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International Journal of Advanced Engineering Technology E-ISSN 0976-3945
IJAET/Vol.II/ Issue II/April-June, 2011/118-128
Research Article
NUMERICAL STUDY OF FLOW BEHAVIOUR IN A MULTIPLE
INTAKE PUMP SUMP Tanweer S. Desmukh & V.K Gahlot
Address for Correspondence
Civil Engineering Department, M.A.N.I.T, Bhopal
E-mail: tanweer_sultanah@yahoo.com, gahlot_vk@rediffmail.com
ABSTRACT In large pumping stations the sumps generally have multiple intakes. The flow in such sumps is always complicated due to
interference of individual intakes, especially when all the pumps are not working. The flow conditions in such sumps are difficult
to predict and have to be evaluated on a case to case basis usually through model studies. Computational fluid dynamics (CFD)
analysis is a tool that can be used to provide insight into flow phenomena and hydraulic designs of an intake structure. In this
paper the flow through a pump sump model with 3 pump intakes has been analysed using commercially available CFD software
CFX. The flow conditions for three different horizontal angles of the sump have been discussed.
KEY WORDS: Sump, numerical simulation, streamline pattern, velocity vectors, swirl.
1. INTRODUCTON
It is a well established fact that a number of problems
faced by pumps in a pumping station are related to
sump design rather than to pump design. Vibration,
Cavitation, rough running, lower than expected
efficiency and reduced pump life can generally, be
traced to undesirable flow conditions in the sump and
its approach area. These problems have been studied
extensively by various investigators (Fraser1953,
Tullis1979, Fletcher1979, Sweeney et al 1982,
Padmanabhan1982, Dicmas1987, Hecker1984etc.).
Almost all the studies report the causes of pump
problems and deficiencies to be – (1) Uneven flow
distribution in the sump causing flow circulation the
pump column (2) Large scale turbulence generated in
the approach to sump (3) Vorticity generated by flow
past pier noses, screen gates and other structural
members (4) Vorticity generated at fluid shear zones
formed at discontinuous flow boundaries in the
vicinity of the pump (5) Vorticity generated in the
boundary layer at the sump walls and
floor.(6)Vorticity generated by the flow past the
pump column.
Although the flow conditions causing pump problems
are well established, there are no specific solutions to
eliminate them. Although some design guidelines for
configuration of pump sumps are available, no
definite design criteria which may have general
applicability have been developed so far. The major
reason for this being site specific geometrical and
hydraulic constraints. The solutions developed for
any project are not universal and different solutions
are needed for the conditions present in different
cases. The most common solution to solving potential
problems in new designs and rectifying problems
observed in existing installations is to construct a
scaled model in the laboratory, observe the flow (by
dye injection) and propose modifications to intake
geometry. The limitation of cost and time, inhibit
trying out of many variants before reaching an
optimized design for an intake
With rapid progress in Computational Fluid
Dynamics (CFD), numerical simulation is being
regarded as an effective tool in solving fluid
problems in simulating the flow of water in pump
sumps. It reduces the cost as well as time associated
with finalizing the design of a pumping station, by
reducing the number of options to be tested in a
laboratory. Although the flow in the sump of the
pump station with one water intake has been studied
using the Navier-Stokes equations and turbulence
model by Constantinescu[1,2] , Xu[20] , Lu[10], and
Guo[6]. In the work of Matahel[11], the inviscid flow
in the sump with two water intakes was studied, on
the basis of successful simulation of the flow in the
sump with one intake. Moreover these studies
pertain to development of CFD code for simple
rectangular sump having one and two pumps. These
codes cannot be used as general software for CFD
analysis of any intake structure. Desmukh et.al [3]
showed that commercially available CFD software –
ANSYS CFX can be used to predict the flow
conditions in a pump sump. In the present work, flow
conditions in a pump sump with three intakes have
been analysed and the effect of horizontal angle of
forebay on the flow conditions studied, using
ANSYS –CFX software.
