Search

Highlights • A numerical gas-contributed thermal conductivity model of aerogels is proposed. • The structure of "an idealized aerogel" is reconstructed numerically. • The effect of the solid–gas coupling heat transfer in aerogels is quantified. • Cracks strongly influence the gas-contributed thermal conductivity of aerogels. Abstract Due to the complex microstructure in aerogels and the intricate heat transfer mechanism of solid-gas coupling heat conduction, modeling of the gas-contributed thermal conductivity of this type of material is quite difficult. The present work introduces a novel numerical methodology for computing the gas-contributed thermal conductivity of aerogels by analyzing their microstructural characteristics and heat transfer mechanism of the thermal coupling between the gas phase and the solid backbone of the system. Specifically, structures of aerogels are reconstructed by an improved three-dimensional diffusion-limited cluster-cluster aggregation (DLCA) method, and the contribution of the solid-gas coupling heat transfer to the gas-contributed thermal conductivity of aerogels is quantified. The present numerical model is fully validated by the available experimental data for different aerogels with porosity ranging from 78% to 97.7%. The proposed numerical method is flexible and versatile because it is capable to account for both the geometrical and topological details of the aerogel structure. [ABSTRACT FROM AUTHOR]

This paper provides an insight into the heat transfer in the solid backbone of aerogel and an effective approach to model the apparent solid thermal conductivity of aerogels. First, a model for the thermal conductivity of aerogel solid backbone is developed based on thermal constriction resistance between interconnected nano-particles by taking into account the size effect of a single particle, and validated by the numerical solutions of the gray Boltzmann transport equation (BTE). Then, combined with the analytical expression derived based on the Laplace heat conduction equation, the proposed model is used to predict the apparent solid thermal conductivity of aerogels, and the predictions are in good agreement with the available experimental data of different kinds of aerogels. [ABSTRACT FROM AUTHOR]

In this paper, a modified model for predicting the gaseous thermal conductivity in aerogel pores is proposed based on the kinetic theory of gases and the data from Direct Simulation Monte Carlo (DSMC) method. With the proposed modified model and the classical Effective Medium Theory (EMT) model, a gas-contributed thermal conductivity model which includes the equivalent gaseous thermal conductivity and the solid-gas coupling thermal conductivity is derived. The present gas-contributed model is validated by available experiment results for different types of aerogels. Comparisons between the present model and existing models show that the present gas-contributed model has a higher accuracy without complex calculations and assumptions. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, large eddy simulations with the dynamic Smagorinsky eddy viscosity model are performed for a fully developed turbulent flow and heat transfer in a square duct with unstable natural convection on the cross section at a turbulent Reynolds number of 400, a Prandtl number of 0.7 and different Grashof numbers from 105 to 5×107. The MPI parallel programming language is implemented to speed up the calculations. The influences of the buoyancy force on the mean flow and heat transfer, turbulent intensity and Reynolds stresses are analyzed. The results show that the mean velocity decreases while the subgrid viscosity, turbulent intensity and heat transfer increase obviously with the increase of Grashof number. The turbulent intensity in the streamwise direction decreases considerably in the near-wall region while the turbulent intensity in the spanwise direction increases clearly in almost whole regions with an increase in Grashof number. The spatial distribution of Reynolds stresses are mainly influenced by the thermal buoyancy force. The turbulence production rate and the buoyancy force production term of Reynolds stress component 〈v′v′〉 increase with the increase of Grashof number. [Copyright &y& Elsevier]

Abstract: Direct numerical simulations (DNS) of turbulent flow and convective heat transfer in a square duct with axial rotation were carried out. The pressure-driven flow is assumed to be hydrodynamically and thermally fully developed, for which the Reynolds number based on the friction velocity and hydraulic diameter is kept at constant (Reτ =400). In the finite length duct, two opposite walls are perfectly insulated and another two opposite walls are kept at constant but different temperatures. Four thermal boundary conditions were chosen in combination with axial rotation to study the effects of rotation and Grashof number on mean flow, turbulent quantities and momentum budget. The results show that thermal boundary conditions have significant effects on the topology of secondary flows, profiles of streamwise velocity, distribution of temperature and other turbulent statistic quantities but have marginal effects on the bulk-averaged quantities; Coriolis force affects the statistical results very slightly because it exerts on the plane normal to main flow direction and the rotation rate is low; Buoyancy effects on the turbulent flow and heat transfer increase with the increase of Grashof number (Gr), and become the major mechanism of the development of secondary flow, turbulence increase, and momentum and energy transport at high Grashof number. [ABSTRACT FROM AUTHOR]

