# Suppression of Performance Curve Instability of an Axial-Flow Pump by Using a Double-Inlet-Nozzle

Authors: P. Pérez Flores; G. Kosyna; D. Wulff
Journal: International Journal of Rotating Machinery
 (2008)

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## Abstract

It has been shown that the sudden decrease of pump head in an axial flow pump caused by stall can be overcome by means of casing treatment. Flat axial grooves in front of the impeller break the swirl of the near-casing backflow. The disadvantage of this method is strong cavitation at the inlet of the grooves for flow rates below the stall point. In this paper another improved method to stabilize the performance curve will be presented, using a double-inlet-nozzle. At the onset of stall the initial near-casing backflow with its high swirl is lead off into the gap between both nozzles. At design conditions this double-inlet-nozzle is working as an injector, supporting the near-casing-inflow. The function of the double-inlet-nozzle is demonstrated by PIV-measurements.


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## Body

## 1. Introduction

The operating range of axial-flow pumps is often
limited by the onset of stall.  If the flow rate  is reduced approximately below
70% of the design flow rate the characteristics will become unstable with
a sudden drop of pump head. Already before stall, theNPSH3-values
are rising and cavitation cause severe vibration. When stall inception is
caused by tip- or wall-stall, it is possible to overcome the negative effects
of stall by using casing treatment.Kurokawa et al. [1] were the first who stabilized the performance
curve of a mixed flow pump without decreasing the maximum of the pump
efficiency. They formed shallow grooves within the casing wall, parallel to the
pressure gradient.Goltz [2] adapted this method successfully for a
single-stage axial-flow pump. But she mentioned as well a disadvantage, the
occurrence of strong cavitation at the inlet of the grooves for flow rates
below the stall point.This cavitation could not be extracted from theNPSH3-values
but can lead to a destruction of the grooved casing. A continuous operation for
flow rates below the stall point is hence not possible.Therefore, another method to stabilize the
performance curve will be presented in this paper.


## 2. Previous Work

Goltz et al. [2–6] showed the
flow structures of a single-stage axial-flow pump from stable operation until
deep stall. After the onset of stall, a short transient takes place. During the
transient Goltz observed spiral-type vortex-breakdown (see Figure 2). The
kinking of the vortex lead to an enormous blockage so that the inlet flow is
forced to separate from the casing, and hence the backflow could suddenly
expand upstream of the rotor. The expansion of the back flow is not only
increased in axial direction but also in radial direction.  The backflow has a
large circumferential component which is transmitted to the inlet flow and as
a consequence the outer 30% to 40% span is affected by strong
preswirl, which is the cause for the sudden drop of head.The
grooves used by Goltz [2] in an axial flow pump, see Figure 1 and the
grooves used by Kurokawa et al. [1] in a mixed flow pump are both able to absorb
the strong angular momentum of the back flow. When reaching the grooves, the backflow is forced to change the direction and loses
swirl due to friction and forming vortices. Hence the inlet flow is less
affected by the back flow.Figure 1
Flat axial grooves in front of the rotor used by Goltz [6].Vortex breakdown of the tip clearance vortex observed by Goltz [4].
(a)(b)Figure3 shows the axial and circumferential components of
the time mean absolute velocities in front of the rotor at deep stall measured
by Goltz [2]. The casing wall is on a
radius r=170mm, the hub on a radius r=60mm.
It can be seen that the radial extension of the backflow has increased to
approximately 20% of blade span.  These results are confirmed by numerical investigations,
see Benra et al. [7]. By using the grooved casing the circumferential
component is well reduced.Axial and circumferential components of the time mean absolute velocities in front of the rotor in deep stall,Q/QN=0.48, NPSHA=35m, Kosyna et al. [8].
(a)(b)


