Effective soil thermal conductivity (λ
eff) is a critical parameter for environmental and earth science as well as engineering applications. Models to predict λ
eff are required in diverse global and ...community land surface schemes as well as climate models to investigate coupled water and heat transport in soils and heat exchange at the earth surface. Among the many soil thermal conductivity models, models based on the normalized concept are most often developed and utilized for estimating λ
eff. However, at present no systematic study has been performed to investigate the origin and evolution of the normalized thermal conductivity models, nor to evaluate their performance with large datasets. The objectives of this study were to: (a) review the development and evolution of the normalized thermal conductivity models, and (b) assess their performance with datasets consisting of soils with a full range of water saturation and a wide range of soil textures and bulk densities. A total of 38 normalized thermal conductivity models were critically reviewed and their relationships were clearly outlined. Their performance was evaluated by five categories according to model characteristics with a compiled dataset consisting of 71 soils and 669 tests collected from nine studies. Our analysis demonstrated key roles of the quartz content, solid thermal conductivity and choice of the Kersten functions in the model applicability and accuracy of estimating λ
eff. The results showed that the Y2018, CK2005, CK2006, J1975, L2007 and T2009 models have the best performance among the models without fitting parameters, but further improvements are required to apply them universally. Although the models of H2017, LD2015, M2006 and K2007 are the best performing models with fitting parameters, approaches to calculate these parameters are required so they can be easily applied. Future studies on parametrization of currently well‐performing models for wider and more accurate application, development of a soil thermal conductivity database for model evaluation and calibration purposes, and connecting soil thermal conductivity models to hydraulic properties are recommended.
Highlights
The history and evolution of normalized thermal conductivity models and the potential Kersten (K
e) functions are collated and synthesized.
A total of 38 models were reviewed and their performance was evaluated with a total of 71 soils and 669 tests from nine studies.
The Y2018, CK2005, CK2006, J1975, L2007 and T2009 are the best ranked models without fitting parameters.
The models of H2017, LD2015, M2006 and K2007 are the best ranked models with fitting parameter.
A review of experimental/computational studies to enhance the thermal conductivity of phase change materials (PCM) that were conducted over many decades is presented. Thermal management of ...electronics for aeronautics and space exploration appears to be the original intended application, with later extension to storage of thermal energy for solar thermal applications. The present review will focus on studies that concern with positioning of fixed, stationary high conductivity inserts/structures. Copper, aluminum, nickel, stainless steel and carbon fiber in various forms (fins, honeycomb, wool, brush, etc.) were generally utilized as the materials of the thermal conductivity promoters. The reviewed research studies covered a variety of PCM, operating conditions, heat exchange and thermal energy storage arrangements. The energy storage vessels included isolated thermal storage units (rectangular boxes, cylindrical and annular tubes and spheres) and containers that transferred heat to a moving fluid medium passing through it. A few studies have focused on the marked role of flow regimes that are formed due to the presence of thermally unstable fluid layers that in turn give rise to greater convective mixing and thus expedited melting of PCM. In general, it can be stated that due to utilization of fixed high conductivity inserts/structures, the conducting pathways linking the hot and cold ends must be minimized.
The quest for advanced superionic materials requires understanding their complex atomic dynamics, but detailed studies of the interplay between lattice vibrations and ionic diffusion remain scarce. ...Here inelastic and quasielastic neutron scattering measurements in the superionic argyrodite Cu7PSe6 are reported, combined with molecular dynamics (MD) based on ab initio and machine‐learned potentials, providing critical insights into the atomistic mechanisms underlying fast ion conduction. The results reveal how long‐range Cu diffusion is limited by intercluster hopping, controlled by selective anharmonic phonons of the crystalline framework. Further, the Green–Kubo simulations reproduce the ultralow lattice thermal conductivity and identify contributions from mobile ions, phonons, and their cross‐correlations. The mode resolved analysis shows that the thermal conductivity is dominated by low‐energy acoustic phonon modes of the overall crystal framework. The analysis of mode‐resolved spectral functions further show that vibrational modes with significant Cu contributions are strongly damped, corresponding to the breakdown of associated phonon quasiparticles. These results highlight the importance of strongly anharmonic effects in superionic systems, in which the traditional quasiharmonic phonon picture is insufficient, and pave the way toward combining machine‐learning accelerated simulations with neutron scattering experiments to rationalize the complex atomic dynamics underlying ionic and thermal transport.
