- Email: [email protected]
Experimental investigations on thermal performance enhancement and effect of orientation on porous matrix filled PCM based heat sink
International Communications in Heat and Mass Transfer 46 (2013) 27–30
Contents lists available at SciVerse ScienceDirect
International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
Experimental investigations on thermal performance enhancement and effect of orientation on porous matrix filled PCM based heat sink☆ Rajesh Baby 1, C. Balaji ⁎ Heat Transfer and Thermal Power Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e
i n f o
Available online 5 June 2013 Keywords: Phase change material (PCM) Thermal management Orientation Heat sink Enhancement ratio
a b s t r a c t Thermal performance in terms of enhancement ratios and the effect of orientation of a copper porous matrix filled phase change material (PCM) based heat sink are experimentally studied in this paper. N-eicosane is used as the phase change material. A copper open cell metal foam, press fitted into an aluminium casing is the thermal conductivity enhancer. In PCM based heat sinks, low thermal conductivity associated with PCMs makes the use of enhancement techniques inevitable for better thermal performance. A plate heater with an overall dimension of 60 × 42 mm2 with 2 mm thickness is used to mimic the heat generation in electronic chips. The effect of orientation of the heat sink on thermal performance is studied by developing a tracking system, capable of placing the heat sink at any specified orientation. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Leapfrogging in thermal management solutions is essential in order to achieve headway in the packing density challenges associated with rapid growth of electronics. Phase change material based cooling is a passive thermal management technique with a tremendous potential and is important when considering the global concerns about environmental impact. When a PCM is selected for a particular application, its melting temperature should be below the maximum operating temperature of the device. PCM based cooling methodology can be applied in many areas and also to thermal management of portable electronic equipment such as mobile phones [1], laptops, personal digital assistants and so on. In the recent past, PCM based cooling has attracted the interest of many researchers because of the large latent heat storage capacity of the PCMs and the tremendous prospects it enjoys in terms of applications. In PCM based heat sinks, a base material with high thermal conductivity is used in conjunction with the PCM in order to compensate for the very low thermal conductivity of PCMs. The use of the base material (also known as thermal conductivity enhancer (TCE)) can be in the form of different types of fins [2–6]. Fukai et al. [7] used carbon fibers distributed in PCMs in order to enhance the thermal conductivity of energy storage media. In order to improve the thermal energy storage, Nomura et al. [8] experimentally studied the impregnation of porous material with PCM. Prieto et al. [9] reported the thermal properties of graphite flakes/ metal composites for thermal management applications. From ☆ Communicated by A.R. Balakrishnan and T. Basak. ⁎ Corresponding author. E-mail address: [email protected] (C. Balaji). 1 Research scholar. 0735-1933/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.icheatmasstransfer.2013.05.018
the experimental study using metal matrix and expanded graphite as the TCE, Zhou and Zhao [10] observed that during melting, the metal foams can double the heat transfer rate when they used in conjunction with the PCMs such as paraffin wax RT 27 and calcium chloride hexahydrate. The effect of orientation on the performance of PCM based heat sinks has been investigated by various researchers for finned heat sinks [4,11]. Wang et al. [12], numerically, studied the effect of orientation on the thermal performance of PCM based plate fin heat sink using a two-dimensional physical model. Paraffin wax was used as the PCM. The evolution of melt fraction with time for various orientation angles was used to quantify the effect on heat transfer performance. The preceding review reiterates the fact that experimental studies on the thermal performance of porous matrix filled PCM based heat sinks are scarce and even fewer experimental studies that analyze the effect of orientation on the performance of such heat sinks are reported in literature. In this paper, we experimentally studied the effect of orientation on heat transfer performance in terms of time to reach a set point temperature for a copper porous matrix filled PCM based heat sink by extensive experimentation for different orientations of the heat sink using a tracking system developed in house. Furthermore, the enhanced thermal performance of the PCM based composite heat sink is quantified in terms of two enhancement ratios. 