• Microchannel heat sink with cavities and longitudinal vortex generators is proposed and evaluated based on three-dimensional numerical simulation. • The main effects of cavities and longitudinal vortex generators geometric parameters on thermal resistance and pumping power of the microchannel heat sink are studied. • The optimal results based on the primary geometric parameters are obtained by the multi-objective optimization algorithms. • The optimal solutions from 0.2594 K/W to 0.4176 K/W for thermal resistance and 0.0117 W to 0.047 W for pumping power, respectively. Microchannel heat sink (MCHS) is an innovative cooling device for high-powered integrated circuits. Cavities and longitudinal vortex generators (LVGs) are two kinds of effective heat transfer enhancement techniques for MCHS. It has been demonstrated that cavities can enhance the heat transfer but generate the dead zone of flow, while longitudinal vortex generators (LVGs) have the advantage of low pressure drop. In this paper, an integrated microchannel heat sink combing cavities and LVGs (MC-LVG-C) is proposed. The impacts of geometric parameters of cavities and LVGs on the thermal-hydraulic performance of proposed MCHSs are analyzed and evaluated by CFD when the Reynolds number (Re) ranges from 150 to 325. Further, the multi-objective configuration optimization based on Non-dominated Sorted Genetic Algorithm-II (NSGA-II) is performed at Re= 325 to minimize the thermal resistance and pumping power of MC-LVG-C simultaneously. It is found that the proposed scheme (MC-LVG-C) has clear superiority in the thermal-hydraulic performance over MCHS with cavities only (MC-C) and MCHS with LVGs only (MC-LVG). Geometric parameters analysis reveals that the height (h l) and length (L) of LVGs and the spacing (L 2) between LVGs and cavities have significant impact on the thermal-hydraulic performance. The NSGA-II optimization indicates that optimal thermal-hydraulic performance values for MC-LVG-C are divided into four design zones. For optimal solutions, better heat transfer can be obtained by increasing h l and L , while smaller resistance can be provided by increasing h l and L 2. In practical design, the reasonable geometric parameters can be chosen from the proposed four design zones by the design requirement. [ABSTRACT FROM AUTHOR]

This paper numerically investigated the coupled flow and heat transfer of a parabolic trough collector (PTC), with the non-uniform heat flux boundary condition on the absorber wall and the rarefied gas effects in the annular vacuum gap being taken into consideration. A fully coupled cross-sectional heat transfer model is established with Direct Simulation Monte Carlo (DSMC) method for the rarefied gas flow and heat transfer in the vacuum annual gap. The PTC tube efficiency can be obtained from the above simulation for a given HTF temperature. Such simulation is conducted for several specified HTF temperature and different efficiency data are obtained. These data are fitted by an equation. This equation is then used to advance the HTF temperature in the axial direction. In such a way a simplified 3D model for the design of a PTC receiver is obtained. Cross-sectional simulation results show that when the gas pressure is less than 0.1 Pa further decrease in pressure makes no further contribution to reduce the heat loss. The effects of periphery non-uniform distribution of heat flux, coating material emissivity, envelope diameter and HTF inlet velocity on the PTC efficiency are discussed. An operation variant is proposed by using the 3D model by which the total PTC tube length can be reduced for a given thermal load. [ABSTRACT FROM AUTHOR]

This paper is aiming at numerically demonstrating the interrelationship and consistency between field synergy principle (FSP) via the field synergy number (Fc) and the entransy dissipation extremum principle (EDEP). Numerical simulation is conducted by using the FLUENT software and the user defined function programs (UDF) for fin-and-tube surfaces (plain plate and slotted fins) and composite porous materials. The thermal boundary conditions include given heat flux and given surface temperature. The flow includes laminar and turbulent. The air properties may be constant or vary with temperature. Based on the numerical data the analyzed results from the FSP via Fc are totally consistent with the results analyzed by the EDEP for all the cases studied. Such consistency between the FSP and the entransy theory can be regarded as a kind of demonstration of the reliability and correctness of both the FSP and the entransy theory. [ABSTRACT FROM AUTHOR]