This paper discusses laminar mixed convection of air flow through vertical tubes. Calculations were performed by solving the Navier-Stokes and energy equations for a number of heating lengths. The Reynolds number based on the fluid bulk velocity and diameter of the tube is Re = 500 and Grashof number based on wall heat flux is Gr = 106. The numerical results have been obtained using a finite-volume code which solves the governing equations in axi-symmetric coordinate system. The pressure field is obtained with the SIMPLE algorithm. The HYBRID scheme is used for the convective terms. The computer code was validated by comparing its predictions with the reported analytical, numerical and experimental results. For various heating lengths the values of Nusselt number and friction coefficient are presented and the effects of heating length on these parameters are studied. It was found that for the buoyancy-aided convection, the velocity in the vicinity of the wall increases while decreases in the core region. These result in an enhancement of wall heat transfer coefficient.
A pore-network model is adopted to simulate fluid flow in porous media. The model is examined for two different hydrophobic loadings. A new algorithm for displacing the fluid inside the porous medium is developed. The algorithm uses an effective pressure as a combination of capillary pressure and hydrodynamic pressure. The capillary pressure is a function of pore size distribution and the wettability characteristics of the medium while the hydrodynamic pressure is correlated to the local saturation of the pores. Thus, the algorithm automatically switches between the capillary driven and the viscous displacement mechanisms based on the value of the local capillary number. The results are compared with the available experimental and numerical data.
Pore network modeling is a relatively new approach for simulation of fluid flows in porous media of the fuel cells. The porous media is simulated with a network of pores connected to each other through throats. The radius of the pores and throats, the length of the throats and other geometric specifications of the network is extracted with respect to the tomography of the porous media under study. The flow of the fluid though the network is then studied using simplified Navier-Stokes equation (usually Darcy’s law). In this paper, a pore-network approach is adopted in order to study the motion of liquid water inside GDL of PEM fuel cells. The effect of wettability of the medium is examined on the movement of water droplets inside the GDL.
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