Showing papers in "Numerical Heat Transfer Part A-applications in 2008"
TL;DR: In this paper, a two-dimensional steady and laminar natural convection in an air-filled (Pr = 0.7) rectangular enclosure is investigated numerically, where the horizontal walls are thermally insulated and the vertical side walls have two spatially varying sinusoidal temperature distributions of different amplitudes and phases.
Abstract: A two-dimensional steady and laminar natural convection in an air-filled (Pr = 0.7) rectangular enclosure is investigated numerically. The horizontal walls are thermally insulated and the vertical side walls have two spatially varying sinusoidal temperature distributions of different amplitudes and phases. The governing equations in primitive variables are discretized by the finite-volume method and solved by the SIMPLE algorithm. The fluid flow and heat transfer characteristics are systematically investigated over a wide range of Rayleigh number (Ra = 103–106), amplitude ratio ( ), phase deviation (φ = 0 − π), and aspect ratio (Ar = 0.25–4). The results show that the natural-convection heat transfer in enclosures with two sinusoidal temperature distributions on the side walls is superior to that with a single sinusoidal temperature profile on one side wall.
98 citations
TL;DR: In this article, the application of the lattice Boltzmann method (LBM) to the analysis of natural convection in the presence of volumetric radiation in a square cavity containing an absorbing, emitting, and scattering medium is discussed.
Abstract: This article deals with the application of the lattice Boltzmann method (LBM) to the analysis of natural convection in the presence of volumetric radiation in a square cavity containing an absorbing, emitting, and scattering medium. Separate particle distribution functions in the LBM are used to calculate the density and velocity fields and the thermal field. The radiative term of the energy equation is computed using the finite-volume method. Streamlines, isotherms, and Nusselt number are analyzed for the effects of different parameters such as Rayleigh number, convection-radiation parameter, extinction coefficient, and scattering albedo.
96 citations
TL;DR: In this article, a thermal wave model of bioheat transfer, together with a seven-flux model for light propagation and a rate process equation for tissue damage, is presented to investigate thermal damage in biological tissues.
Abstract: To ensure personal safety and improve treatment efficiency in laser medical applications, one of the most important issues is to understand and accurately assess laser-induced thermal damage to biological tissues. Biological tissues generally consist of nonhomogeneous inner structures, in which heat flux equilibrates to the imposed temperature gradient via a relaxation phenomenon characterized by a thermal relaxation time. Therefore, it is naturally expected that assessment of thermal damage to tissues could be inaccurate when a classical bioheat conduction model is employed. However, little attention has been given to studying the impact of the bioheat non-Fourier effect. In this article, a thermal wave model of bioheat transfer, together with a seven-flux model for light propagation and a rate process equation for tissue damage, is presented to investigate thermal damage in biological tissues. It is shown that the thermal damage assessed with the thermal wave bioheat model may differ significantly from ...
72 citations
TL;DR: In this article, heat transfer characteristics in the single slot jet impinging cooling process of constant heat flux surface are numerically investigated by using Galerkin finite element method by employing five two-equation turbulence models based on Reynolds-averaged Navier-Stokes approach.
Abstract: In this study, heat transfer characteristics in the single slot jet impinging cooling process of constant heat flux surface are numerically investigated. It is assumed that the flow is turbulent, two-dimensional and in steady state. Governing equations are solved by using Galerkin finite element method by employing five two-equation turbulence models based on Reynolds-averaged Navier-Stokes (RANS) approach. Although the most satisfactory results are obtained with nonlinear algebraic stress model of Shih-Zhu-Lumley in stagnation region, overall performance of RNG and standard k − e models are better in comparison with other models by considering entire region. Subsequent computations are performed with RNG and standard k − e models for nozzle to plate spacing and Reynolds numbers in the ranges of 4 ≤ z/D h ≤ 10 and 4000 ≤ Re ≤ 12000, respectively. Also, inlet turbulence intensity and heat flux boundary conditions effects on heat transfer are investigated. Property variation and buoyancy effects are consid...
