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Based on the fixed-step random walk procedure a Monte Carlo algorithm for the solution of anisotropic heat conduction is presented. It is shown that the Monte Carlo solution is attainable only for a specified range of solid thermal conductivities. It is also illustrated that by following two simple clues considerable reduction in computation time may be achieved. Finally, steady-state temperature distribution, obtained by the Monte Carlo calculations, is presented for a two-dimensional anisotropic solid having simple geometry and boundary conditions.

This paper investigates the unsteady characteristics of flow in a specific type of microvalve with sudden expansion shape. The geometry of the channel is such that the flow resistance caused by vortex structures is different in forward and backward flow directions. This introduces the geometry as a good nominee as a microfluidic rectifier with application in micropump systems and MEMS-based devices. A time-varying sinusoidal pressure was set at the inlet of the microchannel to produce unsteadiness and simulate the pumping action. The existence of block obstacle and expansion shoulders leads to various sizes of vortex structures in each flow direction. All simulation results are based on the numerical simulation of two-dimensional, unsteady, incompressible and laminar Navier-Stokes equations using finite element algorithm. Two fundamental parameters are varied to investigate the vortices growth throughout the time: the frequency of the inlet actuating mechanism and the amplitude of the inlet pressure. The frequency of the inlet pressure was varied in the range of 1 Hz to 1000 Hz to cover the frequency range in many micropump applications. In this way, one can see the effect of actuation mechanism on onset of separation and follow the size and duration of the vortex growth. In order to better understand the effect of geometry and frequency on flow field, the pressure contours are studied through one cycle. Finally, Strouhal number is calculated for frequency to obtain a measure of unsteadiness of the flow. A critical value of f = 250Hz is found for St = 1. The obtained results give a deep insight into the physics of unsteady flow in valveless microvalves and lead to better use of current design as a part of microfluidic system.

In this paper, pulsatile flow through a microchannel with step expansion shape and working as valveless microvalve is investigated. The design of the microchannel is such that it causes the production of different vortex structures in the flow field and near corners. Our primary objective is to investigate how geometrical parameters affect the performance of the microvalve under different flow conditions. Unsteady, two-dimensional axisymmetric flow of an incompressible Newtonian fluid with laminar regime was simulated numerically using finite element approach. In order to imitate the actuation mechanism of a micropump, a time-varying sinusoidal pressure was set at the inlet region of the microchannel. Two basic dimensions were found to have crucial effect on the flow field configuration and vortex generation: the expansion ratio of the channel and the distance of the obstacle from the expansion region. The frequency of the actuation mechanism was varied from 1Hz to 1000Hz to cover the working range for many micropump applications. As the pressure is a key factor and should be manipulated in the right way, the pressure contours were studied. Vortex growth was captured through a single cycle and their size and existence time was compared for different geometrical inputs. At last, Strouhal number which is a measure of unsteadiness of the flow was calculated. This way we can investigate the sensitivity of the flow to changes in block obstacle distance from the expansion section and the expansion ratio. The obtained results lead to better understanding of the physics of pulsatile flows through microchannels and help in improving the efficiency of the existing configuration in designing more efficient micropump systems.

The inverse radiative boundary design problem in a hexagonal furnace is numerically investigated. The aim of this study is to find the strength of heaters in a two-dimensional (2D) enclosure to produce the desired temperature and heat flux distribution on the design surface. Conjugate gradients method is chosen to perform the iterative search procedure for obtaining the optimal solution. The medium is considered participating and both gray and nongray cases are studied. FTn finite volume method (FVM) with nonorthogonal grid is employed to solve the radiative transfer equation in the furnace. In nongray cases, the medium is filled with combustive gas products. To predict nongray behavior properly, the SLW model is used. In problems with gray medium, the effects of wall emissivity, absorption coefficient, and scattering albedo are studied. For nongray problems, the effects of gas concentration and temperature of single species and gas mixtures are analyzed. In all cases, heater power can be estimated precisely and as the results show, estimated heat flux on the design surface is very close to the desired value. For this reason, the maximum root mean square error is less than 1.6%.

In this paper, the inverse radiation boundary problem is solved using a simplified backward Monte Carlo method (MCM) for cases in which radiation is the dominant mode of heat transfer (i.e., radiative equilibrium). For an N-surface enclosure, N2 radiative transfer factors are required to carry out the radiant exchange calculations. In this paper, it is shown that when the enclosure is comprised of some adiabatic surfaces (as is nearly always the case in radiative furnaces), this number can be reduced considerably. This reduction in the required number of distribution factors causes a clear simplification in the formulation of the inverse problem and a substantial reduction in the computational time. After presenting the formulation for the inverse problem, standard test cases are solved to demonstrate the efficiency and the accuracy of the proposed method.

