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Effect of inclination on the thermal response of composite phase change materials for thermal energy storage
Applied Energy 238 (2019) 22–33
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Effect of inclination on the thermal response of composite phase change materials for thermal energy storage Xiaohu Yanga,b, Zengxu Guoa, Yanhua Liua, Liwen Jina, Ya-Ling Heb, a b
T
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Institute of the Building Environment & Sustainability Technology, School of Human Settlements and Civil Engineering, Xi’an Jiaotong University, Xi’an 710049, China Key Laboratory of Thermal Fluid Science and Engineering of MOE, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China
H I GH L IG H T S
experimental system is built to study the orientations effect on phase change. • Visual melting interface is observed in the experiment. • The angle significantly affects heat transfer in pure phase change material. • Inclination • Heat sink is designed using composite PCM without considering installation angle.
A R T I C LE I N FO
A B S T R A C T
Keywords: Phase change material Open-cell metal foam Inclination angle Natural convection Experimental measurement
Using paraffin as phase change material, the melting behaviors of pure phase change material and phase change material embedded in open-cell metal foams (composite phase change material) at different inclination angles are studied. Experimental system enabling continuous rotating is designed and built to explore the influence of inclination angles on phase interface evolution and temperature responses inside pure and composite phase change materials. Constant wall temperature is applied on one surface while others are thermally insulated. Experiments are carried out at angles of 0°, 30°, 60°, and 90° respectively. The effects of inclination on the melting behavior of phase change materials are analyzed by visualizing the solid-liquid interface and analyzing the temperature responses. Results demonstrate that the inclination angle has a great influence on the formation and development of natural convection during melting of pure phase change material, affecting the solid-liquid interface propagation and heat transfer rate. Compared with the case at 90°, the full melting time is reduced by 12.28%, 22.81% and 34.21% at 0°, 30° and 60°, respectively. However, when phase change material is melted in open-cell metal foam, heat conduction dominates, and inclination angle has little influence. Melting fractions at different inclination angles are the same and temperature curves at given points overlaps with each other.
1. Introduction According to the World Resources Institute (WRI), China surpassed the United States as the world's largest carbon emitter in 2009, and carbon dioxide emissions are growing rapidly due to the economic development and the increased energy consumption [1]. It was recently reported by National Energy Administration that China's energy consumption per unit GDP was on the rise, putting greater pressure on carbon emissions [2]. At present, fossil fuels still occupy a major part of the global energy supply. In 2016, fossil fuels (coal, oil and natural gas) accounted for 85.5% of the total global energy consumption [3]. In addition to looking for renewable energy as an alternative energy, there is still room for improvement of waste heat utilization [4,5].
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The latent heat storage system based on phase change materials (PCMs) can store thermal energy from industrial waste heat and release it to industrial process [6], favoring improving the overall energy utilization efficiency [7]. Besides this, PCMs have been widely used in solar power generation [8], solar energy heat collection [9], building [10], electronic heat management [11] and etc. However, their low thermal conductivity (e.g., paraffin is ∼0.2 W m−1 K−1 [12]) significantly limits the thermal storage/release efficiency [13]. Regarding this issue, continuous studies have been conducted to improve the heat transfer during phase change process. Adding fins [14], heat pipes [15], porous media [16], and nanoparticles into PCM [17] are the main strategies. Amongst the aforementioned techniques, embedding PCMs into open-cell metal foam has competitive advantages due mainly to
Corresponding author. E-mail address: [email protected] (Y.-L. He).
https://doi.org/10.1016/j.apenergy.2019.01.074 Received 14 October 2018; Received in revised form 4 January 2019; Accepted 11 January 2019 0306-2619/ © 2019 Published by Elsevier Ltd.
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Nomenclature
Symbols
Abbreviation
fm Fo Ra Ste k
PCM MF PPI WRI GDP
phase change material metal foam pore per inch world resources institute gross domestic product
melting fraction Fourier number Rayleigh number Stefan number thermal conductivity (W m−1 K−1)
Greek symbols θ
inclination angle
embedded in metal foam are mainly at fixed configuration without inclination. The intensity of natural convection in liquid PCM depends mainly on the geometric size of the container [35], inclined configuration and several dimensionless numbers, namely Ra, Ste and Fo number [36,37]. Investigations on the inclination effect upon melting of pure PCMs proved that melting rate as well as temperature response were significantly influenced by tilted configuration [38]. Literature review has shown that the effect of inclination angle on the phase change process is mainly concentrated in pure PCM and few research has been done on composite PCMs. Moreover, there do exist conflicts between the rare published results on the contribution of inclination angle to the melting heat transfer: Lafdi et al. [39] argued that orientation of the composite radiator have a significant effect on the system performance for stable heat source power. However, Baby and Bablji [40] commented that orientation has limited influence on the heat transfer performance of the radiator. Studying the orientation of the phase change heat storage container relative to the gravity field is of great significance for its practical application. On the one hand, the orientation affects the intensity of natural convection in the gravitational field, which in turn affects the efficiency of heat storage and release in the thermal storage system [41,42]. On the other hand, phase change cooling systems are also used for thermal management of portable electronic devices, and the direction of mobile devices is bound to change from time to time [43,44]. Therefore, the effect of orientation must be checked before the practical application of such a hybrid cooling system. In addition, in applications for solar collectors, the orientation of the thermal storage system is also adjustable [45]. In many applications of thermal storage systems, such as solar thermal storage and thermal management of electronic devices, the orientation of the heating surface was adjusted, and this will affect the efficiency of heat storage/release. However, the mechanism of the effect of orientation on heat transfer efficiency remains elusive, especially for composite PCMs. The systematical comparisons (temperature field and melting front visualization) between pure PCM and composite PCM during melting in a tilted cavity have not been reported in open literature. To this end, the purpose of this paper is to experimentally study the orientation effect of pure and composite PCMs on the heat transfer efficiency of thermal storage device. By comparing the pure PCM with composite one with respect to melting phase change behavior, the present study aims to provide benchmark and experimental reference for the practical applications of thermal storage system, especially in the aspect of configuration orientation. A well-designed experimental system enabling melting front visualization is built to study the effect of different inclination angles on heat transfer and melting behavior of pure paraffin and paraffin embedded in metal foam. The test section is hold by a continuously rotating plate. Particular attention is placed upon exploring the physical mechanisms underlying melting front evolution and justifying the contribution of natural convection to phase change behaviors for pure and composite PCMs subjected to various inclined configurations. Besides, 15 T-type thermocouples along x, y and z direction are located inside the PCM domain to get physical insights into the inclination effect on temperature responses.
