Hydraulic Transient Simulation Research Articles

Thermodynamic behaviour and heat transfer characteristics of air vessels in water delivery systems

Air vessels can effectively prevent water hammer caused by pump failure; however, the thermodynamic behaviour and accurate model of air in air vessels are rarely discussed, and an empirical polytropic index of 1.20 is commonly used for air vessel design. This study calculated the polytropic index based on the improved rational heat transfer (RHT) model using the multiphase volume of fluid (VOF) and k-ε turbulence model. The influence of dynamic airflow and heat transfer in the air vessel on the water delivery system pressure was studied. The effects of inlet and outlet flows, initial pressure, and initial air ratio on heat transfer were investigated. The polytropic indices of the expansion and compression processes in this study were between 1.10 and 1.40 and between 1.05 and 1.40, respectively. Curve fitting was applied to analyse the polytropic indices and substituted into the method of characteristics (MOC) for air vessel design. A hydraulic transient simulation of a practical water delivery project was conducted to compare the variable and fixed polytropic indices. The results showed that variable polytropic indices can better describe the dynamic processes of air. Using a fixed index of 1.20 for a low flow rate is risky. By contrast, the system can be optimised at high flow rates.

Hydraulic Transient Simulation of Pipeline-Open Channel Coupling Systems and Its Applications in Hydropower Stations

Hydraulic systems may involve both pipelines and open channels, which challenges the hydraulic transient analysis. In this paper, a method of characteristics (MOC)-finite volume method (FVM) coupling method has been developed with the pipeline modelled using the MOC and the open channel modelled using the FVM. The coupling boundaries between these two simulation regions are developed based on Riemann invariants. The simulated parameters can be transmitted from the MOC region to the FVM region and in the reverse direction through the coupling boundaries. Validation of the method is conducted on a simple tank-pipe system by comparing the simulated result using 3D computational fluid dynamics (CFD) analysis. The new method is then applied to a prototype hydropower station with a sand basin located between the upstream reservoir and the turbines. The sand basin is modelled as an open channel coupled with the pipes in the system. The transient processes are also simulated by modelling the sand basin as a surge tank. The comparison with the results by the MOC-FVM coupling method shows the new coupling method can provide more reliable and accurate results. This is because the flow velocity in the horizontal direction in the sand basin is considered in the coupling method but neglected when the sand basin is modelled as a surge tank in the MOC.

A hydraulic simulation-optimization model of the joint operation of multiple devices in long-distance water diversion systems under the pumps shutdown process using a parallel NSGA-II approach

Abstract The operation of a long-distance water diversion system in the transient process is a rather complicated problem requiring the joint operation of multiple devices. In this study, the joint operation of multiple hydraulic devices in the pumps shutdown process is expressed as a multi-objective optimization problem, and the hydraulic simulation-optimization model is proposed. The model is a bi-level framework, where the optimization model comprehensively considering various safety risks and efficiency through three objective functions is coupled with the MOC-based hydraulic transient simulation model. The parallel NSGA-II approach is proposed to solve the model. Besides, a process for effectively handling the constraints of the joint optimal operation of multiple hydraulic devices is proposed. Finally, the proposed model and approach are applied to a real long-distance water diversion project. The results show that the proposed model can find a set of feasible Pareto front solutions. The parallel approach greatly improves the computational efficiency. For the Pareto front schemes, the hydraulic devices are adjusted less frequently and the total regulation time is only 1/8.92–1/11.49 of that of the current operation scheme. Thus, this study provides an effective approach to formulate the joint operation scheme of multiple devices of long-distance water diversion systems.

