluid dynamics simulation using LES shows good agreement with the experimental results of the performance curve, sound pressure spectra, and the flow field obtained by using a time-resolved particle image velocimetry (PIV) system. Consequently, it was found that the appearance of an unexpected significant flow, which recirculated from the scroll casing, passed through the impeller, and returned to the scroll casing again to exit the scroll casing in a specific region, was similar to the flow suggested in the experimental results, which were obtained utilizing PIV measurement. The fluid flow related to the fluctuation, which was remarkably not included in the impeller main flow, existed beside the outer main flow. A certain amount of fluctuated flow bifurcated near the scroll end, one regurgitating into the impeller, and another colliding with the tongue. The former became a driving source of noise and the latter a direct noise-generating factor. Consequently, it was demonstrated that a significant fluctuation owing to flow collision with the tongue propagated through the impeller and formed a loop.

View full abstract

Over the last years, several breakdowns in hydropower plants with low specific speed Francis runners have been reported. One of the main excitation forces in such runners is the pressure fluctuations originating from the rotor stator interaction. In this paper, the rotor-stator interaction has been analyzed utilizing pressure sensors onboard the runner. The pressure sensors were flush mounted in the hub of the runner and the signals were transmitted through a slip-ring system. The measurements have been analyzed relative to the runner angular position by utilizing an angular position sensor mounted to the shaft end. Measurements with different guide vane angle have been compared in order to study the potential flow interaction and the viscous wake effects for the pressure inside the runner. The results from the onboard pressure measurements found that the phase of the guide vane passing pressure seen by the onboard pressure sensors was independent of the guide vane opening. Hence, the potential flow interaction was found to be the dominant effect and no evidence from the viscous wake effect was found on the onboard pressure.

View full abstract

From the aspect of the applicability of a code for compressor surge analysis to practical industrial plants utilizing axial flow compressors, a simulation code developed by the author was applied to a rather complicated system, in which a nine-stage axial flow compressor was operated in a closed-loop duct including an air cooler (heat exchanger) and a long side-branch line having a large volume, and other duct elements. The results showed reasonable agreements of macroscopic values such as surge frequencies, etc. in comparison with a few experimental data in the situation. The results also described reasonably the detailed behaviors of flows in both of the compressor and the duct. It could be said that the application of the code to practical situations is possible with a fair degree of accuracy, in the environment of sufficient approximations to the compressor and the flowpath configurations and preparations of suitable data for the simulations. At the same time, several requirements have been found for future improvements.

View full abstract

In this paper, numerical investigations are performed to analyze the flow behavior over NACA0015 and the efficiency of vortex generators VGs (triangular and rectangular in shape) and continuous jet as passive and active control devices are evaluated. The unsteady Reynolds-averaged Navier-Stokes equations of the turbulent flow are solved using, k-ω SST provided by the ANSYS CFX-CFD code. The model presented in this paper is a comprehensive representation of the information available in the literature. Validation shows a qualitatively good agreement with previous researches, and almost all flow structures are well reproduced by the computation. The results confirm that the vortex generators can influence flow separation and thus improve the aerodynamic performance of NACA0015 airfoil. The results show that the triangular VGs that include jet are more efficient than the triangular and rectangular VGs.

View full abstract

Numerical analysis was carried out to investigate the influence of the setting angle and chord length on the pressure distribution especially for the shroud span of an axial fan. The setting angle was referred to as AOA (angle of attack). The aerodynamic performance of an axial fan with the change of setting angle and chord length was presented, and the unstable pressure distribution was discussed in detail. The airfoil of an axial fan was based on the NACA 3512. The influence of the setting angle was observed with the designed sets which were rotated based on the center of gravity for the blade. The chord length was adjusted while maintaining the setting angle. For each design parameter, 5 sets were designed to conduct the single-factor analysis, respectively. The setting angle had a sensitive effect on the aerodynamic performance of an axial fan. The pressure distribution became unstable related to the setting angle of the shroud span. On the other hand, the chord length was only proportional to the pressure rise and had a little effect on efficiency.

