Experimental investigation of the effect of perforated fins on thermal performance enhancement of vertical shell and tube latent heat energy storage systems

Highlights

• Perforated fins are used in a finned shell and tube latent heat energy storage system.

• Perforated fins outperform the solid fins by enhancing the natural convection.

• 30% enhancement in Nusselt number by applying perforated fins.

• 7% reduction in melting time by perforated fins instead of solid fins.

Abstract

A key challenge in the deployment of practical latent heat energy storage systems employing phase change materials is the inherent low thermal conductivity of these materials. The present research is motivated by the need to intensify the buoyancy-driven convection flow in the phase change material to enhance the thermal performance of the system. In this paper, for the first time, the effect of applying perforated fins on the thermal performance enhancement of a vertical shell and tube latent heat energy storage heat exchanger is experimentally investigated and the results are compared with those of the unfinned and solid finned heat exchangers as the base cases. Lauric acid as the phase change material is placed in the shell side and the water is passed through the inner tube. The shells of the heat exchangers were made of transparent Plexiglas tubes to enable the visual comparison of the melting processes. The fins and tubes were made of copper. The melting process of phase change material is studied under different inlet water flow rates (0.5 and 1 l/min) and temperatures (55 and 65 °C). The experimental results showed that the time-averaged Nusselt number of the perforated finned heat exchanger is about 30% higher than that of the solid finned heat exchanger due to the minor hindering effect of the perforated fins on the development of the convection flows. Moreover, the total melting time of the perforated finned heat exchanger is about 7% lower than that of the solid finned heat exchanger.

Introduction

The continuous increase in energy demand and global warming due to the greenhouse gas emissions have motivated intensive research for efficient use of energy and development of energy storage systems [1]. Thermal energy storage (TES) which stores heat in a material and releases it when it is needed is one of the efficient techniques to reduce the gap between energy supply and demand. There are many practical applications for TES including solar thermal systems, waste heat recovery, district heating and cooling, thermal power plants and buildings [2]. There are three kinds of TES: sensible heat storage, latent heat storage, and reversible thermochemical reaction. Among different types of TES, the use of phase change materials (PCMs) for latent heat thermal energy storage (LHTES) systems is receiving growing attention due to the absorb and release of thermal energy at an almost constant temperature during the solid–liquid phase change processes [3].

PCMs have a broad variety of applications such as solar heating systems [4], [5], temperature regulation of photovoltaic panels [6], [7], buildings [8], [9], electronics cooling [10], [11], temperature control of the batteries [12], [13], refrigeration [14] and waste heat recovery [15]. However, a significant downside of PCMs is their low thermal conductivity, which decreases the heat transfer rate and restricts the deployment of PCMs in large-scale LHTES units [16]. In order to enhance the thermal performance of the LHTES, many techniques have been established to improve the thermal conductivity of PCMs such as using fins [17], [18], porous structures with high thermal conductivity [19], [20], heat pipes [21] and nanoparticles [22].

Among the above-mentioned enhancement techniques, the utilization of high conductivity fins is one of the most efficient, cost-effective and dependable methods for heat transfer augmentation in LHTES systems. However, fins may have an undesirable influence on the development of natural convection flows in the liquefied PCM. The restriction of natural convection flow suppresses the positive effect of fins on heat transfer enhancement. Therefore, the geometric parameters of the fins have been the subject of numerous studies to develop the LHTES systems with higher efficiency, lower weight, and smaller size [23]. Shell and tube heat exchangers (HXs) are attractive candidates for integration with practical LHTES units due to their simplicity in design, good effectiveness and ease of manufacturing. Hence, the focus of this investigation is to study the solid–liquid phase change rate in the finned shell and tube HXs. There are several studies in the literature on the effect of fin geometry including longitudinal fins [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], pin fins [34], helical fins [35], and annular fins [36], [37], [38], [39] on the thermal behavior of the PCM in the horizontally or vertically oriented shell and tube HXs.

