気泡モデルキャビテーション流れ解析による遠心ポンプ羽根車内の気泡挙動の予測(S25-1 気泡力学に関する基礎解析とその応用(1),S25 気泡力学に関する基礎解析とその応用)

We have developed a numerical simulation code that can predict the cavitation erosion and the residual stress of a material, which are both closely related to plastic deformation. The shock wave generated at the cavitation bubble collapse hits the material surface and the impact energy causes plastic deformation which changes the residual stress. When the impact energy is high, mass loss (i.e., erosion) occurs after the plastic deformation stage. We numerically simulated impulsive bubble pressures that varied on the order of a microsecond in the cavitating flow with the ‘bubble flow model’. The bubble flow model simulates the abrupt time-variations in the radius and inner pressure of bubbles based on the Rayleigh-Plesset equation. Although the shock wave propagation was not simulated, the impact energy was estimated based on the bubble pressure. The predicted impact energy was compared with the distribution of plastic deformation pits, which were observed on the impeller blade surface of a centrifugal pump. The predicted impact energy was also compared with the distribution of residual stress measured on a stainless steel plate after a cavitating jet impinged on the plate. The distribution of the impact energy corresponded qualitatively to that of the residual stress improvement caused by the plastic deformation. High correlation between the predicted impact energy and the plastic deformation of material was confirmed, and we found that our numerical method is relevant for the prediction of cavitation erosion and residual stress on a material surface.

In this work, the cavitation erosion (CE)-corrosion behaviors of high nitrogen austenitic stainless steel (HNSS) Fe-19Cr-15Mn-0.66 N were investigated under both static and CE conditions in 0.5 mol l−1 NaCl solution. For the sample after 8 h CE, the cumulative mass loss increased by 15% in 0.5 mol l−1 NaCl solution compared to distilled water, indicating that the corrosive media promoted mass loss. The corrosion behavior of the samples under static and CE conditions was evaluated using electrochemical techniques. The results showed that compared with the samples under static conditions, the samples under CE had a higher corrosion rate, and the corrosion resistance of the samples gradually decreased with the prolongation of C E time.The open circuit potential and potentiodynamic polarization tests showed a higher corrosion rate under CE conditions. The electrochemical impedance spectroscopy results revealed a decrease in corrosion resistance with increasing CE time. The XPS results showed that CE has a significant effect on the composition of the passive film, promoting a notable increase in Cr2O3 and Fe2O3.

Numerical prediction of cavitation erosion is a great scientific and technological challenge. In the past, many attempts were made—many successful. One of the issues when a comparison between a simulation and erosion experiments is made, is the great difference in time scale. In this work, we do not attempt to obtain quantitatively accurate predictions of erosion process but concentrate qualitatively on cavitation mechanisms with quantitative prediction of pressure pulses which lead to erosion. This is possible, because of our recent experimental work on simultaneous observation of cavitating flow and cavitation erosion by high speed cameras. In this study, the numerical simulation was used to predict details of the cavitation process during the vapor collapse phase. The fully compressible, cavitating flow simulations were performed to resolve the formation of the pressure waves at cavitation collapse. We tried to visualize the mechanisms and dynamics of vapor structures during collapse phase at the Venturi geometry. The obtained results show that unsteady Reynolds-averaged Navier–Stokes (URANS) simulation of cavitation is capable of reproducing four out of five mechanisms of cavitation erosion, found during experimental work.

Passivation effects on corrosion and cavitation erosion resistance of UNS S32205 duplex alloy in 3.5% NaCl

The numerical simulation of a flow through a duct requires an externally specified forcing that makes the fluid flow against viscous friction. To this end, it is customary to enforce a constant value for either the flow rate (CFR) or the pressure gradient (CPG). When comparing a laminar duct flow before and after a geometrical modification that induces a change of the viscous drag, both approaches lead to a change of the power input across the comparison. Similarly, when carrying out direct numerical simulation or large-eddy simulation of unsteady turbulent flows, the power input is not constant over time. Carrying out a simulation at constant power input (CPI) is thus a further physically sound option, that becomes particularly appealing in the context of flow control, where a comparison between control-on and control-off conditions has to be made. We describe how to carry out a CPI simulation, and start with defining a new power-related Reynolds number, whose velocity scale is the bulk flow that can be attained with a given pumping power in the laminar regime. Under the CPI condition, we derive a relation that is equivalent to the Fukagata–Iwamoto–Kasagi relation valid for CFR (and to its extension valid for CPG), that presents the additional advantage of naturally including the required control power. The implementation of the CPI approach is then exemplified in the standard case of a plane turbulent channel flow, and then further applied to a flow control case, where a spanwise-oscillating wall is used for skin-friction drag reduction. For this low-Reynolds-number flow, using 90 % of the available power for the pumping system and the remaining 10 % for the control system is found to be the optimum share that yields the largest increase of the flow rate above the reference case where 100 % of the power goes to the pump.

Prediction of Cavitation Erosion on Stainless Steel Components in Pumps

The influence of two different shapes impeller blades on performance of centrifugal water pump is experimentally investigated under different operational conditions. Test rig is designed and used to operate the required pump during different conditions. The centrifugal water pump is tested under different flow rate with two types of impellers each one consists different blades angle. The experimental results showed that the impeller blade different geometry could improve the water pump mechanical efficiency, the head, the power, and the volumetric efficiency by 11.7, 15.38, 24.2, and 12.9% respectively. The experimental results showed better centrifugal water pump performance when impeller blade with higher outlet angle is used.

