Effects Of Thermal Conductivity Ratio Research Articles

A generalized correlation for predicting thermal conductivity of filled composite with high filler content

The heat conductive materials with high filler content are widely used in various heat-dissipating fields. However, previous theoretical models and empirical correlations cannot accurately predict the thermal conductivity of composite with high filler content. In this study, the effects of filler contact, interface thermal resistance, thermal conductivity ratio and filler content are investigated. The results indicate that the effect of the thermal conductivity ratio on the thermal conductivity of composite is related to the filler content. The difference that caused by thermal conductivity ratio is increased rapidly with filler content. When the filler content is greater than 40%, the effects of thermal conductivity ratio cannot be ignored even the thermal conductivity ratio greater than 100. The filler content of 40% is the percolation threshold of composite that has filler contact. The effect of interfacial thermal resistance decrease rapidly with interfacial thermal resistance. A generalized correlation is derived to predict the effective thermal conductivity of filled composite with high filler content. This correlation is not only used widely (0 ≤ Vf ≤ 60%), but also has higher consistency with experimental values than other theoretical models and empirical correlations.

High-efficient heat flux manipulation of micro-scale thermal metamaterials with facile functional unit design

Based on the novel macro-thermophysical properties exhibited by its peculiarly artificially design structure, thermal metamaterials have been widely used in the harvest and management of thermal energy. Herein, a facile micro-scale thermal functional unit structure is proposed assisted by an advanced picosecond laser technology to realize the fabrication of thermal metamaterials with thermal shielding and thermal concentrating. To obtain the optimal design criterion of thermal functional units, the effects of thermal conductivity ratio, layer orientation angle, and laminated thickness on thermal conduction are investigated. Both simulation and experiment validate that thermal metamaterials assembled by diverse thermal functional units with different orientation angles exhibit completely neoteric performances of thermal concentrating and shielding. Besides, the results verify that the micro-scale thermal metamaterials exhibit better heat flux regulation capability and higher thermal concentration efficiency. Additionally, the effect of inevitable air convection on the thermal performance of these metamaterials has also been evaluated. This work provides an effective strategy to achieve small-scale thermal energy harvesting and protection by using thermal metamaterials with micro-structure.

MHD conjugate natural convection in a porous cavity involving a curved conductive partition and estimations by using Long Short-Term Memory Networks

In this study, MHD conjugate free convection of a porous cavity having a curved shape conductive partition is numerically analyzed by using the Galerkin weighted residual finite element method. The numerical simulation is performed for different values of pertinent parameters: Rayleigh number (between $$10^4$$ and $$10^6$$), Hartmann number (between 0 and 60), Darcy number (between $$5 \times 10^{-4}$$ and 0.05), porosity of the medium (between 0.25 and 0.75), curvature of the partition (minor axis radius of the horizontal ellipse, between 0.01H and 0.3H) and conductivity ratio (between 0.05 and 50). It was observed that the heat transfer rate enhances locally and in average for higher values of Rayleigh number, Darcy number, porosity of the medium and conductivity ratio, whereas the impact is opposite for higher values of Hartmann number. The amount of average Nusselt number reduction is obtained as $$22\%$$ when Hartmann number is changed from 0 to 60 at Rayleigh number of $$10^5$$. Curvature and conductivity of the curved partition affect the variation in fluid flow and heat transfer characteristics. Maximum of $$7\%$$ variation in the average Nusselt number is achieved when the curvature of the conductive partition is varied but the effects of thermal conductivity ratio on heat transfer rate are higher. Long Short-Term Memory Networks are used for estimation of the velocity and temperatures in the computational domain for various values of pertinent input parameters variation in the system which includes conjugate heat transfer mechanism in a porous enclosure with complex-shaped conductive partition under the effects of magnetic field.

Effect of Thermal Conductivity Ratio on Laminar Double-Diffusive Free Convection in a Porous Cavity

This work presents a study on double-diffusive free convection in a porous square cavity using the thermal equilibrium model. Transport equations are discretized using the control-volume method, and the system of algebraic equations is relaxed via the SIMPLE algorithm. The effect of ks/kf on average Nusselt and Sherwood values was investigated. Results show that increasing ks/kf affects Nuw and Shw boosting mass transfer at the expense of reducing overall heat transport across the enclosure.

Mesoscopic pore-scale simulations of natural convection of porous media in closed square cavity by using LBM

A lattice Boltzmann model is applied to simulate natural convection of porous media in closed square cavity. The numerical simulation is conducted to investigate the influences of porosity on fluid flows and the effect of thermal conductivity ratio of solid matrix to fluid on heat transfer. Micro X-CT has been used to obtain 2D optical image of various slice with different location. A construction porous model has been established accurately by 2D optical micro X-CT image of porous media. A fluid –solid conjugate heat transfer model has been applied to the established construction porous model by using Lattice Boltzmann method.This paper confirms the reliability and the computational efficiency of the lattice Boltzmann method in simulating natural convection in porous media at mesoscopic pore-scale.The temperature field is simulated by a simplified thermal energy distribution function which neglects the compression work done by the pressure and the viscous heat dissipation. A comprehensive parametric study of natural convective flows is carried out for various values of Rayleigh number.

