Heat Transfer on Electrical Components by Radiation R. Haller.

Heat Transfer on Electrical Heat Transfer on Electrical Components by RadiationComponents by Radiation

R. Haller

Department of Electrical Power and Environmental Engineering Prof. Dr.- Ing. habil. Rainer Haller, Dr.sc.techn.

Motivation and Introduction

Physical basic relations

Determination of Radiant Power

Fields of Application

Conclusions

TopicsTopics


Motivation

Is the radiation in the low temperature range

(< 100 … 150 °C) important for thermal

calculation or not ?


conduction

convection

radiation

Heat transfer (heating, cooling) is possible by

IntroductionIntroduction


design- tasks for electrical components design- tasks for electrical components (indoor)(indoor)


Influence of heat power on outdoor components

caused by sun and sky radiation

design- tasks for electrical componentsdesign- tasks for electrical components(outdoor)(outdoor)

sky






Conclusions

TopicsTopics


type of electromagnetic radiation

thermal radiation 0.1 … 400 µm



radiation processes are different from those by

conduction or convection

no transfer medium is necessary

transferred heat power is mainly determined by

the object temperature T and the interactions

between the radiated areas (absorption, emissivity)

what can lead to a difference of temperature T ) and

can´t be described by classical thermodynamics


generation of thermal radiation

(modelling)

thermal radiation will be generated by changes of atomic dipol moment caused by thermal induced oscillations


classification

Objects with a temperature T > 0 K emit electromagnetic radiation and, therefore, are able to give away energy to other objects

The radiation takes place from the surface of solid and fluid objects and/ or from the volume of gases

area radiator (radiators are described by their radiation area)

volume radiator (physical basic processes)

the heating by radiation will be generated by inner atomic processes into the radiated object (near the surface, absorption)


intensity of radiation

infrared range


intensity of thermal radiation

[Wien]

[Planck]



[Stefan- Boltzmann´s law]

emissivityarea of radiation


thermal radiation balance



The ratio of emission is equivalent to the ratio of absorption

(thermal balance Kirchhoff´s law

Ratio of emission for real radiators:

radiation of real object with T1

radiation of „black radiator“ with T1 ε =



classification of radiators:

black: all radiations will be absorbed (ά = ε = 1)

white: all radiations will be reflected (ρ = 1)

gray: all radiations will be absorbed/ emitted in the

same ratio about all wave lengthes (ε < 1)


radiation characteristic for planes

[Lambert´s law]






Conclusions

TopicsTopics


determination of emissivity ε

ε ≠ f (T)

ε = f (surface material, ..)

determination of surfaces O

determination of power P and/ or temperature T

Determination of Radiant Parameters

exact calculation

numerical calculation (thermal- grid, … .)

measurement (thermografy, ..)


exact calculation

= geometrical faktor


radiation characteristic for two planes


emitted radiation between two planes with significant area difference


emitted radiation between two planes with significant area difference


emissivity

material Al- busbar, oxidized (indoor) 0,25

Al- busbar, oxidized (outdoor) 0,6 … 0,9

Cu- busbar, oxidized (indoor) 0,25

Cu- busbar, oxidized (outdoor) 0,7 … 0,95

colours, varnishes 0,8 … 0,9

mineralic materials 0,7 … 0,85

usually determined by measurement


numerical calculation

[Stefan- Boltzmann]


numerical calculation

analogous to the thermodynamic relation

[Newton]


determination of heat power

(radiation + convection)

[Newton]

with = k + S1,2


determination of heat power

heat power parts must be determined separately


heat transfer by convection

KoKoKo OP

exact calculation of Navier- Stokes- Equations

numerical calculation (FEM, CFD, …)

theory of similarity


simulation by FEM- program (ANSYS)

boundary conditions results

α = αkonv + αrad



free convection:

forced convection:



free convection forced convection


theory of similarity f r e i e K o n v e k t i o n

s e n k r e c h t e W a n d

Z e l l e n s e i t en w ä n d e i n n e n u n d a u ß e n

3 PrGr 15,0 Nu 108 102 PrGr 107,1

w a a g e r e c h t e W a n d , W ä r m e a b g a b e n a c h o b e n , W ä r m e a u f n a h me v o n u n t e n

