The paper critically reviews the verification and validation (VV) techniques applied to investigate hydraulic turbines. Although there are well-established standards such as AIAA G-077-1998 and ERCOFTAC guide for turbulence modelling, majority of studies conducted on the turbines are lacking of systematic VV. Results without proper VV serve no purpose for safe and reliable designs of turbines. Available standards/guide are for general-purpose industrial applications and have limited scope. Customized VV procedure for the turbine applications is essential to create trust on the obtained results. The present review discusses how available standards/guide can be used to determine uncertainty/error and to demonstrate the credibility of results. The review includes several aspects of VV such as effect of discretization schemes, iterative error, convergence criteria, time-step sizing and impact of passage modeling approaches on the results. Further, how numerical results mislead the user and its implications are addressed. In the last, open questions on turbine modelling and recommendations on prospective numerical studies are discussed.
The bi-directional impulse turbine and the bi-directional flow collector for tidal energy conversion is investigated in this paper. The bi-directional impulse turbine with fixed guide vanes is adopted because the turbine has a high efficiency and an advantage of maintenance. The turbine characteristics of the combined system of impulse turbine and collector are investigated experimentally by using the water tunnel. This system is proved to produce the power by a tidal flow experimentally. Three types of flow collector A, B and C are investigated, where the maximum radius of collector A is smaller than the ones of collector B and C. The velocity ratio of collector A is much smaller than the one for the cases of collector B and collector C, and the output power of collector A is very small compared to the other collectors. Nevertheless, the effect of flow collector is large because velocity ratio of collector A is much larger than the one without collector. Among three cases of 0.5, 0.6 and 0.7 of hub-to-tip ratio, the difference of turbine performance is not so large, but it is observed that the case for0.5 of hub-to-tip ratio has inferior performance and the case for 0.6 of hub-to-tip ratio has the performance among them. Further, the comparison of circulating water tank test and towing tank test was done to show the effect of choking ratio of cross sectional area of channel.
The diameter of the ventricular assist pump and cooling pump for electrical devices like servers is about a dozen millimetres and these kinds of pumps belong to the mini pump. The internal flow condition of the mini centrifugal pump has to be clarified because there are so many types of mini pumps according to special specifications and shapes. Therefore, in this research, an open impeller was adopted as a mini centrifugal pump with 55mm impeller diameter in consideration of reduction in impeller manufacturing cost and its internal flow was investigated. Since the outlet angle of the test impeller is large, the maximum efficiency flow rate moves to a large flow rate and the relatively high head and efficiency were obtained in large flow rate region. In the present paper, the velocity and vorticity distribution near the volute tongue are shown and the internal flow in the mini centrifugal pump is clarified based on the PIV measurement results.
According to the requirement of IEC60041, the paper introduces various turbine efficiency measurement methods, and analyzes their feature and difficulties, then gives a detailed introduction for the principle and requirement of thermodynamic test method. With an example of turbine efficiency test implemented in Costa Rica, the paper gives a complete test procedure and data calculation for thermodynamic method. Finally, a comparison and verification among thermodynamic method, ultrasonic method and Winter-Kennedy method has been made, the result shows that it has great advantage and effectivity for thermodynamic method when using in flow measurement of high water head turbine.
It is requested that the hydroturbine be a small size and have high performance. Therefore, we adopted contra-rotating rotors, which can be expected to achieve radial compactification and high performance. However, when the rotors become smaller, the output power of conventional contra-rotating rotors, which are composed of axial flow rotors, is low. In order to achieve small size, high performance and high output power simultaneously, we propose a new type of contra-rotating rotors, which are composed of a hybrid rotor and a centrifugal rotor. In the present paper, we focus on deflection angle of the hydroturbine composed of these rotors as a first step of this research, and investigate performance by the numerical analysis. As a result, we clarified that there is a possibility that even a negative deflection angle functions as a hydroturbine unlike the conventional contra-rotating rotors and leads to best performance.
Liquid droplet impingement erosion occurs at the elbows in steam pipes where droplets impinge at high speed. In the actual pipe wall which numerous droplets always impinge, it is predicted that both a liquid film exists on a pipe wall surface and, on the other hand, this surface is also eroded by repeated droplet impingement. Therefore, the liquid film and roughness on the material surfaces are considered to exist mixed on the actual impinged point of droplets. In this study, by using an in-house fluid/material two-way coupled numerical method that considers reflection and transmission on the fluid/material interface, the numerical analysis of the phenomenon of liquid droplet impingement on a pitted surface with a water pool is conducted. From the analysis results, the impinged pressure at the moment of impingement is reduced by a water pool. However, as the cavitation bubbles are generated in the bottom and top of the droplet after the impingement and then the cavitation bubble of the bottom side collapses, the collapse pressure which greatly exceeds the pressure of the droplet impingement occurs, and the equivalent stress also increases greatly there. Therefore, this analysis result may indicate one reason why the erosion progresses deeply at the pit part in an actual pipe wall thinning.
