International Journal of Fluid Machinery and Systems
Online ISSN : 1882-9554
ISSN-L : 1882-9554
Volume 13, Issue 1
(January-March)
Displaying 1-25 of 25 articles from this issue
Review papers
  • Johannes Junginger, Oliver Kirschner, Stefan Riedelbauch
    Article type: Review paper
    2020 Volume 13 Issue 1 Pages 1-11
    Published: 2020
    Released on J-STAGE: January 20, 2020
    JOURNAL FREE ACCESS
    This paper describes a method for determining the sequence of transient model test experiments in a closed loop test rig, using previous 1D simulations. An existent power plant with reversible pump turbine in the Austrian Alps serves as baseline. First, real measurement data of power jumps are reproduced in 1D simulations. These simulation results were transferred to the scaled model size via similarity laws. Of particular importance is the choice of model to prototype speed ratio whose influence has been studied more closely. Subsequently, the transient processes are simulated in a closed loop test rig. The simulations are controlled by a temporal power variation of the service pump. An iteration loop with optimizer has adapted the power in order to achieve similar conditions as in the prototype simulation. Due to the good reproducibility of the power jumps in the closed loop model test rig more demanding processes are investigated in the next step. A fast transition with load rejection and guide vane closing has been simulated, as it is state of the art. Furthermore, it was assumed that this power plant also has a full size frequency converter for fast transition from pump mode to turbine mode and vice versa applying linear speed variation. The findings of the pump behavior as well as the test rig behavior can then be used for the transient model test experiments.
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Original papers
  • F Doussot, G Balarac, J Brammer, Y Laurant, O Métais
    Article type: Original Paper
    2020 Volume 13 Issue 1 Pages 12-20
    Published: 2020
    Released on J-STAGE: January 22, 2020
    JOURNAL FREE ACCESS
    Hydraulic machines are designed to operate in flow conditions close to the best efficiency point. However, to respond to the increasing demand for flexibility mainly due to the integration of renewable energy in the electric grid, the operating range of Francis turbines has to be extended towards smaller discharge levels without restriction. When Francis turbines are operated typically between 30% and 60% of the rated output power, the flow field is characterized by the appearance of inter blade vortices in the runner. In these off design operating conditions and due to these phenomena, dynamic stresses level can increase, and potentially lead to fatigue damage of the mechanical structure of the machine. The objective of this paper is to present investigations on the dynamic behaviour of the inter blade vortices and their impact on the runner by using numerical simulations. Computations were performed with different turbulence modelling approaches to assess their relevance and reliability: Reynolds Averaged Navier Stokes (RANS) and Large Eddy Simulation (LES). Computations aimed to better understand the emergence condition of the inter blade vortices. The analysis showed that vortices can be generated due to poor inlet adaptation at part load, however other vortices can also be due to a local backflow in the runner. The competition between these both phenomena leads to various topologies of the inter blade vortices. The numerical results were compared to experimental visualizations performed on scaled model as well as to previous numerical studies results. The impact of these inter blade vortices on the runner were also investigated by considering the pressure fluctuations induced on the blades. The dynamic loading on the blade has to be known in order to evaluate the lifetime of the runner by mechanical analysis. Different operating conditions have been simulated to understand how the pressure fluctuations depend on the operating conditions. The localization of the pressure fluctuations and their consequences on the frequency signature of the torque fluctuations have been analyzed. This article is presenting a part of the work presented at the 29th IAHR Symposium on Hydraulic Machinery and Systems, Kyoto, 2018 [1], and presents another vortex topology and a comparison of LES results of several operating conditions.
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  • Quentin Chatenet, Martin Gagnon, Emmanuel Remy, Mitra Fouladirad, Anto ...
    Article type: Original Paper
    2020 Volume 13 Issue 1 Pages 21-29
    Published: 2020
    Released on J-STAGE: January 24, 2020
    JOURNAL FREE ACCESS
    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.
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  • Yasuyuki Nishi, Kentaro Hatano, Takashi Okazaki, Yuichiro Yahagi, Teru ...
