Narrow Results

The human body regularly produces mucus, and its presence does not mean that something bad is going on. The boundary tissues of mammals, including the nose, throat and lungs, serve as a barrier against pollution and are produced by our respiratory system. Our work describes the flow comprehension of the components moulding the fluid elements of respiratory irresistible infections. A mathematical model is introduced to concentrate on the mucus fluid flow driven by natural convection through asymmetric channel. This study also summarized biological disorder of human lung build-up fluid. The nonlinear governing equations contain importance of heat transfer and fluid motion of mucus fluid. The analytic solutions of unsteady mucus flow through an isothermal channel with thermal conduction are accomplished adopting Laplace transform method. Significance of mucus fluid heat source thermal conductivity on momentum distribution and energy distribution is enumerated. The comparison of various dimensionless parameters is shown graphically with the help of MATLAB software. In order to provide some understanding on the behavior of mucus fluids and heat transfer mechanisms, extension of this study explores at how temperature influences mucus flow properties.

This paper explores natural convection heat removal performance in accordance with the variation of the baffle size and position in a skewed cavity. In this skewed cavity, the top and bottom walls are considered to be adiabatic. The inclined left wall is deliberated at a sinusoidal cool temperature, and the other wall is treated at a hot temperature. The baffle is connected to the hot temperature wall. The dimensionless governing equations will be solved by the Galerkin weighted residual (GWR) technique of the finite element method. The influence of Rayleigh number (${10}^3\le \mathrm{\text{Ra}}\le {10}^6$), baffle sizes ($L=0.20$, 0.35, and 0.50), and baffle positions ($B=0.25$, 0.50, and 0.75) in a fluid with $\mathrm{\text{Pr}}=1.41$ were investigated in this research. The comparisons between the outcome of this work and previously published work in a literature review by Elatar et al.⁸ have been produced to examine the reliability and consistency of the data. The results of the simulation are represented by streamlines, isotherms, local and mean Nusselt numbers, mean fluid temperature, and baffle effectiveness. The results demonstrate that as the Rayleigh number grows, the heat removal performance rate continues to develop in this study. Also, the results revealed that the heat transport rate decreased when gradually raising the baffle length. Baffles can significantly improve the mixing of fluid inside the enclosure, which can mean reductions in reaction times and operating costs, along with increases in heat exchange and efficiency.

The project focuses on simulating natural convection in a tilted quarter-elliptical chamber filled with ${\mathrm{\text{Al}}}_2{\mathrm{\text{O}}}_3$–Cu/Water hybrid Nanofluid, influenced by a magnetic field (MF) at various angles. The chamber’s elliptical shape is modelled with a constant height-to-length ratio of 2. The chamber’s curved wall is cold, while one smooth wall is adiabatic, and the larger wall undergoes three types of heating. The Hartmann number (Ha), MF angle ($\lambda$), chamber wall heating type, inclination angle ($\mathrm{\Gamma })$, and Rayleigh number (Ra) are studied. Results indicate that increasing Ra leads to enhanced convection and a higher average Nusselt number. At $\mathrm{\text{Ra}}=10^4$, heat transmission is primarily through conduction, resulting in the lowest flow power. The presence of a MF slows down heat transportation, especially at $\mathrm{\text{Ra}}=10^4$. The MF’s impact is most significant when applied at a $90^{\circ }$ angle. Constant temperature chamber wall heating yields 75% and 85% higher average Nusselt values compared to sinusoidal and linear heating, respectively. The worst scenario occurs at $\mathrm{\Gamma }=-90^{\circ }$, where the computed Nusselt values and current power are lowest, highlighting the MF’s influence. According to the study, when thermal boundary circumstances and MF angles are just right, the ${\mathrm{\text{Al}}}_2{\mathrm{\text{O}}}_3$–Cu/water hybrid nanofluid greatly improves the transfer of natural convection heat in a tilted cavity. This suggests that thermal management in cooling structures, electronics, and energy-efficient buildings may be improved.