2. Experimental setup The heat sink is made from an aluminium slab with an overall dimension of 80 × 62 mm2 base with height of 25 mm using the conventional milling process. The thickness of the side wall is 7 mm and the internal cavity is available for a height of 20 mm. As the same copper metal foam [13] is used in the present study, the same data is used for
28
R. Baby, C. Balaji / International Communications in Heat and Mass Transfer 46 (2013) 27–30
the characterization of the metal foam. For the copper metal matrix considered for the present study, number of pores per inch (PPI) = 10, thickness = 20 mm, porosity = 0.86, pore diameter = 4.73 mm and the fiber diameter = 0.71 mm. A photograph of the copper metal foam fitted into the cavity made of aluminium used in the present study is given in Fig. 1. Because of the difficulties and high cost associated with brazing the metal foams [14], only press fitting of the metal foam is employed in this study. The phase change material used is n-eicosane with a melting temperature of 36.5 °C. As the porosity is 0.86, the volume fraction of the thermal conductivity enhancer i.e., the ratio of the total volume of copper metal foam to the empty heat sink volume is 14%. Except the top surface of the heat sink, the other sides of the heat sink are insulated with cork. A perspex sheet with an overall dimension of 80 × 62 mm2 with a thickness of 5 mm is kept at the top of the heat sink. On all the sides of the perspex sheet and in the area of contact between the top perspex sheet and the metal casing, a silica sealant is employed, in order to prevent any possible leakage of the PCM during tilting of the heat sink. The solidification and melting patterns of the PCM are clearly seen with the use of perspex sheet. The sheet is attached to the heat sink using six bolts, each with a diameter of 4 mm. In the 2 mm recess provided at the heat sink base, a plate heater with an overall dimension of 60 × 42 mm2 with 2 mm thickness is used to mimic the heat generation in electronic chips. The plate heater is made up of a standard coil-type nichrome wire wound over mica sheet. Before fixing the plate heater, a thermal paste is employed, in order to avoid contact resistance. A heat sink filled with PCM, without a metal matrix and a copper porous matrix filled heat sink without any PCM are used for baseline comparisons. A total of 11 chromel-alumel Ktype calibrated thermocouples are used to measure the temperature at various locations of the heat sink and these thermocouples are fixed in position using araldite epoxy. Two thermocouples are kept at the heat sink base near the plate heater, in order to record the temperature of the heat producing component, as the plate heater mimics the heat generation in electronic equipment. Four thermocouples are kept at the heat sink side wall and the another four are kept inside the PCM by making suitable openings in the perspex sheet and these openings are sealed with araldite
epoxy. All the thermocouples are connected to a PC- based data acquisition unit. The power input to the plate heater is supplied from an independently controlled DC power unit. The uncertainties in the voltage and the current measurements are ±0.1 V and ±0.01 A respectively. The uncertainty in the temperature measurement is ±0.2 °C. The uncertainty in the derived quantities is estimated, based on the uncertainty of primary quantities [15]. For heat input, the resulting uncertainty is found to be ±3.2%.
Fig. 1. A photograph of the heat sink with copper porous matrix used in the present study.
Fig. 2. A photograph of the tracking mechanism used in the present study.