## 3. Experimental Setup

### 3.1. Test Rig and Test Pump

The test pump is a scaled-down model of
the axial-flow pump used by Goltz [2, 3] with a
specific speed of nq=150 and a casing diameter of
D=150mm.This pump was also already used by
Goltz [4].
However, unlike the
former horizontal installed pump, the pump is now installed vertically and
takes in water from an open intake chamber (Figure 4). Another difference
is that the pump casing has no spherical part but is entirely cylindric. The
chamber is made of plexiglass in order to observe the flow and to conduct
measurements with particle image velocimetry (PIV). Figure 5 shows the
head characteristics of the vertically installed pump in comparison with the
test pump used by Goltz.Figure 4
Intake chamber and pump with double inlet nozzle.Figure 5
Head characteristic without casing treatment of model-pump (DN 150) installed vertically in intake chamber and horizontal tubular casing test-pump (DN 350) used by Goltz.


### 3.2. Double-Inlet-Nozzle

Since the grooves have the disadvantage
of strong cavitation at the grooves inlet area as mentioned above another
method for stabilizing was conceived. Based on the knowledge of the flow
structures, the idea is to discharge the backflow with its high swirl. Hence the mixing of
backflow and inlet flow and the transfer of angular momentum is prevented.
Therefore, a so called double-inlet-nozzle was built (see Figure6).
Between the two nozzles, there is a gap which serves as an additional inlet
during stable operation and serves as discharge for the extended backflow when
operating in the stall regime.Figure 6
Design drawing of double-inlet-nozzle.All measurements at this test rig were performed at a rotational speed ofn=11501/min⁡.


### 3.3. Test Procedure

In order to show the flow field at the
inlet of the double-inlet-nozzle, a Dantec PIV-System was used (type New Wave
Gemini Nd:YAG PIV-laser).For these measurements, the Planar-PIV-method was
applied, camera and laser were installed perpendicular to each other (Figure7).
The water was seeded with polyamide particles with an average diameter of
20 μm. The pulsed laser exposed 2 images which are recorded by a 12-bit
CCD-camera with 1024×1280 pixels. For post processing, an adaptive cross correlation
was used followed by a statistical analysis of 14 single correlations and
finally a moving average validation has been applied.Position of the measuring windows for the examination of the inlet flow of the double-inlet-nozzle (above) and sketch from below (below).
(a)(b)The laser produces a light sheet below
the inlet nozzle. The measuring plane is located in the machine axis. It is
divided into 6 single measuring windows. Each window has the lengthx=84mm and y=106.5mm. They overlap each by 4 mm.An additional window is situated at the
gap between the two nozzles. Its dimension is68.8×109.2⁢mm.
Because of the holder from the double-inlet-nozzle, that hinders the light
sheet, the measuring plane for this window is chosen 20 mm outside of the
machine axis (Figure 7).The flow field was also measured at the
inlet of the original smooth casing. Because the outer diameter of the
inlet-nozzle here is smaller than with the double-inlet-nozzle, only 5
measuring windows (84×106⁢mm) form the measuring plane.For investigating the prerotation at 4
axial chord-lengths upstream of the rotor a tilted mirror was used
(Figure11). The laser is located at the opposite site of the plexiglass box.
The post processing used was the same as before. All PIV-measurements have been
carried out by Hauschke [9].Besides  the PIV-measurement also the
head performance and the efficiency of the pump were recorded. The efficiency was measured by a torque measuring shaft.


## 3.1. Test Rig and Test Pump

The test pump is a scaled-down model of
the axial-flow pump used by Goltz [2, 3] with a
specific speed of nq=150 and a casing diameter of
D=150mm.This pump was also already used by
Goltz [4].
However, unlike the
former horizontal installed pump, the pump is now installed vertically and
takes in water from an open intake chamber (Figure 4). Another difference
is that the pump casing has no spherical part but is entirely cylindric. The
chamber is made of plexiglass in order to observe the flow and to conduct
measurements with particle image velocimetry (PIV). Figure 5 shows the
head characteristics of the vertically installed pump in comparison with the
test pump used by Goltz.Figure 4
Intake chamber and pump with double inlet nozzle.Figure 5
Head characteristic without casing treatment of model-pump (DN 150) installed vertically in intake chamber and horizontal tubular casing test-pump (DN 350) used by Goltz.


