Thermal Conductivity Heat Transfer Research Articles

Influence of casting-warm pressing process on the thermal conductivity of micro–nano-Al2O3 substrate

Alumina substrates are increasingly used for high-power integrated circuits due to their high thermal conductivity, low thermal expansion coefficient, and excellent insulation properties. However, pores in the green tape from the tape casting process reduce the thermal conductivity and permittivity of the sintered ceramic substrate. Researchers have attempted to minimize ceramic porosity with chemical additives or by sintering pure alumina at temperatures above 1650 °C, but these methods often degrade thermal conductivity or quality of substrate evenness. This study proposes a low-cost casting-warm pressing process to densify pure alumina ceramic substrates using micro–nano-mixed alumina powders and sintering at relatively low temperatures. The results indicate that the relative density of the pure alumina ceramic substrate prepared at 1500 °C is 93%, a 4.4% improvement over the tape casting process. Additionally, the thermal conductivity of the alumina substrate from the casting-warm pressing process reaches 15.89 W/(m K), which is 1.4 times higher than that of the tape casting process. Microstructure analysis shows that the casting-warm pressing process with micro- and nano-multi-scale mixed alumina powders forms a novel thermal conduction enhancement mechanism. Large particles in the green tape overlap, while small particles fill the spaces between the large particles. The connected micrometer-sized particle skeletons form high thermal conduction net channels in the substrate, improving the thermal conductivity for heat transfer.

Theoretical study on the effective thermal conductivity of silica aerogels based on a cross-aligned and cubic pore model

Abstract Aerogel nanoporous materials possess high porosity, high specific surface area, and extremely low density due to their unique nanoscale network structure. Moreover, their effective thermal conductivity is very l1ow, making them a new type of lightweight and highly efficient nanoscale super-insulating material. However, predicting their effective thermal conductivity is challenging due to their uneven pore size distribution. To investigate the internal heat transfer mechanism of aerogel nanoporous materials, this study constructed cross-aligned and cubic pore model (CACPM) based on the actual pore arrangement of SiO2 aerogel. Based on the established cross-aligned and cubic pore model (CACPM), the effective thermal conductivity expression for aerogel was derived by simultaneously considering gas-phase heat conduction, solid-phase heat conduction, and radiative heat transfer. The derived expression was then compared with available experimental data and the Wei structure model. The results indicate that, according to the model established in this study for the derived thermal conductivity formula of silica aerogel, for powdery silica aerogel under the conditions of T=298K, a 2=0.85, D 1=90μm, ρ=128kg/m3, within the pressure range of 0-105Pa, the average deviation between the calculated values and experimental values is 10.51%. In the pressure range of 103-104Pa, the deviation between calculated values and experimental values is within 4%. Under these conditions, the model has certain reference value in engineering verification. This study also makes a certain contribution to the research of aerogel thermal conductivity heat transfer model and calculation formula.

Stability analysis of GO‐EG and GO‐blood base nanofluids by implementing of thermal radiation phenomena

AbstractThis work leaves to explore the stability of a 2‐D, time free, thermal boundary layer of graphene oxide ethylene glycol (GO‐EG,) where GO‐blood is taken as base nanofluids over a stretchable surface by executing a magnetic field (MHD), coupled stress effects, heat source/sink, and sun‐oriented radiations. This work is done due to the reason that nanofluids have a higher heat conductivity level rather than regular liquids. Nonlinear partial differential equations (N‐PDEs) are modeled utilizing the notable boundary layer equations. Dimensionless ordinary differential conditions (ODEs) are made from the created NPDEs by utilizing dimensional analysis and group theoretic approach. We examine the obtained ODEs through the notable homotopy analysis technique (HAM). The impacts of different parameters on temperature distribution and velocity profiles are shown graphically. However, some tables are constructed in support of the presented study to confirm the stability and convergence of the solutions. The main intentions in this work are to underline the enhancement of traditional heat transfer's thermal conductivity.

