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Combined effects of inclination angle and fin number on thermal performance of a PCM-based heat sink
Applied Thermal Engineering 159 (2019) 113956
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Combined effects of inclination angle and fin number on thermal performance of a PCM-based heat sink
T
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Mustafa Yusuf Yazici, Mete Avci , Orhan Aydin Department of Mechanical Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey
H I GH L IG H T S
effects of inclination angle and fin number on the thermal performance are investigated. • Combined of the solid-liquid interface is photographed during the cooling process. • Dynamic fin number beyond a certain value results a limitation on the performance. • Increasing time can be extended by 83.4% as the inclination angle increased from 0° to 60°. • Operating • Heat sink with inclination angle of 60° and three fins performs the best performance.
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermal management Electronics cooling PCM-based heat sinks Inclination angle Fin number
Combined effects of fin number and inclination angle on the thermal performance of a PCM-based heat sink with longitudinal plate fins are investigated experimentally. Five different heat sink geometries with one, two, three, four and five fins are tested under different inclination angles varied from 0° to 90° at a constant thermal load of 16 W for the cases of with and without PCM inclusion. N-eicosane is used as the phase change material. Simultaneous temperature measurements and solid-liquid interface imaging are made to evaluate cooling performance of each heat sink configuration. The results reveal that the inclination angle and fin number plays a critical role on the formation of convective cells in the liquid PCM domain and consequently on the heat transfer and operating time. It is disclosed that the heat sink configuration with an inclination angle of 60° and three fins performs the optimal result in terms of operating time.
1. Introduction The miniaturization of electronic circuits, higher power levels per chip and increased packaging densities in the electronic industry have imposed stringent requirements not only on the physical design and fabrication but also on the reliability and performance of the electronic systems. The main criteria that determine the expected life or performance of an individual electronic device depends on its recommended operating temperature. Excessive temperatures beyond this safe limit can degrade the performance of device and also cause logic errors. In order to efficiently prevent thermal-induced-failure, the heat generated within an electronic device must be transferred to the environment via an adequate cooling technique such as conventional air cooling, impingement jets and heat exchangers with single phase coolant and thermoelectric coolers. However, most of these techniques require power and volume, and they generate noise. In this regard, application
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of PCMs in cooling of electronic devices is an alternative effective and promising solution considering life time, performance and safety, while at the same time yielding weight, volume, power consumption, noise and esthetic savings. This cooling option is simple, which involves cavities filled with PCMs as the heat storage mediums. Therefore, PCMbased cooling systems are commonly used for various applications such as spacecraft and avionic thermal control [1,2], military equipment (weapon systems, missile electronics, surveillance radars, communication system etc.) [3,4], power electronics [5,6], battery thermal management [7], personal computing and communication equipment [8], wearable computers [9] etc. Although having numerous advantages including high latent heat capacity and controllable temperature stability, most PCMs suffer from the problem of low thermal conductivity. In order to overcome this drawback, various techniques/applications are proposed in the literature, including using fins, graphite/metal foam matrix and high conductivity nanoparticles. Among these
Corresponding author. E-mail address: [email protected] (M. Avci).
https://doi.org/10.1016/j.applthermaleng.2019.113956 Received 31 December 2018; Received in revised form 21 April 2019; Accepted 14 June 2019 Available online 14 June 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
Applied Thermal Engineering 159 (2019) 113956
M.Y. Yazici, et al.
Their results showed that the pin fin arrangement exhibited longer operation times for all power levels than that of the plate-fin arrangement. As a continuation work, they conducted an experimental study to optimize the fin number and PCM volume fraction for the pin-fin arrangement case [29]. Gharbi et al. [30] conducted an experimental research on PCM based heat sinks with various arrangements namely pure PCM, PCM/silicon matrix, PCM/graphite matrix and plate fins. It was observed that the PCM/graphite matrix arrangement showed longer operating times than that of silicon matrix counterpart. Kamkari and Shoukoumand [31] performed an experimental study to evaluate the melting process of PCM inside a rectangular enclosure with plate fins. Their results showed that the total melting time decreased remarkably with an increase in the fin number. They also proposed an empirical correlation in order to predict the surface averaged Nusselt number. This work was further extended by Kamkari and Groulx [32] to study the effect of the inclination angle on the melting behavior of PCM for the same geometry. They observed that the orientation of enclosure played a crucial role in the development of convection cells during the initial stage of melting process. They also reported that the melting rate was enhanced as the orientation of the enclosure moved from vertical (θ = 90°) to horizontal (θ = 0°) position. Arshad et al. [33–38] presented a series of experimental study on the thermal performance of PCM-based pin fin heat sinks with various parameters namely pin fin geometry (square, round and triangle), dimensional change (thickness and diameter), configuration (staggered and inline array), volume fraction of PCM, and PCM materials. Comparison of the results among the fin geometries showed that the triangular shaped pinfins exhibited relatively the best thermal performance. It was also shown that lower surface temperatures were obtained by using circular inline arrangements compared to the rounded one under the same heat loads. The above literature review reveals that very few studies have been reported on the thermal management of PCM-based heat sinks, taking into consideration the combined effects of fin number and inclination angle. Moreover, most of them are numerical. Motivated by this fact, a detailed experimental investigation is carried out to explore the combined effects of fin number and inclination angle on the cooling performance of a PCM-based heat sink with longitudinal plate fins. A significant database is also created for validation of numerical studies, not only in terms of temperature distributions but also detailed solid-liquid interface visualizations.
techniques, using fins is the most common ones because of its low cost and ease of manufacture. Hence, many studies have been devoted to improve the thermal performance of finned PCM-based heat sinks, which include the effects of power load condition (constant or variable load), power level, geometrical parameters (fin type/number, height, orientation etc.) and PCM type and quantity. For a better view, the readers are referred to see comprehensive reviews by Ling et al. [10], Sahoo et al. [11], Garimella [12], Sundaram [13] and Murshed [14]. Here, some of the recent experimental and numerical studies focusing on the thermal management of PCM-based heat sinks are described as follows: Nayak et al. [15] proposed a generalized mathematical formulation and a numerical model to evaluate the thermal performance of PCMbased heat sink with some common type TCEs namely porous matrix, plate and rod-type of fins. Simulations were performed at different power levels. Their analysis showed that the rod-finned type arrangement kept the hot spot temperature lower and maintained the temperature distribution more uniform in the system than those of matrix and plate-finned types. Akhilesh et al. [16] numerically evaluated the thermal performance of composite PCM-based heat sinks with plate type fins. They proposed an empirical correlation for critical dimensions of heat sink in terms of quantity of PCM. Kandasamy et al. [17] carried out both experimental and numerical studies on various designs of PCM-based heat sinks with plate fins under constant power levels. They reported that the PCM inclusion enhanced the thermal performance of heat sinks. Wang et al. [18,19] performed a numerical study to evaluate the effect of inclination angle, PCM volume fraction and PCM type on the cooling performance of finned heat sinks under cyclic and constant heat loads. Their results showed that the inclusion of PCM in the cavities of heat sink significantly improved the cooling performance of system while the effect of inclination angle was much less significant. Fok et al. [20,21] experimentally tested the effects of fin number and inclination angle on the base temperature distribution of a PCM-based heat sinks used for cooling electronic components having variable load cycles (frequent, heavy and light). They pointed out that the maximum base temperature was effectively reduced by increasing the fin number. It was also shown that the inclination angle had an insignificant effect on the cooling performance of device. Saha et al. [22] experimentally and numerically studied the effects of fin number and thickness on the thermal design of PCM-based heat sinks with plate type fins. They obtained that the base temperature reduced by decreasing the fin thickness and increasing the fin number. As a continuation work, they extended their numerical analysis by taking the effects of aspect ratio and heat flux into consideration [23]. They proposed new correlations for Nusselt number in terms of Rayleigh number based on the characteristic length scale. Hosseinizadeh et al. [24] analyzed the thermal behavior of PCM-based heat sinks with different configurations of internal fins. The research parameters examined were the fin number, height and thickness and the power level. They found that an increase in the fin number and fin height led to a significant increase on the overall performance of the system while an increase in the fin thickness was much less significant. Jaworski [25] performed a numerical study on the thermal performance of pin fin heat spreaders. They reported that the inclusion of PCM into the fins remarkably enhanced the heat dissipation rate and reduced the operating temperature of the electronic component. Comparison of various heat sink arrangements, PCM materials and power levels on the thermal behavior of PCM based heat sinks were experimentally investigated by Mahmoud et al. [26]. Parallel- and cross-shaped fin and honeycomb arrangements were tested under steady-state power inputs. They reported that the increase of fin number and inclusion of low melting PCM at moderate power levels had significantly extended the operation time of heat sink. Similar results were presented by Fan et al. [27] for cross-finned heat sink designs exposed to high power levels. Baby and Balaji [28] experimentally evaluated the thermal performances of plate-fin and pinfin heat sinks filled with PCM under various steady-state power levels.
