A STUDY ON ENHANCING HEAT TRANSFER EFFICIENCY OF LED lamps

Temperature profiles of heat sink and air for Model 1 heat sink configuration 0 10 20 30 40 50 o C Fin length of LED, mm Numerical Experimental Figure 4.. Comparison between numerical

The 2012 International Conference on Green Technology and Sustainable Development (GTSD2012)

A STUDY ON ENHANCING HEAT TRANSFER EFFICIENCY OF LED

LAMPS

Thanhtrung Dang1, Vanmanh Nguyen1, Nhatlinh Nguyen1, Tansa Nguyen1, Quocdat Vu1, Dinhvu Tran1,

Vanchung Ha1, Jyh-tong Teng2, and Ngoctan Tran2

1

Ho Chi Minh University of Technical Education, Vietnam

2

Chung Yuan Christian University, Taiwan

ABSTRACT

This paper presented investigations for enhancing heat transfer efficiency of LED Lamp, using numerical and experimental methods The solver of numerical simulations – COMSOL – was developed by using the finite element method The results obtained from numerical simulation were in good agreement with those obtained from the experimental data, with the maximum percentage error being less than 8% In addition, an optimization on heat transfer phenomena of LED lamps was also done in the study

KEYWORDS: Temperature, heat transfer, efficiency, heat sink, LED

1 INTRODUCTION

Nowadays, light emitting diode (LED) has

become more popular because it needs only

low consumption in electricity, but it can

provide high luminosity LEDs are more

energy efficient than other conventional

lamps for two reasons - they require less

energy to operate than incandescent and

fluorescent bulbs and they supply more

lighting capability per watt than

incandescent bulbs The increased

efficiency equates to lower energy costs

and less environmental impacts However,

LED's working temperature should be

accounted for It is estimated that

approximately 70-85% LED power is

converted into heat High operating

temperature would reduce the LED lifetime

and brightness With high power LEDs,

they could generate more heat Many

cooling methods have been used to

dissipate heat from LED lamps The normal

methods are using natural convection by

adding additional surface area to be in

contact with the environment which is at

lower temperature One effective way to

increase the contact area is by attaching a

heat sink to the heat source; in this case, the

heat source is the LED lamp [1] Heat sinks

are devices which enhance heat dissipation from a hot surface, usually for the case of a heat generating component, to a cooler ambient Alvin et al [1] studied thermal resistance of extruded fin heat sink on LED lamp In their study, the most significant factor affecting the thermal resistance value between LED and heat sink is the heat sink mounting pressure, followed by thermal interface material and heat sink materials However, the study did not compare the influence of heat sink configurations on the overall thermal resistance for the LED system Luo et al [2] presented temperature estimation of high-power light emitting diode street lamp by a multi-chip analytical solution In their study, the fin-heat-sink is still the predominant method used in the lighting industry due to its highest reliability and lowest cost Heat pipe [3, 4]

is becoming a good option for emerging high power LEDs Thermal analysis of high power LED array packaging with microchannel cooler was done by Yuan et

al [5] Weng [6] studied advance thermal enhancement and management of LED packages by using the FEM modeling technique for simulating the LED package with different heat slug, PCB, cooling condition and chip size In [7, 8], liquid

metals were used as the coolants to enhance

heat transfer for heat sinks Liu [9]

presented structural optimization of a

microjet based cooling system for high

power LEDs Several numerical and

experimental investigations were done in

[10-12] on the behaviors of heat transfer

and pressure drop for microchannel heat

sinks and heat exchangers In their study,

DI water was used as a working fluid

Based on reviews of the above literatures, it

is essential to study the heat transfer

behaviors of the LED heat sink, using both

numerical and experimental methods For

the present study, air was used as the

working fluid and the influence of

configuration of LED heat sink on heat

transfer characteristics was investigated In

the following sections, two cases will be

discussed for the LED heat sink: (1) the

case with natural convection and (2) the

case with forced convection

2 METHODOLOGY

2.1 Numerical simulation

The governing equations in this system

consist of the continuity equation,

momentum equations, and energy equation

[10-12] The equations can be expressed by

 u/  t+  (u  )u=  [-pI+  (  u+(  u)T)]+F (1)

 u = 0 (2)

 Cp  T/  t+  (-  T)=Qi-  Cpu  T (3)

where  is dynamic viscosity,  is density,

u is velocity field, p is pressure, I is the unit

diagonal matrix, F is body force per unit

volume (F x = F y = F z = 0 N/m3), Q i is

internal heat generation, T is temperature,

C p is specific heat at constant pressure, and

 is thermal conductivity

Numerical study of the behavior of the

LED heat sinks with 3D heat transfer was

done by using the COMSOL Multiphysics

software, version 3.5 The algorithm of this

software was based on the finite element

method The generalized minimal residual

(GMRES) method was used to solve for the

present case and shown in more detail in [1,

10-12] For this study, air was used as the

working fluid No internal heat generation was available Boundary condition for the heat sinks was a constant room temperature

at 30 ºC There are three models to be used for simulation of LED heat sink, as shown

in Fig 1: (1) without any crevice, (2) with one crevice, and (3) with two crevices The substrate material for heat sinks is aluminum having the thermal conductivity

of 237 W/(mK), density of 2,700 kg/m3, and specific heat of 904 J/(kgK) [13, 14].

