Graphene enhanced paraffin nanocomposite based hybrid cooling system for thermal management of electronics

Applied Thermal Engineering 163 (2019) 114342

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Graphene enhanced paraffin nanocomposite based hybrid cooling system for thermal management of electronics Mathew Joseph, V. Sajith

T

⁎

School of Materials Science and Engineering, National Institute of Technology Calicut, Kerala, India

HIGHLIGHTS

GRAPHICAL ABSTRACT

of Hybrid cooling system for • Study electronics under pulsed/uniform thermal loads.

of thermal properties of par• Study affin/graphene composites. in thermal conductivity • Enhancement up to 60% for 0.5 wt% of graphene in paraffin.

in fan operation time with • Reduction paraffin/graphene inclusion. savings for paraffin/graphene • Energy heat sink up to 23%.

ARTICLE INFO

ABSTRACT

Keywords: Graphene Liquid-phase exfoliation Phase change material Hybrid heat sink Pulsed heat loads

Hybrid cooling systems combining forced convection with passive phase change material (PCM) based heat sink is an ideal solution for long-term cooling of high power electronics. The effectiveness of composite PCM with graphene nanofillers on transient performance of a hybrid thermal control system was investigated experimentally under uniform and periodic pulsed heat loads. Graphene was synthesized by liquid-phase exfoliation and paraffin/graphene composite was prepared for various concentrations. The thermo-physical properties of paraffin/graphene composite were measured. Transient thermal responses of heat sink with paraffin/graphene composite (HS-Gr/PCM) and paraffin (HS-PCM) were compared to heat sink without paraffin (HS) for passive and hybrid cooling mode configurations. The thermal performance of HS-Gr/PCM was far superior to HS and HSPCM in all thermal loading scenarios. In passive mode, maximum of 6 °C lower steady-state temperature was attained for HS-Gr/PCM relative to HS under uniform thermal load. In hybrid mode, time to reach fan onset temperature was enhanced by 109%, 122% and 110% for HS-Gr/PCM over HS, corresponding to periodic heat pulses with duty factors 0.5, 0.79 and 0.9 respectively. Experiments with hybrid cooling system showed 11–23% enhancement in fan energy savings for HS-Gr/PCM as compared to HS at different periodic heat pulses.

1. Introduction Most of the electronic systems are subjected to transient periodic thermal loads and efficient heat removal is very critical for the prolonged operation of its components. In addition, miniaturization has not only increased the power densities but also imposed necessity to develop compact thermal management systems capable of maintaining ⁎

the device under safe operating temperature limits. In many electronic applications like motor drives of 3D printers, antilock brake controllers and power steering controllers for automobiles, the thermal loads are periodic as well as transient. Solid-liquid phase change materials (PCMs) based heat sinks can be used to absorb sudden release of heat in these devices and dissipate the stored energy after the end of peak cycles. However, local overheating of components may occur for two

Corresponding author. E-mail address: [email protected] (V. Sajith).

https://doi.org/10.1016/j.applthermaleng.2019.114342 Received 11 April 2019; Received in revised form 19 August 2019; Accepted 4 September 2019 Available online 05 September 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature PCM FLG DSC SEM TEM XRD TGA

TPS transient plane source HS heat sink without paraffin HS-PCM heat sink with paraffin HS-Gr/PCM heat sink with paraffin/graphene composite ESI energy savings index w/PCM with PCM w/o PCM without PCM k thermal conductivity t time

phase change material few-layer graphene differential scanning calorimetry scanning electron microscope transmission electron microscope X-ray diffraction thermo-gravimetric analysis

reasons, once the PCM melts completely and when the time scale of heat pulse is short as compared to the response time of the heat sink system [1,2]. Phase change materials for electronics cooling, such as paraffin and non-paraffin waxes possess inherently low thermal conductivity which increases the thermal resistance of heat sink system resulting in increased response time. Accordingly, to enhance the effective thermal conductivity and heat transfer rate metallic fins are introduced into PCM, but the manufacturing constraints, increased weight and reduction in PCM volume fraction are major concerns that limit their use [3,4]. Incorporation of metallic foams and carbon-based porous foams like expanded graphite in PCM resulted in drastic reduction of latent heat of composite PCM [5]. Various metal oxide fillers like Al2O3 [6], CuO [7], etc. have been used to enhance the thermal conductivity of PCM, but they adversely affect the buoyant fluid movement and melt front propagation during melt phase due to increased viscosity. The composite PCMs with much higher thermal conductivity can be obtained by the incorporation of highly conductive carbon-based nanoparticles like GNF, CNT, GNP, and Graphene. Studies have reported thermal conductivity enhancement for PCMs with expanded graphite [8], GNF [9] and CNT [10,11]. Kim and Drzal [12] and Kalaitzidou et al. [13] prepared paraffin/ exfoliated graphite nanoplatelets (xGnP) composite with high thermal conductivity. Later Xiang and Drzal [14] showed that particle size of xGnP has greater influence on thermal conductivity enhancement of the paraffin composite. Yu and Haddon [15] proposed that thinner graphene nanosheets can improve thermal conductivity of polymers significantly on account of its large in-plane thermal conductivity ( 5300 W/mK). Warzoha and Fleischer [16] and Fan et al. [17] proposed that monolayer graphene sheets tend to fold in matrix giving rise to large number of interfaces resulting in phonon scattering thus increasing the thermal interface resistance and reducing the thermal conductivity enhancement, especially at higher loadings. Yu et al. [18], Harish et al. [19] and Li et al. [20] found that graphene nanoplatelets with two-dimensional planar structure have low thermal interface resistance and greater thermal conductivity enhancement than CNTs or CNFs. Liu and Rao [21] and Shi et al. [22] compared the thermal conductivity enhancements with graphene and exfoliated graphite sheets. Recently, graphene have been used as thermal conductivity promoters for different types of PCMs [19,21,23–25]. Investigations by Fang et al. [26] and Fan et al. [27] reported high viscosity increase for PCM/xGnP or GNP composites with larger flake size at higher concentrations (>1 wt%). Functionalized graphene can be used to reduce the thermal interface resistance and improve the filler reinforcement with PCM matrix [28]. Li et al. [29] used sulfonated graphene, Mehrali et al. [30], Akhiani et al. [31] used reduced graphene oxide and Mehrali et al. [32] used nitrogen doped graphene to improve dispersion, compatibility and shape stability with PCM matrix. Qi et al. [33] and Yang et al. [34,35] developed novel methods to obtain 3-D conducting network in PCM matrix using hybrid graphene aerogels in which graphene oxide nanosheets provides the supporting network for shape stabilization and graphene nanoplatelets forms conducting pathways through the network. More recently, Yang et al. [36] prepared novel segregated

