In recent decades, reverse flow analysis in mixed convection flow has attracted the attention of many researchers owing to its applications in the design of medical and engineering systems. The presence of reverse flow is unfavorable in many respects; therefore, it is crucial to find values of critical parameters affecting the reverse flow to eliminate it. In this paper, the thermal and hydrodynamic behavior of MWCNT-Fe3O4 hybrid nanofluid is explored in a Vertical cylindrical annulus and in the adjacency of radial magnetic field by achieving the results of the exact solution. Furthermore, the effective factors on the reverse flow are investigated, considering the effects of wall movement and suction/injection on it. The range of changes of the governing parameters includes constant velocity of cylinders’ walls A=0−10, B=0−10, Mixed convection parameter η=-1500−1500, dimensionless temperature difference ratio ξ=0−1, Hartman number Ha=0−50, Suction/injection S=-6−6, nanocomposite particles concentration φ=0−0.3% and radios ratio λ=2−10. The results reveal that hybrid nanofluid enhances the heat transfer rate. Moreover, by changing the above-mentioned parameters and selecting the appropriate values for them, the flow, heat transfer and occurrence of reverse flow can be optimally controlled. Meanwhile, such parameters as Ha, S and ξ perform better in eliminating reverse flow.

The unsteady mixed convection flow within a Vertical concentric cylindrical annulus with time-dependent moving walls is investigated. Fluid is suctioned/injected through the cylinder walls of the annulus. The effects of wall movement on flow and heat transfer characteristics inside the cylindrical annulus are sought. An exact solution of Navier-Stokes and energy equations is obtained in this problem, for the fi rst time. Here, the heat transfer occurs from the walls of the hot cylinder with constant temperature to the moving fluid with a cooling role. It is interesting to note that the results indicate that the time-dependency of cylinder wall movements has no effect on the temperature pro le. The results also indicate that usage of an inverse of time for velocity movement causes a signifi cant increase in velocity, stress tensor, and Nusselt number in the vicinity of the moving wall. Also, increasing the mixed convection and suction/injection parameters increases the Nusselt number and decreases the stress tensor on the inner and outer cylinders, respectively; no matter what velocity function is chosen for the moving wall. Therefore, using the velocity function of the wall movement and the change in other nondimensional governing parameters, flow and heat transfer can be controlled.

In this study, a new scheme is suggested to find the analytical approximating solutions for a two-dimensional transient natural convection in a horizontal cylindrical concentric annulus bounded by two isothermal surfaces. The new methodology depends on combining the algorithms of Yang transform and the homotopy perturbation methods. Analytical solutions for the core, the outer layer and the inner layer at small times are found by a new method. Also, the effect of Grashof number, Prandtl number and radius proportion on the heat transfer and the flow of fluid (air) at different values was studied. Moreover, the study calculates the mean of the Nusselt number along with the effect of the Grashof number and radius proportion on it as parameters which acts as clues for heat transfer calculations of the natural convection for the annulus. The results, obtained by using the new method, prove that it is efficient and has high exactness compared to the other methods, used to find the analytical approximate solution for the transient natural convection in a horizontal cylindrical concentric annulus. The convergence of the new method was also discussed theoretically by referring to some theorems, and experientially by a verification of the solutions resulting from the simulations of the convergent condition. Furthermore, the graphs of the new solutions show the veracity, utility and exigency of the new method, and come in line with solutions offered by previous studies.

In this paper, a modified two-fluid model has been adopted to simulate the process of upward Vertical subcooled flow boiling of refrigerant R-113 in a Vertical annular channel at low pressure. The modified model considers temperature dependent properties and saturation temperature variation and was validated against a number of published low-pressure subcooled boiling experiments. The results show good agreement with experimental data including radial profiles of void fraction, phase velocities and liquid temperature. A sinusoidal axial distribution of wall heat flux was applied as well as constant wall heat flux. Results show that by increasing the wall heat flux, the bubble boundary layer will become thicker and the profiles of axial liquid velocity will gradually depart from those of singlephase flow.

Natural convection of an electrically conducting fluid under the different directions of magnetic field in a horizontal cylindrical annulus is numerically studied using lattice Boltzmann method (LBM). The inner and outer cylinders are maintained at uniform temperatures. Detailed numerical results of heat transfer rate, temperature and velocity fields have been presented for salt water with Pr=13.2, Ra=105 to 106 and Ha=0 to 100 for three directions of magnetic field. The computational results reveal that in horizontal cylindrical annulus the flow and heat transfer are suppressed more effectively by direction and intensity of magnetic field. It is also found that the flow oscillations can be suppressed effectively by imposing an external horizontal magnetic field. The average Nusselt number increases with rise in radius ratio but decreases with the Hartmann number. Furthermore, it can be observed that there is a good agreement between the present result and those predicted by available benchmark solutions under the limiting conditions.