Al-Qadisiyah Journal for Engineering Sciences

Abstract This work focuses on a numerical study of the natural convection of a non-Newtonian viscoplastic fluid within a cubic enclosure. The viscoplastic behavior is described by the Bingham model. The considered three-dimensional convective flow is confined within a cavity, subjected to a horizontal temperature gradient, where the vertical walls have two imposed temperatures while the rest of the walls are adiabatic. The Navier-Stokes equations, along with the mass and energy conservation equations, are numerically solved. Fluid flow and heat transfer characteristics are systematically studied over a wide range of Rayleigh numbers Ra (10³ - 10⁶) and Bingham number Bn (0 - 20). Finally, comparisons were made with previous results obtained in two dimensions in order to analyze the existence of a three-dimensional effect on the flow of the Bingham fluid. The results show that the Nusselt number decreases with the increase of the Bingham number, and for the large values ​​of the latter the heat transfer is done by conduction. It is also noteworthy that the critical Bn of the 2D model is higher than that of the 3D model, which confirms the existence of the three-dimensional effect. This is attributed to the presence of a wall along the Z axis which hinders and limits the flow of fluid within the enclosure.

Abstract Researchers in heat transfer are paying close attention to nanofluids because of their potential as high-performance thermal transport media. In light of natural convection's enormous significance, the addition of nanoparticles significantly enhances the thermophysical properties of the nanofluids compared to the base fluid. In this study, experimental work was used to evaluate the influence of CuO nanoparticles on natural convection with magnetohydrodynamic (MHD) flow in a square cavity. The cavity’s left and right vertical walls were maintained at different temperatures, and the top and bottom walls of the cavity were insulated. This experimental study applied a horizontal magnetic field with uniform strength. Results were obtained for a variety of Hartmann numbers ranging from 0–300, Rayleigh numbers going from 2.76E+8 to 6.89E+8, and solid volume fractions ranging from 0 to 1.5%Vol. Results showed that the heat transfer coefficient and Nusselt number values decreased with the increase in the values of the Hartmann number, except for the heat transfer coefficients at Ha=100 and 150 are larger than the heat transfer coefficients at Ha= 0. The maximum heat transfer coefficient and Nusselt number enhancement were 40.8% and 28.5%, respectively, at a 1.5% volume concentration of CuO-water nanofluid, Ra= 6.7E+8 and Ha=100 compared with pure fluid (water) at Ha=0.

Abstract The natural convection heat transfer has many applications in engineering like solar collectors, cooling of electronic equipment and geothermal engineering. The present work demonstrates the recent publications in the last ten years in this specific subject for a body located in complex shapes like rhombic, wavy, trapezoidal, elliptical and Parallelogrammic enclosure. Many parameters like Ra, Nu, number of undulations, the position of the inner body had been addressed and discuss to draw the main conclusions and recommendations.  It is worthy to mention that wavy enclosure had been investigated less than the other simple enclosure shapes due to its complexity. Beside that entropy generation should be included in the future studies in complex shapes of enclosure as this will helps the researchers to extended their studies. The inner bodies inside trapezoidal, parallelogrammic enclosure are very limited and more investigation should be done.

Abstract Numerical investigate of double-diffusive natural convection in an inclined porous square. Two opposing walls of the square cavity are adiabatic; while the other walls are, kept at constant concentrations and temperatures. The Darcy–Forchheimer–Brinkman model is used to solve the governing equations with the Boussinesq approximation. A code written in FORTRAN language developed to solve the governing equations in dimensionless forms using a finite volume approach with a SIMPLER algorithm. The results presented in U-velocity and V-velocity, isotherms, iso-concentration, streamline, the average Nusselt number, and the average Sherwood number for different values of the dimensionless parameters. A wide range of these parameters have been used including; Darcy Number, modified Rayleigh number, Lewis number, buoyancy ratio, and inclination angle.  The results show that for opposite buoyancy ratio (N≤-1), the Nu decreases when the Le increases and the Sh increase when the Le increases. For an (N>0), the Nu increases when the Le increases until Le is equal to 1 and then it decreases, also Sh increases when the Le increases