The influence of host dynamics on the ionic and thermal transport properties in Cu7PSe6 is investigated via neutron scattering measurements and machine‐learned molecular dynamics simulations. In the absence of host dynamics, the intercluster Cu+ hopping is strongly suppressed, reducing long‐range Cu+ diffusion. The anharmonic low‐energy Cu dominated phonon modes are overdamped at high temperatures and lead to ultralow thermal conductivity.
•Unlike the traditional routine, the Cattaneo–Christov heat flux in formulation is employed.•Variable thermal conductivity is accounted.•Surface with variable thickness is considered.•Double ...stratification is analyzed.•Nonlinear stretching phenomenon is imposed.
Here temperature dependent thermal conductivity in stagnation point flow toward a nonlinear stretched surface with variable thickness is considered. Heat flux in formulation is based upon Cattaneo–Christov theory. Double stratification and chemical reaction effects are further retained. Convergent series solution for flow of Jeffrey fluid and heat and mass transfer are developed. Residual errors are calculated for the velocity, temperature and concentration equations and results are discussed through graphs. Influences of skin friction coefficient is also studied. It is observed that temperature profile decreases for higher thermal relaxation parameter.
•Keff correlations for both motionless fluids and in motion are reviewed.•Reviewed correlations for axial flow reported differences of one order of magnitude.•There are no proper correlations for ...low-velocity fluids in dual thermocline tanks.•No proper correlations for packed beds containing PCMs can be found in literature.•For most cases, conduction characteristic time is much lower than convection one.
This paper reviews the extensive literature about the different experimental correlations for effective thermal conductivity in packed beds. The review covers correlations for stagnant thermal conductivity (with a motionless fluid) keff0, including thermal radiation heat transfer, keff0,rad. In addition, correlations for effective conductivity with the fluid in motion for both axial, keffa, and radial, keffr, directions are reviewed together with expressions for effective conductivity in packed beds filled with encapsulated phase change materials keffpcm.
High discrepancies of one order of magnitude in the correlations available in the literature for axial effective thermal conductivity, keffa, are observed. Nevertheless, a sensitivity analysis concludes that the influence of keffa on a packed bed performance is negligible for most cases because the conduction characteristic time is usually much larger than that for convection. Only for very low flow rates (as in thermocline tanks) can it influence the charging and discharging times.
The review concludes that more research and new correlations are necessary for thermocline tanks and for beds filled with encapsulated PCMs. In addition, new correlations for the axial effective thermal conductivity obtained when the fluid flows in the same direction as the heat flux are necessary. The numerical models available in the literature used correlations that were not obtained for the working conditions of these energy storage systems, which may result in large discrepancies in the results of these models.
•Metal and carbon based materials are used to enhance the heat transfer rate in PCM.•Melting point of the PCMs becomes slightly lower for PCM/foam composites.•Higher porosity foams are suitable for ...convection heat transfer.•Latent heat and specific heats are reduced for PCM/foam composites.•Charging and discharging period of composite PCMs become lower.
Phase change material (PCM) is promising media for thermal energy storage owing to its extensive value of latent heat (140–970 KJ/Kg). However, thermal conductivity of PCMs is too low which obstructs energy storage and retrieval rate. In recent days, thermally enhanced PCMs are considered promising materials for efficient heat transfer in many applications. This article designates the review on improved thermal properties and heat transfer of PCMs by using porous materials. Enhanced heat transfer of PCMs can be achieved using extended surfaces (triangular, conical, square, and rectangular fins), heat pipes, and addition of highly conductive nanoparticles (e.g. Cu, Al2O3, Au, SiC, SiO2 and TiO2). Major focus of this article is to study the enhanced heat transfer of PCMs through metallic (copper, nickel, and aluminum) and carbon based (carbon, graphite and expanded graphite) porous materials/foams. Effects of porosity and pore density on heat transfer, thermal conductivity, specific heat, latent heat and charging/discharging time are critically reviewed. Porous materials/foams are reported to be efficient for heat transfer/thermal conductivity enhancement by 3–500 times. Furthermore, correlations to find the effective thermal conductivity of PCM/foam are reported. Important applications of PCM/foam reported by different researchers are also discussed in this paper. Finally, conclusions and recommendations are presented to highlight the research gap in this area.