A rigorous argument based on the characteristic of a monotonously increasing function is presented to establish the well-known fact of higher LMTD value of counter flow heat exchangers as compared to parallel flow ones.

This paper aims to study the effect of particle migration on flow and heat transfer of nanofluids flowing through a circular pipe. To do this, a two-component model proposed by Buongiorno (2006, “Convective Transport in Nanofluids,” ASME J. Heat Transfer, 128, pp. 240–250) was used and a numerical study on laminar flow of alumina-water nanofluid through a constant wall temperature tube was performed. The effects of nonuniform distribution of particles on heat-transfer coefficient and wall shear stress are shown. Obtained results illustrate that by considering the particle migration, the heat-transfer coefficient increases while the wall shear stress decreases, compared with uniform volume fraction. Thus, it can be concluded that the enhancement of the convective heat transfer could not be solely attributed to the enhancement of the effective thermal conductivity, and beside other reasons, which may be listed as this higher enhancement, particle migration is proposed to be an important reason.

In this paper the Sequential Function Specification Method is used to estimate the transient heat flux imposed on the rake face of the cutting tool during the cutting operation. The cutting tool is modeled as a three dimensional object. The capabilities of the geometric modeling, mesh generation as well as solver of the commercial software ANSYS is utilized in order to reduce the time expended for modeling and direct heat conduction solution. This way the inverse heat conduction algorithm employs ANSYS as a subprogram through the ANSYS Parametric Design Language (APDL). The stability as well as accuracy of the heat flux estimation is investigated using familiar triangular as well as step heat flux test cases. The effect of noise level, the number of temperature sensors as well as their locations are investigated in order to arrive at an optimal experimental procedure. Finally, a typical temperature data during the working condition are used to recover the heat flux at the cutting tool surface.Copyright © 2010 by ASME

The problem of natural convection radiation with the presence of participating fluid in a tilted square cavity has been investigated numerically. Two vertical walls are at uniform different temperatures while the others are adiabatic. The working fluid is taken as grey, absorbing, emitting and non-scattering. The finite volume method is used to solve the dimensionless governing equations and SIMPLE algorithm is applied for pressure velocity coupling. The radiative heat flux gradient is estimated by finding radiative intensities from the radiative transfer equation (RTE). A very recent method, called the QL method, is utilized to solve RTE. In this study the effects of the inclination angle, Rayleigh number and optical thickness on the heat transfer and flow characteristics are studied. A great accuracy in the results was observed in the prediction of flow contours and average radiative and convective Nusselt numbers at walls.Copyright © 2010 by ASME

In order to study the heat transfer behavior of the nanofluids, precise values of thermal and physical properties such as specific heat, viscosity and thermal conductivity of the nanofluids are required. There are a few well-known correlations for predicting the thermal and physical properties of nanofluids which are often cited by researchers to calculate the convective heat transfer behaviors of the nanofluids. Each researcher has used different models of the thermophysical properties in their works. The aim of the present paper is to study the convective heat transfer of nanofluids containing low volume concentration of Al2O3 nanoparticles with a regard to the migration of nanoparticles due to Brownian diffusion and thermophoresis. To do this, a two-component model has been used and a numerical study on laminar flow of alumina-water nanofluid through a constant wall temperature tube has been performed. Two different models have been adopted for predicting the thermophysical properties of nanofluids. All of the properties are assumed to be temperature as well as particle concentration dependent. The effects of these models on the predicted value of the convective heat transfer of nanofluid and the migration of nanoparticles have been discussed in detail.

In this study obtaining a uniform heat flux over a target surface was investigated by means of using characteristics of jet impingement heat transfer. Conjugate Gradients Method (CGM) was utilized to minimize the objective function defined on the basis of the squared differences between the target heat flux and the calculated ones. Design variables were taken to be jets’ Reynolds numbers, separation distance between the exit plane of the jets and the target plate, as well as inter-jet spacing. Air single phase jets were used in this study. The problem was solved for the cases of 4 and 6 jets. Temperature difference between the jet exit and the target plate is 100°C, and a steady state condition was assumed. The Finite Volume Method and an unstructured mesh were used for direct solution of the jet impingement heat transfer problem for a laminar jets impingement to a flat plate with constant temperature.

