Numerical study and experimental validation of the effects of orientation and configuration on melting in a latent heat thermal storage unit
Introduction
In recent decades, most countries have intensively used fossil fuels as their primary energy source. The International Energy Association (IEA) reported in 2014 that the total annual primary energy supply was the equivalent of 13,699 billion tons of oil. Only 69%, or about 9.4 billion tons, of this was actually consumed while the reminder was lost during the conversion processes [1]. The main challenge currently facing the world is how to reduce the dependence on energy generated by burning fossil fuels, given the many environmental consequences such as emissions of greenhouse gases like CO2, which contribute to climate change. Thus, emphasis has recently shifted towards considering environmentally friendly energy sources such as solar, wind and geothermal power. Among these sources, solar energy has received increasing attention and has been exploited widely for direct electrical generation using, for instance, photovoltaic (PV) or thermal systems, such as solar water heaters [2]. However, the deployment of solar energy is impeded as it is only available during daylight hours. Methods have therefore been proposed to effectively store solar energy during the day and use it at night. Thermal energy storage is a key technology to enable sustainable implementation of solar energy. Phase change phenomena offer a perfect means to store this solar thermal energy [3]. To this end, paraffin wax has often been considered as a phase change material (PCM) for utilisation in latent heat thermal storage units (LHSUs). The wax has unique properties such as its melting temperature, high latent heat, chemical stability and non-corrosive nature [4] that make it an attractive choice. Its low thermal conductivity, however, remains as one of its main disadvantages. Therefore, investigations are currently focusing on enhancing heat transfer in the PCM by using different methods [5,6], such as using fins on heat transfer surfaces and adding nanomaterials to improve heat transfer.
The change of phase in the PCM in multiple configurations of bare tube latent heat thermal storage units has been extensively studied theoretically [[7], [8], [9], [10], [11], [12], [13], [14]]. These works have become the foundation of current work in the field, which has been resurgent due to issues surrounding energy generation. Zhang et al. [15] investigated experimentally the melting of n-Octadecane as a PCM under a constant heating rate from one side of an enclosure. They found that heat conduction controls the early stages of the melting process, while natural convection increasingly dominated as the PCM melted. The importance of natural convection in the PCM melting process in a cylindrical geometry was confirmed numerically by Ng et al. [16], Regin et al. [17] and Jones et al. [18]. They carried out experimental studies and constructed numerical models of the PCM melting in cylinder heated from the vertical wall side. Their results confirmed that conduction controls the early stages of the melting process while the natural convection dominates later. Lamberg et al. [19] investigated numerically the melting of PCM in a rectangular enclosure with and without the present of natural convection. Their results demonstrated that the inclusion of natural convection reduced the melting time of the PCM predicted by their model by 50%. Stritih [20] measured and developed an analytical model of the movement the phase change interface during PCM melting in a rectangular LHSU. A good agreement between the experimental data and the analytical results was found only at the beginning of the melting process. In 2005, Erek et al. [21] investigated numerically and verified experimentally the effect of fin geometry and spacing on the thermal performance of a horizontal shell-and-tube latent heat thermal energy storage system. Their results showed that the total stored energy increases with the increase of the fin radius and decreasing the fin space. Akgun et al. [22] noted experimentally that heat conduction and natural convection controlled the melting of the PCM (paraffin) in a vertical shell and tube configuration LHSU. Medrano et al. [23] investigated the thermal performance of five different LHSUs. They used RT35 paraffin as a phase change material in all the units that they compared, measuring mean power per unit area and per average temperature gradient. The double pipe unit with a graphite matrix was found to have the highest value amongst those they investigated.
