2023, Vol. 30
,
Praveen Bhaskaran Pillai3,
Sachin Salunkhe1,
J. Paulo Davim4
2. Department of Mechanical Engineering, Kalasalingam Academy of Research and Education, Krishnankoil 626126, Tamil Nadu, India;
3. Department of Mechanical and Aeronautical Engineering, Clean Energy Research Group, University of Pretoria, Pretoria 0002, South Africa;
4. Department of Mechanical Engineering, University of Aveiro, Campus Santiago, Aveiro 3810-193, Portugal
LED is one of the many innovations in lighting systems introduced to the world. Unlike its competing products, i.e., conventional lighting systems like filament bulbs and fluorescent tubes, the LED has advantages like superior light intensity, ability to generate monochromatic light, small size, high service life, low energy consumption, high brightness effectiveness (136 lm/W) [1], little heat radiation, and high life span (i.e., over 25000 h) [2]. These benefits are most desired in the current scenario where the demand for energy has been increasing day by day. The LED is a semiconductor that finds applications in residences, office buildings, laboratories, dentistry, indoor lights, outdoor lighting, street lamps, reading lamps, and automotive lighting system. It is experimentally proven that only 20% of electricity is transformed into light. However, the remaining 80% is converted into waste heat energy [3]. The heat energy thus stored in the finite-size LED unit is huge. Because of this, the service life of the LED gets reduces [4]. The peak temperature of LED at its junction should be maintained at a temperature of less than 125 ℃. This is achieved using suitable cooling methods so as to prolong its service life and expand the service effectiveness of the LED. Hence, the efficacy of thermal management is a serious need for the LED lighting system. However, LED lighting systems have a major drawback, i.e., it generates excessive heat during the conversion of energy from electricity to light. This drawback makes it difficult to use the LED system for an extensive duration in laboratories, dentistry, reading lamps, and small rooms.
In general, LED cooling is achieved using passive and active approaches [5]. The active cooling method is typically costlier than the passive cooling method. Furthermore, this cooling approach does not possess any movable components [6]. Several research studies [7-11] investigated the process of cooling in heat sinks using passive methods for increasing the effectiveness of heat dissipation. The heat transfer capability of three different configurations of heat sinks was investigated experimentally as well as numerically by Li and Byon [12] in three different orientations i.e., 0°, 90°, and 180°. Heat transfer rates were improved with concentrated ringed fins of radially aligned heat sinks with 90° orientation as compared with 180° orientations. Feng et. al [13] evaluated the influence of aspect ratios in the flow boiling of micro channels with no uniform heat fluxes and simulated it numerically. Yan et al. [14] compared the performance of cross-finned heat sinks and thin-fins positioned horizontally for natural convective heat transfer. The cross-finned heat sink performed better in terms of convective heat transfer than conventional heat sinks by 15%. Ye [15-16] reported a numerical simulation of electrode nonuniformity based on the thermal simulation of electrode nonuniformity. Şevik and Özgür [11] presented a battery dryer that was capable of rapidly drying a lithium-ion battery and offering high-uniformity surface temperatures. The numerical model was solved using the finite volume method in order to solve it. Ye et al.[17] developed and validated a thermal model to describe the unsteady generation of heat in a rectangular power battery. A traverse with modified wire coils was analyzed by Hong[18] and Du [19] in order to determine the flow characteristics. The main disadvantage of the metal-made heat sink is its low latent heat capacity. Recent research proved that it can be overcome with a PCM-incorporated heat sink[20-23]. A PCM is a substance, the huge quantity of thermal energy stored in PCM during phase changing (liquid to solid) at a certain temperature. In recent years, studies were reported for PCM application in thermal energy storage[24-27], energy-efficient building[28-30], heat recovery application[31-32]. The PCM materials have gained such as sufficient latent heat, medium density, and high specific heat capacity [33]. Wu et. al [34] performed an experimental study on PCM heat sink embedded with heat pipe for LED application and the LED temperature can reduce from 32 ℃ to 16 ℃ at 30 W. This is because the heat absorbing capacity of the PCM may be increase its lifetime and improve operating time of LED.