    Article type: Original paper
    2020 Volume 13 Issue 1 Pages 30-41
    Published: 2020
    Released on J-STAGE: January 29, 2020
    JOURNAL FREE ACCESS
    Undershot cross-flow turbines are applicable to shallow open channel flows and are characterized by a simple structure free of guide vanes or a casing. Since these water turbines operate in a free surface flow field, the optimum inlet blade angle and outlet blade angle are considered to vary with the flow rate in the open channel; however, guidelines for setting these angles are yet to be established. In this study, a method to improve the performance of an undershot cross-flow water turbine based on shock loss reduction was proposed. First, by varying the blade angle of the straight blades of the turbine, the inlet blade angle and outlet blade angle which reduced shock loss in the first-stage inlet region and second-stage inlet region were investigated through free surface flow analysis. From the results, it was shown that by taking the inlet and outlet blade angles which minimize the first-stage and second-stage shock losses for straight blades and applying them to curved blades, shock loss is reduced, thereby improving turbine efficiency and turbine output.
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  • Lei DAI, Lei GU, Jun WANG, Xiang AO, Xiaobo ZHU
    Article type: Original paper
    2020 Volume 13 Issue 1 Pages 42-54
    Published: 2020
    Released on J-STAGE: January 29, 2020
    JOURNAL FREE ACCESS
    Low efficiency is an urgent problem to be solved for vortex pumps due to large number of vortices and backflows. This paper concentrates on the performance improvement of vortex pumps caused by changing the indent distance of the impeller. First, a complete numerical model was established by selecting the optimal length of inlet and outlet and appropriate mesh quantity. Then, the reliability of this model was validated by comparing simulation results with experimental data. Based on this model, the internal flows of vortex pumps with different impeller indent distances were observed in simulations and the fitted curves of head and efficiency were created. Finally, the cause of the performance improvement was demonstrated by analyzing the flow field in vortex pumps.
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  • Cong Wang, Yongxue Zhang, Hucan Hou, Zhiyi Yuan, Ming Liu
    Article type: Original paper
    2020 Volume 13 Issue 1 Pages 55-67
    Published: 2020
    Released on J-STAGE: February 08, 2020
    JOURNAL FREE ACCESS
    To improve the energy conversion efficiency and cavitation performance of the ultra-low specific-speed centrifugal pump (ULSSCP), the impeller-volute interaction has been studied. Blade outlet setting angle (β2), wrap angle (φ), volute inlet width (b3) and throat area (St) were addressed as the design parameters. The entropy production at 0.5 Q0 and NPSHc (critical net positive suction head) at 1.5 Q0 were selected as the target to characterize the energy loss and cavitation performance. The L9 (34) orthogonal matrix was established via Taguchi method. Results show that the contribution ratio of φ on S/NS is the most vital, followed by b3 and St, while the influence of β2 is relatively small, and the design parameter combination with β2=19°, φ=220°, b3=12mm and St=190mm2 is the best choice for the lowest entropy production at 0.5Q0 and NPSHc. At last, the optimization design reduces the loss greatly before and after cavitation by alleviation of vortex generation and backflow intensity.
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  • Yasuyuki Nishi, Tomoyuki Kobori, Yutaka Kobayashi, Terumi Inagaki, Nor ...
    Article type: Original paper
    2020 Volume 13 Issue 1 Pages 68-78
    Published: 2020
    Released on J-STAGE: February 09, 2020
    JOURNAL FREE ACCESS
    Ultra-small axial flow hydraulic turbines, which are of the size of your palm, are a type of turbine that can be applied to the low heads of existing pipelines and open channels. However, due to their compact size, they are more likely to malfunction in case of foreign body contamination. In our study, we observed the passage of foreign bodies through an ultra-small axial flow hydraulic turbine and their encounter with the blocking mechanism of the turbine. We selected polyethylene ropes of varying lengths with a wire diameter of 5 mm to serve as foreign bodies. By varying the length of the rope, we were able to visually observe the movement of the foreign body. The turbine’s blocking mechanism can be broadly classified as guide vanes or runners. In the case of runner, blocking occurs when foreign bodies are bent and are caught at the leading edge of the blade. The passage rate through the hydraulic turbine is largely dependent on the passage rate at the runner section, which decreases proportionally with the length of the foreign body and the rotational speed of the blades.