International Journal of Advanced Engineering Technology E-ISSN 0976-3945
IJAET/Vol.II/ Issue II/April-June, 2011/118-128
2. PUMP SUMP GEOMETRY
The computational study was done for the pumping
system of a cooling tower having three pumps. The
prototype layout includes leading channel, approach
channel, forebay, pump sump and intake. The
pumping unit has two leading channels carrying
60,000m3/hr, to an approach channel of 6.0m width.
The approach channel leads to a forebay through a
sloping rectangular channel of length 15.30m having
a slope of 13.5° in the vertical plane. This slope
provides a drop of 4.371m from the approach channel
level to the floor level. The forebay is 24.4m long
with an expansion angle of 15° ending in a
rectangular sump of 20.25m x 17.2m. The sump has
three pump bays separated by piers of length 15.4m
and width 1m. Of these the two end pumps were
working while the central pump was a non working
standby pump. A model of the above discussed
prototype at a scale of 1:11 has been taken for the
present computational study. A schematic diagram of
the model in plan and elevation showing all the basic
dimensions is given in Fig 1, where X1 = 0.567m, X2
= 1.390m, X3 = 2.218m, X4 = 1.840m, X5 = 0.500m,
X6 = 1.400m, Z1 = 0.303m, Z2 = 0.364m, Z3 =
0.667m. The model discharge was 0.137m3/s.
3. COMPUTATIONAL MODEL
Fig. 2 shows the solution domain for the numerical
calculation. The computational investigations were
performed using ANSYS ICEM CFD 10.0 and
ANSYS CFX-10.0 softwares. ANSYS ICEM CFD
10.0 was used for modelling and mesh generation
while the analysis was done using ANSYS CFX10.0.
The inputs and outputs of both the softwares are in
easily accessible formats enabling full integration
with any CFD software. For the CFD model in the
present study, volumetric meshing with unstructured
tetra meshing option was adopted for grid generation
in the pump sump geometry. In general the mesh
generated for different variants had about 8 to 12 lakh
elements with the number of nodes varying from 1 to
2 lakhs. The physics of the simulation domain was
defined in CFX-Pre, the preprocessing module of
ANSYS CFX. The domain was specified as a Non-
Buoyant, Stationary Fluid domain with working fluid
as water at 25°C and reference pressure as 1atm. The
turbulence model selected was K-Є model. The inlet
section at the entrance to the sump was specified as
inlet boundary with mass flow rate as 137kg/s and
flow direction as normal to the boundary.
Fig 1 Geometry of original sump model (scale = 1:11)
International Journal of Advanced Engineering Technology E-ISSN 0976-3945
IJAET/Vol.II/ Issue II/April-June, 2011/118-128
Fig 2 Computational model of the Sump
The outlets of the three pipes were specified as outlet
boundaries with relative average static pressure as
0atm. The flow regime at both inlet and outlet
boundaries was specified as subsonic. All the outer
walls of the flow region and the internal walls (pump
columns below free surface) were specified as the
boundary type wall with flow condition as - no slip.
The free surface of fluid was specified as a symmetry
boundary i.e. a stress free plane of symmetry or
surface across which no flow takes place. The
ANSYS CFX-Solver module of ANSYS CFX-10 was
used to obtain the solution of the CFD problem. The
solver control parameters were specified in the form
of solution scheme and convergence criteria. Upwind
scheme was specified for the solution while for
convergence the residual target for RMS values was
specified as 10-6. With the above specified
convergence criteria it took about 48hrs for the
solution of one simulation variant.
