Experimental study on the melting characteristics of n-octadecane with passively installing fin and actively applying electric field

https://doi.org/10.1016/j.icheatmasstransfer.2021.105570Get rights and content

Highlights

  • Experiments on the combination of active electric field and passive fin on PCMs' melting inside a cavity are conducted.

  • The Coulomb force rather than the dielectric force plays the dominant role.

  • The free space charges are generated by the conduction mechanism.

Abstract

This work extends our recent study (Sun et al., 2021 [1]) on the n-octadecane's melting with electric field inside a cavity. The combined heat-transfer enhancement technique involving a passive fin and active electric field is used to experimentally investigate the n-octadecane's melting inside a vertically heated cavity. The effects of magnitude and direction of applied voltage on phase-change heat transfer are studied with and without the fin. Results are provided for the real-time evolution of the melting front, the liquid fraction, as well as the velocity fields and the current-voltage curve. It is found that the adding of fin structure can solely reduce 40.0% total melting time in a 40.0 mm × 40.0 mm × 40.0 mm cavity. For the cavity without fin, a +20.0KV applied voltage from the left wall or right wall of cavity can separately increase 40.0% or decrease 33.3% melting time, respectively. But for a cavity with fin, a left or right side applied +20.0KV voltage will induce a 3.7% decrease or a 48.1% increase of melting time. It is also found that Coulomb force plays a dominant role on melting after the initial stage, and free space charges are mainly produced by the conduction mechanism.

Introduction

Latent-heat thermal storage (LHTS) alleviates the mismatch of energy supply and requirements with respect to time, space, and intensity. It is characterized by smaller volume fluctuations, greater energy storage capacity, and a nearly isothermal behavior during solid-liquid phase changes [2,3]. These characteristics enable a rich range of LTHS applications, such as solar energy [4], waste-heat recovery [5], energy savings in buildings [6], and thermal management of electronic devices [7]. According to their chemical composition, phase-change materials (PCMs) for LHTS can be classified as eutectic, inorganic, and organic [3]. Organic PCMs like paraffin waxes, fatty acids, and alcohols are extensively used in low and intermediate-temperature LHTS because of their intrinsic advantages of low melting points, with no supercooling or phase separations. However, the poor thermal conductivity of organic PCMs increases charging and discharging times, which significantly restricts the LHTS performance.

Various passive and active enhancement methods have been proposed to compensate for the poor thermal conductivity of organic PCMs [8]. Passive techniques mainly include an extension of the heat-exchange surface, the addition of high-conductivity nanoparticles or metal foams, an arrangement of multiple PCMs, encapsulation of the PCMs, and close contact melting [9,10]. Installing fins are the usual means of heat-exchange surface augmentation. Ordinary fins own some attractive features such as simple structure, easy to assemble, and low costs and have been favored by a lot of researchers. For example, Oliveski et al. extensively analyzed the influence of single fin's structural parameters on the lauric acid's melting process. Their fins' tests were the combinations of 9.0 fin aspect ratios and 9.0 fin-to-cavity area fractions. They concluded that the melting time would always be decreased by reduction of fins' aspect ratios [11]. Joneidi et al. investigated the melting behavior of PCMs in horizontal rectangular enclosures at different configurations of plate fins experimentally. The effects of fins' number, fins' height, and fins' thickness are investigated. Their results shown that by adding the number of fins a more uniform temperature distribution would be generated which accelerates the melting process more than other parameters [12]. Biwole et al. analyzed the size and distribution of fins for phase-change heat transfer inside a cavity and found that the surface area plays the most important effect on in increasing the heat transfer rate [13]. Tian et al. analyzed the role of fin's material on the melting of PCMs within a rectangular enclosure numerically and proposed a guideline for selecting fin's material [14]. Furthermore, the effect of strips fins [15] and angled fins [16] on phase-change heat transfer were studied, separately. Meanwhile, novel fins have been developed and implemented into LHTS devices under the urgent demand of energy storage. Skaalum et al. compared the effect of heat transfer between branching and non-branching fins and reported that bifurcated fins would transfer less heat than straight fins during melting process because of a reduced possibility to form convective flows in the liquid region [17]. Al-Mudhafar et al. innovatively proposed the tee shaped fins to enhance the performance of LHTS. With equipment of the tee fins, the melting time is reduced by 33.0% compared to the longitudinal fins [18]. In addition, Deng et al. reported LHTS devices with tree-like fins and suggested that the melting time would be shortened by 40.3% for an optimized topology [19]. Furthermore, the enhancement effects of stepped [20,21], multiple-spiral [22], and twisted [23] fins on accelerating melting were investigated, respectively.

