efficiently determination of the heat exchanger...

EFFICIENTLY DETERMINATION OF THE

HEAT EXCHANGER RTCF 200 kW FOR

ELECTRIC POWER TRANSFORMERS

USING THE FINITE ELEMENT METHOD

M. BICĂ1

M. IOLU 2 Ş. IOLU

3

Abstract: This paper presents the results of analysis using finite element

method with aid of FLUENT 6.3.26 programme concerning the efficiency of

heat exchanger RTCF of 200 kW used for high power electrical transformers

in compared with the results of tests carried experimental.

Key words: heat exchanger, thermal efficiency, FLUENT, MEF.

1 Dept. of Road Vehicle, 2 Dept. Applied Mechanics, Mechanic Faculty, University of Craiova; 3 Dept. of Descriptive Geometry end E.G., Mechanic Faculty, Technical University of Cluj-Napoca;

1. Constructive and Functional Analysis

of the Oil Radiator Type Comb

RTCF200kW

Determination of the numerical calculation

(using M.E.F.) concerning the efficiency of

the heat exchanger with forced cooling circuit with oil and air by type RTCF 200

kW, mounted on the high power electric

transformers, fig.1 (whose cost price can be

between 0,5 ÷ 1,5 million euros) is going to design execution of the transformer, to the component materials and to the extreme

operating conditions prescribed by the

designer (I.C.M.E.T. Craiova). Constructive cooling battery consists of

six identical modules positioned in front of

three fans, united intro metal casing, Fig.2.

In particular, each module contains some 36 identical cooling circuits, fig.3.

Forced cooling system of transformer is

composed of a hydraulic circuit through the oil is cooled into aluminum elements of

which are attached on the outside the

cooling fins.

This circuit is placed inside the another

circuit system with forced air cooling.

Fig.1

Fig.2

Conferinţa Naţională de Termotehnică cu Participare Internaţională CNT 17

34

Fig.3

2. The Spatial Modeling of Cooling

Circuit of Heat Exchanger

Overall in a simulation with M.E.F. at

natural scale of model as result of

discretization is obtain an extremely large number of finite elements, which will lead

to an effort to account very high.

In practice, and consequently without

significantly altering the final outcome, to seek a way to eliminate this inconvenience.

Considering the symmetry of both

constructive cooling circuit and the symmetry phenomenon of flow was

proposed to analyze the efficiency of heat

exchanger heat starting at study of halfelement cooling associated with their

disipative wings and the final result of

study is multiplied with the total number of

simultaneously working halfelements which exist in the cooling circuit.

The actual cooling element is presented

in fig. 4, it has broken halfelement cooling in fig.5 and in fig.6 is shown in the

theoretical model of halfelement subject of

to study in the simulation with M.E.F. A whole section of halfelement cooling

which is survey analysis with M.E.F.

presenting elements attached proper

cooling system is shown in fig.7.

In this section were marked by means of fluid flow in the cooling system of the

hydraulic and pneumatic systems.

Fig.4 Fig.5 Fig.6

3. The Factors that Generate the Errors

of Calculation and Affect the

Quantitative and Qualitative the Heat

Exchange

As will be shown in comparison between

theoretical calculation and the experimental measurements, the errors occur in acceptable

limits are based on the following

justifications: - the important deformation of cooling

wings from assembly fig.4, which change

the flow aerodynamics and the heat exchange;

- the imperfect adjustment of the radiating

wing of halfelement contact cooling

which increase the contact surface due the stik;

- the partial closure of slit input in

hydraulic circuit of halfelement cover by

welding fig.8, which disrupt the hydraulic flow.

Fig. 8

BICĂ M.ş.a.: Efficiently determination of the heat exchanger rtcf 200 kw for electric power… 35

4. The property Thermotechnics of

Execution Materials and for Fluids

Used in Cooling Circuits

Material execution of the radiating

element is an alloy of aluminum with the

graphical following dependencies, which

in relation to the variation of temperature

for specific heat is given in fig.9 and for the coefficient of thermal conductivity in

fig.10.


36

Fig. 9 Fig. 10

Regarding the variation of oil properties

for transformer cooling in relation to

variation of temperature the density is

given in fig.11, the dynamic viscosity in

fig.12 to the specific heat in fig.13 and the

thermal conductivity coefficient in fig.14.

Fig. 11 Fig. 12

Fig. 13 Fig. 14

Dependencies for air properties with

temperature variation are taken automatically from the library materials

program analysis with the M.E.F.

5. Analysis of the Efficiency of Heat

Exchanger Using the M.E.F.

5.1. The Initial Data of Simulation In this paper was analyzed by numerical

simulation the heat exchange efficiency

considering the stationary work of electric

transformer, using the following input data

measured experimentally:

- oil at the entrance channel in slot

cooling element has the following

characteristics: Kt u 352| = ,

upkPa |54 = ,s

mvu 033,1= .

- air to enter the cooling circuit presents:

Kt a 294| = ,s

mva 83,6= and

Pap a 101325| = .

- exterior surfaces roughness element

and cooling coil is radiating mRz µ=100 .