This article presents an optimal shape design methodology for diffuse-walled radiant enclosures. In this study, the shape of the enclosure is parameterized by means of non-uniform rational B-spline (NURBS) surfaces, and their control points represent the design variables. The enclosure geometry is discretized by choosing the parameters of NURBS surfaces as generalized curvilinear coordinates, and the radiosity equation is solved using the infinitesimal-area analysis technique developed by Daun and Hollands [1]. The simplified conjugate-gradient method (SCGM) is used as the optimization method to obtain the optimal shape and adjust the design variables intelligently. The methodology is demonstrated by optimizing the shape profile of a cavity with the objective of enhancing the apparent emittance.Copyright © 2009 by ASME

This article presents an optimal shape design methodology for heat convection problems. In this study, the shape of the convective medium is parameterized by means of non-uniform rational B-spline (NURBS) surfaces, and their control points represent the design variables. The convective domain is discretized by choosing the parameters of NURBS surfaces as generalized curvilinear coordinates, and the conservation equations are solved using the finite difference method. The simplified conjugate-gradient method (SCGM) is used as the optimization method to obtain the optimal shape and adjust the design variables intelligently. The methodology is demonstrated by optimizing the shape profile of a natural convective cavity with the objective of reducing the maximum wall temperature.

This paper is concerned with the conduction heat transfer between two parallel plates filled with a porous medium with uniform heat generation under a nonequilibrium condition. Analytical solution is obtained for both fluid and solid temperature fields at constant porosity incorporating the effects of thermal conductivity ratio, porosity, and a nondimensional heat transfer coefficient at pore level. The two coupled energy equations for the case of variable porosity condition are transformed into a third order ordinary equation for each phase, which is solved numerically. This transformation is a valuable solution for heat conduction regime for any distribution of porosity in the channel. The effects of the variable porosity on temperature distribution are shown and compared with the constant porosity model. For the case of the exponential decaying porosity distribution, the numerical results lead to a correlation incorporating conductivity ratio and interstitial heat transfer coefficient.

An inverse heat conduction problem (IHCP) for identification of flaw parameters in a two-dimensional body is investigated. This is done through the application of a relevant heat load to the body, and acquiring the temperatures on the outer surface of the body. The geometrical shape and number of the flaws is assumed to be known at the outset. To calculate the parameters pertinent to the flaws, use is made of the ANSYS software. This software acts as a subroutine in the inverse algorithm. To conclude, a number of sample cases, with the flaws in the form of cavities or cracks, are solved to highlight the efficiency of the method.

The increasing power dissipation and decreasing dimensions of microelectronic devices have emphasized the demand for extremely efficient compact cooling technology. Microchannel heat sinks are of particular interest due to high rates of heat transfer, which have become known as one of the effective cooling technologies. In the present work, numerical simulation of incompressible flow in two dimensional microchannels by implementing nonuniform electrokinetic forces is performed using finite volume method. The velocity field and the heat transfer rate are influenced by the wall potential variations through the microchannel. Nondimensional parameters of heat transfer and fluid flows, Debay Huckel length, microchannel size and wall charge potential distribution, have major roles in this investigation. For fixed values of Reynolds number and microchannel size, the patterns of wall potentials are optimized to enhance the heat transfer rate. Velocity profiles are computed and temperature distribution and Nusselt number are obtained for uniform wall heat flux boundary condition. Average and local Nusselt numbers are illustrated for different wall potential configurations and Reynolds number. Velocity vectors and pressure drop are presented for different zeta potentials and Reynolds numbers. Finally, results of nonuniform electrical force are compared to uniform ones.Copyright © 2008 by ASME

The increased power dissipation and reduced dimensions of microelectronics devices have emphasized the need for highly efficient compact cooling technologies. Microchannel heat sinks are of particular interest due to the very high rates of heat transfer they enable in conjunction with greatly reduced heat sink length scales and coolant mass flow rate. Therefore, in the present work, optimization of laminar convective heat transfer in the microchannel heat sinks is investigated for uniform heat flux and different cross sectional areas of different aspect ratios. Three-dimensional numerical simulations of general form of energy equation were performed to predict Nusselt number in the laminar flow regime. Using these results, an optimum forced convective heat transfer coefficient was computed for several cross sectional areas and Reynolds numbers, utilizing the univariable search method. Different aspect ratios have different influences on Nusselt number in thermally developing and fully developed regions for different cross sectional areas and Reynolds numbers. There exists an optimum Nusselt number for each Reynolds number and cross sectional area by varying aspect ratio. Thus, optimized state is computed and related graphs are presented.Copyright © 2008 by ASME