Methane hydrate formation enhanced by thermally expanded graphite with multi-sized pores

Methane hydrate has been endowed with great potential in energy storage and transportation, but the sluggish formation kinetics restrict its industrial application. Herein the thermally expanded graphite (EG), with multi-sized pore structure that can be effectively controlled by expanding and grinding processes, is used to promote the nucleation and growth of methane hydrates. The results demonstrate that the EG exhibits a remarkable promotion in the formation of methane hydrate from water, with high gas storage capacity of 143 V/V, short induction time of 10 min, and rapid gas storage rate of 1 V·V·min−1, which are attributed to its well-designed multi-sized pore structure. Thereinto, the nano-sized pores can effectively adsorb guest molecules, increasing the guest–host contact area; meanwhile, the micron-sized pores with moderate scale not only provide abundant channels for mass transfer of reactants but also enable good thermal conductivity for heat transfer to soundings. This work provides an insight into the promotion effect of multi-sized porous medium in hydrate formation, and offers an effective strategy for fabricating high-performance promotors through pore designing.

Mutual impact of thermal radiations and temperature dependent thermal conductivity on non-Newtonian multiple flow regimes

The transfer of thermal energy is a specialized discipline of thermal engineering and hence own practical applications in an industrial thermal systems like refineries, air-conditioning plants, automotive engines, power plants, jet engines, refrigeration, food processing, and chemical processing plants to mention just a few. Owning such importance we have reported the collective impact of thermal radiations and temperature-dependent thermal conductivity for heat transfer in stagnation point magnetized Casson fluid flow field. The thermal flow regime of Casson fluid is manifested with a viscous dissipation effect as well. The flow equations are tackled numerically by the use of the shooting method. The outcomes are debated comparatively by considering various flow regimes namely (I) Thermal flow regime with thermal radiations (II) Thermal flow regime without thermal radiations (III) Heat flow with heat generation and (IV) Heat flow in the absence of heat generation. We believe that the present thermal comparative analysis will be helpful for the preceding studies which own thermal engineering standpoints for the transfer of thermal energy by means of fluid flow.

A numerical model for full and partial penetration hybrid laser welding of thick-section steels

Full and partial penetration welding using high power lasers exhibit different melt pool geometries. The main difference is that full penetration welds tend to widen at the root side. In recent years, full penetration laser welding has been modeled using sophisticated multiphase numerical models. However, less computationally-complex thermo-mechanical models that are capable of accounting for the root widening phenomenon in full penetration laser welding are lacking. In this study, hybrid laser welding was performed for full and partial penetration modes on butt joints of structural steel. A numerical model was presented that used a three-dimensional transient Finite Element (FE) analysis and thermal conduction heat transfer for calculating the temperature fields in both full and partial penetration welding modes. For this purpose, a double-conical volumetric heat source was developed based on the three-dimensional conical (TDC) heat source in the literature. The model was validated and calibrated with different experiments. The results show that the model is capable of calculating the transient temperatures for the common melt pool geometries obtainable by the full and partial penetration hybrid laser welding of thick-section steels. The model can potentially be employed as the basis for predicting e.g. microstructural properties or residual stresses for a given welding procedure.

Experimental investigations of charging/melting cycles of paraffin in a novel shell and tube with longitudinal fins based heat storage design solution for domestic and industrial applications

