Study of Flow Phenomena in Spiral Casing and Double Circular Cascade of a Francis Turbine

Kazuyoshi Miyagawa,Yutaka Kawata,Nobuhide Fukuda 대한기계학회 1998 KSME/JSME THERMAL and FLUID Engineering Conference Vol.1 No.1

Stochastic Impact Model: Poisson Processes and Copulas to Model Cavitation Erosion Impacts

Gabriel Taillon,Kazuyoshi Miyagawa 한국유체기계학회 2019 International journal of fluid machinery and syste Vol.12 No.4

A statistical model that represents impacts caused by cavitation bubble collapse on material surfaces is developed, as one step in the development of a novel cavitation erosion model. This model uses stochastic processes, for example the non-homogeneous Poisson process (NHPP), to represent impact events as points randomly distributed in time and space. Then, marks which represent the impact properties are computed using empirical distributions. This model is based on experimental data and empirical observations using a high-speed pressure sensor in a vibratory cavitation ap-paratus based on the ASTM G32 standard. A total of 41581 impacts were measured over 100 realizations of 4ms long recording. The rate of the non-homogeneous Poisson is shown to be a sinusoidal function, but the process is over-dispersed. The spatial distribution of points is independent from the temporal portion, so modeled independently as a Poisson cluster process. Two marks were measured for each impact event: the impact duration and amplitude. These mark’s joint marginals are modeled as gamma distributions, determined by a Kolmogorov-Smirnov (KS) test. The marks are shown to be dependent, prompting the use of copulas to model their joint distribution. The best-fitting copula is a highly asymmetric Tawn copula, in the class of so-called extreme value copulas. Almost no impacts were observed with a combination of a high amplitude (>12MPa) and low duration (<5µs). An extension of the KS test to two dimensions demonstrated that the copula is a better fit compared with a joint distribution of independent marginals. The combina-tion of the spatio-temporal processes, with the mark’s distributions combine to produce the stochastic impact model for cavitation erosion in the vibratory apparatus

Study on Flow Instability and Countermeasure in a Draft tube with Swirling flow

Nakashima, Takahiro,Matsuzaka, Ryo,Miyagawa, Kazuyoshi,Yonezawa, Koichi,Tsujimoto, Yoshinobu Korean Society for Fluid machinery 2015 International journal of fluid machinery and syste Vol.8 No.4

The swirling flow in the draft tube of a Francis turbine can cause the flow instability and the cavitation surge and has a larger influence on hydraulic power operating system. In this paper, the cavitating flow with swirling flow in the diffuser was studied by the draft tube component experiment, the model Francis turbine experiment and the numerical simulation. In the component experiment, several types of fluctuations were observed, including the cavitation surge and the vortex rope behaviour by the swirling flow. While the cavitation surge and the vortex rope behaviour were suppressed by the aeration into the diffuser, the loss coefficient in the diffuser increased by the aeration. In the model turbine test the aeration decreased the efficiency of the model turbine by several percent. In the numerical simulation, the cavitating flow was studied using Scale-Adaptive Simulation (SAS) with particular emphasis on understanding the unsteady characteristics of the vortex rope structure. The generation and evolution of the vortex rope structures have been investigated throughout the diffuser using the iso-surface of vapor volume fraction. The pressure fluctuation in the diffuser by numerical simulation confirmed the cavitation surge observed in the experiment. Finally, this pressure fluctuation of the cavitation surge was examined and interpreted by CFD.