Effect of thermal conductivity ratio on flow features and convective heat transfer

ABSTRACTIn this paper, we numerically investigate the steady laminar natural convection in a water-filled 2D enclosure containing a rectangular conducting body. Computations are performed for a wide range of dimensionless parameters including the Rayleigh number Ra, the thermal conductivity ratio k, and the location of the inner body δ, while the value of the Prandtl number is maintained constant at 6.8. This simulation spans four decades of Rayleigh number, Ra, from 103 to 106. The influence of these various dimensionless parameters on the flow behavior is investigated. Correlations of the averaged Nusselt numbers are obtained as a function of two parameters (Ra and δ) for each working thermal conductivity ratio. The results indicate that the heat transfer rate through the enclosure can be controlled by the position of the rectangular body. A comparative study was also carried out between a horizontal and a vertical conducting shape inside the enclosure. It is proved that a vertical position leads to a better heat transfer compared to the horizontal case.

MHD natural convection flow in cavities filled with square solid blocks

Purpose – The purpose of this paper is to study the influence of magnetic field on natural convection inside the enclosures partially filled with conducting square solid obstacles. Also, the effect of thermal conductivity ratio between the solid and fluid materials is investigated for different number of solid blocks. Design/methodology/approach – The dimensionless governing equations are transformed into sets of algebraic equations using finite volume method and momentum equations are solved by the SIMPLE algorithm with the hybrid scheme. The validation of the numerical code was conducted by comparing the results of average Nusselt number with previously published works. Findings – The results indicate that both the magnetic field and solid blocks can significantly affect the flow and temperature fields. It is shown that for a given Rayleigh number, variation of Nusselt number might be increasing or decreasing with change in solid-to-fluid thermal conductivity ratio depending on magnetic field strength and number of solid blocks. Originality/value – No work has been reported previously on the effect of magnetic field on natural convection flow in a cavity partially filled with square solid blocks. The numerical analysis of conductivity ratio between the solid and fluid materials under the effect of magnetic field have been carried out for the first time.

Influence of solid phase thermal conductivity on species separation rate in packed thermogravitational columns: A direct numerical simulation model

In this work, a direct numerical simulation model has been proposed to study the influence of porous matrix thermal properties on the separation rate in a model of packed thermogravitational column saturated by a binary mixture. The coupled flow, heat and mass dimensionless equations and boundary conditions have been derived in pore-scale and then solved over a vertical column containing fluid and solid phases. The results show that the separation rate is changed significantly by the conductivity ratio of the solid/fluid phases. The classical maximum separation at optimal Rayleigh number increases by decreasing the solid thermal conductivity. We obtained that the influence of the solid thermal conductivity for small Rayleigh number is not considerable but for intermediate Rayleigh number the separation rate initially decreases with increasing the thermal conductivity ratio and then reaches an asymptote. As the Rayleigh number increases, convection dominates and the effect of thermal conductivity ratio on separation rate becomes completely inversed.

Constructal structure of nanofluids

The present work considers the problem of further enhancing nanofluids performance inside practical systems through designing their inside structures. Constructal analysis is performed up to the two-branching level on the classical blade-configured disk-shaped domain with central heat sink, adiabatic periphery and uniform heat generation. Transition point from one-branching-level to two-branching-level blade configuration is found. The effects of thermal conductivity ratio and overall volume fraction of blade material on nondimensional constructal overall temperature difference are found to appear always in the form of their product. The number of tributaries does not affect the system performance. Constructal robustness is also observed. Compared with the dispersed configuration under same conditions, this orderly-arranged blade configuration generates much smaller constructal overall temperature difference and thus performs much better for the heat conduction domain considered. Therefore, searching for better and better heat-flow structure is significant to performance.

Visualization of heat flow using Bejan’s heatline due to natural convection of water near 4 °C in thick walled porous cavity

A numerical study on natural convection heat transfer of cold water near 4 °C in a thick bottom walled cavity filled with a porous medium has been performed. It is assumed that the cavity is isothermally heated from the outside of the thick bottom wall and cooled from ceiling. The finite-difference method has been used to solve the governing partial differential equations of heat and fluid flow. Effects of thermal conductivity ratio, Rayleigh number and bottom wall thickness on heat transfer from the bottom to the ceiling have been studied. The heatline visualization technique has been used to demonstrate the path of heat transport through the enclosure. Moreover, streamlines and isotherms have been used to present fluid flow and temperature distributions. The obtained results show that multiple circulation cells are formed in the cavity and the local Nusselt numbers at the bottom wall and solid–fluid interface are highly affected by formed cells. The increase of Rayleigh number and thermal conductivity ratio increases heat transfer through the cavity. However, the increase of thickness of the bottom wall reduces the mean Nusselt number. Almost one-dimensional conduction heat transfer is observed in the solid bottom wall of the cavity.

