Heat transfer enhancement and entropy generation are investigated in a nanofluid, stagnation-point flow over a cylinder embedded in a porous medium. The external surface of cylinder includes non-uniform transpiration. A semi-similarity technique is employed to numerically solve the three-dimensional momentum equations and two-equation model of transport of thermal energy for the flow and heat transfer in porous media. The mathematical model considers nonlinear thermal radiation, magnetohydrodynamics, mixed convection and local thermal non-equilibrium in the porous medium. The nanofluid and porous solid temperature fields as well as those of Bejan number are visualised, and the values of circumferentially averaged Nusselt number are reported. The results show that thermal radiation significantly influences the temperature fields and hence affects Nusselt and Bejan number. In general, more radiative systems feature higher Nusselt numbers and less thermal irreversibilities. It is also shown that changes in the numerical value of Biot number can considerably modify the predicted value of Nusselt number and that the local thermal equilibrium modelling may significantly underpredict the Nusselt number. Magnetic forces, however, are shown to impart modest effects upon heat transfer rates. Yet, they can significantly augment frictional irreversibility and therefore reduce the value of Bejan number. It is noted that the current work is the first systematic analysis of a stagnation-point flow in curved configurations with the inclusion of nonlinear thermal radiation and local thermal non-equilibrium.

This paper investigates forced convection of heat and mass from the catalytic surface of a cylinder featuring non-uniform transpiration and impinging flows in porous media. The non-equilibrium thermodynamics including Soret and Dufour effects and local thermal non-equilibrium are considered. Through employing appropriate change of variables, the governing equations in cylindrical coordinate are reduced to nonlinear ordinary differential equations and solved using a finite difference scheme. This results in the calculation of the temperature and concentration fields as well as the local and surface-averaged Nusselt and Sherwood numbers. The conducted analyses further include evaluation of the rate of entropy generation within the porous medium. It is shown that internal heat exchanges inside the porous medium, represented by Biot number, dominate the temperature fields and Nusselt number. This indicates that consideration of local thermal nonequilibrium is of highly important. It is also demonstrated that Dufour and Soret effects can significantly influence the development of thermal and concentration boundary layers and hence modify the values of Nusselt and Sherwood numbers. In particular, it is shown that small variations in Soret and Dufour numbers can lead to noticeable changes in the average Nusselt and Sherwood numbers. Such modifications are strongly dependent upon the type of transpiration and characteristics of the impinging flow. The present work is the first analysis of non-equilibrium effects upon transport by stagnation flows around the curved surfaces embedded in porous media. Keywords Stagnation-point flow Á Local thermal non-equilibrium Á Coupled heat and mass transfer Á Similarity solution Á Soret effect Á Dufour effect Á Entropy generation

A finite difference/front tracking method is used to study the motion of three-dimensional deformable drops suspended in plane Poiseuille flow at non-zero Reynolds numbers. A parallel version of the code was used to study the behavior of suspension on a reasonable grid resolution (128×128×128 grids). The viscosity and density of drops are assumed to be equal to that of the suspending medium. The effect of Capillary number, the Reynolds number, and volume fraction are studied in detail. It is found that drops with small deformation behave like rigid particles and migrate to an equilibrium position about half way between the wall and the centerline (the Segre-Silberberg effect). However, for highly deformable drops there is a tendency for drops to migrate to the middle of the channel, and the maximum concentration occurs at the centerline. The concentration profile obtained across the channel is in agreement with that measured by Kowalewski (T. A. Kowalewski, “Concentration and velocity measurement in the flow of droplet suspensions through a tube,” Exp. Fluids 2, 213 (1984)) experimentally for viscosity ratios less than or equal to one. The effective viscosity of suspension decreases with Capillary number in agreement with the creeping flow limit. Also, the effective viscosity increases with the Reynolds number of the flow.

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In the present work, the effect of the angle of a triangular vortex generator on the heat transfer of a flat plate is investigated. Incompressible three-dimensional fluid flows in a channel with constant temperature boundary condition. The results showed that the Nusselt number increases and the pressure decreases with the Reynolds number. It is demonstrated that the increase in the angle from 30° to 90° has a significant effect on the Nusselt number. The pressure drop remains constant at a 60° angle with increasing Reynolds number. The results revealed that the longitudinal vortices that have an important effect on the heat transfer become stronger at larger angles of the vortex generator.