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  • Jean-David Buron, Sébastien Houde, Claire Deschênes
    Article type: Original paper
    2020 Volume 13 Issue 1 Pages 79-89
    Published: 2020
    Released on J-STAGE: March 19, 2020
    JOURNAL FREE ACCESS
    The three dimensional unsteady swirling flow inside the draft tube of a model bulb turbine is investigated using timeresolved stereoscopic particle image velocimetry. The flow is analysed around the best efficiency points for twoN11 values. At N11 = 170, the draft tube flow dynamics are linked to a known efficency drop of the turbine while at N11 = 130, this phenomenon does not occur. This paper presents the experimental setup and provides a complete overview of the measurement campaign based on statistical and spectral analyses of the velocity fields and data from pressure sensors distributed across the draft tube. After the efficiency drop, the intermittency and skewness of the draft tube pressure recovery are found to be reflected onto the velocity fields. For both runner rotation speeds, spectral analyses reveal the presence of a coherent and non-axisymmetric structure that rotates at the same frequency as the runner.
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  • Kyung Ho Lee, Tae Woong Ha
    Article type: Original paper
    2020 Volume 13 Issue 1 Pages 90-102
    Published: 2020
    Released on J-STAGE: March 19, 2020
    JOURNAL FREE ACCESS
    The hybrid brush seal in which brush seal replaces one or two labyrinth teeth is preferred in turbomachinary to improve the efficiency and rotordynamic stability. Precise prediction method of leakage and rotordynamic coefficients is required in designing an effective hybrid brush seal. In this paper, the 3D CFD analysis method for rotordynamic coefficients is developed based on the seal dynamic analysis method using the relative coordinate system. The complex shape of the brush seal made of bristle wire pack is assumed to be a porous medium in this model. To evaluate the effect of the position of brush and clearance of brush seal on the rotordynamic coefficients, SSB and BSS type of hybrid brush seal are analyzed using FLUENT with cold clearance = 0 and 0.31mm. The rotordynamic coefficients(K, k, and C) obtained by the prposed analysis method are comparble to that of Pugachev's experimental results. The SSB seal shows better stability than the BSS seal when compaing the whirling frequency ratio as a result of brush position and clearance effect. Both SSB and BSS seal shows 30% reduction of leakage and better stability at cold clearance = 0 than at 0.31 mm. These results indicate that the position of the brush seal should be located at the end of hybrid brush seal, while maintaining the contact between the brush seal and rotor.
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  • Dmitry Platonov, Andrey Minakov, Dmitry Dekterev, Alexandra Maslenniko ...
    Article type: Original paper
    2020 Volume 13 Issue 1 Pages 103-113
    Published: 2020
    Released on J-STAGE: March 19, 2020
    JOURNAL FREE ACCESS
    The paper studies the pulsation characteristics of the flow when operating a medium-scale hydrodynamic test bench with a Francis turbine. The pressure fluctuations measurements and flow structure visualization were performed for all operation modes of the hydraulic unit. To control and suppress pulsations in the flow path, a series of experiments on air injection to the hydraulic unit were carried out. Comparison of pressure pulsations for single-phase and two-phase flow experimentation has shown that for modes with maximum loads, the pressure pulsations can be reduced almost to a half by means of air injection. The effect of air injection is also studied with respect to the efficiency of the hydraulic turbine.