61 citations
TL;DR: In this article, the authors explored the exploration of the lattice Boltzmann method (LBM) and the finite-volume method (FVM) in conjunction with the GA for estimation of unknown parameters in an inverse transient conduction-radiation problem.
Abstract: This article deals with the exploration of the lattice Boltzmann method (LBM) and the finite-volume method (FVM) in conjunction with the genetic algorithms (GA) for estimation of unknown parameters in an inverse transient conduction-radiation problem. The conducting-radiating planar participating medium is absorbing, emitting, and scattering. Its boundaries are diffuse gray. In both the direct and inverse methods, the energy equations are solved using the LBM, and the FVM is employed to compute the radiative information. In the inverse method, the optimization is achieved using the GA. For a given set of parameters, first a direct problem is solved using the LBM-FVM, and temperature fields are estimated at various time levels, which, in the inverse problem, are taken as exact. Effects of measurement errors are also considered. With temperature fields known from the direct method, in the inverse method, too, the LBM-FVM combination is used to solve the energy equation involving volumetric radiation, and th...
53 citations
TL;DR: In this paper, a numerical procedure for multiple objectives to optimize staggered elliptic-shaped short pin-fin arrays is presented, where the multiobjective problem is to achieve an acceptable compromise between augmentation of turbulent heat transfer and reduction in friction loss.
Abstract: This work presents a numerical procedure for multiple objectives to optimize staggered elliptic-shaped short pin-fin arrays. The multiobjective problem is to achieve an acceptable compromise between augmentation of turbulent heat transfer and reduction in friction loss. Four nondimensional variables, pin-fin height-to-channel height ratio, major axis length-to-channel height ratio, minor-axis length-to-channel height ratio, and pin-fin pitch-to-channel height ratio are chosen as design variables. The D-optimal method is used to determine the training points. The response surface method is used to approximate the Pareto optimal front with Reynolds-averaged Navier-Stokes analysis of the flow and heat transfer using the shear stress transport (SST) turbulence model. The Pareto-optimal solutions are obtained using a combination of an evolutionary algorithm and a local search.
50 citations
TL;DR: In this article, a three-dimensional simulation of the turbulent reactive flow and radiative heat transfer problem in a walking-beam-type slab reheating furnace was performed by the commercial software STAR-CD.
Abstract: Three-dimensional of the turbulent reactive flow and radiative heat transfer problem in a walking-beam-type slab reheating furnace simulations are successfully performed in the present study by the commercial software STAR-CD. The geometric model built herein considers the whole furnace, including the burners, the walking-beam system with skid buttons, the slab, the dams, and the down-take outlet. The results show that the difference between the predicted and measured heating efficiencies is 6.8%, and that the predictions of the temperatures at selected checking points differ from the measurements within only 10%. The present study also shows that the walking-beam system causes significant radiation shielding to the bottom surface of the slab, resulting in skid marks.
50 citations
TL;DR: In this article, the authors show numerical results for a jet impinging against a flat plane covered with a layer of a porous material, which is maintained at a higher temperature than the incoming fluid.
Abstract: This work shows numerical results for a jet impinging against a flat plane covered with a layer of a porous material, which is maintained at a higher temperature than the incoming fluid. Parameters such as permeability and thickness of the porous layer and thermal conductivity ration are varied in order to analyze their effects on the local distribution of Nu. The macroscopic equations for mass, momentum, and energy are obtained based on a volume-average concept. The numerical technique employed for discretizing the governing equations was the control volume method with a boundary-fitted nonorthogonal coordinate system. The SIMPLE algorithm was used to handle the pressure-velocity coupling. Results indicate that inclusion of a porous layer decreases the peak in Nu avoiding excessive heating or cooling at the stagnation point. Also found was that the integral heat flux from the wall is enhanced for certain range of values of layer thickness, porosity, and thermal conductivity ratio.
48 citations
TL;DR: In this article, the effects of channel size, contact angle, and wall superheat on the bubble growth and heat transfer in a microchannel were investigated by solving the equations governing conservation of mass, momentum, and energy in the liquid and vapor phases.
