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In this paper, the performance of a solar still equipped with a heat exchanger using nanofluids has been studied both experimentally and theoretically through three key parameters, i.e., freshwater yield, energy efficiency and exergy efficiency. First, experiments are performed on a set-up, which is mainly composed of two flat plate solar collectors connected in series, and a solar still equipped with a heat exchanger. After heated in the collectors, the nanofluid enters the heat exchanger installed in the solar still basin to exchange heat with brackish water. The research question is to know how much the effect of nanofluids on the evaporation rate inside the solar desalination system is. The experiments are conducted for different nanoparticle volume fractions, two sizes of nanoparticles (7 and 40 nm), two depths of water in the solar still basin (4 and 8 cm), and three mass flow rates of nanofluids during various weather conditions. It is found that the weather conditions (mainly the sun radiation intensity) have a dominant influence on the solar still performance. To discover the effects of nanofluids, a mathematical model is developed and validated by experimental data at given weather conditions. The results reveal that using the heat exchanger at temperatures lower than 60 o C is not advantageous and the corresponding yield is smaller than that of solar still without the heat exchanger; although in such a case, using 2 nanofluids as the working fluid in the heat exchanger can enhance the performance indices about 10%. At higher temperatures (e.g. 70 o C), the use of heat exchanger is beneficial; however, using nanofluids instead of water can augment the performance indices marginally i.e. just around 1%. In addition, it is found that in high temperatures using SiO 2 /water nanofluids, which have a lower effective thermal conductivity than that of Cu/water nanofluids, provides higher performance indices.Keywords: Nanofluids; Solar desalination; Heat exchanger; Freshwater yield 1-IntroductionNowadays, "Nano" and "Energy" have been two hot keywords, not only in the scientific community but also in our daily life. During recent decades, researchers have attempted to apply nanotechnology to various energy and power systems such as electric generators, fuel cells, batteries, and solar cells [1][2][3][4][5]. Nanotechnology has also been implemented to enhance the heat transfer potential of common liquids like water and oil to ameliorate the efficiency of thermal systems; this can be done through adding solid nanoparticles (particles with a size of 1-100 nm) to the liquids. The mixture of nanoparticles and conventional liquids is named "nanofluid" [6].Despite some limitations such as relatively high preparation cost and stability issues, extensive attempts have been made to develop the applications of nanofluids in energy systems such as solar energy based devices [7][8][9][10][11][12][13], cooling and thermal management of electronic equipment [14,15], grinding and drilling, absorption systems, medicine...

Exploiting nanofluids in thermal systems is growing day by day. Nanofluids having ultrafine solid particles promise new working fluids for application in energy devices. Many studies have been conducted on thermophysical properties as well as heat and fluid flow characteristics of nanofluids in various systems to discover their advantages compared to conventional working fluids. The main aim of this study is to present the latest developments and progress in the mathematical modeling of nanofluids flow. For this purpose, a comprehensive review of different nanofluid computational fluid dynamics CFD approaches is carried out. This study provides detailed information about the commonly used formulations as well as techniques for mathematical modeling of nanofluids. In addition, advantages and disadvantages of each method are rendered to find the most appropriate approach, which can give valid results.Keywords: nanofluid, CFD, numerical simulation, mathematical modeling, singleand two-phase methods . IntroductionIn general, the assessment of the thermal performance of a system through numerical simulations is much affordable compared to experimental studies with high expenses of material and equipment. The significance of a numerical study is highlighted when a nanofluid is utilized as the working fluid. High costs for the production of nanofluids and difficulties in preparing stable nanofluids are the main barriers to perform experiments with nanofluids. Therefore, numerical modeling of nanofluids, where a suitable approach is selected to simulate the flow, could be the best solution for problems involved with nanoparticle suspensions.However, in spite of considerable developments in computing power and methods, literature review reveals that there is no comprehensive study to conclude the best technique for the modeling of nanofluids. In particular, due to the ultrafine size of nanoparticles, the governing terms in multiphase models are still not entirely identified. In the present work, latest studies on numerical simulations of nanofluid flow are reviewed with a particular focus on different multiphase schemes. . Numerical methods for nanofluids' flow simulationNanofluid computational fluid dynamic CFD modeling can be classified into two main groups single-phase and two-phase models. However, there are few other models that may not be included in these categories, such as Lattice-Boltzmann method LBM . Moreover, different numerical approaches have been employed to solve models mentioned above to predict thermal and hydraulic characteristics of nanofluids flow. Finite volume method FVM and finite element method FEM are two main approaches for solving the governing equations of nanofluid problems. However, finite difference method FDM , control volume-based finite element method, and some novel numerical approaches such as homotopy analysis method HAM and smoothed particle hydrodynamics SPH methods have also been utilized in the previous studies. In this study, a comprehensive review of various numerical ...

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