The improved efficiency of heat sink containing PCM is also proved for electronics components likened to the effectiveness of heat sink performance during the heat transfer with and without PCM (n-eicosance)[35]. Baby et al. found that the PCM upsurges the operational span of the heat sink when it is operated between 45 ℃ and 55 ℃. Ashraf et al.[36] examined the effectiveness of the heat sink with fins arranged in two different configurations (inline and staggered) and six different PCM materials. They observed superior heat dissipation for heat sinks with staggered pin fin arrangement without PCM, additionally, SP-31 PCM exhibited a high enhancement ratio compared to other PCM materials. Shahsavar et al.[27] reported an experimental study on heat sinks having three different fin thicknesses (1, 2, and 3 mm) and four-volume concentrations of PCM (0.00, 0.33, 0.66, and 1.00). They concluded that a PCM-based 2 mm fin thickness heat sink exhibited better thermal efficiency at a PCM of 1 vol. %. Few investigators[37-39] carried out experimental studies on the performance of heat sinks containing PCM for electronic components. They studied Neopentyl Glycol and paraffin as PCM. Their results revealed the fact that the presence of PCM in the heat sink could improve the perforce by enhancing the heat sink heat storage capacity. The PCM inside the heat sink helps to reduce the operating temperature and thereby the device temperature is maintained at a safe operational range. In addition, the PCM can manage the sudden spike in temperature to avoid device failure. Praveen and Suresh[40] conducted an experimental study on a heat sink containing plate fin and n-Eiconse PCM. When they used intermittent input power, they noticed a significant temperature reduction in the PCM-based fin heat sink. Baby and Balaji[41] explored the PCM melting efficiency of four different fins (no fins, two fins, four fins, and six fins) heat sinks experimentally. As the fin number increases, the heat sink temperature decreases due to increased heat sink surface area. It was also observed that PCM stores more heat energy during the post-melting process period. Deng et al.[42] made experiments to investigate the effects of six different (one cavity, three cavities, nine cavities, one cavity insert honeycomb, six cavities, and 36 cavities) configurations of PCM-based heat sinks on thermal heat dissipation performance. Honeycomb structure with a single cavity PCM-based heat sink exhibited better thermal performance at 5 W input power due to PCM lowest melting temperature.
From the many pieces of literature, it is found that the LED lighting system has been subjected to intense research work around the globe. Attempts have been made to increase its service life by reducing the heat generated in it. This paper presents the findings made to reduce the heat stored in the LED lighting system. In the current study, the deviation in the thermal properties of high-intensity LED with PCM-based heat sinks is discussed by conducting experiments with three types of heat sinks by varying the fin design with and without PCM. A basic design (H-1) shows the combined effect of PCM and natural convection through an outside curved surface. With second design (H-2), shows the effect of conduction by radial fins inside. The design (H-3) reveals the effect of conduction by radial fins on the PCM and is extended to the exterior to get combined convection from the curved and fins surface. The performance of the system is discussed with junction temperature of the LED, luminous flux and thermal resistance of the heat sink have been investigated experimentally. The discussions are focused on the effect of fin design and PCM on the key parameters.
1 Experimentation Process 1.1 Heat Sink DesignThe design of the heat sink plays a vital role in proper working of the LED system.An improper design will lead to ineffective heat transfer to the surrounding atmosphere. This will drastically affect its performance and reduce its service life. Apart from this excessive heat will be stored in the heat sink, making it difficult to handle delicate operations like dentistry, laboratories, reading, and toys. In this experimental work, heat sinks with three different configurations were designed and fabricated. Heat sinks were named as follows: (1) heat sink without fin (H-1), (2) three internal radial fin heat sink (H-2) and (3) internal radial fins which are extended to outside of heat sink (H-3). The configurations of the designed heat sinks are shown in Fig. 1. The overall diameter and height of heat sinks are 50 and 25 mm respectively. The fin thickness is 2 mm. The fin volume fraction (∅) of heat sinks is calculated with Eq. (1).
