Hydraulic Transients Research Articles

Numerical studies on applications of cavitation models in water hammer-induced cavitating flows in pipelines

Hydraulic transients caused by a fast-closing needle valve and accompanying water hammer-induced cavitation-vortex flow are numerically investigated using the shear stress transport (SST) k–ω model with a pressure–density equation considering the weak compressibility of water and the new cavitation models based on the Rayleigh–Plesset equation that takes into account the surface tension and kinematic viscosity effect on the bubble radius change rate, respectively. The predicted pressure evolutions exhibit reasonable agreement with the experimental data, and the interfacial surface tension-based model is the most accurate in predicting transient pressure evolutions, followed by the kinematic viscosity-based model and the Singhal cavitation model, based on three proposed error indices with minimum of 0.0515, 0.1337, and 0.0035. The wave speed in the upper part of the cavitation trailing edge is higher than that in the lower part, resulting in the evolution of inclination angle of the trailing edge. According to the convective transport mechanism of vorticity, the change rate of vorticity in non-cavitation regions is mainly dominated by the stretching–twisting term and the turbulent fluctuation–diffusion term. In cavitation regions, the dilatation–shrink term makes significant contributions to the change rate of vorticity, and the baroclinic-torque term mainly acts near the edge of the cavitation region.

Avoiding water hammer and other hydraulic transients

AbstractWater hammer may occur upon rapidly closing a water pipe valve. Oscillating pressure waves generated in the system may cause significant equipment damage, including joint leaks and even pipe rupture. Similar phenomena can occur when starting and stopping pumps. As various situations can cause potentially damaging pressure waves, water hammer and related phenomena are grouped as “hydraulic transients.” The phenomena are not limited to water pipes and may occur in any liquid, including condensed gases such as chlorine, ammonia, and liquefied natural gas (LNG). Hydraulic transients deserve more coverage in college courses and textbooks on process safety, while at an industrial level, the topic should be considered in design and hazard reviews. This article provides a brief introduction to the topic with a focus on mitigation methods. Available options should be evaluated during the design process rather than applied retroactively after a problem becomes evident.

On the starting and operational transients at the core of the RMB based on 2D multigroup diffusion

RMB is a multipurpose reactor to be built by CNEN. As a multipurpose facility, unlike a nuclear power plant, some operations may result in transients, whether during the movement of materials from remote irradiation, extraction of neutrons through beam holes, reactor starting up, control rod accident, etc. For that, a methodology to visualize some variables like neutron fluxes, temperatures, power, etc., was developed. In this one present some preliminary results based on multigroup diffusion theory, thermal hydraulics feedbacks, and numerical methods, systematized in the DINUCLE code. DINUCLE couples a numerical code of multigroup spatial kinetics, for calculations of the flux of neutrons in the core of reactors, with a numerical code for the analysis of thermo hydraulic transients in rods and plates, considering the refrigerant always single-phase. Use was made of a programming in VBA (Visual Basic for Applications) from Excel, for data visualization in the transient.

1D-3D coupled simulation method of hydraulic transients in ultra-long hydraulic systems based on OpenFOAM

Transients in hydraulic systems are normally simulated by one-dimensional (1D) method of characteristics (MOC). When local three-dimensional (3D) flow patterns have crucial impacts on the hydraulic system characteristics, 3D computational fluid dynamics (CFD) simulation should be incorporated. Since simulating an entire long system by the 3D CFD method is unrealistic, we have proposed several 1D-3D coupled simulation methods. However, the coupled methods are difficult to apply and popularise because most CFD codes are inaccessible and implementation details are unclear. Consequently, we utilised the open source toolbox OpenFOAM to develop a 1D-3D coupled simulation method and proved its feasibility by applying it to analysing practical problems. This paper describes the implementations of water compressibility, 1D-3D coupling, gas-liquid flow simulation, and dynamic mesh; shows their verifications of accuracy by benchmark cases; demonstrates the simulations of the closing and opening processes of a 194 km long water conveyance system. The gas-liquid flow patterns in the gate chamber and the maximum/minimum water head envelopes along the tunnels, which support the design optimisation of the project, are presented. It is shown that the method is computationally efficient and accurate, and can be applied to hydraulic transient analyses for water conveyance systems, pumped-storage power stations, etc.

