Multiple orifice plates results

In tables 4.14 and 4.15 the results of the CFD calculation for the multiple orifice configuration is given. There have been done significantly less results since the time was limited and the needed time for the simulations was much more then the 2D and quater model simulations. The choice has been made to only perform the simulations with one massflow. The construction of the mesh and the preperation of the model has been done in the same manner as the earlier simulations.

The model that has been used is a half model. In the single orifice has an infinite number of symmetry planes since it is a single hole installed in a tube, this made the use of 2D axial symmetric model possible. The combination of orifice plates is too wide to be able to be fitted in the center of the tube. The complete assembly was therefore mounted excentric. This means that the first used orifice plate is also mounted excentric so this simulation was also done. The excentric mounted hole however only has one symmetry plane so a half model was used. The combination also has one symmetry plane so this was also modelled and evaluated with a half model.

In figure 4.8 the velocity contours for the different configurations can be seen. The scales in all figures are the same for convenience. It is clear to see than when the number of stages increases, and the smallest crosssectional area decreases the peak velocity increases. This is what one would expect, if the mass of volume flow is constant and the flow passage becomes smaller then via the law of mass conservation the velocity must increase. In the figures is also clear to see the increase in velocity near the inlet, the regions of increases velocity radiate out from the center of the entrance. At the exit the increased velocity is smeared out over outlet portion in an elipse form or a jet. Interesting to see in the creation of an vortex like velocity region in the lower side of the outlet. The size of this affected area differs with the number of stages used and thus the peak velocity. The highspeed jet in the top portion of the outlet tries to carry fluid with it towards the outlet, this creates the wake region. When the velocity of the jet increases with an increase in stages we would expect the wake region also to increase. From the velocity contours we can see that this is not the case. There are two possible reasons for this. One reason is that the position of the jet changes with an increase in the number of stages and the second is that the walls of the tube are relatively close to the jet and the wake area. Both possible causes limit the size of the wake region

The influence of the orifice being placed excentric in the tube is inves- tigated by running an additional simulation. The obtained results in axial- symmetric and quater model orifice of 15mm placed cocentric in the tube are seen in table 4.12. The average orifice coefficient for the cocentric mounted orifice is 0.805, for the not concentric mounting the obtained result is 0.801. The orifice coeffient is thus slightly smaller in a non concentric mounting. In the velocity contourplot the jet at the oulet is very close to the wall, but the influence seems negiable. An important differnce can be seen with the wake formation. In the concentric configuration a wake is formed all around the jet, however in the non-concentric mounting this is only seen in the lower part of the outlet tube. The influence of this concentric mounting seems negiable, however this is only for this situation. The roughness of the wall is not explicitly modelled, the no-slip boundary condition was applied, but the wall is still assumed to be smooth. The second remark is that the reynolds number for this flow is relatively small, maybe at higher reynolds number

the influence of the wall will become a bigger contributing factor, as well as the formation of the wake.

The influence of the wall in this simulations is not very large, but is does have some significant influence on the results. For the calculation of the orifice coefficients of the stage combinations this is not ideal since the performance is obviously affected by the pipe wall. However the aim is still to find the performance characteristics of the multi-stage trim of a control valve. In a control valve the wall is not as near to the stage exit as in this simulation but other holes might be. The presence of a wall can mimic the influence that another jet in the near area has on the performance of one multi-stage hole. When we keep this in mind the presence of the wall is somewhat less inconvenient then one would think initially.

Configuration plates installed Mass flowrate [kg/s] Reynolds Number [-] Inlet pressure [Pa] 5 0.135 2499 448.082 4-5 0.135 2499 729.954 3-4-5 0.135 2313 2151.24 2-3-4-5 0.125 2313 4229.63 1-2-3-4-5 0.125 2313 1,0002.8

Table 4.14: Data of the 3D Multiple orifice simulations with Normal offset

Configuration plates installed Mass flowrate [kg/s] Cd [-] Cv [-] 5 0.135 0.801 8.41 4-5 0.135 0.943 6.58 3-4-5 0.135 0.874 3.83 2-3-4-5 0.125 0.861 2.53 1-2-3-4-5 0.125 0.847 1.64

Figure 4.8: Velocity Contours plot of the different configurations with nor- mal offset