Due to vulnerability of solar energy based technologies to weather fluctuations and variations in solar thermal irradiance, thermal energy storage (TES) systems with their high thermal storage capacity offer a sustainable solution. In this article, experimental investigations are conducted to identify thermal performance of latent heat storage (LHS) unit in connection with flat plate solar collector during charging cycles. LHS unit is comprised of novel geometrical configuration based shell-and-tube heat exchanger with longitudinal fins, paraffin as thermal storage material and water as heat transfer fluid (HTF). In order to overcome the effect of low thermal conductivity of paraffin, the effective surface area and overall thermal conductivity for heat transfer is significantly improved by employing longitudinal fins. Moreover, the vertical orientation of longitudinal fins supports natural convection, which can assure rapid charging of paraffin in LHS unit. In experimental tests, the focus is on probing the heat transfer mechanism and temperature distribution in entire novel LHS unit, the influence of inlet temperature and volume flow rate of HTF on phase transition rate and mean power. Experimental results revealed that natural convection significantly influences the phase transition rate. Therefore, enthalpy gradient is noticed between paraffin at top, central and bottom positions in LHS unit. Likewise, the phase transition rate and mean power of LHS unit is significantly increased by a fraction of 50.08% and 69.71% as the inlet temperature of HTF is increased from 52°C to 67°C, respectively. Similarly, it is concluded that volume flow rate of HTF has a relatively moderate influence on thermal performance; however the influence declines with an increase in inlet temperature of HTF. Due to significant enhancement in thermal performance, the novel geometrically configured LHS unit can accumulate about 14.36MJ of thermal energy in as less as 3hours. Furthermore, a broad range of domestic and commercial energy demands can be fulfilled by simply assembling several LHS units in parallel sequence.

On Thermo-viscoelasticity with Variable Thermal Conductivity and Fractional-Order Heat Transfer

The equations of generalized thermo-viscoelasticity for an isotropic medium with variable thermal conductivity and fractional-order heat transfer are given. The resulting formulation is applied to a half-space subjected to arbitrary heating which is taken as a function of time and is traction free. The Laplace transform technique is used. A numerical method is employed for the inversion of the Laplace transforms. Numerical results for temperature, displacement, and stress distributions are given and illustrated graphically for the problem. The effects of the fractional order and the variable thermal conductivity for heat transfer on a viscoelastic material such as poly(methyl methacrylate) (Perspex) are discussed.

Effect of Hygrothermal Transfer on Multilayer Walls Behavior, Assessment of Condensation Risk

Good insulation of envelopes in new or rehabilitated buildings represents one of the major issues in reducing the energy consumption in the construction sector. In order to ensure better insulation, a material compatible with the wall must be chosen to guarantees both good thermal insulation and reduction of humidity risk.This study reviews the risks related to moisture, particularly those related to condensation one occurring in buildings. Two stages were conducted during the study. Firstly, a coupled heat, air and moisture transfer model in a multilayer wall was developed. This model took into account both the discontinuity at the interfaces between the wall layers and the evolution of material properties depending on the water content. Then, applications to assess hygrothermal behavior of multilayer walls and predict of condensation risk into them were examined.Secondly, a parametric study was achieved on hygrothermal properties of insulation materials. The objective of this second stage was to evaluate sensitivity magnitude of different material properties on hygrothermal behavior of multilayers walls.This study focused on the most influential thermal and hydric properties in hygrothermal transfer model. Thus, thermal conductivity for heat transfer, total moisture permeability and moisture capacity to hydric transfer were selected.Finally, simulation results were analyzed and conclusions were derived on the effect of key parameters on the hygrothermal behavior in the walls and the conditions causing condensation risk to increase.