2. Experimental study The solid model and photograph of the experimental facility, mainly consisting of a test section, an adjustable-tilt mechanism, a DC power system, a data acquisition system and a flow visualization system are shown in Fig. 1(a) and (b), respectively. The test section is a rectangular enclosure with inside cavity dimensions of 48 mm in width, 34 mm in depth and 100 mm in height. The left wall of the enclosure acted as the heat sink is made of 1.5 mm thick Aluminum −6082 (k = 172 W/m2 K) while the other walls are made of 10 mm thickness polycarbonate transparent material in order to facilitate imaging of the phase change inside the enclosure. Five different heat sink geometries with one, two, three, four and five plate fins are tested in the present study. The heat sinks are manufactured from aluminum slabs (Al-6082) by EDM technique in order to ensure no thermal contact resistance between the fins and the base plate. Each fin has dimensions of 12 mm in depth, 100 mm in height and 1.2 mm in width. The isometric view and overall dimensions of the heat sink configurations are given in Fig. 2, respectively. To simulate the heat source, a thin flexible heater with dimensions of 48 mm × 80 mm is attached to a copper sheet (0.2 mm) with a thermally conductive silicone paste (Omegatherm 201) as in Fig. 3. Nine evenly spaced rectangular slots with dimensions of 0.5 mm × 1 mm are milled on the back surface of the heat sink to place thermocouples to monitor the base 2
Applied Thermal Engineering 159 (2019) 113956
M.Y. Yazici, et al.
Fig. 1. Experimental setup: solid model (a), and photograph (b) [39].
personal computer, which records the temperatures at 10 s intervals. The required power input (16 W) to the thin film heater is provided by a DC power supply unit (GW Instek GPS-4303) with an accuracy of ± 0.01% (FS). For attaching components together, nylon screws and low conductive liquid seals are preferred on the contact surfaces in order to minimize the conduction heat transfer between the walls and heat sink and prevent any leakage of the liquid PCM from the enclosure, respectively. For further reduction of heat loss to the environment, the heater and walls of the enclosure are also insulated with 30 mm
temperature distribution. After mounting nine T-type thermocouples inside these slots, the copper sheet is mounted on the back side of the heat sink, again using the same thermal conductive paste in order to minimize the thermal contact resistance. Except these nine thermocouples, fifteen additional thermocouples are also used to follow the transient melting of PCM inside the enclosure. Schematic diagrams with thermocouple locations and labels are depicted in Figs. 2 and 3. All thermocouples are calibrated in range of 10–100 °C with an accuracy of ± 0.5 °C. A data acquisition system (Keithley 2701) is connected to a 3
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Fig. 2. Schematic view of fin based heat sink: isometric view (a) and overall dimension (b).
base temperature reached 70 °C. During this process, the insulation layer is temporally removed from the lateral wall of the enclosure and the images are recorded at 3 min intervals. Each test is initiated by first verifying that all the thermocouples on the base plate and within the enclosure have the same value as the ambient temperature of 25 °C. The experiments are repeated at least twice with maximum deviations of ± 2%. By following the approach of Kline & McClintock, the maximum uncertainties in the temperature and heat output are found to be ± 2.2% and ± 3.22%, respectively. More details on the experimental set up and measurement procedure can be found in Ref. [39].
polystyrene foam sheet. For all the cases considered, the maximum heat loss from the back side of the heater is found to be 3% of the total power input by using the temperature data of five thermocouples located on the outside surface of the polystyrene foam layer. A commercially available paraffin wax known as n-eicosane with an average melting point of 37 °C is used as PCM. Thermo-physical properties of PCM used are given in Table 1. For all the cases, considering the volume change based on the density variation between the solid and liquid transition of PCM and fin addition, 107 g of paraffin is included to the enclosure. In order to address the effect buoyancy forces on the thermal performance of heat sink, simultaneous imaging and temperature measurements are performed at various inclination angles from 0° (vertical) to 90° (horizontal) at 15° intervals. For this purpose, three digital cameras with a resolution of 6528 × 3672 pixels are mounted on an adjustable tilting mechanism which inclines the test section at desired angular positions. For each run, the image recordings are captured from the beginning of the heating process and continued until the maximum
3. Experimental results and discussion The present research is aimed at better understanding of the effects of fin number and inclination angle on the thermal performance of a PCM-based heat sink with longitudinal plate fins. To achieve this aim, five different heat sink geometries with one, two, three, four and five 4
Applied Thermal Engineering 159 (2019) 113956
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temperatures sharply rise to the maximum allowed temperature (T = 70 °C) in short operating times, being relatively independent of inclination angle. The results also show that an increase in the fin number has a limited effect on the rate of temperature rise for each inclination angle. A plausible explanation for these behaviors is the low thermal heat conductivity and heat storage capacity of air, which results in a poor heat transfer. However, for the case with PCM, the plots show that the rate of temperature change is a function of both fin number and inclination angle. As a typical character, in the initial stage of heating process (t < 3 mins.), the base temperature shows a sharp increase with time and quickly passes to the melting temperature of PCM, which is in the range of 35–37 °C. In this period, PCM is in the solid phase and the heat diffused through PCM via conduction. After reaching the melting point temperature (t > 3 mins.), phase change starts and a thin layer of liquid PCM is initially formed around the base plate and fins. Thus, more heat is absorbed by PCM in the latent form and hence, the temperature rise slows down. Note that, in this period, the buoyancy forces are relatively weaker than the viscous forces and the dominant heat transfer mechanism is conduction. As time elapses, the buoyancy forces overcome the viscous forces with the development of melt fraction and results in formation of convective cells in the liquid domain. This, in turn, directly enhances the heat transfer within PCM domain and causes more decrease in the temperature rise as compared with the earlier period. After liquid fraction becomes dominant relative to the solid fraction, the base temperature again increases with time since the heat is stored in sensible heat form. These trends in base temperature can easily be seen in Fig. 6d.
