Research Article NUMERICAL STUDY OF FLOW BEHAVIOUR IN A

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: [email protected], [email protected]

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.



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)



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.



Fig. 3 Velocity vector plot at junction of forebay and rectangular sump

Fig 4 Velocity profiles



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



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



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



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



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



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



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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Documents

Research Article NUMERICAL STUDY OF FLOW BEHAVIOUR IN A