Stability of mass oscillations in hydropower plants with brook intakes

The need for more energy storage in the transition to a renewable energy system leads to increasing interest in upgrading existing hydropower plants to pumped storage plants. Such upgrades result in stronger hydraulic transients in the tunnel system, including increased mass oscillation amplitudes. In this study, a generalized tunnel system with multiple brook intakes/surge tanks is analysed to assess the influence of the brook intakes on the mass oscillation stability. The variables that are assessed are the number of brook intakes, cross section area, the amount of inflow, and throttling of the brook intakes/surge tanks. The study is carried out using frequency-response analysis, where the excitation is the oscillation of the guide vane position, and the response is the product of the measured head and discharge (equivalent to the hydraulic power). The results show that brook intakes are generally beneficial for the stability of the mass oscillations. Existing hydropower plants with brook intakes may have sufficient stability margins to allow upgrade and reconstruction without the need for upgrading of the surge tank with regard to mass oscillation stability.

Implementation and Evaluation of a Complex Pumped-Storage Hydropower Plant with Four Units, Common Penstock, and Surge Tank in a Real-Time Digital Simulator

The demand for energy storage systems is rising together with the proportion of renewable energy sources (RES) in power systems. The highest capacity among the various energy storage systems in power systems is provided by pumped-storage hydropower (PSH). In this paper, the ability of the real-time digital simulator (RTDS), e.g., dSpace–SCALEXIO, to emulate a complex pumped-storage hydropower plant with four units, two common penstocks, a surge tank, and a long headrace tunnel is investigated. The RTDS is the smart brain of an advanced lab setup called power hardware in the loop (PHIL), which is an extremely safe and useful lab system for electrical power system research and testing hardware and methods under various conditions. In this research, the capability of an RTDS to emulate the behavior of a pumped-storage hydropower plant including four Francis pump-turbines, four short penstocks, two common penstocks, a surge tank, and a long headrace tunnel is evaluated. Francis pump-turbines are modelled based on the hill chart-based interpolation and waterways including penstocks and headrace tunnel are modelled based on the polynomial approximation of a hyperbolic function. Finally, the results from the RTDS are presented and discussed. According to the results of the paper, we confirm that the RTDS can accurately emulate the hydraulic, mechanical, and electrical transients of a pumped-storage hydropower plant with a complex configuration.

Free-Surface Vortices Mitigation using Anti-Vortex Plates in Dam Intakes through CFD

By recording parameters such as velocity and volume fraction by contour plots or plane, a CFD model enables to analyse flow patterns in the model, such as free-surface vortices A free-surface vortex, a common problem may indeed be observed in a variety of submerged water intakes, notably shallow basins and low head intakes. These FSVs are likely to form an air-core vortex, eventually entrapping detritus and air pockets in the water intake system and causing further vibration and damage to the downstream turbine. When paired with a high velocity, the formation of vortices in the system been known to produce hydraulic transients, which cause unwanted operation or pressure changes. The model of the 1:100 scale dam reservoir was generated, computationally meshed, and modelled in FLUENT under ANSYS 2019 R3 at two different water levels to observe the FSV formations. To mitigate those FSV formations, anti-vortex plates with two distinct plates—square and wedge—were used. From the findings square plates outperform wedge plates because square it lowers the speed of a fast-flowing fluid and reduces it into a laminar flow rather of a turbulent flow, which benefits vortex class deterioration. Data from the simulation and experimental shows a strong agreement in terms of velocity at outlet 1 from both water levels with relative errors of 3.0% and 14.1% respectively