3.1.2. Effects of fin number and inclination angle For a given inclination angle, as it is seen from Figs. 4–6, the base temperature decreases with increasing value of fin number, as expected. A plausible cause for this dependency is the higher extended heat transfer surface area which enhances the thermal conductivity of PCM and also induces the flow mixing in the liquid PCM. Thus, more heat is absorbed by PCM with the heat transfer modes of conduction and convection. As an example, for the vertical orientation of the heat sink (θ = 0°) at t = 28 mins., it can easily be seen from Fig. 4d and h that the maximum base temperature is approximately % 10 lower for five fins compared to the one fin case. Among these cases, it is also seen that the time required to reach the maximum allowable maximum temperature (operating time) is increased by 80% for five fins compared to the case with one fin. However, as the plots show, the increment in the operating time becomes less prominent for high values of fin number. As stated earlier, except the initial stage of heating process (t < 3 mins.), natural convection is the dominant heat transfer mode that governs the melting rate and heat transfer within PCM domain. In this mode, heat transfer strongly depends on the circulation of PCM (convection cell) formed between the adjacent fins. For higher values of fin number, the magnitude of this fluid motion is weakened due to the small spacing between adjacent two fins, which causes high flow resistance. As a result, heat transfer rate decreases. Figs. 4–6 show the effect of inclination angle on the base temperature variation for the cases with zero, one, three and five plate fins. As compared to the vertical orientation (θ = 0°), at a given time and fin number, it can clearly be seen that the base temperature gets lower values with an increase in the inclination angle. This decrement can be attributed to two main reasons. The first is the increase in the magnitude of buoyancy-induced flow which impinges large solid PCM domains with an increase in inclination angle and the second is the expansion of liquid PCM zone which allows the growth of convective cells formed with in PCM domain. The effect of inclination angle is more clearly evident for one and three fins as compared to the case with five fins. As discussed above, this behavior can be attributed to the decrease of flow intensity with fin insertion.
Fig. 3. Thermocouple locations and labels: back side of the heat sink surface (a), within enclosure (b) [39]. Table 1 Thermo-physical properties of n-eicosane [15]. Formula
C20H42
Melting range, °C Density, kg/m3 Specific heat, kJ/kg °C Latent heat, kJ/kg Thermal conductivity (W/mK)
35–37 810 (solid), 770 (Liquid) 1.9 (solid), 2.2 (Liquid) 241 0.39 (solid), 0.157 (Liquid)
fins are tested under different inclination angles varied from 0° to and 90°. All the results are also compared with the un-finned case of the same geometry studied by Avci and Yazici [39].
3.1. Temperature distribution on the base plate 3.1.1. Effect of PCM inclusion As mentioned above, the temperatures are measured at nine evenly spaced locations (Th1-h9) on the heater surface (base temperature) to evaluate thermal behavior of each heat sink configuration with and without PCM. The time history of temperature variations on these locations are depicted in Figs. 4–6 for different inclination angles of heat sink with various values of fin number (NoF). For the case without PCM, it is clearly seen that at a given value of fin number, the base 5
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Fig. 4. The time history of temperature variations on the heater surface with various fin numbers for θ = 0° : without PCM, 0 fin [39] (a), 1fin (c), 3 fin (e), 5 fin (g); with PCM, 0 fin [39] (b), 1fin (d), 3 fin (f), 5 fin (h).
3.2. Temperature distribution within the PCM medium
the mid-axial plane of the PCM domain are given in Figs. 7 and 8 at different values of fin number for both the vertical and horizontal orientations of the heat sink. As it seen from Fig. 7, for the vertical
The time history of PCM temperature at some specified locations in
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Fig. 5. The time history of temperature variations on the heater surface with various fin numbers for θ = 45° : without PCM, 0 fin [39] (a), 1fin (c), 3 fin (e), 5 fin (g); with PCM, 0 fin[39] (b), 1fin (d), 3 fin (f), 5 fin (h).
buoyancy force is parallel to the heater. However, for the horizontal orientation (Fig. 8), the temperature variations are relatively similar due the formation of three dimensional Benard convection cells which induces effective mixing within the liquid PCM. This mechanism was
orientation, the temperatures in the upper half of the PCM domain are higher than those of in the lower half. The reason for this trend is the higher contribution of buoyancy which drives the high temperature liquid PCM to the upward direction. Here, it should be noted that the 7
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Fig. 6. The time history of temperature variations on the heater surface with various fin numbers for θ = 90° : without PCM, 0 fin [39] (a), 1fin (c), 3 fin (e), 5 fin (g); with PCM, 0 fin [39] (b), 1 fin (d), 3 fin (f), 5 fin (h).
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Fig. 7. Time history of the PCM temperature at some specified locations in the mid-axial plane of the PCM domain for θ = 0° : 0 fin [39] (a), 1 fin (b), 3 fin (c), 5 fin (d).
buoyancy acts vertically, the inclination angle causes important changes on the generation of buoyancy-induced flow and its extent within the PCM as given in Fig. 10. For the vertical orientation case, as it is seen from Fig. 10a, the formation of convective cells are firstly initiated near the top right corner of the PCM domain and then expands opposite to the heated wall with an increase in time. This flow motion locally enhances heat transfer in the upper half of the PCM domain and causes a fast movement of the solid-liquid interface than those in the lower half part, which creates a concave interface shape. Similar observations were also found by Kamkari et al. [40] and Kamkari and Amlashi [41] who studied the convection-driven melting of phase change material in an inclined rectangular enclosure experimentally and numerically. Due to the small spaces between the base plate/fin and solid PCM, the conduction mode is still dominant in the lower part of PCM domain. As the inclination angle changes from 0° to 45° (Fig. 10b), the convection cell expands to the lower half of the PCM domain because more of hot PCM separates from the heated wall and impinges to the solid PCM. This strong convection motion enhances the melting rate and changes the solid-liquid interface profile as convex. Similar observations were found by Kamkari and Amlashi for the formation of convection cells. For the horizontal orientation (θ = 90°), as it is seen from Figs. 9c and 10c, the interface inside the PCM domain progresses in a near linear fashion, expect some inconsistencies when moving along the base plate. This melting trend is attributed to the formation of three dimensional Benard convection cells which induces effective mixing within the liquid PCM. The images also reveal that, at the respective time, the change in the fin number has not any considerable influence
discussed in our earlier study [39] for the un-finned case of heat sink. Results also show that for all inclination angles, the value of PCM temperature in each selected location decreases with increasing values of fin number.
3.3. Solid-liquid interface evaluation To better understand the effects of fin number and inclination angle on the melting and heat transfer behavior in the PCM, the dynamic of the solid-liquid interface is photographed from the lateral surface of the heat sink at twelve-minute-intervals during the heating process (Figs. 9–11). Final period of melting process (Fig. 11) is also visualized for each configuration at the time to reach 70 °C. In these images, the black and white colors correspond to the liquid and solid phases, respectively. In the early stage of heating process (t = 12 mins.), as it is seen from Fig. 9, the movement of solid liquid interface is relatively uniform along the base plate for all cases, except small changes in the upper half of PCM domain for the case of θ = 0° at lower values of fin number. As stated earlier, in this period, the buoyancy forces are not high enough to overcome the viscous forces due to the insufficient thickness of melted PCM layer adjacent to the base plate and fins, which makes the conduction as the dominant heat transfer mechanism. The temperature distributions given in Figs. 4 and 7 also confirm this heat transfer mechanism. As the liquid layer becomes thicker with time (t = 24 mins.), the buoyancy force takes an active role on the heat transfer and dictates the movement of the solid-liquid interface within PCM domain. As the 9
Applied Thermal Engineering 159 (2019) 113956
M.Y. Yazici, et al.