The Taylor series expansion- and least squares-based lattice Boltzmann method (TLLBM) was used in this paper to extend the current thermal model to an arbitrary geometry so that it can be used to solve practical thermo-hydrodynamics in the incompressible limit. The new explicit method is based on the standard lattice Boltzmann method (LBM), Taylor series expansion and the least squares approach. The final formulation is an algebraic form and essentially has no limitation on the mesh structure and lattice model. Numerical simulations of natural convection in a square cavity on both uniform and nonuniform grids have been carried out. Favorable results were obtained and compared well with the benchmark data. It was found that, to get the same order of accuracy, the number of mesh points used on the nonuniform grid is much less than that used on the uniform grid.

Natural convection in an enclosure with different ratios are investigated with the lattice Boltzmann method, and double distribution functions (DDF) are proposed to simulate the velocity and the temperature fields. Meanwhile, compared with other existing results, we studied the effect of the different aspect ratios on heat transfer, and 2D numerical simulation of natural convection flow in a square cavity are performed at Rayleigh numbers 10³–10⁶ with fixed Prandtl number 0.71 in detail. The numerical results of the Nusselt number along the two sidewalls and the maximum velocities along the horizontal and vertical lines through the cavity center are in good agreement with existing results, which shows the accuracy of the present model.

An improved lattice Boltzmann model is proposed for thermal flows in which the viscous heat dissipation and compression work by the pressure can be neglected. In the improved model, the whole complicated gradient term in the internal energy density distribution function model is correctly discarded by modifying the velocity moments' condition. The corresponding macroscopic energy equation is exactly derived through Chapman–Enskog expansion. In particular, based on the improved thermal model, a double-distribution-function lattice BGK model is developed for two-dimensional Boussinesq flow, which is a typical flow with negligible viscous heat dissipation and compression work. A two-dimensional plane flow and the natural convection of air in a square cavity with various Rayleigh numbers are simulated by using the double-distribution-function lattice BGK model. It is found that there is excellent agreement between the present results with the analytical or benchmark solutions.

A finite-difference lattice Boltzmann (LB) algorithm is described on general body-fitted coordinate systems. An alternative treatment for the implicit collision term of the Boltzmann–Bhatnagar–Gross–Krook equation is used, which completely removes the implicitness of the numerical scheme through using the characteristic of collision invariants. LB simulations are carried out for a two-dimensional supersonic viscous flow past a circular cylinder and the natural convection heat transfer in a circular enclosure with an inner hexagonal cylinder for the first time. The pressure coefficient distribution along the surface of the circular cylinder and the Nusselt number in the natural convection obtained from the simulations agree well with previous experimental measurements and/or classical computational fluid dynamics simulations. Comparisons of detailed flow patterns with other studies are also satisfactory.

This study reports the modeling of the turbulent natural convection in a double air-channel solar chimney (SC-DC) and its comparison with a single air-channel solar chimney (SC-C). Prediction of the mass flow and the thermal behavior of the SC-DC were obtained under three different climates of Mexico during one summer day. The climates correspond to: tropical savannah (Mérida), arid desert (Hermosillo) and temperate with warm summer (Mexico City). A code based on the Finite Volume Method was developed and a $k-\omega$ turbulence model has been used to model air turbulence in the solar chimney (SC). The code was validated against experimental data. The results indicate that during the day the SC-DC extracts about 50% more mass flow than the SC-C. When the SC-DC is located in Mérida, Hermosillo and Mexico City, the air-changes extracted along the day were 60, 63 and 52, respectively. The air temperature at the outlet of the chimney increased up to 33%, 38% and 61% with respect to the temperature it has at the inlet for Mérida, Hermosillo and Mexico City, respectively.

In this paper, the magnetic field effects on natural convection of power-law non-Newtonian fluids in rectangular enclosures are numerically studied by the multiple-relaxation-time (MRT) lattice Boltzmann method (LBM). To maintain the locality of the LBM, a local computing scheme for shear rate is used. Thus, all simulations can be easily performed on the Graphics Processing Unit (GPU) using NVIDIA’s CUDA, and high computational efficiency can be achieved. The numerical simulations presented here span a wide range of thermal Rayleigh number ($10^4\le \mathrm{\text{Ra}}\le 10^6$), Hartmann number ($0\le \mathrm{\text{Ha}}\le 20$), power-law index ($0.5\le n\le 1.5$) and aspect ratio ($0.25\le \mathrm{\text{AR}}\le 4.0$) to identify the different flow patterns and temperature distributions. The results show that the heat transfer rate is increased with the increase of thermal Rayleigh number, while it is decreased with the increase of Hartmann number, and the average Nusselt number is found to decrease with an increase in the power-law index. Moreover, the effects of aspect ratio have also investigated in detail.

