جستجوی مقالات مرتبط با کلیدواژه "water hammer" در نشریات گروه "فنی و مهندسی"

Using pressure reducing valves to reduce the pressure in water distribution systems to the minimum of required value is one of the most effective ways for leakage reduction. Valve opening/closing, switching a pump on/off and water consumption fluctuation by a large consumer cause transient flows. Interference between transient flow and a PRV with constant needle-valve setting may intensify pressure waves in WDS. Applying smart PRVs can limitpressure fluctuation. In this research, Input-output feedback linearization method has been used for smart PRV control. The results of this method have been compared with PRV with CNVS and proportional-integral-derivative controller. A theoretical network taken from references was used to evaluate the proposed methods. Network demands include normal consumers and an industrial large consumer. Water hammer caused by consumption variations, PRV with CNVS operation, IOFL method and PID controllers were modeled in Simulink. PRV outlet head fluctuation in PRV with CNVS is 18 to 28 m in PID controller and in IOFL method are 26 to 28 m. Also, the results showed that the IOFL method has smoother and less fluctuation than PID. Root-mean-square error for PRV outlet head in CNVS, PID controller and IOFL is 1.6, 0.32 and 0.28, respectively. Therefore, IOFL method has less error and better performance than PID. Also, this method has simpler computational operations by converting nonlinear system equations to linear equations.

In this paper, the water hammer resulting from the fast closure of the valve in a pipeline is simulated using numerical solution of continuity and Navier-Stokes equations. Simulation has been performed for a high-viscosity oil and for water. The initial flow regime for water is turbulent and for the oil is laminar. The obtained results are compared with the experimental results and for both fluids, a good agreement between the simulation and experimental results at different times is obtained. Velocity contours at different times show two regions with different behavior in transient flow. The wall region and the pipe core region. In the wall region, the effects of fluid viscosity are dominant, the velocity gradients are sharper, and flow changes more rapidly. While the pipe core region is affected by fluid inertial forces. As the fluid viscosity decreases and the Reynolds number increases, the core region becomes larger. In addition, a parametric study has been conducted and the effect of different parameters on water hammer has been studied. The results show that by reducing the thickness or length of the pipe, reducing the mass flow rate, or using a pipe with a lower elastic modulus, the water hammer effects can be significantly reduced. For example, by reducing the length of the pipe from 60 meters to 18 meters, the maximum pressure decreases by 11%.

Transient flow in a pipe would cause interaction between the fluid and the pipe wall. This interaction can be studied by a mathematical model in which the pipe considered as a beam under an applied fluid pressure loading. This model is a fluid filled pipeline that is connected to a tank in the upstream and to a valve in the downstream and undergoes forces of water hammer when there is a sudden closure in the valve. The aim is to study the possibility of instability in this pipeline when there are large lateral displacements and in the meantime small constrains. As conventional dynamic analysis models in beams which are based on the infinitesimal constrain theories (ε=∂u⁄∂x) cannot reflect the effect of large lateral displacements in the governing equations. In the governing equations the axial stresses are modeled as linear stresses and constrains are modeled by so called von Karman constrains as nonlinear constrains. The resulting partial differential equations are solved by the method of finite elements in the time domain. On the other hand the linearized equation of lateral vibration of a pipeline becomes dimensionless and by drawing the dimensionless frequencies versus the dimensionless fluid velocity, the stability of the pipeline is studied in the frequency domain. The results show that in shortly supported spans the frequency domain criterion (dimensionless fluid velocity π and 2π for pinned-pinned and fixed-fixed configurations accordingly) determines the pipeline stability behavior and so does not the critical bulking pressure.

Unlike previous studies in Non-Newtonian fluids that use complex two-dimensional models to calculate the velocity gradient in this research one-dimensional models have been used to calculate non-residual losses that can be implemented faster And have higher execution speeds. The main objective of this research is to investigate the phenomenon of water hammer in Non-Newtonian fluids of power type (Power Law) using Brunon and Zeilke models. In order to calculate the shear stress in relation to the momentum of the Zeilke and Brunon model, and to solve the equations, the line characteristics of the nonlinear fluid solution have been used. The Brunon model is based on the assumption that the shear stress of the wall changes due to the acceleration of the acceleration, proportional to the acceleration of the fluid. Zilck's method for calculating unsteady friction coefficient presents a model based on the analytic integral of convolution. The velocity gradient in steady state is used to obtain the velocity gradient in the Zeilke model. Finally, numerical results are compared with the results of other papers to ensure the accuracy of the solution algorithm. The results of non-Newtonian fluid modeling show significant changes in pressure values. The proposed formulas, similar to the two-dimensional models, can simulate these changes. As expected in the same continuous flow conditions, the maximum pressure decreases with decreasing viscosity of the fluid. In other words, by decreasing the viscosity of the fluid, the amount of drop across the pipe path will be reduced.