their characteristics of high porosity for housing PCMs [18], highly conducting ligaments for transporting heat [19], large specific area for exchanging heat [20] and relatively low density for compact design [21]. A series of studies demonstrated that metal foam can significantly enhance phase change heat transfer. They have proved that the melting of PCM in an enclosed space is a transient process controlled by heat conduction and natural convection [22]. Particular attention has been paid to justify the contribution of natural convection to enhancing the overall heat transfer rate during phase change process. Yang et al. [23] studied the solidification behavior in a metal foam composite PCM for cold storage numerically and experimentally. The results showed that local natural convection in the unsolidified phase caused a remarkable promotion of the interface evolution. Tian and Zhao [24,25] numerically studied the enhancement of phase change heat transfer by metal foam. The results showed that the thermal conductivity of PCM was improved significantly by adding metal foam, but the natural convection was suppressed. Li et al. [26] employed seven high porosity copper foams to study the melting heat transfer of the embedded paraffin. The effects of porosity and pore density on the temperature response were studied. Zhang et al. [27] set up an experimental device to study the melting characteristics of composite PCMs made of copper foam and paraffin. The evolution of melting front and temperature change were observed and compared with the double-energy model. It was found that the heat transfer between paraffin and copper foam has thermal non-equilibrium effect. A detailed preparation procedure for fabricating composite PCM (metal foam/paraffin) was reported by Xiao et al. [28–30]. They prepared several nickel and copper foams with saturated by paraffin. The thermal behavior of composite PCMs were analyzed by differential scanning calorimeter (DSC). It indicated that the thermal conductivity of the composite PCM was remarkably higher than that of pure paraffin. Recently, Yang et al. [31] symmetrically compared volume-averaged method with direct numerical simulation on the melting heat transfer in PCM embedded in open-cell copper foam. Two-temperature model accounting for temperature difference between PCM and metallic ligaments was employed and the direct numerical simulation was performed based on the reconstructed 3D microstructures. Results revealed the feasibility and necessity of applying the two-temperature model. The pore-scale features of local temperature distribution and natural convection flow field were also observed by three-dimensional temperature and velocity field. Chen et al. [32] employed infrared microscope and optical microscope to measure the temperature field to capture the melting evolution of PCM embedded in metal foam. Pore-scale numerical simulation was performed for comparison. They found that the local natural convection was suppressed but heat conduction was greatly enhanced. Overall, metal foam contributed remarkably to accelerate the melting front propagation. A series of studies showed that although natural convection was suppressed by adding metal foam into PCM, it also had a great influence on the formation of the slant shape of phase interface and the heat transfer during melting [33,34]. Previous studies regarding the phase change process in PCM 23
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2. Experimental measurement As shown in Fig. 1, a cuboid block of copper foam with a size of 28 mm (length) × 68 mm (width) × 68 mm (height) is prepared by means of wire cutting. The pore density of copper foam is 10 PPI (pore per inch) and porosity is 0.96. In order to facilitate the replacement of copper foam specimens and make them in close contact with the heating substrate, thermal conductive adhesive (thermal conductivity 25 W m−1 K−1) was used to bond the copper sheets with a thickness of 2 mm to the copper foam. In this experiment, liquid paraffin with melting range of 47–60 °C and thermal conductivity of 0.2 W m−1 K−1 is filled in copper foam under a vacuum environment of 94 Pa to ensure that the voids in the metal foam are completely filled. After paraffin is cooled and solidified, the excess part is removed and the preparation of copper foam PCM composite is done (see Fig. 1(b)). In order to study the effect of different inclination angles and natural convection on the melting process of paraffin, a visualized phase change heat transfer experimental system is set up. The experimental system is mainly composed of a rotating angle device, a constant temperature water tank, a phase change test section, a high definition camera and a data acquisition device, as shown in Fig. 2. The constant temperature water tank uses distilled water as heating medium. Considering the melting point of paraffin and the experiment time, the constant wall temperature of 70 °C is provided for the melting of paraffin in this experiment. In order to ensure the stability of test section, the angle of the whole test section is determined by the universal angle ruler (precision is 0.02°) and the bolt is fixed on the bracket of the rotating angle device. The thermal storage process of pure PCM and the composite PCM at angles of 0°, 30°, 60° and 90° are studied respectively. Details on testing cases refer to Table 1. The water tank simulates a heat source in which the heat transfer fluid flows out, and the heat transfer fluid and the phase change material exchange heat through the plate heat exchanger. The experimental system can simulate an industrial waste heat recovery system or cooling of the electronic devices. In these phase change thermal storage devices, heat conduction and natural convection are the main modes of heat transfer, and the proportion of radiative heat transfer is small, considering the low temperature range in experiments. For the test section, metal foam sample is housed by a Perspex (k ∼ 0.2 W m−1 K−1) container with a thickness of 20 mm. Perspex is purposely used not only to enable easy visualization of melting front during the whole phase change process, but also to reduce the heat loss
Fig. 2. Schematic illustration of experimental system for visualization of melting front in metal foam-paraffin composite PCM.
from the side walls. Metal foam sample is bonded to a plate heat exchanger where high temperature fluid flows through to keep a constant thermal boundary. Three T-type thermocouples are arranged on the surface of the heating plate to monitor boundary wall temperature. A heat conductive adhesive with thermal conductivity of 25 W m−1 K−1 is filled in the gap between the test specimen and the microchannel plate heat exchanger to reduce the interfacial contact thermal resistance. To reduce the heat loss from the backward of the plate heat exchanger, the plate is embedded in polyurethane foam with thermal conductivity of 0.02 W m−1 K−1. Besides, the whole test section is covered by an insulation cotton with thermal conductivity of 0.02 W m−1 K−1 to thermally insulate from the ambient environment. To quantitatively estimate the overall heat loss from test section during experiment, Fourier heat conduction equation is employed provided the temperature of internal and external surfaces for the insulated cotton are measured. It reveals a 1.5% of heat loss against the total stored energy. To obtain the transient temperature distribution in PCM, 15 T-type thermocouples are used to record the temperature, as shown in Fig. 3(a). The bottom gray surface denotes the heating surface and the other five surfaces can be regarded as adiabatic. Photos are taken on the right side. To house the thermocouples, five holes are drilled on the upper surface of the Perspex container. Each is filled with a wooden rod with a diameter of 2 mm, who carries three T-type thermocouples (see Fig. 3(b)). T-type thermocouples are tied to the fixed position (7 mm,
Fig. 1. Test sample: (a) size demonstration; (b) sample with infiltrated by paraffin. 24
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Table 1 Test cases under different inclination angles. Case
Case1
Case2
Case3
Case4
Case5
Case6
Case7
Case8
Specimen Angle
PCM 0°
PCM 30°
PCM 60°
PCM 90°
PCM + MF 0°
PCM + MF 30°
PCM + MF 60°
PCM + MF 90°
Note: Here PCM stands for phase change material, MF denotes metal foam.
measuring points are basically the same, and there is little difference among them for the four scenarios. The maximum temperature difference appears in Fig. 4 (a), reaching a maximum temperature difference of 1.6 °C near 2000 s and a relative temperature difference of 3.4%. The maximum root temperature difference is 0.6 °C in Fig. 4(a). The group comparisons for other test points along y axis in different x-y planes (marked as b and c points in Fig. 3(a)) under different inclination angles (30° and 60°) are also conducted. Limited to space, this article only enumerates four sets of results in Fig. 4.
14 mm and 21 mm from the bottom heating surface) of the small wooden rods by thin wires and the thermocouple head is exposed. Series 2 is located in the center, and the distance between the other four series to series 2 is 17 mm. Experimental procedure is as follows: first open valve V1 and close valve V2, V3; and then turn on the heating function of thermostatic bath until the water temperature reaches the set one (70 °C); Second, open valve V2, V3 and close valve V1, so that high-temperature fluid flows through the plate heat exchanger. Simultaneously, turn on the data acquisition device for temperature recording; set the acquisition time interval as every 30 s and trigger on image capturing. The realtime position of the phase interface is recorded by a high-definition camera in every three minutes to show the transient phase transition until paraffin is completely melt.
3.2. Propagation of melting front Fig. 5 shows the real-time phase interface positions of PCM at angles of 0°, 30°, 60° and 90°. The white opaque part of the picture depicts solid paraffin, and the black part represents liquid paraffin. Number (1)–(16) separately denotes the image of melting front at different time and inclination angles. Fig. 5(1)–(4) indicate the melting process of paraffin in the horizontal (θ = 0°) enclosure. At the beginning, the interface is macroscopically horizontal, and heat transfer is mainly heat conduction. As time goes on, the interface gradually appears serrated, as shown in Fig. 5(2). This is because of the complex natural convection in the liquid region, which is a form of Rayleigh-Bernard convection. In the Bernard convection unit, the hot liquid paraffin rises perpendicular to the heating wall in the central region of the unit, and falls back at the edge of the unit after washing the solid paraffin. As can be seen from Fig. 5(3)–(4), the number of serrations at the interface is decreasing. This is because the liquid region grows thicker, the Bernard cell merges with the adjacent convective cell and becomes larger, and the homogeneity of the Bernard cell decreases. For the case at 30°, the tilted configuration leads to a long and narrow room for the melt phase to flow. Bernard cells are significantly reduced. A portion of fluid flows upwardly in parallel to the heating wall and the rest is still washing the solid-liquid interface. Observing in Fig. 5(5)–(8), it can be found that the dark area is smaller than that for the case at 0°. As θ is further increases, more fluid flows in parallel to the heating wall but less washes against the interface. Thus it can be
3. Results and discussion 3.1. Simplification of dimensions (temperature distribution along y axis) The visualization experiment in this paper is carried out in a cuboid container, which is a three-dimensional situation. Given the symmetry nature of the cuboid geometry and the uniform temperature on the heating face (constant temperature boundary), three-dimensional case may be reduced to a two-dimensional one. This helps to calculate melting rate (the ratio of melt volume to the whole one) according to the two-dimensional (x-z plane) images of melting front captured by the high-resolution camera. Section 3.2 will illustrate the details for the two-dimensional experimental photographs for melting front and the calculated melting rate. To this end, 15 T-type thermocouples are placed inside the specimen to monitor the real time temperature of PCM for both cases during melting process. Among them, three points (2a, 4a, 5a) with the same x and z coordinates but different y coordinates (17, 34 and 51 mm) are selected. Fig. 4 shows the temperature changes of three measuring points for the cases of PCM and PCM-MF under inclination angle of 0° and 90°. It can be seen that the temperature trends of the three
Fig. 3. (a) Design of thermocouples locations; (b) real thermocouples used in the experiments. 25
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Fig. 4. Temperature histogram at fixed points along y axis for the case of PCM ((a) and (b)) and PCM + MF ((c) and (d)).