Micromixers have received much interest as essential part of microfluidic devices. Therefore, enhancement of mixing quality has gained a lot of attention in recent years. In the present study, improvement of mixing quality for two different miscible liquids is considered in passive micromixers. Numerical approach is based on a second order finite volume Jameson scheme in order to solve two dimensional incompressible Navier-Stocks and mass transport equations by implementing artificial compressibility. Mixing quality is influenced by Reynolds and Schmitt numbers as well as size and location of the ribs. Diffusion mechanism has the main role for mixing in micro scale fluid flows; therefore, increasing Peclet number leads to extend mixing time. In order to enhance mixing quality, ribs are used in different locations through the microchannel which cause more instability in the fluid flow and leads to a better mixing. The Reynolds number is constant while the Schmitt number is in the range of 10 to 100. However, in order to laminar fluid flow, ribs just have an influence near itself and faraway, mixing mechanism return to earlier state. Therefore, in low Reynolds numbers they have no effective influence. When Reynolds number increase, flow instability that is created by different ribs leads to a better mixing.Copyright © 2008 by ASME

In the present paper a new scheme is presented which combines the Sequential Function Specification Method (SFSM) of Beck and the Dual Reciprocity Boundary Element Method (DRBEM). In this scheme the unknown boundary condition is estimated sequentially by using two transformation matrices. One of the matrices takes into account the initial condition, while the other incorporates the measured temperature data. The matrices defined in the direct heat conduction calculations by the Dual Reciprocity Boundary Element Method are used as a basis for the definition of the transformation matrices, and the mathematical derivations for the inverse estimation are in accordance with the Sequential Function Specification of Beck. A two-dimensional problem with unknown heat flux components, both temporally and spatially, is considered as a test case. The inactive surfaces are all adiabatic. In order to compare the speed and the accuracy of the method with the existing SFS methods, the exact analytical solution of a simulated test case is utilized. Results indicate an impressive improvement in the efficiency as well as the accuracy of this scheme as compared with the conventional ones.

Turbulent flow convective heat transfer and friction factor characteristics of Al 2 O 3 /water nanofluid flowing inside a uniformly heated horizontal circular tube are conducted in this Letter. To do this, Al 2 O 3 nanoparticles of 40 nm size were characterised and dispersed in distilled water to form stable suspension containing 0.1, 0.5, 1.0, 1.5 and 2.0 volume concentrations of nanofluids. Results indicate that heat transfer coefficients increase with nanofluid volume concentration and do not change with Reynolds number. The enhancement of the Nusselt number is about 22 at Re 13 500 using 2 alumina nanoparticles compared to distilled water. Experimental results are also compared with the existing correlations of convective heat transfer coefficient and pressure drop in turbulent regime. The measured pressure loss when using nanofluids is almost equal to that of the base fluid.

Using nonequilibrium molecular dynamics simulations, we show if a multi-walled carbon nanotube (MWNT) is heated asymmetrically, an evident thermal rectification can be detected. We attribute the observed rectification to the asymmetric radial thermal transport between constructing layers in MWNT. The underlying physics is explained by calculating temperature distribution of MWNT layers and phonon power spectrum. Thermal rectification in this carbon nanotube based thermal rectifier does not diminish by increasing the system size, and exists in a wide range of temperatures. These results open a door in the applicability of MWNTs in nanoscale engineering of thermal transport devices.

International audience; We prove that interfacial asperity sharpness allows for tuning superlattice in-plane thermal conductivity below or above the limit of high roughness derived from the Lucas-Ziman (LZ) model. Whereas LZ’s model predicts molecular dynamic (MD) results of Lennard-Jones superlattices for small asperities, it has to be modified with a roughness- and sharpness-dependent layer thickness to remain relevant at higher roughness. For the case of sharpest asperities, the modified LZ model still fails, and ray-tracing computations matching MD data reveal a phonon-trap effect in the asperity valleys. This behavior scales with the Knudsen number and should appear at the micron scale in large mean-free-path crystals, such as silicon.

The thermal conductivity of hybrid graphene-graphane nanoribbons (GGNRs) have been investigated using nonequilibrium molecular dynamics simulations. The interface between graphene and graphane leads to a Kapitza resistance with strongly dependence on the imposed heat flux direction. We introduce GGNRs as promising thermal rectifiers at room temperature. By calculating phonon spectra, underlying mechanisms were investigated.