thermally conducting structure with increased latent heat using graphene microencapsulated paraffin. Various methods were reported for the synthesis of graphene sheets [37], among which liquid phase exfoliation of graphite powder in organic solvents is an economical and easy method to obtain clean, oxygen-free mono/few-layer graphene [38,39]. Shang et al. [40] reported that long exfoliation time produces thinner graphene sheets and reduces the thermal conductivity enhancement of PCM composite. On the other hand, since the graphene surface area, structure, layer thickness and size vary with the synthesis methods, there are contradictions regarding the phase change phenomena and thermal conductivity enhancements exhibited by graphene-based composite PCMs. High loading of graphene (more than 1 wt%) increases the viscosity of the composite PCM and hence detrimental for electronics thermal management applications [41]. Weinstein et al. [42] used 0.25, 1 and 5 wt% paraffin/GNF composite in a heat sink cavity of 50.8 × 50.8 × 50.8 mm3 under uniform heat load of 3 W and 7 W. The composite PCM at low fiber loading levels (0.25 wt%) effectively conducted heat and delayed the attainment of steady state. Shaikh and Lafdi [43] designed a thermal control system (75 × 50 × 50 mm3) with PCM/CNT composite and reported a reduction in junction temperature for pulsed heating (200 s heater ON/ 100 s OFF) at 24 W, 36 W and 48 W. Sansui et al. [44] used 10 wt% paraffin/GNF composite in a heat sink module subjected to heat flux of 4 W/cm2 and 20 W/cm2 and reported a reduction in the cooling time of the module. Chintakrinda et al. [2 45] used 11 wt% paraffin/GNF in a 50.8 × 50.8 × 50.8 mm3 heat sink under power input in the range 50–750 W and observed that the effect of particle loading is less significant at higher powers. Fan et al. [46] compared the effect of CNT and GNPs at various loadings in the range 0.3–3 wt% in a heat sink (80 × 80 × 30 mm3) subjected to power input of 40 W, 80 W and 120 W. The best performance of heat sink was reported for 0.3 wt% of GNP in paraffin and the overall performance of the heat sink with CNT/paraffin composite was found to be inferior to GNP/paraffin composite. It was also proposed that improved thermal conductivity and low viscosity of composite PCM are the key parameters responsible for the decrease in the heat sink base temperature. In order to curtail temperature overshoots at chip-heat sink interface after complete melting of PCM, hybrid cooling system by combining active and passive thermal management techniques were proposed. Alimohammadi et al. [47] used fan-cooling after the melting of salt hydrate in a plate-fin heat sink. The hybrid cooling system was able to reduce the chip temperature and ensured long period usage. Sahoo et al. [48] compared the conventional fan-cooled heat sinks with and without PCM for power surging situation in electronic devices. A reduction in power consumption for the fan in PCM based heat sink was reported. Forced convection cooling along with PCM cooling can accommodate continuous thermal loads and maintain the chip at safe operating temperature for longer duration [49,50]. Yoo and Joshi [51] investigated the thermal responses of metallic PCM incorporated plate fin and pin fin heat sinks with periodic loads under forced convection cooling. They reported 5.4–12.4% reduction in cooling fan energy consumption in a PCM based heat sink as compared to heat sink without PCM. Kozak et al. [52] observed that forced convection cooling 2

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of PCM heat sinks using fan can significantly reduce the chip temperature even after the end of melting process of PCM. To conclude, hybrid cooling systems combining forced convection with PCM heat sinks can be effectively used for continuous cooling of devices even after the PCM melts thus preventing temperature overshoot at the interface. The thermal conductivity of PCM should be increased without affecting the viscosity so as to enhance the thermal responses in PCM based heat sinks. Among the carbon-based nanomaterials, graphene conducting fillers have the highest intrinsic thermal conductivity and hence considered as one of the best thermal conductivity enhancer for PCM. The incorporation of very low concentration of paraffin/graphene composite (less than 0.5 wt% graphene) in a hybrid cooling system has not been studied in the literature, which in fact is the novelty in the present work. Moreover, the studies pertaining to hybrid cooling systems subjected to transient periodic pulsed heating are scarce in the literature. In the present work, the thermal performance of paraffin/graphene composite PCM in a hybrid cooling system subjected to uniform and periodic pulsed heating have been investigated. Graphene was synthesized by liquid phase exfoliation technique and the thermal properties of graphene/paraffin nanocomposite were studied. The transient thermal responses of heat sinks with paraffin wax (HS-PCM) and paraffin/graphene composite (HS-Gr/PCM) were compared to heat sink without PCM (HS) for two cases: (1) Passive cooling mode (cooling fan OFF) under continuously applied uniform heat load (2) Hybrid cooling mode (cooling fan ON after complete melting of PCM) under uniform as

well as periodic pulsed heat loads. Finally, the energy savings based on fan operation time for HS-PCM and HS-Gr/PCM were compared with respect to HS under uniform and periodic thermal loads. 2. Materials and methods 2.1. Synthesis of graphene and paraffin/graphene composite Graphene was synthesized by liquid phase exfoliation method. In this method, 0.26 mg/mL of graphite powder (150 mesh, NICE chemicals) was mixed with 500 mL of N, N-Di-methyl formamide (≥99% pure, Emplura, Merck chemicals) which was used as organic solvent. This mixture was kept in an ultra-sonication bath (LabmanTM Scientific Instruments) maintained at 45 °C, for a duration of 15 h. The dispersed solution thus obtained was kept overnight for the settlement of larger unexfoliated particles. To remove further macroscopic aggregates of graphite, the solution was centrifuged at 10,000 rpm in a centrifuge (Eltech, MP 400 R) for 15 min. The resulting supernatant was collected and filtered using a vacuum pump. The filtered particles were washed and dried at 80 °C in a hot air oven to obtain graphene. The schematic of the synthesis procedure is shown in Fig. 1(a). The paraffin/graphene composite was synthesized by two-step technique consisting of dispersion and solidification steps. Paraffin wax (m.p 58–60 °C, Merck chemicals) was used as PCM. A fixed quantity of paraffin wax was initially melted by heating it on a hot plate cum magnetic stirrer apparatus maintained at 70 °C. Graphene of various

Fig. 1. (a) Graphene synthesis using liquid phase exfoliation (b) Preparation of paraffin/graphene nanocomposite. 3

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mass fractions was mixed with the molten PCM by means of a magnetic stirrer for 15 min at a speed of 450 rpm. The paraffin/graphene suspension was then transferred to an ultra-sonication bath maintained at 70 °C, which was well above the melting point of PCM and sonicated for 1 h. In the second step, the suspension was poured into cylindrical cavities (16 mm dia.) and cooled at room temperature to obtain solid paraffin/graphene nanocomposite. The synthesis procedure of the nanocomposite is shown in Fig. 1(b).

heat sink was supplied by means of a plate heater placed below the aluminium heat sink to simulate the heat dissipation from electronic components. The heater was made from standard coil-type nichrome wire wound over a mica sheet (42 mm × 42 mm) and a thermal paste was used between the heat sink and heater to avoid the contact resistance. The power input to the heater was controlled by means of an autotransformer and the voltage and current were measured by means of a multimeter. The heat sink with heater assembly was insulated from both sides and bottom by inserting the base of the heat sink in the cavity of a PTFE block. Five K-type thermocouples (T1, T2, T3, T4, and T5) were used to measure the base temperature of the heat sink and one thermocouple was used for the measurement of ambient temperature. The thermocouples were connected to a data acquisition device (Agilent, 34972A) and the readings were captured in an interval of 10 s. The fan was powered by a DC power supply and the input power to fan and hence the fan speed was kept constant (6 V and 0.15 A) for all the experiments. The DC power supply was connected through Arduino UNO board to the computer thereby controlling the fan operation. The Arduino was programmed to switch ON the fan when the temperature reaches a high set point value and switch OFF the fan when it reaches a low set point value. To simulate periodic power inputs, the heater power supply was connected to the computer through the UNO board and controlled using an Arduino code which provided different periodic power input patterns.

2.2. Characterisation of graphene and paraffin/graphene composite The morphology of graphene was observed using SEM (Hitachi SU 6600, Field Emission SEM), after gold sputtering using E-1010 Gold Ion Sputter. The dispersion of graphene in paraffin wax was studied using TEM (JOEL2100). The formation of graphene and paraffin/graphene composite was analysed using a Raman spectrometer (Alpha300RA, WITec). A diode-pumped solid-state laser with a wavelength of 532 nm and a maximum power of 70 mW was used to excite the sample. The crystal structure of graphene and graphite was studied using RigakuMiniflex600 diffractometer with Cu Kα radiation of wavelength 1.5406 Å. 2.3. Thermo-physical property measurements of paraffin/graphene composite 2.3.1. Differential scanning calorimetry and thermo-gravimetric analysis The melting and solidification temperatures and enthalpies of pure paraffin wax and paraffin/graphene composite were estimated using a Differential Scanning Calorimeter (DSCQ20, TA instruments). The DSC test consists of heating and cooling cycles between 0 °C and 90 °C at a temperature ramp rate of 5 °C/min. The thermo-gravimetric analysis of paraffin and paraffin/graphene composite was done in a thermal analysis system (STA 7200, Hitachi). The samples were heated from 30 °C to 600 °C in alumina pan at a ramp rate of 5 °C/min and the weight losses were estimated.