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  • Yasuyuki Nishi, Takahiro Noji
    Article type: Original paper
    2020 Volume 13 Issue 1 Pages 114-125
    Published: 2020
    Released on J-STAGE: March 21, 2020
    JOURNAL FREE ACCESS
    As centrifugal pumps with a single blade can secure large passed particle size, they are widely used as sewage pumps transporting liquid containing foreign bodies. Not only dos the internal flow of centrifugal pumps with a single blade vary significantly according to the flow rate, but it also varies significantly according to the rotation of the impeller, and therefore it is extremely complex. Elucidation of this complex and unsteady internal flow is extremely important for enhancing efficiency by reducing loss and realizing high reliability by reducing the fluid force. This research has involved a structural survey of flow in the casing of closed-type centrifugal pump with a single blade through PIV measurement and CFD analysis. The study has showed that various vortexes were produced inside the casing by maximum efficiency point flow rate, and that a large scale vortex structure which varies significantly along with rotation of the impeller is formed. This suggests that increase or decrease of mixing loss inside the casing produced by rotation of the impeller does not depend on increase or decrease of vortex area inside the casing, but is rather produced by increase or decrease of vortexes produced near the tongue part and the beginning of the volute winding.
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  • Hao-cen HONG, Bin ZHANG, Min YU, Qi ZHONG, Hua-yong YANG
    Article type: Original paper
    2020 Volume 13 Issue 1 Pages 126-135
    Published: 2020
    Released on J-STAGE: March 21, 2020
    JOURNAL FREE ACCESS
    The periodic flow ripples and pressure pulsations in a fixed-displacement axial-piston pump will directly impact its dynamic characteristics. To reduce the discharge flow ripples and pressure pulsations is one of the design challenges. An optimizing method is investigated herein using numerical models. The U-shaped damping groove at the discharge kidney groove is the primary optimized object. The numerical models of a fixed-displacement axial-piston pump with nine pistons are proposed and developed by MATLAB/Simulink. The effects of width radius, groove depth and groove length angle on flow dynamics are discussed. The optimization models based on the root-mean-square, the pulsation and the suction-extrusion ratio are proposed. As results show that the optimized structure can decrease the flow ripples from 31.08% to 20.33%.
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  • Kesar M Kothari, R. Udayakumar, Ram Karthikeyan, Vishweshwar S, Nikith ...
    Article type: Original paper
    2020 Volume 13 Issue 1 Pages 136-149
    Published: 2020
    Released on J-STAGE: March 21, 2020
    JOURNAL FREE ACCESS
    Hydraulic manifolds are metal cuboids machined to realize the compact circuit layout within them. They are introduced in the hydraulic machines to fit the large and complex hydraulic system layouts in narrow spaces available in the machines. Therefore, designing of the manifold in fact is more oriented towards achieving minimum size and weight. But the use of manifold, may introduce high pressure losses in the system. Efficiency of the system decreases and temperature of the fluid increases with pressure drop. This present research work focuses on understanding the pressure losses in the most common channel connections used in the manifold to realize the hydraulic circuit and to understand the efficiency of the manifold at different flow values. To achieve these objectives, a real-time case study is considered, where a manifold for a cable pulling winch machine is modified to reduce the pressure drop and increase the efficiency of the machine. Simple channel models are considered and analyzed using semi empirical equations available in the literature and are compared with results obtained from Computational Fluid Dynamics (CFD) analysis. Various geometric bend models are drafted in Solid Works and then exported to do the CFD software to obtain the pressure drop with different flows. The values obtained from CFD and the characteristic of the valves from the manufacturer’s catalogue are used to create the manifold in Matlab Simulink to predict the performance of the manifold at different flows. Therefore, with these results, the overall hydraulic efficiency of the winch is determined.
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  • Wei Ma, Ting Pan
    Article type: Original paper
    2020 Volume 13 Issue 1 Pages 150-159
    Published: 2020
    Released on J-STAGE: March 22, 2020
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    To explore the dynamic instability characteristic of pressure relief valve, the experimental research was performed by considering the influence factors of flow rate, set pressure and inlet pipe length. Data analyses for typical chatter instability conditions are conducted. Theoretical and experimental analyses of water hammer effect during chatter instability were conducted. The results showed that the valve core hits the seat with high frequency in chatter instability. The lift, velocity and acceleration of the valve core increase with the increase of the flow rate. It is reliable to judge the water hammer pressure rise according to the closing time of valve and the transit time of reflected wave.