Numerical Investigation of Column separation and Bubbles Growth in Water Hammer Maryam Mousavifard‎, Assistant Professor, Department of Civil Engineering‎, Faculty of engineering‎, Fasa University, Fasa, Iran Reza Roohi‎, Assistant Professor, Department of Mechanical Engineering‎, Faculty of engineering‎, Fasa University, Fasa, Iran Introduction In modern hydraulic systems, when a sudden change occurs in fluid velocity because of any reasons (sudden pump stroke, valve closure, etc.), it will be followed by intense pressure fluctuations in the system, which is referred to water hemmer. If the fluctuations decrease to less than the vapor pressure of the fluid, the separation column will occur. In general, two cases can be distinguished: either the pressure drops below the saturation pressure but keeps above the vapor pressure, or the pressure drops to the vapor pressure of the liquid. In the former case, gaseous cavitation takes place, characterized by the presence of a large number of gas nuclei. When the pressure drops suddenly, a significant gas release may occur. In the latter case, vaporous cavitation takes place, and when the fluid pressure drops to its vapor pressure, a sudden growth of the nuclei containing vapor occurs. In general, there are two basic assumptions to describe the phenomenon of cavitation: discrete and distributed cavitation. In discrete cavitation, the vapor or gas cavities create discontinuity in fluid. In this case, it is assumed that the vapor and gas cavities occur at computational nodes, when the pressure reaches less than the vapor pressure of the fluid or the saturation pressure. In distributed cavitation, the two phases of the liquid and the vapor (or the gas) are simultaneously solved, and the vapor (or gas) cavities are continuously exist all over the fluid. Methodology In this paper, the column separation in pressurized pipes affected by water hammer is numerically investigated using one-dimensional and quasi-two-dimensional models of gas cavity. A one-dimensional model is modeled based on the method of characteristics, and the energy dissipation is simulated using the summation of the Brunone unsteady friction and quasi-steady friction. In the proposed quasi-two-dimensional model, the characteristic equations along the pipeline axis and the finite difference equations along the pipe radius are used to simulate water hammer, and then the governing equations of discrete gas cavity model is coupled with the primary model, and the five-layer turbulence model was also used to simulate the energy dissipation. It should be noted that, in the quasi two dimensional model of separation column, like the one-dimensional model, the velocity on the upstream and downstream of each computing node will not be equal. Free gas distribution throughout liquid in a homogeneous mix in a pipeline yields a wave propagation velocity that is strongly pressure dependent. Results and Discussion After verifying the developed models by experimental data, the total shear stress is studied in some initial cycles of water hammer. In the second part of the paper, the dynamics of bubble growth and the process of temperature and pressure variation within the bubbles are also analyzed using the Rayleigh-Plesset equation. The growth of bubbles in the fluid is a function of a variety of variables such as applied pressure, fluid surface tension, evaporation pressure in fluid temperature and viscosity. Conclusion - the quasi-two-dimensional model has been more successful in calculating energy depreciation, especially in the final cycles of water hammer. - Regarding the head oscillation shape, it can be evaluated that the quasi 2D DGCM improved the energy losses reproduction, and reproduces the experimental spikes successfully even in final cycles of experimental runs. - The 1D model of DGCM reproduce the first oscillations of the experimental data successfully, but in final cycles, does not predict the shape of head oscillations successfully and does not stay in phase with experimental data. - According to the shear stress diagrams it can be concluded that in high pressure pulses, the total shear stress is negative and it is positive in low pressure pulses. - The difference between the minimum and maximum radius is about 0.5 μm. Considering the average radius of the bubbles and the relationship between the radius of the bubbles and the gas phase volume, the 85 to 125% increase in the volume of the gas phase in the fluid during the periodic pressure fluctuations process, is visible. - It should be noted that in spite of the fact that the maximum pressure inside the bubbles (the driving force for the growth of bubbles) is greater than the external pressure, the high dependence of the internal gas pressure on the radius of the bubbles and the dynamic behavior of the system, lead to periodic changes. In other words, the strong growth of the resistive force by reducing the radius of the bubbles prevents significant contraction in them, and again, by reducing the pressure head in the fluid, the growth of the bubbles will be renewed to reach the initial radius. Keywords: Water hammer; Column separation; unsteady friction; quasi two Dimensional model; shear stress; Rayleigh Plesset equation

Due to the importance of water hammer in pipe network and pipe containing liquids in this paper, the diameter of pipe and the velocity of fluid were considered experimentally in laboratory and simulated the problem in HAMMER software. In this study, the tests were carried out with six different types of materials (Brass, Carbon Steel, Copper, Five layer and PVC). It was assumed that valve which causes water hammer was closed fast. Also, how valve closing, is another important issue for this phenomenon. The tests in two modes slow and fast closing valve results showed if it takes more time to close, water hammer is less and the waves would fade faster. Also by examining the results indicate that the diameter of the pipes increasing pipe's diameter, increases the mess flow and fluid's velocity therefore effect of water hammer increases. This phenomenon shows that effect of increasing fluid velocity is more powerful than increasing pipe diameter.