2.5. Experimental procedure In the present study, three different types of heat sinks; heat sink without paraffin (HS), heat sink with paraffin (HS-PCM) and heat sink with paraffin/graphene composite (HS-Gr/PCM) were analysed for different thermal loading scenarios. Paraffin and paraffin/graphene composite were initially made in the form of small wires (diameter 2 mm) using aluminium moulds. These wires were then filled in the cavities of the heat sink which was melted by heating the sink followed by re-solidification by cooling under ambient condition. The paraffin and the composite were filled in the cavities of the heat sink, layer by layer, thus preventing air entrainment within PCM. The cooling system was tested for two modes of operation: (1) Passive cooling mode in which continuous and uniform thermal load corresponding to four different power levels applied to each type of heat sinks (2) Hybrid cooling mode with DC powered fan mounted on the top of the heat sink, programmed to start when the heat sink base temperature reaches a particular set point temperature. The fan onset time for HS, HS-PCM, and HS-Gr/PCM were estimated and compared for uniform and periodic thermal loads. The schematic of a typical periodic power input pattern and the duty factor used in the present study is given in Fig. 4. The transient responses of each heat sink corresponding to three different periodic duty factors (0.5, 0.79 and 0.9) were compared. Finally, the cyclic operation of HS, HS-PCM and HS-Gr/PCM were studied for uniform and periodic pulsed thermal loads and the energy saving of fan with HS-PCM and HS-Gr/PCM were estimated. The range of applied heat fluxes for each heat sink are given Table 2.

2.3.2. Thermal conductivity, specific heat and viscosity measurements The thermal conductivity, thermal diffusivity and volumetric specific heat of pure paraffin and paraffin/graphene composite were estimated by transient plane source technique (ISO 22007-2) using Thermal constant analyser (TPS 500 S, Hot DiskTM). The analyser consists of a two-sided nickel foil sensor (20 μm thick) with double spiral pattern sandwiched between 25 μm thick sheets of Kapton insulation. The sensor strip was kept between the pair of sample and was firmly supported by a sample holder made of stainless steel as shown in Fig. 2. The heating power and measurement time were optimized based on the probing depth, which was fixed as 3 mm. The heating power was optimized typically between 25 and 30 mW for all the measurements. Initial calibration of the thermal constant analyser was done with SIS2343 mild steel. The absolute viscosity of liquid paraffin and paraffin/graphene composite was measured using Rheometer (MCR 102, Anton Paar). The temperature was varied from 62 to 100 °C and the measurements were taken at different shear stress after keeping the shear rate constant at 600 s−1. 2.4. Experimental set up: Hybrid cooling system The experimental set up basically comprises of a cooling fan mounted on heat sink assembly with allied controls. The heat sink geometry was selected based on previous studies in the literature [48,49,51]. The heat sink was machined from aluminium by CNC milling. The schematic diagram of the heat sink is shown in Fig. 3(a) and the dimensions are listed in Table 1. The PCM was filled in three cavities provided between plate fins. The schematic and photograph of the experimental set up are shown in Fig. 3(b)–(c) respectively. The experimental set up consists of a DC powered fan mounted on the heat sink. The constant heat flux to the

Fig. 2. Schematic representation of TPS measurements. 4

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Fig. 3. (a) Schematic of heat sink (b) Layout of the experimental set up (c) Photograph of the experimental set up.

2.6. Uncertainty analysis

0.7 °C and 0.6 °C for the thermocouples connected to data acquisition and temperature amplifier respectively. Thereafter, these biases were corrected in the temperature measurements for the purpose of calibration. The uncertainty in primary measurements and derived variables were estimated based on standard procedure [53]. The uncertainties associated with each measurement are listed in Table 3. The uncertainties associated with different thermophysical property measurements were determined based on three sets of experiments conducted on three batches of samples. The uncertainty in

The uncertainties in measurements were estimated by the calibration. Six K-type thermocouples were connected to a data acquisition system and one K-type thermocouple was connected to temperature amplifier of Arduino UNO board. The calibration was done using a constant temperature bath with a standard mercury in glass thermometer (temperature range from 0 to 100 °C and 0.01 °C resolution). The average temperature bias against the thermometer was found to be 5

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TEM images confirms the formation of a very thin coating of paraffin over the flakes. The TEM image in Fig. 6(c) shows graphene with 3–5 layers with a thickness of 5 nm dispersed in paraffin. The selected area diffraction image (SAED pattern) of paraffin/graphene composite in Fig. 6(d) shows (0 0 2) and (0 0 4) Miller indices planes which are characteristic to graphene.

Table 1 Dimensions of heat sinks. Heat sink parameters

Notation

Dimension (mm)

Length Width Height Fin height Fin thickness Fin spacing Cavity thickness Cavity width

L1 W1 H2 H1 S1 S2 S3 W2

42 42 32 26 2 6 6 38

3.2. Raman spectroscopy and X-ray diffraction studies The comparison of Raman spectra of graphite powder and graphene are shown in Fig. 7(a). The spectra of graphite exhibit two principal bands at 1569 cm−1 and 2700 cm−1 designated as G and 2D band respectively. The spectrum of graphene consists of G band at 1577 cm−1 and 2D band at 2714 cm−1 respectively. G-band position was found to be shifted to higher wave numbers in the case of graphene due to decrease in bond strength. The G-band position is a function of the number of layers and as the number of layer decreases the position shifts to higher wavenumbers [54]. If the intensity ratio, IG/I2D, is between 0.5 and 1, then graphene is single layer or bilayer and if it is between 1.5 and 3 then graphene obtained will be few layered (3–5 layers) [55]. From the Raman spectra for graphene IG/I2D is 1.6 which confirms the formation of few-layered graphene. A subtle third band with weak intensity is exhibited by exfoliated graphene at 1344 cm−1 indicating the disorder band or defect band (D-band) corresponding to the apparent edge effects from sp2 carbon rings. The peak at 3239 cm−1 for graphite and graphene corresponds to the 2D' band caused by overtone (second order excitations) due to scattering from grain boundaries or edges. The increase in intensity for 2D' band in case of graphene is mainly due to the increased edge effects in the few layered graphene. The Raman spectrum of paraffin/graphene nanocomposite is shown in Fig. 7(b). The Raman shift region from 889 cm−1 to 1294 cm−1 corresponds to medium CeC bending vibrations of paraffin wax molecules. The shift at 1373 cm−1 is the characteristics of medium eCH3 symmetrical stretching vibrations while the peak at 1440 cm−1 corresponds to medium eCH2 in-plane bending vibrations of paraffin. The region from 2750 cm−1 to 2880 cm−1 corresponds to strong CeH bending vibration of paraffin molecules. The Raman shift corresponding to graphene cannot be identified due to the low loading levels in paraffin. The XRD spectrum of graphite and graphene are shown in Fig. 7(c). From this it is clear that the peak intensity is reduced by more than 50% for graphene as compared to graphite, clearly indicating distortion of atomic planes due to exfoliation.

Fig. 4. Periodic power input pattern and duty factor.

thermal conductivity, dynamic viscosity and latent heat measurements are estimated to be ± 2.4%, ± 2.73% and ± 0.6% respectively. Two sets of experiments were performed for each heat flux and the absolute uncertainty in transient temperature measurements of heat sink (HS) system were determined as shown in Fig. 5 (corresponding to uniform flux of 5669 W/m2). The uncertainty in transient temperature measurements was less than ± 1.3%, indicating the reliability of experimental results. 3. Results and discussion 3.1. Microscopic characterization of graphene and paraffin/graphene composite The SEM image of graphene obtained by the liquid phase exfoliation of graphite powder is given in Fig. 6(a) which clearly shows the formation of graphene flakes due to exfoliation, resulting in a transition from 3-D to 2-D structure. SEM image shows that the in-plane size of the sheet is much higher than the out-of-plane size. The larger in-plane size promotes phonon transport more efficiently giving rise to higher inplane thermal conductivity for graphene [15]. The size of the flakes is non-uniform as seen from the SEM image, however, the average inplane size of the flakes is within 1–3 μm. The TEM images of paraffin/graphene composite are shown in Fig. 6(b)–(c). The dispersion of few layered graphene nanoflakes in paraffin can be observed from the TEM image shown in Fig. 6(b). The

3.3. Phase change characteristics and thermal stability of paraffin/graphene composite The DSC curves for paraffin wax and paraffin/graphene composite (0.05, 0.2 and 0.5 wt%) are presented in Fig. 8(a) and the consolidated observations for different concentration of graphene in composite are given in Table 4. To eliminate uncertainties in heating of specimen, for each concentration, three different melting/solidification cycles were performed with three different batches of the sample. From the DSC curves, two transition peaks (at around 43 °C and 60 °C) are observed for both paraffin wax and paraffin/graphene composite during melting.