Study on fluid–solid coupling heat transfer in fractal porous medium by lattice Boltzmann method

Abstract A lattice Boltzmann model is applied to simulate fluid–solid coupling heat transfer in fractal porous medium. The numerical simulation is conducted to investigate the influences of pressure drop and porosity on fluid flows and the effect of thermal conductivity ratio of solid matrix to fluid on heat transfer. The simulation results indicate that fluid flows still obey Darcy’s Law in the range of flow and pressure level in this paper, and that both velocity field and temperature evolution conform to the local structural characteristics of porous medium. The comparison of temperature results from lattice Boltzmann model against those from the finite-volume method (FVM, one of the conventional CFD methods) is also presented to demonstrate the reliability of LBM. The present results agree well with those from FVM, All these indicate the feasibility and the reliability for the lattice Boltzmann model to be used to reveal the phenomenon and rules of fluid–solid coupling heat transfer in complex porous structures.

Numerical Study of Natural Convection Heat Transfer of Porous Cavity contains Partitions and Square body by Using Thermal Non-Equilibrium Model

In this research, natural cnvective heat transfer between hot walls of cavity and a cold body with square section concentric in cavity is studied numerically. The cavity is filled with a saturated porous medium with thermally non-equilibrium between the solid and fluid phases. Two adiabatic partitions attached to the walls of cavity .The governing equations include continuity, momentum and energy equations are solved by using finite deference method with Gauss-Seidle iterative method. This investigation was performed under the effect of four non-dimensional groups which defined as : modified Rayleigh number (100≤Ra*≤1000), scaled heat transfer coefficient (0.1≤H≤100), thermal conductivity ratio (0.1≤Kr≤100) and the location of the partitions with respect to the length of the cavity (Yp*=0.25,0.5,0.75). The results were presented in terms of streamlines and isotherms of fluid and solid phases and the relations between Nusselt number variation with modified Rayleigh number .The Nusselt number in each phases(solid and liquid) and total Nusselt number are found to be increased proportionally due to the increasing in modified Rayleigh number. The effect of thermal conductivity ratio on Nusselt number was comprehensive while scaled heat transfer coefficient was confined to solid phase Nusselt number. Also the results showed that when the two partitions moved towards upper part of cavity , the quantity of heat transfer is to be larger and the maximum heat transfer is to be when the horizontal partitions in the upper part of the cavity in all cases .

Analytical Solution of Nonequilibrium Heat Conduction in Porous Medium Incorporating a Variable Porosity Model With Heat Generation

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.

Drag on a ceramic particle being pushed by a metallic solidification front

The drag force experienced by a ceramic particle being steadily pushed by a metallic solidification front is calculated using a sharp interface numerical technique. The effects of thermal conductivity ratio between the particle and the melt (less than one for the ceramic–metal system) are accounted for in obtaining the drag expression. The drag expression is cast in two forms, the first expresses the drag as a function of the thermal conductivity ratio between the particle and the melt and the second as a function of the radius of curvature of the solid–liquid interface.

Natural convection in an inclined porous cavity with variable porosity and thermal dispersion effects

The variable porosity and thermal dispersion effects on natural convection in an inclined porous cavity are investigated numerically. The wall effect on porosity is approximated by an exponential function and its effect on thermal dispersion is modeled in terms of a dispersive length. Numerical results show that both variable porosity and thermal dispersion effects increase the temperature gradient adjacent to the wall resulting in the enhancement of surface heat flux. These effects become important when the dimensionless particle diameter is increased. The variable porosity effect increases the fluid velocity near the wall, consequently enhancing convective heat transfer. The Prandtl number effect on the Nusselt number is small for Prandtl number greater than one, but increases as the Prandtl number decreases below one. The effect of thermal conductivity ratio on the Nusselt number is greater at low Rayleigh numbers where conduction heat transfer is predominant. A comparison between theoretical and experimental results shows that the calculated Nusselt numbers which take into account variable porosity and thermal dispersion effects have the best agreement with experimental data.

TRANSIENT THERMAL STRESSES DUE TO PERIODICALLY MOVING LINE HEAT SOURCE OF COMPOSITE HOLLOW CYLINDER

Abstract The transient thermal stress distribution of a finite composite hollow cylinder, which is heated by a periodically moving line source on its inner boundary and cooled convectively on the outer surface, is analyzed in this paper. The heat sources are assumed to be axisymmetric, moving along the axis of the hollow cylinder with constant velocity. To solve the temperature distribution of the hollow cylinder, the Laplace transform and eigenfunction expansion methods are used. The associated thermal stress distributions are then obtained by solving the thermoelastic displacement function and the Love function. Finally, a numerical scheme, the Fourier series technique, is utilized to calculate the inverse transform. The numerical results of the temperature and thermal stress distribution are presented, which demonstrate the effects of thermal conductivity ratio, shear modulus ratio, Biol number, and period of the moving heat source.