This study was initiated to investigate the use of a porous structure in some parts of a pipeline to protect it against water hammer pressure wave. This study was inspired by the utilization of porous breakwaters which are commonly used to protect coastal areas. To fulfill the main objective of the study, different porous structures were installed in an experimental pipeline fed by a pump. The water hammer was induced by an upstream valve. The homogeneous porous structures were made up of spherical beads or broken rocks in different sizes and filled them in pipes with different lengths and subject them under different water hammer conditions. Results obtained from this study showed that during the first cycle of water hammer wave, the ratio of maximum pressure downstream of porous structure to the corresponding value in upstream is between 0.6 to 0.95, respectively. This pressure difference decreased drastically in next cycles. Generally, it can be conclude that an insignificant protection gained from a porous structure in downstream from water hammer; however, the overall pressure fluctuations in the pipe filled with a porous structure was less than a fully open pipe under the same discharge rates and the same water hammer condition. The results showed that porous structures in particular condition could be used to control pressure oscillations in piplines and it is proposed to consider practical aspects of this idea in future researches.

In this paper, the hydraulic modeling of a simple surge tank with two assumptions of with or without friction condition at both gradually and suddenly valve closing is studied. The four different kinds of the finite difference method called the Explicit Euler, Implicit Euler, Predictor-Corrector Euler and Rung-Kutta methods are used to solve hydraulic equations and the results are presented and compared. Comparison of the results shows that in all cases, by using the Implicit Euler method, the fluid level variation is less than other available methods. In other words, the fluid level variation obtained using Implicit Euler is about 3 percent less than that obtained using Rung-Kutta method at without friction condition. In addition, at friction condition, the fluid level variation obtained using Implicit Euler is about 4, 3 and 2 percent less than that obtained using Explicit Euler, Predictor-Corrector Euler and Rung-Kutta methods, respectively. Moreover, the results converge faster than the other methods at without friction condition using Implicit Euler method and finally the results converge faster than the other methods at friction condition using Rung-Kutta method.

Optimization and modeling of the pressurized irrigation system in other to reduce their construction cost has attracted many scientists up to the present time. Optimization of these irrigation systems has often been conducted by using available commercial codes or toolbox's of conventional evolutionary algorithms combined with hydraulic models. Developed code that is based on genetic algorithm assigns an integer numeric to each available diameter. Then, by applying the cross-over, mutation and reinsertion with elitism approach on set of chromosomes optimum diameter of pipes are selected. Optimum diameters are sent to another subroutine that has been developed to simulating water hammer pressure in which maximum pressures due to water hammer phenomenon in all pipe are calculated. Then, calculated maximum pressures are sent to optimization subroutine again where constrains of working pressure are substituted by the bursting pressure constrains. Exchange of information between optimization and hammer models continues so that the calculated optimal diameters have not change. The results showed that optimized design by the present model reduces cost of implementation of pipelines of Ismailabad irrigation network from 825935.28$ to 730958.37$ which is equivalent to 11.5% of the cost of implementing of the pipelines.Also the results showed that by changing diameter of 3 pipes of 16 existing pipes we can control excess pressure of water hammer due to rapid closing of all valve at the end of pipelines during less than 1 second. In this case the cost of implementing of the pipelines increases 6.1% e.g. from 730958.37$ to 775511.5$.

Under transient flow condition, the behavior of water conveyance system varies according to their characteristics. In the present study, the pressure was measured using a fast and sensitive pressure gauge in Bukan and Piranshahr water conveyance system. The pressure simulation was conducted using Bentley Hammer software. The friction head loss was calculated by different methods. The results showed that Unsteady Vitkovsky method had minimum error comparing with other methods. Wave velocity increase had direct effect on maximum pressures while velocity decrease affected minimum pressures. In a shorter water conveyance system, the reduction of wave velocity had direct effect on maximum pressure. Destruction to the long conveyance system was more probable and maximum and minimum pressures occurred during the first period. Shorter conveyance system had more pressure fluctuations and the minimum pressure did not occur in the first period. Coincidence of periods happened at the beginning and continued untill the end of data recording in the longer conveyance system. However, as time passed by, such coincidence did not occure in shorter conveyance system.