Sensitivity Analysis of Hydraulic Transient Simulations Based on the MOC in the Gravity Flow

The purpose of this study was to evaluate the sensitivity of input parameters to output results when using the method of characteristics (MOC) for hydraulic transient simulations. Based on a gravity flow water delivery project, we selected six main parameters that affect the hydraulic transient simulation and selected maximum pressure as the output parameter in order to perform a parameter sensitivity analysis. The Morris sensitivity analysis (Morris) and the partial rank correlation coefficient method based on Latin hypercube sampling (LHS-PRCC) were both adopted. The results show that the sensitivity of each parameter is the same except for the friction factor. The flow rate and Young’s modulus are positively correlated with the maximum pressure, whereas the pipe diameter, valve closing time, and wall thickness are negatively correlated. It is discussed that the variability of the friction factor comes from the function of the flow and pressure regulating valve. When other conditions of the gravity flow project remain unchanged, the maximum pressure increases with the increase in the friction factor. The flow rate, pipe diameter, and valve closing time are the key parameters that affect the model. Meanwhile, Morris and LHS-PRCC proved to be effective methods for evaluating parameter sensitivity in hydraulic transient simulations.

Hydraulic Transients in Viscoelastic Pipeline System with Sudden Cross-Section Changes

It is well known that the water hammer phenomenon can lead to pipeline system failures. For this reason, there is an increased need for simulation of hydraulic transients. High-density polyethylene (HDPE) pipes are commonly used in various pressurised pipeline systems. Most studies have only focused on water hammer events in a single pipe. However, typical fluid distribution networks are composed of serially connected pipes with various inner diameters. The present paper aims to investigate the influence of sudden cross-section changes in an HDPE pipeline system on pressure oscillations during the water hammer phenomenon. Numerical and experimental studies have been conducted. In order to include the viscoelastic behaviour of the HDPE pipe wall, the generalised Kelvin–Voigt model was introduced into the continuity equation. Transient equations were numerically solved using the explicit MacCormack method. A numerical model that involves assigning two values of flow velocity to the connection node was used. The aim of the conducted experiments was to record pressure changes downstream of the pipeline system during valve-induced water hammer. In order to validate the numerical model, the simulation results were compared with experimental data. A satisfactory compliance between the results of the numerical calculations and laboratory data was obtained.

Numerical simulation of hydraulic transients in hydropower station with complicated water conveyance system

With the construction of Jinping II hydropower station with extra-large diversion tunnel, Baihetan hydropower station with complicated tailwater system, Yangfanggou hydropower station with complicated surge chamber, High requirements are put forward for the calculation and analysis of hydraulic transient process. In the water transmission and power generation system design of these projects, the following technical problems was solved by Power China Huadong Engineering Corporation Limited: simulation of variable impedance coefficient of differential surge chamber, interface tracking analysis of free-surface-pressure flow system, narrow upper chamber open channel effect and simulation of trigeminy surge chamber. The solutions to those technical problems and were summarized, and the hydraulic transients simulation and calculation software of complex waterway system - Hysim was developed. Based on the field test of Jinping II Hydropower Station, Baihetan hydropower station and Yangfanggou hydropower station, the simulation calculations were carried out. The accuracy of the hydraulic transients simulation software is verified by comparing the inversion results with the test values.

Turbine mode start-up simulation of a FSFC variable speed pump-turbine prototype – Part I: 1D simulation

Variable speed hydroelectric units equipped with full size frequency converter (FSFC) offer high operational flexibility enabling fast operating point transitions which increase grid regulation capacities. The XFLEX HYDRO H2020 European research project aims to demonstrate flexibility of such technology at prototype scale. The Z’Mutt pumping station, part of the Grande Dixence hydroelectric scheme located in Switzerland, is one of the demonstrators focused on the FSFC technology with a new 5 MW reversible Francis pump-turbine which will be commissioned in 2021. This paper, divided in two parts, aims to simulate the turbine mode fast start-up sequence made possible with the use of a FSFC and to assess the unit damage by means of 1D and 3D CFD simulations. The part I of this paper presents the 1D hydraulic transient simulation results of start-up sequences of unit U5 considering both conventional fixed speed technology and variable speed technology. The time evolution of the unit’s operating point is used as input data for 3D CFD simulations of part II, aiming to assess the impeller damage. Different control strategies to use the FSFC for turbine mode start-up sequence are analysed. Advantages and limits of each strategy are discussed, and recommendation is made for the Z’Mutt prototype demonstrator.