Fig. 8. Time history of the PCM temperature at some specified locations in the mid-axial plane of the PCM domain for θ = 90° : 0 fin [39] (a), 1 fin (b), 3 fin (c), 5 fin (d).
play a dominant role on the convection mode. As the fin number increases beyond a certain value where the spacing between adjacent two fins is too small to permit a formation of convection cell, the magnitude of flow motion is strongly weakened and causes a decrease in heat transfer. Similar observations are made by Saha et al. [22] for different pin fin arrangements. Their findings for the optimal value of fin volume fraction (8%) agree very well with our results for the three-fin case, which is nearly 9%. For the case without PCM inclusion, as it is seen from Fig. 12b, the operating time increases with an increase in the fin number, being relatively independent of inclination angle.
on the interface profile. The final forms of the solid-liquid interface profiles are given Fig. 11. Comparison of images show that while the trends of interface profiles are relatively similar with the previous period, the melt fraction increases with an increase in both the fin number and inclination angle. This behavior is clearly visible in the lower half of PCM domain at low inclination angles. 3.4. Effects of fin number and inclination angle on the thermal performance of heat sink
4. Conclusions
The cooling performance of each heat sink configuration with and without PCM is compared in terms of operating time. As it is seen from Fig. 12 a, at a given value of fin number, the operating time increases with increasing inclination angle until it reaches a maximum at 60°, then it decreases with further increasing of inclination angle for the finned case with PCM. As mentioned earlier, this trend in the operating time is due to the stronger buoyancy-induced flow and larger liquid PCM domains for the convective cells to grow. Comparison of the results show that the operating time can be extended by 83.4% and 79.2% for the one fin case as the inclination angle increased from 0° to 60° and 0° to 90°, respectively. The plots also show that, the operating time is a function of fin number. As it is seen from the figure, an increase in the fin number extends the operating time. This extension is more dominant until a certain value of a fin number (3), and then it becomes less prominent with further increasing of fin number. This trend can be attributed to the change in the intensity and magnitude of convection cells which
In this study, the combined effects of fin number and inclination angle on the thermal performance of PCM-based heat sinks with longitudinal fins are investigated experimentally. The major findings of the present study can be summarized as follows:
• More uniform temperature distribution is achieved with an increase of fin number and inclination angle at the base surface. • The orientation of the heat sink significantly affects the melting rate • • 10
and movement of solid-liquid interface due to the variations in the convective currents. For the vertical orientation case (θ = 0°), the operating time can be extended by 586% as the number of fin increases from 0 to 5. The operating time can be increased up to 79.2% by changing the orientation of heat sink from vertical (θ = 0°) to horizontal (θ = 0°) for the one fin case.
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Fig. 9. Solid-liquid interface progression at 12mins. under different inclination angles: θ = 0° (a), θ = 45° (b), θ = 90° (c).
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Fig. 10. Solid-liquid interface progression at 24mins. under different inclination angles: θ = 0° (a), θ = 45° (b), θ = 90° (c).
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Fig. 11. Solid-liquid interface progression at final the period under different inclination angles: θ = 0° (a), θ = 45° (b), θ = 90° (c).
• An increase in the fin number beyond a certain value (3) results a
limitation on the thermal performance of heat sink. This is due to the fact that adding fins although increases the heat transfer area, but it simultaneously increases the flow resistance of liquid PCM
• 13
which hampers the convective flow and thus the heat transfer within the PCM. Among the seven inclination angles were tried, heat sink with inclination angle of 60° showed the best result in terms of operating
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Fig. 12. The effect of fin number and inclination angle on the thermal performance of heat sink: with PCM (a), without PCM (b).
time.
change materials in finned heat sinks, Int. J. Therm. Sci. 49 (2010) 109–117. [21] G. Setoh, F.L. Tan, S.C. Fok, Experimental studies on the use of a phase change material for cooling mobile phones, Int. Commun. Heat Mass Transfer 37 (2010) 1403–1410. [22] S.K. Saha, K. Srinivasan, P. Dutta, Studies on optimum distribution of fins in heat sink filled with phase change materials, J. Heat Transfer 130 (2008) 1–4. [23] S.K. Saha, P. Dutta, Heat transfer correlations for PCM-based heat sinks with plate fins, Appl. Therm. Eng. 30 (2010) 2485–2491. [24] S.F. Hosseinizadeh, F.L. Tan, S.M. Moosania, Experimental and numerical studies on performance of PCM-based heat sink with different configurations of internal fins, Appl. Therm. Eng. 31 (2011) 3827–3838. [25] M. Jaworski, Thermal performance of heat spreader for electronics cooling with incorporated phase change material, Appl. Therm. Eng. 35 (2012) 212–219. [26] S. Mahmoud, A. Tang, C. Toh, R. Al-Dadah, S.L. Soo, Experimental investigation of inserts configurations and pcm type on the thermal performance of PCM based heat sinks, Appl. Energy 112 (2013) 1349–1356. [27] L.W. Fan, Y.Q. Xiao, Y. Zeng, X. Fang, X. Wang, X. Xu, Z.T. Yu, R.H. Hong, Y.C. Hu, K.F. Cen, Effects of melting temperature and the presence of internal fins on the performance of a phase change material (PCM)-based heat sink, Int. J. Therm. Sci. 70 (2013) 114–126. [28] R. Baby, C. Balaji, Experimental investigations on phase change material based finned heat sinks for electronic equipment cooling, Int. J. Heat Mass Transfer 55 (2012) 1642–1649. [29] R. Baby, C. Balaji, Thermal optimization of PCM based pin heat sink: an experimental study, Appl. Therm. Eng. 54 (2013) 65–77. [30] S. Gharbi, S. Harmand, S.B. Jabrallah, Experimental comparison between different configurations of PCM based heat sinks for cooling electronics components, Appl. Therm. Eng. 87 (2015) 454–462. [31] B. Kamkari, H. Shokouhmand, Experimental investigation of phase change material melting in rectangular enclosures with horizontal partial fins, Int. J. Heat Mass Transfer 78 (2014) 839–851. [32] B. Kamkari, D. Groulx, Experimental investigation of melting behavior of phase change material in finned rectangular enclosures under different inclination angles, Exp. Therm Fluid Sci. 97 (2018) 94–108. [33] M.J. Ashraf, H.M. Ali, H. Usman, A. Arshad, Experimental passive electronics cooling: parametric investigation of pin-fin geometries and efficient phase change materials, Int. J. Heat Mass Transfer 115 (2017) 251–263. [34] A. Arshad, H.M. Ali, M. Ali, S. Manzoor, Thermal performance of phase change material (PCM) based pin-finned heat sinks for electronic devices: effect of pin thickness and PCM volume fraction, Appl. Therm. Eng. 112 (2017) 143–155. [35] A. Arshad, H.M. Ali, W. Yan, A.K. Hussein, M. Ahmadlouydarab, An experimental study of enhanced heat sinks for thermal management using n-eicosane as phase change material, Appl. Therm. Eng. 132 (2018) 52–66. [36] H.M. Ali, A. Arshad, Experimental investigation of n-eicosane based circular pin-fin heat sinks for passive cooling electronic devices, Int. J. Heat Mass Transfer 112 (2017) 649–661. [37] A. Arshad, H.M. Ali, S. Khushnood, M. Jabbal, Experimental investigation of PCM based round pin fin heat sinks for thermal management of electronics: effect of pin-fin diameter, Int. J. Heat Mass Transfer 117 (2018) 861–872. [38] H.M. Ali, M.J. Ashraf, A. Giovannelli, M. Irfan, T. Irshad, H.M. Hamid, F. Hassan, A. Arshad, Thermal management of electronics: an experimental analysis of triangular, rectangular and pin-fin heat sinks for various PCMs, Int. J. Heat Mass Transfer 123 (2018) 272–284. [39] M. Avci, M.Y. Yazici, An experimental study on effect of inclination angle on the performance of a PCM-based flat-type heat sink, Appl. Therm. Eng. 131 (2018) 806–814. [40] B. Kamkari, H. Shokouhmand, F. Bruno, Experimental investigation of the effect of inclination angle on convection-driven melting of phase change material in a rectangular enclosure, Int. J. Heat Mass Transfer 72 (2014) 186–200. [41] B. Kamkari, H.J. Amlashi, Numerical simulation and experimental verification of constrained melting of phase change material in inclined rectangular enclosures, Int. Commun. Heat Mass Transfer 88 (2017) 211–219.