This three-dimensional (3D) numerical work based on the volume control method quantifies the convective heat transfer occurring in a hemispherical cavity filled with a ZnO–H₂O nanofluid saturated porous medium. Its main objective is to improve the cooling of an electronic component contained in this enclosure. The volume fraction of the considered monophasic nanofluid varies between 0% (pure water) and 10%, while the cupola is maintained isothermal at cold temperature. During operation, the active device generates a heat flux leading to high Rayleigh number reaching $7.29\times 10^{10}$ and may be inclined with respect to the horizontal plane at an angle ranging from 0${}^{\circ }$ to 180${}^{\circ }$ (horizontal position with cupola facing upwards and downwards, respectively) by steps of 15${}^{\circ }$. The natural convective heat transfer represented by the average Nusselt number has been quantified for many configurations obtained by combining the tilt angle, the Rayleigh number, the nanofluid volume fraction and the ratio between the thermal conductivity of the porous medium’s solid matrix and that of the base fluid. This ratio has a significant influence on the free convective heat transfer and ranges from 0 (without porous media) to 70 in this work. The influence of the four physical parameters is analyzed and commented. An empirical correlation between the Nusselt number and these parameters is proposed, allowing determination of the average natural convective heat transfer occurring in the hemispherical cavity.

In this paper, natural convection of power-law Al₂O₃-water nanofluids with temperature-dependent properties in a square enclosure with a circular cylinder is studied. The governing equations of the flow and temperature fields are solved by the lattice Boltzmann method (LBM), and the curved velocity and thermal boundary conditions are treated by immersed boundary method (IBM). The effects of Rayleigh number, power-law index, nanoparticle volume fractions, radius of circular cylinder, nanoparticle diameter and temperature difference on flow and heat transfer characteristics are discussed in detail. The results indicate that the heat transfer rate is increased with the increases of Rayleigh number, radius of circular cylinder and temperature difference, while it generally decreases with an increase in power-law index and nanoparticle diameter. Additionally, it is observed that there is an optimal volume fraction at which the maximum heat transfer enhancement is obtained, and the value of it is found to increase slightly with decreasing the nanoparticle diameter, and to increase remarkably with increasing the temperature difference.

This paper performs a numerical analysis of the natural convection within two-dimensional enclosures (square enclosure and enclosures with curved walls) full of a H₂O-Cu nanofluid. While their vertical walls are isothermal with a cold temperature $T_c$, the horizontal top wall is adiabatic and the bottom wall is kept at a sinusoidal hot temperature. The working fluid is assumed to be Newtonian and incompressible. Three values of the Rayleigh number were considered, viz., 10³, 10⁴, 10⁵, the Prandtl number is fixed at 6.2, and the volume fraction $\varphi$ is taken equal to 0% (pure water), 10% and 20%. The numerical simulation is achieved using a 2D-in-house CFD code based on the governing equations formulated in bipolar coordinates and translated algebraically via the finite volume method. Numerical results are presented in terms of streamlines, isotherms and local and average Nusselt numbers. These show that the heat transfer rate increases with both the volume fraction and the Rayleigh number, and that the average number of Nusselt characterizing the heat transfer raises with the nanoparticles volume fraction.

This paper reports a computational fluid dynamics and experimental study to analyze the effect of surface thermal radiation on the turbulent natural convection in a closed cubic cavity. Experimental and numerical results are compared for low and high wall emissivities. Experimental temperature profiles were obtained at six different depths and heights consisting of 14 thermocouples each. Several turbulence models were evaluated against experimental data. It was found that renormalized $k$-${ϵ}_t$ and standard $k$-$\omega$ turbulence models present the best agreement with the experimental data for emissivities of walls of 0.98 and 0.03, respectively. Thus, the numerical results of temperature fields and flow patterns were obtained with these models. From the results, it was found that the effect of thermal radiation on experimental heat transfer coefficients is significantly, increased between 48.7% ($\mathrm{\text{Ra}}=4.06\times 10^{11}$) and 50.16% ($\mathrm{\text{Ra}}=1.85\times 10^{11}$), when the emissivity of the walls increases from 0.03 to 0.98. Therefore, the radiative exchange should not be neglected in heat transfer calculations in cubic enclosures, even if the temperature difference between heated wall and cold wall is relatively small (between 15 and 30K).

