Heat Transfer on Electrical Components by Radiation R. Haller.

transcript

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)

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

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

K o K o

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

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

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

K o K o

2*Re²Re Re'

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

radiation influence on outdoor components

influence of sky radiation

comparison with convective transferred heat power

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 > 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)

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

surr_temperature [°C]

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

Verlustle istung ²RI

Sonnen- und H im m elsstrahlung P SH

W ärm eleitungswiderstand

konvektiver W ärm eübergangswiderstand

W ärm estrahlungswiderstände

W ärm kapazität

Verbindung Leiter

V L L L L L L

I²R V

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

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

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

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 50 100 150 200 250 300 350 400 450

cross section [mm**2]

Heat Transfer on Electrical Components by Radiation R. Haller.

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