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.113956. References [1] D.V. Hale, M.J. Hoover, M.J. O’Neill, Phase change materials handbook, High Series Contractor Report, NAS8-25183, Marshal Space Flight Centre, Alabama, September 1971. [2] A.J. Fosset, M.T. Maguire, A.A. Kudirka, F.E. Mills, D.A. Brown, Avionics passive cooling with microencapsulated phase change materials, ASME J. Electron. Packag. 120 (1998) 238–242. [3] D.C. Price, A review of selected thermal management solutions electronics for military system, IEEE Trans. Compon. Pack. Technol. 26 (2003) 26–39. [4] R. Kumar, M.K. Misra, R. Kumar, D. Gupta, P.K. Sharma, B.B. Tak, S.R. Meena, Phase change materials: Technology status and potential defence application, Def. Sci. J. 61 (2011) 576–582. [5] T.J. Lu, Thermal management of high power electronics with phase change cooling, Int. J. Heat Mass Transfer 34 (2000) 2245–2256. [6] A.G. Evans, M.Y. He, M. Hutchinson, Temperature distribution in advanced power electronics and effect of phase change materials on temperature suppression during power pulses, ASME J. Electron. Packag. 123 (2001) 211–217. [7] H. Liu, Z. Wei, W. He, J. Zhao, Thermal issues about Li-ion batteries and recent progress in battery thermal management systems: a review, Energy Convers. Manag. 150 (2017) 304–330. [8] M. Hodes, R.D. Weinstein, S.J. Pence, J.M. Piccini, L. Manzione, C. Chen, Transient thermal management of handset using phase change material (PCM), J. Electron. Packag. 124 (2002) 419–426. [9] M.J. Vesligaj, C.H. Amon, Transient thermal management of temperature fluctuations during time varying workloads on portable electronics, IEE Trans. Compon. Packag. Technol. 22 (1999) 541–550. [10] Z. Ling, Z. Zhang, G. Shi, X. Fang, L. Wang, X. Gao, Y. Fang, T. Xu, S. Wang, X. Liu, Review on thermal management systems using phase change materials for electronics component, Li-ion batteries and photovoltaic modules, Renew. Sust. Energy Rev. 31 (2014) 427–438. [11] S.K. Sahoo, M.K. Das, P. Rath, Application of TCE - PCM based heat sink for cooling of electronic components: A review, Renew. Sust. Energy Rev. 59 (2016) 550–582. [12] S. Garimella, Advances in mesoscale thermal management technologies for microelectronics, Microelectron. J. 37 (2006) 1165–1185. [13] S.S. Anandan, V. Ramalingam, Thermal management of electronics: a review of literature, Thermal Sci. 12 (2008) 5–26. [14] S.M.S. Murshed, C.A.N. Castro, A Critical Review of traditional and emerging techniques and fluids for electronics cooling, Renew. Sust. Energy Rev. 78 (2017) 821–833. [15] K.C. Nayak, S.K. Saha, K. Srinivasan, P. Dutta, A numerical model for heat sinks with phase change materials and thermal conductivity enhancers, Int. J. Heat Mass Transfer 49 (2006) 1833–1844. [16] R. Akhilesh, A. Narasimhan, C. Balaji, Method to improve geometry for heat transfer enhancement in PCM composite heat sinks, Int. J. Heat Mass Transfer 48 (2005) 2759–2770. [17] R. Kandasamy, X.C. Wang, A.S. Mujumdar, Transient cooling of electronics using phase change material (PCM)-based heat sinks, Appl. Therm. Eng. 28 (2008) 1047–1057. [18] 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, Int. Commun. Heat Mass Transfer 34 (2007) 801–808. [19] X.Q. Wang, C. Yap, A.S. Mujumdar, A parametric study of phase change material (PCM)based heat sinks, Int. J. Therm. Sci. 47 (2008) 1055–1068. [20] S.C. Fok, W. Shen, F.L. Tan, Cooling of portable hand-held electronic devices using phase
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Combined effects of inclination angle and fin number on thermal performance of a PCM-based heat sink
T
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Mustafa Yusuf Yazici, Mete Avci , Orhan Aydin Department of Mechanical Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey
H I GH L IG H T S
effects of inclination angle and fin number on the thermal performance are investigated. • Combined of the solid-liquid interface is photographed during the cooling process. • Dynamic fin number beyond a certain value results a limitation on the performance. • Increasing time can be extended by 83.4% as the inclination angle increased from 0° to 60°. • Operating • Heat sink with inclination angle of 60° and three fins performs the best performance.
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermal management Electronics cooling PCM-based heat sinks Inclination angle Fin number
Combined effects of fin number and inclination angle on the thermal performance of a PCM-based heat sink with longitudinal plate fins are investigated experimentally. Five different heat sink geometries with one, two, three, four and five fins are tested under different inclination angles varied from 0° to 90° at a constant thermal load of 16 W for the cases of with and without PCM inclusion. N-eicosane is used as the phase change material. Simultaneous temperature measurements and solid-liquid interface imaging are made to evaluate cooling performance of each heat sink configuration. The results reveal that the inclination angle and fin number plays a critical role on the formation of convective cells in the liquid PCM domain and consequently on the heat transfer and operating time. It is disclosed that the heat sink configuration with an inclination angle of 60° and three fins performs the optimal result in terms of operating time.
1. Introduction The miniaturization of electronic circuits, higher power levels per chip and increased packaging densities in the electronic industry have imposed stringent requirements not only on the physical design and fabrication but also on the reliability and performance of the electronic systems. The main criteria that determine the expected life or performance of an individual electronic device depends on its recommended operating temperature. Excessive temperatures beyond this safe limit can degrade the performance of device and also cause logic errors. In order to efficiently prevent thermal-induced-failure, the heat generated within an electronic device must be transferred to the environment via an adequate cooling technique such as conventional air cooling, impingement jets and heat exchangers with single phase coolant and thermoelectric coolers. However, most of these techniques require power and volume, and they generate noise. In this regard, application
⁎
of PCMs in cooling of electronic devices is an alternative effective and promising solution considering life time, performance and safety, while at the same time yielding weight, volume, power consumption, noise and esthetic savings. This cooling option is simple, which involves cavities filled with PCMs as the heat storage mediums. Therefore, PCMbased cooling systems are commonly used for various applications such as spacecraft and avionic thermal control [1,2], military equipment (weapon systems, missile electronics, surveillance radars, communication system etc.) [3,4], power electronics [5,6], battery thermal management [7], personal computing and communication equipment [8], wearable computers [9] etc. Although having numerous advantages including high latent heat capacity and controllable temperature stability, most PCMs suffer from the problem of low thermal conductivity. In order to overcome this drawback, various techniques/applications are proposed in the literature, including using fins, graphite/metal foam matrix and high conductivity nanoparticles. Among these
Corresponding author. E-mail address: [email protected] (M. Avci).