2.1. Tracking mechanism A tracking mechanism is designed and fabricated, in order to study the effect of orientation on heat transfer performance. A photograph of the tracking mechanism used in the present study is given in Fig. 2. The tracking mechanism is made of aluminium. Four rubber mountings are used at the base for better stability and also to make the levelling easy. The levelling is aided by the use of a spirit level. The heat sink assembly is mounted on the shaft, provided at the top of the tracking mechanism and two bearings are kept inside the block to enable smooth rotation of the shaft. The rotation of the shaft is calibrated in terms of orientation angles with the help of the angle indicator ring fixed to the aluminium block. The rectangular base of the tracking mechanism is made of mild steel and is fixed to the aluminium block with the help of four M4 screws. The heat sink is kept at the required orientation with the help of a tightening screw. 3. Results and discussion 3.1. Effect of metal foam and PCM on heat transfer performance In order to evaluate the effect of metal foams and PCM on the heat transfer performance of the heat sink, two enhancement ratios are defined. The enhancement ratio is defined as ratio of the operation time of the heat sink with metal matrix filled with PCM to that of a heat sink, without the metal matrix, but filled with PCM. The PCM
R. Baby, C. Balaji / International Communications in Heat and Mass Transfer 46 (2013) 27–30
55
7
Enhancement ratio PCM enhancement ratio
6
Tset = 520C
Temperature (0C)
8
Enhancement ratio
29
5 4
45 40 35
3
30
2
25 4
5
6
7 8 Power level (W)
9
10
Heating phase
0
11
00 90 0 180 0
5W
50
100
Cooling phase
200
300
400
500
Time (min)
Fig. 3. Enhancement ratios of the heat sink at various power levels to reach a set point temperature of 52 °C.
Fig. 5. Depiction of the temperature–time profile for 5 W at different orientations (0°, 90° and 180°) of the heat sink.
enhancement ratio is the ratio of the operating time of the metal matrix filled PCM based heat sink to that of a heat sink with the metal matrix, but without PCM. The two enhancement ratios for a power level ranging from 5 to 10 W (corresponding to 1.98 and 3.97 kW/m2) for a set point temperature of 52 °C are shown in Fig. 3. The average value of the two thermocouple readings kept at the heat sink base is used to quantify the heat transfer performance. From Fig. 3, it is seen that PCM enhancement ratio does not vary significantly with the increase in the power level and a maximum enhancement of 3 in the operation time is achieved for a power level of 7 W. The enhancement ratio, though, varies significantly with the power level and the enhancement is high for higher power levels. An enhancement of 7.5 in operation time is achieved at 10 W. Similarly, experiments were also done for a 10 PPI, 20 mm thick aluminium metal foam filled PCM based heat sink with a porosity of 0.94, and an enhancement of 15 in operation time was obtained for a power level of 8 W when the set point temperature is 47 °C. The good thermal performance of the metal matrix at higher power levels is significant considering the fact that the heat rejection rate of the portable electronic devices are increasing day by day because of new add on features as a result of market competition.
5 W is shown in Fig. 4. Fig. 5 depicts the temperature–time history in the melting (160 min) and solidification regions at 5 W for 3 different orientations (0, 90 and 180°) of the heat sink. The temperatures recorded at the heat sink base at the end of 160 min for the orientations (0, 90 and 180°) are 49.92, 50.42 and 51.19 °C respectively. Better performance is observed when the orientation angle of the heat sink is 0°. The thermal stratification due to the change in the heating pattern is responsible for the slightly inferior thermal performance of the heat sink when it is kept at at an orientation of 180°. A comparison of the operating time to reach different set point temperatures of 40, 45 and 50 °C at 5 W is given in Table 1. All the experimental results corroborate the fact that the orientation of the heat sink assembly does not play a significant role as far as the usage of the porous matrix filled PCM based heat sinks are concerned.
