Papers by Keyword: Mixed Convection

Abstract: The mixed convection of fluid flow and heat transfer in a discretely heated square lid-driven cavity has been numerically investigated using the lattice Boltzmann method. The fluid inside the inclined cavity is a water-based nanofluid, enhanced with Al₂O₃ nanoparticles. The cavity is discretely heated from the left and bottom walls and cooled from the right wall, while the top wall is adiabatic and moves at a constant velocity. Simulations have been performed to analyze the effects of key controlling parameters, including the Richardson number (Ri), inclination angle (θ), and the solid volume fraction of nanoparticles (ϕ). The results indicate that increasing the inclination angle enhances heat transfer on the left wall but reduces it on the bottom wall. Furthermore, to achieve the lowest mean fluid temperature, an inclination angle of 90° is recommended, regardless of the Richardson number and nanoparticle volume fraction. Additionally, the introduction of nanoparticles into the base fluid improves the heat transfer rate and increases the average temperature within the cavity. Nomenclature

Abstract: In this work, we present a numerical study of mixed convection flows around large-scale heat sinks. It is based on the Cascade Lattice Boltzmann Method (LBM) for values of the Rayleigh number, in transitional regime, in the range 5×10⁷≤Ra≤5×10⁸ and for a Reynolds value fixed at Re=1000. The study is carried out in a rectangular cavity of dimension H subjected to periodic thermal and dynamic boundary conditions on its vertical walls. Two heat sources of (L', l', H/2) with a hot temperature T_h, are placed on the bottom wall of the cavity to simulate heat sinks. Fresh air (for cooling these heat sinks) is injected at a temperature T_c< T_h from the bottom of the cavity through two openings of length L''. The hot air is extracted through an opening (2L'' long) managed on the upper horizontal wall. The preliminary results, presented in this paper, are in the form of streamlines, isotherms and thermal profiles in the range of the Rayleigh number considered. Heat transfer is studied in terms of the average Nusselt number calculated over the entire surface of the two heat sources.

Abstract: In the present study, we have performed a numerical investigation of the effect of aspect ratio (AR), solid-to-fluid volume fraction (χ) of the heated rectangular block and Richardson number on mixed convection heat transfer in a lid-driven square cavity having centered rectangular heated block inside. The vertical walls of the cavity are exposed to the cold temperature while the horizontal walls are kept at adiabatic, with top wall moving to the right with a constant velocity. The cavity is filled with air (Pr = 0.71) as working fluid. A wide range of Ri (0.01 ≤ Ri ≤ 100) by varying Reynolds number at fixed Rayleigh number Ra = 10⁴, aspect ratio (0.5 ≤ AR ≤ 2) and the solid-fluid volume fraction of the block (20% ≤ χ ≤ 50%) are considered. The obtained results indicate that the total average Nusselt number depends strongly on the Richardson number, the aspect ratio and the solid-to-fluid volume fraction, which reaches its maximum for higher values of χ and for AR = 2 (horizontal rectangular block), at low values of Ri. Additionally, it is observed that more effective cooling of the cavity is generally achieved in the scenario where the aspect ratio is 1 (square heated block) and the solid-fluid volume fraction is 20%. In addition, for Ri = 1, when changing the volume fraction of solid-fluid from 20% to χ = 35% / (χ = 50%), an increase around 37.94% /(90.42%) of Nu is achieved at AR = 1. Nomenclature

Abstract: The present study explores combined free-forced convective flow and entropy generation in a constant-volume double lid-driven trapezoidal cavity. All configurations of the isosceles trapezoidal cavity were meticulously designed to possess identical leg lengths and constant volume, ensuring that the same amount of heat is transferred from the cavity’s legs. The cavity has left and right lid-driven walls capable of oscillating upward and downward, while all other domain boundaries remain stationary. The left wall is sustained at a consistently high temperature, whereas the right wall is kept at a stable low temperature, and the upper and lower horizontal walls are thermally insulated. The modelling of this problem was carried out based on the finite volume technique. The obtained results were carefully validated against existing literature related to similar problems. The influence of relevant parameters such as Richardson number (0.01 ≤ Ri ≤ 100), aspect ratio (0.4 ≤ AR ≤ 1) and three distinct moving arrangements (Case-A, Case-B and Case-C) were examined. The findings revealed that heat transport was restricted at high Ri for all the presented aspect ratios, especially for Case-C. For all the presented aspect ratios and cases, entropy generation decreases as Ri increases, with the lowest values ​​observed for Case-A. Trapezoidal cavities with AR = 0.4, 0.6, and 0.8 generate lower entropy than the square cavity at high Ri, but higher entropy at low Ri.

Abstract: This article examines laminar mixed convection of a nanofluid within a square cavity that contains a vertical rectangular obstacle serving as a vortex promoter. Employing Buongiorno's theory, the dimensionless governing equations are numerically solved using the finite element method to analyze the distributions of velocity, temperature, nanoparticle concentration, and entropy generation. Attention is paid to the entropy generation. Results are presented and discussed, showing that increasing the Reynolds number generates a large vortex near the obstacle, which diminishes reverse flow, enhances heat conduction, and increases entropy generation. Moreover, thermophoresis drives tiny nanoparticles from hot to cold regions, affecting heat transfer. Indeed, nanoparticle concentration decreases with higher thermophoresis (N_T) and Brownian motion (N_B) constraints, as these parameters are inversely related to the concentration profile.

