1 Chapter 5 Effects of Guide Vanes on PAT In this Chapter design, development and installation aspects of movable guide vane mechanism in PAT are discussed. The experimental investigations carried out at different speeds at various guide vane positions are presented. The comparison of best efficiency parameters before and after installation of guide vanes is made. 5.1 Necessity of Flow Controlling Mechanism Centrifugal pump is hydraulically similar to Francis turbines. But, the part load efficiency of Francis turbine is relatively higher than that of PAT due to presence of guide vanes. In Francis turbine at part load, flow is properly guided by guide vanes which led to decrease in hydraulic losses and hence increase in efficiency.
Transcript
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Chapter 5
Effects of Guide Vanes on PAT
In this Chapter design, development and installation aspects of movable guide vane
mechanism in PAT are discussed. The experimental investigations carried out at different
speeds at various guide vane positions are presented. The comparison of best efficiency
parameters before and after installation of guide vanes is made.
5.1 Necessity of Flow Controlling Mechanism
Centrifugal pump is hydraulically similar to Francis turbines. But, the part load efficiency of
Francis turbine is relatively higher than that of PAT due to presence of guide vanes. In
Francis turbine at part load, flow is properly guided by guide vanes which led to decrease in
hydraulic losses and hence increase in efficiency.
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Few investigators have studied the effects of installation of guide vanes on PAT performance.
Shi et al. (2012) numerically studied the impact of inlet and outlet angles of guide vane on the
hydraulic performance of PAT. Four combinations of length, inlet angle and outlet angle were
adopted and concluded that the inlet and outlet angle have great impact on the turbine
performance and there exist best combination of these angles. Chauhan (2014) numerically
studied the effects of two different airfoil shaped guide vanes viz. National Advisory
Committee for Aeronautics (NACA)-2605 and NACA-4414 at different angles on PAT
performance. Among the various options, seven number of guide vanes (NACA 4414 profile)
at an angle of 20° led to 8-10% improvement in efficiency. Giosio et al. (2015) installed guide
vane assembly consisted of thirteen hydrofoil shaped vanes in PAT which led to marginal
increase in efficiency. It was found that, most of the researchers have studied the effects of
guide vanes on PAT performance with numerical approach and very few researchers have
analyzed the effects experimentally. Hence, there exist a scope for further research in this
area.
5.2 Selection of Guide Vane Profile
In Francis turbine, additional space is available in the casing for the provision of guide vanes.
But, in centrifugal pumps such space is not available in the casing. As discussed in Chapter 4
(Section 4.6), the part load performance of PAT was improved with 10% trimmed blade
rounded impeller i.e. φ225 mm impeller. The trimmed impeller also facilitated space for
provision of guide vanes in the casing. Hence, in the present study guide vanes were provided
in PAT with φ225 mm impeller.
Patel (2015) numerically studied the effects of four different NACA profile guide vanes viz.
2605, 4414, 5520 and 6520 on the performance of PAT. Among these, the PAT performance
was found better with NACA 6520 vanes. Hence, it was selected for guide vane profile. Due
to volute passage in the casing, as water travels the space available for flow keeps on
decreasing. Considering the space availability, five guide vanes were provided in the first and
second quadrants of casing (first quadrant starts from tongue region and moving along flow
direction). The coordinate details of NACA 6520 profile are given in Table 5.1
(http://airfoiltools.com/airfoil/naca4digit). The chord length and maximum thickness of guide
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vane were taken as 55 mm and 10.92 mm respectively. The impeller blade width was 22 mm.
Accordingly, the guide vane width was kept as 25 mm.
Table 5.1 Coordinates of NACA 6520 profile guide vane
(http://airfoiltools.com/airfoil/naca4digit).