## 3.2. Double-Inlet-Nozzle

Since the grooves have the disadvantage
of strong cavitation at the grooves inlet area as mentioned above another
method for stabilizing was conceived. Based on the knowledge of the flow
structures, the idea is to discharge the backflow with its high swirl. Hence the mixing of
backflow and inlet flow and the transfer of angular momentum is prevented.
Therefore, a so called double-inlet-nozzle was built (see Figure6).
Between the two nozzles, there is a gap which serves as an additional inlet
during stable operation and serves as discharge for the extended backflow when
operating in the stall regime.Figure 6
Design drawing of double-inlet-nozzle.All measurements at this test rig were performed at a rotational speed ofn=11501/min⁡.


## 3.3. Test Procedure

In order to show the flow field at the
inlet of the double-inlet-nozzle, a Dantec PIV-System was used (type New Wave
Gemini Nd:YAG PIV-laser).For these measurements, the Planar-PIV-method was
applied, camera and laser were installed perpendicular to each other (Figure7).
The water was seeded with polyamide particles with an average diameter of
20 μm. The pulsed laser exposed 2 images which are recorded by a 12-bit
CCD-camera with 1024×1280 pixels. For post processing, an adaptive cross correlation
was used followed by a statistical analysis of 14 single correlations and
finally a moving average validation has been applied.Position of the measuring windows for the examination of the inlet flow of the double-inlet-nozzle (above) and sketch from below (below).
(a)(b)The laser produces a light sheet below
the inlet nozzle. The measuring plane is located in the machine axis. It is
divided into 6 single measuring windows. Each window has the lengthx=84mm and y=106.5mm. They overlap each by 4 mm.An additional window is situated at the
gap between the two nozzles. Its dimension is68.8×109.2⁢mm.
Because of the holder from the double-inlet-nozzle, that hinders the light
sheet, the measuring plane for this window is chosen 20 mm outside of the
machine axis (Figure 7).The flow field was also measured at the
inlet of the original smooth casing. Because the outer diameter of the
inlet-nozzle here is smaller than with the double-inlet-nozzle, only 5
measuring windows (84×106⁢mm) form the measuring plane.For investigating the prerotation at 4
axial chord-lengths upstream of the rotor a tilted mirror was used
(Figure11). The laser is located at the opposite site of the plexiglass box.
The post processing used was the same as before. All PIV-measurements have been
carried out by Hauschke [9].Besides  the PIV-measurement also the
head performance and the efficiency of the pump were recorded. The efficiency was measured by a torque measuring shaft.


## 4. Results and Discussion

### 4.1. Pump Head Characteristic and Efficiency

Figure8 shows the pump head
characteristics and the pump efficiencies of the original casing, the casing
with grooves and the  double-inlet-nozzle.Pump head (above) and pump efficiency (below) of original smooth casing, grooved casing and double-inlet-nozzle.
(a)(b)The head characteristic shows that the
last point of the stable branch of the original casing is located at
approximatelyQ/Qopt=0.7. Reducing the mass flow even further will
lead to the drop in pump head. But using the casing with grooves or the
double-inlet-nozzle the head characteristics show a small saddle and than the
pump head grows again.The efficiency of all the three casings
remains the same
until the onset of stall. From there, the efficiency with the grooved casing
and with the double-inlet-nozzle is higher. It is remarkable that for the
latter the blockage due to the additional nozzle has negative influences neither on the head
performance nor on the efficiency.