A thermodynamically consistent model for hydraulic transients in metallic pipes undergoing elasto-viscoplastic deformations

This work presents an extension of the fluid transient analysis to include the elasto-viscoplastic behavior of the pipe material. A previously developed quasi-2D flow model is employed as a basic framework, and the pipe’s elasto-viscoplastic behavior is added by considering constitutive equations formulated via an internal variables theory. The resulting governing equations form a non-linear hyperbolic system of partial differential equations whose approximated solutions are obtained by employing the method of characteristics followed by the Newton–Raphson method. The model is thermodynamically consistent, a feature that is consistently detailed in this work to uphold the application of energy-based theoretical analyses of the phenomenon. Taking as reference the transient responses obtained from an experimental installation that simulates an accident, the present approach is found to be fully consistent and valuable for simulating fast hydraulic transients in metallic materials experiencing plastic deformation. By comparing the pressure fluctuations of models with and without elasto-viscoplasticity, more attenuation and dispersion of the pressure responses are found when such behavior is considered. The elasto-viscoplasticity effect on the pressure responses is better understood when the connection between the viscoplastic mechanisms and energy dissipation is exposed. During the fluid transient, the pipe dissipates meaningful amounts of energy that cause a significant alteration in the whole fluid–pipe dynamics. Another numerical example, based on a hypothetical water hammer flow that causes plastic deformation in the pipeline, is also addressed. Such a case puts in evidence that not only the anelastic behavior of the pipe must be taken into account but also that an accurate description of the fluid friction may be significant to portray realistically fluid transient responses. At last, an assessment of the pipe integrity by means of a scalar damage variable theory is carried out to emphasize the application prospects of the present research.

Post-extrapolation for specified time-step results without interpolation in MOC-based 1D hydraulic transients and gas release computations

The goal of the paper is to present a supplementary step called postextrapolation. When applied to the well-known method of characteristics (MOC), this assures the continuous use of the specified time steps or regular numerical grid without interpolations during computations of transients in 1D 2-phase flow in straight elastic pipes. The new method consists of two steps, the first being a typical MOC step, where the C− and C+ characteristics start from regular nodal points, allowing for the point of intersection to differ from a regular one. After defining the variables there the method transforms it corresponding to the near regular grid point, using the first derivatives contained in the original, nonlinear, governing equations, as evaluated numerically from the variables got earlier in the neighboring nodes. The procedure needs no interpolations; it deals with grid-point values only. Instead of the Courant-type stability conditions, shock-wave catching and smoothing techniques help to assure numerical stability between broad limits of parameters like the closing time of a valve and the initial gas content of the fluid. Comparison by runs with traditional codes under itemized boundary conditions and measurements on a simple TPV (tank-pipe-valve) setup show acceptable scatter.

Thermo-hydraulic coupled analysis of long-distance district heating systems based on a fully-dynamic model

The long-distance district heating pipeline (LDHP) system is a key solution for transporting the renewable heat from the suburban to urban areas. In LDHP, thermal and hydraulic characteristics are coupled. Developing effective thermo-hydraulic co-simulation method is vital to enhance LDHP’s operation performance and to enhance its flexibility. As the pressure wave travels much faster than the hot water in the district heating (DH) pipeline, the research in this regard mainly focuses on pseudo-dynamic model, in which the calculation of thermal dynamics only adopts the static hydraulic conditions. In this paper, a novel fully-dynamic model considering both the hydraulic transients and thermal dynamics is developed. The model is solved using the method of characteristic (MOC). The optimal spatial and temporal step sizes for the thermal dynamic modeling are analyzed and studied. Next, we compared our proposed model with the conventional pseudo-dynamic model on a 20 km LDHP. Results show that the simulated temperature fluctuations of the two models are close to each other, but the predicted temperature arriving at the heat exchange station is up to 28 s earlier than the pseudo-dynamic model, indicating that hydraulic transients have some influence thermal dynamics. Moreover, the fully-dynamic model has capability to predict the temperature and flow rate transport of the system in both disturbance state and steady state, which is important for the rapid and accurate analysis of the operation of LDHP.