Table 2 Range of applied thermal loads to each heat sink. Passive cooling mode

Hybrid cooling mode 2

Power (W)

Flux (W/m )

Thermal loading

Power (W)

Flux (W/m2)

Thermal loading

5 10 15 20

2835 5669 8503 11,338

Uniform Uniform Uniform Uniform

10 20 20 20 20

5669 11,338 11,338 11,338 11,338

Uniform & continuous Uniform & continuous Periodic ON/OFF @ 90 s/10 s Periodic ON/OFF @ 90 s/24 s Periodic ON/OFF @ 90 s/90 s

& & & &

continuous continuous continuous continuous

6

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Table 3 Uncertainty in experimental measurements. Sl. No.

Parameter/property

Unit

Device

Range

Accuracy (%)

Uncertainty

1 2 3 4 5 6 7 8 9

Voltage Current Length scale Temperature Weight Power Thermal conductivity Dynamic Viscosity Latent heat

V A mm °C g W W/mK mPa.s kJ/kg

Digital multimeter Digital multimeter Vernier caliper K-type thermocouple Electronic balance – Hot Disk apparatus Rheometer Differential Scanning Calorimetry

0–100 0–1 0–50 0–120 0–20 0–25 0–0.5 0–15 0–210

±5 ±5 ±3 ±2 ±2 – ±5 ±5 ±5

± 0.221 ± 0.010 ± 0.100 ± 0.314 ± 0.0002 ± 0.533 ± 0.011 ± 0.273 ± 1.2

melts into the liquid state. Thus during melting there exists solid-solid (γ-rotator) and solid-liquid (rotator-liquid) phase transitions and similar mechanisms are also present during solidification [56]. The average melting and solidification enthalpies of pure paraffin were estimated to be 204.9 J/g and 203.3 J/g respectively as shown in Table 4. The melting enthalpy of paraffin/graphene composite was found to be decreased up to 12.35% for a graphene loading of 0.5 wt%. Fang et al. [26] reported decrease in melting enthalpy for paraffin/graphene composite with particle loading. From their studies, the decrease in melting enthalpy was more for smaller particle size (<5 μm), which is true with the present work also. Xiang and Drzal [14] and Shang et al. [40] also observed an inverse relation between graphene loading and melting enthalpy. The volume expansion of paraffin during solid-liquid phase change is restricted by the presence of graphene which increase the pressure in the pores and decrease the molecular heat transfer. Generally, with graphene addition the molecular structure of paraffin experiences a strain resulting from interfacial liquid layering [57]. At the interface, the van der Waals force cause the paraffin molecules to create a more ordered densely packed layer thus inducing strain in the neighbouring molecules. Consequently, only less energy is required to overcome the molecular bonds in the strained region which in turn reduces the latent heat. Further, the Brownian motion of particles increases the swept volume of weakened bond structure, further contributing to the reduction of latent heat. With graphene loading, the smaller particles with high surface energy can aggravate particle clustering, further reducing the latent heat of composite [24]. It can be seen that with increase in graphene loading the composite solidification temperature decreases, due to incomplete crystallization in the presence of graphene. The mass fraction of crystallized paraffin in nanocomposite (ω) can be estimated as,

Fig. 5. Absolute uncertainty in transient temperatures of heat sink (HS) for 5669 W/m2.

=

Endothermic latent heat of composite PCM × 100% Endothermic latent heat of pure PCM

(1)

The mass fraction of crystallized paraffin in nanocomposite was estimated to be 98.5%, 97.2%, 93.5%, 90.6%, and 87.6% for graphene loading of 0.1, 0.2, 0.3, 0.4, and 0.5 wt%, respectively. The average peak melting and solidification temperatures of paraffin wax were found to be 59.6 °C and 55.5 °C respectively. There is a slight shift in the melting and solidification peak with increase in graphene loading. For graphene loading of 0.5 wt%, the peak melting and solidification temperature was found to be decreased by 2.52%. As loading increased, the clustering of graphene and weakened bond structure reduced the availability of active nucleating sites and inhibited complete crystallization [34]. Hence, due to the change in the local steric hindrance of paraffin molecules induced by graphene fillers and incomplete crystallization [17,31,56] the solidification enthalpy of the composite was found to be slightly lower than the melting enthalpy. In present work, the latent heat of composite PCM was reduced by 12.35% at low loading of 0.5 wt% due to weakened bond structure. For Li et al. [29], similar reduction in latent heat occurred at 1 wt% of sulfonated graphene. However, Yang et al. [36] reported an increase in crystallinity with graphene microencapsulated paraffin which

Fig. 6. (a) SEM image of exfoliated graphene (b) & (c) TEM image of graphene/ paraffin composite (d) SAED pattern of graphene dispersed in paraffin wax.

During the melting transition the γ phase crystal structure of paraffin does not directly proceed to the liquid phase, instead it transforms into an intermediate rotator phase and on further heating the rotator phase 7

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Fig. 7. (a) Raman spectra of graphene and graphite (b) Raman spectra of paraffin/graphene nanocomposite (c) XRD spectra of graphene and graphite.

enhanced the latent heat of composite with particle loading. Yang [34] and Qi [33] also found that crystallinity improved with hybrid graphene aerogels and also improved the latent heat of composite. The lack of strong supporting network structure reduced the latent heat with loading in the present case. The TGA curves of 0.5 wt% paraffin/graphene composite and pure paraffin are shown in Fig. 8(b). Although the decomposition temperature increases with graphene addition as reported by many authors [24,26], no significant change in the decomposition temperature and weight loss was observed for paraffin/graphene composite owing to the low loading levels in the present work. Nevertheless, high thermal stability of graphene and the synergistic effect between graphene and paraffin molecules in addition to homogeneous distribution of graphene can impede the diffusion of volatile decomposition products in the

composite. Accordingly, the thermal stability of paraffin was not degraded by the addition of graphene. 3.4. Thermal conductivity, specific heat and viscosity of paraffin/graphene composite The variation of thermal conductivity and thermal diffusivity of paraffin and paraffin/graphene composite for different loadings are given in Fig. 9(a)–(b) respectively. For repeatability of results, the thermal properties of paraffin/graphene composite were estimated for three samples from different synthesis batches. The experiments were performed thrice for each sample. It was found that thermal conductivity and thermal diffusivity of paraffin/graphene increased linearly with increase in graphene loading. The thermal conductivity of 8

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Fig. 8. (a) DSC curves of paraffin and paraffin/graphene composite (b) TGA curves of paraffin and 0.5 wt% of paraffin/graphene composite. Table 4 Consolidated DSC observations. PCM

wt.%

(a) Melting enthalpy and temperature Paraffin – Paraffin/graphene 0.05 0.1 0.2 0.3 0.4 0.5 PCM

wt.%

(b) Solidification enthalpy and temperature Paraffin/graphene 0.05 0.1 0.2 0.3 0.4 0.5

Avg. melting enthalpy (J/g)

Rel. change (%)

Avg. melting temp. (°C)

Rel. change (%)

204.9 202.9 202.0 199.2 191.7 185.8 179.6

– 0.98 1.42 2.78 6.44 9.32 12.35

59.6 59.4 59.2 58.8 58.5 58.3 58.1

– 0.33 0.67 1.34 1.84 2.18 2.52

Avg. solidification enthalpy (J/g)

Rel. change (%)

Avg. solidification temp. (°C)

Rel. change (%)