3.2. Effect of orientation on heat transfer performance For studying the effect of orientation on the heat transfer performance of the copper metal foam filled PCM based heat sink, experiments were carried out for different orientations (0, 25, 45, 60, 75, 90, 110, 130, 150, 180 and 210°) of the heat sink at a power level of 5 W. At the end of 90 min of operation of the heat sink, the maximum variation in the heat sink base temperature for all the orientations considered in the present study is found to be less than 0.5 °C and at the end of 160 min, the variation is seen to be 1.84 °C. The temperature–time histories at various orientations of the heat sink at a power level of
4. Conclusions For quantifying the heat transfer performance of a PCM based copper metal foam, an experimental study is carried out and the enhancement is presented in the form of two enhancement ratios. The enhancement ratio defined as ratio of the operation time of the heat sink with metal matrix filled with PCM to that of a heat sink, without the metal matrix, but filled with PCM is 7.5 at 10 W and the maximum PCM enhancement ratio defined as the ratio of the operation duration of the metal matrix filled PCM based heat sink to that of a heat sink with the metal matrix, but without PCM is 3 at 7 W for a set point temperature of 52 °C. Identical results were also obtained for aluminium metal foam filled PCM based heat sink. From the extensive experimental studies for various orientations on the heat sink, it is found that (i) heat transfer performance is comparable and (ii)orientation does not have any significant impact on the heat transfer performance of the metal foam filled PCM based heat sinks.
55
Temperature (oC)
00 250 450 600 750 900 1100 1300 1500 1800 2100
5W
50 45 40 35
Table 1 Comparison of operating time (s) to reach various set point temperatures at different orientations of the heat sink.
30 25 0
40
80
120
160
200
Time (min) Fig. 4. Temperature–time histories of various orientations of the heat sink at a power level of 5 W.
Sl No
Orientation
Time to reach 40 °C (s)
Time to reach 45 °C (s)
Time to reach 50 °C (s)
1 2 3 4 5 6 7 8 9 10 11
0° 25° 45° 60° 75° 90° 110° 130° 150° 180° 210°
1640 1635 1485 1565 1590 1520 1400 1505 1390 1595 1505
7430 7055 6880 7090 7060 6810 6385 6625 6200 6530 6460
9650 9425 9385 9560 9590 9245 8645 8995 8230 8570 8605
30
R. Baby, C. Balaji / International Communications in Heat and Mass Transfer 46 (2013) 27–30
References [1] G. Setoh, F.L. Tan, S.C. Fok, Experimental studies on the use of a phase change material for cooling mobile phones, International Communications in Heat and Mass Transfer 37 (2010) 1403–1410. [2] K.C. Nayak, S.K. Saha, K. Srinivasan, P. Dutta, A numerical model for heat sinks with phase change materials and thermal conductivity enhancers, International Journal of Heat and Mass Transfer 49 (2006) 1833–1844. [3] R. Baby, C. Balaji, Experimental investigations on phase change material based finned heat sinks for electronic equipment cooling, International Journal of Heat and Mass Transfer 55 (2012) 1642–1649. [4] S.C. Fok, W. Shen, F.L. Tan, Cooling of portable hand-held electronic devices using phase change materials in finned heat sinks, International Journal of Thermal Sciences 49 (2010) 109–117. [5] V. Shatikian, G. Ziskind, R. Letan, Numerical investigation of a PCM-based heat sink with internal fins, International Journal of Heat and Mass Transfer 48 (2005) 3689–3706. [6] M. Gharebaghi, I. Sezai, Enhancement of heat transfer in latent heat storage modules with internal fins, Numerical Heat Transfer, Part A: Applications 53 (2007) 749–765. [7] J. Fukai, M. Kanou, Y. Kodama, O. Miyatake, Thermal conductivity enhancement of energy storage media using carbon fibers, Energy Conversion and Management 41 (2000) 1543–1556.