Previously, the research on thermal conductivity of ceramic thermal barrier coatings mainly focused on phonon and photon thermal conductivity (thermal radiation effect). However, electrical ...conductivity is remarkable in some systems. Hence, the contribution of phonon, photon and electronic heat conduction to thermal conductivity of high-entropy systems was evaluated in this study. The (La0.2Gd0.2Y0.2Yb0.2Er0.2)2(Zr1-xCex)2O7 (x = 0–0.5) high-entropy ceramics with single defective fluorite structure were successfully prepared via a solid reaction method. Below 600 °C, the thermal conductivities decrease with increasing temperature for x = 0.1–0.5 components, then reveal a drastic temperature dependent increase. Moreover, the composition dependent thermal conductivities are also unusual based on the conventional phonon thermal conduction mechanism. The increased electronic thermal conductivity, improved photon thermal conductivity (at high temperatures) and reduced phonon-grain boundary scattering should be responsible for the unusual thermal conductivity behavior. This can be verified by the significantly increased electrical conductivity, optical transmittance and grain size, as well as reduced emissivity for (La0.2Gd0.2Y0.2Yb0.2Er0.2)2(Zr1-xCex)2O7 high-entropy ceramics. The present study also broadens the way to investigate the thermal conductivity of ceramic thermal barrier coatings, and is helpful to design thermal barrier coatings with low thermal conductivity.
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•A series of novel high-entropy ceramics were prepared via a solid reaction method.•The temperature and component-dependent thermal conductivity show unusual behavior.•This study evaluated the effects of phonon, photon and electron on heat conduction.•The unusual heat conduction is mainly attributed to photon and electronic effects.•This study broadens the route for studying the thermal conductivity of TBCs.
•Expanded graphite can improve thermal conductivity of RT44HC by 20–60 times.•Thermal conductivity of PCM/EG composites keeps constant before/after melting.•Thermal conductivity of PCMs nearly ...doubled during phase changing.•Thermal conductivity of composite PCM increases with density and percentage of EG.•The simple model predicts thermal conductivity of EG-based composites accurately.
This work studies factors that affect the thermal conductivity of an organic phase change material (PCM), RT44HC/expanded graphite (EG) composite, which include: EG mass fraction, composite PCM density and temperature. The increase of EG mass fraction and bulk density will both enhance thermal conductivity of composite PCMs, by up to 60 times. Thermal conductivity of RT44HC/EG composites remains independent on temperature outside the phase change range (40–45°C), but nearly doubles during the phase change. The narrow temperature change during the phase change allows the maximum heat flux or minimum temperature for heat source if attaching PCMs to a first (constant temperature) or second (constant heat flux) thermal boundary. At last, a simple thermal conductivity model for EG-based composites is put forward, based on only two parameters: mass fraction of EG and bulk density of the composite. This model is validated with experiment data presented in this paper and in literature, showing this model has general applicability to any composite of EG and poor thermal conductive materials.
Phonon scattering by nanostructures and point defects has become the primary strategy for minimizing the lattice thermal conductivity (κL) in thermoelectric materials. However, these scatterers are ...only effective at the extremes of the phonon spectrum. Recently, it has been demonstrated that dislocations are effective at scattering the remaining mid‐frequency phonons as well. In this work, by varying the concentration of Na in Pb0.97Eu0.03Te, it has been determined that the dominant microstructural features are point defects, lattice dislocations, and nanostructure interfaces. This study reveals that dense lattice dislocations (≈4 × 1012 cm−2) are particularly effective at reducing κL. When the dislocation concentration is maximized, one of the lowest κL values reported for PbTe is achieved. Furthermore, due to the band convergence of the alloyed 3% mol. EuTe the electronic performance is enhanced, and a high thermoelectric figure of merit, zT, of ≈2.2 is achieved. This work not only demonstrates the effectiveness of dense lattice dislocations as a means of lowering κL, but also the importance of engineering both thermal and electronic transport simultaneously when designing high‐performance thermoelectrics.
Eu‐doping effectively converges the valence bands of PbTe, while Na‐doping enables dense lattice dislocations, leading to an extremely low lattice thermal conductivity (κL) of <0.4 W m−1 K−1. This contributes to a high zT of ≈2.2, opening new possibilities for advancing thermoelectrics through dislocation and band‐engineering approaches.
A haloscope of the QUAX– a γ experiment composed of an oxygen-free high thermal conductivity-Cu cavity inside an 8.1 T magnet and cooled to ∼ 200 mK is put in operation for the search of galactic ...axion with mass ma ≃ 43 μ eV . The power emitted by the resonant cavity is amplified with a Josephson parametric amplifier whose noise fluctuations are at the standard quantum limit. With the data collected in about 1 h at the cavity frequency νc = 10.40176 GHz , the experiment reaches the sensitivity necessary for the detection of galactic QCD-axion, setting the 90% confidence level limit to the axion-photon coupling gaγγ < 0.766 × 10−13 GeV−1.
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