200.8 198.1 197.0 190.9 183.7 177.3

1.23 2.56 3.18 6.09 9.64 12.69

55.2 54.9 54.6 54.4 54.3 54.1

0.54 1.08 1.62 1.98 2.16 2.52

composite was found to be 0.334, 0.377, 0.396, 0.452, and 0.457 W/m K corresponding to 0.1, 0.2, 0.3, 0.4, and 0.5 wt% of graphene loading. A comparison of thermal conductivity enhancement obtained in the present work with the Maxwell model in Eq. (2) is given in Fig. 9(c).

keff = kPCM

kf + 2kPCM + 2 (kf

kPCM )

kf + 2kPCM

kPCM )

2 (k f

0.2 wt% and 0.5 wt% paraffin/graphene composite respectively, at 62 °C. Even though the viscosity of paraffin wax was found to be increased with graphene loading, the effect of viscosity was predominant only up to 65 °C, thereafter the variations were found to be negligible with temperature. Fan et al. [46] reported 16.5% increase in viscosity for GNP based PCM composite at 80 °C corresponding to 0.3 wt% loading. They also reported 3 and 7-fold increase in viscosity with 1 wt% and 3 wt% GNP loading respectively and 115% increase in viscosity for PCM composites with 0.3 wt % CNT loading. On the other hand, in the present study, increase in viscosity was found to be less than 10% for 0.5 wt% graphene loading at 80 °C. The viscosity increase of paraffin/graphene composite is less pronounced than CNT or any other carbon-based nanofillers, primarily on account of its 2D-planar structure and lesser size. The planar geometry allows the shear-induced realignment and inter-sliding of graphene flakes, enhancing the lubrication effect with neighbouring flakes [18]. Since higher loading levels of nanofillers can increase the viscosity of PCM composites drastically, for thermal management applications, low loadings are preferred, befitting to the fact that increase in viscosity reduces the natural convection coefficient in melting phase [26,27].

(2)

where kf and kPCM represents thermal conductivity of filler and PCM respectively, and is the volume concentration of filler. This illustrates that much higher relative enhancement in thermal conductivity of composite in the range 18–59.5% is obtained with graphene loading in the range 0.1–0.5 wt%. The increased thermal conductivity and diffusivity for paraffin/graphene composite are mainly attributed to the 2Dstructure of the graphene and low Kaptiza resistance at the grapheneparaffin interface [14,15,19,22,24]. The variation of specific heat with loading is shown in Fig. 9(d). At room temperature, the specific heat capacity of graphene is 0.7 J/g K [58] and that of paraffin is in the range 2.0 J/g K (solid) to 2.15 (liquid) J/g K. According to effective medium theory, the specific heat capacity of paraffin/ graphene decreases with increase in graphene loading as shown in Fig. 9(d). The variation of absolute viscosity with temperature for liquid paraffin and paraffin/graphene composite (0.2 wt% and 0.5 wt%) are shown in Fig. 9(e). The viscosity was found to be increased by 13% and 24% for

3.4.1. Effect of thermal interface resistance on thermal conductivity of paraffin/graphene composite In the case of ellipsoidal particles like platelets, the thermal interface resistance at low loading (< 1wt . %) can be predicted by the 9

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Fig.9. Thermo-physical properties of paraffin and paraffin/graphene composite (a) thermal conductivity (b) thermal diffusivity (c) thermal conductivity enhancement factor (d) volumetric heat capacity (e) absolute viscosity.

10

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Maxwell–Garnett type effective medium approach proposed by Nan et al. [59]. Using this model the thermal conductivity ratio (keff / kPCM ) can be estimated as,

keff kPCM

=

3+f(

x

+

f

x

3

z)

;

x

=

2(k11 g kPCM ) ; (k11 g + kPCM )

z

=

k33 g kPCM

1

(>1 wt%), the phonon-phonon scattering and phonon-boundary scattering increased the thermal interface resistance as observed by Xiang and Drzal [14], Yu et al. [15], and Fang et al. [26]. However, in the present work the low particle loading reduced the number of interfaces, lowered the phonon scattering rate and thermal interface resistance significantly and hence improved the enhancement ratio. On the other hand, recent work of Yang et al. with graphene aerogels [34], Li et al. with sulfonated graphene [29] could further lower the Kapitza resistance due to the superior thermally conducting network structure, thus proving to be more effective than graphene or xGnP. Table 5 shows the comparison of thermal conductivity and latent heat of paraffin/graphene composites in the present study with that of the PCM composites reported in literature. In the present work, the paraffin/graphene composite shows enhancement in thermal conductivity at low loading and the melting enthalpies of the composite are found to be in the range with that of various paraffin composites employed for latent heat storage applications.

(3)

where f is the volume fraction of filler materials. Assuming that the graphene flake is coated with a very thin interfacial thermal barrier layer, the transverse and longitudinal thermal conductivities of the flake is given by,

k11 g =

kf

1+

( ) 2ak kf

tkPCM

; k33 g =

kf

1+

(

2ak kf dkPCM

)

(4)

where t and d are thickness and diameter of the graphene flake taken as 5 nm and 3 μm respectively. The in-plane and out-plane thermal conductivities of graphene (kf ) are taken as 3000 W/mK and 10 W/mK respectively [14]. The Kapitza radius (ak ) is obtained as,

ak = Rk kPCM

3.5. Thermal performance of heat sinks

(5)

3.5.1. Passive cooling mode under uniform thermal load The thermal response of HS, HS-PCM and HS-Gr/PCM for various power inputs of 5 W, 10 W, 15 W and 20 W (corresponding heat fluxes of 2835 W/m2, 5669 W/m2, 8503 W/m2 and 11,338 W/m2 respectively) during the passive cooling mode operation are shown in Fig. 11. Even though the increase of graphene loading led to increase in the thermal conductivity of the composite, the latent heat was found to be decreased considerably beyond 0.2 wt% as per the DSC studies. Hence, the concentration of composite was fixed as 0.2 wt% in HS-Gr/PCM. In the case of HS (see Fig. 11(a)–(d)), during the initial stages of heating for all heat fluxes, temperature rises sharply and attain a steady state. The heat dissipation from the source was only through natural convection from the fin surface. The instantaneous temperatures were lowered in case of HS-PCM for heat flux of 5669 W/m2, 8503 W/m2 and 11,338 W/m2 on account of the solid-liquid phase transition of paraffin, whereas at 2835 W/m2, this was due to the solid-solid phase transition of paraffin before melting. The instantaneous temperatures were further lowered in case of HS-Gr/PCM due to the improved thermal conductivity of paraffin/graphene composite. The transient temperature variations for HS-PCM and HS-Gr/PCM in Fig. 11(b)–(d) is divided into three stages based on the phase change of paraffin and the composite: the pre-melting (conduction dominated heat transfer), the melting (latent heat transfer), and the post-melting

where Rk is the thermal interface resistance (Kapitza resistance). The value of thermal interface resistance was obtained as 1~ 3 × 10 8 m2 K/W , from the curve fitting result using this model as shown in Fig. 10(a). The Kapitza resistance obtained is very close to that across graphene nanoplatelets, reported by Harish et al. [19] (1.6 × 10 8 m2 K/W ) and lower than that reported by Xiang and Drzal for xGnP [14] (7~ 8 × 10 8 m2 K/W ). A comparison of thermal conductivity ratio of paraffin composites (keff / kPCM ) with various mass fractions of carbon fillers in literature is shown in Fig. 10(b). The thermal conductivity enhancement in present case is slightly higher at low loading than those reported in similar works with carbon based materials. Shi et al. [22] proposed that for smaller size flakes at low loading levels, the uniform dispersion of filler particles in matrix creates evenly distributed interconnected thermal pathways in the composite. Since the paraffin is absorbed into the spaces between graphene flakes and held together by capillary forces and surface tension, the thermal interface resistance is reduced thereby enhancing the thermal energy transport. The excellent thermal coupling at grapheneparaffin interface resulting from the atomic level ordering of alkane molecules across graphene faces lowered the interface thermal resistance in the composite. However, the major heat carriers in the composite are low frequency acoustic phonons, which undergoes phonon boundary scattering at paraffin-graphene interfaces. When loading levels are high

