cfd simulation ofat t-junction

8/10/2019 Cfd Simulation Ofat T-Junction

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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



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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)


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

cfd simulation ofat t-junction

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