Abstract: The bubble dynamics and heat transfer associated with nucleate boiling in a microchannel is studied numerically by solving the equations governing conservation of mass, momentum, and energy in the liquid and vapor phases. The liquid-vapor interface is tracked by a level set method which is modified to include the effects of phase change and contact angle. Also, the method is coupled with a simple and efficient model for predicting the evaporative heat flux from the liquid microlayer. The effects of channel size, contact angle, and wall superheat on the bubble growth and heat transfer in a microchannel are investigated.
48 citations
TL;DR: In this article, the authors applied the lattice Boltzmann method (LBM) to solve the energy equation of a combined radiation and non-Fourier conduction heat transfer problem.
Abstract: This article concerns the application of the lattice Boltzmann method (LBM) to solve the energy equation of a combined radiation and non-Fourier conduction heat transfer problem. The finite propagation speed of the thermal wave front is accounted by non-Fourier heat conduction equation. The governing energy equation is solved using the LBM. The finite-volume method (FVM) is used to compute the radiative information. The formulation is validated by taking test cases in 1-D planar absorbing, emitting, and scattering medium whose west boundary experiences a sudden rise in temperature, or, with adiabatic boundaries, the medium is subjected to a sudden localized energy source. Results are analyzed for the various values of parameters like the extinction coefficient, the scattering albedo, the conduction-radiation parameter, etc., on temperature distributions in the medium. Radiation has been found to help in facilitating faster distribution of energy in the medium. Unlike Fourier conduction, wave fronts have b...
45 citations
TL;DR: In this article, the coupling between natural convection and surface radiation in a square cavity with its vertical walls submitted to different heating models is studied numerically using a finite-difference procedure.
Abstract: The coupling between natural convection and surface radiation in a square cavity with its vertical walls submitted to different heating models is studied numerically using a finite-difference procedure. The temperature of the left vertical surface (hot temperature) is varied sinusoidally with time, while that of the opposite surface (cold temperature) is maintained constant. The remaining horizontal walls are considered adiabatic. The parameters governing the problem are the emissivity of the walls (0 ≤ e ≤ 1), the amplitude (0 ≤ a ≤ 1) and the period (0.001 ≤ τ ≤ 1) of the variable temperature, the Rayleigh number ( Ra = 106 ), and the Prandtl number ( Pr = 0.72). The effect of these parameters on heat transfer and fluid flow within the cavity is examined. Streamlines, isotherms, and total Nusselt numbers are presented for various typical combinations of the governing parameters. The results obtained show that the heat transfer could be significantly enhanced, with respect to the case of a constant heati...
TL;DR: In this paper, a combination of finite element (FE) and differential quadrature (DQ) methods was employed for solving the direct inverse heat conduction problem of functionally graded materials (FGMs).
Abstract: Two-dimensional transient inverse heat conduction problem (IHCP) of functionally graded materials (FGMs) is studied herein. A combination of the finite element (FE) and differential quadrature (DQ) methods as a simple, accurate, and efficient numerical method for FGMs transient heat transfer analysis is employed for solving the direct problem. In order to estimate the unknown boundary heat flux in solving the inverse problem, conjugate gradient method (CGM) in conjunction with adjoint problem is used. The results obtained show good accuracy for the estimation of boundary heat fluxes. The effects of measurement errors on the inverse solutions are also discussed.
TL;DR: In this paper, the authors quantify the behavior of popular turbulence models in regions where the flow is laminar according to the accepted Reynolds number criteria, and evaluate three models: 1) standard k-e, 2) k-ω, and 3) SST.
Abstract: Attention is focused on physical situations in which separate zones of laminar and turbulent flow coexist within a given device. The specific motivating application for this study is a complex fluid-flow manifold used in the thermal management of electronic equipment. The adopted approach to the solution of this fluid-flow problem was to employ a turbulent-flow model for the entire solution space but to use several turbulence models as inputs. The goal of the study was to quantify the behavior of popular turbulence models in regions where the flow is laminar according to the accepted Reynolds number criteria. Three models were considered: 1) standard k-e, 2) k-ω, and 3) SST. Evaluation of the models was based on the values of the ratio μ t /μ in nominally laminar regions, where μ t and μ are, respectively, the turbulent and molecular viscosities. Values of this ratio well below one in a nominally laminar region would indicate that a turbulence model reduces, in effect, to a laminar model. It was found tha...