a) LED heat sink without crevice (Model 1)

b) LED heat sink with one crevice (Model 2)

c) LED heat sink with two crevices (Model 3)

The 2012 International Conference on Green Technology and Sustainable Development (GTSD2012)

2.2 Experimental setup

The experimental system includes a power

supply, a temperature measurement unit, a

fan, and a velocity measurement unit, as

shown in Fig 2 The heat dissipation

patterns – fin aluminum heat sink - were

tested under different heat transfer modes:

natural convection and forced convection

The LED with a power supply of 7W was

used in this study Accuracies and ranges of

testing apparatuses are listed in Table 1

Table 1 Accuracies and ranges of testing

apparatuses

Testing apparatus Accuracy Range

Thermocouples  0.1  C 0  100  C

Velocity meter  1 % 0  50 m/s

Figure 2 A photo of the experimental system

Experimental data obtained from the LED

heat sinks were under the constant room

temperature condition of 30 ºC For the

case with natural convection, air velocity

was measured at 0.1 m/s; for forced

convection, air velocity was measured at

1.2 m/s At the middle fin of the heat sink,

five thermocouples were soldered on the

top of fin to obtain the temperature

readings

3 RESULTS AND DISCUSSION

3.1 Natural convection condition

a For Model 1 Heat Sink Configuration

For experiments carried out in this study,

with LED capacity of 7 W and air velocity

of 0.1 m/s, heat transfer from the LED through the heat sink was constant; the bottom temperature of heat sink was measured to be 49 ºC Fig 3 shows temperature profiles of heat sink and air for model 1 heat sink configuration

Figure 3 Temperature profiles of heat sink and

air for Model 1 heat sink configuration

0 10 20 30 40 50

o C

Fin length of LED, mm

Numerical Experimental

Figure 4 Comparison between numerical and

experimental results for Model 1 heat sink

configuration

Comparison between numerical and experimental results for Model 1 heat sink configuration is shown in Fig 4 It is observed that the results obtained from the numerical simulation are in good agreement with those obtained from the experimental data, with the maximum diffrence of 4.6% The difference is due to the error in temperature measurements caused by temperature sensors which were soldered at the outer rims of the fins while

the numerical results indicated more exact

phenomena taken place in the air

surrounding the heat sink

b For Model 2 Heat Sink Configuration

For the same experimental condition above,

with air velocity of 0.1 m/s, the bottom

temperature of heat sink was measured to

be 50.4 ºC Temperature profiles of heat

sink and air for Model 2 heat sink

configuration are shown in Fig 5 Fig 6

shows the comparison between numerical

and experimental results

Figure 5 Temperature profiles of heat sink and

air for Model 2 heat sink configuration

0

10

20

30

40

50

o C

Fin length of LED, mm

Numerical Experimental

Figure 6 Comparison between numerical and

experimental results for Model 2 heat sink

configuration

c For Model 3 Heat Sink Configuration

With the same conditions, the bottom

temperature of heat sink was measured to

be 49.7 ºC The Fig 7 shows temperature profiles of heat sink and air for Model 3 heat sink configuration

Figure 7 Temperature profiles of heat sink and

air Model 3 heat sink configuration

0 10 20 30 40 50

o C

Fin length of LED, mm

Numerical Experimental

Figure 8 Comparison between numerical and

experimental results for Model 3 heat sink

configuration

Comparison between numerical and experimental results for Model 3 heat sink configuration is shown in Fig 8 It is also indicated that the numerical and experimental results are in good agreement From Figs 3-8, for the natural convection case, it is observed that the bottom temperature of heat sink for Model 1 heat sink configuration was the lowest It is due

to the fact that Model 1 heat sink configuration has the largest heat transfer area

The 2012 International Conference on Green Technology and Sustainable Development (GTSD2012)