Modelling heat transfer in steel coils

During annealing, heat transfer within steel coils is complicated by the different conductivity in radial and axial directions due to small air gaps between the steel layers in the radial direction. Here an analytic solution augmented with numerical calculations illustrates some of the fundamental behaviour of the system, including calculations of the time lag. The beneficial effects of additional heating on the curved surfaces are shown. References M. Abramowitz and I. Stegun. Handbook of Mathematical Functions, Dover, New York, 1970. http://mintaka.sdsu.edu/faculty/wfw/ABRAMOWITZ-STEGUN/ G. Chen and M. Gu. Simulation of steel coil heat transfer in a high performance hydrogen furnace. Heat Transf. Eng., 28, 25--30, 2007. doi:10.1080/01457630600985568 M. Cozijnsen and W. Y. D. Yuen. Stress distributions in wound coils. 2nd Biennial Austr. Eng. Math. Conf., 1996, 117--124. http://search.informit.com.au/documentSummary;dn=716511683510327;res=IELENG G.F. Harvey. Mathematical simulation of tight coil annealing. Metallurgical forum 9, Math. Models, 22, 28--37, 1977. R. Hickson, S. I. Barry and G. N. Mercer. Heat transfer across multiple layers. ANZIAM J (E), submitted, 2008. Y. Jaluria. Numerical study of the thermal processes in a furnace. Num. Heat Transf., 7, 211--224, 1984. doi:10.1080/01495728408961820 M. McGuinness, W. L. Sweatman, D. Baowan and S. I. Barry. Annealing Steel Coils. Proc. MISG 2008, in press, 2008. S. S. Sahay, A. M. Kumar and A. Chatterjee. Development of integrated model for batch annealing of cold rolled steel. Ironmaking and Steelmaking, 31, 144--151, 2004. doi:10.1179/030192304225010990 M. R. Sridhar and M. M. Yovanovicht. Review of elastic and plastic contact conductance models: Comparison with experiment. J. Thermophysics Heat Transfer, 8, 633--640, 1994. http://www.aiaa.org/content.cfm?pageid=406&gTable=japaperimportPre97&gID=592 U. O. Stikker. Numerical simulation of the coil annealing process, Math. Models in Metallurgical process development, Iron and Steel Institute, Special report. 123, 104--113, 1970. W. Wu, F. Yu, X. Zhang and Y. Zuo. Mathematical model and its application of radial effective thermal conductivity for coil heat transfer in a HPH furnace. J. Thermal Science 11, 134--137, 2002. doi:10.1007/s11630-002-0033-1 X. Zhang, F. Yu, W. Wu and Y. Zhao. Application of radial effective thermal conductivity for heat transfer model of steel coils in HPH furnace. Int. J. Thermophysics, 24, 1395--1405, 2003. doi:10.1023/A:1026115521232

Theoretical Analysis of Radiative Effects on Transient Free Convection Heat Transfer past a Hot Vertical Surface in Porous Media

The purpose of the present investigation deals with the unsteady free convective flow of a viscous incompressible gray, absorbing-emitting but non-scattering, optically-thick fluid occupying a semi-infinite porous regime adjacent to an infinite moving hot vertical plate with constant velocity. We employ a Darcian viscous flow model for the porous medium. The momentum and thermal boundary layer equations are non-dimensionalized using appropriate transformations and then solved subject to physically realistic boundary conditions using the Laplace transform technique. Thermal radiation effects are simulated via a radiation-conduction parameter, Kr, based on the Rosseland diffusion approximation. The influence of Grashof (free convection) number, radiation-conduction parameter (Kr), inverse permeability parameter (Kp) and dimensionless time (t) are studied graphically. We observe that increasing thermal radiation parameter causes a noticeable increase in the flow velocity, u. Temperature, θ, is significantly increased within the boundary layer with a rise in Kr since the latter represents the relative contribution of thermal radiation heat transfer to thermal conduction heat transfer. Increased radiation therefore augments heat transfer, heats the fluid and increases the thickness of the momentum and thermal boundary layers. Velocity is found to decrease with an increase in Kp (inverse permeability parameter) as are shear stress function ( ∂u/∂y | y=0) magnitudes owing to greater resistance of the porous medium for lower permeabilities, which decelerate the flow. An increase in Kr however boosts the shear stress function magnitudes i.e. serves to accelerate the flow. Temperature gradient, ∂θ/∂y | y=0 is also positively affected by an increase in thermal radiation (Kr) and with time. The present study has applications in geological convection, forest fire propagation, glass heat treatment processes at high temperature, metallurgical processing etc.

Effective Thermal Conductivity for Heat Transfer Through Semitransparent Solids

The steady‐state effective thermal conductivity of fused quartz was examined analytically and experimentally for two classes of boundary conditions. These results are compared with several effective conductivity models which have been suggested for semitransparent solids. Mean effective conductivity results illustrate the effect of sample thickness, temperature, and boundary conditions as well as serving to evaluate the