TL;DR: In this paper, a three-dimensional simulation is performed for the turbulent reactive flow and radiactive heat transfer in the walking-beam-type slab reheating furnace using STAR-CD software.
Abstract: In the present study, a three-dimensional simulation is performed for the turbulent reactive flow and radiactive heat transfer in the walking-beam-type slab reheating furnace using STAR-CD software. The geometric model takes care of all components of the furnace. To obtain a steady solution, the walking beams are assumed fixed in the furnace and the slab is modeled as a laminar flow having a very high viscosity and thus moving at a nearly constant speed. The temperature distributions of the slab and the gas mixture are obtained through a coupled calculation. The simulation results successfully predict the temperature distribution inside the slab and the heat flux on the slab surfaces, providing an opportunity for a full exploration of the influence of the walking beam system on the skid marks. The simulation results show that the radiative shielding by the static beams is the main cause of the skid marks. The heat loss through the skid button to the cooling system worsens the skid marks.
TL;DR: In this paper, the effect of acceleration, deceleration, and centrifugal force on mist transport and cooling effectiveness in the entrance duct of a gas turbine inlet is investigated.
Abstract: The output and efficiency of gas turbines are reduced significantly during the summer. Gas turbine inlet air-cooling is considered a simple and effective method to increase the power output as well as thermal efficiency. Among various cooling schemes, fog cooling (a direct evaporative cooling) has gained increasing popularity due to its simplicity and low installation cost. During fog cooling, water is atomized to micro-scaled droplets (or mist) and introduced into the inlet airflow. The inlet air temperature is reduced through water evaporation. To investigate the mist transport in the entrance duct, numerical study is performed in this article. Different fundamental geometries are considered first, which include a straight tunnel, a diffuser, a contraction, and a 90° bend. These geometries are used to investigate the effect of acceleration, deceleration, and centrifugal force on mist transport and cooling effectiveness, respectively. Lastly, a duct representing a real application is used. The effects of...
TL;DR: In this paper, the authors present a numerical investigation of unsteady laminar mixed convection heat transfer in a two-dimensional square cavity, where one of the vertical walls is cooled and slides either with a constant speed or with a sinusoidal oscillation.
Abstract: This article presents a numerical investigation of unsteady laminar mixed convection heat transfer in a two-dimensional square cavity. The cavity is configured such that one of the vertical walls is cooled and slides either with a constant speed or with a sinusoidal oscillation. A portion of the opposite stationery wall is heated by a constant temperature heat source while, the remaining walls of the cavity are thermally insulated. Different configurations of sliding wall movement and a series of Richardson numbers and Strouhal numbers are tested. The results indicate that the direction and magnitude of the sliding wall velocity affect the heat transfer rate. At low Richardson numbers, the average heat transfer rate for the cavity with an oscillating wall is found to be lower compared to that for the cavity with a constant velocity wall. In addition, at a fixed Richardson number, as the Strouhal number decreases the oscillation frequency of average Nusselt number on the vertical walls decreases; however, ...
TL;DR: In this paper, a finite-volume code suitable for unstructured grids has been developed to simulate the flow and heat transfer around a square cylinder at incidence (α = 0°−−45°).
Abstract: A numerical investigation of flow and heat transfer around a square cylinder at incidence (α = 0° − 45°) is presented for a range of Reynolds numbers ( Re = 60 − 150). A finite-volume code suitable for unstructured grids has been developed to simulate the flow. The unstructured grid has been generated using the Delaunay triangulation algorithm. A modified pressure-velocity correction scheme with semi-explicit time-stepping is implemented to solve the Navier-Stokes equations. Collocated grid arrangement has been used for the dependent variables. Convective terms have been discretized using a second order upwind least squares scheme. The formation of Karman vortex street has been captured and the Strouhal number associated with the wake has been determined. The dependence of Strouhal number, force coefficients (drag and lift), moment coefficient and average Nusselt number on Reynolds number, and angle of incidence for a fixed blockage ratio has been reported and analyzed.