View full abstract

This paper investigates a novel casing treatment method, called recirculation-bleeding channels, which combine a recirculation channel with additional bleeding channels. The system consists of 36 channels distributed around the blades and located on rotor shroud surface of a single-stage transonic axial compressor. This study focuses on its effects on aerodynamic performance of a single-stage transonic axial compressor, NASA Stage 37. Validation of numerical model of NASA Stage 37 was performed using experimental data for the single-stage transonic axial compressor. A common drawback of flow recirculation and air bleeding is the reduction in efficiency; however, numerical results showed that with the presence of recirculation-bleeding channels, both stall margin and adiabatic efficiency of the single-stage transonic axial compressor were increased as compared to the smooth casing with small penalty in pressure ratio. A parametric study of the recirculation-bleeding channels was performed for six geometric parameters. With recirculation-bleeding channels, the compressor could reach the stall margin of 13.85% at maximum while still retaining an increase in peak adiabatic efficiency. It is also showed that proper adjustments of the channels design can eliminate the deficiency in pressure ratio at peak efficiency condition.

View full abstract

The dynamic behavior of the internal gear motor and pump is significantly influenced by the stiffness and damping characteristics of the oil lubricating film. This study addresses the stability problem of the internal gear motor and pump. The single-valued analysis is firstly used to predict the stability of internal gear motor and pump under the effect of the oil film coefficients. However, the oil lubricating film characteristics change according to operating conditions. Moreover, the internal ring gear and housing parameters are known with a certain tolerance interval owing to the manufacturing inaccuracy and poor assembly process. Consequently, the geometric parameters of the internal gear motor and pump are given as a range of values rather than single.valued quantities. This leads to significantly changes in dynamic behavior as well as stability of the internal gear motor and pump. Therefore, the interval analysis technique must be used to analyze the stability of the internal gear motor and pump. The numerical simulation is performed to prove the applicability and importance of this method. With this approach, it allows designers to evaluate the stability of internal gear motor and pump as well as analyze the effect of geometric and operating parameters on stability status of the internal gear motor and pump at the early design stage. This is the basic to expand the stable operating area for internal gear motor and pump.

View full abstract

In this paper, Finite Difference Method (FDM) is used to calculate the pressure distribution in lubricated oil film inside the internal gear motor and pump. The effects of geometric and working parameters on the pressure distribution is then analysed. The simulation results point out that the pressure distribution as well the maximal pressure in film thickness greatly depends on the position of oil supporting nuts. Based on the analysis of the pressure distribution, a new concept for oil supporting system for the internal gear motor/pump is proposed. The simulation results point out that the new concept of oil supporting system allows the IGMaP to avoid the negative effect of nut position on pressure distribution even when the internal gear motor and pump operates in 4 quadrants.

View full abstract

Essential features of compressor surge phenomena are found to be described by the following two non-dimensional parameters; a resonance excitation frequency and a surge frequency parameter. The former is the number of times of excitations by the system resonance frequency given to the fluid particle in the time required for the particle to pass through the whole flowpath, or, in short, the resonance frequency multiplied by the passing time. The latter is the number of surge cycles repeated in the particle passing time. When multiplied by corresponding Mach number of the compressor tip speed, both parameters become nearly independent of the compressor speeds. Both parameters construct numerical-experimentally a whole picture of the surge behaviors on the basis of simulation results. It shows the behaviors of surge frequencies and the stall stagnation limits affected by the resonance excitation frequency under the influence of various factors, such as flowpath configurations, relative locations of the compressor in the flowpath, numbers of compressor stages, compressor speeds and operating conditions, pressure ratios, etc. The growth or decay of surge actions and the stagnation occurrences are found to be controlled essentially by the resonance excitation frequency. For less than some specified value of the resonance excitation frequency, stall stagnations will occur. On the other hand, sufficient magnitude of the parameter value will develop deep surges. It could be employed also as a relatively simple and reasonable criterion for the stall-stagnation boundary.

View full abstract

Reaction turbines, of which Francis turbines, constitute a large proportion of low and medium head turbines installed in hydropower plants. Managing these machines represents a real challenge in terms of efficiency, competitiveness and demands on the energy market. Turbines runner blades exhibit loss of performance from damage due to several reasons. One common source of damage is erosion due to the cavitation phenomenon. Indeed, at a given operating region, rapid changes of velocity can create bubbles in the water flow due to local low pressures. When cavitation bubbles reach pressure recovery, they collapse and may induce wear or erosion in these regions. Even if this phenomenon has been intensively studied in the past decades, cavitation erosion is not fully understood as it is driven by several parameters such as flow dynamic, turbine design, environment, or material properties. Some of these parameters can be studied in laboratory to compare materials resistance between each other. This article aims to model the cavitation by a stochastic model using erosion experimental data observed in the laboratory. The benefit of such models is to consider both the uncertainties and natural fluctuations of the phenomenon. With the proposed framework, the study will highlight the differences observed in cavitation erosion experiments of two common materials used to manufacture Francis’s runners. This study is the first step in a project aiming at the prediction of turbines mass loss due to cavitation erosion on actual operating Francis turbines.