Study on Flow Instability and Countermeasure in a Draft tube with Swirling flow

Takahiro Nakashima,Ryo Matsuzaka,Kazuyoshi Miyagawa,Koichi Yonezawa,Yoshinobu Tsujimoto 한국유체기계학회 2015 International journal of fluid machinery and syste Vol.8 No.4

The swirling flow in the draft tube of a Francis turbine can cause the flow instability and the cavitation surge and has a larger influence on hydraulic power operating system. In this paper, the cavitating flow with swirling flow in the diffuser was studied by the draft tube component experiment, the model Francis turbine experiment and the numerical simulation. In the component experiment, several types of fluctuations were observed, including the cavitation surge and the vortex rope behaviour by the swirling flow. While the cavitation surge and the vortex rope behaviour were suppressed by the aeration into the diffuser, the loss coefficient in the diffuser increased by the aeration. In the model turbine test the aeration decreased the efficiency of the model turbine by several percent. In the numerical simulation, the cavitating flow was studied using Scale-Adaptive Simulation (SAS) with particular emphasis on understanding the unsteady characteristics of the vortex rope structure. The generation and evolution of the vortex rope structures have been investigated throughout the diffuser using the iso-surface of vapor volume fraction. The pressure fluctuation in the diffuser by numerical simulation confirmed the cavitation surge observed in the experiment. Finally, this pressure fluctuation of the cavitation surge was examined and interpreted by CFD.

Cavitation Surge in a Small Model Test Facility simulating a Hydraulic Power Plant

Yonezawa, Koichi,Konishi, Daisuke,Miyagawa, Kazuyoshi,Avellan, Francois,Doerfler, Peter,Tsujimoto, Yoshinobu Korean Society for Fluid machinery 2012 International journal of fluid machinery and syste Vol.5 No.4

Model tests and CFD were carried out to find out the cause of cavitation surge in hydraulic power plants. In experiments the cavitation surge was observed at flow rate, both with and without a surge tank placed just upstream of the inlet volute. The surge frequency at smaller flow rate was much smaller than the swirl mode frequency caused by the whirl of vortex rope. An unsteady CFD was carried out with two boundary conditions: (1) the flow rate is fixed to be constant at the volute inlet, (2) the total pressure is kept constant at the volute inlet, corresponding to the experiments without/with the surge tank. The surge was observed with both boundary conditions at both higher and lower flow rates. Discussions as to the cause of the surge are made based on additional tests with an orifice at the diffuser exit, and with the diffuser replaced with a straight pipe.

Cavitation Surge in a Small Model Test Facility Simulating a Hydraulic Power Plant

Koichi Yonezawa,Daisuke Konishi,Kazuyoshi Miyagawa,François Avellan,Peter Doerfler,Yoshinobu Tsujimoto 한국유체기계학회 2012 International journal of fluid machinery and syste Vol.5 No.4

Model tests and CFD were carried out to find out the cause of cavitation surge in hydraulic power plants. In experiments the cavitation surge was observed at flow rates higher and lower than the swirl free flow rate, both with and without a surge tank placed just upstream of the inlet volute. The surge frequency at smaller flow rate was much smaller than the swirl mode frequency caused by the whirl of vortex rope. An unsteady CFD was carried out with two boundary conditions: (1) the flow rate is fixed to be constant at the volute inlet, (2) the total pressure is kept constant at the volute inlet, corresponding to the experiments without/with the surge tank. The surge was observed with both boundary conditions at both higher and lower flow rates. Discussions as to the cause of the surge are made based on additional tests with an orifice at the diffuser exit, and with the diffuser replaced with a straight pipe.

Dynamic Characteristics of the Radial Clearance Flow between Axially Oscillating Rotational Disk and Stationary Disk

Horiguchi, Hironori,Ueno, Yoshinori,Takahashi, Koutaro,Miyagawa, Kazuyoshi,Tsujimoto, Yoshinobu Korean Society for Fluid machinery 2009 International journal of fluid machinery and syste Vol.2 No.2

Dynamic characteristics of the clearance flow between an axially oscillating rotational disk and a stationary disk were examined by experiments and computations based on a bulk flow model. In the case without pressure fluctuations at the inlet and outlet of the clearance, parallel and contracting flow paths had an effect to stabilize the axial oscillation of the rotating disk. The enlarged flow path had an effect to destabilize the axial oscillation due to the negative damping and stiffness for outward and inward flows, respectively. It was shown that the fluid force can be decomposed into the component caused by the inlet or outlet pressure fluctuation without the axial oscillation and that due to the axial oscillation without the inlet or outlet pressure fluctuation. A method to predict the stiffness and damping coefficients is proposed for general cases when the device is combined with an arbitrary flow system.