BICĂ M.ş.a.: Efficiently determination of the heat exchanger rtcf 200 kw for electric power…

37

With observation that the rate of flow of oil to average input channel slot cooling

element was determined as follows:

- the flow of the pump

is: smQu /0125,0 3= ;

- the section of slot of channel inside a

cooling element

is: 230 610,5028,0002,0 mS =×= ;

- the number of elements in the battery

cooling are: 36=n ;

- the number of batteries cooling: 6=N ;

- the average oil velocity at the entrance of

the channel slot cooling element

is:s

mnNSQv uu 033,1)/( 0 == .

Denote by L the linear coordination of measured along the axis of symmetry

halfelement cooling (with the origin fixed

at the entrance section of the oil in the first segment in oil flow) and the H section in

the thickness direction of flow of air (as

see in fig.15, fig.16 and fig.17).

L = 800÷1600

H = 30 ÷ 60

input air

of section 1

heated air output

to section 3

entry heating oil

from section 1

output oil cooled

by section 3

Fig.16

section 2

H = 0 ÷30

L = 0÷ 800

heated air output to section 2

input air

atmospheric

entry heating oil

from vat transformer

output oil cooled

to section 2

Fig.15

section 1


38

Linear sections are between the following limits:

Table 1 sections L [mm] H [mm]

section 1 0 ÷ 800 0 ÷ 30

section 2 900 ÷ 1800 30 ÷ 60

section 3 1900 ÷ 2700 60 ÷ 90

Are excluded from the calculation for

results cover the heat losses that occur in elbows mounted between the linear

sections.

5.2. The Results of Simulation with Aid

of M.E.F.

In tab.2, tab.3 and tab.4 are presented the

average temperature of the surface oil in

flow channel section at mmL 100=∆ over

the linear length L coordinating and for distance of spread the air temperatures on

the channel corresponding aerodynamic

means spread. The simulation is made with aid of the FLUENT 6.3.26 programme

output cooled oil

by vat transformer

L = 1600÷ 2400

H = 60 ÷ 90

Input air in section 2 heated air

output to fans

entry heating oil

from section 2

Fig.17

section 3


39

Graphical representation of oil

temperature decline over the coordinating L

is presented in fig.18, and the air temperature variation over the L

coordinating on each segment is presented

in graphics of fig.19 to fig.21 and as

according to tconsHL tan, = in fig.22 to

fig.29.

Fig.18

Fig. 19

Fig. 20

Fig. 21

Fig. 22

Fig. 23

Fig. 24

Fig. 25


40

Fig.26

Fig.28

Fig.27

Fig.29

Table 2

transformer oil air

section 1 T input T output section 1 H T input T output ∆T air

[mm] [K] [mm] [K]

L = 0…100 352 351.181 L= 50 0...30 294 303.863 9.863

L =100…200 351.181 350.374 L= 150 0…30 294 303.729 9.729

L= 200…300 350.347 349.579 L= 250 0…30 294 303.595 9.595

L= 300…400 349.579 348.796 L= 350 0…30 294 303.461 9.461

L= 400…500 348.796 348.025 L= 450 0…30 294 303.327 9.327

L= 500…600 348.025 347.266 L= 550 0…30 294 303.193 9.193

L= 600…700 347.266 346.519 L= 650 0…30 294 303.059 9.059

L= 700…800 346.519 345.785 L= 750 0…30 294 302.953 8.953

Table 3 transformer oil air

section 2 T input T output section 2 H T input T output ∆T air

[mm] [K] [mm] [K]

L = 800… 900 345.785 345.458 L= 850 30...60 302.953 305.137 2.184

L = 900…1000 345.458 345.264 L= 950 30…60 303.059 305.221 2.162

L =1000…1100 345.264 345.072 L= 1050 30…60 303.193 305.338 2.145

L= 1100…1200 345.072 344.881 L= 1150 30…60 303.327 305.456 2.129

L= 1200…1300 344.881 344.691 L= 1250 30…60 303.461 305.605 2.144

L= 1300…1400 344.691 344.502 L= 1350 30…60 303.595 305.695 2.1

L= 1400…1500 344.502 344.314 L= 1450 30…60 303.729 305.816 2.087

L= 1500…1600 344.314 344.128 L= 1550 30…60 303.863 305.927 2.064


41

Table 4

transformer oil air

section 3 T input T output T input section 3 T input T output ∆T air

[mm] [K] [mm] [K]

L =1600…1700 344.128 343.613 L= 1650 60...90 305.927 313.995 7.668

L =1700…1800 343.613 343.106 L= 1750 60…90 305.816 313.402 7.586

L =1800…1900 343.106 342.603 L= 1850 60…90 305.695 313.197 7.572

L= 1900…2000 342.603 342.481 L= 1950 60…90 305.605 313.160 7.555

L= 2000…2100 342.481 342.318 L= 2050 60…90 305.456 312.997 7.541

L= 2100…2200 342.318 341.972 L= 2150 60…90 305.338 312.876 7.538

L= 2200…2300 341.972 341.515 L= 2250 60…90 305.221 312.746 7.525

L= 2300…2400 341.515 341.029 L= 2350 60…90 305.137 312.627 7.490

For mmL 1000 ÷= the velocity and temperature

field distribution obtained with M.E.F. are

given:

- in cross section in fig.30 and fig.31; on halfelserpentine radiating surfaces in fig.32

and fig.33;

- the flow pathways in fig.34 and fig. 35;

In the last section between

length mmL 24002300 ÷= field distribu-

tion of velocity and temperature: - in

cross section in fig.36 and fig.37; - on

halfserpentine radiating surfaces in fig.38 and fig.39; - and the flow

pathways in fig.40 and fig.41;

Fig.30 Fig.31

Fig.32

Fig.33

Fig.34 Fig.35


42

6.Conclusions concerning the results of

simulation using M.E.F.

