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In order to improve the performance of a solar collector in low heat transfer rate zones, a 3-D numerical study of the effects of partially moving wall sections and the use of a water nanofluid (CNT) in a tilted parallelepiped solar collector was performed. Equations governing the mixed convection phenomena occurring in the cavity are developed based on the 3-D potential-vorticity formulation and solved using the finite volume method. Two cases related to the direction of the moving surfaces are considered and compared to the base case (no driven walls). The results are presented in term of flow structures, temperature fields and local and average Nusselt numbers. The Richardson number is varied from 0.001 to 10 and the CNT volume fraction from 0 to 0.045. The results showed that for low Richardson values (less than 1), the motion direction of the moving surfaces has no significant effect on heat transfer rates and becomes effective for higher values. The highest rates of heat transfer are found for high Richardson values and CNT volume fractions, while the enhancement ratio (compared to the base case) occurs for low Richardson values. [ABSTRACT FROM AUTHOR]

This study presents 2-D simulations of a flow-through a sudden expansion/contraction micro-channel with the existence of obstacles. The bottom wall is maintained at constant flux, while the other walls are adiabatic. Rectangular adiabatic obstacles are mounted before the expansion region on the upper and lower wall of the channel used. The finite element method was used to discretize the equations that govern the physical model. Results indicate the apparition of a separate vortex, situated in the corner after the sudden expansion of the micro-channel for low Reynolds numbers. For higher values and expansion ratios, the vortex separation length increases. The obtained results show that the obstacles have a considerable effect on the dynamics of the flow and enhancement of heat transfer. [ABSTRACT FROM AUTHOR]

Numerical simulation of the natural convection and entropy generation in an airfilled cubical cavity with active lateral walls is performed in this work. Both the lateral front and right sidewalls are maintained at an isothermal cold temperature. While an isothermal hot temperature is applied for both the lateral back and left sidewalls. The upper and lower walls are kept adiabatic. Entropy generation rates due to the fluid friction and the heat transfer are simulated by using the Second law of thermodynamics. Results are illustrated for Rayleigh numbers varied from (103 ≤ Ra ≤ 106). It was shown that the increase in the Rayleigh number leads to increase the average Nusselt number and to decrease the Bejan number. Also, it was found that both, Sth, and Stot, increase slightly with the increase in Rayleigh number until they reach (Ra = 105) and then begin to jump after this value. After (Ra = 105), the increase in both, Stot, and Sfr, is greater than Sth. Moreover, it was observed that iso-surfaces of Stot are similar to Sth at (103 ≤ Ra ≤ 105), while they are similar to Sfr at high Rayleigh number. [ABSTRACT FROM AUTHOR]

Effect of magnetic field on combined surface tension and buoyancy convection in an enclosure with partially active vertical walls is investigated numerically. The active part of the left side wall is at a higher temperature than the active part of the right side wall. The bottom and the inactive parts of the side walls are adiabatic and capillary forces occur at the top free surface. The governing equations are discretized by the finite volume method. The results are obtained for Pr = 0.054, 0 ≤ Ha ≤ 100, 0 ≤ Ma ≤ 10000, and 2.104 ≤ Gr ≤ 2.106. The flow structure and temperature field were presented by streamlines and isotherms respectively. The surface tension effect of is manifested by increasing Marangoni number. The application of magnetic field was found to control the flow and to oppose the capillary effects. [ABSTRACT FROM AUTHOR]

In the present study, the performance of a proton exchange membrane fuel cells is studied under cathode flooding conditions. A 2-D model of water and heat management based on the laws of conservation and electrochemical equations is used. The performance of the proton exchange membrane cell is evaluated on the basis of the computed average current density and its distribution along the channels. Operating parameters are optimized with the objective of maximizing average current density while minimizing its variations. The problem is formulated into a multi-objective form that is solved by the non-dominated sorting genetic algorithm-II to find the optimal Pareto front. The results of the base case are compared to those of the optimized cell. A 38.94% increase in average current density and a 38.8% decrease in standard deviation are obtained. [ABSTRACT FROM AUTHOR]

A computational analysis of the natural ventilation process and entropy generation in 3-D prismatic greenhouse was performed using CFD. The aim of the study is to investigate how buoyancy forces influence air-flow and temperature patterns inside the greenhouse having lower level opening in its right heated façade and also upper level opening near the roof top in the opposite cooled façade. The bottom and all other walls are assumed to be perfect thermal insulators. Rayleigh number is the main parameter which changes from 10³ to 106 and Prandtl number is fixed at Pr = 0.71. Results are reported in terms of particles trajectories, iso-surfaces of temperature, mean Nusselt number, and entropy generation. It has been found that the flow structure is sensitive to the value of Rayleigh number and that heat transfer increases with increasing this parameter. Also, it have been noticed that, using asymmetric opening positions improve the natural ventilation and facilitate the occurrence of buoyancy induced upward cross air-flow (low-level supply and upper-level extraction) inside the greenhouse. [ABSTRACT FROM AUTHOR]

A numerical study of the 3-D double-diffusive natural convection in an inclined solar distiller was established. The flow is considered laminar and caused by the interaction of thermal energy and the chemical species diffusions. The governing equations of the problem, are formulated using vector potential-vorticity formalism in its 3-D form, then solved by the finite volumes method. The Rayleigh number is fixed at Ra = 105 and effects of the buoyancy ratio and inclination are studied for opposed temperature and concentration gradients. The main purpose of the study is to find the optimum inclination angle of the distiller which promotes the maximum mass and heat transfer. [ABSTRACT FROM AUTHOR]

In this article we studied the effect of radiative transfer and the aspect ratio on the 3-D natural convection. Prandtl and Rayleigh numbers are fixed at 13.6 and 105, respectively. Equations of natural convection are expressed according the vorticity-stream function formulation. This equations and radiative transfer equation are descritized by volume control method and the FTn finite volume method, respectively. Obtained simulation show that the principal flow structure is considerably modified when the radiation-conduction parameter was varied. However, the peripheral spiraling motion is qualitatively insensitive to these parameters. [ABSTRACT FROM AUTHOR]