Research and Application on Numerical Simulation of Hydraulic Transients in Complex Water Conveyance System

Accurate simulation of hydraulic transients is crucial and complicated in the design of hydropower station and pumped storage power station. In this paper, key technologies for hydraulic transient simulation calculation in complex water conveyance system were studied. The structural matrix method with advantages of high accuracy and fast speed for numerical calculation was introduced. Complicated elements in the conveyance system were refined, such as high precision simulation of differential surge chamber, simulation of long and narrow upper chamber, new interpolation method for characteristic curve of pumped storage unit. Based on Jinping II hydropower station with special long headrace system and differential surge chamber, Baihetan hydropower station with complex tailrace system and large surge chamber and Xianyou pumped storage power station with complex unit characteristics and various working conditions, the application of the simulation software HYSIM in field test and inversion calculation was illustrated. The paper providing references for the design of similar projects.

Hydraulic transient modeling and analysis of the district heating network

Abstract The hydraulic transients are obvious during the operation and adjustment of large-scale district heating (DH) network. Hydraulic transient modeling and simulation methods can provide basis for efficient operation of the DH network. In this paper, two hydraulic transient models, the distributed parameter model (DPM) and lumped parameter model (LPM), were established for DH network. Efficient numerical algorithms were presented for the two models. The proposed DPM and LPM were both applied to a DH network to simulate and analyze the network hydraulic transients caused by the valve and pump operations. Results indicate that with the increase of the distance from the adjustment location, the transient time for the flow rate and pressure gets longer. As the adjustment duration decreases, the transient time increases. Comparison of the simulation performances between the two models indicates that the DPM can capture the high-frequency hydraulic transients subtly. With the decrease of adjustment duration, more severe high-frequency hydraulic transients will occur. However, only the average hydraulic fluctuations can be described by LPM. The computation time of LPM is shorter than DPM because of the vectorizable calculation procedure and the less data to record, since the LPM neglects the pressure waves and the fluid compressibility.

Hydraulic transient simulation of primary cooling circuit in Egyptian second research reactor

Hydraulic transient is a flow situation where the velocity of the flow and the pressure change rapidly with time in pipes networks filled with fluid. Water hammer is one of the consequences that may occur accidentally in the reactors cooling system. The pressure level can exceed the limits given by the manufacturer, and it leads to the failure of the pipes integrity or nuclear fuel elements fixation in the core. Cooling system involves many components, and when a flow control component changes abruptly its status (for example, a valve closing or pump trip), it causes rapid changes in velocity and propagates pressure pulses in short time. The cooling system is equipped with flapper valve and flywheel which overcomes the transient issues. A pump or check valve may be stopped or closed abruptly and may cause bad impacts on the system which is simulated with soft trip using flywheel and without flywheel in case of its failure for any accidental reasons. In addition the abrupt valve closure is deemed as a major issue in the system if occurred. AFT Impulse 5.0 software is used to simulate and study the transient of each case. The simulation appears that the reactor cooling system may suffer from surge that may occur during the sudden closure of check valve but in other hand it can be avoided by regular maintenance of flapper valve to open in timely manner.

Dynamic Closing Modelling of the Penstock Protection Valve for Pipe Burst Simulations

The penstock protection valve equipped with downstream air valve is a safety component aiming to cut off the discharge in case of pipe burst. The hydraulic transient simulation of pipe burst scenario is challenging since the closing time results from the system dynamics, air is admitted through the downstream air valve and cavitation may occur in the penstock. To perform such simulation, a dynamic model of the penstock protection valve is developed and presented in this paper. Closing time, maximum flow rate reached during the pipe burst or maximum oil pressure in the cylinder of servomotor during the non-linear closing law are results derived with this model. Moreover, influence of cavitation in the valve and in the penstock are considered in the model. It is showed that simulation of pipe burst should consider i) cavitation model in the penstock protection valve, ii) water column separation in the penstock and iii) air admission by the air valves located downstream to the valve to obtain realistic results.

