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CFD-SIMULATION OF A T-JUNCTION -
BORIS HUBER
Institute of Hydraulic and Water Resources Engineering, Department of Hydraulic
Engineering, Vienna University of Technology, Austria
1. INTRODUCTION
In this paper the hydraulic properties of a T-junction are investigated. The T-junction,
planned in the Kops 2 high head power station by the Austrian Vorarlberger Illwerke AG, is
located in the duct system between each turbine- and pump conduit of the three machine units.
The projected pump power-plant Kops 2 is designed to equalize peaks of energy-fluctuationsmainly. Therefore fast regulation processes are necessary and flow conditions are changing.
To determine the flow characteristics and head losses the T-junction was investigated in a
physical model test as well as in a CFD-simulation. The results of the hydraulic model tests
were compared with the numeric calculation in order to assess the results. Then an alternative
design of the junction was simulated to find out which junction has better flow properties or
lower head-losses respectively.
2. INVESTIGATED OPERATION CONDITIONS
Following 5 operating conditions which differ in regard to the direction of flow were
investigated: turbine operation, pumping and 3 special operating conditions for fast regulation
processes in a hydraulic short-circuit flow (coming from the pumps, going through the
turbines and then through the pumps again):
CASE 1 Turbine operation:
res
pu
tu
C1
Flow approaches the T-junction from the reservoir and leaves
through the turbine; there is no flow in the branch to the pump.
The maximum flow through each of the 3 turbines is 26.67 m/s in
nature.
res
pu
tu
C2
CASE 2 Pump operation:
Flow comes from the pump and exits to the reservoir while there
is no flow through the turbines. The maximum pump discharge is
19.33 m/s per pump.
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CASE 3 Hydraulic short-circuit
mp) diverts into the two
re
ASE 4 - Hydraulic short-circuit
w enters from the pump
an
ASE 5 - Hydraulic short-circuit
ax. 19.33 m/s per pump)
th
3. PHYSICAL MODEL
Experiments were conducted w :9.9. The diameters of the pipes
w
Fig. 2 Sketch and photo of the T-junction in th
res
pu
C3
res
pu
C4
res
pu
tu
C5
tu
tuFlow from the pump (19.33 m/s per pu
maining branches. The ratio of the split flow is variable.
C
In this special operating condition flo
d leaves through the turbine; there is no flow in the branch to the
reservoir.
C
In addition to flow from the pump (m
ere is a combining flow from the reservoir which amounts up to
Qres = - 7.33 m/s for each turbine (flow direction is definednegative).
Fig. 1 Operation Conditions
ith a length-scale of l= 1
ere 192 mm in the reservoir and turbine branches and 172 mm in the branch coming from
the pump. The T-junction and the adjacent pipes were made of Plexiglas. In nature there is a
90 bending between the pump and the T-junction. In order to reproduce the resulting flow-
pattern correctly, a 90 bow was used in the experimental setup also (see Fig. 4). The bendingturned out to be of significant influence on the experimental results.
e experimental setup (dimensions in mm)
192
192
172
192
590
400
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The experimental conduit was mounted on the laboratory wall at a height of 2.5 m and the
T-junction was tilted with an angle of 16 to the vertical axis corresponding to the prototype.
The complete experimental setup consisted of pipes, valves and measuring instruments:
inductive flow meters (IDM) as well as pressure gauges, a difference pressure transmitter and
piezometric tubes (see Fig. 3 and 4).
Fi
Piezometric
tubes
Computer
IDM
Pressure gauges
Difference pressure
sensor
Piezometric
tubes
Computer
IDM
Pressure gauges
Difference pressure
sensor
g. 3 Hydraulic model in the laboratory
Pressure was measured at 3 sections: M1 and M2 located in the reservoir-turbine branch,
2.08 m from the intersection point of the pipe-axes away and M4 in the branch to the pump
with a distance of 0.61 m from the axes intersection point (see Fig. 4).
Fig. 4 Sketch of experimental setup (dimensions in cm)
IDM
IDM
200 59 200
M1 M2
M4
21
200
40
21 21M2bM2aM1a M1b M4bM4a
M4c
M1 M2
208208
inflow (pu)
outflow (tu)
inflow (res)
outflow (res)
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The head-loss coefficient of the T-junction is determined by:
g2v 2
m
ij
jtoifromfrictionpipejbranchinurepresstotalibranchinpressuretotalK
=
e loss coefficient is based on the pump-velocity (vpu). However, as there is no
fl
ded to the measured
o
paratively small the medium head
res-tu
: there is a sudden change of
flow direction combined with strong swirl flow resulting from the junction itself on the onehand and from
pu-res
Generally th
ow in the pump in case 1 here the loss coefficient is based on vres(or vtuwhich is the same in
case 1).
To obtain the total pressure the local velocity-head (v2/2g) was ad
(static) pressure. The friction head loss of the straight pipe section between two measuring
sections was calculated with a roughness of k=0.003 mm (Plexiglas).
