Selection of suitable heat transfer fluid for heat dissipation is an important consideration in the design of heat exchanging systems. Nanofluid, a colloidal mixture made of a base fluid and a nanoparticle, is a new generation of heat transfer fluids becoming a high potential fluid in heat transfer applications due to enhanced thermal conductivity. Research studies about nanofluids are on the rise owing to the mounting interest and demand for nanofluids as heat transfer fluids in a wide variety of applications. Recently, nanofluid technology has a new dimension of impregnating two or more nanoparticles in base fluids, namely hybrid or composite nanofluids. This paper reviews the preparation of metal and metal oxides nanofluids and hybrid nanofluids and the various techniques used to study the physical and chemical characteristics of nanofluids. Thermo-physical and heat transfer properties of nanofluids including the improved thermal conductivity, viscosity and specific heat models for nanofluids are presented. Finally, various application areas of nanofluids, such as transportation, electronic cooling, energy storage, mechanical applications etc. are discussed.

A review on preparation, characterization, properties and applications of nanofluids

Economic reasons (material and energy saving) leads to make efforts for making more efficient heat exchange. The heat transfer enhancement techniques are widely used in many applications in the heating process to make possible reduction in weight and size or enhance the performance of heat exchangers. These techniques are classified as active and passive techniques. The active technique required external power while the passive technique does not need any external power. The passive techniques are valuable compared with the active techniques because the swirl inserts manufacturing process is simple and can be easily employed in an existing heat exchanger. Insertion of swirl flow devices enhance the convective heat transfer by making swirl into the bulk flow and disrupting the boundary layer at the tube surface due to repeated changes in the surface geometry. An effort has been made in this paper to carry out an extensive literature review of various turbulators (coiled tubes, extended surfaces (fin, louvered strip, winglet), rough surfaces (Corrugated tube, Rib) and swirl flow devices such as twisted tape, conical ring, snail entry turbulator, vortex rings, coiled wire) for enhancing heat transfer in heat exchangers. It can be concluded that wire coil gives better overall performance if the pressure drop penalty is considered. The use of coiled square wire turbulators leads to a considerable increase in heat transfer and friction loss over those of a smooth wall tube.

Review of heat transfer enhancement methods: Focus on passive methods using swirl flow devices

A review on how the researchers prepare their nanofluids

are the liquid density and surface tension).This law is also observed to hold on partially wettable surfaces, provided that liquidsof low viscosity (such as water) are used. The law is interpreted as resulting fromthe eﬀective acceleration experienced by the drop during its impact. Viscous dropsare also analysed, allowing us to propose a criterion for predicting if the spreading islimited by capillarity, or by viscosity.

Maximal deformation of an impacting drop

Three-dimensional fully developed turbulent mixed convection heat transfer of Al2O3/synthetic oil nanofluid in a trough collector tube with a non-uniform heat flux was numerically studied. The effect of AL2O3 particle concentration in the synthetic oil on the rate of heat transfer from the absorber tube was also investigated. Various nanoparticle concentrations (

Heat transfer enhancement in parabolic trough collector tube using Al2O3/synthetic oil nanofluid

Study on heat transfer and friction factor characteristics of γ-Al2O3/water through circular tube with twisted tape inserts with different thicknesses

Droplet impact: Viscosity and wettability effects on splashing.

Experimental investigation of hydrodynamics and heat transfer characteristics of γ-Al2O3/water under laminar flow inside a horizontal tube

In this paper, a systematic study was performed to understand the drop impact on hydrophilic and hydrophobic surfaces that were moving in the horizontal direction. Drops (D0 = 2.5 mm) of liquids with three different viscosities were used. Wide ranges of drop normal velocity (0.5 to 3.4 m s-1) and surface velocity (0 to 17 m s-1) were studied. High speed imaging from the top and side was used to capture the impact phenomena. It was found that drop impact behavior on a moving surface significantly differs from that on a stationary surface at both the lamella extension stage (i.e. t ≤ tmax) and the retraction stage (t > tmax). Starting with the lamella extension stage, it was observed that the drop spreads asymmetrically over a moving surface. It was also found that the splashing behavior of the drop upon impact on a moving surface, unlike the understanding in the literature, is azimuthally different along the lamella contact line. In the case of the drop spreading over a moving surface, the surface movement stretches the expanded lamella in the direction of the surface motion. For hydrophilic surfaces, the stretched lamella pins to the surface and moves with the surface velocity; however, for hydrophobic surfaces, the lamella recoils during such stretching. A new model was developed to determine the splashing threshold of the drop impact on a moving surface. The model is capable of describing the azimuthally different behavior of the splashing which is a function of normal capillary and Weber numbers, surface velocity, and surface wettability. It was also found that the increase of the viscosity decreases the splashing threshold. Finally, comprehensive regime maps of the drop impact outcome on a moving surface were provided for both t ≤ tmax and t > tmax stages.

Understanding the drop impact on moving hydrophilic and hydrophobic surfaces

Study of the spreading of an impacting drop onto a surface has gained importance recently due to applications in printing, coating, and icing Limited studies are conducted to understand asymmetric spreading of a drop seen upon drop impact onto a moving surface; there is no relation to describe such spreading Here, we experimentally studied the spreading of a drop over a moving surface; such study also provides insights for systems where a drop impacts at an angle relative to a surface, ie, drop has both normal and tangential velocities relative to the surface We developed a model that for the first time allows prediction of time evolution for the asymmetric shape of the lamella during spreading The developed model is demonstrated to be valid for a range of liquids and surface wettabilities as well as drop and surface velocities, making this study a comprehensive examination of the topic We also found out how surface wettability can affect the recoil of the drop after spreading and explained the rol

H. Almohammadi

Papers

Study on heat transfer and friction factor characteristics of γ-Al2O3/water through circular tube with twisted tape inserts with different thicknesses

Droplet impact: Viscosity and wettability effects on splashing.

Experimental investigation of hydrodynamics and heat transfer characteristics of γ-Al2O3/water under laminar flow inside a horizontal tube

Understanding the drop impact on moving hydrophilic and hydrophobic surfaces

Asymmetric Spreading of a Drop upon Impact onto a Surface