https://doi.org/10.1016/j.applthermaleng.2019.113956 Received 31 December 2018; Received in revised form 21 April 2019; Accepted 14 June 2019 Available online 14 June 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
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Their results showed that the pin fin arrangement exhibited longer operation times for all power levels than that of the plate-fin arrangement. As a continuation work, they conducted an experimental study to optimize the fin number and PCM volume fraction for the pin-fin arrangement case [29]. Gharbi et al. [30] conducted an experimental research on PCM based heat sinks with various arrangements namely pure PCM, PCM/silicon matrix, PCM/graphite matrix and plate fins. It was observed that the PCM/graphite matrix arrangement showed longer operating times than that of silicon matrix counterpart. Kamkari and Shoukoumand [31] performed an experimental study to evaluate the melting process of PCM inside a rectangular enclosure with plate fins. Their results showed that the total melting time decreased remarkably with an increase in the fin number. They also proposed an empirical correlation in order to predict the surface averaged Nusselt number. This work was further extended by Kamkari and Groulx [32] to study the effect of the inclination angle on the melting behavior of PCM for the same geometry. They observed that the orientation of enclosure played a crucial role in the development of convection cells during the initial stage of melting process. They also reported that the melting rate was enhanced as the orientation of the enclosure moved from vertical (θ = 90°) to horizontal (θ = 0°) position. Arshad et al. [33–38] presented a series of experimental study on the thermal performance of PCM-based pin fin heat sinks with various parameters namely pin fin geometry (square, round and triangle), dimensional change (thickness and diameter), configuration (staggered and inline array), volume fraction of PCM, and PCM materials. Comparison of the results among the fin geometries showed that the triangular shaped pinfins exhibited relatively the best thermal performance. It was also shown that lower surface temperatures were obtained by using circular inline arrangements compared to the rounded one under the same heat loads. The above literature review reveals that very few studies have been reported on the thermal management of PCM-based heat sinks, taking into consideration the combined effects of fin number and inclination angle. Moreover, most of them are numerical. Motivated by this fact, a detailed experimental investigation is carried out to explore the combined effects of fin number and inclination angle on the cooling performance of a PCM-based heat sink with longitudinal plate fins. A significant database is also created for validation of numerical studies, not only in terms of temperature distributions but also detailed solid-liquid interface visualizations.
techniques, using fins is the most common ones because of its low cost and ease of manufacture. Hence, many studies have been devoted to improve the thermal performance of finned PCM-based heat sinks, which include the effects of power load condition (constant or variable load), power level, geometrical parameters (fin type/number, height, orientation etc.) and PCM type and quantity. For a better view, the readers are referred to see comprehensive reviews by Ling et al. [10], Sahoo et al. [11], Garimella [12], Sundaram [13] and Murshed [14]. Here, some of the recent experimental and numerical studies focusing on the thermal management of PCM-based heat sinks are described as follows: Nayak et al. [15] proposed a generalized mathematical formulation and a numerical model to evaluate the thermal performance of PCMbased heat sink with some common type TCEs namely porous matrix, plate and rod-type of fins. Simulations were performed at different power levels. Their analysis showed that the rod-finned type arrangement kept the hot spot temperature lower and maintained the temperature distribution more uniform in the system than those of matrix and plate-finned types. Akhilesh et al. [16] numerically evaluated the thermal performance of composite PCM-based heat sinks with plate type fins. They proposed an empirical correlation for critical dimensions of heat sink in terms of quantity of PCM. Kandasamy et al. [17] carried out both experimental and numerical studies on various designs of PCM-based heat sinks with plate fins under constant power levels. They reported that the PCM inclusion enhanced the thermal performance of heat sinks. Wang et al. [18,19] performed a numerical study to evaluate the effect of inclination angle, PCM volume fraction and PCM type on the cooling performance of finned heat sinks under cyclic and constant heat loads. Their results showed that the inclusion of PCM in the cavities of heat sink significantly improved the cooling performance of system while the effect of inclination angle was much less significant. Fok et al. [20,21] experimentally tested the effects of fin number and inclination angle on the base temperature distribution of a PCM-based heat sinks used for cooling electronic components having variable load cycles (frequent, heavy and light). They pointed out that the maximum base temperature was effectively reduced by increasing the fin number. It was also shown that the inclination angle had an insignificant effect on the cooling performance of device. Saha et al. [22] experimentally and numerically studied the effects of fin number and thickness on the thermal design of PCM-based heat sinks with plate type fins. They obtained that the base temperature reduced by decreasing the fin thickness and increasing the fin number. As a continuation work, they extended their numerical analysis by taking the effects of aspect ratio and heat flux into consideration [23]. They proposed new correlations for Nusselt number in terms of Rayleigh number based on the characteristic length scale. Hosseinizadeh et al. [24] analyzed the thermal behavior of PCM-based heat sinks with different configurations of internal fins. The research parameters examined were the fin number, height and thickness and the power level. They found that an increase in the fin number and fin height led to a significant increase on the overall performance of the system while an increase in the fin thickness was much less significant. Jaworski [25] performed a numerical study on the thermal performance of pin fin heat spreaders. They reported that the inclusion of PCM into the fins remarkably enhanced the heat dissipation rate and reduced the operating temperature of the electronic component. Comparison of various heat sink arrangements, PCM materials and power levels on the thermal behavior of PCM based heat sinks were experimentally investigated by Mahmoud et al. [26]. Parallel- and cross-shaped fin and honeycomb arrangements were tested under steady-state power inputs. They reported that the increase of fin number and inclusion of low melting PCM at moderate power levels had significantly extended the operation time of heat sink. Similar results were presented by Fan et al. [27] for cross-finned heat sink designs exposed to high power levels. Baby and Balaji [28] experimentally evaluated the thermal performances of plate-fin and pinfin heat sinks filled with PCM under various steady-state power levels.
2. Experimental study The solid model and photograph of the experimental facility, mainly consisting of a test section, an adjustable-tilt mechanism, a DC power system, a data acquisition system and a flow visualization system are shown in Fig. 1(a) and (b), respectively. The test section is a rectangular enclosure with inside cavity dimensions of 48 mm in width, 34 mm in depth and 100 mm in height. The left wall of the enclosure acted as the heat sink is made of 1.5 mm thick Aluminum −6082 (k = 172 W/m2 K) while the other walls are made of 10 mm thickness polycarbonate transparent material in order to facilitate imaging of the phase change inside the enclosure. Five different heat sink geometries with one, two, three, four and five plate fins are tested in the present study. The heat sinks are manufactured from aluminum slabs (Al-6082) by EDM technique in order to ensure no thermal contact resistance between the fins and the base plate. Each fin has dimensions of 12 mm in depth, 100 mm in height and 1.2 mm in width. The isometric view and overall dimensions of the heat sink configurations are given in Fig. 2, respectively. To simulate the heat source, a thin flexible heater with dimensions of 48 mm × 80 mm is attached to a copper sheet (0.2 mm) with a thermally conductive silicone paste (Omegatherm 201) as in Fig. 3. Nine evenly spaced rectangular slots with dimensions of 0.5 mm × 1 mm are milled on the back surface of the heat sink to place thermocouples to monitor the base 2
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Fig. 1. Experimental setup: solid model (a), and photograph (b) [39].