Numerous experiments with different flow and pressure conditions were carried out. In
every case flow was gradually increased within a range where a constant loss coefficient was
btained. Discharge varied between 30 and 100 l/s.
3.1. EXPERIMENTAL RESULTS
In case 1 the T-junction works like a local expansion. The flow pattern can be compared
with the one of a straight pipe and the head losses are com
was 0.141.loss coefficient K
In the other cases the flow pattern was significantly different
the bows in the inlet conduit on the other hand. Due to the asymmetrical
approach flow (caused by the 90 bow at the inlet section) case 2 and case 4 was not mirror-
inverted. The head loss between the pump and the reservoir in case 2 (K = 0.83)2was
lower than the head loss between the pump and the turbine in case 4 respectively (Kpu-tu=
1.08)2. The determined head losses for case 1, 2 and 4 - compared with values obtained by
Miller (1978) for a sharp-edged T-junction - are summarized in Table 1. Loss coefficients for
case 3 and 5 are depicted in Fig. 5 and 6.
loss coefficient K (90) loss coefficient K (0)
min max mean Miller min max mean Miller
case 1 Kpu-tu -0.41 -0.39 0.40 -1.00 Kres-tu 0.15 0.13 0.14 0.05
case 2 Kpu-res 0.78 0.89 0.83 0.70 Kres-tu -0.15 -0.21 0.18 -0.33
case 4 Kpu-tu 0.99 1.15 1.08 0.70 Ktu-res -0.17 -0.30 0.24 -0.33
Table 1 Loss coefficient for cases 1, 2 and 4 (based on v resin case 1, else on vpu)
1based on vresor vturespectively2Based on vpu
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loss-coefficient case 5 (based on v pu)
Fig. 5 and 6 Loss coefficients (based on vpu) for cases 5 (left) and 3(right)
As mentione before s observe visualize swirl
pressure air wa in ated. I n rl was n king in the
direction of
To quant e r q a ur institute
was used. I s e n d l oted in the
nter of the
spheres and if there is swirl in the flow, the swirl-meter turns accordingly. With an optical
sensor applied at the pipe outside the passing-by of the spheres is recorded and the tangential
velocity can then be derived from the number of revolutions per minute. The ratio of
tangential and axial velocity ranges from 0.7 to 1.0.
d , strong swirl flow wa d in case 2 to 5. To
s fl n the turbi e branch swi orie ted clockwise loo
flow.
ify swirl, a sp cial measu ing-e uipment which w s developed by o
t consists of 2 steel pher s con ecte with a rod ike a bar-bell, piv
middle. Due to the shape of the spheres fluid forces always run through the ce
-2.00
-1.50
-1.00
-0.50
0.00
0.50
1.00
1.50
2.00
-2.00 -1.80 -1.60 -1.40 -1.20 -1.00 -0.80 -0.60 -0.40 -0.20 0.00
q=Q(res)/Q(pu)
loss-coeffic
ientK
K pu-res K pu-res case 4
K tu-res K tu-res case 4K pu-tu K pu-tu case 4
loss-coefficient case 3 (based on v pu)
-2.00
-1.50
-1.00
-0.50
0.00
0.50
1.00
1.50
2.00
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
q=Q(res)/Q(pu)
loss-coefficientK
K pu-res case 4 K pu-res K pu-res case 2
K tu-res case 4 K tu-res K tu-res case 2
K pu-tu case 4 K pu-tu K pu-tu case 2
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4. CFD-SIMULATION
The numerical simulation (3-dimensional, steady) was conducted with the program
package FLUENT 6.1.22. At first the system was modelled in nature scale and compared with
a simulation in model scale. As there were no significant differences the further simulation
was conducted in model scale, because the simulation in full scale demanded extremely long
computation times.
The simulation was carried out in consideration of gravity (except for case 1). The origin of
the coordinate system was located at the intersection point of the pipe-axes and the z-axis ran
in direction of the axis of the branch to the pump (thus deviating from the gravitational axis
under 16).
4.1 MESH GENERATION
The mesh generation was carried out with GAMBIT. Depending on the particular case,about 1 - 2 Million cells had been used. Especially in the near-wall region and zones of high
pressure or velocity gradients the mesh had to be very dense and therefore computation time
amounted about 24 hours on a machine with 4 parallel processors. As turbulence models the
Standard-k--Model and the RNG-k--Model (swirl dominated flow) were used.
Fig. 7 (a and b) T-junction with mesh
4.2 MODIFIED T-JUNCTION GEOMETRY
If the results of the CFD-simulation turned out to be satisfactory, a modified T-junction
geometry should be simulated in order to find out which one had better hydraulic properties.
By the use of the numeric simulation instead of another physical model test with the
alternative design it was possible to save costs. The dimensions of the original T-junction and
the variant are shown in Fig. 8.
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the top (see Fig. 9). In the dividing branch velocity is approximately zero. The flow pattern of
the variant showed no significant differences.