### 4.2. PIV-Measurement of the Inlet Flow Near Design

Figure9 shows the inlet flow near
design conditions for the double-inlet-nozzle and for the original smooth
casing. The size of the vectors and also the colors illustrate the absolute
velocity c composed of the two velocity-components cax and cr in the image plane. The pictures are each composed of 7 and 5 windows,
respectively, which represent again an average of 14 individual measurements.
For a better overview, only every 4 vector in the x- and y-direction is shown.
For the window at the gap only every 6 vector is shown.Inlet flow of the double-inlet-nozzle (a) and of the original casing (b) near design conditions,Q/Qopt=1.06, c=(cax2+cr2)1/2.
(a)(b)The flow
approaches the pump from the right side.For the
double-inlet-nozzle, the inlet flow is nearly symmetrically to the machine
axis. For the smooth casing without grooves this does not apply well. One
reason could be that the outer diameter of the inlet-nozzle of the smooth
casing is smaller than the outer diameter of the double-inlet-nozzle and that
because of this the flow under the double-inlet-nozzle has more space to get
uniform.Besides the symmetry there can hardly
be found any differences between the inlet flows of both casings.In the upper figure, the small inlet
flow into the gap between both nozzles can be seen. About 13% of the mass flow
enters the pump through the gap.


### 4.3. PIV-Measurement of the Inlet and  Outlet Flow at Deep Stall

Figure10
shows the inlet flow at deep stall conditions.  The discharged near wall
backflow of the original casing without grooves (Figure 10(b)) is
deflected by the approaching flow and reenters into the inlet nozzle. This
causes a vortex over the whole circumference, so that the vortex has the shape
of a torus. At Figure 10(b) the cross section view of the vortex in the
machine axis can be seen. The core of the vortex shows a blockage which can be
seen in the middle.Inlet flow of the double-inlet-nozzle (a) and of the original casing (b) at stall conditions and discharge of the backflow at the gap in (a), Q/Qopt=0.3, c=(cax2+cr2)1/2.
(a)(b)Figure 11
Optical path for investigation of the prerotation.However the inlet flow at the
double-inlet-nozzle (Figure10(a)) is still smooth. The discharged flow is
ejected off the gap. According the principle of angular momentum, the
circumferential velocity component will be reduced on the way outwards. At a
radius r=127mm the maximum velocity of cr is about 20%
of the circumferential velocity u2.


### 4.4. PIV-Measurement of the Prerotation in Deep Stall

Figures12
and 13 show a comparison of the prerotation at 4 times the length of
span L upstream of the rotor. In order to know the position of the impeller,
the border of the rotor is shown. The size of the vectors and also the colors
show the composed velocity of the two velocity-components in the image plane.
The occurrence of prerotation is found at both casings. But with the
double-inlet-nozzle the maximum velocity cu is approximately
20% lower than with the original casing.Figure 12
Prerotation at original casing at stall conditions,Q/Qopt=0.3, 4L upstream of the rotor. cu=(u2+v2)1/2.Figure 13
Prerotation at double-inlet-nozzle at stall conditions,Q/Qopt=0.3, 4L upstream of the rotor.cu=(u2+v2)1/2.


## 4.1. Pump Head Characteristic and Efficiency

Figure8 shows the pump head
characteristics and the pump efficiencies of the original casing, the casing
with grooves and the  double-inlet-nozzle.Pump head (above) and pump efficiency (below) of original smooth casing, grooved casing and double-inlet-nozzle.
(a)(b)The head characteristic shows that the
last point of the stable branch of the original casing is located at
approximatelyQ/Qopt=0.7. Reducing the mass flow even further will
lead to the drop in pump head. But using the casing with grooves or the
double-inlet-nozzle the head characteristics show a small saddle and than the
pump head grows again.The efficiency of all the three casings
remains the same
until the onset of stall. From there, the efficiency with the grooved casing
and with the double-inlet-nozzle is higher. It is remarkable that for the
latter the blockage due to the additional nozzle has negative influences neither on the head
performance nor on the efficiency.