X (mm) Y upper (mm) Y lower (mm)
0.00 0.0000 0.0000
0.04 0.4714 -0.4499
0.71 1.9374 -1.5989
1.60 2.9398 -2.1944
2.82 3.9367 -2.6518
4.37 4.9008 -2.9717
6.21 5.8022 -3.1576
8.33 6.6109 -3.2167
10.71 7.2995 -3.1606
13.30 7.8450 -3.0050
16.08 8.2301 -2.7690
19.00 8.4443 -2.4745
22.04 8.4839 -2.1442
25.15 8.3520 -1.8003
28.29 8.0574 -1.4628
31.41 7.6146 -1.1483
34.49 7.0423 -0.8688
37.48 6.3629 -0.6317
40.33 5.6022 -0.4396
43.02 4.7885 -0.2912
45.51 3.9521 -0.1824
47.76 3.1247 -0.1071
49.75 2.3383 -0.0580
51.45 1.6244 -0.0285
52.83 1.0124 -0.0123
53.89 0.5283 -0.0044
55.00 0.0000 0.0000
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The 2D drawing of NACA 6520 profile guide vane is shown in Appendix IV. Its 3D model
and actual guide vane fabricated from Stainless Steel (SS) 304 are shown in Figure 5.1(a and
b).
Figure 5.1. NACA 6520 profile guide vane (a) 3D model and (b) fabricated from SS 304.
5.3 Movable Guide Vane Mechanism
The velocity triangles for PAT at rated and part load conditions are shown in Figure 5.2. It
can be seen that, at part load as Q decreased V1, Vf1 and Vw1 decreased. This led to increase in
Vr1 and hence Vr2. As V1 and Vf1 decreased, V2 and Vf2 also decreased such that Vw2
increased. It can be noted that angle β1 decreased to β1’; which may led to hydraulic losses
due to flow separation at blade inlet. Whereas, at outlet increase in Vw2 may lead to whirling
of fluid at blade outlet. Hence, at part load head acting on PAT and energy transferred to the
impeller decreased (according to Eq. 4.1) which led to decrease in efficiency. Fixed guide
vane may improve the PAT performance in the narrow range of discharge only. To improve
the performance in the entire discharge range, the water needs to be guided at different
discharge conditions. This can be done by provision of movable guide vanes. In the present
study, movable guide vane mechanism was provided to vary the guide vane angle depending
on requirement.
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Figure 5.2. Effects of part load on velocity triangles (a) at PAT inlet and (b) at PAT outlet.
( at rated discharge, ----- at part load)
The movable guide vane mechanism was consisted of five major components viz. guide vane
pin, connecting link, motion ring with gear teeth, meshing gear and supporting drum with
flange. The details of major component are given in Table 5.2. The 2D drawings of these
components are shown in Appendix IV. The 3D models of these components are shown in
Figure 5.3. The guide vanes were connected with guide vane pin using locking pin. The guide
vane pins were connected with motion ring using connecting links. The supporting drum with
flange was fixed on exit flange of the casing to give support to the motion ring. To rotate the
motion ring by certain angle, gear teeth were provided on it. The gear teeth were in mesh with
the spur gear provided with support on the casing. Hand wheel was provided on the other end
of the gear shaft to rotate the gear. When gear is rotated through certain angle by hand wheel,
its movement was transmitted to motion ring by gear meshing. The movement of motion ring
was transmitted to guide vane pin through connecting link to rotate the guide vane.
The assembly of different components is very important to obtain precise guide vane rotation.
The gear movement was calibrated with guide vane rotation and the markings were done on
the gear shaft. The guide vane angle was considered as the angle between the chord line and
the line tangent to the impeller diameter. From the calibration, it was found that when gear is
rotated through 28° the guide vane angle changes by 20°. The assembly drawing of movable
guide vane mechanism is shown in Appendix IV. The 3D solid model and photograph of
movable guide vane mechanism are shown in Figures 5.4 and 5.5 respectively.
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Table 5.2 Details of different components of movable guide vane mechanism.
Name of component Details
Guide vane Material: Stainless Steel 304
Numbers: 5
Guide vane pin Material: Stainless Steel 304
Numbers: 5
Connecting link Material: Mild Steel
Numbers: 5
Motion ring Material: Mild Steel
Gear Material: Mild Steel
Type: spur
Module: 2 mm,
Number of teeth: 24
Pressure angle: 20°
Supporting drum with flange Material: Mild Steel
Figure 5.3. 3D model of different components (a) guide vane pin (b) connecting link (c) drum
with flange and (d) motion ring.
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Figure 5.4. 3D model of movable guide vane mechanism (a) front view and (b) back view.