Fig. 10. (a) Thermal interface resistance of paraffin/graphene composite using Nan’s model (b) Comparison of thermal conductivity enhancement ratio with graphene enhanced composites in literature. 11

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Table 5 Comparison of thermal conductivity and latent heat of different PCM composites. PCM composite

Loading

Thermal conductivity (W/m K)

Melting enthalpy (J/g)

Reference

Paraffin/expanded graphite Paraffin/graphene

20 wt% 0.5 wt% 2 wt% 0.5 wt% 2 wt% 10 wt% 1 wt% 20 vol% 52 wt% 1 wt% 1 wt% 2 wt% 0.464 wt% 17 wt% 1 wt% 0.2 wt% 0.5 wt%

7.654 0.35 0.46 0.32 0.41 0.936 0.32 7.2 0.932 0.35 0.35 0.8 0.4 2.43 0.34 0.376 0.457

147.4 141.1 137.8 141.6 143.8 181.8 157.2 314.53 63.11 190 232.4 177 180.2 170.72 199.65 199.2 179.6

[60] [21]

Paraffin/exfoliated graphite Paraffin/nano-graphite Paraffin/EP/CNTa Paraffin/graphene Paraffin/GOb sheets Paraffin/GNPc PEG@graphene MEPCMd PEG/sulfonated graphene PEG/GNP + GO Palmitic acid/graphene Palmitic acid/NDGe Paraffin/graphene Paraffin/graphene a b c d e

[21] [61] [10] [23] [30] [17] [36] [29] [34] [24] [32] Present work Present work

Expanded perlite/carbon nanotube. Graphene oxide. Graphene nanoplatelets. Micro encapsulated PCM. Nitrogen doped graphene.

(convection dominated heat transfer) stages. In HS-PCM, the paraffin close to the vicinity of base melts initially and reduces the heat transfer to solid layers above it due to increased temperature gradients on account of low thermal conductivity. In HS-Gr/PCM, due to the improved thermal conductivity of paraffin/graphene composite, the heat is conducted uniformly through the composite thereby decreasing the internal temperature gradients as compared to paraffin in HS-PCM. Hence the instantaneous temperature rise was found to be lowered for HS-Gr/ PCM as compared to HS-PCM, during the initial stages of heating. During the melting stage, paraffin as well as paraffin/graphene composite starts to melt and latent heat absorption occurs at the melting temperature (Fig. 11(b)–(d)). The slopes of temperature profiles in HSPCM and HS-Gr/PCM is lowered and the temperature rise is delayed as compared to HS case. However, the improved thermal conducting network and better dispersion of graphene results in uniform melting of paraffin/graphene composite in HS-Gr/PCM and thus the slope of temperature profiles was found to be lower than HS-PCM at all heat fluxes. At higher heat flux of 11,338 W/m2 (Fig. 11(d)), the rate of heat addition to PCM is high enough producing large difference between melting temperature and source temperature. Therefore, as compared to lower heat fluxes (Fig. 11(b)–(c)), the melting time was reduced, the premelting stage was shortened and PCM melted quickly thus increasing the slope of instantaneous temperature in both HS-PCM and HS-Gr/PCM. In Fig. 11(b)–(d), at the end of the melting stage, there is an inflection point for the temperature profiles in HS-PCM and HS-Gr/PCM which marks the beginning of the post-melting stage. In this stage, the natural convection in the melt-phase overweighs the conduction heat transfer. On account of increased conduction resistance, the superheating of paraffin near to the base results in sharp increase of base temperature in HS-PCM. Fan et al. [46] reported that for paraffin/ graphene composites, the microscale heat transfer in liquid phase is enhanced due to Brownian motion and thermophoretic motions of graphene particles leading to reduced conduction resistance and less superheating at the base as compared to liquid paraffin. In addition, for paraffin/graphene composite, the lower viscosity increase of the meltphase does not inhibit the Rayleigh-Benard convection currents, thereby lowering the instantaneous temperature rise in HS-Gr/PCM. The steady-state temperature attained for the HS was 57 °C, 70 °C, 85 °C and 101 °C corresponding to 5 W, 10 W, 15 W, and 20 W respectively. In the case of HS-PCM, the steady temperature was found to be decreased by 1.8%, 4.2%, 2.3% and 1.9% for heat fluxes of 2835 W/m2, 5669 W/m2, 8503 W/m2 and 11,338 W/m2 respectively, as compared to HS. Similarly,

in the case of HS-Gr/PCM, the steady temperature was decreased by 5.2%, 8.5%, 5.8% and 4.9% corresponding to heat fluxes of 2835 W/m2, 5669 W/m2, 8503 W/m2 and 11,338 W/m2 respectively, as compared to HS. For a heat flux of 2835 W/m2, the resulting steady-state temperature was found to be below the melting temperature of paraffin wax. The PCM based heat sinks delay the time to reach a particular set temperature as compared to a conventional heat sink without PCM. The increase in operation time to reach 65 °C (with respect to HS) for HS-PCM and HS-Gr/PCM is expressed in terms of relative enhancement factor as shown in Fig. 11(e). In the case of HS-PCM and HS-Gr/PCM, the PCM absorbs latent heat at its melting point which provides an additional thermal stabilization period without any increase in temperature leading to an extended operating period before reaching the set temperature. When compared to heat flux of 5669 W/m2, a sudden variation in the rate of temperature rise was observed for HS-PCM and HS-Gr/PCM at 8503 W/m2 and 11338 W/m2 due to faster melting of PCM. As a result, with increase in heat flux, the relative enhancement factor is reduced for both HS-PCM and HS-Gr/PCM. The superior performance of HS-Gr/PCM is attributed to the enhanced heat diffusion rate through the paraffin/ graphene composite in the pre-melting stage and melting stage. Hence, for higher heat fluxes it is recommended to use PCM with higher melting point so that the melting time is extended further. 3.5.1.1. Performance evaluation of heat sink. The performance of heat sink is evaluated based on the effective thermal resistance Rth . eff defined as,

Rth . eff =

T (t )

Tamb P

(6)

where T (t ) is the instantaneous temperature of heat sink base, Tamb and P represents ambient temperature (°C) and heating power (W) respectively. The HS-PCM and HS-Gr/PCM are compared based on the time averaged thermal resistance during heating. An increase in Rth . eff indicates ineffective heat dissipation and rapid increase in base temperature. In the present work, the effective thermal resistance of HS-Gr/PCM was found to be lower than HS-PCM at different heat loads thereby improving the thermal response time of paraffin/graphene nanocomposite based heat sink. Table 6 shows the comparison of PCM composites based heat sinks in literature with the present work. An increase in effective thermal resistance was observed for the cases with high filler loading ( > 1 wt%) [2,42,46,47], as large number of filler particles did not 12

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Fig. 11. Transient temperature variations of HS, HS-PCM and HS-Gr/PCM in passive cooling mode for heat flux of (a) 2835 W/m2 (b) 5669 W/m2 (c) 8503 W/m2 (d) 11338 W/m2 (e) Relative enhancement in time (w.r.t HS) to reach set temperature of 65 °C.

13

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undergo phase change thus reducing the latent heat of composites significantly. Consequently, only less heat was absorbed from the source, while the thermal stabilization period was shortened and base temperature of heat sink was increased. In the present work, the more efficient thermal conducting network through composites reduces the temperature gradients and enhances heat diffusion through it. Also, the latent heat variation of PCM composite is negligible in the present case due to low filler loading. Fan et al. [27], Sebti et al. [62], Khodadadi et al. [63] reported that at higher particle loading, the suppression of Rayleigh–Benard convection in the liquid-phase superheats the viscous mixture near the base thereby increasing the base temperature rapidly. In the present work, at low filler loading (0.2–0.3 wt%), the viscosity increase is negligible and thus the convection currents enhance the liquid-phase heat transfer as compared to high filler loading.