TL;DR: In this article, the authors investigated the transient thermoelastic stresses in a rotating non-homogeneous micro-engineering anisotropic solid and found that the effects of inhomogeneity and isotropy are very pronounced.
Abstract: The present article concerns the investigation of the transient thermoelastic stresses in a rotating non-homogeneous micro-engineering anisotropic solid. The system of fundamental equations is solved by means of a boundary element method (BEM) and the numerical calculations are carried out for temperature, displacements and stresses. The results indicate that the effects of inhomogeneity and isotropy are very pronounced.
TL;DR: In this article, the authors study laminar impinging jets on surfaces covered with a layer of a porous material and derive a set of equations that are valid for the entire computational domain, including both the porous layer attached to the surface and the fluid layer over the porous substrate.
Abstract: This work aims at studying laminar impinging jets on surfaces covered with a layer of a porous material. This contribution may provide insight into the design and optimization of heat and mass transfer processes over surfaces. Numerical simulations are presented and the porous substrate is treated as a rigid, homogeneous, and isotropic medium. Macroscopic transport equations are written for a representative elementary volume (REV), yielding a set of equations that is valid for the entire computational domain, including both the porous layer attached to the surface and the fluid layer over the porous substrate. These equations are discretized using the control-volume method and the resulting system of algebraic equations is relaxed using the Strongly Implicit Procedure (SIP) methods. The SIMPLE algorithm is used to handle the pressure–velocity coupling. Results for flow, in both clear and porous flow domains, are given in terms of streamlines patterns, velocity profiles, pressure contours, and friction coe...
TL;DR: In this article, a velocity-vorticity form of Navier-Stokes equations is used to predict the characteristic parameters of flow, temperature, and solutal concentration fields using a much coarser mesh compared to the mesh used in a stream function-Vorticity formulation.
Abstract: In this article, convection driven by combined thermal and solutal concentration buoyancy effects in a lid-driven square cavity is examined using velocity-vorticity form of Navier-Stokes equations. The governing equations consist of vorticity transport equation, velocity Poisson equations, energy equation, and concentration equation. Validation results are discussed for convection due to heat and mass transfer in a lid-driven square cavity at Re = 500, Le = 2, and GRT = GRS = 100. These results indicate that the present velocity-vorticity formulation could predict the characteristic parameters of flow, temperature, and solutal concentration fields using a much coarser mesh compared to the mesh used in a stream function-vorticity formulation. The capability of the proposed algorithm to handle complex geometry is demonstrated by application to mixed convection in a lid-driven square cavity with a square blockage. The effect of buoyancy ratio on the convection phenomenon is discussed for buoyancy ratio var...
TL;DR: In this paper, numerical simulations for fluid flow and heat transfer in triangular ducts are carried out, where flow is considered to be laminar, hydrodynamically, and thermally developing.
Abstract: Numerical simulations for fluid flow and heat transfer in triangular ducts are carried out. Flow is considered to be laminar, hydrodynamically, and thermally developing. Heat transfer by both forced and natural convection is taken into account. Simulations are carried out for constant wall temperature cases which are at a higher temperature than the inlet temperature of the fluid. The effect of Rayleigh number on bulk mean temperature and Nusselt number is studied. Isotherm and secondary velocity profile formed because of natural convection is shown at different locations with varying Rayleigh number. The effect of the apex angle of the triangular duct on Nusselt number and bulk mean temperature is studied. Results are compared with the cases of mixed convective heat transfer in a square duct.
TL;DR: In this article, the authors investigated the surface Nusselt number distributions of heat sinks with smooth, ribbed, and dimpled surfaces in laminar flow using the numerical code FLUENT 6.16.