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Fig.1 Isometric and orthogonal views of heat sinks (Unit: mm) |
| $ \emptyset=\frac{V_{\text {fin }}}{V_{\text {withoutfin }}-V_{\text {fin }}} $ | (1) |
where, Vfin and Vwithoutfin denote the volume of the fin (mm3) and volume of the heat sink (without fin in mm3) respectively. Previous research study recommends that the volume fraction of the fin should be 9 % to achieve high thermal performance for heat sinks with PCM[43]. The heat sinks are fabricated in aluminium (6063) material by vertical milling machining.
1.2 Phase Change MaterialThe Phase Change Materials (PCM) are wide class of materials that undergo phase change at relatively low temperatures. Phase change is a process in which the material gets converted from one state of matter into the other. Processes like boiling, evaporation, melting, and condensation are phase change processes. Phase change typically occurs due to heat transfer. The latent heat energy exhibited during phase change process has enabled the PCMs for applications in heat storage and heat transfer. Some of the notable PCMs are water, petrol, naphthalene, champers, and paraffin wax.
In this research study, the paraffin was preferred as the PCM. Paraffin wax offers benefits like low density, low melting point, easy to store and handle, non-combustible, and safe to the environment. Table 1 displays the thermo-physical properties of PCM (Sigma-Aldrich, 327212), found with the help of Differential Scanning Calorimeter (DSC) Make: TA Instruments, Waters Austria-Model: Q200. The DSC test was carried out for the thermal cycles ranging from 30 ℃ to 90 ℃. Using a PCM mass of 2.5 g, a heating and cooling rate of 5 ℃/min was maintained throughout the test.
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Table 1 PCM thermophysical properties [44] |
1.3 Experimental Facility
A schematic illustration of the experimental facility assembled for this study is depicted in Fig. 2. The experimental facility comprises a heat sink, white LED, dual adjustable DC power supply, USB data acquisition system, Lux meter, K-type thermocouple and computer. The input power is supplied using a dual adjustable DC power supply (KUSAM-MECO, 302D, India) to the LED module. The input power levels in this experiment were set to 4 W and 10 W, respectively, and the corresponding heat flux at the two power levels was 0.011 W/mm2 and 0.27 W/mm2, respectively.Presented here is a white LED module (LUSTRON LL610F - Cool white), which has a size of 19 mm×19 mm and a cable length of 9 m. During this experiment, the specifications of this module, including the DC current, voltage, and maximum junction temperature of 900 mA, 40 V, and 125 ℃ were preferred to the specifications of the standard module. The LED is mounted on the heat sink surface using M3 screw. The thermal paste as a Thermal Interface Material (TIM) (HALNZIYE Shenzhen) was applied in the interface of the heat sink and LED module (heat source). TIM helps to reduce the interface thermal resistance (Kapitza resistance) at the junction by filling the micro voids to ensure good thermal contact. The Lux meter was placed 600 mm from the LED module. The (Tc) was recorded using the a K-Type thermocouple one end connected to the (-)ve terminal of the LED module and other end connected to the data acquisition system (Make: KEYSIGHT Technologies -Model: 34973A). The readings were taken for 90 min, when the temperatures become steady enough or the completion of transient temperature variation, with an interval of 60 s. The junction temperature (Tj) was measured using Eq. (2) and the thermal resistance was calculated using Eq. (3).
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Fig.2 Schematic representation of the experimental facility |
| $ T_j=T_c+\left(R_{j-c} \times P_d\right) $ | (2) |
| $ R_h=\left(T_b-T_a\right) / P_d $ | (3) |
where Tj, Tc, Rj-c, Pd, Rh, Tb, and Ta denote the junction temperature of LED (℃), LED case temperature (℃), thermal resistance at the junction (℃/W), dissipated power (W), thermal resistance associated with the heat sink (℃ /W), the temperature of interface material (℃) and ambient temperature respectively. The procedure used in this study is that the change time was limited to 90 min and the discharging time was limited to 120 min, so the discharging cycle ended when the output temperature reached the same temperature as the surrounding environment.