View full abstract

Based on computational fluid dynamics (CFD) simulation, this study investigates the effect of leakage flow on the performance of centrifugal compressor, which helps to showcase the flow phenomenon considering the leakage flow and the aerodynamic load on the impellers. We consider four types of compressor models including the normal stage with hub and shroud leakages (SWT), the normal stage with hub leakage (SWH), the normal stage with shroud leakage (SWS), and the normal stage without leakages (SOT)). The results indicate that the leakage flow obviously affects the compressor performance, especially the flow field around the leading/trailing edge of the impeller. The aerodynamic load at the leading/trailing edge of the impeller was obtained for the models through unsteady simulation. The dominant frequency and the pulse amplitude obtained by Fast Fourier transforms revealed that the main frequencies of the aerodynamic load present different distributions under different leakage flows.

View full abstract

This study investigates the flow characteristics of pulp suspensions flowing through a 90° bend, which is used in the production system of an actual paper making machine, e.g., for transporting the pulp suspensions from its stock reservoir to the header. Experiments were conducted on pulp suspensions with a fiber concentration Cs of 0.3 and 0.6 wt% and a bend with a diameter of 22 mm and a curvature radius ratio of 4.0. Flow visualization and measurements of the distributions of time-averaged fiber concentration Ca and axial velocity U were performed with a light section method and the particle image velocimetry (PIV) method, respectively, at representative bulk velocities and at various longitudinal stations. The influence of the flow rate on the changes of their distributions in the streamwise direction was examined. The flow characteristics of the pulp suspension depend on the flow pattern in the upstream straight tube and are greatly different from those of the single-phase water flow. For a low flow rate, the flocculated pulp fibers move without getting disentangled in the bend and the distribution of Ca shows a shape bias towards the inner wall side. As the flow rate is increased, Ca becomes larger in the outer part of the bend, and it changes into a rather flat distribution in the downstream tangent.

View full abstract

Current-meters are well-known water flowrate measuring instruments in industry, and for many years, they have been used to measure flow velocity in open channels and closed conduits. Despite the extent of their application, there is not much information on basic equations of propeller-type current-meters. This paper is dedicated to present a novel design method for propeller-type current-meters. First of all, the basics of current-meters are theoretically investigated through momentum and airfoil theory approaches. Then, the essential design parameters are discussed to shape the design method. The current-meters are divided into three groups by considering their axial pitch values: low-, medium-, and high-pitch; and for each group, the design procedure is demonstrated thoroughly. In addition, the influence of axial pitch value is numerically studied on performance of current-meters via an experimentally validated CFD model. It is shown that low-pitch current-meters are more sensitive to velocity changes, and high pith meters are more reliable in different flow conditions.

View full abstract

To investigate the gas-liquid two-phase flow characteristics in the reversing bucket of water jet propulsion, the numerical simulations based on the Euler-Euler inhomogeneous model was conducted to the internal flow field of the reversing bucket under 1%~12% different inlet gas volume fractions (α). Based on that, the gas distribution, gas-liquid velocity streamlines and thrust changing were obtained to discuss the gas-liquid flow characteristics and the influence on the thrust of the reversing bucket. The calculation results show that the gas mainly distributes in the inner part of the reversing bucket flow passage under different inlet gas volume fractions. On the condition that the inlet gas volume fraction is more than 2%, the gas accumulation has occurred in the inlet section of the flow passage. With the increasing of inlet gas volume fraction, average velocity and pressure increase gradually at each cross-section of reversing bucket flow passage, and at the higher gas concentration in each cross-section, where the vortex will exist, indicating that the gas concentration has a great relationship with the vortex aggregation in the reversing bucket flow passage. When the inlet gas volume fraction is 7%, the thrust reaches the maximum. It shows that the appropriate gas can effectively improve the thrust efficiency of the reversing bucket.