Abstract The characteristics of the conjugate natural convection of (Al₂O₃-water) nanofluid inside differentially heated enclosure is numerically analyzed using COMSOL Multiphysics (5.3a). The enclosure consists of two vertical walls, the left wall has a thickness and maintain at a uniform hot temperature, while the opposite wall at cold temperature and the horizontal walls are isolated. A high thermal conductivity thin baffle has been added on the insulated bottom wall at a different inclination angles. The effect of the volume fractions of nanoparticles (f), Rayleigh number (Ra), solid wall thermal conductivity ratio (Kr), baffle incline angles (Ø) and the thickness of solid wall (D) on the isothermal lines, fluid flow patterns and the average Nusselt number (Nu)  has been investigated. At low Rayleigh number (Ra=10³ to 10⁴) the Isothermal lines are parallel with the vertical wall which is characteristic of conduction heat transfer. on the other hand, when Rayleigh number increase to (Ra=10⁶),  the isotherms lines distribution in the inner fluid become parallel curves with the adiabatic horizontal walls of the enclosure and smooth in this case convection heat transfer becomes dominant. As the Rayleigh number further increases, the average Nusselt number enhance because of buoyancy force become stronger. In addition, the fluid flow within the space is affected by the presence of a fin attached to the lower wall that causes blockage and obstruction of flow near the hot wall, hence the recirculation cores become weak and effect on the buoyant force. The maximum value of the stream function can be noticed in case of nanofluid at (Ø=60), whereas they decrease when (Ø > 60), where the baffle obstruction causing decreases in flow movement. So that the left region temperature increases which cause reduction of the convective heat transfer by the inner fluid temperatures. This is an indication of enhancing of insulation. When the inclination angle increases (Ø >90), the baffle obstruction on flow and fluid resistance becomes smaller and the buoyancy strength increase, as a result, the heat transfer is increasing in this case. As a result of increasing the thermal conductivity from 1 to 10, an increase in the amount of heat transferred through the solid wall to the internal fluid have been noticed. This change can be seen in the isothermal lines, also, there was growth and an increase in the temperature gradient. The increasing of wall thickness from (D=0.1 to 0.4) leads to reduce the intensive heating through the solid wall as well as small heat transferred to the inner fluid. Therefore, it can be noticed that when the wall thickness increases the stream function decrease.

Abstract In this work, a numerical simulation of natural convection in an oblique enclosure filled with silver-water nanofluid is obtained for different values of Rayleigh numbers, volume fraction and inclination angle of sloping walls. The considered oblique enclosure with left and right side walls are maintained at constant cold temperature (Tc). The horizontal top wall of enclosure is kept insulated, but the bottom is maintained at constant hot temperature (Th). The present work is utilized to obtain results in the range of Rayleigh number (103-106) , volume fraction of nanofluid varied from (0-0.2) and inclination angle of side walls are (-60o, -30o, 0o, 30o, 60o). The Prandtle number is 6.0. The governing equations in the two-dimensional are solved numerically by using finite-difference technique. Comparisons with other works are performed and the results are found to be in good agreement. The obtained results are shown in the form of stream function, isothermal lines and average Nusselt numbers. It observed from results that acute shaping wall and Ag-nanoparticles with high concentration are effective to enhance the rate of heat transfer, also; the average Nusselt number for all range of inclination angle increases with increase in the Rayleigh number and the solid volume fraction of the Nanofluid.

Abstract Natural convection of an air filled partitioned rectangular enclosure is studied numerically. Top and bottom of the enclosure are adiabatic; the two vertical walls are isothermal. Two perfectly insulated baffles were attached to its horizontal walls at symmetric position. The flow is assumed to be two-dimensional. The discretized equations were solved by finite volume method.The study was performed for different values of Rayleigh numbers Ra ( ), baffles length and position ( )(0-0.8,0.2-0.8) and aspect ratios of the enclosure. The effect of ( and on heat transfer and flow were addressed.Two different patterns of the flow field were observed. The first is the flow circulate in single primary vortex strangled by two trapped fluids and the second pattern is the flow consist of two vortexes separated by one trapped fluid.With increasing of Ra heat transfer rate(Nusselt number)increased and for increasing baffles length the heat transfer rate decreases. The numerical results of the values of average Nusselt number and maximum absolute stream function have been confirmed by comparing it with similar previous works using the same boundary conditions. Good agreement was obtained.