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  • Jun Du, Junfang Xia, Hao Wu, Wenqiang Xu
    Article type: Original Paper
    2020 Volume 13 Issue 1 Pages 160-166
    Published: 2020
    Released on J-STAGE: March 22, 2020
    JOURNAL FREE ACCESS
    The agricultural digging device for lotus root make waterjet scour away the soil around lotus root to help it float up to water surface by buoyancy on harvesting. The performance of the waterjet was investigated by CFD-DEM simulation and experiment for its optimization. The Hertz-Mindlin with JKR model was used as the soil around lotus root was cohesive. The simulation shoes that the digging depth increases with the increase of the jetting angle and the decrease of the diameter of nozzle. The preliminary trial verified this trend, and it revealed that the digging depth determined the digging ratio. In the field experiment, the effects of type of nozzle, diameter of nozzle and jetting angle on digging depth, digging ratio, breaking ratio and surface damage ratio were studied. The results showed that a larger diameter with a larger jetting angle for a suitable type of nozzle can obtain a high digging ratio with a lower breaking ratio and surface damage ratio.
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  • Fei Yan, Lei Sun, Ziyu Wang, Rui Zhu
    Article type: Original paper
    2020 Volume 13 Issue 1 Pages 167-176
    Published: 2020
    Released on J-STAGE: March 24, 2020
    JOURNAL FREE ACCESS
    Low-temperature pressure drop experiments take a long time, in this study, computational fluid dynamics (CFD) and a general regression neural network (GRNN) are used to predict the pressure drop in a wind power lubrication system to serve as an alternative to experiments. The simulation results show a clear increase in the yield stress as the temperature decreases, especially under -35℃. The factors that affect the pressure of lubricating grease transport are as follows in decreasing order of importance: temperature, high-pressure pipe diameter, and flow rate. The general regression neural network can be used to effectively predict the pressure of lubricating grease transport under different conditions with a mean relative error of 8.1%.
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  • S. M. Gunadal, M. Govardhan
    Article type: Original paper
    2020 Volume 13 Issue 1 Pages 177-189
    Published: 2020
    Released on J-STAGE: March 24, 2020
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    The type of diffuser used has a strong influence on the performance of a centrifugal compressor. However, the knowledge about the influence of partial vaned diffusers is limited. In the present study two types of partial vaned diffusers, one with partial vanes fixed to the hub wall (hub vaned diffuser), and the other with the partial vanes fixed to the shroud wall (shroud vaned diffuser) are explored and their influence on the compressor stage performance is analyzed numerically. Besides the vane height is varied and the performances investigated over the entire operating range. The results show that, the performance of the shroud vaned diffuser is better than both the vaneless diffuser and the hub vaned diffuser.
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  • Taku Iwase, Hideshi Obara, Yoshinobu Yamade, Chisachi Kato
    Article type: Original paper
    2020 Volume 13 Issue 1 Pages 190-202
    Published: 2020
    Released on J-STAGE: March 25, 2020
    JOURNAL FREE ACCESS
    In this study we calculated flow fields and aerodynamic noise in centrifugal fan of air conditioner by large eddy simulation (LES). The numerical simulation code employed throughout the LES was FrontFlow/blue (FFB). We compared calculation results for 10 million grids (10M-grid), 60 million grids (60M-grid) and 500 million grids (500M-grid) to investigate the influence of grid resolution on the prediction accuracy. The prediction accuracy of the aerodynamic noise by the 500M-grid was improved compared to the 10M-grid and 60M-grid calculation results. The grid resolutions between the shroud and the bell mouth and between one blade and an adjacent blade were important. Moreover, we found the proper capturing of the streaks contributed to better prediction.
