Modeling and analysis of simultaneous effects of watter hammer and cavitation impact on pipe

Document Type : Research Article

Authors

1 Department of Engineering South Tehran, Azad Branch

2 Master of Water Engineering and Hydraulic Structures, Department of Civil Engineering, Islamic Azad University, South Tehran Branch

10.30482/jhyd.2021.291024.1535

Abstract

Transient currents occur when phenomena change with time, and ram impact is one of these phenomena. Since the ram impact phenomenon is a transient and damping phenomenon, it can therefore be called a non-continuous damping current, which occurs between two flow regimes. In this study, in order to investigate the ram impact phenomenon along with cavitation, a flow model in pipes in two-dimensional (quasi-two-dimensional) space has been developed. This numerical modeling has been done in a cylindrical coordinate system and the finite element numerical solution method has been used to solve the equations. This model has been used to calculate the shear stress between different layers of the flow, for each type of flow (quiet or turbulent) of its own relations. In this modeling, the continuity equation is explicitly solved and the momentum equation is implicitly solved. In order to find the best model, Araya laboratory conditions were modeled in both ANSYS and Fluent models and the output results were considered as the basis for selecting the best model. The output results indicate that the model built in Fluent is closest to the laboratory results. In the second stage, numerical modeling has been done in MATLAB software with the best model selected and laboratory conditions of Araya. At this stage, all three results were closely related.A numerical model of ram shock is the solution of simplified Navira-Stokes equations in the space of a tube and a cavitation model involves solving two-phase equations with shock and ram equations which are continuous. For numerical solution of both ram impact and cavitation models, the finite difference numerical solution method has been used. In these equations, the variable u is a function of r, x and t, while H is a function of t, x, so this model is a quasi-two-dimensional model. Wardi and Wang (1991) showed that for both slow and turbulent currents, the maximum radial velocity is between 10 and 20 μm / s. Along the pipe, the normal stress value at all points is assumed to be equal to the pressure head, so the values σr, σx and σθ are assumed to be equal to zero.Conclusion and selection of appropriate software for analyzing the water hammer impact phenomenon:
In this section, three categories of results will be compared:
1. Experimental data obtained from vote experiments
2- Results of CDF 3. Results from UDF2.The quantities of steady state velocity, initial maximum pressure and the number of pressure wave oscillations generated during the seconds of recording the results have been selected as criteria for comparing these three groups of results. The values of these three quantities should be given in the table number table for the three groups of results. They have been compared. The values of these three quantities should be given in the table number for the three groups. The mentioned results have been compared. As can be seen, the results of UDF2 are not consistent with other results. Due to the inappropriate answers of UDF2, it can be concluded that this software has not modeled the water hammer impact phenomenon properly. As can be seen, the results of UDF2 are not consistent with other results. Considering the inappropriate answers of UDF2, it can be concluded that this software has not modeled the ram impact phenomenon properly.Transient conditions are created due to sudden changes in a hydraulic system. These changes are usually due to changes in flow by valves, turbines, etc. and cause sudden changes in pressure in pipelines. This increase or decrease in pressure can damage the pipes of hydraulic systems valves.two-dimensionality of the model and consequently segmentation of the pipe section.To different layers and separate calculation of each layer, the modeling is closer to the real state of the phenomenon and as a result the results are more realistic. The two-dimensionality of the model and as a result of dividing the pipe cross section into different layers and calculating each layer separately, has brought the modeling closer to the real state of the phenomenon (compared to one-dimensional models) and as a result the results are more realistic. Also, the performed modelings show that omitting the radial velocity component of the fluid in the pipes has no effect on the accuracy of the problem.Transient conditions are created due to sudden changes in a hydraulic system. These changes are usually due to changes made in the flow by valves, turbines, etc. and cause sudden changes in pressure in pipelines. This increase or decrease in pressure can damage the pipes of hydraulic systems valves.

Keywords


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  • Receive Date: 08 July 2021
  • Revise Date: 06 November 2021
  • Accept Date: 14 November 2021
  • First Publish Date: 22 June 2022