3.2 Forced convection condition

Experiments for forced convection

condition were done on Model 3 heat sink

configuration by using a fan with an air

velocity of 1.2 m/s

For this case, the bottom temperature of heat

sink was measured to be 38.5 ºC Figure 9

shows the comparison between numerical

and experimental results for the case with

forced convection It is also indicated that

the numerical and experimental results are

in good agreement, with the maximum

discrepancy of the temperature estimated to

be less than 8 %

From Figs 4-9, it is shown that the heat

transfer capability obtained from the case

with forced convection is higher than that

obtained from the case with natural

convection case: at the same room

temperature condition and LED power

supply capacity, the bottom temperature of

LED heat sink is reduced from 49.7 to 38.5

ºC

0

10

20

30

40

50

o C

Fin length of LED, mm

Numerical Experimental

Figure 9 Comparison between numerical and

experimental results for Model 3 heat sink

configuration with forced convection case

4 CONCLUSION

Numerical and experimental studies have

been performed on three LED heat sinks

with different configurations In natural

convection case, the heat transfer capability

obtained from the heat sink without crevice

was higher than those obtained from the

heat sinks with crevice or crevices The

heat transfer capability obtained from the case with forced convection is higher than that obtained from the case with natural convection case: at the same room temperature condition and LED power supply capacity, the bottom temperature of LED heat sink is reduced from 49.7 to 38.5

ºC Furthermore, the results obtained from the experiments were in good agreement with those obtained from the numerical simulations, with the maximum discrepancy of the temperature estimated to

be less than 8 %

5 ACKNOWLEDGEMENTS

The supports of this work by (1) the projects (Project Nos 54-11-CT/HD-CTTB and 38- 12-CT/HĐ-CTTB) sponsored by New Product & Technology Center (NEPTECH) – Hochiminh City Department

of Science and Technology of Vietnam, (2) the project (Project No T2012-16TĐ /KHCN -GV) sponsored by the specific research fields at Hochiminh City University of Technical Education, Vietnam, (3) the project (Project Nos NSC 99-2221 -E-033-025 and NSC 100-2221 -E-033-065) sponsored by National Science Council of the Republic of China in Taiwan, and (4) the project (under Grant No CYCU-98-CR -ME) sponsored by the specific research fields at Chung Yuan Christian University, Taiwan, are deeply appreciated

6 REFERENCES

[1] Christian Alvin, Jyh-tong Teng, andThanhtrung Dang, Thermal Resistance Analysis of Extruded Fin Heat Sink on LED Lamp,The International Electron Devices and Materials Symposium 2011 (IEDMS2011), Taipei, Taiwan, Nov 17-18, 2011, P-C-19, pp 1-4

[2] X.B Luo, W Xiong, T Cheng, and S Liu, Temperature estimation of high-power light emitting diode street lamp by a multi-chip analytical solution, IET Optoelectron, 3, 2009, pp 225–232

[3] L Kim, J.H Choi, S.H Jang, and M.W

Shin, Thermal analysis of LED array

system with heat pipe, 6th Symposium

of the Korean Society of

Thermophysical Properties, Seoul,

2006, pp 21–25

[4] Zirong Lin, Shuangfeng Wang, Jiepeng

Huo, Yanxin Hu, Jinjian Chen,

Winston Zhang, and Eton Lee, Heat

transfer characteristics and LED heat

sink application of aluminum plate

oscillating heat pipes, Applied Thermal

Engineering, 31, 2011, pp 2221-2229

[5] L.L Yuan, S Liu, M.X Chen, and X.B

Luo, Thermal analysis of high power

LED array packaging with

microchannel cooler, 7th International

Conference on Electronics Packaging

Technology, Shanghai, 2006, pp

574–577

[6] C.J Weng, Advanced thermal

enhancement and management of LED

Communications in Heat and Mass

Transfer, 37, 2009, pp 245–248

[7] Y Deng and J Liu, A liquid metal

cooling system for the thermal

management of high power LEDs

International Communications in Heat

and Mass Transfer, 37, 2010,

pp.788–791

[8] K.Q Ma and J Liu, Liquid metal

cooling in thermal management of

computer chips, Front Energy Power

Eng China, 1, 2007, pp 384–402

[9] S Liu, J.H Yang, Z.Y Gan and X.B

Luo, Structural optimization of a

microjet basedcooling system for high power LEDs, Int J.Therm Sci 47,

2008, pp 1086–1095

[10] Thanhtrung Dang, Ngoctan Tran and Jyh-tong Teng, Numerical and Experimental investigations on heat transfer phenomena of an aluminium microchannel heat sink, Applied Mechanics and Materials, 145, 2012,

pp 129-133 [11] Ngoctan Tran,Thanhtrung Dang and Jyh-tong Teng, Numerical and experimental studies on pressure drop and performance index of an aluminum microchannel heat sink, 2012 IEEE International Symposium on Computer,

Control(IS3C2012), June 4-6, 2012, Taichung City, Taiwan, pp 252-257 [12] Thanhtrung Dang and Jyh-tong Teng, Comparison on the heat transfer and pressure drop of the microchannel and minichannel heat exchangers, Heat and Mass Transfer, 47, 2011, pp 1311-1322

[13] J.P Holman, Heat transfer, Ninth Edition, McGraw-Hill, New York,

2002 [14] COMSOL Multiphysics version 3.5 (2008) – Documentation

Contact

Thanhtrung Dang, Ph.D

Tel: +84913606261 Email: trungdang@hcmute.edu.vn