View full abstract

The mathematical model of the change in oil pressure in the piston chamber of the radial piston pump pre-compression area and the CFD geometric model for field simulation were set up considering the compressibility of oil, and the variation curves for pressure were obtained by the two methods. The conclusion obtained from the variation curve and the contours of pressure and velocity vector is that the oil pressure is gradually increased under the action of mechanical compression and flow from the damping groove, and the oil flow to the piston chamber from the outlet port before reaching the discharge oil pressure. It is found that the trend change of the two curves is the same, and the value of the relative error between the CFD model and the mathematical model is large in the middle, small at both ends, and less than 5%. Then two oil pressure gradients obtained from the two methods, respectively, are calculated for the pre-compression area and the curves are plotted. It can be seen that the pressure gradient of the two curves is equal at 10.1 degrees, and the relations are different before and after 10.1 degrees. Finally, the influence of the bottom edge length of the cross section of the triangular groove and the span angle of the triangular groove on the oil pressure in the pre-compression area is analyzed. The law in effect is that the two parameters for oil pressure in the piston chamber are opposed such that the value of the length of the bottom edge and the span angle of triangular groove should be comprehensively considered in order to decrease the impact of the pressure. The value of the length of the bottom edge can not be too large and the value of the span angle can not be too small.

View full abstract

In order to solve the problem of generating distortion elements in the mapping from parameter space to real space, and the boundary coincidence of the mesh generated by the software quality, an approach for parametric surface mesh generation based on Riemannian metric, combined with Delaunay triangulation and AFT is proposed. In our algorithm, the boundary curves are discretized based on the proximity and curvature of the curves in the model after derivation the correlation of curve length between parametric space and real space. Background meshes of parametric space were generated by using improved AFT, and could improve the efficient of the algorithm and control element sizing and metric values. When background mesh of parametric space were refined, to counteract mapping distortion, the traditional Delaunay incremental insertion kernel is replaced by inserting the center of triangle circumscribed ellipse, and the algorithm for locating ellipse center and judging whether nodes within ellipse. In this paper, the details of the surface mesh generated by the algorithm are introduced in detail. The algorithm proposed in this paper has the characteristics of reliable algorithm, high mesh generation efficiency and mesh quality. Finally, the reliability of the proposed algorithm is verified by an example of surface mesh generation.

View full abstract

An essential parameter governing the surge behaviors in compressor systems was found on the basis of numerical-experimental results by surge simulations. It is named “flowpath-average reduced resonance frequency”, consisted of the resonance frequency, the total flowpath length and the average velocity. The reduced frequency normalized by the compressor tip Mach number tends to be basically about 1.5 at the stall stagnation boundaries for compressors having few stages and low pressure-ratios, although the value could vary to some extent affected by various circumstantial factors. It means that the stall stagnations could be caused by insufficient number of times of excitations by the resonant frequency on the fluid particles in passing through the whole flowpath. Deep surges tend to occur for the parameter values greater than around 1.5. On the foundation of the normalized flowpath-average reduced resonance frequency can be formed a basic framework for the surge behaviors in terms of non-dimensional surge frequencies and stall-stagnation points. In reference to the framework, several general features of the surge behaviors are described in a relatively unified manner, including the behaviors of surge frequencies and stall stagnation boundaries, which are affected in a complicated manner by compressor conditions of speeds, pressure ratios, and number of stages, and flowpath geometries, relative compressor locations in the flowpath, etc. The framework contributes much to clarification of the characteristic behaviors in surges hitherto unexplained and could provide keys to further surge studies. It could also be useful to devise countermeasures against stall stagnation problems.

View full abstract

In order to avoid throttling loss and recover differential pressure energy, braking energy and gravitational potential energy without changing the original hydraulic system, the four-port hydraulic transformer (FHT) is proposed. Its theoretical and experimental efficiency characteristics are researched. The basic structure and principle of the FHT are explained. Besides, the mathematical model of its efficiency is established. The results show that the pressure ratio can be changed by adjusting the control angle of valve plate, and the efficiency characteristics of the FHT match the efficiency characteristics of the hydraulic pump/motor. The total efficiency increases first and decreases afterwards with the increment of cylinder speed, and increases with the control angle of valve plate, while decreases with the recycling pressure difference.

View full abstract

Interference in blood flow at bifurcation after stent implantation is one of the important causes of thrombosis. Therefore, it is necessary to research the effects of stent structure and implantation position on hemodynamics. Computational fluid dynamics and experimental comparative research methods were used to analyze the influence of stent structure and position of stent on exit velocity, pressure difference between inlet and outlet, higher and lower wall shear stress areas and contralateral flow field. It is indicated in the research that the effect of iliac vein stent implantation on the flow velocity and pressure distribution at the outlet is not obvious, but stent implantation has an influence on the proportion of low wall shear stress and high wall shear stress area in the iliac vein. The low wall shear stress area is inversely proportional to the height of the stent. After stent is implanted, proportion of the high wall shear stress area will decrease, but the degree of decline is not obvious. As the height of stent is increased, the disturbing effect of stent on contralateral blood flow becomes more pronounced. In addition, it is confirmed by the experiments in vitro and in vivo that stent with crown would effectively reduce the interference to the contralateral flow field and the risk of thrombosis and restenosis. Finite element analysis provides effective theoretical support for studying the influence of iliac vein stent implantation on blood flow field.