personal computer, which records the temperatures at 10 s intervals. The required power input (16 W) to the thin film heater is provided by a DC power supply unit (GW Instek GPS-4303) with an accuracy of ± 0.01% (FS). For attaching components together, nylon screws and low conductive liquid seals are preferred on the contact surfaces in order to minimize the conduction heat transfer between the walls and heat sink and prevent any leakage of the liquid PCM from the enclosure, respectively. For further reduction of heat loss to the environment, the heater and walls of the enclosure are also insulated with 30 mm
temperature distribution. After mounting nine T-type thermocouples inside these slots, the copper sheet is mounted on the back side of the heat sink, again using the same thermal conductive paste in order to minimize the thermal contact resistance. Except these nine thermocouples, fifteen additional thermocouples are also used to follow the transient melting of PCM inside the enclosure. Schematic diagrams with thermocouple locations and labels are depicted in Figs. 2 and 3. All thermocouples are calibrated in range of 10–100 °C with an accuracy of ± 0.5 °C. A data acquisition system (Keithley 2701) is connected to a 3
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Fig. 2. Schematic view of fin based heat sink: isometric view (a) and overall dimension (b).
base temperature reached 70 °C. During this process, the insulation layer is temporally removed from the lateral wall of the enclosure and the images are recorded at 3 min intervals. Each test is initiated by first verifying that all the thermocouples on the base plate and within the enclosure have the same value as the ambient temperature of 25 °C. The experiments are repeated at least twice with maximum deviations of ± 2%. By following the approach of Kline & McClintock, the maximum uncertainties in the temperature and heat output are found to be ± 2.2% and ± 3.22%, respectively. More details on the experimental set up and measurement procedure can be found in Ref. [39].
polystyrene foam sheet. For all the cases considered, the maximum heat loss from the back side of the heater is found to be 3% of the total power input by using the temperature data of five thermocouples located on the outside surface of the polystyrene foam layer. A commercially available paraffin wax known as n-eicosane with an average melting point of 37 °C is used as PCM. Thermo-physical properties of PCM used are given in Table 1. For all the cases, considering the volume change based on the density variation between the solid and liquid transition of PCM and fin addition, 107 g of paraffin is included to the enclosure. In order to address the effect buoyancy forces on the thermal performance of heat sink, simultaneous imaging and temperature measurements are performed at various inclination angles from 0° (vertical) to 90° (horizontal) at 15° intervals. For this purpose, three digital cameras with a resolution of 6528 × 3672 pixels are mounted on an adjustable tilting mechanism which inclines the test section at desired angular positions. For each run, the image recordings are captured from the beginning of the heating process and continued until the maximum
3. Experimental results and discussion The present research is aimed at better understanding of the effects of fin number and inclination angle on the thermal performance of a PCM-based heat sink with longitudinal plate fins. To achieve this aim, five different heat sink geometries with one, two, three, four and five 4
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temperatures sharply rise to the maximum allowed temperature (T = 70 °C) in short operating times, being relatively independent of inclination angle. The results also show that an increase in the fin number has a limited effect on the rate of temperature rise for each inclination angle. A plausible explanation for these behaviors is the low thermal heat conductivity and heat storage capacity of air, which results in a poor heat transfer. However, for the case with PCM, the plots show that the rate of temperature change is a function of both fin number and inclination angle. As a typical character, in the initial stage of heating process (t < 3 mins.), the base temperature shows a sharp increase with time and quickly passes to the melting temperature of PCM, which is in the range of 35–37 °C. In this period, PCM is in the solid phase and the heat diffused through PCM via conduction. After reaching the melting point temperature (t > 3 mins.), phase change starts and a thin layer of liquid PCM is initially formed around the base plate and fins. Thus, more heat is absorbed by PCM in the latent form and hence, the temperature rise slows down. Note that, in this period, the buoyancy forces are relatively weaker than the viscous forces and the dominant heat transfer mechanism is conduction. As time elapses, the buoyancy forces overcome the viscous forces with the development of melt fraction and results in formation of convective cells in the liquid domain. This, in turn, directly enhances the heat transfer within PCM domain and causes more decrease in the temperature rise as compared with the earlier period. After liquid fraction becomes dominant relative to the solid fraction, the base temperature again increases with time since the heat is stored in sensible heat form. These trends in base temperature can easily be seen in Fig. 6d.
3.1.2. Effects of fin number and inclination angle For a given inclination angle, as it is seen from Figs. 4–6, the base temperature decreases with increasing value of fin number, as expected. A plausible cause for this dependency is the higher extended heat transfer surface area which enhances the thermal conductivity of PCM and also induces the flow mixing in the liquid PCM. Thus, more heat is absorbed by PCM with the heat transfer modes of conduction and convection. As an example, for the vertical orientation of the heat sink (θ = 0°) at t = 28 mins., it can easily be seen from Fig. 4d and h that the maximum base temperature is approximately % 10 lower for five fins compared to the one fin case. Among these cases, it is also seen that the time required to reach the maximum allowable maximum temperature (operating time) is increased by 80% for five fins compared to the case with one fin. However, as the plots show, the increment in the operating time becomes less prominent for high values of fin number. As stated earlier, except the initial stage of heating process (t < 3 mins.), natural convection is the dominant heat transfer mode that governs the melting rate and heat transfer within PCM domain. In this mode, heat transfer strongly depends on the circulation of PCM (convection cell) formed between the adjacent fins. For higher values of fin number, the magnitude of this fluid motion is weakened due to the small spacing between adjacent two fins, which causes high flow resistance. As a result, heat transfer rate decreases. Figs. 4–6 show the effect of inclination angle on the base temperature variation for the cases with zero, one, three and five plate fins. As compared to the vertical orientation (θ = 0°), at a given time and fin number, it can clearly be seen that the base temperature gets lower values with an increase in the inclination angle. This decrement can be attributed to two main reasons. The first is the increase in the magnitude of buoyancy-induced flow which impinges large solid PCM domains with an increase in inclination angle and the second is the expansion of liquid PCM zone which allows the growth of convective cells formed with in PCM domain. The effect of inclination angle is more clearly evident for one and three fins as compared to the case with five fins. As discussed above, this behavior can be attributed to the decrease of flow intensity with fin insertion.
Fig. 3. Thermocouple locations and labels: back side of the heat sink surface (a), within enclosure (b) [39]. Table 1 Thermo-physical properties of n-eicosane [15]. Formula
C20H42
Melting range, °C Density, kg/m3 Specific heat, kJ/kg °C Latent heat, kJ/kg Thermal conductivity (W/mK)
35–37 810 (solid), 770 (Liquid) 1.9 (solid), 2.2 (Liquid) 241 0.39 (solid), 0.157 (Liquid)
fins are tested under different inclination angles varied from 0° to and 90°. All the results are also compared with the un-finned case of the same geometry studied by Avci and Yazici [39].
3.1. Temperature distribution on the base plate 3.1.1. Effect of PCM inclusion As mentioned above, the temperatures are measured at nine evenly spaced locations (Th1-h9) on the heater surface (base temperature) to evaluate thermal behavior of each heat sink configuration with and without PCM. The time history of temperature variations on these locations are depicted in Figs. 4–6 for different inclination angles of heat sink with various values of fin number (NoF). For the case without PCM, it is clearly seen that at a given value of fin number, the base 5
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Fig. 4. The time history of temperature variations on the heater surface with various fin numbers for θ = 0° : without PCM, 0 fin [39] (a), 1fin (c), 3 fin (e), 5 fin (g); with PCM, 0 fin [39] (b), 1fin (d), 3 fin (f), 5 fin (h).