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  • Yingkun Zhang, Kunhang Li, Yunlong Li, Jingyin Li
    Article type: Original paper
    2020 Volume 13 Issue 1 Pages 203-213
    Published: 2020
    Released on J-STAGE: March 25, 2020
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    The effects of the guiding tank on flow characteristics inside a double-suction squirrel-cage fan for the range hood were numerically investigated in this paper. Flow analysis was conducted by solving the three-dimensional steady Reynolds-averaged Navier-Stokes equations using the realizable k- turbulence model. Numerical results were validated with the experimental data for total pressure. Differences in flow characteristics between the fan model and the range hood model at the nearly maximum mass flow rate of 0.3214 kg/s were analyzed in detail. Numerical simulation results show that the incoming flow of the squirrel-cage fan will be distorted due to the guiding tank. The inlet distortion will deteriorate the aerodynamic performance of the squirrel-cage fan and cause obvious differences in the velocity field, the pressure field and the vortex structures inside the fan.
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  • Maofei Geng, Yuntao Song, Anyi Cheng, Hansheng Feng
    Article type: Original Paper
    2020 Volume 13 Issue 1 Pages 214-221
    Published: 2020
    Released on J-STAGE: March 26, 2020
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    To improve the efficiency of the Experimental Advanced Superconducting Tokamak (EAST) subcooled helium cryogenic system, a centrifugal cold compressor (CC) was investigated as an alternative to the oil ring pump. A numerical simulation method was applied to analyse variations in CC performance parameters, pressure ratio and polytropic efficiency. Results indicated that the performance curve of the CC and warm compressor (WC) were similar. The tip clearance at the design point was 0.2 mm and increased to 0.231 mm when the CC was operated at 4 K. This increase in tip clearance resulted in a 0.86% drop in the pressure ratio and a 0.54% drop in polytropic efficiency. To verify the simulation model, experiments were done using cold nitrogen.
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  • Yan Yan, Zhichuan Guan, Cai Yang, Han Yan
    Article type: Original Paper
    2020 Volume 13 Issue 1 Pages 222-232
    Published: 2020
    Released on J-STAGE: March 26, 2020
    JOURNAL FREE ACCESS
    In order to improve cleaning efficiency of dual-diameter PDC bit, the total flow area, inclination angle and azimuth angle of bit’s pilot and reaming nozzles were analyzed. By numerical simulation, the velocity, pressure and particle distribution of bottomhole are obtained. The results show that low pressure area of pilot downhole decreases while low pressure of reamer downhole increases with the increase of total flow area. The mud velocity of central bottomhole has the negative relationship with inclination angle and positive relationship with azimuth angle. Best cleaning efficiency can be achieved when total flow area is 12 cm2, inclination angles of pilot and reamer are respectively 15° and 25°, and azimuth angles of pilot and reamer are respectively 60° and 30°.
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  • Yong Sang, Xudong Wang, Weiqi Sun
    Article type: Original paper
    2020 Volume 13 Issue 1 Pages 233-240
    Published: 2020
    Released on J-STAGE: March 28, 2020
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    Hydraulic poppet pressure relief valve, which has the advantages of fast response and good sealing performance, is extensively used in the hydraulic industry. In this paper, a two-dimensional axisymmetric geometry model of the hydraulic poppet safety relief valve with a small size is established. Dynamic mesh, along with UDF in FLUENT software, is used in this study. The dynamic characteristics of a hydraulic poppet safety relief valve using CFD are investigated under different conditions. For incompressible hydraulic oil, a sharp force rise and collapse caused by a redirection of the fluid flow due to inertia forces is depicted, and the variation of the poppet motion under the influence of the back pressure is analyzed based on the flow information. The dynamic behavior of the hydraulic poppet relief valve under different set pressure is investigated. In the end, a numerical study on the instability of the poppet relief valve supplying a constant volumetric flow rate is performed, and the chatter frequency is obtained.
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  • De Zhang, Yusuke Katayama, Satoshi Watanabe, Shin-Ichi Tsuda, Akinori ...