View full abstract

In order to study the effect of balance drum clearance on flow distribution in back sidewall gap of last-stage impeller, numerical calculation of a multistage pump was carried out. The accuracy of the numerical calculation method was verified with the external characteristic test. The axial distribution of the radial velocity at the inlet and the bottom of the back sidewall gap at different angles was plotted. The results showed that the radial velocity near the impeller flows radially to the outlet of the impeller, and the radial velocity near the pump shell flows radially to the hub. Influenced by the high-turbulent energy fluid at the outlet of the impeller, there are obvious eddy current movement at the inlet of the back sidewall gap, and the radial velocity increases with the increase of the balance drum clearance. When the flow rate increases from 0.5Q to 1.5Q, the radial velocity decreases gradually at the same angle. The results can provide a reference for the pump design and optimization.

View full abstract

Jet fish pump is an efficient tool to transport fish. Due to the operation principle, shear flow cannot be avoided in jet fish pumps. Shear flow is a potential risk for fish damage in the fluid machinery. Therefore, it is valuable to evaluate the probability of hydrodynamic shear damage for fish passing through jet fish pumps. In the present study, the numerical investigation was performed on the distribution of dangerous region, where exposure strain rate was above 500 s-1, and probability of hydrodynamic shear damage for fish. Experiments were conducted to check the damage probability calculated by numerical simulation and the injury of fish was analyzed. The results showed that the dangerous region is usually distributed in the near wall region. As the rise of flow ratio, the range of dangerous region gradually became wider and the dangerous region extended into the throat. In the same inlet boundary conditions, the damage probability is lower for a jet fish pump with a bigger area ratio. With the increase of flow ratio, both probability of hydrodynamic shear damage and effect of area ratio grow. Compared with the results of experiments, the probability of hydrodynamic shear damage can be overestimated. The main reasons lie in the extremely short transit time for fish, independent movement of fish and the velocity difference between fish and flow.

View full abstract

Various types of centrifugal impellers have been proposed to achieve low flow rates and high heads. To improve the performance and suppress an upward-sloping unstable characteristic in the head curve, this paper suggested a new impeller in which radial and circumferential flow passages are regularly and geometrically arranged, and a one-dimensional performance prediction method is also proposed. As a result, the impeller suppressed an upward-sloping unstable characteristic in the head curve, and the best efficiency was found to be about 40% for a flow rate of 0.061 m3/min. Furthermore, the results clearly indicate that a one-dimensional performance-prediction method can easily calculate the total loss of the impeller from the best-efficiency-point flow rate to the excess-flow rate.

View full abstract

The hydrodynamic characteristics of pump annular seals are closely related to the internal flow characteristics and the pump operation stability. Previous studies of annular seals texture focus on damp rather than stiffness. Here a new spiral groove texture on cylindrical surface of annular seals is proposed. Based on the N-S equations, CFD method considering cavitation is used to simulate the internal flow of the spiral groove annular seals. Eccentricity is ensured by the moving mesh technology. The flow characteristics are studied, and comparatively analyzed with that of the smooth annular seals. Results show that: the spiral groove can generate the dynamic pressure effect and the pumping effect. Obvious changes in the pressure field appear. With eccentricity, the spiral groove can respectively enhance the stiffness, reset force and reduce the offset angle, which indicates that the spiral groove is helpful to acquire better rotor concentricity. The spiral groove also reduces the leakage and has little impact on the frictional torque. The above results demonstrate that the spiral groove is beneficial to improve the hydrodynamic characteristics of the annular seals and the concentricity of the rotor, which also reduces friction and wear.

View full abstract

An error was found in Vol. 12, No. 4, page 383. The legend in Fig. 10 was not shown correctly due to a misprint. The corrected figure is shown in PDF.

View full abstract

An error was found in Volume 13, Issue 1, page 42. The information of the corresponding author in the footnote was not correct. The corrected information is:
accepted for publication October 17 2019.

View full abstract

There was an error in Volume 13, Issue 1, page 116. The correction is shown in PDF.

View full abstract

There were errors in the review paper. The corrections are shown in PDF.

View full abstract

There were errors in the original paper. The corrections are shown in PDF.

View full abstract

There were errors in the original paper. The corrections are shown in PDF.

View full abstract