The cooling or discharging period during fan operation represents the recovery capability of heat sinks for consecutive use. In Fig. 12(a)–((b), the temperature drop in HS was sharp as compared to HS-PCM or HS-Gr/PCM due to forced convection heat transfer. In fact, the temperature drop in HSPCM and HS-Gr/PCM was gradual and less steep relative to HS because of the presence of additional cooling load in terms of latent heat. However, the extended delay in attainment of critical temperature overshadowed the increased cooling time of PCM based heat sinks over HS at both heat fluxes. At low flux (5669 W/m2), the cooling curve of both PCM and composite based heat sinks were steep until 3–4 °C below the solidification point of PCM. However, at high heat flux (11,338 W/m2) the slopes were less steep till solidification point and then decreased considerably resulting in slower cooling. At higher heat fluxes, the difference in source and melting temperature is large, which resulted in faster heat addition. But the rate of heat release was constant for both fluxes, which increased the cooling time at higher fluxes. Therefore, both HS-PCM and HS-Gr/PCM takes more time to cool as the heat flux is increased.

3.5.2. Hybrid cooling mode under uniform heat load In the hybrid cooling mode, the fan was started during the postmelting stage of paraffin. The instantaneous temperature variations of HS, HS-PCM and HS-Gr/PCM subjected to fan cooling for heat fluxes of 5669 W/m2 and 11,338 W/m2 are shown in Fig. 12(a)–(b). The cooling fan was programmed to start when the base temperature of the heat sink reaches the critical operating temperature. For heat flux of 5669 W/m2, the critical temperature was arbitrarily fixed at 65 °C as the maximum steady temperature attained was 70 °C. Preliminary experiments shows no significant enhancement in time beyond 68 °C corresponding to a heat flux of 11338 W/m2 (Fig. 11(d)). Thus the fan start temperature was set at 65 °C for 5669 W/m2 and 68 °C for 11,338 W/m2. From Fig. 12(a)–(b), it is clear that the onset of fan operation was delayed for HS-PCM and HSGr/PCM as compared to HS at both heat fluxes. In case of HS-PCM at 5669 W/m2, the fan onset time was delayed by 1630 s as the latent heat of melting was absorbed by paraffin which lowers the instantaneous temperature rise thus delaying the attainment of critical temperature. In the case of HS-Gr/PCM, this delay was increased to 2920 s due to enhanced conduction heat transfer in pre-melting stage followed by latent heat transfer during melting of paraffin/graphene composite. Similarly, for a heat flux of 11,338 W/m2, the fan onset was delayed by 460 s for HS-PCM relative to HS and for HS-Gr/PCM the fan onset was delayed by 600 s. At higher heat fluxes, sharp rise in source temperature led to melting of PCM in minimum time and hence the critical temperature was attained quickly for HS-PCM and HS-Gr/PCM.

3.5.3. Hybrid cooling mode under periodic pulsed heat load The instantaneous temperature variations for hybrid cooling mode with respect to HS, HS-PCM and HS-Gr/PCM for three periodic pulsed heat loads are shown in Fig. 13(a)–(c). The thermal responses of the three heat sinks with respect to periodic duty factor of 0.9 is illustrated in Fig. 13(a). It can be seen that relative to HS the fan starting time was delayed by 500 s and 650 s for HS-PCM and HS-Gr/PCM respectively. Similarly, from Fig. 13(b), for a duty factor of 0.79 the fan starting time was delayed by 750 s and 930 s for HS-PCM and HS-Gr/PCM respectively. For a duty factor of 0.5, the delay in fan onset time was 1436 s and 2845 s respectively as seen in Fig. 13(c). The relative increase in time (with respect to HS) to reach the set temperature obtained for HSPCM and HS-Gr/PCM for periodic power input patterns are shown in Fig. 13(d). For a duty factor of 0.9, the time to reach the set temperature was found to be increased by 77% and 110% for HS-PCM and HSGr/PCM respectively. Similarly, for duty factor of 0.79, the respective enhancements were 98% and 122% for HS-PCM and HS-Gr/PCM. For 0.5 duty factor, 56% and 109% enhancement in time was observed for HS-PCM and HS-Gr/PCM respectively. The time enhancement was found to more for HS-Gr/PCM as compared to HS-PCM. Due to the increased thermal conductivity of paraffin/graphene composite together with sufficient heater idle time, the

Table 6 Comparison of the effective thermal resistance of various PCM composite based heat sinks in literature. Reference

PCM composite @ loading

Rth . eff (°C/W) HS-PCM

HS-PCMC

Paraffin/GNF @11 wt%

0.62

0.55

0.74

0.77

Alimohammadi et al. [47]

Mn(No3)2/Fe3O4 @ 1 wt%

1.27

1.51

Fan et al. [46]

Hexadecanol/CNT @ 0.3 wt% Hexadecanol/CNT @ 1 wt% Hexadecanol/GNP @ 0.3 wt% Hexadecanol/GNP @ 1 wt% Paraffin/GNF(p) @0.25 wt% Paraffin/GNF(p) @1 wt% Paraffin/graphene @ 0.2 wt%

0.840 0.840 0.840 0.840 3.21

0.904 0.914 0.838 0.841 2.45

3.21

3.23

3.1

2.8

2.96

2.71

2.82

2.67

Chintakrinda et al. [2]

Weinstein et al. [42]

Present work

Heat flux @ Power

Heat sink geometry (L × B × H ) (mm)

15600 W/m2 @50 W 58000 W/m2 @50 W 3000 W/m2 @ 17 W

50. 8×50. 8 × 50. 8 (Cube w/o fins)

12500 W/m2 @80 W

80 × 80 × 30 (Cavity w/o fins)

2713 W/m2 @7W

50. 8×50. 8 × 50. 8 (Cube w/o fins)

5669 W/m2 @10 W 8503 W/m2 @15 W 11338 W/m2 @20 W

42× 42× 25 (heat sink with fins)

75× 75× 40 (heat sink with fins)

HS-PCM: heat sink with PCM, HS-PCMC: heat sink with composite PCM, GNF(p): graphite nanofiber platelet, GNP: graphite nanoplatelets, CNT: carbon nanotubes. 14

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Fig. 12. Transient temperature variation of HS, HS-PCM and HS-Gr/PCM in hybrid cooling mode at heat flux of (a) 5669 W/m2 (b) 11,338 W/m2.

Fig. 13. Transient temperature variations for HS, HS-PCM and HS-Gr/PCM for (a) duty factor 0.9 (b) duty factor 0.79 (c) duty factor 0.5 (d) Relative enhancement in fan onset time for different periodic loading.

15

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heat diffusion through composite was uniform and hence the instantaneous temperature rise was lowered in HS-Gr/PCM than HS-PCM for different periodic duty factors. As a result, HS-Gr/PCM reached the fan onset temperature slowly as compared to HS-PCM. The time for premelting, melting and post-melting stages of both HS-Gr/PCM extended considerably as the periodic duty factors changed from 0.9 to 0.5. For short heater pause time, the periodic heat pulses were more or less identical to uniform heating and the sharp rise in source temperature decreased the melting time of composite PCM. As such, the fan onset temperature is attained quickly and time enhancement was less for 0.9 duty factor as compared to 0.79 duty factor. At 90 s heater pause time, the heat sink without PCMs (HS) performed better owing to very slow heating. This eventually improved the heat dissipation rate of HS and increased the time to attain the critical temperature. Consequently, the relative enhancement in time for HS-PCM and HS-Gr/PCM was found to be lowest in the case of 0.5 duty factor. Therefore, the enhancement in time to reach critical temperature depends on the periodic ON-OFF duration of heater. The HS-PCM and HS-Gr/PCM exhibited slightly more enhancement in time for 90 s/24 s input power pattern (0.79 duty factor) as compared to 90 s/10 s (0.9 duty factor) and 90 s/90 s (0.5 duty factor).