[8] T. Nomura, N. Okinaka, T. Akiyama, Impregnation of porous material with phase change material for thermal energy storage, Materials Chemistry and Physics 115 (2009) 846–850. [9] R. Prieto, J. Molina, J. Narciso, E. Louis, Fabrication and properties of graphite flakes/metal composites for thermal management applications, Scripta Materialia 59 (2008) 11–14. [10] D. Zhou, C.Y. Zhao, Experimental investigations on heat transfer in phase change materials (pcms) embedded in porous materials, Applied Thermal Engineering 31 (2011) 970–977. [11] R. Kandasamy, X.Q. Wang, A.S. Mujumdar, Application of phase change materials in thermal management of electronics, Applied Thermal Engineering 27 (2007) 2822–2832. [12] X.Q. Wang, A.S. Mujumdar, C. Yap, Effect of orientation for phase change material (PCM)-based heat sinks for transient thermal management of electric components, International Communications in Heat and Mass Transfer 34 (2007) 801–808. [13] P.M. Kamath, C. Balaji, S.P. Venkateshan, Convection heat transfer from aluminium and copper foams in a vertical channel–an experimental study, International Journal of Thermal Sciences 64 (2013) 1–10. [14] X.H. Han, Q. Wang, Y.G. Park, C. T’Joen, A. Sommers, A. Jacobi, A review of metal foam and metal matrix composites for heat exchangers and heat sinks, Heat Transfer Engineering 33 (2012) 991–1009. [15] S.P. Venkateshan, Mechnical measurements, first ed. Ane Books India, New Delhi, 2008.
Contents lists available at SciVerse ScienceDirect
International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
Experimental investigations on thermal performance enhancement and effect of orientation on porous matrix filled PCM based heat sink☆ Rajesh Baby 1, C. Balaji ⁎ Heat Transfer and Thermal Power Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e
i n f o
Available online 5 June 2013 Keywords: Phase change material (PCM) Thermal management Orientation Heat sink Enhancement ratio
a b s t r a c t Thermal performance in terms of enhancement ratios and the effect of orientation of a copper porous matrix filled phase change material (PCM) based heat sink are experimentally studied in this paper. N-eicosane is used as the phase change material. A copper open cell metal foam, press fitted into an aluminium casing is the thermal conductivity enhancer. In PCM based heat sinks, low thermal conductivity associated with PCMs makes the use of enhancement techniques inevitable for better thermal performance. A plate heater with an overall dimension of 60 × 42 mm2 with 2 mm thickness is used to mimic the heat generation in electronic chips. The effect of orientation of the heat sink on thermal performance is studied by developing a tracking system, capable of placing the heat sink at any specified orientation. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Leapfrogging in thermal management solutions is essential in order to achieve headway in the packing density challenges associated with rapid growth of electronics. Phase change material based cooling is a passive thermal management technique with a tremendous potential and is important when considering the global concerns about environmental impact. When a PCM is selected for a particular application, its melting temperature should be below the maximum operating temperature of the device. PCM based cooling methodology can be applied in many areas and also to thermal management of portable electronic equipment such as mobile phones [1], laptops, personal digital assistants and so on. In the recent past, PCM based cooling has attracted the interest of many researchers because of the large latent heat storage capacity of the PCMs and the tremendous prospects it enjoys in terms of applications. In PCM based heat sinks, a base material with high thermal conductivity is used in conjunction with the PCM in order to compensate for the very low thermal conductivity of PCMs. The use of the base material (also known as thermal conductivity enhancer (TCE)) can be in the form of different types of fins [2–6]. Fukai et al. [7] used carbon fibers distributed in PCMs in order to enhance the thermal conductivity of energy storage media. In order to improve the thermal energy storage, Nomura et al. [8] experimentally studied the impregnation of porous material with PCM. Prieto et al. [9] reported the thermal properties of graphite flakes/ metal composites for thermal management applications. From ☆ Communicated by A.R. Balakrishnan and T. Basak. ⁎ Corresponding author. E-mail address: [email protected] (C. Balaji). 1 Research scholar. 0735-1933/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.icheatmasstransfer.2013.05.018