propagation of melting front. Both heat conduction and the natural convection work together to transport heat from heating wall to the solid-liquid phase interface.
expected that the melting rate of paraffin at 60° is further reduced. For the case at 90° (Fig. 5(13)–(16)), the dark area (melting rate) is even lower than the previous three cases. Unlike Fig. 5(9), the liquid region in Fig. 5(13) is thinner at the same time, especially at the lower part of the liquid region. Except the case at 0°, the melting front shapes are similar for the other three cases. This can be explained, taking 90° for example, as follows: at the initial stage of melting, a thin layer of melted paraffin appears near the heating wall, where heat conduction is dominant. As time goes on, the layer of liquid paraffin is thickened, favoring sufficient room for natural convection to develop. The temperature in the liquid region near the heating wall is higher and moves upwardly, while the one with relatively low temperature near the phase interface moves downwardly. Under the action of natural convection, the upper part of the liquid region melts rapidly, and eventually the phase interface tilts. With time elapsed, the phase interface continues to tilt, showing the state as illustrated in Fig. 5(15) and (16) until the end of melting. Fig. 6 visualizes the melting process of paraffin embedded in metal foam at angles of 0°, 30°, 60° and 90°. When the inclination angle is 0°, the solid-liquid interface is basically horizontal, and there is also a serrated interface. This phenomenon can be seen in Fig. 6(1)–(4). This is due mainly to the formation of Bernard flow. Fig. 6(5)–(8) and Fig. 6(9)–(12) show the melting process of paraffin embedded in metal foam at inclination of 30° and 60° respectively. The interface shows similar slope shape, reflecting the contribution of natural convection to the melting process. Compared with the corresponding pictures in Fig. 5, we can find that the phase interface in Fig. 6(5)–(16) is almost a slanted line, i.e. very small curvature, while the interface curvature in Fig. 5 is larger. This can be explained as follows: when pure paraffin melts, liquid paraffin caused by natural convection significantly washes the solid paraffin, and the upper part receives a quantity of thermal energy transported by natural convection. As for the composite PCM, heat conduction through metal skeleton contributes a lot to the
3.3. Melting fraction The melting fraction of PCM refers to the volume ratio of liquid PCM to the whole one. As mentioned above, the two-dimensional phase interface can represent the three-dimensional one. It is therefore reasonable to use the area ratio of the desired interface to represent the volume ratio. Fig. 7 compares the variation of melting fraction with time for PCM at 0° 30°, 60°, and 90°. It can be seen that the eight curves separately in Fig. 7(a) and (b) show similar trend of fm as a function of time. When pure paraffin melts, the full melting time of PCMs becomes shorter and shorter with the decrease of inclination angle θ. This is in consistence with the observation of melting front in Fig. 5. Setting the case at 90° as the comparison basis, the full melting time of paraffin at 0°, 30° and 60° decreases by 12.28%, 22.81% and 34.21%, in respective. This may be due mainly to the different intensity of natural convection. The shortest path of natural convection of liquid paraffin is at 0°, and the washing effect on solid paraffin is the strongest. On the contrary, liquid paraffin moves in parallel to the heating wall at 90°, and the washing effect on solid paraffin is weak. The situations at 30° and 60° are between these two limits. Fig. 7(b) compares the melting fraction of PCM embedded in metal foam at 0° 30°, 60°, and 90°. It is found that the four curves overlaps and the full melting time of paraffin embedded in metal foam has no obvious difference at different inclination angles, where the maximum deviation is 4.35%. This can be also confirmed by the observations in Fig. 6. Possible explanations can be made as follows: The inclined configuration affects the development of natural convection in a cavity. On one hand, the involvement of metal foam significantly suppresses the natural convection in the melt phase; on the other hand, highly26
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Fig. 5. Melting front of pure paraffin at four selected time under inclination angle of 0°, 30°, 60° and 90°.
part. Liquid PCM has the highest temperature at the upper part, resulting in heat accumulation in the upper part. Fig. 8(b) demonstrates the temperature change at point 2b in the middle part. Similar to the temperature trend in Fig. 8(a), the temperature in PCM increases monotonically. It shows that the temperature rises fastest at 0° and becomes slower at 30°, 60° and 90°. The difference among the four temperature curves becomes larger than that in Fig. 8(a). The main heat transfer mode at middle part at 0° is natural convection. As the configuration is further inclined (θ is increasing), the heat transfer mode is gradually turning from convection-dominated to conduction-dominated, thus leading to larger difference among the four temperature curves. This is more obvious in Fig. 8(c) for the lower part. For the composite PCM, Fig. 8(d)–(f) demonstrate the temperature changes at points 1b, 2b and 3b at different inclination angles. Exactly the same monitoring points are chosen as the ones in pure PCM configuration. It is evident that the three groups of curves overlap very well in the early and middle stages of melting, and there is no obvious difference among the four curves at each points. This indicates that heat conduction occupies the majority of heat transfer in the early and middle stage of melting. At the later stage of melting, the temperature curves at different angles begin to separate, especially at position 3b; and it finally shows that the temperature rises the fastest at 0° followed by 30°, 60° and 90°. In the experiments with different inclination angles, the heat conduction of the metal skeleton is basically the same (the same specimen). It is the natural convection that eventually leads to the separation of the temperature curves. Compared with the larger difference in pure PCM, the difference among the four curves is much smaller. This may be due mainly to the conduction-dominated heat transfer mode when metal foam is involved.
conducting metal ligaments greatly help to transport thermal energy deeply into the unmelt region, enjoying the dominate role in the overall melting process. Therefore, when the effect of natural convection is no longer dominated, the melting rate of paraffin in metal foam structure will not dramatically affected by different inclination angles. 3.4. Temperature response Using thermocouples to record temperature data can provide accurate information for studying the effect of different inclination angles on the thermal response of PCM in melting process. As the configuration is inclined, the responses of temperature under different inclination angles are different. Three typical points locating at upper, middle and lower region (referring to 90°) are selected. As can be seen in Fig. 8(a), it is demonstrated that the temperature rises the fastest at inclination of 90°, followed by 60°, 30° and 0°. This is distinctively the opposite to those of points in the middle and lower parts (Fig. 8(b) and (c)), i.e. temperature at 0° rises the fastest and the one at 90° is the slowest. Nevertheless, the difference among the four curves in Fig. 8(a) is not larger compared with the ones in Fig. 8(b) and (c). This can be explained as follows: although the overall natural convection is the strongest at 0° and the melting front evolves the fastest among the four inclination angles (0°, 30°, 60° and 90°), the local natural convection at the upper part of the PCM is also strong at 90°. As can be seen in the solid-liquid interface images in Fig. 5(2), (6), (10) and (14), the local melting fraction at the upper part (highlighted in red circle) at 90° is the highest. When the inclination angle is not 0° (horizontal), the local natural convection of the upper PCM is stronger than that of the lower part, and heat transfer is correspondingly larger than that of the lower 27
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Fig. 6. Melting front of composite PCM (paraffin embedded in metal foam) at four selected time under inclination angle of 0°, 30°, 60° and 90°.
