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In the present work, the effect of natural convection caused by immiscible intermediate fluid on melting process in a horizontal shell-and-tube latent heat storage is examined numerically. The intermediate fluid is placed between the section filled with phase change material and the storage outer hot wall (as heat source). Water is considered as the intermediate fluid to transfer heat from the hot wall to PCM (n-octadecane) by the buoyancy impact of heated water. Water and the liquid phase of the n-octadecane are immiscible. Numerical analysis is performed employing the enthalpy-porosity method with the assistance of a commercial CFD code, ANSYS FLUENT 18.2, based on the finite volume method. The present work consists of two parts. In the first part, a simple 2D latent heat storage with various volume proportions of water as intermediate fluid, including; 25, 50, 75, and 100%, is considered. In the second part, the effect of the number and arrangement of heat transfer fluid channels is evaluated and analyzed. In all investigated models, the volume of the storage is constant and the amount of the PCM varies. The solid PCM was subcooled to 1 ℃, and the temperature of the surface of the storage was maintained at 329 K. The pertinent dimensionless parameters, including; Prandtl, Rayleigh, and Stefan numbers, are 59.5, 6.3 × 108, and 0.27, respectively. The results depicted that the maximum saved energy inside the proposed latent heat storage belongs to the case with 50% of storage volume capacity filled by water. Moreover, the case with four heat transfer fluid tubes indicated the highest absorbed energy, and among them, the four tubes with diamond pattern arrangement stored the highest amount of energy. [ABSTRACT FROM AUTHOR]

Enhancement in properties of thermal storage materials improves their performance and contributes to reducing the greenhouse gas emissions. The enhancement can be made in a passive way, which is cost-effective and hardly requires management. For decades, phase change materials (PCMs) have been used in many applications for thermal storage, thermal control and thermal insulation purposes. PCM can store a huge amount of energy with low or no temperature swing. However, the major drawback of PCM is the low thermal conductivity. Techniques of different scales (macroscopic, microscopic and nanoscopic) have been adopted in thermal engineering to enhance the thermal conductivity of PCM. This paper presents a comprehensive review of the literature dealing with micro/nano enhancement techniques of PCMs. Enhancement effects as well as limitations of each technique are discussed in detail. Moreover, direction for future research and possible challenges are pointed out along with conclusions drawn from the studies. [ABSTRACT FROM AUTHOR]

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Copyright of Journal of Central South University is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In the present work, a parabolic trough solar collector with inner helical axial fins as a swirl generator is considered and analyzed. All the numerical outcomes are obtained by utilizing the finite volume method's commercial code, ANSYS Fluent 18.2. The discretization of turbulence kinetic energy, turbulence dissipation rate, and energy equations as well as the spatial momentum equation have been done by a second-order upwind scheme. Hybrid nanofluid is utilized as a working fluid. This work consists of two sectors. In the first one, the influence of hybrid nanofluid types and, in the next sector, the impact of the volume concentration of selected hybrid nanofluid on the turbulence thermal efficiency are appraised numerically. The two considered hybrid nanofluids here contain multi-wall carbon nanotubes–iron oxide/water and silver and graphene nanoparticles/water. Obtained results showed that utilizing hybrid nanofluids causes more heat exchange rate than pure water. Also, among the evaluated hybrid nanofluids, multi-wall carbon nanotubes/iron oxide indicated the highest thermal performance. At the lowest studied Re number (Re = 5000), multi-wall carbon nanotubes/iron oxide presented higher thermal performance than silver–graphene/pure water by about 9.66 and 13.68%, respectively. Moreover, the peak thermal performance belongs to the case with volume concentration equal to 4% (φnf,1 = φnf,2) by an 18.5% growth in thermal performance. [ABSTRACT FROM AUTHOR]

Utilizing passive methods is an efficient way in order to improve the thermal performance of heat exchangers. The novelty of the present paper is that two kinds of passive techniques for heat transfer enhancement, including a corrugated tube equipped with a coiled wire as turbulator, are combined in the present study. Heat transfer and pressure drop in a corrugated tube with a wire coil are analyzed numerically by a commercial CFD software. The range of the considered Reynolds number is 5000–20,000, which guarantees that the flow regime is fully turbulent. The effects of two different geometrical parameters and of two flow operating parameters, namely the Reynolds and Prandtl numbers, are numerically investigated. The conservative equations for momentum are closed using the k-ε realizable model. Results indicate that the geometrical and fluid flow parameters have significant effects on the thermal performance of the system. When one increases the pitch ratio of the wire coil (P/d), the heat transfer enhancement ratio NuER decreases. The maximum decrease is obtained for P/d = 5 with − 7.78% at Re = 20,000 in comparison with the baseline case P/d = 2. The maximum increase in the performance evaluation criterion (PEC) is observed at Re = 5000: + 11.16% and + 17.4% for P/d = 3 and P/d = 5, respectively. The configuration with a roughness height equal to e = 3 mm exhibits the best thermal performance with a maximum enhancement in terms of NuER of + 3.3% at Re = 10,000 in comparison with the baseline case e = 0.5 mm. By considering the PEC number, increasing the parameter e has a negative effect on the thermal performance of the proposed system. [ABSTRACT FROM AUTHOR]

In this study, the effect of using twisted tapes with various geometries in both tubes of a double-pipe heat exchanger is numerically investigated. The twisted tapes are inserted in both sides of double-pipe heat exchanger to improve the heat transfer where the fluid is water in both sides. The influence of geometrical parameters including the number of twisted tape and creating hollow on twisted tape with different aspect ratios is investigated numerically. The obtained results are analyzed by calculating the outlet temperature of both sides, pressure drop, Nusselt number, and coefficient of performance. Numerical simulations are performed by a commercial CFD code, ANSYS FLUENT 18.2. The results indicate that increasing the number of twisted tape from one-fin to four-fin leads to an increase in the Nusselt number 3.1%, pressure drop 64%, and coefficient of performance 63.9%. Eventually, based on outcomes, four-fin twisted tapes provided better coefficient of performance. In the second part of the study, the effect of creating hollow on the four-fin twisted tape with different aspect ratios (AR = W/H) is studied. Three different aspect ratios including 2.25, 1, and 0.44 are considered. The results show that creating the hollows with AR = 1 on the twisted tape leads to better coefficient of performance. [ABSTRACT FROM AUTHOR]