the experimental study using metal matrix and expanded graphite as the TCE, Zhou and Zhao [10] observed that during melting, the metal foams can double the heat transfer rate when they used in conjunction with the PCMs such as paraffin wax RT 27 and calcium chloride hexahydrate. The effect of orientation on the performance of PCM based heat sinks has been investigated by various researchers for finned heat sinks [4,11]. Wang et al. [12], numerically, studied the effect of orientation on the thermal performance of PCM based plate fin heat sink using a two-dimensional physical model. Paraffin wax was used as the PCM. The evolution of melt fraction with time for various orientation angles was used to quantify the effect on heat transfer performance. The preceding review reiterates the fact that experimental studies on the thermal performance of porous matrix filled PCM based heat sinks are scarce and even fewer experimental studies that analyze the effect of orientation on the performance of such heat sinks are reported in literature. In this paper, we experimentally studied the effect of orientation on heat transfer performance in terms of time to reach a set point temperature for a copper porous matrix filled PCM based heat sink by extensive experimentation for different orientations of the heat sink using a tracking system developed in house. Furthermore, the enhanced thermal performance of the PCM based composite heat sink is quantified in terms of two enhancement ratios. 2. Experimental setup The heat sink is made from an aluminium slab with an overall dimension of 80 × 62 mm2 base with height of 25 mm using the conventional milling process. The thickness of the side wall is 7 mm and the internal cavity is available for a height of 20 mm. As the same copper metal foam [13] is used in the present study, the same data is used for
28
R. Baby, C. Balaji / International Communications in Heat and Mass Transfer 46 (2013) 27–30
the characterization of the metal foam. For the copper metal matrix considered for the present study, number of pores per inch (PPI) = 10, thickness = 20 mm, porosity = 0.86, pore diameter = 4.73 mm and the fiber diameter = 0.71 mm. A photograph of the copper metal foam fitted into the cavity made of aluminium used in the present study is given in Fig. 1. Because of the difficulties and high cost associated with brazing the metal foams [14], only press fitting of the metal foam is employed in this study. The phase change material used is n-eicosane with a melting temperature of 36.5 °C. As the porosity is 0.86, the volume fraction of the thermal conductivity enhancer i.e., the ratio of the total volume of copper metal foam to the empty heat sink volume is 14%. Except the top surface of the heat sink, the other sides of the heat sink are insulated with cork. A perspex sheet with an overall dimension of 80 × 62 mm2 with a thickness of 5 mm is kept at the top of the heat sink. On all the sides of the perspex sheet and in the area of contact between the top perspex sheet and the metal casing, a silica sealant is employed, in order to prevent any possible leakage of the PCM during tilting of the heat sink. The solidification and melting patterns of the PCM are clearly seen with the use of perspex sheet. The sheet is attached to the heat sink using six bolts, each with a diameter of 4 mm. In the 2 mm recess provided at the heat sink base, a plate heater with an overall dimension of 60 × 42 mm2 with 2 mm thickness is used to mimic the heat generation in electronic chips. The plate heater is made up of a standard coil-type nichrome wire wound over mica sheet. Before fixing the plate heater, a thermal paste is employed, in order to avoid contact resistance. A heat sink filled with PCM, without a metal matrix and a copper porous matrix filled heat sink without any PCM are used for baseline comparisons. A total of 11 chromel-alumel Ktype calibrated thermocouples are used to measure the temperature at various locations of the heat sink and these thermocouples are fixed in position using araldite epoxy. Two thermocouples are kept at the heat sink base near the plate heater, in order to record the temperature of the heat producing component, as the plate heater mimics the heat generation in electronic equipment. Four thermocouples are kept at the heat sink side wall and the another four are kept inside the PCM by making suitable openings in the perspex sheet and these openings are sealed with araldite
epoxy. All the thermocouples are connected to a PC- based data acquisition unit. The power input to the plate heater is supplied from an independently controlled DC power unit. The uncertainties in the voltage and the current measurements are ±0.1 V and ±0.01 A respectively. The uncertainty in the temperature measurement is ±0.2 °C. The uncertainty in the derived quantities is estimated, based on the uncertainty of primary quantities [15]. For heat input, the resulting uncertainty is found to be ±3.2%.
Fig. 1. A photograph of the heat sink with copper porous matrix used in the present study.