    Article type: Original Paper
    2020 Volume 13 Issue 1 Pages 241-252
    Published: 2020
    Released on J-STAGE: March 28, 2020
    JOURNAL FREE ACCESS
    It is known that higher efficiency can be achieved with reduced specific speed design of rear rotor in contra-rotating axial flow pump. To investigate the mechanism of increased efficiency with the reduced specific speed of rear rotor, three models with different specific-speed rear rotors are designed with the conventional method, and the flow fields are simulated by unsteady RANS simulation. To analyze the loss generation mechanism, two loss evaluation methods based on the entropy production rate and the material-derivative of rothalpy are employed. It is found that, although the both methods qualitatively estimate the total loss through the rear rotor, the derivative of rothalpy can give much better quantitative prediction of the losses. Two distinct flow features are observed in the rear rotor, the corner separation at the hub corner of blades and the tip leakage vortex, both of which are responsible for the loss generation. With the evaluation of local loss generation based on the material derivative of rothalpy, the loss contribution of corner separation is found to be very small compared with that due to the tip leakage vortex. The tip leakage vortex structure in high specific speed rear rotor shows the strong interaction with the leading edge of adjacent blade, which seems to strengthen the blockage effect in the tip region. This is relieved in the lower specific speed rear rotor, resulting in the achievement of higher efficiency with it.
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  • Ryota Kobayashi, Hiroaki Terakado, Kotaro Sato, Junya Taniguchi, Koich ...
    Article type: Original Paper
    2020 Volume 13 Issue 1 Pages 253-265
    Published: 2020
    Released on J-STAGE: March 29, 2020
    JOURNAL FREE ACCESS
    This paper presents a fundamental study of jet directional control, i.e., thrust vectoring, using a synthetic jet generated by an asymmetric stepped slot. We present and discuss the results for a synthetic jet generated by a one-sided asymmetric stepped slot and focus on the relationship between the traveling direction of the formed jet and the offset ratio of the stepped slot and/or dimensionless frequency of the synthetic jet, within a jet deflection angle range of 0–90°. We demonstrate that there is an optimum offset ratio for jet deflection under the same dimensionless frequency because the traveling direction of the synthetic jet is dependent on the offset ratio of the stepped slot and the dimensionless stroke length (dimensionless frequency).
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  • Nobuyuki Yamaguchi
    Article type: Original Paper
    2020 Volume 13 Issue 1 Pages 266-280
    Published: 2020
    Released on J-STAGE: March 29, 2020
    JOURNAL FREE ACCESS
    The author has proposed in the preceding study on the behaviors of infinitesimal perturbations of surge in simplified compressor systems that the occurrences of stagnations could be closely related with the travelling wave conditions of the surge oscillations through the phase-differences and the propagations along the flowpaths. The concept was applied to real situations of systems of compressors and flowpaths in the present study. It resulted in a non-dimensional parameter named flowpath-average reduced resonance frequency, fRav, which is an equivalent of the phase-propagation parameter proposed in the preceding study. It is finally a product of the resonance frequency of the system and the time required for the fluid particles to pass through the whole flowpath, namely, the total number of excitations that the fluid particles suffer in the time. A number of numerical experiments on compressor surges have demonstrated that the fRav parameter could explain the cause of surge stagnations very simply and clearly. It has values of 1.5 . 3 at the stagnation boundaries for variations in flowpath configurations and compressors having a single-stage to nine stages. It means that the stagnations occur for the total number of the excitation cycles less than 1.5-3. It could be the most essential cause of the stagnations. In other words, deep surges could develop only after the fluid particles have experienced more number of cycles of the excitations in the passing time. In further, the fRav parameter is made more comprehensive by multiplication of the compressor tip Mach number Mt, i.e., fRav×Mt, which gives a nearly constant value of 1.5 over a wide range of flowpath configurations and tip speeds of compressors, for relatively small number of stages. Those parameters not only suggest the essential cause of surge stagnations, but also are the simplest and clearest measurers of stagnation boundaries. It is much easier to apply in real situations of the systems than any of the parameters proposed so far. However, for multi-stage compressors in long delivery flowpaths and in off-design operations, the parameters appear to deviate somewhat from the constant value.
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