Darzi et al. [26] numerically investigated the influence of adding the number of longitudinal fins on the melting and solidification times of n-eicosane in a horizontal shell and tube HX. It was concluded that both the melting and solidification times reduce when the fins are implemented. However, the fins were found to be more effective during the solidification process. This was attributed to the hampering effect of fins on the evolution of buoyancy-driven flows during the melting process. The same thermal behavior was also reported by Kamkari and Shokouhmand [27] for the melting of PCM in a finned rectangular LHTES enclosure. Yuan et. al [28] studied the impact of the installation angle of the longitudinal fins on the melting of PCM in a horizontal double pipe LHTES unit. It was concluded that the angle of fins should be selected so that it minimizes the obstruction effect of fins on the development of natural convection flows. Sodhi et al. [29] numerically analyzed the melting and solidification of PCM in horizontal conical shell-based LHTES systems. The optimum cone angle was obtained and also the effects of longitudinal fins with different geometries on the charging and discharging process were investigated. Mahdi and Nsofor [30] numerically investigated the solidification of PCM in a horizontal triplex-tube LHTES system by applying longitudinal fins and nanoparticles and validated with experimental data. It was proposed that the utilization of fins alone results in a better enhancement than applying either nanoparticles alone or both of the nanoparticles and fins. The solidification time was reduced by up to 55% using fins alone, 8% using nanoparticles alone and 30% using the combination of fins and nanoparticles. In another study, Mahdi et al. [33] numerically investigated the simultaneous charging and discharging processes in a triplex-tube heat exchanger. A comparison between the fin system and that containing nanoparticles showed that the insertion of fins with the recommended structure is more favorable for achieving higher thermal performance.

Besides the use of longitudinal fins, some researches have been conducted to enhance the thermal performance of shell and tube LHTES systems using different fin geometries including annular, pin and helical shapes [35]. Tay et al. [34] numerically compared the effectiveness of annular and pin fins in LHTES systems. The analysis showed that using the annular fins is a more effective heat transfer enhancement method than applying the pin fins. Ogoh and Groulx [36] investigated the melting of paraffin wax in a vertical shell and tube HX with different number of annular fins. Yang et al. [37] conducted a numerical investigation on the melting process of paraffin in a vertical shell and tube LHTES unit with annular fins. It was found that by adding the number of fins, the conduction heat transfer through the PCM is improved but the formation of natural convection flows is limited due to the decrease in fin spaces. Therefore, an optimum fin number was introduced beyond which any increase reduced the heat transfer rate. Parsazadeh and Duan [38] numerically investigated the melting process of nano-PCM in a vertical shell and tube HX with angled annular fins. It was noticed that the fins with upward angles are more favorable to improve the heat transfer rate due to the natural convection intensification. Also, the results indicated that the thermal conductivity enhancement with nanoparticles was not able to overcome the suppression of natural convection caused by higher viscosity of the nano-PCM which leaded to a longer melting time. Shahsavar et al. [39] numerically evaluated the effects of fin arrangement and diameter on melting and solidification processes in a vertical finned shell and tube LHTES system. It was found that the optimized non-uniform fin array reduces the melting time by 23.9% compared to the uniform fin distribution. Also, increasing the fin diameter beyond a certain value showed an adverse effect due to suppressing the natural convection effect.

Based on the above literature review, it is found that applying the annular fins on the inner tube of a vertical shell and tube LHTES systems effectively improves the heat transfer rate. However, the fins restrict the development of convection flows inside the liquid PCM. Therefore, the conflict between the enhancement in heat diffusion through the PCM and the simultaneous decrease in the buoyancy effect due to adding fins should be considered by the designers through the selection of the optimum fin geometry to maximize the fin effect.

Some previous studies have already shown that the use of perforated fins instead of solid fins may enhance the single-phase convective heat transfer coefficient [40], [41], [42]. Awasarmol and Pise [43] experimentally investigated the natural convection heat transfer enhancement from perforated rectangular fin arrays using air as the heat transfer fluid. It was concluded that perforated fins can increase the heat transfer coefficient by up to 32% while decreasing the fin mass by 30%. The same concept can be used to improve the thermal performance of LHTES heat exchangers, which has not been investigated before. The motivation of the present study is the enhancement of the convective heat transfer coefficient between the liquid PCM and the heat transfer surface by using the perforated annular fins instead of the solid fins. To the best of authors’ knowledge, this is the first time that the effect of applying perforated fins on the melting process of PCM has been investigated. For this purpose, an experimental setup has been constructed to measure the instantaneous heat transfer and energy storage rates in unfinned, solid finned and perforated finned HXs during the melting process of the PCM under the different working conditions. The results of the perforated finned HX are compared with those of the unfinned and solid finned HXs as the based cases. The findings provide useful information on the performance improvement of the LHTES systems. The total weight of the perforated fins is 16% less than that of the solid fins, which leads to a lighter and more cost-effective HX while enhancing the thermal performance of the system