melts at 30°, 60° and 90°, in respective. Different from the trend of the three curves in Fig. 9 (a), the curve separation trend in Fig. 9(b), (c) and (d) is more obvious. As the inclination angle increases, the separation tendency of the three curves in Fig. 9(b), (c) and (d) is also increasing. This can be understood as: as inclinational angle increases, the uniformly-intensive natural convection is gradually weakening. When θ = 90°, non-uniformity natural convection intensity can be confirmed by the slanted solid-liquid interface in Fig. 5. Fig. 10 compares the temperature variations at three selected points along x axis for composite PCM at an inclination of 0°, 30°, 60° and 90°. It is found that the four groups of temperature curves demonstrate a
3.5. Temperature distribution
1.0
1.0
0.8
0.8
0.6
0.6 PCM
0.4
= 0° = 30° = 60° = 90°
0.2 0.0 0
fm
fm
3.5.1. Temperature distribution along x axis Fig. 9(a) shows the temperature change of the measuring points 1a, 2a and 3a when pure paraffin melts at 0°. It is found that the temperature changes of the measuring points 1a, 2a and 3a are substantially the same. This can be also confirmed by the fact that the solidliquid phase interface in each of Fig. 5(1)–(4) is parallel to the heating wall. The natural convection intensity is basically the same thanks to the formation of Bernard flows. Fig. 9(b), (c) and (d) demonstrate the temperature changes of the points 1a, 2a and 3a when pure paraffin
1000
2000
3000
4000
5000
6000
PCM+MF = 0° = 30° = 60° = 90°
0.4 0.2
7000
0.0 0
1000
2000
3000
t/s
t/s
(a)
(b)
4000
5000
Fig. 7. Melting fraction for two cases (a) pure paraffin and (b) composite PCM under four inclination angles of 0°, 30°, 60° and 90°. 28
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Fig. 8. Temperature histograms at selected points for the cases of pure PCM ((a)–(c)) and composite PCM ((d)–(f)) under different inclination angles.
large temperature difference does exist with subjected to non-zero inclined angles.
similar variation trend like conduction-dominated temperature curve, where the whole heat storage process can be clearly divided into three stages: the initial stage is the sensible heat storage stage; the temperature rises linearly, then the paraffin begins to melt and enters the latent heat storage stage. Paraffin melts and absorbs a large amount of heat, with the curve slope becoming smaller. After paraffin is melted, it enters the third stage and returns to the sensible heat storage stage. As it can be seen, the temperature at three points begins to separate within the third stage under the inclination angle of 30°, 60° and 90°. This may be understood by the fact that natural convection intensity is significantly increased in the third stage since there is sufficient room for natural convection after a large portion of solid PCM is melted. Recalling the situations in the pure PCM, no extra resistance like porous matrix exists to hinder the development of natural convection, where
3.5.2. Temperature distribution along z axis Fig. 11 illustrates the temperature variation as a function of time at three points along z axis. The three points have the same x and y coordinates. The depths away from the heating surface are 7 mm, 14 mm, and 21 mm, in respective. There is large difference in temperature at the three points under four inclined angles. Compared the case of 0° in Fig. 9(a), large difference does exist along z axis. When pure PCM melts at 0°, the root mean square temperature difference between 2b and 2c is 4.8 °C and the one between 2a and 2b is 6.9 °C. And the local melting times at points 2a, 2b, and 2c are 1020 s, 720 s, and 600 s, respectively When the composite PCM melts at 0°, the root mean square temperature 29
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Fig. 9. Temperature histograms of pure PCM at selected points along x axis under four inclination angles: (a) 0°; (b) 30°; (c) 60°; (d) 90°.
Fig. 10. Temperature histograms of composite PCM at selected points along x axis under four inclination angles: (a) 0°; (b) 30°; (c) 60°; (d) 90°. 30
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Fig. 11. Temperature histograms of pure PCM at selected points along z axis under four inclination angles: (a) 0°; (b) 30°; (c) 60°; (d) 90°.
Fig. 12. Temperature histograms of composite PCM at selected points along x axis under four inclination angles: (a) 0°; (b) 30°; (c) 60°; (d) 90°.
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to qualitatively and quantitatively underline the thermophysics for phase change heat transfer through analyzing the distributions of flow and temperature field, and optimization will be performed for practical engineering applications.
Table 2 Uncertainties for different variables. Parameters
Uncertainty
Size of PCM and composite PCM (mm) Temperature measured by thermocouples (°C) Porosity of copper foam (%) Melt fraction (%)
± 0.02 ± 0.2 ± 0.5 ± 0.24
Acknowledgement This work was supported by the National Natural Science Foundation of China (51506160), Natural Science Foundation of Shaanxi Province (2017JQ5007) and the fundamental research funds for central universities (xjj2016042).
Note: here PCM stands for phase change material.
differences between 2a and 2b, 2b and 2c are 3.7 °C and 3.5 °C respectively, which are smaller than those of pure PCM. It shows that the uniformity of PCM is improved after the addition of metal foam. And the local melting times at points 2a, 2b, and 2c are 570 s, 540 s, and 540 s, respectively (see Fig. 12).
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4. Uncertainty analysis The uncertainty of experimental results is often affected by the inevitable errors in experimental measurement, and depends on the uncertainty of measuring instruments. All thermocouples are connected to a temperature scanner (Agilent 34970A) to monitor their temperature changes throughout the phase transition process. And these are calibrated by Omega CL3515R calibration system before testing. The uncertainty of measurement is estimated to be ± 0.2 °C. The uncertainties of all the parameters are listed in Table 2. 5. Conclusions In this paper, the melting behaviors of pure phase change material and phase change material embedded in open-cell metal foam are experimentally studied. Melting experiments are carried out at 0°, 30°, 60° and 90° inclination angles, subjected to a single-sided constant wall temperature boundary in a cuboid container housing phase change materials. The effect of inclination angle on the melting process of phase change material is demonstrated by analyzing the visible solidliquid interface and the temperature field in phase change material. Main conclusions can be drawn as follows: (1) Compared with the case at 90°, the full melting time for pure phase change material is reduced by 12.28%, 22.81% and 34.21% at 60°, 30° and 0°, respectively. However, little influence (maximum is 4.35%) is found in full melting time in composite phase change material under inclined situations. (2) Melting front visualization demonstrates that the curvature for pure phase change material is larger than that for the composite one. The visualized solid-liquid phase interface clearly demonstrates that natural convection significantly affects heat transfer in pure phase change material but contributes little to composite phase change material for an inclined configuration. (3) For pure phase change material, local melting time decreases as an increase in distance away from heating boundary for all inclined configurations (the local melting times at points 2a, 2b, and 2c are 1020 s, 720 s, and 600 s, respectively); while for composite phase change material, there is little difference. (4) The root mean square temperature difference between different points in vertical to the heating boundary wall is smaller for composite phase change material than that for pure phase change material. This indicates that a better uniformity is obtained with metal foam involved. (5) The results can be applied to phase-change thermal management systems with adjustable orientation, such as cooling of portable electronic devices, solar thermal collection (tracing solar direction), and phase change thermal storage devices for industrial waste heat recovery. For future work, numerical simulations will be conducted
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[35] El Omari K, Kousksou T, Le Guer Y. Impact of shape of container on natural convection and melting inside enclosures used for passive cooling of electronic devices. Appl Therm Eng 2011;31:3022–35. [36] Sheikholeslami M. Influence of magnetic field on nanofluid free convection in an open porous cavity by means of Lattice Boltzmann method. J Mol Liq 2017;234:364–74. [37] Sheikholeslami M, Rokni HB. Melting heat transfer influence on nanofluid flow inside a cavity in existence of magnetic field. Int J Heat Mass Transf 2017;114:517–26. [38] Kamkari B, Shokouhmand H, Bruno F. Experimental investigation of the effect of inclination angle on convection-driven melting of phase change material in a rectangular enclosure. Int J Heat Mass Transf 2014;72:186–200. [39] Lafdi K, Mesalhy O, Elgafy A. Merits of employing foam encapsulated phase change materials for pulsed power electronics cooling applications. J Electron Packag 2008;130:021004–21008. [40] Baby R, Balaji C. Experimental investigations on thermal performance enhancement and effect of orientation on porous matrix filled PCM based heat sink. Int Commun Heat Mass Transf 2013;46:27–30. [41] Yang Y-T, Wang Y-H. Numerical simulation of three-dimensional transient cooling application on a portable electronic device using phase change material. Int J Therm Sci 2012;51:155–62. [42] Kamkari B, Groulx D. Experimental investigation of melting behaviour of phase change material in finned rectangular enclosures under different inclination angles. Exp Therm Fluid Sci 2018;97:94–108. [43] Wu W, Zhang G, Ke X, Yang X, Wang Z, Liu C. Preparation and thermal conductivity enhancement of composite phase change materials for electronic thermal management. Energy Convers Manage 2015;101:278–84. [44] Rao Z, Wang Q, Huang C. Investigation of the thermal performance of phase change material/mini-channel coupled battery thermal management system. Appl Energy 2016;164:659–69. [45] Papadimitratos A, Sobhansarbandi S, Pozdin V, Zakhidov A, Hassanipour F. Evacuated tube solar collectors integrated with phase change materials. Sol Energy. 2016;129:10–9.