Numerical investigation of three-dimensional cavitation evolution and excited pressure fluctuations around a twisted hydrofoil

Bin Ji,Xianwu Luo,Yulin Wu,Kazuyoshi Miyagawa 대한기계학회 2014 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.28 No.7

Unsteady cavitating turbulent flow around a twisted hydrofoil was analyzed to illustrate the physical mechanism of the cavitygeneratedpressure fluctuations. The numerical simulations of cavitating flow were based on the Partially-Averaged Navier-Stokes(PANS) method and a mass transfer cavitation model. The validity of PANS model has been evaluated and confirmed in cavitation simulationsby present authors using three different cases, 2D hydrofoil (Ji et al. 2012 [37]), 3D hydrofoil (Ji et al. 2013 [31]) and marine propeller(Ji et al. 2012 [38]), which shows that the PANS model with fk = 0.2 and fe = 1 can obtain more accurate estimates of unsteadycavitating flows with large-scale fluctuations at a reasonable cost. In present paper we intended to shed light on the physical process responsiblefor the pressure fluctuations excited by cavitation. The cavity volume was analyzed to illustrate the relationship between thecavitation evolution and the pressure fluctuations. The results show that the cavity volumetric acceleration curve tracks remarkably wellwith the main features of the time-dependent pressure fluctuations except for the high frequency component. Thus, the cavity volumetricacceleration is the main source of the excited pressure fluctuations by cavitation. It is noted that the cavitation induced pressure fluctuationsare transmitted along the suction surface of the hydrofoil and are synchronized with those on the pressure surface at the midplane ofthe twisted hydrofoil. Further, the pressure fluctuations on the pressure surface decrease towards the center from both the leading andtrailing edges of the hydrofoil, with a minimum at 60% chord length from the leading edge.

Study of flow instability in off design operation of a multistage centrifugal pump

Akiha Shibata,Hideto Hiramatsu,Shutaro Komaki,Kazuyoshi Miyagawa,Manabu Maeda,Shun Kamei,Ryota Hazama,Takeshi Sano,Masamichi Iino 대한기계학회 2016 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.30 No.2

In recent years, attempts have been made to make multistage centrifugal pumps smaller in size and more efficient. However, such designs are known to cause positive-slope phenomena in the Q-H curve, especially under low-flow conditions. These phenomena, which have thus far been studied experimentally and numerically, stem from flow instability in the pump. However, their mechanisms have not yet been clarified because it depends on various parameters. In this study, we focused on diffuser rotating stall, observed in positive Q-H characteristics. This study elucidates the mechanism of positive-slope generation through experimental results and two-dimensional numerical analysis.

Experimental Analysis of Diffuser Rotating Stall in a Three-Stage Centrifugal Pump

Takamine, Taiki,Furukawa, Daichi,Watanabe, Satoshi,Watanabe, Hiroyoshi,Miyagawa, Kazuyoshi Korean Society for Fluid machinery 2018 International journal of fluid machinery and syste Vol.11 No.1

Rotating stall phenomenon limits the operation range of turbomachines, therefore it is important to understand the crucial parameters of this phenomenon. In the present study, the diffuser rotating stall in a three-stage centrifugal pump was experimentally studied. Examined main parameter was an axial offset of rotor against the stationary part, which might be unavoidable due to accumulation of geometrical tolerances and assembling errors. The effect of leakage flow rate at the balance drum section employed as a thrust balancing device, which increases the thru-flow rate at the first and second stages diffusers, was also studied. The effect of rotor axial offset was clearly observed and, with the rotor axial offset to the suction side, the rotating stall appeared only at the third stage diffuser. By setting the balance flow rate set to zero, the onset range of rotating stall became wider in the first and second stage diffusers, which was well explained by the decrease of the thru-flow rate.