Water hammer is one of the destructive hydrodynamic phenomena which established in most pumping stations, transmission lines and hydroelectric power plants. Fluid distribution systems and hydropower plants can be severely damaged by water hammer. Water hammer is the forceful slam, bang, or shudder that occurs in pipes when a sudden change in fluid velocity creates a significant change in fluid pressure. Since this phenomenon can force destructive effects on transmission lines and hydroelectric power plants (Turbines and pipelines), so it is essential to investigate the influence of water hammer (positive and negative pressure waves) along the transmission lines. One of the methods to reduce the effect of water hammer effects is to establish a surge tank in hydraulic pipelines. Surge problems are encountered in connection with unsteady state of flow of fluids in pipelines. In general a surge tank is designed to reduce the distance between the free water surface and turbine thereby reducing the water hammer effects on penstock and also protect upstream tunnel from high pressure rises. The other function is to serve as a supply tank to the turbine when the water in pipe is accelerating during increased load conditions and as a storage tank when the water is decelerating during reduced load conditions. In the present study, while describing the performance of different surge tanks and their role in adjustment the negative and positive pressures in transmission lines of hydroelectric power, two types of surge tanks (i.e., simple and differential) are selected and investigated their action on variations of flow pressure and velocity along the pipe lines. Also, regarding to the economic considerations, the dimensions of surge tanks have been optimized. Finding the optima position of tanks according the oscillations of water surface is another evaluated of items in present study. For analyzing the equations by finite difference technique, discretization of equations has been conducted in Matlab. The results revealed that the differential surge tanks are more efficient and economical than simple ones up to 40%. Also the half of transmission length at upstream of turbines are determined as the optima position of surge tanks.

Whenever and for any reason the velocity of the fluid ceases in the transfer line or water distribution networks, pressure waves will form in the pipe lines, which can produce a pressure several times more than a pressure pump’s output and lead to a great deal of tension on network’s components, which in turn will cause the water hammer phenomenon. Today, in every water transfer project, a detailed study on the water hammer phenomenon is essential, so that with a full recognition of this process’s adverse effects, the adequate measures would be taken to deal with it.This study uses Water Hammer V8i software to analyze the water hammer of Mehran city’s border terminal water conveyance pipe. The study was executed in 3 phases of simulation with protective equipment, without the protective equipment and with consultant’s recommended equipment. The results indicate that in the phase without the protective equipment, a great deal of negative pressure builds up along the water conveyance pipe line that needs to be controlled. In the next step, various combinations of Hydropneumatic tank and air valves were suggested. The results of several simulations showed that a 2 square meter Hydropneumatic tank and 3 fifty millimeter air valves will be capable of controlling the negative pressure along the pipe line.

In recent years، the optimal design of pipeline systems has become increasingly important in the water industry. In this study، the two methods of genetic algorithm and mathematical optimization were employed for the optimal design of pipeline systems with the objective of avoiding the water hammer effect caused by valve closure. The problem of optimal design of a pipeline system is a constrained one which should be converted to an unconstrained optimization problem using an external penalty function approach in the mathematical programming method. The quality of the optimal solution greatly depends on the value of the penalty factor that is calculated by the iterative method during the optimization procedure such that the computational effort is simultaneously minimized. The results obtained were used to compare the GA and mathematical optimization methods employed to determine their efficiency and capabilities for the problem under consideration. It was found that the mathematical optimization method exhibited a slightly better performance compared to the GA method.

Sudden changes in the boundary conditions of water transmission systems, such as sudden opening and closing of valves or abrupt on and off switching of pumps and turbines cause a transient flow called ‘water hammer’. In this study, comparisons were made between the effective parameters including pipeline material, on the one hand, and the equipment and tools available for reducing the effects of water hammer, on the other. For this purpose, a practical example of a water transmission line from a pumping station located near Shahid Shirdom Residential District to the upstream reservoir in Tehran was used for modeling by the Bentley Hammer XMV: 8 software. The results obtained for the different parameters and options were compared and it was revealed that, regarding the pipe material, GRP pipes reduced pressure by 49.1 Kpa compared to the Asbestos cement pipes and by 50.3 Kpa compared to the iron pipes. Comparison of the results for the protective systems indicated that the surge tank outperformed the other alternatives in controlling pressure such that maximum pressure was reduced by 3.9 bar when using surge tanks compared to the flywheel and by 5 bar compared to the check valve. Finally, it was found that the concurrent use of the surge tank and the flywheel would be the most ideal method for controlling the water hammer effects.