4. Numerical Analysis of Flow through Sump
Model
The results of the computational simulation can be
analyzed using number of variables. In this study it
has been restricted to the comparison of results based
on the pattern of streamlines of flow, velocity vectors
and the velocity profiles. The major problem revealed
through the study of the streamlines of flow is the
formation of a large rotating fluid mass, in the central
bay with the non working pump. Velocity vectors in
horizontal planes as well as planes perpendicular to
the flow direction (Fig 3) show a uniform expansion
of flow both along the depth as well as the width of
the sump, caused by the expansion of the sump in
these directions. This results in a concentration of
flow along the boundaries (i.e. sidewalls and bottom
of the channel) which in turn creates high velocity
zones along the boundaries and a low velocity zone at
the center. As the cross sectional area of sump
increases there is a corresponding decrease in the
magnitude of velocities in all the zones. This is
depicted in the velocity profiles at sections S1,S2 and
S3 (Fig. 4 ). It can be seen that in the sloping section
the minimum velocity reduces from 0.78m/s at
section S1 to 0.29m/s at S2 while the maximum
velocity reduces from 1.16m/s to 0.61m/s. At section
S3 there is a further decrease in the magnitudes - the
maximum velocity reducing to 0.34m/s and minimum
velocity to 0.06m/s. The streamline pattern in the
vertical plane parallel to the sump axis (Fig. 5) shows
that a very large rotating mass of fluid is created in
the rectangular portion of the sump. This rotating
mass intrudes about 0.8m into the expansion portion
and on the downstream side it just intrudes into the
pump bays. The extent of the rotating mass is
maximum along the centerline and it reduces
gradually as we move towards the boundaries. This is
because in the central bay there is a non working
standby pump. In the vertical direction the rotating
mass extends from the free surface to about 0.2m
above the sump bottom. It is seen that the rotating
mass occurs mostly in the central low velocity zone
(which has been continuing right from the starting of
the vertical slope) in the rectangular portion of the
sump just ahead of the central bay with the non-
working pump. The streamline pattern also shows
that as the high velocity flows along the bottom of
the channel reach near the pump column, the flow
moves towards the free surface (in the sudden empty
space) and then turns towards the backwall to form
another rotating mass in the central bay. In the side
bays, as the extent of the rotating mass is less the
distance of upward movement as well as the velocity
of flow moving upward is less and the second vortex
is not formed. The direction of rotation in the
upstream vortex is clockwise. Hence, the streamlines
of flow in the upper half of the vortex (upto 0.15m
from free surface) have an upstream direction and
oppose the normal flow in the sump. Thus when both
these flows meet, the streamlines of the vortex are
forced to turn back in the horizontal plane. This is
clearly seen in streamline pattern in the horizontal
plane (Fig. 6). This turned flow then enters the side
bays compressing the directly coming streamlines
towards the sidewalls resulting in an increase in
velocities at the boundaries. This is reflected in the
velocity profiles in the YZ (Fig. 7.) plane along
section S4. The velocity plots for all the depths show
a sharp increase in the velocities in a short distance of
about 0.105m near the boundaries, with the
maximum velocity ranging from 0.27m/s to 0.29m/s.
International Journal of Advanced Engineering Technology E-ISSN 0976-3945
IJAET/Vol.II/ Issue II/April-June, 2011/118-128
Fig. 3 Velocity vector plot at junction of forebay and rectangular sump
Fig 4 Velocity profiles
International Journal of Advanced Engineering Technology E-ISSN 0976-3945
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Along centerline of sump
Along pump axis in end bay
Fig 5. Streamline pattern in vertical longitudinal planes in Original Model
Horizontal Plane at free surface
Horizontal Plane at 0.3m from bottom of sump
Fig. 6. Streamline pattern in horizontal planes in Original Model
International Journal of Advanced Engineering Technology E-ISSN 0976-3945
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Velocity profile in YZ plane along section S4
Fig. 7 Velocity profiles in original model
The velocity plot also shows that the velocity in the
central portion of the sump reduces from about
0.17m/s at free surface i.e.0.66m depth of flow to a
minimum of 0.015m/s at 0.5m from the bottom of the
channel and then again increases as we move towards
the bottom of the sump. This is because center of the
rotating mass where the flow is nearly stagnant lies at
about 0.48m from the bottom of the channel. The
depths lower than 0.5m depth correspond to the
lower half of the rotating mass where the streamlines
of the rotating mass are in the same direction as those
of the flow coming from upstream. Hence more flow
enters the end bays directly, and the flow conditions
become more and more uniform as we move down.