Abstract: Steady flow characteristics and surface Nusselt number distributions of heat sinks with smooth, ribbed, and dimpled surfaces are investigated in laminar flow using the numerical code FLUENT 6.2.16. Presented are local and spatially averaged surface Nusselt numbers, as well as distributions of numerically predicted flow characteristics within the heat sink passages. These include distributions of static pressure, velocity, and streamwise vorticity in flow cross-sectional planes. These results are valuable because of the design information provided to optimize heat sink thermal performance, without the use of costly and time-consuming experiments.
TL;DR: In this paper, a robust automated method for the design of two-dimensional enhanced surfaces is presented, where multi-objective optimization algorithms are employed; the competing objectives addressed are the maximization of the heat transfer and the minimisation of the pressure drop for Re = 1,000 and Pr = 0.74.
Abstract: Heat transfer enhancing surfaces are of interest for a wide range of industrial applications. The aim of this article is to provide a robust automated method for the design of two-dimensional enhanced surfaces. Multiobjective optimization algorithms are employed; the competing objectives addressed are the maximization of the heat transfer and the minimization of the pressure drop for Re = 1,000 and Pr = 0.74. The surfaces are parameterized with Bezier curves, and a finite-volume solver is used for the computational fluid dynamics analysis. The optimization is based on different algorithms used sequentially. Finally, a robust design assessment analysis is carried out on two configurations.
TL;DR: In this article, the generalized model of the momentum equation, known as the Forchheimer-Brinkman extended Darcy model, was used in representing the fluid motion inside the porous layer.
Abstract: The present numerical investigation addresses non-Darcian effects on the mixed convection heat transfer in a metallic porous block with a confined slot jet. The generalized model of the momentum equation, which is also known as the Forchheimer-Brinkman extended Darcy model, was used in representing the fluid motion inside the porous layer. The local thermal equilibrium condition was assumed to be valid for the range of the thermophysical parameters considered in the present investigation. The transport equations were solved using the finite element formulation based on the Galerkin method of weighted residuals. The validity of the numerical code used was ascertained by comparing our results with previously published results. Our results revealed that the heat transfer performance of the slot jet was 2.4 times as large as that without the presence of a porous block. In addition, the average Nusselt number was found to increase with a decrease in porosity and an increase in the thermal conductivity ratio. T...
TL;DR: In this article, a composite-supported liquid membrane (CSLM) was used to recover both sensible heat and moisture from the exhaust air stream from a building, and the sensible and moisture-recovery effectiveness were obtained with effectiveness-NTU (number of transfer units) methodology.
Abstract: The membrane-based total heat exchanger is a device to recover both sensible heat and moisture from the exhaust air stream from a building. Heat and mass transfer intensification has been undertaken simultaneously in two directions: air-side augmentation with cross-corrugated triangular ducts and material-side augmentation with a novel composite-supported liquid membrane (CSLM). Performance of heat and mass transfer intensification has been investigated numerically. As a first step, the convective heat and mass transfer coefficients in the flow passages are calculated. Then, the heat and moisture diffusion resistance through the CSLM itself is estimated. Finally, the sensible and moisture-recovery effectiveness are obtained with effectiveness-NTU (number of transfer units) methodology. It is found that the new concept of cross-corrugated triangular ducts with CSLM has a 14% higher sensible effectiveness and a 46% higher latent effectiveness in comparison with a traditional total heat exchanger of parallel...
TL;DR: In this article, a numerical model based on finite differences is developed to solve the mass, momentum, and energy equations in two-dimensional parabolic cavities heated from below with insulated walls and flat top and bottom walls.
Abstract: The interaction of natural convection and surface radiation in two-dimensional parabolic cavities heated from below with insulated walls and flat top and bottom walls is studied numerically. The shape of the cavities arises from the design of non-imaging optics-based compound parabolic concentrators (CPC). A numerical model based on finite differences is developed to solve the mass, momentum, and energy equations. A coordinate transformation is used to map the parabolic shape into a rectangular domain where the governing equations are solved. The results show that surface radiation significantly changes the temperature distribution and local Nusselt number inside the parabolic enclosure. Solutions are obtained for two different parabolic geometries that correspond to two different levels of concentration.