2 Uncertainty AnalysisThe uncertainties related to the base parameters are linked to the voltage and current, which are the least counts of respective measuring devices. The uncertainty of the input power calculation using Eq.(4). The thermocouple acts as an accurate way of measuring the temperature of the LED case and is capable of accuracy to within a half degree Celsius or so. The ammeter and voltmeter accuracies are ±0.01 A and ±0.05 V respectively. The uncertainty results are shown in Table 2. The calculated uncertainty of power is less than 5%.
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Table 2 Uncertainty results |
| $ \sigma_p= \pm \sqrt{\left(\frac{\partial P}{\partial V} \sigma_V\right)^2+\left(\frac{\partial P}{\partial I} \sigma_I\right)^2} $ | (4) |
Fig. 3 (a) and (b) show the LED transient junction temperature variation in the heat sink for the case of with and no PCM at different input power respectively. The H-1 heat sink devoid of PCM has a lower heat transfer performance at 4 W and 10 W. The maximum (after 90 min) junction temperatures were 59.5 ℃, 57.5 ℃, and 55.5 ℃ for H-1, H-2 and H-3 heat sinks without PCM respectively at 4 W input power. Meanwhile, junction temperatures of 57.5 ℃, 55.5 ℃, and 54.5 ℃ were obtained for H-1, H-2 and H-3 heat sinks with PCM respectively. H-3 heat sink with PCM tends to have significantly lower temperatures than the further two heat sinks. Because of the combined effect of PCM's heat storage capacity and convection, the heat sink was kept at a lower temperature. Fig. 3 (b) shows a similar effect for 10 W input power and comparative study shows in Table 3. This means that the heat sink H-3 with PCM performed superiorly as compared to the heat transfer of the H-1 and H-2 heat sinks. This significant reduction in junction temperature was due to natural convection and increased heat absorbing capacity of the heat sink with the PCM material [45]. The heat sink H-3 has extended fin surface at outside, while leads to improve the convection at outside and conduction in the bulk PCM at inside. This combined effect leads to enhance the thermal performance of the heat sink.
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Fig.3 Junction temperature-time history for with and without PCM heat sinks |
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Table 3 Comparative study |
3.2 LED Luminous Flux
The reduction of the luminous flux was observed due to the raise in the input power as well as the junction temperatures in the case of a LED. The present study was focused on measuring the LED luminous flux while using a lux meter (HTC, LX-101 A, India) and range from 0.1 lux to 200000 lux. Fig. 4 represents the comparison between the luminous flux and the input power of the PCM with and without a heat sink. The results revealed that the overall luminous flux of the PCM based heat sinks have marked best performance than without PCM heat sinks. For the observed luminous flux, it was also found the heat sink H-3 with PCM had the highest luminous flux 351 lux at 4 W and 686 lux at 10 W. This happened because of the reduction in the junction temperature. The H-3 heat sink which contains PCM exhibited superior luminous performance.
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Fig.4 LED luminous flux |
3.3 Heat Sink Thermal Resistance
Fig. 5 shows the heat sink thermal resistance of LED with input power. Eq. (2) has been used to calculate the thermal resistance of heat sink. The results revealed that the heat sink H-3 with PCM and the heat sink thermal resistance were low compared to others heat sinks. In the case of 10 W input power, the results revealed that for the heat H-3 with PCM thermal resistance was reduced by 18.21 % than H-1 with the PCM heat sink. Therefore heat sinks containing PCM have more heat performance.