Macroscopic behaviors of the frequencies of deep-surges in systems of multi-stage compressors and flowpaths could be evaluated very roughly by a constant value of an effective reduced surge frequency fRsmeff, as described in Part I ofthe present study. However, some deviations from the rough average value could often appear, depending on the situations of the concerned systems. In the present study, the causes of the deviations are made clear by examinations on the numerical results. The deviations are concerned with the following three conditions; (1) a multi-stage compressor having a sufficiently high design pressure-ratio, (2) changes in the compressor speeds through near the design speed, and (3) a sufficiently short delivery plenum. In the situations affected by combined effects of the above three, the compressor could be located in a zone of the surge flow mode where the amplitude could vary noticeably in the axial direction. When the stall-responsible stages relocate from the front stages to the rear ones within the zone during the speed increase through near the design speed, the small difference in the relative location of the stall-responsible stages in the surge mode could sensitively affect the surge behaviors and lower the surge frequency. In the situations, incomplete stall recoveries, and sometimes subharmonic surges, tend to occur additionally, elongating the deep-surge periods and lowering the surge frequencies. It should be emphasized from the aspects of numerical-experimental observations that very local events as such could happen to have significant effects on the surge phenomena.
Essential causes of stall stagnations in systems of a compressor and flowpaths were examined with emphasis on axially travelling wave conditions of small disturbance oscillations in a simplified model of the system. The situation could be measured in terms of both the total phase differences across the whole flowpath and the phase differences experienced by the fluid particles in passing through the whole flowpath. In the situation where the total phase difference is larger, the average accelerations of the fluid mass tend to be smaller. On the other hand, when the phase difference of the passing fluid particles is smaller, the fluid particles tend to be excited by two frequencies separated much by modulations similar to Doppler shift effect. The situation tends to disorder the intrinsically synchronous oscillations of the flow and to suppress the fluid motion amplitudes. Both effects tend to result in the stall stagnations in the system. The behaviors of the phase-related parameters were compared with those of the stall-stagnation boundaries predicted by simulations on single-stage compressors. From the fairly good coincidences between both behaviors in the neighborhood of stagnation boundaries, the phase phenomena could be considered as one of possible candidates for the essential cause of the stall stagnations.
In the many industries centrifugal pumps consume a lot of energy. So the efficiency optimization of these pumps is so important. In this study shape of impeller of centrifugal pump was optimized to increase efficiency and head. The evolutionary algorithm based on modified artificial neural network (ANN), particle swarm optimization (PSO) and validated CFD data were used to optimized shape of centrifugal impeller. The pump experimentally investigated in test rig and measured data were used to verify the numerical results. Finally, to verify optimization results the complete numerical characteristic curves of the initial pump were compared with the optimized pump. The numerical results of optimized pump depict that efficiency improved 3.2% and head increase 5.52(m).
In the present study, the mechanism of the vortex downhole tool (VDT) to improve gas well production efficiency is investigated and the optimal geometric parameter combination of VDT is obtained by an orthogonal experiment. Results show that VDT has the potential to reduce the critical gas velocity only when its helix angle is less than 63°, and the theoretical reduction will not exceed 21.5%, compared to the Turner model. Under the action of VDT, the friction factor of the gas-liquid two-phase flow is reduced, as well as the total flow pressure drop. The field testing results show that the proposed structure optimization method of VDT is feasible and the optimal VDT has greatly improved the production conditions of well A.
Investigation of the thermodynamic effect of tip-leakage-vortex cavitation on a two-dimensional hydrofoil designed with a tip clearance for hot water is reported herein. During unsteady cavitation of water maintained at 90 °C (hereinafter referred to as hot water), the observed decrease in temperature in the tip-leakage region was greater compared to that in the mid-span region. In contrast, during supercavitation of hot water, the situation was reversed (i.e., the temperature decrease in the tip-leakage region was smaller than that in the mid-span region). The cavitation-pattern map for hot water was observed to be largely similar to that for water maintained at 30 °C (hereinafter referred to as tepid water). On the other hand, the amplitude of the dominant frequencies of unsteady cavitation of the hot water was observed to be greater than that of the cavitation of tepid water owing to the sudden collapse of cloud cavitation.
Rotordynamic fluid forces and a mechanism of their occurrence were examined in experiments and computations for
a closed type centrifugal impeller in whirling motion. The rotordynamic fluid force on a front shroud was a main
component of the rotordynamic fluid force. In a clearance between the front shroud and a casing, velocity disturbances
were caused by an eccentricity of the impeller and a squeeze effect due to the whirling motion of the impeller. Appling
the Bernoulli equation to disturbed swirling flow in the clearance in a relative coordinate fixed on the whirling motion,
the pressure disturbance which generated the rotordynamic fluid force could be reasonably explained.
This paper presents results of the numerical analysis of two Pelton turbines: a 6-jet turbine for middle head and a 2-jet turbine for high head. For the 6-jet turbine losses in manifold, quality of the jets and turbine efficiency were predicted and validated with the experimental data. Additional improvement of accuracy of efficiency prediction was obtained with cavitation modelling. It was also checked that there was no interaction between the evacuating water sheets and the incoming jets. For a 2-jet turbine cavitation prediction was done. Small vapour cavity at the inner side and a larger one at the back side of the bucket were observed. Detailed analysis of cavitation and condensation processes showed that the conditions for cavitation pitting were not fulfilled.
