Purpose The purpose of this paper is to investigate analytically the steady general three-dimensional stagnation-point flow of an aqueous titania-copper hybrid nanofluid past a circular cylinder that has a sinusoidal radius variation. Design/methodology/approach First, the analytic modeling of hybrid nanofluid is presented, and using appropriate similarity variables, the governing equations are transformed into nonlinear ordinary differential equations in the dimensionless stream function, which is solved by the well-known function bvp4c from MATLAB. Findings The current solution demonstrates good agreement with those of the previously published studies in the special cases of regular fluid and nanofluids. Graphical results are presented to investigate the influences of the titania and copper nanoparticle volume fractions and also the nodal/saddle indicative parameter on flow and heat transfer characteristics. Here, the thermal characteristics of hybrid nanofluid are found to be higher in comparison to the base fluid and fluid containing single nanoparticles. An important point to note is that the developed model can be used with great confidence to study the flow and heat transfer of hybrid nanofluids. Originality/value Analytic modeling of hybrid nanofluid is the important originality of present study. Hybrid nanofluids are potential fluids that offer better heat transfer performance and thermophysical properties than convectional heat transfer fluids (oil, water and ethylene glycol) and nanofluids with single nanoparticles. In this investigation, titania (TiO2, 50 nm), copper (Cu, 20 nm) and the hybrid of these two are separately dispersed into the water as the base fluid and analyzed.

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The development of easy-to-implement 3D analytical models of the wake is highly desirable, due to the critical role played by the prediction of the wake velocity profile in the estimation of power generation and the layout optimization of wind turbines in wind farms. Herein, a 3D analytical model of the wake (based on a 2D analytical model) has been proposed, which is easy to implement thanks to using the Gaussian function for the description of the velocity profile and assuming an isotropic value for the wake decay coefficient. Validation of the model was carried out using six cases of wind tunnel, field measurement, and simulation data in horizontal and vertical planes, where the model was compared with three 3D analytical models of the literature. The predictions of the model for the normalized wind velocity typically exhibited mean absolute percentage error (MAPE) values of MAPE ≤3 at downwind distances of x ≥ d t (d t is the rotor diameter), which are the economically-and power harvesting-optimized inter-turbine spacing in wind farms. In addition to its theoretical significance, the model bears utility potentials in applications concerning solving practical problems in wind farms.

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This article reports a numerical study of natural convection heat transfer in a differentially heated enclosure filled with a Al2O3–water nanofluid. Fluent v6.3 is used to simulate nanofluid flow. Simulations have been carried out for the pertinent parameters in the following ranges: the Rayleigh number, Ra = 106, 107, and the volumetric fraction of alumina nanoparticles, ϕ = 0 − 4%. The effect of Brownian motion on the heat transfer is considered and examined. The numerical results show a decrease in heat transfer with an increase in particle volume fraction. Similar to experimental results, the Nusselt number increases with the Rayleigh number in the numerical results. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (http://wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21121

A 2D analytical model has been proposed for predicting the wind velocity distribution inside the wake by coupling the Jensen wake model and the Gaussian function using the conservation of mass and momentum. More realistic approaches were adopted in choosing the control volume and calculation of the wake initial radius at the formation and a new modified expression was employed for the wake turbulence intensity to be used for computation of the revised wake decay. Validation of the model was performed using a body of experimental and numerical data, where the model was thoroughly compared with five analytical models of the literature. The model was successful in predicting double-interacting wakes, and gave more accurate single wake velocity profiles at the far wake region, especially at downwind distances of x ≥ 7d t (d t is the rotor diameter), which are the economically-and power harvestingoptimized spacing between the turbines in wind farms.