Hydraulic Transients in Water Distribution Systems

Hydraulic transients in water distribution systems (WDS) is a field of research that has advanced greatly in the last few decades [...]

Investigation of quench flow boiling heat transfer correlations for liquid oxygen feed line chill-down with outlet contraction

Cryogenic feed lines used for ground-based liquid transfer, thermal management in space flight, as well as the one with an injector at the outlet used for supplying propellants to the cryogenic engine combustion chamber, undergo an initial transient cool-down operation before vapor free liquid supply can be established through them. Accurate prediction of the thermal and hydraulic transients during this period has critical importance in the safe operation and efficient utilization of the cryogen. In the present work, a one-dimensional non-homogeneous two-phase model is used for simulating the chill-down operation in feed lines with and without outlet contraction. For transport feed lines without outlet constriction, the obtained transient wall temperature profiles indicate good prediction when the heat transfer correlations proposed based on recent chill-down experiments are employed as constitutive relations. Subsequently, the numerical model is applied for predicting the wall temperature transients during outlet contracted LOX feed line chill-down. It was noted that built-in heat transfer correlations in thermal–hydraulic code SINDA/FLUINT need revision, especially for rewetting point and heat transfer correlation during post rewetting regimes, for accurate thermal transient predictions during end constrained feed line chill-down. Hence, recent chill-down correlations reported in the literature and the correlations employed in previous chill-down numerical models are tested for their applicability. The recent cryogenic chill-down correlations reported in the literature performed well when applied within the correlation range. However, these correlations over predict the heat transfer at a very high mass flux (order of 3412 kg/m2 s). Hence, modified correlations are proposed for film and nucleate boiling heat transfer coefficients and rewetting temperature.

Fluid transient analysis by the method of characteristics using an object-oriented simulation tool

Abstract This paper presents the development of an efficient, interactive and fully extendable computer code to analyse hydraulic transients – water hammer – using a generic object-oriented modelling and simulation (OOMS) tool to simplify and shorten the development process. OOMS tools provide continuous simulation features by means of ordinary differential equation (ODE) and differential algebraic equation (DAE) solvers, and discrete simulation features by means of event handling algorithms. Previous OOMS tool applications to simulate fluid flow in pipe networks primarily used semi-discrete methods with continuous solvers. A novel aspect of this work is the application of the method of characteristics (MOC) using the discrete simulation features of the OOMS tool. When compared to a code developed with an OOMS tool using a semi-discrete method and to commercial software that uses the MOC, the new code shows much higher accuracy and performance than the former and is similar to the latter in accuracy and calculation time. It has a graphical user interface, and its modularity makes it easily extendable with new components and algorithms. An academic version is available on github.

Does intermittent supply result in hydraulic transients? Mixed evidence from two systems

Abstract Pressure transients can cause severe damage in continuous water supply pipe networks, but little is known about pressure transients in intermittent networks. Published examples of high-frequency pressure monitoring in intermittent networks are lacking. Intermittent supply can be caused by poor network condition and is associated with delivering less water, less frequently, and with poorer quality than continuous supply. Given the frequency with which intermittent systems drain, fill, and change supply regimes, pressure transients have been hypothesized to be common and to be one mechanism by which intermittent supply further degrades network condition. We present supply start-up data from two very different intermittent systems: a low-pressure, intermittent network in Delhi, India, and a higher-pressure intermittent network in Arraiján, Panama. Across monitoring locations at both sites, we did not detect substantial pressure transients due to pipe filling. In Arraiján, pump start-ups, pump shutdowns, and pipe bursts were associated with potentially problematic transients. We conclude that pipe filling in intermittent supply does not always result in concerning pressure transients. The largest risks to pipe conditions we observed were due to pumping changes in close succession; hence, we recommend that utilities operating intermittent (and continuous) systems leave adequate dissipation time between changes in pump operation.

