CFD modeling of the conjugate heat

transfer inside a door of gas ovens

Jhon F. HincapiéManuel García

Grupo de Investigación Mecánica Aplicada Universidad EAFITMedellín Colombia

PRESENTATION TOPICS

• Introduction

• Formulation of the problem

• Computing domain

• Validation model

• Design parameters

• Temperature profiles

• Thermal transmittance – View factor

• References

• Questions

Introduction

The oven door has double glass, which allowyou to view the cooking of food without openingthe door. The transfer of heat energy through theglass door is the main source of heat loss byconduction, convection and radiation.

The thermal resistance which ensures two doorglasses is increased by the air gap betweenthese, the low thermal conductivity of air givesinsulation effect.

FigFigFigFig 1.1.1.1. Gas ovenFuente: http://www.haceb.com

Formulation of the problem

Cavity

��

��

��,� ��,�

��

�����

����

��

��,���,�

��,�

FigFigFigFig 2.2.2.2. Schematic of seccion gas oven door

The door uses two parallel glasses, a glass near the oven cavity in direct contact withhigh temperature air and an outer glass surrounded by air at ambient temperature, thetwo glasses are separated by a air space. Through glasses flowing air coming from thebottom of the door and exits the top.

Computing domain

Cavity

GLASS 1

Nodes 41040

Elements 26878

GLASS 2

Nodes 40770

Elements 26700

AIR

Nodes 297992

Elements 281255

Air

(External enviroment)

Glass 1

Glass 2

GLASS

Thermal Conductivity [w/mK] 1,4

Density [kg/m3] 2500

Specific Heat Cp [kJ/kg K] 750

Emissivity [] 0,863 0,1

Refractive Index [] 1,5

Absorption Coefficient [1/m] 62,11 47,47

AIR

Dynamic Viscosity [Pa s] Sutherlands

Thermal Conductivity [w/mK] Sutherlands

Density [kg/m3] Gas ideal –Buoyant

Specific Heat Cp [kJ/kg K] Polynomial

FigFigFigFig 3.3.3.3. Computing domain

TabTabTabTab 1. 1. 1. 1. Mesh statistics

TabTabTabTab 2. 2. 2. 2. Properties domains

Resistance network

1

ℎ�

��,�

��

��

��,�

��

��

��,�

1

ℎ�

��,�

1

ℎ�

�� ��

���������

John P. Abraham & Ephraim M. Sparrow

radccc hhh ,cov, +=

3

, 4 ccradc Th σε=

2

3 Pr

νβ c

a

THgR

H

∆=

( )[ ] 935.007.125.083.3545.0 +=

HaRNu

c

cH

Nukh 1

cov, =

ih

1radiii hhh ,,cov +=

212

2

1

1

3

, 111

4

−

+−

+−

=

F

Th radt

εε

εε

σ

( )να

β 3dTTgR

da∞−

=

75.0

35exp1

24

−−=

d

H

RH

dRNu

g

ag

a

s

s

**Elenbaas (1942)

dNuh a

t

λ=cov,

g

ac

HNuh

λ=

2

27/816/9

6/1

Pr

492.01

387.0825.0

+

+= HgaR

Nu

( )να

β 3

g

a

HTTgR

Hg

∞−=

**Churchill & Chu (1975)

oic hk

e

hk

e

h

U111

1

2

2

1

1 ++++=

Model validation

3,0

3,2

3,4

3,6

3,8

4,0

4,2

4,4

4,6

0,005 0,010 0,015 0,020 0,025 0,030 0,035 0,040

The thermal transmittance

Analytical model ANSYS-CFX

Width of the air space d [m]

Th

erm

al

tra

nsm

itta

nce

[m

2K

/w]

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

0,005 0,010 0,015 0,020 0,025 0,030 0,035 0,040

The convective heat transfer coefficient

Analytical Model ANSYS-CFX

Width of the air space d [m]

Co

nve

ctiv

e c

oe

ffic

ien

t [w

/m2K

]FigFigFigFig 3.3.3.3. Validation of the numerical simulation model.

The thermal transmittance.FigFigFigFig 4.4.4.4. Validation of the numerical simulation model. The convective

heat transfer coefficient between the glasses of the door.

Design parameters

Op-1 Op-2 Op-3 Op-4 Op-5

Emissivity ε1 0.863 0.1 0.863 0.863 0.863

Emissivity ε2 0.863 0.863 0.1 0.863 0.863

e 1 0.004 0.004 0.004 0.006 0.004

e 1 0.004 0.004 0.004 0.004 0.006

Abs. Coef. G,1 62.11 62.11 62.11 41.41 62.11

Abs. Coef. G,2 62.11 62.11 62.11 62.11 41.41

0.005 < d < 0.04

3,30

3,80

4,30

4,80

5,30

5,80

6,30

6,80

7,30

0,005 0,010 0,015 0,020 0,025 0,030 0,035 0,040

Analysis of variables

Op-1 Op-2 Op-3 Op-4 Op-5

Width of air space d [m]

Co

nve

ctiv

e c

oe

ffic

ien

t [w

/m2K

]

TabTabTabTab 3. 3. 3. 3. Design parameters for the simulation of double glazed oven door

FigFigFigFig 5.5.5.5. The convective heat transfer coefficient for design parameters.

