Msc Project1 3s473g

MSc project, Advanced Mechanical Engineering.

Computational Fluid Dynamics simulation on a large diameter, Square knife gate valve.

University Of Leicester Student: Raef Kobeissi Supervised by: Dr. Shian Gao 29th of July 2012

Table of Contents Abstract.......................................................................5 1. Introduction......................................................................6 1.1 Background..................................................................................6 1.2 Case Study................................................................................7 1.3 Objective...................................................................................8 2. Theory.............................................................................. 9 2.1.1 Navier Stokes equations...........................................................9 2.1.2 RANS - Reynolds Averaged Navier Stokes equations..............10 2.2........................................................ Large Eddy simulation - LES ....................................................................................................12 3. Pre-processing................................................................14 3.1 Geometry......................................................................................... 14 3.2 Meshing........................................................................................... 18 3.3 ANSYS Fluent – Pre-processing.........................................................23

4. Results...........................................................................25 4.1 Velocity and Pressure fields...............................................................25 4.2 Statistical Data................................................................................33 3.1Results Analysis.................................................................................... 40

5. Conclusion......................................................................43 Appendix A.............................................................................42 References.............................................................................. 45

Abstract Based on differences in flow measurement readings on a constant diameter pipe line, the project herein establishes a connection between a partially closed square knife gate valve and the behaviour of a downstream flow utilising experimental data collected by a variety of Ultrasonic metering devices. The pipe line is located in one of Severn Trent Water treatment’s facilities. Two computational fluid dynamic simulations were carried out to study the levels of turbulence in the down-stream side of the pipe. The results obtained from the two models (LES / K-EPSILON) were investigated and compared. Fluent 13.0 from ANSYS was the chosen package for the computational processing, the results obtained were post processed in Fluent and CFD post. Further improvements were suggested at the end of this dissertation to improve the quality of the simulation and to get generalised results that can be based on for any future study case of similar nature.

5. Conclusion The simulations that were carried out on the square knife gate valve show high turbulence levels in the downstream side of the main (30 to 60 %). An analysis for its intensity has concluded that even at two meters away from the valve and just at the inlet side of a bend the turbulence did not decay but instead the highest levels were ed at that location. High levels of turbulence and the presence of vortices are considered the main for different velocity readings in separate locations. The position of the valve near a bend contributes in amplifying the turbulence and creating swirls ahead down-stream. Due to swirl formation, the valve is not recommended to be installed near a bend and it is advised to keep a fair distance between them (50 X Diameter) so that the turbulence levels decrease and the velocity profile restores its developed shape before it reaches a metering device. (Figure 4.28). The LES simulation gave better results in predicting the behaviour of the flow with respect to time and that was evident in the show case presented in this dissertation. Large eddies and separate chaotic circulations that were moving from the centre and the upper side along the pipe were captured by the LES method. The video submitted with this dissertation gives a better description of the flow. This method is more preferable for deep analysis and realistic perspective of the flow’s turbulent nature. Although the advantages of LES are numerous unlike the time averaged RANS method, it is known to be much more expensive than the RANS method due to its long processing time and the difficulty in applying an appropriate mesh with a convenient near wall damping function. The implementation of Van Driest near wall damping function is a futuristic complementary improvement for this project. For a better understanding of the relationship between high turbulence and the measurement data, the case studied herein should be extended to include a longer section of the pipe downstream with all the existing bends.

The gate valve modelled for this simulation has an irregular profile (Figure 3.12) due to corrosion, it is recommended to make a comparison study between a normal valve and a corroded one to study the effect of corrosion and the geometrical shape of the valve on the nature of the flow.

Figure -3.12- gate valve corrosion – Video monitoring

Figure – 4.28 – Velocity profile in y direction (LES), line at y=1m