Building on previous studies of finned tubes, Lacroix [24] studied numerically the time- dependent performance of a shell and tube LHSU with both bare and finned tube configurations. The numerical model was validated with experiments with the aim of being able to simulate the effects of different parameters, such as the shell radius, the mass flow rate of heat transfer fluid (HTF) and the presence of fins, on the thermal performance of the LHSU. His results revealed that annular fins were effective at moderate mass flow rates and low HTF inlet temperatures. A similar configuration, but including the effect of the fin height, was investigated numerically by Zhang and Faghri [25]. They observed that the liquid fraction of the PCM during the melting process increases as the length of the fins increases. The thermal performance of a high temperature PCM mounted in a shell and tube heat exchanger which could be used for a solar power plant in space was studied numerically by Seeniraj et al. [26]. They neglected natural convection because of the microgravity that would be associated with space. Their results showed that when bare tubes were implemented, a fraction of the PCM near the exit of the tube remained unmelted. This could be solved by adding a small number of fins. In addition to fins can significantly increases the total energy stored in the LHSU.
Aiming for further improvement in LHSU performance, different longitudinal fin designs, other than the traditional rectangular shape, was proposed by different authors. These designs included tree-shaped fins [27], snowflake fins [28] triangular fins [29], V- shape fins [30] and finally, high conducting branch-shape fins [31]. These studies all reported that the innovative fin design shortened the PCM melting time by about 10–25% in comparison to the rectangular fin design. Rahimi et al. [32] compared the thermal performance of finned and non-finned tubes in LHSUs with different initial temperatures and flow rates of the heat transfer fluid (HTF), using RT35 paraffin as a phase change material. They observed that the melting time in the finned tube was shorter than that in the non-finned one. Additionally, increasing the inlet temperature of the HTF led to a significant decrease in the melting time in both types of storage system, while the average temperature of the PCM increased more rapidly in the case of the finned tube. The influence of the flow rate of the HTF was found to have more significance on solidification time. Liu and Groulx [33] experimentally investigated heat transfer in a horizontal longitudinal finned double pipe heat exchanger. Straight and angled finned configurations were tested under various HTF inlet temperatures and flow rates during both melting and solidification processes. The results indicated that the HTF inlet temperature had a stronger effect than the HTF flow rate on the melting time. Manish and Banerjee [34] enhanced the performance of LHSUs experimentally using three longitudinal fins in a shell-and-tube thermal storage unit for both melting and solidification processes under different HTF operational conditions. They noticed that increasing the HTF temperature had more impact than increasing the flow rate on the augmentation of the heat transfer during the melting process. Agarwal and Sarviya [35] performed an experimental investigation to evaluate the heat transfer and thermal characteristics of a horizontal shell-and-tube solar dryer latent heat storage system. The study found that the charging time was reduced by 20% when the HTF temperature was decreased from 90 °C to 80 °C. In addition, they found that the charging process was almost as short as the discharging process. Caron-Soupart et al. [36] developed a new numerical model and used an experimental technique to investigate the performance of a vertical finned tube thermal energy storage unit. Different fin types (circular and longitudinal) were tested and the behaviour of the PCM during charging process was visualised at several melting times. Their results indicated that an efficient heat exchange process could be achieved by increasing the heat transfer area (using a finned tube). Further investigations were suggested regarding PCM and fin types to obtain optimal conditions. Martinelli et al. [37] studied experimentally the heat transfer enhancement in a LHTS unit based on both the PCM and HTF. Their tests involved two different radially and externally finned tubes. Two fin arrangements, with 7 and 10 fin. per inch, were tested. They found that in the presence of fins, the flow regime became laminar, and heat was transferred only by convection, which allowed the melting front to move more uniformly through the unit. In 2017, Wang et al. [38] evaluated the thermosphysical properties of a LHSU unit using a vertical shell and tube during the melting and solidification process of the PCM. Temperature contours were plotted that showed more detail of the melting temperature gradient with time. They concluded that natural convection was the dominant factor during the melting process. However, for the solidification process, natural convection played the main role initially, after which conduction became dominant. Khan and Khan [39] studied the thermal performance of a longitudinal finned shell