Fig. 2. A photograph of the tracking mechanism used in the present study.
2.1. Tracking mechanism A tracking mechanism is designed and fabricated, in order to study the effect of orientation on heat transfer performance. A photograph of the tracking mechanism used in the present study is given in Fig. 2. The tracking mechanism is made of aluminium. Four rubber mountings are used at the base for better stability and also to make the levelling easy. The levelling is aided by the use of a spirit level. The heat sink assembly is mounted on the shaft, provided at the top of the tracking mechanism and two bearings are kept inside the block to enable smooth rotation of the shaft. The rotation of the shaft is calibrated in terms of orientation angles with the help of the angle indicator ring fixed to the aluminium block. The rectangular base of the tracking mechanism is made of mild steel and is fixed to the aluminium block with the help of four M4 screws. The heat sink is kept at the required orientation with the help of a tightening screw. 3. Results and discussion 3.1. Effect of metal foam and PCM on heat transfer performance In order to evaluate the effect of metal foams and PCM on the heat transfer performance of the heat sink, two enhancement ratios are defined. The enhancement ratio is defined as ratio of the operation time of the heat sink with metal matrix filled with PCM to that of a heat sink, without the metal matrix, but filled with PCM. The PCM
R. Baby, C. Balaji / International Communications in Heat and Mass Transfer 46 (2013) 27–30
55
7
Enhancement ratio PCM enhancement ratio
6
Tset = 520C
Temperature (0C)
8
Enhancement ratio
29
5 4
45 40 35
3
30
2
25 4
5
6
7 8 Power level (W)
9
10
Heating phase
0
11
00 90 0 180 0
5W
50
100
Cooling phase
200
300
400
500
Time (min)
Fig. 3. Enhancement ratios of the heat sink at various power levels to reach a set point temperature of 52 °C.
Fig. 5. Depiction of the temperature–time profile for 5 W at different orientations (0°, 90° and 180°) of the heat sink.
enhancement ratio is the ratio of the operating time of the metal matrix filled PCM based heat sink to that of a heat sink with the metal matrix, but without PCM. The two enhancement ratios for a power level ranging from 5 to 10 W (corresponding to 1.98 and 3.97 kW/m2) for a set point temperature of 52 °C are shown in Fig. 3. The average value of the two thermocouple readings kept at the heat sink base is used to quantify the heat transfer performance. From Fig. 3, it is seen that PCM enhancement ratio does not vary significantly with the increase in the power level and a maximum enhancement of 3 in the operation time is achieved for a power level of 7 W. The enhancement ratio, though, varies significantly with the power level and the enhancement is high for higher power levels. An enhancement of 7.5 in operation time is achieved at 10 W. Similarly, experiments were also done for a 10 PPI, 20 mm thick aluminium metal foam filled PCM based heat sink with a porosity of 0.94, and an enhancement of 15 in operation time was obtained for a power level of 8 W when the set point temperature is 47 °C. The good thermal performance of the metal matrix at higher power levels is significant considering the fact that the heat rejection rate of the portable electronic devices are increasing day by day because of new add on features as a result of market competition.
5 W is shown in Fig. 4. Fig. 5 depicts the temperature–time history in the melting (160 min) and solidification regions at 5 W for 3 different orientations (0, 90 and 180°) of the heat sink. The temperatures recorded at the heat sink base at the end of 160 min for the orientations (0, 90 and 180°) are 49.92, 50.42 and 51.19 °C respectively. Better performance is observed when the orientation angle of the heat sink is 0°. The thermal stratification due to the change in the heating pattern is responsible for the slightly inferior thermal performance of the heat sink when it is kept at at an orientation of 180°. A comparison of the operating time to reach different set point temperatures of 40, 45 and 50 °C at 5 W is given in Table 1. All the experimental results corroborate the fact that the orientation of the heat sink assembly does not play a significant role as far as the usage of the porous matrix filled PCM based heat sinks are concerned.