Hydraulic Oscillation Analysis of the Hydropower Station with an Equivalent Circuit-Based Hydraulic Impedance Scheme

The concept of hydraulic impedance is widely used for periodic or oscillating flow scenarios in applied hydraulic transients. This paper proposes an equivalent circuit-based discrete hydraulic impedance model for the hydraulic system of the power station. The equivalent hydraulic circuit is established by an analogy between the water hammer wave propagation in pressurized pipes and the electromagnetic wave propagation in conductors. The discrete hydraulic impedance at a given location is obtained by calculating the total specific impedance of the series and parallel circuits from one end to the other. In addition, the numerical solution process to obtain the natural frequencies of the system via the proposed model is designed in detail. Furthermore, oscillation characteristics of the pipelines in the hydropower station are investigated. The variation trends of the decay coefficients of different orders of oscillation of the system and the influence of turbine impedance on the oscillation frequencies are discussed, respectively.

Contribution for the roadmap of hydraulic short circuit implementation: Case of Grand-Maison pumped storage power plant

The objectives of the 2050 energy policy based on the decarbonization of the electric power networks generate drastic changes for grid balancing with a massive integration of non-dispatchable Renewable Energy Sources. Hydroelectric power plants already significantly support electricity power system flexibility with innovative solutions such as variable speed units, fast frequency control, fast generating to pumping modes transition, high ramping rate, inertia emulation, etc.For pumped storage power plants (PSP), a quick solution to increase the flexibility without large investment is to operate the power plant in hydraulic short circuit (HSC) mode. This technological solution is simple to implement, but requires an in-depth study of various technical aspects among which the hydraulic transients of the new operating modes is of high importance from the installation’s safety perspective. In the framework of XFLEX HYDRO H2020 European research project, the exploitation of this solution is under implementation at Grand-Maison PSP. Located in the French Alps, Grand-Maison PSP is equipped with 8 reversible multi-stage Francis pump-turbines and 4 Pelton turbines, for a total installed capacity of 1800MW, thus being the largest PSP in Europe and one of the major PSP in the world. The waterway includes a headrace tunnel, a headrace surge tank, 3 parallel penstocks feeding the 12 units operated under a maximum gross head of 955mWC.In this paper, after a description of the general HSC considerations, the 1D model of the Grand Maison PSP and the related validation are presented. Finally, the most critical load cases in HSC operation are described to identify the potential hydraulic transient issues, such as extreme water levels in the upstream surge tank, maximum static pressure along the pressure shaft and minimum static pressure along the tunnels. The analysis performed for Grand Maison PSP is a contribution to the roadmap for the implementation of HSC operation in pumped storage power plant and will be made available as a public deliverable of the XFLEX HYDRO H2020 European research project.

Comprehensive application analyses of elastic models and viscoelastic models in transient flows in polymeric pipelines

Abstract Based on the MOC (Method of Characteristics), the applicabilities of both the elastic models of the SF (Steady Friction), the CB-UF (Convolution-Based Unsteady Friction) and the MIAB-UF (Modified Instantaneous Acceleration-Based UF), and the viscoelastic models of the SF-VE (SF-Viscoelasticity), the CB-UF-VE (Convolution-Based Unsteady Friction-Viscoelasticity) and the MIAB-UF-VE (Modified Instantaneous Acceleration-Based UF-Viscoelasticity), are investigated for hydraulic transients induced by a downstream rapid valve closure in a polymeric pipeline. The predicted results by the elastic models are very different from the experimental data, whereas the predicted pressure peaks and phases by the viscoelastic models agree well with the experimental data because considering the pipe–wall creep effect, the CB-UF-VE and the MIAB-UF-VE generate better results than the SF-VE. The creep effects near the first pressure peak are captured well by the viscoelastic models. The analyses of the contributions of different factors to pressure attenuation show that for transient flows in polymeric pipelines, the effect of VE is greater than that of the CB-UF and MIAB-UF. The spectrum analyses show that the pressure amplitudes and harmonic frequencies by the elastic models match badly with the experimental data, whereas those by the viscoelastic models match well with the experimental data. The harmonic frequencies by the MIAB-UF-VE are the best, followed by the CB-UF-VE, and are worst by the SF-VE.