This causes the velocities in the central zone to
increase progressively with lowering depths. Also
with the flow becoming more uniform the velocity
profiles become more flat with nearly constant
velocities in the central region – about 0.06m/s at
0.4m depth, 0.1m/s at 0.3m depth and 0.12m/s at
0.2m depth. Beyond this at 0.1m flow depth, the
velocity shows a large increase reaching a maximum
of 0.29m/s in the central bay. This is because the
flow here corresponds to the high velocity zone at the
bottom of the channel as already explained.
The effect of a sudden change from an expanding
section to a rectangular section is evident through the
streamline pattern in the horizontal plane. As the
diverging streamlines of the expansion portion are
suddenly forced to move inwards, the flow starts
leaving the end walls and moves towards the inner
walls causing a concentration of flows along the pier
walls. This results in creation of a separation near the
outer corner and concentration of flows towards the
inner corners of the side bays. When this high
velocity flow along the pier wall reaches the
backwall it turns following the boundary, causing
formation of backwall vortices as well as rotation of
flow around the pump column. This creates swirl at
entry to the suction pipe of the pump, which can be
seen clearly in the streamline pattern in a horizontal
plane at the entry level. The velocities at the pipe
entrance vary from about 1.5 to 2m/s. The streamline
pattern in the vertical plane behind the pumps (Fig. 8)
shows the formation of bottom corner vortices and
free surface vortices.
Variant-1 (H=12.61°, V=14.68°)
The flow pattern in the original model showed
divergence of streamlines towards the sidewalls in
the forebay. Assuming that this might be due to the
large angle of horizontal expansion in this variant the
angle has been reduced from 13.5° to 12.61° by
increasing the length of the forebay. Study of the
streamline pattern along the vertical plane parallel to
sump axis (Fig. 9) shows that there is only one
rotating mass in the central pump bay as compared to
International Journal of Advanced Engineering Technology E-ISSN 0976-3945
IJAET/Vol.II/ Issue II/April-June, 2011/118-128
the two in the original model. The rotating mass now
covers whole of the central bay extending right upto
the pump column and on the upstream side it intrudes
about 1m into the forebay. In the vertical direction
also the extent has increased covering nearly the full
depth of the sump. Consequently the center of the
rotating mass has also shifted down and now lies at
about 0.4m from the bottom. These changes are
reflected in the velocity profiles.
The velocity profiles in the YZ plane along section
S4 (Fig. 10) show a similar trend as in the original
model. However, the velocity distributions have
become more uniform and the profiles are no longer
skewed. This is because the rotating mass is
symmetrical about the sump axis. It is seen that the
velocity in the central portion of the sump reduces
from 0.24m/s at free surface to 0.12m/s at 0.5m and
0.045m/s at 0.4m from channel bottom and then
increases at lower flow depths reaching 0.07m/s at
0.3m and 0.14m/s at 0.2m. This is because the center
of the rotating mass lies near 0.4m where the velocity
is least. The velocity at 0.1m flow depth reaches
0.23m/s.
Thus it can be seen that the velocities in the rotating
mass have increased as compared to the original
model. However, below it i.e. near the bottom of the
channel the velocity has decreased. The velocities at
the boundary are in the same range i.e. 0.27 to
0.3m/s.
Fig. 8 Streamline pattern near backwall in Originalmmodel
Fig. 9. Streamline pattern in vertical plane along sump axis in Variant-1
International Journal of Advanced Engineering Technology E-ISSN 0976-3945
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Fig. 10 Velocity profiles in YZ plane along section S4 in Variant-1
Study of the streamline pattern along the vertical
plane parallel to sump axis (Fig. 9) shows that there
is only one rotating mass in the central pump bay as
compared to the two in the original model. The
rotating mass now covers whole of the central bay
extending right upto the pump column and on the
upstream side it intrudes about 1m into the forebay.
In the vertical direction also the extent has increased
covering nearly the full depth of the sump.