Current investigation deals with influence of inclusion of nanoparticles within the permeable medium within a tank with circular outer wall. The inner surface is hot and the radiative term has been imposed in temperature equation. Vorticity formula helps us to remove the pressure terms from equations and CVFEM was incorporated to calculate the amount of scalars in each node. With correlating the current data from previous paper, verification procedure was done which demonstrates good accuracy. Permeability has crucial role and greater values of Da results in stronger thermal penetration and isotherms become more disturbed. Intensity of cell augments with rise of Da about 70% in absence of Ha. Impose of Rd cannot affect the isotherms too much while it can change the Nu regarding the definition of this factor. When $\text{Da}=0.01$, growth of Ha can decline the strength of eddy about 35%. Given $\text{Ha}=20$, as Da increases, Nu enhances about 10.24% and 0.25% when $\text{Ra}=1$e5 and 1e3, respectively. Replacing platelet with sphere shape can augment the Nu about 0.38% and 0.6% when $\text{Ha}=20$ and 0, respectively.

The magnetic field effect on natural convection flow of power-law (PL) non-Newtonian fluid has been studied numerically using the multiple-relaxation-time (MRT) lattice Boltzmann method (LBM). A two-dimensional rectangular enclosure with differentially heated at two vertical sides has been considered for the computational domain. Numerical simulations have been conducted for different pertinent parameters such as Hartmann number, $\mathrm{\text{Ha}}=0-20$, Rayleigh number, $\mathrm{\text{Ra}}=10^4-10^6$, PL indices, $n=0.6$–1.4, Prandtl number, $\mathrm{\text{Pr}}=6.2\mathrm{\text{(water)}}$, to study the flow physics and heat transfer phenomena inside the rectangular enclosure of aspect-ratio $\mathrm{\text{AR}}=2.0$. Numerical results show that the heat transfer rate, quantified by the average Nusselt number, is attenuated with increasing the magnetic field, i.e. the Hartmann number (Ha). However, the average Nusselt number is increased by increasing the Rayleigh number, $\mathrm{\text{Ra}}$ and decreasing the PL index, $n$. Besides, the generation of entropy for non-Newtonian fluid flow under the magnetic field effect has been investigated in this study. Results show that in the absence of a magnetic field, $\mathrm{\text{Ha}}=0$, fluid friction and heat transfer irreversibilities, the total entropy generation decreases and increases with increasing $n$ and $\mathrm{\text{Ra}}$, respectively. In the presence of the magnetic field, $\mathrm{\text{Ha}}>0$, the fluid friction irreversibility tends to decrease with increasing both the shear-thinning and shear thickening effect. It is noteworthy that strengthening the magnetic field leads to pulling down the total entropy generation and its corresponding components. All simulations have been performed on the Graphical Processing Unit (GPU) using NVIDIA CUDA and employing the High-Performance Computing (HPC) facility.

In this paper, the natural convection flow in a square cavity filled with nanofluid water-${\mathrm{\text{Al}}}_2{\mathrm{\text{O}}}_3$ with a hot circular cylinder in the center of the cavity is numerically analyzed. All the walls are in lower temperatures than the circular cylinder. The Navier–Stokes and energy equations in the primitive variable form are discretized and solved by the finite element method (FEM). The effect of the volume fraction, the radius of the circular cylinder, the temperature and Rayleigh number is considered on the average Nusselt number. For the calculation of the viscosity coefficient and thermal conductivity coefficient of water-${\mathrm{\text{Al}}}_2{\mathrm{\text{O}}}_3$ nanofluid, an experimental model is used which is the function of the volume fraction, temperature and nanoparticles diameter. This model is compared to the Brinkman model for viscosity and Maxwell model for thermal conductivity which are only the functions of volume fraction and are used by many researchers. The results show the experimental model leads to different results in comparison with the Brinkman model and Maxwell model, and indicate that the rate of the heat transfer can increase or decrease with the increase in volume fraction and temperature.