Stress-informed control of medium- and high-head hydropower plants to reduce penstock fatigue

The displacement of conventional generation in favor of stochastic renewable requires increasing regulation duties from the remaining dispatchable resources. In high- and medium-head hydropower plants (HPPs), providing regulation services to the grid and frequently changing the plant's set-point causes water hammer, which engenders pressure and stress transients within the pressurized conduits, especially the penstock, damaging it in the long run. This paper proposes a model predictive control (MPC) that explicitly models the hydraulic transients within the penstock. It achieves to reduce the mechanical loads on the penstock wall, and, consequently, fatigue effectively. Thanks to a suitable linearization of the plant model, the optimization problem underlying the MPC scheme is convex and can be solved with off-the-shelf optimization libraries. The performance of the proposed controller is tested with numerical simulations on a 230MW medium-head HPP with Francis turbine providing primary frequency control. Simulation results show substantially reduced penstock fatigue, existing approaches outperformed, and problem resolution times compatible with real-time control requirements.

Chlorine decay and disinfection by-products transformation under booster chlorination conditions: A pilot-scale study

Booster chlorination was usually employed in water distribution systems with a long hydraulic retention time. The free chlorine decay and disinfection by-products (DBPs) transformation under booster chlorination conditions were investigated within a pilot-scale water distribution system (WDS). Compared with the initial chlorination in water plants, the loss of chlorine was relatively slow and could be described with first-order kinetic model. The rate of chlorine decay and the generation of DBPs in WDS were much greater than those in beaker. High flow rate and the hydraulic transients both promoted chlorine decay and DBPs formation, especially for dichloroacetonitrile (DCAN). The formation of trihalomethanes (THMs) and haloacetic acids (HAAs) was higher in the ductile iron pipe than in the steel pipe. After booster chlorination, THMs, HAAs, and DCAN all climbed up and then declined continuously, but the peak times were different during the reaction process. The results showed the generation period of DBPs followed the order: THMs (27 h) > HAAs (22 h) > DCAN (5 h). DCAN was not stable in WDS and could be decomposed for a long hydraulic retention time (HRT). The decrease of dichloroacetic acid (DCAA) and increase of trichloroacetic acid (TCAA) indicated that DCAA may turn into TCAA. Linear relationships between the free chlorine demand (FCD) and the generation of THMs that considered both buck water and the pipe wall, as well as the different hydraulic conditions, were established to predict the formation of DBPs in WDS after booster chlorination.

Dimensionless impedance method for the transient response of pressurized pipeline system

Modeling an unsteady flow is an important problem in pressurized pipeline systems. Depending on the dimensions and properties of the fluid and pipeline material, the generation of hydraulic transient issues and their spatiotemporal variation patterns can be different. To address the water hammer problem without scale issues in pipeline systems, this study developed a dimensionless impedance method (DIM). The existing partial differential equations of mass, momentum, and energy conservation were converted into corresponding dimensionless equations. The solutions of proposed equations were analytically developed in terms of several operators in the dimensionless frequency domain. A dimensionless water hammer number was introduced to address the similarity in hydraulic transients between different systems. Both numerical and experimental comparisons showed that the DIM well matched the existing methods. Based on comprehensive simulations of water hammer phenomena for ranges of water hammer numbers, a vectorized chart was formulated for a reservoir pipeline valve system. Both simulations and calibrations of the experimental hydraulic transients achieved well-matched results between the DIM computations and chart interpolation results. The proposed DIM method can provide substantial efficiency for both simulation and calibration, as well as generality in transient solutions for pipeline systems.