3.2. Effect of orientation on heat transfer performance For studying the effect of orientation on the heat transfer performance of the copper metal foam filled PCM based heat sink, experiments were carried out for different orientations (0, 25, 45, 60, 75, 90, 110, 130, 150, 180 and 210°) of the heat sink at a power level of 5 W. At the end of 90 min of operation of the heat sink, the maximum variation in the heat sink base temperature for all the orientations considered in the present study is found to be less than 0.5 °C and at the end of 160 min, the variation is seen to be 1.84 °C. The temperature–time histories at various orientations of the heat sink at a power level of
4. Conclusions For quantifying the heat transfer performance of a PCM based copper metal foam, an experimental study is carried out and the enhancement is presented in the form of two enhancement ratios. The enhancement ratio defined as ratio of the operation time of the heat sink with metal matrix filled with PCM to that of a heat sink, without the metal matrix, but filled with PCM is 7.5 at 10 W and the maximum PCM enhancement ratio defined as the ratio of the operation duration of the metal matrix filled PCM based heat sink to that of a heat sink with the metal matrix, but without PCM is 3 at 7 W for a set point temperature of 52 °C. Identical results were also obtained for aluminium metal foam filled PCM based heat sink. From the extensive experimental studies for various orientations on the heat sink, it is found that (i) heat transfer performance is comparable and (ii)orientation does not have any significant impact on the heat transfer performance of the metal foam filled PCM based heat sinks.
55
Temperature (oC)
00 250 450 600 750 900 1100 1300 1500 1800 2100
5W
50 45 40 35
Table 1 Comparison of operating time (s) to reach various set point temperatures at different orientations of the heat sink.
30 25 0
40
80
120
160
200
Time (min) Fig. 4. Temperature–time histories of various orientations of the heat sink at a power level of 5 W.
Sl No
Orientation
Time to reach 40 °C (s)
Time to reach 45 °C (s)
Time to reach 50 °C (s)
1 2 3 4 5 6 7 8 9 10 11
0° 25° 45° 60° 75° 90° 110° 130° 150° 180° 210°
1640 1635 1485 1565 1590 1520 1400 1505 1390 1595 1505
7430 7055 6880 7090 7060 6810 6385 6625 6200 6530 6460
9650 9425 9385 9560 9590 9245 8645 8995 8230 8570 8605
30
R. Baby, C. Balaji / International Communications in Heat and Mass Transfer 46 (2013) 27–30
References [1] G. Setoh, F.L. Tan, S.C. Fok, Experimental studies on the use of a phase change material for cooling mobile phones, International Communications in Heat and Mass Transfer 37 (2010) 1403–1410. [2] K.C. Nayak, S.K. Saha, K. Srinivasan, P. Dutta, A numerical model for heat sinks with phase change materials and thermal conductivity enhancers, International Journal of Heat and Mass Transfer 49 (2006) 1833–1844. [3] R. Baby, C. Balaji, Experimental investigations on phase change material based finned heat sinks for electronic equipment cooling, International Journal of Heat and Mass Transfer 55 (2012) 1642–1649. [4] S.C. Fok, W. Shen, F.L. Tan, Cooling of portable hand-held electronic devices using phase change materials in finned heat sinks, International Journal of Thermal Sciences 49 (2010) 109–117. [5] V. Shatikian, G. Ziskind, R. Letan, Numerical investigation of a PCM-based heat sink with internal fins, International Journal of Heat and Mass Transfer 48 (2005) 3689–3706. [6] M. Gharebaghi, I. Sezai, Enhancement of heat transfer in latent heat storage modules with internal fins, Numerical Heat Transfer, Part A: Applications 53 (2007) 749–765. [7] J. Fukai, M. Kanou, Y. Kodama, O. Miyatake, Thermal conductivity enhancement of energy storage media using carbon fibers, Energy Conversion and Management 41 (2000) 1543–1556.
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