There have been few reports on active enhancement techniques. Wu et al. reviewed the effects of extra fields on the phase change of PCMs and enhancement heat transfer [24]. The external fields mainly include mechanical movement [25], ultrasonic vibrations [26], magnetic fields [27], and electric fields [[28], [29], [30], [31]]. Since organic PCMs are usually dielectric materials, they can sustain strong electric fields. Therefore, the electric field has been considered to actively strengthen or control the solid-liquid phase change heat transfer of various organic PCMs. Dellorusso et al. studied the phase-change of paraffin wax with 15.0 kV applied voltage inside a double-wall fin array configuration. They found that the electric field did not significantly enhance the paraffin's melting because a complex cell design was adopted [28]. In 2015, Nakhla et al. examined the melting of organic PCMs inside a horizontal rectangular module embedded with multi-wire electrodes. The melting time of paraffin wax was decreased by 40.5% when 8.0 kV was applied to the wire electrodes. Because the paraffin wax was a mixture with a wide melting range, the enhancement was attributed to “solid extraction” in the mush zone [29]. For pure n-octadecane, the electric field improved the heat transfer by 8.6 times comparing to the case with no electric field [30]. Coulomb force was considered to be main enhancement mechanism, and the space-free charge was produced by the injection mechanism. When the LHTS module was placed vertically [31], the average heat transfer enhancement factor was 1.7 when 6.0 kV was set. The enhancement was achieved by the reducing size and redistribution of convection cells. However, the information of velocity fields on the studies by Nakhla et al. [39–31] has not been reported. Meanwhile, the melting of dielectric PCMs inside a cavity in the presence of electric field was numerically stimulated by the means of lattice Boltzmann method [32] and finite volume method [33]. Luo et al. [32] and Selvakumar et al. [33] all concluded that the electric governing parameter has significant effect on the rate of melting. The melting process would be manipulated by setting various electric parameter and the melting time can be shortened by around 50.0% at the higher electric Rayleigh number. After that, stability analysis of electroconvection and investigation of electro-thermo-convection with a solid-liquid interface were further studied by Luo et al. [34] and He et al. [35], respectively. They all found that the flow in the liquid region presents the characteristics of subcritical bifurcation by switching electric Rayleigh number.

As mentioned above, adopting the metal fins is one of the most popular ways to extend the heat-exchange surface. There are numerous literature on passively utilizing ordinary straight fins or novel fins to shorten the PCMs' melting time and enhance the performance of LHTS. However, it should be pointed out that the solely installing fins would not dynamically control the performance of LHTS. With the burgeoning reports [[29], [30], [31], [32], [33], [34], [35]] of actively electric field in the field of LHTS, the inherent advantages of electric field have attracted more and more researchers' attention. To the best knowledge, there is no reported literature on the interesting topic of passively mounting fin and actively applying electric field to manipulate the performance of LHTS. And the corresponding velocity fields in the fin-cavity configuration under the action of an electric field have not ever been reported.

Based on our previous experimental study [1], the active electric field is applied to affect the melting of n-octadecane inside a cavity. It was found the n-octadecane's melting would be inhibited or accelerated when the voltage is applied to the cavity's left wall or the cavity's right wall, respectively. Therefore, the main novelty of the present investigation is twofold: (i) to experimentally explore and evaluate the combination effect of passive fin and active electric field on the influence of melting heat transfer of n-octadecane; (ii) to provide the velocity field in the fin-cavity structure under the action of an electric field using the particle image velocimetry (PIV), which may provide a reference for future numerical studies.

Section snippets

Experimental methods

The experimental layout is depicted in Fig. 1(a). Transformer oil (25#) with an electrical conductivity of 4.7 × 10−12 S/m is used as the circulating liquid in a constant-temperature bath (DCM-2020, Hengping, China). Direct current (DC) or alternating current (AC) signals are produced with a waveform generator (AFG1022, Tektronix, USA) and amplified with a amplifier (AMP-30B10, Matsusada, Japan). The voltage in the current is dynamically reported with a high-voltage probe (P6015A, Tektronix,

Results and discussion

The experiments are performed at the ambient temperature 25 °C. In all cases, the left heated wall's temperature is fixed at 35.0 °C, which is higher than the melting point of n-octadecane. Melting experiments without the applied electric field are performed initially and used as a baseline to evaluate heat transfer enhancements. Positive DC voltages are then applied to the left or right walls. Hence, the electric field and the thermal gradient either have the same or opposite direction. The

Concluding remarks

This work presents the results for the first attempt on experimentally studying the melting characteristics of n-octadecane with passively installing fin and actively applying electric field. The combined effects are investigated. The main conclusions are summarized as follows:

  • (1)

    Initial melting is basically unaffected by the electric fields. Effects of electric field occur only at the later stages.

  • (2)

    The presence of the fin reduced the melting time by 40.0% relative to the case without a fin. With

Author statement

Author contributions: Jian Wu designed the research. Zhihao Sun performed the research and wrote the manuscript. Kang Luo and Jian Wu analyzed and discussed some results. All other authors commented on the manuscript.

All supports have been included in the Acknowledgments.

Data and materials availability: All data related to this study can be obtained from the corresponding author (Jian Wu) via email.

Declaration of Competing Interest

The authors declared that there is no conflict of interest in this manuscript.

Acknowledgments

This study is supported by the National Natural Science Foundation of China (Grant No. 11802079, 51906051) and CAST-BISEE innovation fund (Grant No. 2019-012).

References (41)

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