2018;133:252–65. [23] Yang X, Bai Q, Zhang Q, Hu W, Jin L, Yan J. Thermal and economic analysis of charging and discharging characteristics of composite phase change materials for cold storage. Appl Energy 2018;225:585–99. [24] Tian Y, Zhao CY. A numerical investigation of heat transfer in phase change materials (PCMs) embedded in porous metals. Energy 2011;36:5539–46. [25] Han XX, Tian Y, Zhao CY. An effectiveness study of enhanced heat transfer in phase change materials (PCMs). Int J Heat Mass Transf 2013;60:459–68. [26] Li WQ, Qu ZG, He YL, Tao WQ. Experimental and numerical studies on melting phase change heat transfer in open-cell metallic foams filled with paraffin. Appl Therm Eng 2012;37:1–9. [27] Zhang P, Meng ZN, Zhu H, Wang YL, Peng SP. Melting heat transfer characteristics of a composite phase change material fabricated by paraffin and metal foam. Appl Energy 2017;185:1971–83. [28] Xiao X, Zhang P, Li M. Preparation and thermal characterization of paraffin/metal foam composite phase change material. Appl Energy 2013;112:1357–66. [29] Zhang P, Ma F, Xiao X. Thermal energy storage and retrieval characteristics of a molten-salt latent heat thermal energy storage system. Appl Energy 2016;173:255–71. [30] Zhang P, Xiao X, Ma ZW. A review of the composite phase change materials: Fabrication, characterization, mathematical modeling and application to performance enhancement. Appl Energy 2016;165:472–510. [31] Yang X, Bai Q, Guo Z, Niu Z, Yang C, Jin L, et al. Comparison of direct numerical simulation with volume-averaged method on composite phase change materials for thermal energy storage. Appl Energy 2018;229:700–14. [32] Chen Z, Gao D, Shi J. Experimental and numerical study on melting of phase change materials in metal foams at pore scale. Int J Heat Mass Transf 2014;72:646–55. [33] Alipanah M, Li X. Numerical studies of lithium-ion battery thermal management systems using phase change materials and metal foams. Int J Heat Mass Transf 2016;102:1159–68. [34] Esapour M, Hamzehnezhad A, Rabienataj Darzi AA, Jourabian M. Melting and solidification of PCM embedded in porous metal foam in horizontal multi-tube heat storage system. Energy Convers Manage 2018;171:398–410.
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Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Effect of inclination on the thermal response of composite phase change materials for thermal energy storage Xiaohu Yanga,b, Zengxu Guoa, Yanhua Liua, Liwen Jina, Ya-Ling Heb, a b
T
⁎
Institute of the Building Environment & Sustainability Technology, School of Human Settlements and Civil Engineering, Xi’an Jiaotong University, Xi’an 710049, China Key Laboratory of Thermal Fluid Science and Engineering of MOE, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China
H I GH L IG H T S
experimental system is built to study the orientations effect on phase change. • Visual melting interface is observed in the experiment. • The angle significantly affects heat transfer in pure phase change material. • Inclination • Heat sink is designed using composite PCM without considering installation angle.
A R T I C LE I N FO
A B S T R A C T
Keywords: Phase change material Open-cell metal foam Inclination angle Natural convection Experimental measurement
Using paraffin as phase change material, the melting behaviors of pure phase change material and phase change material embedded in open-cell metal foams (composite phase change material) at different inclination angles are studied. Experimental system enabling continuous rotating is designed and built to explore the influence of inclination angles on phase interface evolution and temperature responses inside pure and composite phase change materials. Constant wall temperature is applied on one surface while others are thermally insulated. Experiments are carried out at angles of 0°, 30°, 60°, and 90° respectively. The effects of inclination on the melting behavior of phase change materials are analyzed by visualizing the solid-liquid interface and analyzing the temperature responses. Results demonstrate that the inclination angle has a great influence on the formation and development of natural convection during melting of pure phase change material, affecting the solid-liquid interface propagation and heat transfer rate. Compared with the case at 90°, the full melting time is reduced by 12.28%, 22.81% and 34.21% at 0°, 30° and 60°, respectively. However, when phase change material is melted in open-cell metal foam, heat conduction dominates, and inclination angle has little influence. Melting fractions at different inclination angles are the same and temperature curves at given points overlaps with each other.
1. Introduction According to the World Resources Institute (WRI), China surpassed the United States as the world's largest carbon emitter in 2009, and carbon dioxide emissions are growing rapidly due to the economic development and the increased energy consumption [1]. It was recently reported by National Energy Administration that China's energy consumption per unit GDP was on the rise, putting greater pressure on carbon emissions [2]. At present, fossil fuels still occupy a major part of the global energy supply. In 2016, fossil fuels (coal, oil and natural gas) accounted for 85.5% of the total global energy consumption [3]. In addition to looking for renewable energy as an alternative energy, there is still room for improvement of waste heat utilization [4,5].
⁎
The latent heat storage system based on phase change materials (PCMs) can store thermal energy from industrial waste heat and release it to industrial process [6], favoring improving the overall energy utilization efficiency [7]. Besides this, PCMs have been widely used in solar power generation [8], solar energy heat collection [9], building [10], electronic heat management [11] and etc. However, their low thermal conductivity (e.g., paraffin is ∼0.2 W m−1 K−1 [12]) significantly limits the thermal storage/release efficiency [13]. Regarding this issue, continuous studies have been conducted to improve the heat transfer during phase change process. Adding fins [14], heat pipes [15], porous media [16], and nanoparticles into PCM [17] are the main strategies. Amongst the aforementioned techniques, embedding PCMs into open-cell metal foam has competitive advantages due mainly to
Corresponding author. E-mail address: [email protected] (Y.-L. He).
https://doi.org/10.1016/j.apenergy.2019.01.074 Received 14 October 2018; Received in revised form 4 January 2019; Accepted 11 January 2019 0306-2619/ © 2019 Published by Elsevier Ltd.
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Nomenclature
Symbols
Abbreviation
fm Fo Ra Ste k
PCM MF PPI WRI GDP
phase change material metal foam pore per inch world resources institute gross domestic product
melting fraction Fourier number Rayleigh number Stefan number thermal conductivity (W m−1 K−1)
Greek symbols θ
inclination angle
embedded in metal foam are mainly at fixed configuration without inclination. The intensity of natural convection in liquid PCM depends mainly on the geometric size of the container [35], inclined configuration and several dimensionless numbers, namely Ra, Ste and Fo number [36,37]. Investigations on the inclination effect upon melting of pure PCMs proved that melting rate as well as temperature response were significantly influenced by tilted configuration [38]. Literature review has shown that the effect of inclination angle on the phase change process is mainly concentrated in pure PCM and few research has been done on composite PCMs. Moreover, there do exist conflicts between the rare published results on the contribution of inclination angle to the melting heat transfer: Lafdi et al. [39] argued that orientation of the composite radiator have a significant effect on the system performance for stable heat source power. However, Baby and Bablji [40] commented that orientation has limited influence on the heat transfer performance of the radiator. Studying the orientation of the phase change heat storage container relative to the gravity field is of great significance for its practical application. On the one hand, the orientation affects the intensity of natural convection in the gravitational field, which in turn affects the efficiency of heat storage and release in the thermal storage system [41,42]. On the other hand, phase change cooling systems are also used for thermal management of portable electronic devices, and the direction of mobile devices is bound to change from time to time [43,44]. Therefore, the effect of orientation must be checked before the practical application of such a hybrid cooling system. In addition, in applications for solar collectors, the orientation of the thermal storage system is also adjustable [45]. In many applications of thermal storage systems, such as solar thermal storage and thermal management of electronic devices, the orientation of the heating surface was adjusted, and this will affect the efficiency of heat storage/release. However, the mechanism of the effect of orientation on heat transfer efficiency remains elusive, especially for composite PCMs. The systematical comparisons (temperature field and melting front visualization) between pure PCM and composite PCM during melting in a tilted cavity have not been reported in open literature. To this end, the purpose of this paper is to experimentally study the orientation effect of pure and composite PCMs on the heat transfer efficiency of thermal storage device. By comparing the pure PCM with composite one with respect to melting phase change behavior, the present study aims to provide benchmark and experimental reference for the practical applications of thermal storage system, especially in the aspect of configuration orientation. A well-designed experimental system enabling melting front visualization is built to study the effect of different inclination angles on heat transfer and melting behavior of pure paraffin and paraffin embedded in metal foam. The test section is hold by a continuously rotating plate. Particular attention is placed upon exploring the physical mechanisms underlying melting front evolution and justifying the contribution of natural convection to phase change behaviors for pure and composite PCMs subjected to various inclined configurations. Besides, 15 T-type thermocouples along x, y and z direction are located inside the PCM domain to get physical insights into the inclination effect on temperature responses.