Many consider the Pelton turbine a mature technology, nevertheless the advent of Computational Fluid Dynamics (CFD) in recent decades has been a key driver in the continued design development. Impulse turbine casings play a very important role and experience dictates that the efficiency of a Pelton turbine is closely dependent on the success of keeping vagrant spray water away from the turbine runner and the water jet. Despite this overarching purpose, there is no standard design guidelines and casing styles vary from manufacturer to manufacturer, often incorporating a considerable number of shrouds and baffles to direct the flow of water into the tailrace with minimal interference with the aforementioned. The present work incorporates the Reynolds-averaged Navier Stokes (RANS) k-ɛ turbulence model and a two-phase Volume of Fluid (VOF) model, using the ANSYS® FLUENT® code to simulate the casing flow in a 2-jet horizontal axis Pelton turbine. The results of the simulation of two casing configurations are compared against flow visualisations and measurements obtained from a model established at the National Technical University of Athens. Further investigations were carried out in order to compare the absolute difference between the numerical runner efficiency and the experimental efficiency. In doing so, the various losses that occur during operation of the turbine can be appraised and a prediction of casing losses can be made. Firstly, the mechanical losses of the test rig are estimated to determine the experimental hydraulic efficiency. Following this, the numerical efficiency of the runner can then be ascertained by considering the upstream pipework losses and the aforementioned runner simulations, which are combined with previously published results of the 3D velocity profiles obtained from simulating the injectors. The results indicate that out of all of the experimental cases tested, in the best case scenario the casing losses can be approximated to be negligible and in the worst case scenario ≈3%.
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 apparatus 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 copu-la is a highly asymmetric Tawn copula, in the class of so-called extreme value copulas. Almost no impacts were ob-served 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 combination of the spatio-temporal processes, with the mark’s distributions combine to produce the stochastic impact model for cavitation erosion in the vibratory apparatus.
In many industrial processes such as natural gas processing, seawater desalination et al, there is a large amount of liquid waste pressure energy, which can be converted into electric or mechanical energy by a micro-hydraulic turbine. But its efficiency and stability will deteriorate when the working fluid contains gas. Aiming at improving the hydraulic efficiency of the micro-turbine, the three-dimensional transient two-phase flow model which adopts the shear-stress-transport (SST) model as turbulence model and Eulerian-Eulerian model as two-phase flow model with the commercial code ANSYS-CFX was established and solved to study the two-phase flow characteristics of the micro-hydraulic turbine with fixed guide vane openings under different gas volume fraction (GVF). The numerical simulation results show that the power and efficiency of the turbine decrease by 24% and 21% respectively when the inlet GVF is 0.20. The area of low pressure in each impeller passage increases with the inlet GVF, and the pressure difference between the working and suction surface of the blade decreases. The GVF distribution in impeller passage is asymmetrical, which is caused by the gas-liquid separation in the volute. At the left lower part of impeller passage, the gas accumulates at the suction surface of the blades under the Coriolis force and its GVF reaches 90%. The asymmetrical GVF distribution will result in the asymmetrical pressure distribution in the impeller passage, which leads to the imbalance force on the blades. Moreover, the asymmetrical GVF distribution will reinforce the positive incidence at the lower part of impeller passage, and weaken it at the upper right impeller passage. The volute should be redesigned to ensure the uniform gas distribution at the inlet of each impeller passage.
A computational methodology to predict the cavitation phenomena appearance and evolution in a 5-blades Kaplan turbine scale model was developed. Two different inlet boundary conditions have been tested in both non-cavitating and cavitating regimes: the classical one, the mass flow rate and the new one, the total pressure. The best results were obtained applying a constant total pressure on the inlet. The torque and efficiency drop curves were well-predicted with the proposed calculation methodology and the numerical cavitation structures agreed with experimental observations. Indeed, this new inlet boundary condition allows to keep the machine head constant during the cavitation drop, as in experiments. Unsteady simulations are under investigation to improve the prediction and the analyses of more developed cavitating regimes.
In this study, the characterization of the flow through a safety relief valve (SRV) is performed in presence of cavitation and gas desorption. For this purpose, a transparent safety relief valve model is used on an experimental facility in which the flow conditions (mass flow rate, fluid temperature, and pressure upstream and downstream the valve) are accurately monitored. For six different valve openings, the characteristic curves of the valve are measured while flow visualization is performed on the transparent model. The results show that choked flow conditions are reached for the six valve openings used in this study, and with a remarkable repeatability. In order to take into consideration the gas saturation level of the working fluid, the vacuum system used to adjust the pressure downstream the valve is also used for gas desorption by storing the liquid under vacuum conditions, a process known as vacuum degasification. In saturated liquids the evolved gas bubbles modify the flow properties, such as the speed of sound, and this may have an influence in the cavitation inception and the occurrence of choked flow. This study proves that gas saturated water in standard conditions (atmospheric pressure and 293 K) has the same behavior that fully deaerated water, both under the same cavitating conditions. It is thus not necessary to take the saturation level of the liquid into account when in standard conditions.