From Fig. 14(a), the higher improvement in energy savings associated with HS-Gr/PCM is mainly due to the extended delay incurred in fan onset time. Moreover, a 16% reduction in cooling time was achieved with HS-Gr/PCM for attaining the drop of temperature from 65 °C to 45 °C as compared to HS-PCM. In Fig. 14(b), for a heat flux of 11,338 W/m2, the cooling time corresponding to the temperature drop from 68 °C to 54 °C increased considerably for both HS-PCM and HS-Gr/ PCM and the energy savings index decreased to 7.6% and 10.9% respectively for HS-PCM and HS-Gr/PCM. The superior performance of HS-Gr/PCM over HS-PCM is attributed to the 25% average reduction in cooling time from 68 °C to 54 °C. The difference in cooling time of paraffin and paraffin/graphene composite plays a significant role in enhanced energy savings of HS-Gr/ PCM. The higher heat transfer coefficient due to fan cooling accelerated the sensible heat release in melt-phase leading to a sharp decrease in temperature up to the solidification point of PCM. Since the heat addition was still present, the rate of latent heat release became slower and gradually decreased the slope of cooling curve in case of HS-PCM and HSGr/PCM compared to that of HS. The decrease in slope (indicating slow cooling) was more predominant at 11338 W/m2 than at 5669 W/m2 as the rate of heat addition was large compared to the rate of heat release. From Fig. 14(a)–(b), it is found that the cooling time of HS-Gr/PCM is slightly lower than HS-PCM due to the excellent thermal conducting network offered by graphene. Hence, after the cyclic operation, 16% and 25% overall reduction in cooling time was observed for HS-Gr/PCM relative to HS-PCM at 5669 and 11338 W/m2 respectively.

3.6. Evaluation of energy savings in hybrid cooling mode The energy savings was evaluated by the comparison of total fan operation time for HS-PCM and HS-Gr/PCM with respect to HS after performing the experiment for a duration of 50 h. The cooling fan mounted on the heat sink was operated intermittently between two set temperatures (above and below the melting point of PCM). The associated energy savings are expressed in terms of Energy Savings Index given as,

ESI =

t fan.ON.w/PCM t fan.ON.w/oPCM × 100% t fan.ON.w/oPCM

3.6.2. Energy savings with periodic pulsed heat load The temperature profiles with fan ON/OFF operation for HS, HSPCM and HS-Gr/PCM for periodic power input patterns of 90 s/10 s, 90 s/24 s and 90 s/90 s (duty factors 0.9, 0.79 and 0.5) are illustrated in Fig. 14(c)–(e). Although the experiments were conducted for 50-hour duration, the temperature profiles are shown only for the first few hours for the sake of clarity. The fan was switched ON at 68 °C and OFF at 51 °C for each experiment. The reduction in fan operation time through the usage of paraffin and paraffin/graphene was taken as a measure of energy saving. In periodic heating, the reduction in fan operation time was 8.7%, 10.7% and 14.3% for duty factors of 0.9, 0.78 and 0.5 respectively in the case of HS-PCM (see Table 7). The corresponding energy savings with HS-Gr/PCM was 13.2%, 16.7% and 23.3% for duty factors of 0.9, 0.78 and 0.5 respectively. The delayed fan onset times and reduction in cooling time on account of enhanced thermal conductivity of paraffin/ graphene reduced the fan operation of HS-Gr/PCM compared to HSPCM. The slope of cooling curve for HS-PCM and HS-Gr/PCM varied with periodic duty factor. In Fig. 14(c), since the heater pause time was very short for 0.9 duty factor the HS-PCM and HS-Gr/PCM were cooled gradually and slowly compared to HS. From Fig. 14(d), it was observed that the cooling time for HS-PCM and HS-Gr/PCM was slightly reduced when heater pause time was increased to 24 s (0.79 periodic duty

(7)

The energy savings and total fan operation time realized with different thermal loads are summarized in Table 7. 3.6.1. Energy savings with uniform heat load The temperature profiles of HS, HS-PCM, and HS-Gr/PCM in cyclic operation subjected to two different heat fluxes of 5669 W/m2 and 11,338 W/m2 respectively are depicted in Fig. 14(a)–(b). For brevity, the temperature profiles are shown only for a selected time duration in each case. The fan voltage was kept constant at 6 V for all experiments. The heat sink was cooled to a minimum temperature of 45 °C and 54 °C corresponding to 5669 W/m2 and 11,338 W/m2 respectively. As such, the higher and lower temperatures for fan ON/OFF condition were set as 65 °C and 45 °C for 5669 W/m2 and for a heat flux of 11,338 W/m2 the fan ON/OFF temperature was set at 68 °C and 54 °C. Energy savings of 13.4% and 18.5% were obtained for HS-PCM and HS-Gr/PCM respectively, for a heat flux of 5669 W/m2 (see Table 7). Table 7 Total fan operation time and energy savings. No.

1. 2. 3. 4. 5.

Power input

10 W uniform (5669 W/m2) 20 W uniform (11338 W/m2) 20 W periodic 0.9 duty factor 20 W periodic 0.79 duty factor 20 W periodic 0.5 duty factor

Fan ON/OFF Temp. (°C)

Total fan ON time (min) HS

HS-PCM

HS-Gr/PCM

HS-PCM

HS-Gr/PCM

65/45

979

848

798

13.4

18.5

68/54

2168

2003

1932

7.6

10.9

68/51

1640

1499

1442

8.5

13.2

68/51

1468

1310

1223

10.7

16.7

68/51

514

440

394

14.3

23.3

16

Energy Savings Index (%)

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Fig. 14. Temperature variation with fan ON/OFF operation at (a) 5669 W/m2 (b) 11338 W/m2 (c) duty factor of 0.9 (d) duty factor of 0.79 (e) duty factor of 0.5.

factor). However, relative to HS-PCM an average reduction of 25% and 20% in cooling time was observed for HS-Gr/PCM corresponding to periodic duty factor of 0.9 and 0.79 respectively on account of faster

heat release through thermally conducting fillers. In Fig. 14(e), very steep decrease in temperature was observed for HS-PCM and HS-Gr/ PCM for a periodic duty factor of 0.5 due to longer heater pause time 17

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(90 s heater OFF) and hence for HS-PCM and HS-Gr/PCM the slopes were comparable to that of HS. Also, for this duty factor no significant difference in cooling time was observed between HS-PCM and HS-Gr/ PCM. In the case of 0.5 duty factor, faster cooling was achieved for HSPCM and HS-Gr/PCM and the cooling times were comparable to the heat sink without PCM (HS). This fact along with delayed fan starting time resulted in the high energy savings index with 0.5 periodic duty factor.

[5]

[6] [7]

4. Conclusions

[8]

The present study investigated the enhanced thermal conductivity effect of paraffin/graphene composite in a hybrid cooling system for thermal management of high power electronics subjected to uniform and pulsed heat loads. The hybrid cooling system was designed by combining passive PCM based heat sink with forced convection fan cooling. The major findings of the present work are as follows:

[9]

[10]

• The few-layered graphene synthesised by liquid phase exfoliation • • •

•

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technique have tremendous potential to enhance the thermal conductivity of paraffin even at low loading levels of 0.1–0.5 wt%. The change in viscosity of paraffin/graphene composite relative to paraffin was found to be negligible at temperatures beyond 65 °C, for the concentration in the range 0.2–0.5 wt%. In passive cooling mode, HS-Gr/PCM exhibited a lower instantaneous temperature rise relative to heat sink with paraffin (HSPCM) due to enhanced heat conduction in pre-melting stage and dominance of natural convection in post-melting stage of paraffin/ graphene composite. In hybrid cooling mode, the delay in fan onset time was found to be extended considerably for HS-Gr/PCM in contrast to HS-PCM and HS under both uniform and periodic thermal loading. The extended delay in fan onset time decreased with higher applied heat flux under uniform heating and increased with associated periodic duty factors in case of pulsed heating. The inclusion of paraffin/graphene composite in regular heat sinks can provide energy savings by reducing the fan operation time. By virtue of the extended fan onset and slightly modified cooling times due to enhanced thermal transport in paraffin/graphene composite, the energy savings quantified with HS-Gr/PCM were higher than HS-PCM and HS for all thermal loading scenarios.

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To summarize, irrespective of the applied thermal loads, the transient thermal performance of paraffin/graphene composite based heat sink outperformed the heat sinks with and without paraffin in both passive and hybrid cooling mode configurations. In a long-term perspective, the reduced fan operation can alleviate power consumption and noises in a hybrid cooling system.

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Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.114342.

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