Abstract: In the present work, a numerical investigation of the unsteady mixed convection and entropy generation of a nanofluid in an annular cylindrical space is presented using the Buongiorno’s two-phase flow model. It deals with a concentric tube heat exchanger where the inner cylinder rotates with a constant frequency and is maintained at hot temperature, while the outer cylinder is cold. The aim of the present investigation is to highlight the effects of some parameters on the hydrodynamic, thermal and mass behavior of the considered nanofluid as well as on the system irreversibility, namely: the inertia (1 ⩽ Re ⩽ 20), the buoyancy (0 ⩽ Ri ⩽ 5), the mass diffusion (0.1 ⩽ Le ⩽ 10) and the vertical positions of the inner cylinder (-0.4 ⩽ H ⩽ 0.4). Moreover, at specific parameters, an optimal position in terms of heat transfer has been determined. The flow of the nanofluid is two-dimensional and governed by the equations of continuity, momentum, energy as well as volume fraction conservation. After performing a finite element method mesh test and validation with the literature, the Nusselt number and the entropy generation are discussed. The results show that the heat transfer rate and entropy generation increase with increasing values of Richardson and Reynolds number, especially when positioning the inner cylinder in the lower part. On the other hand, the nanoparticles migration under the thermophoretic diffusion decrease with the increase of the Lewis number, which consequent decrease of the heat transfer rate.

Abstract: This present study is intended for a CFD analysis of hydrodynamic and thermal characteristics of water-based fluid containing TiO₂ or CuO nanoparticles flowing in laminar regime in a 3D uniformly heated horizontal annulus utilizing several. Four distinct models have been developed using various combinations (A, B, C and D) of the available theorical-based and experimental-based thermal conductivity and viscosity correlations. A CFD-Fortran code based on the finite volume technique was elaborated for the numerical solution of the mathematical model of the problem. The implications of Grashof number, volume fraction, and type of nanoparticle on isovelocity, isotherms, mean and wall temperatures, Nusselt number, heat transfer coefficient, pressure drop, and thermal performance evaluation criteria are explored using these different models. The results demonstrate that the Nusselt number and heat transfer coefficient of all developed models improve with the addition of nanoparticles. For 2% of nanoparticles’ concentration, the largest enhancement was reached for model D by about 23.5% with respect to the based liquid, while the smallest enhancement was obtained for model B by about 1.16%. The highest Performance Evaluation Criteria (PEC) are attained by employing model D by about 1.263, followed by model C by about 1.074.

Abstract: A study of the heat transport and fluid flow behaviour around a tilted elliptical cylinder that is located concentrically in a square enclosure whose top horizontal wall is driven by a lid in the positive x-direction is presented. Due to the disparities in the results of convective heat transfer in square cavities in the literature, this study seeks to investigate the combined effects of the Grashof number, Aspect ratio of the geometry, and Elliptical cylinder inclination angle on the dynamics of thermal and flow fields within the geometry investigated. COMSOL Multiphysics 5.5 version was used to resolve the non-dimensional transport equations, while simulations were performed to examine the implications of salient parameters such as the elliptical inclination angle , Grashof number and aspect ratio . The simulation outcomes are displayed as average Nusselt numbers, velocity streamlines, and isothermal contours. Findings from this study show that an increase in aspect ratio resulted in increased heat transfer at the elliptical cylinder wall, with the highest rate of heat transfer occurring when .0. Furthermore, the inclination angle increments when Gr= and led to a reduction in the average Nusselt number of the elliptical cylinder wall. At and AR ranges of , the value of the elliptical cylinder wall increased as the ellipse's inclination angle increased. The findings of this study have found use in heat transfer systems, particularly electronic cooling and nuclear technologies.

Abstract: In this work, the investigation has focused on the unsteady hydromagnetic mixed convection couple stress fluid through an inclined linearly stretching sheet. The model equations governing the flow are converted to ordinary differential equations employing appropriate similarity transformation variables. An efficient technique, Runge-Kutta 4th order (RK4) technique together with shooting method is deployed to tackle the dimensionless equations with relevant boundary conditions. The impacts of various parameters such as unsteadiness parameter , Hartman number , mixed convection parameter , concentration buoyancy parameter , angle of inclination , chemical reaction parameter and Schmidt number are analysed and discussed with plots. Fluid velocity decreases as the unsteadiness, Hartman number, Schmidt number, and chemical reaction parameters rise; while the angle of inclination, mixed convection, and concentration buoyancy parameters speed up the flow. Furthermore, the unsteadiness, angle of inclination and mixed convection parameters reduce fluid temperature, while all the parameters reduce flow concentration.

Abstract: This study investigates the implications of the area ratio (AR) and Grashof number (Gr) on fluid flow properties and heat transfer due to mixed convection around heated trapezoidal blocks located concentrically inside a larger trapezium driven by a lid. The outer trapezium's upper and lower horizontal walls are moving in opposite directions. The model developed was solved using the finite element technique. The inner walls of the trapezium are retained at an isothermal temperature, while the slanted outer walls of the trapezium are perfectly insulated. The upper and lower walls of the enclosure are subjected to normalized sinusoidal temperatures. Grashof number in the range of 10³£Gr£10⁵ and area ratios ( ) of , and were investigated. The simulation outcomes are displayed as stream function, isothermal contours, and local Nusselt number. Considering the interval of for the inner block, the Nusselt number increase with diminishing area ratio for the upper wall, while the response of the lower wall to Gr variation is a function of the AR considered. At the bottom wall of the outer trapezium, results showed that the rate of heat transfer was not significantly affected by changes in area ratio. Furthermore, as the AR reduces, the heat transmission along the top wall of the outer trapezium improves with the Grashof number, with the least and peak heat transfer enhancements occurring at 50 % and 100 % percent of the wall length, respectively.