Recolección de datos

The impeller internal flow analysis, or equivalently the quasi three-dimensional analysis, have a start-up window as shown in figure 8.19. A control panel is located on the left hand side, where input data must be entered. At the bottom a panel reserved for presenting results appear, while two axes aid the display of the calculation findings. These axes are located in the middle of the window and at the bottom right.

In order to run the impeller internal flow analysis the geometry of the impeller must already have been generated. This is done by using the impeller detailed design tool, as described in

the previous section. Thus, by clicking on the button Import detailed impeller geometry, the results of the detailed impeller analysis tools are imported, together with the inlet properties chosen for this design. Of course, these inlet properties may be altered and changed arbitrary by the designer if desired. Also, the desired mass flow and rotating speed are imported; these may also be changed at any time. The three-dimensional geometry is plotted in the main axes located in the middle of the window, giving the user some feedback that the geometry has been fetched. Such a plot looks exactly like the one shown in figure 8.18 and need no further explanation. It is important to note that no changes to the geometry itself are possible using the quasi three-dimensional design tool. This tool is strictly used for the flow analysis itself. If changes to the geometry are needed one must resort to the detailed impeller design analysis. This presents no problem since easy access to both tools are available with aid of the menu bar at the top of the window.

Having set the correct inlet conditions, the quasi three-dimensional flow analysis may be initiated by pressing the Run Quasi-3D analysis button. If the analysis converges without any errors a dialogue box appears as shown to the left in figure 8.20. No results are presented if

Figure 8.20: Different dialogue boxes appears depending on whether the analysis converges or not. To the left is the dialogue box shown after a converged solution and to the right the dialogue box when the quasi three-dimensional analysis did not converge

the solution did not converge, but the dialogue box to the right of figure 8.20 is generated. Different reasons can exist when convergence problems appear, usually one can track the issue to supersonic velocities on some of the blade surfaces. Since the analysis depends on a stream function solution, it will diverge if velocities higher than the sonic ditto appears. Thus, some parameter(s) must be altered. Sometimes it is sufficient to alter some inlet conditions which can be done directly in the quasi three-dimensional analysis tool. In some cases, a complete redefinition of the whole impeller geometry itself is necessary, which must be carried out with the detailed impeller analysis tool. If instead the solution does converge, results are presented and the axes at the bottom right presents the stream surface pattern as found by the calculation. The result panel typically takes the form of figure 8.21. The results are a direct consequence of the analysis, and gives the designer important information of the discharge conditions, and the final mass flow of the impeller. This mass flow may be reduced during the solution process due to choking conditions in the hub-to-shroud solution. A typical stream surface pattern is shown in figure 8.22.

As was seen in figure 8.21 additional options appeared at the right hand side of the result panel. These are plot tools, allowing the designer to determine what velocity results should be visible on the main axes located in the middle of the window. By checking different radio buttons, one can choose between relative velocities or Mach numbers. Also, the choice between blade-to-blade flow velocities or hub-to-shroud velocities (through-flow velocities) can be made with simple radio buttons. A set of check boxes are also available, from which the user can pick certain stream surfaces to be included in the plot result. More than one stream surface can be presented at the same time by checking multiple check boxes. Typical Mach

Figure 8.21: The result panel of a quasi three-dimensional flow analysis showing single parameter results to the left. To the right different options for presenting velocity distributions graphically are available

Figure 8.22: An example of stream surface patterns, whose positions are iteratively updated in the quasi three-dimensional analysis

number distributions in the blade-to-blade plane is shown in figure 8.23. The Mach number or velocity distributions are plotted against the dimensionless blade surface distance, enabling all distributions to be shown in the same axes. Blue lines represent pressure side conditions, while red lines indicate suction side conditions. In this figure, it is seen that the Mach number distributions meet each other at the trailing edge, as implied by using the Kutta condition as one of the boundary conditions.

If instead choosing hub-to-shroud relative Mach numbers results similar to those of figure 8.24 can be expected. Here, Mach numbers based on the meridional through-flow velocity are presented as a function of dimensionless meridional distance. These can show quite large differences between hub and shroud surfaces, the difference stemming from the solution of the momentum equation. As has already been stressed, which stream surfaces to plot is in full control of the designer, in the above example only two of the available seven stream surfaces were shown.

Figure 8.23: Mach number distributions as found by the impeller internal flow analysis. These are blade-to-blade Mach numbers, where red lines indicate the suction side and blue lines the pressure side. The velocity distribution with higher levels of Mach numbers is located at the shroud, while the one with lover levels represents the hub surface

Figure 8.24: Typical meridional Mach number distributions as found by the hub-to-shroud flow analsysis. The top curve represents meridional through-flow Mach numbers at the shroud surface, while the lower corresponds to meridional Mach numbers on the hub surface