Values Of Thermal Conductivity Ratio Research Articles

Three-dimensional simulation of conjugate heat transfer using the hybrid lattice Boltzmann-finite difference method

This paper presents a three-dimensional numerical study of natural convection and conduction thermal phenomena using MRT-lattice Boltzmann and finite difference approaches. Conduction is examined using a simple conjugate heat transfer technique, which is predicated on the assumption of continuity of heat flux and temperature at the solid/fluid interfaces, instead of utilizing the conventional method based on the solution of the conduction equation. Several parameters such as Rayleigh number and thermal conductivity related to common construction materials are also investigated to explore their effects on the evolution of heat transfer. The obtained results show that augmentation of the Rayleigh number increases the quality of heat transfer by around 35–293% for various values of thermal conductivity ratio (solid/fluid). While the increase in thermal conductivity ratio enhances the heat transfer rate by about 212–685%, where the maximum rate of heat exchange is still the same for the thermal conductivity ratio ≥1000. Entropy generation is also examined in this paper to evaluate the efficiency of our studied system. The performed investigations indicate that the rate of entropy generation due to heat transfer, friction, and total entropy generation varies significantly with increasing Rayleigh number and thermal conductivity of the solid.

Magneto-convection in an anisotropic porous cavity due to nonuniform heat flux at bottom wall

In the present article, a comprehensive numerical investigation of the magneto-convection in an electrically conducting isotropic, and hydro-dynamically as well as a thermally anisotropic porous cavity, is presented. The motion of the flow is due to the applied nonuniform heat flux on the bottom wall of the cavity and the insulation of other walls. The non-Darcy model has been adopted that includes Darcy and Brinkman terms in the momentum equations. The coupled governing equations are solved numerically by using the finite volume method (FVM). The impact of periodicity parameter (N), Hartmann number (Ha), and anisotropy parameters on the dynamics of flow as well as heat transfer rate have been investigated. From rigorous numerical experiments, it has been found that the structure of streamlines is unicellular except for the inner kernel in some situations. In general, the profile of local heat transfer rate possesses points of singularity for sinusoidal heat flux with periodicity parameter (N), and the profile of local Nusselt number is not symmetric for even values of N. The heat transfer rate is highly affected by a relatively large value of thermal conductivity ratio () as well as Ha, and the absolute difference between the maximum and minimum temperature of the system increases as a function of as well as Ha, whereas the strength of flow decreases gradually with increasing Ha.

Nanofluid jet impingement heating of a cooled surface with a constant heat flux in the presence of porous layer

PurposeThe purpose of this study is to numerically investigate the confined single-walled carbon nanotube-water nanofluid jet impingement heating of a cooled surface with a uniform heat flux in the presence of a porous layer. The analysis of the convective heat transfer mechanism is introduced considering the buoyancy force effect under local thermal non-equilibrium conditions.Design/methodology/approachThe governing equations for the nanofluid and solid phase are discretized by the finite volume method and the SIMPLE algorithm is used to solve these equations.FindingsIt is observed that there is an increase in a local variation of temperature along the upper wall with increasing Reynolds, Darcy and Grashof numbers. For given parameters, the optimum values of thermal conductivity ratio and porous layer thickness leading to better heating on the upper wall are found as Kr = 1.0 and S = 0.5, respectively. The maximum and minimum values of temperature on the upper wall are obtained in the case of higher nanoparticle volume fraction at Re = 100, however, the temperature values get higher along the upper wall with increasing nanoparticle volume fraction at Re = 300.Originality/valueThe effects of various parameters, such as Reynolds number, Darcy number and Grashof number, on thermal behavior and nanofluid flow are examined to determine the desirable heating conditions for the upper wall. This paper provides a solution to problems such as icing on the surface with a suitable thermal design and optimum geometric configuration.