## 4.2. PIV-Measurement of the Inlet Flow Near Design

Figure9 shows the inlet flow near
design conditions for the double-inlet-nozzle and for the original smooth
casing. The size of the vectors and also the colors illustrate the absolute
velocity c composed of the two velocity-components cax and cr in the image plane. The pictures are each composed of 7 and 5 windows,
respectively, which represent again an average of 14 individual measurements.
For a better overview, only every 4 vector in the x- and y-direction is shown.
For the window at the gap only every 6 vector is shown.Inlet flow of the double-inlet-nozzle (a) and of the original casing (b) near design conditions,Q/Qopt=1.06, c=(cax2+cr2)1/2.
(a)(b)The flow
approaches the pump from the right side.For the
double-inlet-nozzle, the inlet flow is nearly symmetrically to the machine
axis. For the smooth casing without grooves this does not apply well. One
reason could be that the outer diameter of the inlet-nozzle of the smooth
casing is smaller than the outer diameter of the double-inlet-nozzle and that
because of this the flow under the double-inlet-nozzle has more space to get
uniform.Besides the symmetry there can hardly
be found any differences between the inlet flows of both casings.In the upper figure, the small inlet
flow into the gap between both nozzles can be seen. About 13% of the mass flow
enters the pump through the gap.


## 4.3. PIV-Measurement of the Inlet and  Outlet Flow at Deep Stall

Figure10
shows the inlet flow at deep stall conditions.  The discharged near wall
backflow of the original casing without grooves (Figure 10(b)) is
deflected by the approaching flow and reenters into the inlet nozzle. This
causes a vortex over the whole circumference, so that the vortex has the shape
of a torus. At Figure 10(b) the cross section view of the vortex in the
machine axis can be seen. The core of the vortex shows a blockage which can be
seen in the middle.Inlet flow of the double-inlet-nozzle (a) and of the original casing (b) at stall conditions and discharge of the backflow at the gap in (a), Q/Qopt=0.3, c=(cax2+cr2)1/2.
(a)(b)Figure 11
Optical path for investigation of the prerotation.However the inlet flow at the
double-inlet-nozzle (Figure10(a)) is still smooth. The discharged flow is
ejected off the gap. According the principle of angular momentum, the
circumferential velocity component will be reduced on the way outwards. At a
radius r=127mm the maximum velocity of cr is about 20%
of the circumferential velocity u2.


## 4.4. PIV-Measurement of the Prerotation in Deep Stall

Figures12
and 13 show a comparison of the prerotation at 4 times the length of
span L upstream of the rotor. In order to know the position of the impeller,
the border of the rotor is shown. The size of the vectors and also the colors
show the composed velocity of the two velocity-components in the image plane.
The occurrence of prerotation is found at both casings. But with the
double-inlet-nozzle the maximum velocity cu is approximately
20% lower than with the original casing.Figure 12
Prerotation at original casing at stall conditions,Q/Qopt=0.3, 4L upstream of the rotor. cu=(u2+v2)1/2.Figure 13
Prerotation at double-inlet-nozzle at stall conditions,Q/Qopt=0.3, 4L upstream of the rotor.cu=(u2+v2)1/2.


## 5. Conclusion and Future Work

This investigation showed that the double-inlet-nozzle, developed at Pfleiderer-Institut,(i)
reduces the preswirl upstream of the impeller by discharging the backflow when operating in stall regime,(ii)
stabilizes the head performance of an axial-flow pump as well as a grooved casing without suffering from cavitation at the grooves,(iii)
reduces the prerotation upstream of the rotor in deep part load conditions, and(iv)
does not reduce the efficiency although the cross section is partly blocked by the second inlet nozzle.Further investigations are as follows:(i)
variation of the axial position of the second nozzle,(ii)
NPSH-measurements by using a closed intake chamber with variable system pressure,(iii)
measurements of vibrations, shaft stresses, and sound level,(iv)
numerical investigation.



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*Source: 536850-2008-09-02.xml*