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Fig.5 Heat sink thermal resistance |
3.4 Improvement in Performance Time for High Input Power of Heat Sink Containing PCM
The LED maximum operating temperature of 50 ℃, 75 ℃ and 90 ℃ were chosen as critical set point temperatures (SPTs). The critical set point temperature (SPT) is the highest operating temperature that any electric device can withstand without being damaged or breaking down. As per the LED manufacture data sheet, the maximum LED case temperature is 90 ℃. Fig. 6(a) and (b) show the critical SPT of 50 ℃ of 4 W and 10 W respectively. It can be seen that the heat sink H-3 with PCM took more period to obtain SPT of 50 ℃ at 4 W, compared with 10 W. The results revealed that the maximum time taken to obtain the SPT of 50 ℃ was 1 min in the case of heat sink H-3 without PCM at input power at 10 W. However, the heat sink H-3 containing PCM took 31.25 min to reach the SPT at 4 W due to extended fins in contact with ambient. This prolonged duration is due to the combined effect of PCM and convection by the extended fins. It was observed that the PCM filled heat sinks take longer duration to reach the critical SPTs against the heat sinks devoid of PCM. The rapid increase in the junction temperature occurs because of the post-melting phase of PCM in the heat sink H-2 with PCM [46]. This increased the time taken to 29.10 min before attaining the temperature of 75 ℃ as shown in Fig. 6(c). The maximum time taken to achieve the critical SPT in the heat sink H-3 with PCM is comparatively greater than that of its counterpart without PCM shown in Fig. 6(d). The maximum time taken to reach the SPT was 16.10 min, 20.40 min, and 18.30 min in the case of heat sinks H-1, H-2 and H-3 without PCM at input power 10 W. However, the heat sinks H-1, H-2 and H-3 with PCM took 54.12 min, 58.10 min and 68.30 min respectively to reach the SPT. It was found that the heat sink H-3 with PCM required more time to reach the SPT compared to the other heat sinks considered for this study. Hence, the LED performance improved with the heat sink H-3 with PCM.
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Fig.6 Critical time to reach SPTs of (a) 50 ℃ at 4W (b) 50 ℃ at 10 W (c) 75 ℃ and (d) 90 ℃ with and without PCM |
4 Conclusions
The experiment was conducted on three different heat sinks with PCM for LED thermal management. The effective heat transfer performances were discussed below.
1) The LED junction temperature was reduced because of heat sinks containing PCM and this extends the life of the LEDs.
2) The Tj of heat sink H-3 with PCM decreased by 3.1 % when compared with a without PCM heat sink at 10 W.
3) For 4 W, the junction temperature of the H-1 heat sink with PCM and without PCM are 57.5 ℃ and 59.5 ℃ respectively. And heat sink H-1 with PCM A exhibits 2.5 ℃ lower in the junction temperature.
4) Thermal resistance was reduced by 18.2% in H-3 with PCM-based heat sink compared to H-3 without PCM at 10 W.
5) This experimental research revealed that LED heat transfer of PCM based high power LED cooling systems may be suitable for down light applications.
6) The luminous flux of the heat sink H-3
containing PCM was 351 lux at 4 W and 686 lux at 10 W.
7) Enhancement in the operating duration of the heat sink H-3 containing PCM was higher than the rest of the heat sinks at SPT 90 ℃.
8) This experimental research revealed that LED heat transfer of PCM based high power LED cooling systems may be suitable for down light applications.
9) With the addition of PCM, the heat sink's ability to absorb heat increased. The PCM based heat sink absorbs heat energy during phase change (solid to liquid state). As seen here, increasing the heat transfer of the heat sink.
AcknowledgmentThe authors wish to acknowledge Vel Tech Rangarajan Dr. Sagunthala R& D Institute of Science and Technology, Avadi, Chennai, Tamil Nadu, India for providing the required facilities to conduct the experiment. The authors also thank Dr. N. Dilip Raja, Assistant Professor, and Mr. T. Balaji, Assistant Professor, Department of Mechanical Engineering, Vel Tech Rangarajan Dr. Sagunthala R& D Institute of Science and Technology, Avadi, Chennai, Tamil Nadu, India for providing the technical expertise and suggestions that helped to complete the study.
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