Hydraulic transient simulation and characteristic investigation on Kaplan type hydroelectric power Plant with open channel

Kaplan type of turbines are meant for low head hydro development and sometimes are installed in systems with long open channels. Transient flow analysis in Kaplan hydroelectric power plants (HPP) is one of the most important issues for the prevention of undesirable pressure fluctuations in waterways. To keep efficient operation of a Kaplan turbine, the wicket gates and the runner blades position should be regulated as water head changed. Therefore, the hydraulic transients of Kaplan type HPP with open channel is more complex, exhibiting unique hydraulic characteristic. Based on the transient flow theory, both implicit difference method and method of characteristic are introduced to establish the mathematic model of hydraulic transients calculation for Kaplan type HPP with open channel. Then the fluid transients due to load rejection is simulated to investigate the hydraulic behaviors. The results show that different blades movements give different hydraulic behaviors. If the blades are not involved in the regulation, the hydraulic behavior is exactly the same as that of Francis turbines. If the blades adopt closure mode, the maximum spiral case pressure will slightly increase and the maximum rotating speed rising rate will obviously decrease with the increase of closing time. While the blades adopt opening mode, it will give the opposite hydraulic behavior compared to closure mode. Opening of the runner blades during the turbine shutdown can improve over-speed performance and worsen the spiral case pressure, and vice versa. Additionally, taking open channel into consideration, the influence on hydraulic characteristic is enlarged, while the length of the open channel has little effect on it.

Multi-Agent Simulation of Hydraulic Transient Equations in Pressurized Systems

AbstractComputational modeling pervades virtually every industrial process. By using numerical representations of the behavior of elements that constitute a system it is possible to obtain efficient and safe designs. Moreover, system operation can be better defined by using such models, thus enabling greater reliability and control. In this paper the use of agents to solve the equations describing fast transients in water networks is investigated. As the simulation of hydraulic transients in pressurized systems is a naturally distributed problem, the authors argue that a multi-agent based system is very suitable for the solution of this complex engineering phenomenon. A hybrid solution is built by deploying agents to work with sets of equations describing hydraulic transient behavior in pipeline systems. The details necessary to assemble a complete and lubricated machine to model the complex phenomenon of hydraulic transients in pressurized systems are described. This research develops a platform that con...

Innovative Closure Law for Pump-Turbines and Field Test Verification

Based on numerical simulation of hydraulic transients in a pump storage power station (PSP), different closure laws are investigated for use when a high-head pump-turbine load rejection occurs. According to the characteristics of the pump-turbine and main inlet valve (MIV), a novel closure law, combining closure of wicket gates (WG) and MIV, is presented. The proposed closure law can reduce the effects of S-shaped characteristics of a pump-turbine, and the maximum pressure in the spiral case, the maximum rotating speed of the runner, and the minimum pressure in the draft tube during the transient process can be mitigated. A simultaneous load rejection test on a double unit is conducted, and the numerical predictions agree well with the field test data.

Parameter Identification in Pipeline Networks: Transient-Based Expectation-Maximization Approach for Systems Containing Unknown Boundary Conditions

The simulation of hydraulic transients within fluid line networks is important for many applications (for example, water hammer analysis within distribution networks). However, in many instances, modeling efforts are impeded by the fact that the pipeline parameters are either unknown or can vary significantly from their assumed design values. Consequently, research efforts have focused on the development of parameter identification techniques, mapping from measured transient data to pipeline parameter estimates. A limitation of previous works has been the need for systems to have all boundary conditions either measured or known (e.g., transient pressure measurements or reservoir boundary conditions). This paper aims to relax this requirement and presents a parameter identification method for fluid line networks based on transient-state measurements of the hydraulic state variables of pressure and flow, in the presence of unmeasured and unknown boundary conditions. Utilizing a Laplace-domain admittance matrix representation of the system, the contribution to the hydraulic system dynamics from the measured and unmeasured state variables (i.e., boundary conditions) is made explicit. This model is then used as the basis for the development of a parameter estimation methodology based on the expectation-maximization (EM) algorithm. The importance of the EM approach is that it provides a framework for parameter estimation in the presence of unmeasured state variables by effectively integrating out the influence of the unmeasured variables. Numerical examples demonstrate the utility of this method for a network with a range of pipeline models.