3.2. Temperature distribution within the PCM medium
the mid-axial plane of the PCM domain are given in Figs. 7 and 8 at different values of fin number for both the vertical and horizontal orientations of the heat sink. As it seen from Fig. 7, for the vertical
The time history of PCM temperature at some specified locations in
6
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Fig. 5. The time history of temperature variations on the heater surface with various fin numbers for θ = 45° : without PCM, 0 fin [39] (a), 1fin (c), 3 fin (e), 5 fin (g); with PCM, 0 fin[39] (b), 1fin (d), 3 fin (f), 5 fin (h).
buoyancy force is parallel to the heater. However, for the horizontal orientation (Fig. 8), the temperature variations are relatively similar due the formation of three dimensional Benard convection cells which induces effective mixing within the liquid PCM. This mechanism was
orientation, the temperatures in the upper half of the PCM domain are higher than those of in the lower half. The reason for this trend is the higher contribution of buoyancy which drives the high temperature liquid PCM to the upward direction. Here, it should be noted that the 7
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Fig. 6. The time history of temperature variations on the heater surface with various fin numbers for θ = 90° : without PCM, 0 fin [39] (a), 1fin (c), 3 fin (e), 5 fin (g); with PCM, 0 fin [39] (b), 1 fin (d), 3 fin (f), 5 fin (h).
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Fig. 7. Time history of the PCM temperature at some specified locations in the mid-axial plane of the PCM domain for θ = 0° : 0 fin [39] (a), 1 fin (b), 3 fin (c), 5 fin (d).
buoyancy acts vertically, the inclination angle causes important changes on the generation of buoyancy-induced flow and its extent within the PCM as given in Fig. 10. For the vertical orientation case, as it is seen from Fig. 10a, the formation of convective cells are firstly initiated near the top right corner of the PCM domain and then expands opposite to the heated wall with an increase in time. This flow motion locally enhances heat transfer in the upper half of the PCM domain and causes a fast movement of the solid-liquid interface than those in the lower half part, which creates a concave interface shape. Similar observations were also found by Kamkari et al. [40] and Kamkari and Amlashi [41] who studied the convection-driven melting of phase change material in an inclined rectangular enclosure experimentally and numerically. Due to the small spaces between the base plate/fin and solid PCM, the conduction mode is still dominant in the lower part of PCM domain. As the inclination angle changes from 0° to 45° (Fig. 10b), the convection cell expands to the lower half of the PCM domain because more of hot PCM separates from the heated wall and impinges to the solid PCM. This strong convection motion enhances the melting rate and changes the solid-liquid interface profile as convex. Similar observations were found by Kamkari and Amlashi for the formation of convection cells. For the horizontal orientation (θ = 90°), as it is seen from Figs. 9c and 10c, the interface inside the PCM domain progresses in a near linear fashion, expect some inconsistencies when moving along the base plate. This melting trend is attributed to the formation of three dimensional Benard convection cells which induces effective mixing within the liquid PCM. The images also reveal that, at the respective time, the change in the fin number has not any considerable influence
discussed in our earlier study [39] for the un-finned case of heat sink. Results also show that for all inclination angles, the value of PCM temperature in each selected location decreases with increasing values of fin number.
3.3. Solid-liquid interface evaluation To better understand the effects of fin number and inclination angle on the melting and heat transfer behavior in the PCM, the dynamic of the solid-liquid interface is photographed from the lateral surface of the heat sink at twelve-minute-intervals during the heating process (Figs. 9–11). Final period of melting process (Fig. 11) is also visualized for each configuration at the time to reach 70 °C. In these images, the black and white colors correspond to the liquid and solid phases, respectively. In the early stage of heating process (t = 12 mins.), as it is seen from Fig. 9, the movement of solid liquid interface is relatively uniform along the base plate for all cases, except small changes in the upper half of PCM domain for the case of θ = 0° at lower values of fin number. As stated earlier, in this period, the buoyancy forces are not high enough to overcome the viscous forces due to the insufficient thickness of melted PCM layer adjacent to the base plate and fins, which makes the conduction as the dominant heat transfer mechanism. The temperature distributions given in Figs. 4 and 7 also confirm this heat transfer mechanism. As the liquid layer becomes thicker with time (t = 24 mins.), the buoyancy force takes an active role on the heat transfer and dictates the movement of the solid-liquid interface within PCM domain. As the 9
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Fig. 8. Time history of the PCM temperature at some specified locations in the mid-axial plane of the PCM domain for θ = 90° : 0 fin [39] (a), 1 fin (b), 3 fin (c), 5 fin (d).
play a dominant role on the convection mode. As the fin number increases beyond a certain value where the spacing between adjacent two fins is too small to permit a formation of convection cell, the magnitude of flow motion is strongly weakened and causes a decrease in heat transfer. Similar observations are made by Saha et al. [22] for different pin fin arrangements. Their findings for the optimal value of fin volume fraction (8%) agree very well with our results for the three-fin case, which is nearly 9%. For the case without PCM inclusion, as it is seen from Fig. 12b, the operating time increases with an increase in the fin number, being relatively independent of inclination angle.
on the interface profile. The final forms of the solid-liquid interface profiles are given Fig. 11. Comparison of images show that while the trends of interface profiles are relatively similar with the previous period, the melt fraction increases with an increase in both the fin number and inclination angle. This behavior is clearly visible in the lower half of PCM domain at low inclination angles. 3.4. Effects of fin number and inclination angle on the thermal performance of heat sink
4. Conclusions
The cooling performance of each heat sink configuration with and without PCM is compared in terms of operating time. As it is seen from Fig. 12 a, at a given value of fin number, the operating time increases with increasing inclination angle until it reaches a maximum at 60°, then it decreases with further increasing of inclination angle for the finned case with PCM. As mentioned earlier, this trend in the operating time is due to the stronger buoyancy-induced flow and larger liquid PCM domains for the convective cells to grow. Comparison of the results show that the operating time can be extended by 83.4% and 79.2% for the one fin case as the inclination angle increased from 0° to 60° and 0° to 90°, respectively. The plots also show that, the operating time is a function of fin number. As it is seen from the figure, an increase in the fin number extends the operating time. This extension is more dominant until a certain value of a fin number (3), and then it becomes less prominent with further increasing of fin number. This trend can be attributed to the change in the intensity and magnitude of convection cells which
In this study, the combined effects of fin number and inclination angle on the thermal performance of PCM-based heat sinks with longitudinal fins are investigated experimentally. The major findings of the present study can be summarized as follows:
• More uniform temperature distribution is achieved with an increase of fin number and inclination angle at the base surface. • The orientation of the heat sink significantly affects the melting rate • • 10
and movement of solid-liquid interface due to the variations in the convective currents. For the vertical orientation case (θ = 0°), the operating time can be extended by 586% as the number of fin increases from 0 to 5. The operating time can be increased up to 79.2% by changing the orientation of heat sink from vertical (θ = 0°) to horizontal (θ = 0°) for the one fin case.
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Fig. 9. Solid-liquid interface progression at 12mins. under different inclination angles: θ = 0° (a), θ = 45° (b), θ = 90° (c).
11
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Fig. 10. Solid-liquid interface progression at 24mins. under different inclination angles: θ = 0° (a), θ = 45° (b), θ = 90° (c).
12
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Fig. 11. Solid-liquid interface progression at final the period under different inclination angles: θ = 0° (a), θ = 45° (b), θ = 90° (c).
• An increase in the fin number beyond a certain value (3) results a
limitation on the thermal performance of heat sink. This is due to the fact that adding fins although increases the heat transfer area, but it simultaneously increases the flow resistance of liquid PCM
• 13
which hampers the convective flow and thus the heat transfer within the PCM. Among the seven inclination angles were tried, heat sink with inclination angle of 60° showed the best result in terms of operating
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Fig. 12. The effect of fin number and inclination angle on the thermal performance of heat sink: with PCM (a), without PCM (b).
time.
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