Defect and Diffusion Forum Vol. 406

Abstract: In the present numerical study, the convection diffusion phenomena associated with solid-liquid phase transition processes during phase change material (PCM) melting within a rectangular cavity is studied. The cavity is heated from left wall with a sinusoidal temperature distribution. Initially the enclosure was filled by solid gallium at melting temperature 29.78°C. The enthalpy-based lattice Boltzmann method (LBM) with D2Q9 particle velocity model is used to solve density, velocity and temperature fields. Influence of Rayleigh number ranging from 10³ to 4×10⁵ on streamlines, isotherms and liquid fraction is analyzed. The results indicate that natural convection of liquid phase change material (PCM) plays a significant role in the melting heat transfer of PCM. It is found that the rate of the melting increases with the increase in the values of the Rayleigh number.

Abstract: Forced convection in a ventilated enclosure with aspect ratio 2 is studied. Three heat sources simulating electronic component are placed in the bottom wall of the cavity, all walls are kept insulated. With varying the inlet and the outlet location of cold air firstly then swapping the location of the heat sources, the optimal cooling strategy was identified. Consideration was given to steady two-dimensional laminar flow and Reynolds number (Re) in the range 10–1500. The governing equations along with the boundary conditions are solved by using the control volume method. Calculations showed that enhancement in heat transfer occurred, and the results indicate that there exists an optimal location of ventilation ports and an optimal disposition of heat sources for which the heat transfer is maximized for all ranges of Reynolds numbers.

Abstract: In this work, we performed a numerical simulation of laminar forced convection and, in an annular space inside a vertical circular cylinder with an inner coaxial rectangular cylinder having an aspect ratio (height/radius) γ=2, filled with a liquid metal (Pr = 0.0023). Six annular gaps R =0.9, 0.8, 0.7, 0.6, 0.5 and 0.4 were studied. The governing equations are solved using the ANSYS Fluent code which is based on the finite volume method. SIMPLE algorithm is employed for the pressure-velocity coupled momentum equations. Two cases of the rotating parts of the cylinders are investigated and the effect of Reynolds number on the flow are examined. The obtained results of the forced convection show that the increase of the Reynolds number Re affects straightly on the structure of the flow wherever the velocity field are destabilized and the strongest stabilization of the velocity field occurs when the flow generated by the rotating of the circular cylinder and the rectangular cylinder.Keywords: forced convection, annular gap, circular cylinder, rectangular cylinder, co-rotating.

Abstract: Combined free and forced convection in a square cavity filled with a viscous fluid characterized by a small Prandtl number is studied numerically. The left wall is moving with a constant velocity v and is maintained at a local cold temperature Tc, while the right wall is fixed and maintained at a local hot temperature Th (Tc <Th). The top and bottom walls of the cavity is assumed to be adiabatic. The governing Navier-Stokes, and energy equations along with appropriate boundary conditions are solved using the finite-volume method. The flow and temperature fields are presented by stream function and isotherms, respectively. The effects of important parameters such as Reynolds number, Prandtl number, and Grashof number on the transition from forced convection to mixed convection are investigated. Results indicate that increasing Reynolds number results to fluid acceleration and, thus, to flow transition. Results also show that Grashof and Prandtl's numbers influenced the conditions for the transition to the mixed convection regime.

Abstract: This paper studies the effects of heat generation and chemical reaction on the coupled conjugate heat and mass transfer by MHD laminar mixed convective flow along a vertical slender hollow cylinder. The governing boundary layer equations along with the boundary conditions are first cast into a dimensionless form by a non similar transformation and the resulting equations are then solved by the finite difference method using Matlab^@ following the code bvp4c. Numerical results of the velocity, temperature and concentration for different values of the conjugate heat transfer parameter p, the magnetic parameter M, the heat generation Q, and the chemical reaction K are studied. The local skin friction, Nusselt number and Sherwood number are also analyzed and presented graphically. In the numerical ranges of the main parameters, it is found mainly that working with strong conjugate heat transfer or/and all others parameters affects negatively the Nusselt and Sherwood numbers. The same trend is revealed for the skin friction factor.

Abstract: In this work, the heat transfer by conduction and convection mode and effect of fluid flow on the morphology of the weld pool and the welding properties is investigated during Tungsten Inert Gas (TIG) process. In the first part, a computation code under Fortran was elaborated to solve the equations resulting from the finite difference discretization of the heat equation, taking into account the liquid-solid phase change with the associated boundary conditions. In order to calculate the velocity field during welding, the Navier-Stokes equations in the melt zone were simplified and solved considering their stream-vorticity formulation. A mathematical model was developed to study the effect of the melted liquid movement on the weld pool. The evolution of the fraction volume of the liquid and the thermal fields promoted the determination of the molten zone (MZ) and the Heat Affected Zone (HAT) dimensions, which seems to be in good agreement with literature.

Abstract: In the present study, a numerical investigate the transport mechanism of laminar mixed convection in a vented enclosure. The walls of the cavity were kept adiabatic except the right vertical wall which was equipped with three fins dissipating the heat at a constant temperature. The equations of considered phenomenon were established and discretized by the finite difference method. The sweeping method line-by-line and the Thomas Algorithm (TDMA) were used for the resolution of the system of discretized equations. The results obtained showed that both the variations of the Prandtl and Richardson number have important effects on the flow structure and on the heat transfer.

Abstract: The effect of buoyancy ratio on the two dimensional natural convection heat and mass transfer generated in an inclined square bi-L-shaped layered porous cavity filled with Newtonian fluid has been investigated numerically. Each porous layer is considered isotropic, homogeneous and saturated with the same fluid. The cavity is heated and salted from below where as the vertical walls are assumed to be adiabatic and impermeable. The physical model for the momentum conservation equation makes use of the Darcy-Brinkman-Forcheimer model, and the set of coupled equations is solved using a finite volume approach. The power-law scheme is used to evaluate the flow, heat and mass fluxes across each of the control volume boundaries. Tri diagonal matrix algorithm with under-relaxation is used in conjunction with iterations to solve the nonlinear discretized equations. An in-house code developed for this study is validated using previous studies. The results are presented graphically in terms of streamlines, isotherms and iso-concentrations. In addition, the heat and mass transfer rate in the cavity is measured in terms of the average Nusselt and Sherwood numbers.

Abstract: A numerical simulation was performed in four geometries with different boundary conditions; two geometries have top walls moving with a constant horizontal velocity U₀ in two opposite directions, while the other geometries have vertical walls moving in two opposite directions with a constant vertical velocity V₀. These cavities are filled with hybrid nanofluid Al₂O₃-Cu/water, and heated by two constant flow heat sources placed on the left vertical wall. The moving wall and the other walls are respectively maintained at a local cold temperature Tc. The interest of this work is to see the effects generated by incorporation of hybrid nanofluids on the mixed convection flow, and to make an analysis of the entropy production in the mixed convection problem in order to be able to choose the geometry with different boundary conditions among the four geometries with different boundary conditions that will ensure energy efficiency. The finite volume method was used to solve the heat transfer flow equations across the physical domain with the SIMPLER algorithm. The influence of relevant parameters such as Richardson and Reynolds numbers and volume fraction of nanoparticles on entropy generation and heat transfer rate were studied. It was found that entropy generation decreases with increasing Richardson number, Reynolds number and that incorporation of a hybrid Al₂O₃-Cu/water nanofluid in the base fluid improves the high heat transfer rate.