Figure 5.5. (a) Movable guide vane mechanism (b) closer view of casing with guide vanes.
5.4 Experimental Investigations at Different Speeds
As discussed in Chapter 4, the experiments were performed in the speed range of 900-1500
rpm. However, the performance of PAT was found better in the speed range of 1000-1400
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rpm. Hence, after installing five guide vanes in PAT casing experiments were performed in
the range of 1000-1400 rpm. To find the optimum guide vane position at different speeds, the
guide vane angle (GVA) was varied between 10°-21° and the performance is compared with
original impeller (φ250 mm impeller without guide vanes). It was found that, incorporation of
guide vanes led to relatively higher increase in discharge and hence higher shift in the range
of ϕ. Hence, the variations in ψ, π and η were studied against variations of non-dimensional
discharge i.e. ratio of discharge and best efficiency point discharge (Q/QBEP) in place of ϕ.
At 1400 rpm
The comparison of performance curves with and without guide vanes at 1400 rpm are shown
in Figure 5.6. It can be seen that with increase in Q/QBEP; ψ and π also increased. However,
the variation in η was found parabolic. To find out the optimum position of guide vane, its
angle was varied in the range of 10°-21° in five steps viz. 10°, 13°, 15°, 18° and 21°. Due to
guide vanes, ψ and π increased almost in the entire discharge range. Moreover, the increments
in ψ and π were also increased with increase in discharge. As discharge increased the slope of
ψ curves also increased. But, the π varied almost linearly with discharge. Guide vanes led to
increase in η at all the discharges in the GVA range of 13°-18°. This may be due to proper
guidance of water at different discharges i.e. streamlined flow and hence decrease in losses
due to flow separation (Patel, 2015). At other angles (i.e. 10° and 21°), the η improved upto
QBEP then remained nearly same as in case of original impeller. The maximum values of ψ, π
and η were observed at GVA of 15°. The maximum efficiency was found as 66.97% at guide
vane angle of 15°.
The effects of installation of guide vanes on velocity triangles are shown in Figure 5.7. As
discussed, provision of the guide vanes led to increase in discharge at all the loads. It can be
seen that, as Q increased V1, Vf1 and Vw1 also increased. This led to decrease in Vr1 and
hence Vr2. As V1 and Vf1 increased, V2 and Vf2 also increased such that Vw2 decreased.
The overall effect of these variations was increase in head acting on PAT and energy
transferred to the impeller (according to Eq. 4.1) which was resulted in increase in efficiency.
This may be due to the fact that, guide vanes provide additional flow resistance which led to
increase in head acting on PAT.
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Figure 5.6. Performance curves with and without guide vanes at 1400 rpm (a) ψ Vs. ϕ (b) π
Vs. ϕ and (c) η Vs. ϕ.
Figure 5.7. Effects of guide vanes on velocity triangles (a) at PAT inlet and (b) at PAT outlet.
( without guide vanes, ----- with guide vanes)
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The effects of guide vanes on different parameters in comparison with original impeller
(without guide vanes) at QBEP are summarized in Table 5.3. Provision of guide vanes resulted
in wider discharge range and the best efficiency point was shifted towards higher discharge.
At different angles, ϕ increased by 34-45%. This may be attributed to the fact that, provision
of guide vanes resulted in improvement in energy conversion process. Hence, PAT was able
to work with higher discharge compared to earlier. When the guide vane angle was increased
from 10° to 15°, percentage rise in ϕ, ѱ, π and η also increased. But, at GVA of 18° all the
parameters decreased. However, further rise in angle (at 21°) led to increase in ϕ and π;
though ѱ and η decreased. The reason could be, higher guide vane opening facilitate higher
discharge entry (ϕ increased from 36.83% to 45.70%) i.e. increase in energy supply to the
PAT. But, the rate of energy conversion somewhat decreased (i.e. π decreased from 52.81%
to 50.83%). At different GVAs, ѱ and π increased in the range of 9-13% and 37-67%
respectively. The η was increased by 1-10% at different angles. The maximum rise in π and η
were found as 67.00% and 9.84% at GVA of 15°.
Table 5.3 Variations in parameters w.r.t. original impeller at QBEP at 1400 rpm.