their characteristics of high porosity for housing PCMs [18], highly conducting ligaments for transporting heat [19], large specific area for exchanging heat [20] and relatively low density for compact design [21]. A series of studies demonstrated that metal foam can significantly enhance phase change heat transfer. They have proved that the melting of PCM in an enclosed space is a transient process controlled by heat conduction and natural convection [22]. Particular attention has been paid to justify the contribution of natural convection to enhancing the overall heat transfer rate during phase change process. Yang et al. [23] studied the solidification behavior in a metal foam composite PCM for cold storage numerically and experimentally. The results showed that local natural convection in the unsolidified phase caused a remarkable promotion of the interface evolution. Tian and Zhao [24,25] numerically studied the enhancement of phase change heat transfer by metal foam. The results showed that the thermal conductivity of PCM was improved significantly by adding metal foam, but the natural convection was suppressed. Li et al. [26] employed seven high porosity copper foams to study the melting heat transfer of the embedded paraffin. The effects of porosity and pore density on the temperature response were studied. Zhang et al. [27] set up an experimental device to study the melting characteristics of composite PCMs made of copper foam and paraffin. The evolution of melting front and temperature change were observed and compared with the double-energy model. It was found that the heat transfer between paraffin and copper foam has thermal non-equilibrium effect. A detailed preparation procedure for fabricating composite PCM (metal foam/paraffin) was reported by Xiao et al. [28–30]. They prepared several nickel and copper foams with saturated by paraffin. The thermal behavior of composite PCMs were analyzed by differential scanning calorimeter (DSC). It indicated that the thermal conductivity of the composite PCM was remarkably higher than that of pure paraffin. Recently, Yang et al. [31] symmetrically compared volume-averaged method with direct numerical simulation on the melting heat transfer in PCM embedded in open-cell copper foam. Two-temperature model accounting for temperature difference between PCM and metallic ligaments was employed and the direct numerical simulation was performed based on the reconstructed 3D microstructures. Results revealed the feasibility and necessity of applying the two-temperature model. The pore-scale features of local temperature distribution and natural convection flow field were also observed by three-dimensional temperature and velocity field. Chen et al. [32] employed infrared microscope and optical microscope to measure the temperature field to capture the melting evolution of PCM embedded in metal foam. Pore-scale numerical simulation was performed for comparison. They found that the local natural convection was suppressed but heat conduction was greatly enhanced. Overall, metal foam contributed remarkably to accelerate the melting front propagation. A series of studies showed that although natural convection was suppressed by adding metal foam into PCM, it also had a great influence on the formation of the slant shape of phase interface and the heat transfer during melting [33,34]. Previous studies regarding the phase change process in PCM 23
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2. Experimental measurement As shown in Fig. 1, a cuboid block of copper foam with a size of 28 mm (length) × 68 mm (width) × 68 mm (height) is prepared by means of wire cutting. The pore density of copper foam is 10 PPI (pore per inch) and porosity is 0.96. In order to facilitate the replacement of copper foam specimens and make them in close contact with the heating substrate, thermal conductive adhesive (thermal conductivity 25 W m−1 K−1) was used to bond the copper sheets with a thickness of 2 mm to the copper foam. In this experiment, liquid paraffin with melting range of 47–60 °C and thermal conductivity of 0.2 W m−1 K−1 is filled in copper foam under a vacuum environment of 94 Pa to ensure that the voids in the metal foam are completely filled. After paraffin is cooled and solidified, the excess part is removed and the preparation of copper foam PCM composite is done (see Fig. 1(b)). In order to study the effect of different inclination angles and natural convection on the melting process of paraffin, a visualized phase change heat transfer experimental system is set up. The experimental system is mainly composed of a rotating angle device, a constant temperature water tank, a phase change test section, a high definition camera and a data acquisition device, as shown in Fig. 2. The constant temperature water tank uses distilled water as heating medium. Considering the melting point of paraffin and the experiment time, the constant wall temperature of 70 °C is provided for the melting of paraffin in this experiment. In order to ensure the stability of test section, the angle of the whole test section is determined by the universal angle ruler (precision is 0.02°) and the bolt is fixed on the bracket of the rotating angle device. The thermal storage process of pure PCM and the composite PCM at angles of 0°, 30°, 60° and 90° are studied respectively. Details on testing cases refer to Table 1. The water tank simulates a heat source in which the heat transfer fluid flows out, and the heat transfer fluid and the phase change material exchange heat through the plate heat exchanger. The experimental system can simulate an industrial waste heat recovery system or cooling of the electronic devices. In these phase change thermal storage devices, heat conduction and natural convection are the main modes of heat transfer, and the proportion of radiative heat transfer is small, considering the low temperature range in experiments. For the test section, metal foam sample is housed by a Perspex (k ∼ 0.2 W m−1 K−1) container with a thickness of 20 mm. Perspex is purposely used not only to enable easy visualization of melting front during the whole phase change process, but also to reduce the heat loss
Fig. 2. Schematic illustration of experimental system for visualization of melting front in metal foam-paraffin composite PCM.
from the side walls. Metal foam sample is bonded to a plate heat exchanger where high temperature fluid flows through to keep a constant thermal boundary. Three T-type thermocouples are arranged on the surface of the heating plate to monitor boundary wall temperature. A heat conductive adhesive with thermal conductivity of 25 W m−1 K−1 is filled in the gap between the test specimen and the microchannel plate heat exchanger to reduce the interfacial contact thermal resistance. To reduce the heat loss from the backward of the plate heat exchanger, the plate is embedded in polyurethane foam with thermal conductivity of 0.02 W m−1 K−1. Besides, the whole test section is covered by an insulation cotton with thermal conductivity of 0.02 W m−1 K−1 to thermally insulate from the ambient environment. To quantitatively estimate the overall heat loss from test section during experiment, Fourier heat conduction equation is employed provided the temperature of internal and external surfaces for the insulated cotton are measured. It reveals a 1.5% of heat loss against the total stored energy. To obtain the transient temperature distribution in PCM, 15 T-type thermocouples are used to record the temperature, as shown in Fig. 3(a). The bottom gray surface denotes the heating surface and the other five surfaces can be regarded as adiabatic. Photos are taken on the right side. To house the thermocouples, five holes are drilled on the upper surface of the Perspex container. Each is filled with a wooden rod with a diameter of 2 mm, who carries three T-type thermocouples (see Fig. 3(b)). T-type thermocouples are tied to the fixed position (7 mm,
Fig. 1. Test sample: (a) size demonstration; (b) sample with infiltrated by paraffin. 24
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Table 1 Test cases under different inclination angles. Case
Case1
Case2
Case3
Case4
Case5
Case6
Case7
Case8
Specimen Angle
PCM 0°
PCM 30°
PCM 60°
PCM 90°
PCM + MF 0°
PCM + MF 30°
PCM + MF 60°
PCM + MF 90°
Note: Here PCM stands for phase change material, MF denotes metal foam.
measuring points are basically the same, and there is little difference among them for the four scenarios. The maximum temperature difference appears in Fig. 4 (a), reaching a maximum temperature difference of 1.6 °C near 2000 s and a relative temperature difference of 3.4%. The maximum root temperature difference is 0.6 °C in Fig. 4(a). The group comparisons for other test points along y axis in different x-y planes (marked as b and c points in Fig. 3(a)) under different inclination angles (30° and 60°) are also conducted. Limited to space, this article only enumerates four sets of results in Fig. 4.
14 mm and 21 mm from the bottom heating surface) of the small wooden rods by thin wires and the thermocouple head is exposed. Series 2 is located in the center, and the distance between the other four series to series 2 is 17 mm. Experimental procedure is as follows: first open valve V1 and close valve V2, V3; and then turn on the heating function of thermostatic bath until the water temperature reaches the set one (70 °C); Second, open valve V2, V3 and close valve V1, so that high-temperature fluid flows through the plate heat exchanger. Simultaneously, turn on the data acquisition device for temperature recording; set the acquisition time interval as every 30 s and trigger on image capturing. The realtime position of the phase interface is recorded by a high-definition camera in every three minutes to show the transient phase transition until paraffin is completely melt.