This paper describes an experimental investigation of rotor–stator interaction in a centrifugal pump with several vaned diffusers. Steady and unsteady diffuser vane pressure measurements were made for a two–dimensional test impeller. Unsteady impeller blade pressure measurements were made for a second two–dimensional impeller with blade number and blade geometry identical to the two–dimensional impeller used for the diffuser vane pressure measurements. The experiments were conducted for different flow coefficients and different radial gaps between the impeller blade trailing edge and the diffuser vane leading edge (5% and 8% of the impeller discharge radius). The largest pressure fluctuations on the diffuser vanes and the impeller blades were found to be of the same order of magnitude as the total pressure rise across the pump. The largest pressure fluctuations on the diffuser vanes were observed to occur on the suction side of the vane near the vane leading edge, whereas on the impeller blades the largest fluctuations were observed to occur at the blade trailing edge. However, the dependence of the fluctuations on the flow coefficient was found to be different for the diffuser vanes and the impeller blades; on the vane suction side, the fluctuations were largest for the maximum flow coefficient and decreased with decreasing flow coefficient, whereas at the blade trailing edge, the fluctuations were smallest for the maximum flow coefficient and increased with decreasing flow coefficient. Increasing the number of the diffuser vanes resulted in a significant decrease of the impeller blade pressure fluctuations. The resulting lift on the diffuser vanes was computed from the vane pressure measurements; the magnitude of the fluctuating lift was found to be larger than the steady lift.

Cavitation erosion in waters having different surface tensions

An evaluation methodology of a thermal-hydraulics based on a mechanism in light water reactors (LWRs) is needed from a viewpoint of the safety analysis during normal operation and unanticipated transient such as under a severe accident. Currently, the evaluation of safety for the nuclear reactor has been implemented by a best estimate (BE) code and subchannel analysis code. These analysis codes contain models and empirical correlations. Therefore, the full-scale mock-up tests are needed to evaluate the reliability and validation of code. And the model and empirical correlation are allowed to be applied only in the range where the experiments were implemented. The large mock-up tests are once again needed in order to consider the new geometry and boundary conditions when the design of components is changed. Hence, the 3D detailed numerical simulation by the mechanistically based method is expected to be applied for the preliminary analysis to improve the design of fuel assemblies and evaluate the safety. This 3D detailed numerical simulation can reduce the large mock-up tests. The detailed numerical simulation method can provide much information relating to the two-phase flow such as the bubble size, its velocity, and detailed void distribution which, for example, are needed to predict the critical heat flux based on the mechanism. Moreover, JAEA is implementing the development of the nuclear-thermal-coupling code by using a detailed two-phase flow analysis code based on the VOF method like a JUPITER code. In this study, the numerical simulation of two-phase flow in the 4x4 bundle was examined by numerical simulation code JUPITER in order to examine the possibility of the JUPITER code for the large scale two-phase flow analysis. The simulation results are verified by the previous experimental data of two-phase flow.

To analyze the effect of dsfferent impeller type on inner flow in impeller, two low-specific-speed high-speed com- plex impellers with eight blades are investigated whose types are closed and semi-open. Using the Spalart-Allmaras turbu- lence model and the Reynolds-Averaged Navier-Storkes equa- tions, the three-dimensional turbulent flow fields in impellers are analyzed numerically. And the experimental studies on the centrifugal pump are carried out. The simulation results show that there all exists back flow region in impellers, the back flow region is few and the relative flow in clearance is alone the circumferential orientation in the half-open impeller, the static pressure in impellers rises gradually from blade inlet to outlet whose contours are almost alone the circumferential orientation, the static pressure of blade tip is bigger than that of blade root in the same radius of the blade in half-open impeller, and the static pressure coefficient at outlet of closed impeller is larger. The experimental results show that the efficiency of the half-open impeller is higher and the back flow has important influence on the performance of pump.

A mathematical model considering the interaction of bubbles in continuous casting mold of steel

Abstract. Cavitation erosion affects the hydropower plants operation mode, destroys the surface of water-conducting way, affects the efficiency coefficient as well as the turbine output. The most common damage is observed on water-conducting ways having poor streamline shape, various ledges, irregularities along the water flow. Disruptive cavitations are formed initially on the rotation axes of vortexes, and the caverns of disruptive cavitation develop with the period subordinating the Strouhal Law. One of the methods of cavitational erosion field studies is the method of obtaining high-speed cavitational erosion of metal plates, which has low resistance to cavitation mechanical effects. It provides an opportunity to determine erosion degree and intensity at different modes of hydropower plants operation. The paper demonstrates the results of studying the cavitational erosion process of hydroturbine impeller blades of Zhiguli Hydroelectric Station (HS). Cavitation tests are conducted for three operating modes. Technological peculiarities are also described. The paper presents results of turbine blades of Unit 5 for three operation modes. The destruction areas of impeller blades peripheral edges are visualized. The study provides the results of field and laboratory cavitation tests of hydroturbine elements that serve as recommendations for repairing and restoration of damaged hydroelectric units of Zhiguli HS.

The Stagnation Point Flow Experiment is a popular means for studying blood-surface interactions in a surface strain rate gradient. The strain rate distribution on the test surface of the flow chamber used in this experiment was originally assumed to be simply proportional to the radial distance from the stagnation point. A numerical simulation of the flows through the five most frequently used chambers shows that this is true only for one of the chambers, approximately so for another, and not true for the other three. An additional numerical simulation indicates the blood should purge the saline priming liquid within a few seconds at the beginning of the experiment.

Numerical 3D flow simulation of ultrasonic horns with attached cavitation structures and assessment of flow aggressiveness and cavitation erosion sensitive wall zones