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• Nanolayer thickness is calculated with varying silica wall thicknesses. • A modified model is proposed to predict the melting enthalpy of composite PCM. • Heat flux is decomposed to reveal the contributions of nanolayer. Thermophysical properties of composite phase change materials (PCMs) are influenced by the interfacial nanolayer. Molecular dynamics (MD) method is used to study the effect of interfacial nanolayer on thermophysical properties of silica-paraffin composite PCM. The simulation results show that the melting enthalpy is reduced from 197.9 J·g−1 to 29.1 J·g−1, and the thermal conductivity is enhanced from 0.142 W·m−1·K−1 to 0.268 W·m−1·K−1, when the silica thickness is increased from 0 to 15.0 Å. The interfacial nanolayer is observed in composite PCM, and its thickness increases with the increase of silica wall thickness. In the composite PCM with 7.0 Å silica wall, a loss of 62.9% in the melting enthalpy is observed, with 32.6% attributed to the silica mass and 30.3% attributed to the nanolayer. Based on that, a modified model is proposed to predict the melting enthalpy of composite PCMs by considering the effects of both silica mass fraction and nanolayer. To reveal the thermal conductivity enhancement mechanism, the total heat flux is decomposed into three terms (namely, kinetic energy term, potential energy term and interaction energy term). It is found that the interaction energy term has the largest contribution to the total heat flux. The presence of nanolayer strengths the interactions, leading to the thermal conductivity enhancement. [ABSTRACT FROM AUTHOR]

• We have proposed a data-driven machine learning workflow for designing multilayer hyperbolic metamaterials composed of α-MoO 3 , achieving high modulation contrast in near-field thermal radiation. • By combining the multilayer perceptron and bayesian optimization techniques, we optimize the rotation angle, layer thickness, and gap distance of the multilayer metamaterials, resulting in a 97 % improvement in thermal modulation contrast compared to previous single layer structures. • The large thermal modulation contrast is attributed to the controlled alignment and misalignment of hyperbolic plasmon polaritons and hyperbolic surface phonon polaritons in each layer, achieved by manipulating the rotation angles. The thermal modulator based on the near-field radiative heat transfer has wide applications in thermoelectric diodes, thermoelectric transistors, and thermal storage. However, the design of optimal near-field thermal radiation structure is a complex and challenging problem due to the tremendous number of degrees of freedom. In this work, we have proposed a data-driven machine learning workflow to efficiently design multilayer hyperbolic metamaterials composed of α-MoO 3 for near-field thermal radiative modulator with high modulation contrast. By combining the multilayer perceptron and Bayesian optimization, the rotation angle, layer thickness and gap distance of the multilayer metamaterials are optimized to achieve a maximum thermal modulation contrast ratio of 6.29. This represents a 97 % improvement compared to previous single layer structure. The large thermal modulation contrast is mainly attributed to the alignment and misalignment of hyperbolic plasmon polaritons and hyperbolic surface phonon polaritons of each layer controlled by the rotation. The results provide a promising way for accelerating the designing and manipulating of near-field radiative heat transfer by anisotropic hyperbolic materials through the data-driven style. [Display omitted] [ABSTRACT FROM AUTHOR]

Highlights • A 30–50 K deterioration of thermal insulation performance of functionally graded thermal barrier coatings (FGTBCs) compared to traditional TBCs is reported. • Thermophysical properties of Functional Graded Materials(FGMs) are experimentally determined. • A comprehensive heat transfer model was built to predict the stationary temperature distribution and heat flux inside FGTBCs. Conduction heat flux and thermal radiation flux were analyzed separately. Abstract To further study the heat transfer process inside the functionally graded thermal barrier coatings (FGTBCs) beyond the companion part of the present paper concerning their radiative properties, this paper takes both conductive and radiative heat transfer into account and establishes a comprehensive heat transfer model considering both conductive and radiative heat transfer to estimate the thermal insulation performance of FGTBCs. Thermal conductivities of more than thirty yttria-stabilized zirconia (YSZ)/NiCoCrAlY FGTBC samples were determined based on their thermal diffusivities determined by the laser flash analysis. To predict the overall thermal insulation performance of 8YSZ/NiCoCrAlY FGTBCs, a comprehensive heat transfer model based on the two-flux method and multi-band model was built to solve the stationary temperature distribution and heat flux inside thermal barrier coatings (TBCs) and FGTBCs. Results indicate that FGTBCs show worse thermal insulation performance compared to conventional YSZ TBCs; we also investigated the influence of the number of layers. The temperature of engine blade is increased by 30 – 50 K if coated with an FGTBC instead of a conventional TBC. Conduction heat flux is significantly greater than radiative flux for both TBCs and FGTBCs. This paper gives a detailed analysis by simultaneously considering the heat conduction and thermal radiation in FGTBCs, which is helpful for achieving better thermal design of YSZ/NiCoCrAlY FGTBCs. Along with the Part I, this two-part article presents a deep and thorough study on the thermal insulation performance of YSZ/NiCoCrAlY FGTBCs. [ABSTRACT FROM AUTHOR]