3.2. Propagation of melting front Fig. 5 shows the real-time phase interface positions of PCM at angles of 0°, 30°, 60° and 90°. The white opaque part of the picture depicts solid paraffin, and the black part represents liquid paraffin. Number (1)–(16) separately denotes the image of melting front at different time and inclination angles. Fig. 5(1)–(4) indicate the melting process of paraffin in the horizontal (θ = 0°) enclosure. At the beginning, the interface is macroscopically horizontal, and heat transfer is mainly heat conduction. As time goes on, the interface gradually appears serrated, as shown in Fig. 5(2). This is because of the complex natural convection in the liquid region, which is a form of Rayleigh-Bernard convection. In the Bernard convection unit, the hot liquid paraffin rises perpendicular to the heating wall in the central region of the unit, and falls back at the edge of the unit after washing the solid paraffin. As can be seen from Fig. 5(3)–(4), the number of serrations at the interface is decreasing. This is because the liquid region grows thicker, the Bernard cell merges with the adjacent convective cell and becomes larger, and the homogeneity of the Bernard cell decreases. For the case at 30°, the tilted configuration leads to a long and narrow room for the melt phase to flow. Bernard cells are significantly reduced. A portion of fluid flows upwardly in parallel to the heating wall and the rest is still washing the solid-liquid interface. Observing in Fig. 5(5)–(8), it can be found that the dark area is smaller than that for the case at 0°. As θ is further increases, more fluid flows in parallel to the heating wall but less washes against the interface. Thus it can be
3. Results and discussion 3.1. Simplification of dimensions (temperature distribution along y axis) The visualization experiment in this paper is carried out in a cuboid container, which is a three-dimensional situation. Given the symmetry nature of the cuboid geometry and the uniform temperature on the heating face (constant temperature boundary), three-dimensional case may be reduced to a two-dimensional one. This helps to calculate melting rate (the ratio of melt volume to the whole one) according to the two-dimensional (x-z plane) images of melting front captured by the high-resolution camera. Section 3.2 will illustrate the details for the two-dimensional experimental photographs for melting front and the calculated melting rate. To this end, 15 T-type thermocouples are placed inside the specimen to monitor the real time temperature of PCM for both cases during melting process. Among them, three points (2a, 4a, 5a) with the same x and z coordinates but different y coordinates (17, 34 and 51 mm) are selected. Fig. 4 shows the temperature changes of three measuring points for the cases of PCM and PCM-MF under inclination angle of 0° and 90°. It can be seen that the temperature trends of the three
Fig. 3. (a) Design of thermocouples locations; (b) real thermocouples used in the experiments. 25
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Fig. 4. Temperature histogram at fixed points along y axis for the case of PCM ((a) and (b)) and PCM + MF ((c) and (d)).
propagation of melting front. Both heat conduction and the natural convection work together to transport heat from heating wall to the solid-liquid phase interface.
expected that the melting rate of paraffin at 60° is further reduced. For the case at 90° (Fig. 5(13)–(16)), the dark area (melting rate) is even lower than the previous three cases. Unlike Fig. 5(9), the liquid region in Fig. 5(13) is thinner at the same time, especially at the lower part of the liquid region. Except the case at 0°, the melting front shapes are similar for the other three cases. This can be explained, taking 90° for example, as follows: at the initial stage of melting, a thin layer of melted paraffin appears near the heating wall, where heat conduction is dominant. As time goes on, the layer of liquid paraffin is thickened, favoring sufficient room for natural convection to develop. The temperature in the liquid region near the heating wall is higher and moves upwardly, while the one with relatively low temperature near the phase interface moves downwardly. Under the action of natural convection, the upper part of the liquid region melts rapidly, and eventually the phase interface tilts. With time elapsed, the phase interface continues to tilt, showing the state as illustrated in Fig. 5(15) and (16) until the end of melting. Fig. 6 visualizes the melting process of paraffin embedded in metal foam at angles of 0°, 30°, 60° and 90°. When the inclination angle is 0°, the solid-liquid interface is basically horizontal, and there is also a serrated interface. This phenomenon can be seen in Fig. 6(1)–(4). This is due mainly to the formation of Bernard flow. Fig. 6(5)–(8) and Fig. 6(9)–(12) show the melting process of paraffin embedded in metal foam at inclination of 30° and 60° respectively. The interface shows similar slope shape, reflecting the contribution of natural convection to the melting process. Compared with the corresponding pictures in Fig. 5, we can find that the phase interface in Fig. 6(5)–(16) is almost a slanted line, i.e. very small curvature, while the interface curvature in Fig. 5 is larger. This can be explained as follows: when pure paraffin melts, liquid paraffin caused by natural convection significantly washes the solid paraffin, and the upper part receives a quantity of thermal energy transported by natural convection. As for the composite PCM, heat conduction through metal skeleton contributes a lot to the
3.3. Melting fraction The melting fraction of PCM refers to the volume ratio of liquid PCM to the whole one. As mentioned above, the two-dimensional phase interface can represent the three-dimensional one. It is therefore reasonable to use the area ratio of the desired interface to represent the volume ratio. Fig. 7 compares the variation of melting fraction with time for PCM at 0° 30°, 60°, and 90°. It can be seen that the eight curves separately in Fig. 7(a) and (b) show similar trend of fm as a function of time. When pure paraffin melts, the full melting time of PCMs becomes shorter and shorter with the decrease of inclination angle θ. This is in consistence with the observation of melting front in Fig. 5. Setting the case at 90° as the comparison basis, the full melting time of paraffin at 0°, 30° and 60° decreases by 12.28%, 22.81% and 34.21%, in respective. This may be due mainly to the different intensity of natural convection. The shortest path of natural convection of liquid paraffin is at 0°, and the washing effect on solid paraffin is the strongest. On the contrary, liquid paraffin moves in parallel to the heating wall at 90°, and the washing effect on solid paraffin is weak. The situations at 30° and 60° are between these two limits. Fig. 7(b) compares the melting fraction of PCM embedded in metal foam at 0° 30°, 60°, and 90°. It is found that the four curves overlaps and the full melting time of paraffin embedded in metal foam has no obvious difference at different inclination angles, where the maximum deviation is 4.35%. This can be also confirmed by the observations in Fig. 6. Possible explanations can be made as follows: The inclined configuration affects the development of natural convection in a cavity. On one hand, the involvement of metal foam significantly suppresses the natural convection in the melt phase; on the other hand, highly26
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Fig. 5. Melting front of pure paraffin at four selected time under inclination angle of 0°, 30°, 60° and 90°.
part. Liquid PCM has the highest temperature at the upper part, resulting in heat accumulation in the upper part. Fig. 8(b) demonstrates the temperature change at point 2b in the middle part. Similar to the temperature trend in Fig. 8(a), the temperature in PCM increases monotonically. It shows that the temperature rises fastest at 0° and becomes slower at 30°, 60° and 90°. The difference among the four temperature curves becomes larger than that in Fig. 8(a). The main heat transfer mode at middle part at 0° is natural convection. As the configuration is further inclined (θ is increasing), the heat transfer mode is gradually turning from convection-dominated to conduction-dominated, thus leading to larger difference among the four temperature curves. This is more obvious in Fig. 8(c) for the lower part. For the composite PCM, Fig. 8(d)–(f) demonstrate the temperature changes at points 1b, 2b and 3b at different inclination angles. Exactly the same monitoring points are chosen as the ones in pure PCM configuration. It is evident that the three groups of curves overlap very well in the early and middle stages of melting, and there is no obvious difference among the four curves at each points. This indicates that heat conduction occupies the majority of heat transfer in the early and middle stage of melting. At the later stage of melting, the temperature curves at different angles begin to separate, especially at position 3b; and it finally shows that the temperature rises the fastest at 0° followed by 30°, 60° and 90°. In the experiments with different inclination angles, the heat conduction of the metal skeleton is basically the same (the same specimen). It is the natural convection that eventually leads to the separation of the temperature curves. Compared with the larger difference in pure PCM, the difference among the four curves is much smaller. This may be due mainly to the conduction-dominated heat transfer mode when metal foam is involved.