Analysis on Air Valve Placement in Long-Distance Water Conveyance Pipeline System

Water hammer is one of the key factors that can cause accidents by the burst of pressure pipeline. Important features and related reasons of accidents caused by water hammer in long-distance water conveyance pipeline system were introduced, and protection measures were presented. An long-distant raw water conveyance pipeline project was taken as an example to study on the influence of protective effect by different placement schemes of air valves based on hydraulic transient simulation and analysis. Results shows that largest number of air valves does not reach the best and unreasonable placement of air valves might be the cause of an accident. It is critical to select the proper location of air valves, it is necessary to pay more attention on local hump in the front of pipeline near the pump and put exhaust valves instead of inlet valves at the back of pipeline (except some higher hump location).

Simulation of hydraulic transients in hydropower systems using the 1-D-3-D coupling approach

Although the hydraulic transients in pipe systems are usually simulated by using a one-dimensional (1-D) approach, local three-dimensional (3-D) simulations are necessary because of obvious 3-D flow features in some local regions of the hydropower systems. This paper combines the 1-D method with a 3-D fluid flow model to simulate the Multi-Dimensional (MD) hydraulic transients in hydropower systems and proposes two methods for modeling the compressible water with the correct wave speed, and two strategies for efficiently coupling the 1-D and 3-D computational domains. The methods are validated by simulating the water hammer waves and the oscillations of the water level in a surge tank, and comparing the results with the 1-D solution data. An MD study is conducted for the transient flows in a realistic water conveying system that consists of a draft tube, a tailrace surge tank and a tailrace tunnel. It is shown that the 1-D-3-D coupling approach is an efficient and promising way to simulate the hydraulic transients in the hydropower systems in which the interactions between 1-D hydraulic fluctuations of the pipeline systems and the local 3-D flow patterns should be considered.

Investigation on complete characteristics and hydraulic transient of centrifugal pump

An improved method was developed to obtain the complete characteristic of centrifugal pump. The conversion formula of complete characteristics is established based on the normal performance curve. An example was presented to illuminate the new method, and the complete characteristic curves of 14SA-10 centrifugal pump were obtained by the new method. The hydraulic transient of the centrifugal pump failure and start-up was simulated by method of characteristics (MOC), which quote the complete characteristics data. The results show that the inversion method is available to obtain the complete pump characteristic curves provided the normal performance curve. For hydraulic transient simulation, more accurate numerical result can be obtained, if the new model is adopted to convert the experimental normal performance curve to complete characteristics curve of centrifugal pump.

Critical superposition instant of surge waves in surge tank with long headrace tunnel

Superposed mass oscillation that often occurs in a throttled surge tank with a long headrace tunnel is studied. The instant of the worst superposition of mass oscillation in a surge tank is analysed. The analytical formula predicting the worst superposition instant is derived exactly and verified (1) with a numerical solution to the fundamental equations for the surge tank and (2) with a numerical simulation of hydraulic transients in an actual hydropower station. It is shown that the superposed oscillation provides the highest upsurge at the instant that the initial flow rate in the headrace tunnel and initial water level in the surge tank satisfy the judgement formula. The conclusion provides a theoretical and computational basis for the numerical simulation of maximum upsurge in a surge tank.

Application of rectangular integral to numerical simulation of hydraulic transients in complex pipeline systems

The hydraulic oscillation of surge tank was analyzed through numerical simulation. A rectangular integral scheme was established in order to improve the numerical model. According to the boundary control equation of surge tank, the rectangular integral scheme omits the second-order infinitesimal and simplifies the solving process. An example was provided to illustrate the rectangular integral scheme, which is compared with the traditional trapezoid integral scheme. Appropriate numerical solutions were gained through the new scheme. Results show that the rectangular integral scheme is more convenient than the trapezoid integral one, and it can be applied to the numerical simulation of various surge tanks in complex pipeline systems.