TL;DR: In this article, a node-based smoothed point interpolation method (NS-PIM) is formulated to analyze 3D steady-state thermoelastic problems subjected to complicated thermal and mechanical loads.
Abstract: A node-based smoothed point interpolation method (NS-PIM) is formulated to analyze 3-D steady-state thermoelastic problems subjected to complicated thermal and mechanical loads. Gradient smoothing technique with node-based smoothing domains is utilized to modify the gradient fields and to perform the numerical integration required in the weak form formulation. Numerical results show that NS-PIM can achieve more accurate solutions even when the 4-node tetrahedral mesh is used compared to the finite-element method (FEM) using the same mesh, especially for strains and hence stresses. Most importantly, it can produce an upper bound solution of the exact solution in energy norm for both temperature and stress fields when a reasonably fine mesh is used. Together with FEM, we now for the first time have a simple means to obtain both upper and lower bounds of the exact solution to complex thermoelastic problems.
TL;DR: In this paper, the subsonic gas flows through straight rectangular cross-sectional microchannel with patterned microstructures were simulated using the direct simulation Monte Carlo (DSMC) method.
Abstract: The subsonic gas flows through straight rectangular cross-sectional microchannel with patterned microstructures was simulated using the direct simulation Monte Carlo (DSMC) method. An implicit treatment for low-speed inflow and outflow boundaries for the DSMC of the flows in microelectromechanical systems (MEMS) is employed. The 3-D microchannel flows are simulated with the cross-section aspect ratio ranging between 1 and 5. The comparison between 3-D cases and 2-D case shows that when the aspect ratio < 3, the two extra side-walls in the 3-D case have significant effects on the heat transfer and flow properties. When the aspect ratio increases, the flow pattern and heat transfer characteristics tend to approach those of 2-D results. The 2-D simplification is found to be reasonable when the cross-section aspect ratio is larger than 5. The microchannel flows with microstructures are also calculated with three different Knudsen numbers regime cases, and each case is calculated with three different microstru...
TL;DR: In this article, the effects of a heat barrier, located in the ceiling wall of an enclosure, on conjugate conduction/natural convection are investigated numerically, and the effect of the heat barrier on dimensionless heat transfer rate through the enclosure increases as the Rayleigh number decreases.
Abstract: The effects of a heat barrier, located in the ceiling wall of an enclosure, on conjugate conduction/natural convection are investigated numerically. The vertical walls of the enclosure are differentially heated and the horizontal walls are adiabatic. Heatline technique is used to visualize heat transport. The variations of average Nusselt number, dimensionless heat transfer rate through the ceiling wall, and dimensionless overall heat transfer rate are studied. Calculations are performed for different Rayleigh numbers (103 ≤ Ra ≤ 106), thermal conductivity ratios (1 ≤ K ≤ 100), dimensionless locations of the heat barrier (0 < X h < 1),and two dimensionless ceiling wall thicknesses (D = 0.05 and D = 0.20). For high thermal conductivity ratio (K = 100), the heat barrier considerably reduces the dimensionless overall heat transfer rate. The effect of the heat barrier on dimensionless heat transfer rate through the enclosure increases as the Rayleigh number decreases. For low Rayleigh number (i.e., Ra = 103)...
TL;DR: In this article, a numerical procedure to design two-phase periodic microstructural composites with tailored thermal conductivities is presented, where the objective function is formulated in a least-square of the difference between the target and effective conductivities.
Abstract: This article presents a numerical procedure to design two-phase periodic microstructural composites with tailored thermal conductivities, which is generalized as a topology optimization problem. The objective function is formulated in a least-square of the difference between the target and effective conductivities. The effective values are derived from homogenization method with periodic boundaries; whereas, the target points locate in the Milton-Kohn bounds. The bound-based interpolation scheme and nonlinear diffusion technique are explored to regularize the original problem for attaining mesh-independent, edge-preserving, and checkerboard-free results. Various microstructures both in 2- and 3-dimensions are presented to demonstrate such a systematic procedure of conductive material design.