Temperature profiles

300

350

400

450

500

550

Th,1 Th,2 Tc,3 Tc,4

Te

mp

era

ture

[K

]

Glass temperature d= 0.020 [m]

Op- 1 Op-2 Op-3 Op-4 Op-5

d Th,1 Th,2 Tc,3 Tc,4

Op- 10.005 530 489.96 409.05 406.11

0.040 530 477.97 382.54 380.35

Op-20.005 530 509.44 343.18 342.16

0.040 530 501.45 323.75 323.31

Op-30.005 530 516.75 364.56 363.82

0.040 530 498.38 327.65 327.31

Op-40.005 530 491.14 405.44 402.68

0.040 530 476.1 381.91 379.89

Op-50.005 530 490.26 406.42 401.87

0.040 530 483.12 388.48 383.94

TabTabTabTab 4444. . . . Temperature glasses for two wide air space d=0.005 , 0.040 [m]

Glass 1 - Th,2

Glass 2 – Tc,4

Thermal transmittance – View factor

4,22

4,24

4,26

4,28

4,30

4,32

4,34

4,36

4,38

4,40

0,005 0,010 0,015 0,020 0,025 0,030 0,035 0,040

The thermal transmittance analytic - View factor

Without view factor View factor

Width of the air space d [m]

Th

erm

al

tra

nsm

itta

nce

[m

2K

/w]

3,34

3,36

3,38

3,40

3,42

3,44

3,46

3,48

3,50

0,005 0,010 0,015 0,020 0,025 0,030 0,035 0,040

The thermal transmittance ANSYS CFX - View factor

Without view factor View factor

Width of the air space d [m]

Th

erm

al

tra

nsm

itta

nce

[m

2K

/w]

121 =−F

d [m] F1-2

0.005 0.99801

0.010 0.99264

0.015 0.98432

0.020 0.97334

0.025 0.96000

0.030 0.94458

0.035 0.92735

0.040 0.90855

!"

#

!�

−+=− ts

q

p

wF ln

12

1

21 π

212

2

1

1 111

1

−

+−

+−

=

F

effective

εε

εε

ε

FigFigFigFig 7.7.7.7. From a square plate of side Hc toa coaxial square plate of side Hg.

Conclusions

� Heat transfer coefficient varies with changes in the air space between the panes.

� The validation of heat transfer between the CFD simulation and the model ofresistance has good agreement.

� Heat transfer by radiation is greater than the heat transfer by convection, radiation isthe predominant phenomenon.

� Emissivity glass is the most influential term in order to calculate the convectivecoefficient. low emission in glasses lowers the surface temperature of the outerglass-Op 1 and Op-2.

� Thermal transmittance changes when the view factor of the effective emissivitydecreases.

References

• U. Prasopchingchana, “Simulation of Heat Transfer in the Multi-Layer Door of the Furnace,” WordAcademy of Science, Enginnering and Technology, pp. 1060 – 1064, 2011.

• H. Mistry, S. Ganapathisubbu, S. Dey, P. Bishnoi, and J. L. Castillo, “A methodology to model flow-thermals inside a domestic gas oven,” Applied Thermal Engineering, vol. 31, no. 1, pp. 103–111, Jan.2011.

• E. M. Sparrow and J. P. Abraham, “A computational analisys of the Radiative and ConvectiveProcesses that Take Place in Preheated and Non-Preheated Ovens,” vol. 24, no. 5, pp. 25–37, 2003.

• J. Xamán, J. Arce, G. Álvarez, and Y. Chávez, “Laminar and turbulent natural convection combinedwith surface thermal radiation in a square cavity with a glass wall,” Int. J. Therm. Sci., vol. 47, no. 12,pp. 1630–1638, Dec. 2008.

• A. Dhall, A. K. Datta, and K. E. Torrance, “Radiative Heat Exchange Modeling Inside an Oven,” AIChEJournal, vol. 55, no. 9, pp. 2448 – 2460, 2009.

• B. . Shaughnessy and M. Newborough, “Energy performance of a low-emissivity electrically heatedoven,” Applied Thermal Engineering, vol. 20, no. 9, pp. 813–830, Jun. 2000.

• S.-Y. Wong, W. Zhou, and J. Hua, “CFD modeling of an industrial continuous bread-baking processinvolving U-movement,” Journal of Food Engineering, vol. 78, no. 3, pp. 888–896, Feb. 2007.

• H. Naeimi and F. Kowsary, “Simplex ray-object intersection algorithm as ray tracer for Monte Carlosimulations in radiative heat transfer analysis,” International Communications in Heat and Mass

Transfer, vol. 38, no. 5, pp. 646–651, May 2011.

Questions

FigFigFigFig 8.8.8.8. Fuente: http://addedvalues.files.wordpress.com/