and tube LHSU. Their results revealed that the phase transition rate and mean power of the storage was improved by 50.08% and 69.71% when HTF inlet temperature increased from 52 to 67. A comparative study of longitudinal and annular fin LHSU was investigated experimentally and numerically by Afridi et al. [40]. They found that the longitudinal fins were more efficient than the annular fins by 6%. Similarly, Abdulateef et al. [41] observed that the longitudinal fin was more efficient than annular, plate and pin fins. More recently, Kazemi et al. [42] numerically studied the effect of triple fin and double fin inclination angle on the thermal performance of the LHSU. They observed that because of the dominance of natural convection, the upper fin configuration has no significant impact on the total melting time. Amagour et al. [43] investigated experimentally the thermal performance of a compact shell and tube LHSU using Moroccan provenance as a natural PCM and circular fins as effective heat transfer surfaces. The heat transfer phenomena in the storage unit during both the charging and discharging processes were studied under different HTF flow rates. More recently, a numerical study on a novel fin configuration has been carried out by Deng et al. [44] to investigate the improvement of the heat transfer in LHSU. The fins were fixed at the lower part of the storage symmetrical along the vertical centreline with the fins angles of θ = 30°, 60°, 90°, 120°, 150° and 180°. The effect of shell conductivity, dimensionless fin length (l) and HTF temperature on the melting of the PCM were investigated. The results showed that there was an optimal θ for each l and the total melting time can be reduced by 66.7% when l increases from 0.5 to 1.0 with an optimum angle of θ = 30° and θ = 120° respectively. Deng et al. [45] performed a numerical study to try and optimise the fin arrangement in a horizontal concentric tube LHSU using the finite volume method (FVM). The fin arrangements they studied included: bare tube, straight fins, angled fins, lower fins and upper fins. The effect of different parameters such as the fin number (N), the dimensionless fin length (l), the heat transfer fluid temperature and the material of the outlet tube on the melting of the PCM was investigated. Depending on the fin number (N), they observed that the lower arrangement was efficient when , otherwise the angled fins when . The results also showed that the optimum configuration was angled fins when while it was the lower arrangement when .
In the present paper, a numerical study using CFD code, which is validated through comparison with experimental measurements in a shell-and-tube LHSU, is described. Both finned and non-finned configurations in a vertical orientation were considered. The numerical model was first validated through both qualitative and quantitative comparisons with the experimental data. Thus, the model could then be used with confidence to eliminate the need for further experiments, thereby reducing costs. The thermal performance of the LHSU in terms of the temperature and liquid fraction was predicted during the melting (charging) process.
Section snippets
Experimental apparatus
Two separate thermal storage rigs (vertical and horizontal LHSU orientations) were designed, manufactured and installed under the same environmental conditions. The experimental apparatus, as shown in Fig. 1, consisted of a 500 l electrical water bath, cylindrical thermal storage, pipes, valves, thermocouples and data logger. Water was used as the working fluid for the charging process, while commercial paraffin wax was used as the PCM. The thermal energy storage system was a shell-and-tube
Numerical model
To reduce the number of physical experiments required to gain significant insight into this system, a numerical model was developed using CFD. Specifically, ANSYS Fluent R.15 was used. Two different storage designs (Fig. 2) were considered. The enthalpy-porosity method was used to simulate the phase change phenomena of the PCM in the LHSU system [46]. A 3D transient study was performed, in which the Bousinnesq approximation was applied. Thus, the fluid was assumed to be incompressible in all
Validation of numerical model
To achieve accuracy and confidence in the numerical model of the LHSUs, experimental work was performed as described in Section 2.2 above. Qualitative and quantitative comparisons between the CFD simulation and the experimental results were performed, with both the PCM temperature and liquid fraction examined during the melting process. This was done for both non-finned and finned tube configurations arranged vertically.
Fig. 6 (a and b) illustrates the time history of the PCM temperature during
Conclusions
This paper has explored the melting behaviour of commercial paraffin wax, which was used as a PCM in finned and non-finned shell-and-tube LHSUs. An experimental investigation was carried out to validate the numerical model. After validation of the numerical model, the thermal performance of the LHSUs was investigated intensively. The investigation included the temperature history and the liquid fraction of melting process in both non-finned and finned tube configurations implemented in vertical
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