The effect of inclination angle on heat transfer by turbulent natural convection in an inclined open cubic cavity with side length of 1m is experimentally and numerically studied. The wall opposite to the aperture is subjected to uniform heat flux condition with a value of 110W/m², whereas the remaining walls were kept thermally insulated. Six most used two equations turbulence models were tested, with Computational Fluid Dynamics (CFD) software ANSYS FLUENT. The results were obtained for four inclination angle values: 0^∘, 15^∘, 30^∘ and 45^∘. The spatial distribution for temperature, flow pattern and magnitude of velocity are determined and analyzed. An experimental prototype was built to obtain experimental temperature profiles and heat transfer coefficients. The experimental average Nusselt number increases as the inclination angle increases, having a minimum of 203 for 0^∘ case and a maximum of 225 for 45^∘ case. The comparison between experimental and numerical average Nusselt numbers indicates that the minimum average difference is with the realizable $\kappa -{\epsilon }_t$ turbulence model, therefore, this turbulence model is recommended to model this type of systems.

The study of the effect of periodic boundary condition is of primary interest to understand the maximum temperature of the wall and avoid overheating phenomena. Available computational fluid dynamic studies provide information on the effect of the local heat flux or local temperature. This study aims to understand the temperature and fluid flow distribution inside the cavity under such periodic thermal boundary conditions to avoid overheating phenomena. This study reports the numerical results of the natural convection inside an air-filled cavity with its horizontal walls insulated and its vertical walls at different thermal boundary conditions. The heat flux of the left wall and the temperature of the right wall are varied sinusoidally. The Lattice Boltzmann Method (LBM) is used to solve the governing equations with the related boundary conditions. The maximum temperature of the wall under the heat flux and the maximum stream function inside the cavity are investigated. Also, the effect of oscillation period on the maximum temperature of the left wall is studied. The results show that the maximum temperature of the left wall corresponds to the heat flux wavelength of 0.8. Moreover, the stream function value increases intensively by increasing the Rayleigh number.

This paper contains natural convection of Ag–MgO/water micropolar hybrid nanofluid in a hollow hot square enclosure equipped by four cold obstacles on the walls. The simulations were performed by the lattice Boltzmann method (LBM). The influences of Rayleigh number and volume fraction of nanoparticle on the fluid flow and heat transfer performance were studied. Moreover, the effects of some geometric parameters, such as cold obstacle height and aspect ratio, were also considered in this study. The results showed that when the aspect ratio is not large ($\mathrm{\text{AR}}=0.2$ or 0.4), at low Rayleigh number (10³), the two secondary vortices are established in each main vortex and this kind of secondary vortex does not form at high Rayleigh number (10⁶). However, at $\mathrm{\text{Ra}}=10^6$, these secondary vortices occur again in the middle two vortices at $\mathrm{\text{AR}}=0.6$, which is similar to that at $\mathrm{\text{Ra}}=10^3$. At $\mathrm{\text{AR}}=0.2$, the critical Rayleigh number, when the dominated mechanism of heat transfer changes from conduction to convection, is 10⁴. However, the critical Rayleigh number becomes 10⁵ at $\mathrm{\text{AR}}=0.4$ or 0.6. When the cold obstacle height increases, the shape of the vortices inside the enclosure changes due to the different spaces. Besides, at $\mathrm{\text{Ra}}=10^6$, for different cold obstacle heights, the location of the thermal plume is different, owing to the different shapes of vortices. Accordingly, the average Nusselt number increases by increment of the Rayleigh number, nanoparticle volume fraction, cold obstacle height and aspect ratio.

A lattice-Boltzmann scheme for natural convection in porous media is developed and applied to the heat transfer problem of a 1000 kg potato packaging. The scheme has features new to the field of LB schemes. It is mapped on a orthorhombic lattice instead of the traditional cubic lattice. Furthermore the boundary conditions are formulated with a single paradigm based upon the particle fluxes. Our scheme is well able to reproduce (1) the analytical solutions of simple model problems and (2) the results from cooling down experiments with potato packagings.