Experimental evaluation on natural convection heat transfer of microencapsulated phase change materials slurry in a rectangular heat storage tank

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Abstract

The main purpose of this experiment is to evaluate natural convection heat transfer characteristics of microencapsulated PCM (phase change material) slurry (MPCMS) during phase change process in a rectangular heat storage tank heated from the bottom and cooled at the top. The microencapsulated PCM is several material compositions of n-paraffin waxes (mainly nonadecane) as the core materials, outside a layer of a melamine resin wrapped. In the present study, its slurry is used mixing with water. And the specific heat capacity with latent heat shows a peak value at the temperature of about T = 31 °C. We investigate the influences of the phase change process of the MPCMS on natural convection heat transfer. The experimental results indicate that phase change process of the MPCMS promote natural convection heat transfer. The local maximum heat transfer enhancement occurs at approximately TH = 34 °C corresponding to the heated plate temperature. With high mass concentration Cm, the onset of natural convection gets easier for the MPCMS. The temperature gradient is larger near top plate and bottom plate of a rectangular heat storage tank. Heat transfer coefficient increases with the phase change of the PCM. And it summarizes that the phase change process of the PCM promote the occurrence of natural convection.

Highlights

► It gives heat transfer characteristics in a rectangular heat storage tank as the basic unit for reservoir of thermal storage. ► Onset of natural convection gets easier for the MPCMS with a higher mass concentration. ► It enhances the heat transfer ability of natural convection for the MPCMS. ► Obtained the relationship between Ra and Nu of the MPCMS.

Introduction

Latent heat storage technology has been a considerable scientific concern since it can gain high energy storage density, large heat storage capacity and heat energy release at a constant temperature or over a small temperature range [1], [2], [3], [4], [5], corresponding to the phase transition temperature of the PCM. Especially, the fluid with the PCM can enhance heat transfer process and improve thermal storage speed through natural convection [6], [7], [8]. In the thermal engineering applications, the natural convection heat transfer enhancement may be accomplished not only through improving heat transfer surface composition but also utilizing the good flowing media or the specific fluids which are called functionally thermal fluids [7], [8], [9], [10], [11]. The MPCMS is a main kind of the functionally thermal fluids which large number literatures may be found various applications both melting and freezing phenomenon from the fundamentals to applications [7], [12]. To reduce the natural mineral resources consumption, energy efficient utilization and heat storage technology has been aroused people’s wide interests. To raise energy conversion efficiency is an effective means [12]. The thermal storage and heat release are also a kind of effective method in national resources utilization [13], [14], [15], [16]. Among kinds of heat transfer methods, natural convection heat transfer plays a major role in various thermal systems, such as thermal comfort in building [4], thermal energy storage system [15], [16], crystal growth, cooling electronic component and solar collector [16]. Compared to conventional fluids working in the sensible heat temperature range, the MPCMS shows high values of apparent specific heat capacity during the phase change process that enhance the heat transfer rate between fluid and wall [10], [12], [15], [16], [17], [18]. Moreover, tank is often used as the active heat energy storage system to store the energy. Rectangular tank with the geometrical simplicity has been extensively used in practical application. However, movements of the fluid and heat transfer processes in the tanks are greatly complicated due to space limitations and flow of fluid. They not only depend on properties of the fluid and the temperature difference between the hot surface and the cold surface, but also depend on the shape structure and size of space [15], [18]. Heat and mass transfer related to the phase-change process of PCM is of great importance in diverse engineering problems. Especially in recent years, phase-change process using the MPCMS has received considerable attention for its capability to have high energy storage efficiency, its liquidity and some specific characteristics [6], [14], [15], [16], [17], [18]. Natural convection in a tank plays an important role in different applications especially solar thermal receiver system as well as other engineering system. Hence, according the above background, the main purpose in present study is that experimental study on natural convection heat transfer characteristic of the MPCMS in a horizontal rectangular heat storage tank heated from the bottom and cooled at the top. Thermo physical and rheological properties of the MPCMS have been introduced in the previous literature [6], [15], [18].

In recent decades, the characteristics of flow and heat transfer on the MPCMS has been studied in various theoretical and simulation studies [9], [15], [18]. The natural convective heat transfer enhancement is discovered because the PCMs in the MPCMS caused through the phase transient process or melting process in the phase change temperature range [6], [9], [10], [19], [20]. The MPCMS has been analyzed by most of researches as Newtonian fluid [9], [10], [14], [20], [21]. However, it is very little study on the influence of fluid flow and heat transfer as the non-Newtonian fluid with the MPCMS [22], [23].

The experimental investigations on natural convection heat transfer of the MPCMS are accomplished in a horizontal rectangular heat storage tank heated from the bottom and cooled at the top. The key influence parameters on the natural convection heat transfer of the MPCMS are taken into account such as mass concentration of the MPCMS, the temperature difference between heated plate and cooled plate, the Prandtl number Pr, the Rayleigh number Ra, and the aspect ratio (width/height) Ar of the horizontal rectangular heat storage tank. The natural convection heat transfer characteristics of the MPCMS are concluded from predicted conditions of phase transient state or solid–liquid coexistence state. The upper cooled plate is fixed at a constant temperature that phase change of the PCM begin, and the lower heated plate temperature is varied from low value to high value through adjusting the electric power input of an electric plate heater. To emphasis phase change influence, heat plate temperature range is from start to end temperature for phase change of the PCM. Experiment is performed under the thermal steady condition in the MPCMS. And experiments had been carried out for four kinds of tanks of various heights H and different mass concentrations Cm of the MPCMS.

Section snippets

Experimental setup

The MPCMS is filled in a horizontal rectangular tank. The present experimental system consists of three parts including the temperature control system, the experiment test part and data acquisition system shown in Fig. 1. Here, the thermocouples installed in each part of the experimental setup record the data of temperature. An electric plate heater heats the copper plate at the bottom of the tank whose temperature is controlled by adjusting the input voltage of power supply. A constant

Experimental results and discussion

Natural convection heat transfer characteristic experiment of the MPCMS was demonstrated through the vertical temperature distributions of the MPCMS in the center position of tank, the NuRa plot method and the heat transfer coefficient α. The heat transfer coefficient α, Nusselt number Nu and Rayleigh number Ra are defined as follows:q=qmt-qlossα=qTH-TCNu=αHkRa=ρ0gβ¯(TH-Tc)H2n+1Ka0nβ¯=TCTHβ(T)dT

The Rayleigh number Ra was defined in the previous articles [6], [22], [28], [29]. The volumetric

Conclusion

Experimental evaluation on natural convection heat transfer characteristics of MPCMS during a phase change process is performed for a heat storage tank heated from the bottom and cooled at the top, and the MPCMS has complex rheological and thermophysical properties during phase change process in two-phase concomitant state (solid and liquid phase). The effect of phase change process of the PCM on natural convection heat transfer is studied in following three aspects, mass concentration Cm of

Acknowledgments

The author thanks Scientific Research Fund of Hunan Provincial Education Department for providing financial support through Project No. 10B116, Scientific Research Fund of Central South University of Forestry and Technology No. 104-0105, the International Cooperation and Exchange Program from the Ministry of Science and Technology of China (Grant No. 2011DFA60290) and National Natural Science Foundation of China (Grant No. 51146009).

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