CHAPTER 4 Hydrodynamic Design
4.3 Parametric Analysis
4.3.2 Second Stage Analysis
56 hub but had little effect on the loading at the tip, where it is most severe. The reduced number of blades at the inlet also resulted in an increase in the local blade loading probably associated with the 50% lower solidity when compared to a full 8 blade design.
It appears that there is little benefit in adding splitter vanes to this impeller and that the baseline arrangement of 6 full blades should be retained. It is, however, noted that a more comprehensive investigation into splitter design could possibly find an improved solution. A CFD analysis would be useful for appropriately locating the splitter blades at the point of unacceptable divergence between the pressure and suction side streamlines [59].
Figure 4-6 Blading arrangements for a) 6, b) 8 and c) 4-8 bladed impellers.
4.3.1.5 Conclusion of Analysis
The first stage parametric analysis suggests two major changes to the baseline design; a reduction in exit blade angle and a reduction in the exit swirl parameter. The combination of these parameters must, however, be selected to yield suitable diffusion within the impeller. The solution of the initial attempt at using β2b = 22.5° and λ2m = 3.8 did not converge, indicating that this low level of exit swirl is not achievable with such a degree of backsweep. As a compromise an exit blade angle of 25° and an exit swirl parameter of 0.4 were adopted for the revised design. This resulted in a 2.8% increase in efficiency from 81.7% to 84.5%. The gradient of the head characteristic curve increased slightly, indicating a minor stability improvement. The blade-to-blade loading is improved and the secondary flow area fraction is reduced from 0.75 to 0.59, indicating that the impeller is operating closer to optimal diffusion where a value of approximately 0.4 is expected.
57 angle and exit swirl parameter to be investigated. The models used in this analysis were set up using the revised design from the results of the first stage analysis with the addition of a vaneless diffuser and volute. These downstream elements were added as they have a significant effect on the conditions at the impeller exit. It is important that the impeller exit is optimised for the arrangement that will be used in operation.
4.3.2.1 Relationship Between β2b and λ2m
In order to investigate the relationship between the exit blade angle and the exit swirl parameter a matrix of combinations was created, with the meanline calculations for each being calculated in PUMPAL. The range of values to be analysed was based on the result of the first stage analysis which recommended values of β2b = 25° and λ2m = 4.0.
Therefore β2b values between 25° and 30° and λ2m values between 3.8 and 4.2 were used in this analysis.
The exit performance was analysed in terms of four characteristics: size reduction, stability, secondary flow formation and diffusive characteristic. The exit sizing is characterised by the head coefficient. The flow stability can be measured in terms of the magnitude of the negative gradient of its head characteristic (H-Q) [58]. This analysis used a gradient defined as the difference in head between a flowrate of 80% and 120% of the design point. The secondary flow formation in the impeller with respect to the exit design is characterised by secondary zone area ratio (E). The diffusive characteristic is more difficult to assess, this analysis used the blade-to-blade loading profiles to determine the uniformity of the diffusion especially at the exit. These data are compiled in Table E-1.
Figure 4-7 shows the relationship between secondary zone blockage and exit blade angle.
It can be seen that reducing the exit swirl parameter corresponds to a near-linear increase in secondary flow blockages. This also shows an approximately linear relationship between blade angle and secondary blockage that is maintained for all swirl parameters. It should be noted that there is a slight increase in gradient for lower blade angles, indicating a more rapid reduction in blockage for lower β2b values. Similar blockage performance is achievable using 25°/4.0, 26°/3.9 and 27°/3.8.
58 Figure 4-7 Secondary zone blockage vs. exit blade angle for various exit swirl parameters.
The relationship between the H-Q gradient and exit blade angle used to assess stability is shown in Figure 4-8.
Figure 4-8 Head characteristic gradient vs. exit blade angle for various exit swirl parameters.
59 This graph shows a marked reduction in the stability improvements attainable by reducing the blade angle for a given swirl parameter. It also appears that the swirl parameters tend towards an asymptotic value where the differences in stability become minimal. This implies that the best design point would be at the "bend" where a large H- Q gradient can be achieved with minimal backsweep. However, the previous graph shows that there is a significant advantage for decreasing the blade angle. It is therefore optimal to chose a point with a smaller blade angle which falls along the "asymptote" to the left of the bend. This again suggests using 25°/4.0, 26°/3.9 or 27°/3.8
Figure 4-9 shows the linear relationship between head coefficient and exit blade angle. In order to minimise the size of the impeller a large head coefficient is desirable. Thus increasing the exit blade angle reduces the size of the impeller. This is directly opposed to the previous design suggestions. Thus a compromise has been made between impeller sizing and the other design goals.
Figure 4-9 Head coefficient vs. exit blade angle for various exit swirl parameters.
In order to identify the main trends in blade-to-blade loading a set of sample points were chosen across a range of blade angles at λ2m = 4.0. Figure 4-10 shows these B-B loading plots at blade angles of 25°, 27° and 30°.
60 Figure 4-10 Blade-to-blade loading curves for a) 25°/4.0, b) 27°/4.0 and c) 30°/4.0
61 It can be seen that increasing the exit blade angle has a negative effect on the uniformity of the flow field through the impeller. This is due to the reduction in channel length which forces rapid and unstable diffusion. The sharp rise in loading at the exit is of particular concern for the exit optimisation. The magnitude of this exit spike was recorded for all of the β2b/λ2m combinations (see Table E-1) which reveals the presence of the exit spike corresponding to ψ > 0.44, across the range of swirl parameters.
When this limit is applied to the suggested combinations established so far, only 25°/4.0 and 26°/3.9 remain favourable. These two have very similar exit performance, as shown in Table 4-2. It was, however, decided that the 26°/3.9 combination provides the optimal solution as the lower exit swirl reduces the likelihood of stall at the vaneless diffuser (see 4.2.2).
Table 4-2 Comparison of exit performance for the most suitable β2b/λ2m combinations.
25°/4.0 26°/3.9
E 0.534 0.545
H-Q gradient 175.9 175.4
ψ 0.437 0.439