Consequently the center of the rotating mass has also
shifted down and now lies at about 0.4m from the
bottom. These changes are reflected in the velocity
profiles. The velocity profiles in the YZ plane along
section S4 (Fig. 10) show a similar trend as in the
original model. However, the velocity distributions
have become more uniform and the profiles are no
longer skewed. This is because the rotating mass is
symmetrical about the sump axis. It is seen that the
velocity in the central portion of the sump reduces
from 0.24m/s at free surface to 0.12m/s at 0.5m and
0.045m/s at 0.4m from channel bottom and then
increases at lower flow depths reaching 0.07m/s at
0.3m and 0.14m/s at 0.2m. This is because the center
of the rotating mass lies near 0.4m where the velocity
is least. The velocity at 0.1m flow depth reaches
0.23m/s. Thus it can be seen that the velocities in the
rotating mass have increased as compared to the
original model. However, below it i.e. near the
bottom of the channel the velocity has decreased.
The velocities at the boundary are in the same range
i.e. 0.27 to 0.3m/s.
Thus we see that although the rotating mass in the
central bay still exists the reduction in angle of
horizontal expansion has caused the flow conditions
to become more symmetrical. Hence in the next
variant the angle is reduced further.
Variant-2 (H=12.27°, V= 14.77°)
In this variant the horizontal angle of expansion was
reduced from the previous 12.61° in Variant-1 to
12.27°. For this the width of the approach section was
increased from 500 to 600mm. Rounding off the
approach length to1380mm changes the angle of
vertical slope is 14.77°. The results obtained for this
variant are similar to that in the original model but
with the formation of only one rotating mass in the
central bay (Fig. 11). The extent of the rotating mass
has also reduced. It now intrudes only 0.35m into the
forebay
The velocity profiles in the YZ (Fig. 12) plane along
section S4 also show a similar trend as in the original
model but with an overall increase in velocities
except at the boundaries. At the side walls the
velocity has reduced to a maximum of about 0.25m/s
while near the bottom of the channel it has reduced to
0.26m/s. Due to this reduced velocity of the
streamlines at the bottom of the channel the second
vortex does not form properly. It is also seen that in
the rotating mass the velocities at the periphery have
International Journal of Advanced Engineering Technology E-ISSN 0976-3945
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increased by about 0.02m/s as compared to the
original model while in the central portion it is nearly
in the same range i.e. 0.06m/s at 0.4m from the
bottom and 0.11m/s at 0.3m.
A comparison of the vector plots in horizontal plane
in the right bay of the original sump model and the
two variants (Fig. 13) shows that variant-2 shows the
best conditions. The compression of flow towards the
sidewalls is least and flow conditions are most
uniform in this variant. It is also seen that the
velocities inside the pump column in the original
model are lower as compared to that in the two
variants. Both variants 1 & 2 show almost same
range of velocities however variant-2 shows almost
negligible swirl. The streamline pattern near backwall
shows that the bottom corner vortices have also been
eliminated (Fig 14).
Vertical plane along the sump axis
Fig. 11 Streamline pattern in vertical plane along sump axis in Variant-2
Fig. 12 Velocity profiles in YZ plane along section S4 in Variant-2
International Journal of Advanced Engineering Technology E-ISSN 0976-3945
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Model
Variant-1
Variant-2
Fig. 13 Vector plots in right pump bay in horizontal plane
Variant-2
Fig. 14 Streamline pattern near backwall in Variant-2
International Journal of Advanced Engineering Technology E-ISSN 0976-3945
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5. CONCLUSIONS
Following conclusions can be drawn from the present
study:
1. The study confirms that the flow within the
pump sump is greatly affected by the
geometry of the sump.
2. CFD is very helpful in analyzing the effects
of sump geometry on the flow pattern.
3. Commercial softwares like ANSYS CFX
reduce the time of analysis significantly.
4. For the present case reducing the angle of
expansion in the horizontal plane improves
the flow conditions considerably. However
validation through model study will further
confirm the results.
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