conducting metal ligaments greatly help to transport thermal energy deeply into the unmelt region, enjoying the dominate role in the overall melting process. Therefore, when the effect of natural convection is no longer dominated, the melting rate of paraffin in metal foam structure will not dramatically affected by different inclination angles. 3.4. Temperature response Using thermocouples to record temperature data can provide accurate information for studying the effect of different inclination angles on the thermal response of PCM in melting process. As the configuration is inclined, the responses of temperature under different inclination angles are different. Three typical points locating at upper, middle and lower region (referring to 90°) are selected. As can be seen in Fig. 8(a), it is demonstrated that the temperature rises the fastest at inclination of 90°, followed by 60°, 30° and 0°. This is distinctively the opposite to those of points in the middle and lower parts (Fig. 8(b) and (c)), i.e. temperature at 0° rises the fastest and the one at 90° is the slowest. Nevertheless, the difference among the four curves in Fig. 8(a) is not larger compared with the ones in Fig. 8(b) and (c). This can be explained as follows: although the overall natural convection is the strongest at 0° and the melting front evolves the fastest among the four inclination angles (0°, 30°, 60° and 90°), the local natural convection at the upper part of the PCM is also strong at 90°. As can be seen in the solid-liquid interface images in Fig. 5(2), (6), (10) and (14), the local melting fraction at the upper part (highlighted in red circle) at 90° is the highest. When the inclination angle is not 0° (horizontal), the local natural convection of the upper PCM is stronger than that of the lower part, and heat transfer is correspondingly larger than that of the lower 27
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Fig. 6. Melting front of composite PCM (paraffin embedded in metal foam) at four selected time under inclination angle of 0°, 30°, 60° and 90°.
melts at 30°, 60° and 90°, in respective. Different from the trend of the three curves in Fig. 9 (a), the curve separation trend in Fig. 9(b), (c) and (d) is more obvious. As the inclination angle increases, the separation tendency of the three curves in Fig. 9(b), (c) and (d) is also increasing. This can be understood as: as inclinational angle increases, the uniformly-intensive natural convection is gradually weakening. When θ = 90°, non-uniformity natural convection intensity can be confirmed by the slanted solid-liquid interface in Fig. 5. Fig. 10 compares the temperature variations at three selected points along x axis for composite PCM at an inclination of 0°, 30°, 60° and 90°. It is found that the four groups of temperature curves demonstrate a
3.5. Temperature distribution
1.0
1.0
0.8
0.8
0.6
0.6 PCM
0.4
= 0° = 30° = 60° = 90°
0.2 0.0 0
fm
fm
3.5.1. Temperature distribution along x axis Fig. 9(a) shows the temperature change of the measuring points 1a, 2a and 3a when pure paraffin melts at 0°. It is found that the temperature changes of the measuring points 1a, 2a and 3a are substantially the same. This can be also confirmed by the fact that the solidliquid phase interface in each of Fig. 5(1)–(4) is parallel to the heating wall. The natural convection intensity is basically the same thanks to the formation of Bernard flows. Fig. 9(b), (c) and (d) demonstrate the temperature changes of the points 1a, 2a and 3a when pure paraffin
1000
2000
3000
4000
5000
6000
PCM+MF = 0° = 30° = 60° = 90°
0.4 0.2
7000
0.0 0
1000
2000
3000
t/s
t/s
(a)
(b)
4000
5000
Fig. 7. Melting fraction for two cases (a) pure paraffin and (b) composite PCM under four inclination angles of 0°, 30°, 60° and 90°. 28
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Fig. 8. Temperature histograms at selected points for the cases of pure PCM ((a)–(c)) and composite PCM ((d)–(f)) under different inclination angles.
large temperature difference does exist with subjected to non-zero inclined angles.
similar variation trend like conduction-dominated temperature curve, where the whole heat storage process can be clearly divided into three stages: the initial stage is the sensible heat storage stage; the temperature rises linearly, then the paraffin begins to melt and enters the latent heat storage stage. Paraffin melts and absorbs a large amount of heat, with the curve slope becoming smaller. After paraffin is melted, it enters the third stage and returns to the sensible heat storage stage. As it can be seen, the temperature at three points begins to separate within the third stage under the inclination angle of 30°, 60° and 90°. This may be understood by the fact that natural convection intensity is significantly increased in the third stage since there is sufficient room for natural convection after a large portion of solid PCM is melted. Recalling the situations in the pure PCM, no extra resistance like porous matrix exists to hinder the development of natural convection, where
3.5.2. Temperature distribution along z axis Fig. 11 illustrates the temperature variation as a function of time at three points along z axis. The three points have the same x and y coordinates. The depths away from the heating surface are 7 mm, 14 mm, and 21 mm, in respective. There is large difference in temperature at the three points under four inclined angles. Compared the case of 0° in Fig. 9(a), large difference does exist along z axis. When pure PCM melts at 0°, the root mean square temperature difference between 2b and 2c is 4.8 °C and the one between 2a and 2b is 6.9 °C. And the local melting times at points 2a, 2b, and 2c are 1020 s, 720 s, and 600 s, respectively When the composite PCM melts at 0°, the root mean square temperature 29
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Fig. 9. Temperature histograms of pure PCM at selected points along x axis under four inclination angles: (a) 0°; (b) 30°; (c) 60°; (d) 90°.
Fig. 10. Temperature histograms of composite PCM at selected points along x axis under four inclination angles: (a) 0°; (b) 30°; (c) 60°; (d) 90°. 30
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Fig. 11. Temperature histograms of pure PCM at selected points along z axis under four inclination angles: (a) 0°; (b) 30°; (c) 60°; (d) 90°.
Fig. 12. Temperature histograms of composite PCM at selected points along x axis under four inclination angles: (a) 0°; (b) 30°; (c) 60°; (d) 90°.
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to qualitatively and quantitatively underline the thermophysics for phase change heat transfer through analyzing the distributions of flow and temperature field, and optimization will be performed for practical engineering applications.
Table 2 Uncertainties for different variables. Parameters
Uncertainty
Size of PCM and composite PCM (mm) Temperature measured by thermocouples (°C) Porosity of copper foam (%) Melt fraction (%)
± 0.02 ± 0.2 ± 0.5 ± 0.24
Acknowledgement This work was supported by the National Natural Science Foundation of China (51506160), Natural Science Foundation of Shaanxi Province (2017JQ5007) and the fundamental research funds for central universities (xjj2016042).
Note: here PCM stands for phase change material.
differences between 2a and 2b, 2b and 2c are 3.7 °C and 3.5 °C respectively, which are smaller than those of pure PCM. It shows that the uniformity of PCM is improved after the addition of metal foam. And the local melting times at points 2a, 2b, and 2c are 570 s, 540 s, and 540 s, respectively (see Fig. 12).
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4. Uncertainty analysis The uncertainty of experimental results is often affected by the inevitable errors in experimental measurement, and depends on the uncertainty of measuring instruments. All thermocouples are connected to a temperature scanner (Agilent 34970A) to monitor their temperature changes throughout the phase transition process. And these are calibrated by Omega CL3515R calibration system before testing. The uncertainty of measurement is estimated to be ± 0.2 °C. The uncertainties of all the parameters are listed in Table 2. 5. Conclusions In this paper, the melting behaviors of pure phase change material and phase change material embedded in open-cell metal foam are experimentally studied. Melting experiments are carried out at 0°, 30°, 60° and 90° inclination angles, subjected to a single-sided constant wall temperature boundary in a cuboid container housing phase change materials. The effect of inclination angle on the melting process of phase change material is demonstrated by analyzing the visible solidliquid interface and the temperature field in phase change material. Main conclusions can be drawn as follows: (1) Compared with the case at 90°, the full melting time for pure phase change material is reduced by 12.28%, 22.81% and 34.21% at 60°, 30° and 0°, respectively. However, little influence (maximum is 4.35%) is found in full melting time in composite phase change material under inclined situations. (2) Melting front visualization demonstrates that the curvature for pure phase change material is larger than that for the composite one. The visualized solid-liquid phase interface clearly demonstrates that natural convection significantly affects heat transfer in pure phase change material but contributes little to composite phase change material for an inclined configuration. (3) For pure phase change material, local melting time decreases as an increase in distance away from heating boundary for all inclined configurations (the local melting times at points 2a, 2b, and 2c are 1020 s, 720 s, and 600 s, respectively); while for composite phase change material, there is little difference. (4) The root mean square temperature difference between different points in vertical to the heating boundary wall is smaller for composite phase change material than that for pure phase change material. This indicates that a better uniformity is obtained with metal foam involved. (5) The results can be applied to phase-change thermal management systems with adjustable orientation, such as cooling of portable electronic devices, solar thermal collection (tracing solar direction), and phase change thermal storage devices for industrial waste heat recovery. For future work, numerical simulations will be conducted
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