Analysis of the average results

concerning the temperature shows that the oil has a cooling equal

with KToil 97,10= (less with the real

measured value KToil 03,1=∆ ) and for air

equal with KTair 995,19=∆ (less with the

real measured value KTair 2,3=∆ ).

Particularly in special interested the mode in which the oil temperature is cooled. The

result of simulation is realistic because the

simplifying assumptions using in calculation is reasonable.

This assumption taken into account

three major aspects:

Additional measurements with

KT 03,1=∆ of heating with oil may come

from the fact that in collector 1 the oil temperature is coresponding with the

input temperature in radiator

KToil 352= region denoted by A, fig.7,

which surround the elbow in the region B,

where the oil have KT 128,334= , so that

oil will come in section 3 will be further heated by convection-conduction-

convection;

Also elbow C where the temperature

is KT 345785= is surround by the collector 1

with region D where KT 029,341= , the fluid

being cooled by convection-conduction-

convection. These heat flows between the regions: A,

B, C and D was not taken into account and

especially the fact that all the hydraulic

circuits (the oil that circulates through jackanapes) inside their meeting performing

hydraulic nodes.

Fig.36

Fig.37 Fig.38

Fig.39

Fig.40 Fig.41


43

Additional cooling air to KT 2,3=∆ to

exeprimental measurement can come from that was not held to the additional

loss of temperature achieved by:

• radiant surface of the 2 collectors 1,

denoted by E, fig.42;

• radiator covers, noted by F, fig.42;

• metal frame that supports batteries,

denoted by G, and the frame, fig.43.

• radiant coil by 6

1 total battery coil /

radiator, on the edge of each frame

batteries that are not related to items that circulate oil (unheeded assumption of

calculation) denoted by H in fig.44;

• collectors of II, denoted by I, fig.45

which are designed to bend back the flow

of fluid 1800, and they are on the hydraulic

circuit between junction section 1– section 2 and section 2 – section 3;

• the serpentines radiating geometry

(their deformations) fig.4 and how to

stick with heater elements, fig.6 (which

actually change the flow and the exchange of heat from the proposed

model calculation).

It can take account the fact of that experimental measurements of air

temperature show only a medium value,

while the field of spatial distribution of temperature is not uniform over a section so


44

great as that in our case is equal

with 28002840 mmS ×= as the batteries of

the cooling radiator, see fig.42.

One can conclude that the study with aid of M.E.F. based on 3D model proposed

provides a good result concerning the

evaluation of heat exchanger efficiency of

200 kW by type RTCF providing detailed results of graphical and numerical spatial

fields of temperature and velocity.

Fig. 45

Acknoledgments

This work has partly been funded by the

Romanian Ministry of Education, Research and Youth, through The National University

Research Council, Grant PN–II–ID–PCE–

2007–1, code ID_1107, duration 2007 -

2010, “The development of a database with representations of complex surfaces and

objects using engineering graphics.

Applications in art and technique” and Contract Research Program Partenerships in

Priority Areas, funded by the Romanian

Ministry of Education and Research, Program RELANSIN. Contract execution

projects: nr.21070/ 14.09.2007; duration

2007-2009; “Monitoring the efficiency of

the cooling of high power transformers”.

References

1. Bejan, et. al.: Advanced Engineering Thermodynamics, Second edition, John Wiley & Sons, New York. 1997, p. 62-110.

2. Ferzieger, J.L., Peric, M.: Computational Methods for fluid Dynamics, Heidelberg. Springer Verlag, 1996, p. 86-123.

3. Greitzer, E.M., et. al.: Internal Flow. New York. Cambridge Univ. Press,. 2004, p. 23-78.

4. Incropera, F.P.: Fundamentals of heat and Mass Transfer, Second Edition, John Wiley & Sons, New York. 1985, p. 112-156.

5. Kim, S.E., et. al.: Computation of Complex turbulent Flows Using the Comercial Code FLUENT, Presented at Symposium on Modeling Complex Turbulent Flows Hampton, USA, Virginia. 1997.

6. Mills, F.A.: Basic Head & Mass Transfer, In: Pretince Hall, New Jersey. 1999, p.34-82.

7. Moran, M., et. al: Fundamentals of engineering thermodynamics. John Wiley & Sons, New York. 2000, p. 105-128...

the metallic frame of

the battery cooling

the supplimentary

serpentine

lied by the metalic frame

of

H

Fig.44

efficiently determination of the heat exchanger...

Documents