Fig. 9 Contours of Velocity case 1
4.4 CFD-SIMULATION CASES 2-5
In cases 2-5 there is a sudden change of flow direction. The flow pattern can be compared
with an impinging jet - superposed with strong generation of swirl. It is not expected that the
Standard-k--Model performs well here. For flow with a strong swirl for instance the RNG-k-
(swirl dominated) or the Realizable-k--Model or even the Reynolds-Stress-Modell (RSM)
is recommended.
The head losses were determined at the same cross sections M1, M2 and M4 where the
measurements took place in the experiments. At first, the Standard-k--Model was used. As
expected, the loss coefficients were clearly lower then those gained from the experiments. A
significant improvement was achieved by using the RNG-model (short description: RNG-k--
Model, swirl dominated flow, swirl factor 0.07, Standard wall function, 1 Mill. Cells,
roughness of walls k=3e-6 m). Simulations were conducted until full convergence was
reached which was surveyed by monitors showing the time dependent development of total
pressure in a measuring point.
Wit the RNG-M el e h ad l ed from the CFD-sim ere clearly
smaller than those from the experime particular tial velocity was significantly
higher in the experim y er of
bends in the experim ac t fo ig wi bo ry ion at
the pu difie s o con velocity an addition nge el (half
as mu velocity) was applied.
With this modification the ss oefficients Kpu and K -res agreed well with the
experi . b) he los m the pump the e was
h reservoir was high
co
10 c).
h od th e osses deriv ulation w
nts. In tangen
ent than in the CFD-simulation. This is most likel due to the numb
ental setup. To coun r the h her s rl, the unda condit
mp was mo d: in tead f a stant al ta ntial v ocity
ch as the orthogonal
lo c -res tu
ments (see Fig 10a and . T head s fro to turbin K pu-tu
predicted very well in all cases except in case 5 when flow from t e
mpared to flow from the turbine (q = Qres / Qtu). However, when flow from the reservoir
was high in case 5 (q < -0.5) the CFD-simulation drifted away from the experiments (see Fig.
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The loss coefficients of the experiments and CFD-simulations of case 2 to 5 are depicted in
Fig. 14 a to c. In these pictures the loss coefficients of the variant are also included. In general,
the pattern of flow of the variant did not differ significantly from the tested T-junction.
Contour-plo to visualize
case 2 to 4.
Further im esh-
refineme
ts of velocity of the T-junction and the variant as well as path lines
swirl flow are depicted in the following. The loss coefficients of the variant were higher in
provement of the CFD-simulation could possibly be obtained by m
nt until the whole boundary layer is resolved and by the use of the RSM-Model. To
obtain this, a huge number of cells and extremely long computation time would be needed.
Fig. 10 a) Loss coefficient K pu-res
loss-coefficient K pu-res (based on v pu)
0.50
1.50
2.00
-0.5 0 0.5 1
q=Q(res)/Q(pu)
Kp
u
1.00
-3.00
-2.50
-2.00
-1.50
-1.00
-0.50
0.00
-2 -1.5 -1-res
experiment
CFD-Simulation
CFD-simulation variant
Fig. 10 b) Loss coefficient K tu-res
loss-coefficient K tu-res (based on v pu)
-0.50
0.00
0.50
1.00
1.50
2.00
-2 -1.5
Kt
u-res
-3.00
-2.50
-2.00
-1.50
-1.00
q=Q(res)/Q(pu)
-1 -0.5 0 0.5 1
experiment
CFD-Simulation
CFD-simulation variant
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loss-coefficient K pu-tu (based on v pu)
-1.50
-1.00
-0.50
0.00
0.50
1.00
1.50
2.00
-2 -1.5 -1 -0.5 0 0.5 1
Kp
u-tu
experiment
CFD-Simulation
-3.00
-2.50
-2.00
q=Q(res)/Q(pu)
CFD-simulation variant
Fig. 10 c) Loss coefficient K pu-tu
Fig. 11 a to d) Contours of velocity with details: case 2 left) and case 4 (right)
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Fig. 12 Swirl flow in case 4, visualization with path lines
5. CONCLUSIONS
The hydraulic properties of the investigated T-junction proved to be very satisfactory. In
particular the loss coefficients were comparatively small. Due to the asymmetrical approach
flow and the shape of the T-junction considerable swirl was observed in the branch to the
turbine.
The CFD-simulation agreed with the experiments very well. Only in case 5, when flow
from the reservoir compared to flow from the pump was high, there were significant
differences in the loss coefficients.A variant of the T-junction was also investigated in the CFD-simulation. In case of turbine
operation the head losses of the variant were slightly smaller, but in the other cases higher
than the original T-junction.
REFERENCES
Miller, D.S. (1978).Internal Flow Systems,BHRA Fluid Engineering, Vol. 5
Vorarlberger Illwerke AG, leaflet Kopswerk II, Vorarlberger Illwerke AG