ENTROPY GENERATION ANALYSIS OF NATURAL CONVECTION IN A NON-DARCY ANISOTROPIC POROUS CAVITY

The entropy generation associated with the natural convection in a non-Darcy anisotropic porous cavity is numerically analyzed using the finite element method (FEM). Furthermore, it is endeavored to investigate the effects of different anisotropic porous parameters on the flow, thermal field, and entropy generation. The main considered parameters in the study consist of the Rayleigh number (103 ≤ Ra ≤ 109), the Darcy number (10-2 ≤ Da ≤ 10-6), the permeability ratio (K* = 0.1,1,10), the thermal conductivity ratio (k* = 0.1,1,10), the ratio of Forchheimer constant (F* = 0.1, 1, 10), and the inclination angle of principal axis on the flow (θ = 0°, 45°, 90°). It is found that the anisotropic properties have a significant influence on the entropy generation. Change in the inclination of the principal axis θ alters the average Bejan number and the total entropy generation. The effects of ratio of the Forchheimer constant on the entropy generation and the average Bejan number appear clearly at high values of Rayleigh number. Entropy generation has the highest values in the case of high permeability ratio and low ratio of Forchheimer constant. Lower values of the average Bejan number and the total entropy gerneration are found at high values of thermal conductivity ratio. An increase of the Rayleigh number strongly enhances the total entropy generation. The entropy generation due to the fluid friction is dominant over the entropy generation due to heat transfer at high Rayleigh and Darcy numbers.

Study of the meso-structure and its impact on the thermal performance of closed-cell insulation with moisture ingress

Mechanical pipe insulation systems are commonly used around cold pipes and play significant roles in maintaining the efficiency of building HVAC&R systems. However, thermal insulation systems normally become moist after years of service and various types of insulation are found to have different thermal performances even with the same amount of moisture ingress. Such inconsistencies are possibly caused by the internal structure of the materials and the location of water accumulation. In this paper, a full-scale fractal approach was proposed to simulate the thermal performance of closed-cell insulation by considering meso-structure parameters and moisture with different distribution patterns. Based on the validated model, sensitivity analysis was further discussed to determine the impacts of main meso-structure parameters and physical properties, such as fractal dimension, open porosity, size ratio of cells, thermal conductivity of gas and solid matrix, on the values of thermal conductivity ratio (TCR) with moisture ingress.

Three dimensional analysis of natural convection and entropy generation in a sharp edged finned cavity

A computational study has been performed to make a computational analysis of natural convection and entropy generation in a sharp edged finned cavity. Three dimensional analysis has been done by solving governing equations with a written computational code in Fortran. The study is performed for fin inclination angles from −60° to 60°, Ra=105, Pr=0.7, Rc (conductivities ratio) changes from 0.01 to 100 and irreversibility coefficient is taken as φ=10-5. It is observed that higher values of thermal conductivity ratio (Rc⩾1) do not affect entropy generation due to heat transfer and Bejan number.

Thermal analysis of solar central receiver tube with porous inserts and non-uniform heat flux

In this paper, enhancement for convection heat transfer of turbulent flow in a solar central receiver tube with porous medium and non-uniform circumferential heat flux was numerically investigated. A new method was introduced to build different porous medium configurations in a unified grid system. Four kinds of enhanced receiver tubes (ERTs) with different porous insert configurations were modeled to optimize the performance of ERT. Furthermore, parameters including filling ratio of porous medium, thermal conductivity ratio (thermal conductivity of porous medium versus that of working fluid), porosity and Reynolds number were analyzed. The results showed that ERT partially filled with porous medium has better heat transfer performance than that fully filled with porous medium. The configuration of porous insert for optimal thermal or thermo-hydraulic performance is interactively affected by all the parameters discussed in this paper. The thermal conductivity ratio is the most crucial parameter to the thermal or thermo-hydraulic performance of ERT. The value of thermal conductivity ratio should be greater than 100 to obtain a good thermo-hydraulic performance. The ERTs with horizontal cylindrical segment shaped porous inserts and hollow cylinder shaped porous inserts are proposed because they can obtain optimal thermal or thermo-hydraulic performance.

Conduction-combined forced and natural convection in lid-driven enclosures divided by a vertical solid partition

Numerical simulations of the conduction-combined forced and natural convection (mixed convection) heat transfer and fluid flow have been performed for 2-D lid-driven square enclosure divided by a partition with a finite thickness and finite conductivity. Left vertical wall of enclosure has two different orientations in positive or negative vertical coordinate. Buoyancy forces are taken into account in the system. Horizontal walls are adiabatic while two vertical walls are maintained isothermal temperature but the temperature of the left moving wall is higher than that of the right stationary wall. Thus, heat transfer regime between moving lid and partition is mixed convection. Conduction occurs along the partition. And, pure natural convection is formed between the partition and the right vertical wall. This investigation covers a wide range of Richardson number which is changed from 0.1 to 10, thermal conductivity ratio varies from 0.001 to 10. It is observed that higher heat transfer was formed for higher Richardson number for upward moving wall for all values of thermal conductivity ratio. When forced convection becomes effective, the orientation of moving lid becomes insignificant. Heat transfer is a decreasing function of increasing thermal conductivity ratio for all cases and Richardson numbers.