Z e l l e n d a c h I n n e n u n d a u ß e n

3 PrGr 17,0 Nu 98 101,1 PrGr 103,2

w a a g e r e c h t e W a n d , W ä r m e a b g a b e n a c h u n t e n

K a n a l b o d en a u ß e n

3 Pr Gr095,0 Nu 98 102,1 PrGr 103,1

w a a g e r e c h t e Z y l i n d e r

K a b e l u n d R u n d l e i t e r , h o r i z o n t a l v e r l e g t

lw

ROK o

K o K o

1

K ow

N u

l

4 PrGr 54,0 Nu 72 102 PrGr 105

3 PrGr 13,0 Nu 137 10 PrGr 102



e r z w u n g e n e u n d e r z w u n g e n e m i t ü b e r l a g e r t e r f r e i e r K o n v e k t i o n

v e r t i k a l e W a n d V

5,0

66,0

Pr)Gr (91,0 Re*

;Re'16,0 Nu

R e ' 1 0 5

Z y l i n d e r q u e r a n g e s t r ö m t

lw

V

403,0

62,0

Pr) Gr(97,6 Re*

;Re'17,0 Nu

R e ' 1 0 4

E i n f a c h s c h i e n e w a a g e r e c h t , h o c h k a n t

L u f t b e w e -g u n g i m G e r ä t d u r c h L ü f t u n g s -ö f f n u n g e n o d e r L ü f t e r

lw

V

ROK o

K o K o

1

K ow

N u

l

2*Re²Re Re'

42,0

6,0

Pr)Gr (25,2 Re*

;Re'4,0 Nu

R e ' , 4 2 1 0 4


thermal grid method

Gth· θ + Cth· (dθ/ dt) = P

with θ temperature

P heat power

Gth , Cth thermal admittance, capacity

ordinary differential equation system

coefficients are dependent from variable


thermal grid method

DQ = Rth * P (steady state)

with DQ difference of temperatures

P heat power

Rth thermal resistances

procedure:

separation of interesting areas/ volumes into n- parts

determination of heat power sources

calculation of thermal resistances

appropriate network calculation method

(nonlinear, iterative calculations)


thermal grid method

Rth = RL + RKo + RS


thermal grid method


thermal grid method


radiation influence on outdoor components


influence of sky radiation


comparison with convective transferred heat power

I

Tvertical plate with temperature T > T0T0

busbar of switching equipments


comparison of heat transfer

vertical plate (OS1 = 1m2), T1 = 70 °C, T2 = 30 °C

1 = 0,25/ 0,9; 2 = 0,9; OS1 << OS2

S= 1,92 W/m2K

PS= 76,8W/m2

S= 6,9 W/m2K

PS= 276 W/m2

Ko= 5,83 W/m2K

PKo = 233,2 W/m2

free convection

1T 2TPKo

PS

Ps > Pk !



Parameter: I = 2000 A, busbar (10 x 100) mm2, Al

=



heat transfer on busbar(parameter: Al, 10x100, I= 2000A, eps= 0,25/ 0,9)

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

-10 -5 0 5 10 15 20 25 30 35 40

surr_temperature [°C]

Pko

nv,

Pra

d /

Pto

tal

Prad --- (ε = 0,25; 0,9)


measurement of radiant power

measurement principle


measurement of radiant power

working range of IR- measurement systems

transmission ability of measuring distance


application in electrical power engineering


heating up process of an electronic circuit






Conclusions

TopicsTopics


evaluation of thermal conditions for technical

devices design, development, quality inspection, …

studying of physical processes

….

fields of application


technical data:

θ ~ 90 °C, 200 … 700 W

efficiency of radiation heating is mainly determined by ability of absorption/ emissivity of the located objects/ areas but not from the room air !!

radiation heating


Gas Insulated Lines (GIL)


Overhead line joint


Overhead line joint

Hasse

Verlustle istung ²RI

Sonnen- und H im m elsstrahlung P SH

W ärm eleitungswiderstand

konvektiver W ärm eübergangswiderstand

W ärm estrahlungswiderstände

W ärm kapazität

I R V

Verbindung Leiter

V L L L L L L

0

H

I²R V

0°C


Overhead line joint

Messwerte mit ku = 1,8

Rechnung mit ku = 2,0




-0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 m 2,0

90

85

°C

80

75

70

60

65

Tem

pe

ratu

r

Abstand x

Verbinder Leiter






-0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 m 2,0

90

85

°C

80

75

70

60

65

Tem

pe

ratu

r

Abstand x

Verbinder Leiter






-0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 m 2,0

90

85

°C

80

75

70

60

65

Tem

pe

ratu

r

Abstand x

Verbinder Leiter






Conclusions

TopicsTopics


thermal radiation processes are different from those as

convection or conduction

heat transfer should be determined by convection and

radiation processes even in the low temperature range

for determination of heat transfer the method of thermal

grid can be used as an effective engineering tool


Thank you --dekuji za pozornost

Questions ?

Answers !


additional informations





RITTAL- bus- bars with different coefficients of emissivity (0,9/ 0,4)

0

200

400

600

800

1000

1200

0 50 100 150 200 250 300 350 400 450

cross section [mm**2]

limit

of

curr

ent

[A]



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Heat Transfer on Electrical Components by Radiation R. Haller.

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