Highlights • Decreased reflectance of YSZ/NiCoCrAlY Functionally Graded Thermal Barrier Coatings (FGTBCs) compared to that of conventional YSZ TBCs. • Determination of spectral absorption coefficients and spectral scattering coefficient of YSZ TBCs and YSZ/NiCoCrAlY mixture coatings. • The role of ceramic top coating in radiative properties of ceramic-metal FGTBCs. Abstract Functionally graded thermal barrier coatings (FGTBCs) are functional composites with multilayered structures whose mechanical properties are better than those of conventional ceramic thermal barrier coatings (TBCs). Although many theoretical and experimental studies concerning both the mechanical properties and heat transfer including heat conduction and thermal radiation of TBCs have been conducted, little attention was paid to heat transfer process, especially the coupled conduction-radiation heat transfer, inside FGTBCs. This two-part paper systematically studied the heat transfer inside YSZ/NiCoCrAlY FGTBCs. In Part I, we investigated their radiative properties through experimental measurements. In Part II, we presented a study on the thermal conductivity of FGTBCs and established a coupled heat transfer model. In this paper, more than thirty yttria-stabilized zirconia (YSZ)/NiCoCrAlY duplex TBC and multilayered FGTBC samples with different multilayer structures, porosities and thicknesses were fabricated via air plasma spraying. The transmittance and reflectance spectra within the 0.3 – 15 μ m wavelength range were experimentally obtained by UV–visible spectrometry and Fourier transform infrared spectroscopy. A scanning electron microscope analysis was carried out to characterize the microstructures of FGTBCs. Additionally, we studied each layer of the FGTBCs individually; the four-flux method was employed to obtain the radiative properties based on our experimental measurements. Functionally graded materials (FGM) show larger absorption coefficients and smaller scattering coefficients compared to pure YSZ. The FGTBCs show lower reflectance than YSZ TBCs. We also study the influence of the number of layers on reflectance. The results indicate that FGTBCs have worse thermal insulation ability against radiative flux tconventional ceramic TBCs of the same thickness. This paper provides practical guidance to improve the design of YSZ/NiCoCrAlY FGTBCs and other FGM coatings to reach a balance between good thermal insulation and mechanical strength. [ABSTRACT FROM AUTHOR]

• Topological phonon polaritons (TPhPs) are shown to be sustained by a family of onedimensional quasiperiodic silicon carbide nanoparticle chains. • TPhPs can significantly improve radiative heat transfer, evidenced by longitudinal and transverse heat spectra and eigenmode decomposition. • The complicated interplay among topological order, long-range quasiperiodic order and nearfield electromagnetic interactions can lead to substantial modulations of radiative heat transfer. Topological phonon polaritons (TPhPs) are localized boundary modes arising from the combination of topological photonics and phonon polaritons in micro/nanostructures made of polar dielectrics, which are capable of mediating long-range radiative heat transfer and infrared sensing in a robust manner. This work shows TPhPs can be sustained in a family of one-dimensional quasiperiodic silicon carbide nanoparticle (NP) chains, and can significantly improve radiative heat transfer for these arrays. This family of 1D quasiperiodic lattices is a continuous interpolation between the two paradigmatic limits, viz., the off-diagonal Aubry-André-Harper (AAH) chain and the Fibonacci chain. For different interpolation parameters τ , the band structures are calculated with respect to the modulation phase ϕ as the wavenumber in a synthetic dimension, where a series of gaps and midgap edge states can be observed. These edge states are shown to be topologically protected by applying the gap-labeling theorem and the principle of bulk-boundary correspondence. By means of many-body radiative heat transfer theory for a group of electric dipoles, it is shown quasiperiodicity and topological property can have an important impact on radiative heat transfer. These results can give useful insights on the interplay among topological order, long-range quasiperiodic order, near-field electromagnetic interactions and radiative heat transfer. [ABSTRACT FROM AUTHOR]

In the last a few decades, the approximate nature of radiative transfer equation (RTE) leads to a bunch of considerations on the effect of dependent scattering in random media, especially in particulate media composed of discrete scatterers. This effect usually indicates those deviations of RTE from experimental and exact numerical results due to electromagnetic wave interference. Here we theoretically and numerically demonstrate the effect of dependent scattering on absorption in disordered media consisting of highly scattering scatterers. By making comparisons between the independent scattering approximation-radiative transfer equation (ISA-RTE) approach and the full-wave coupled dipole method (CDM), we find that deviations between the two approaches increase as the scatterer density increases. The discrepancy also grows with the optical thickness of the whole random media. To quantitatively take dependent scattering effect into account, we develop a theoretical model of the dependent-scattering corrected radiative properties, based on the path-integral diagrammatic technique and the quasi-crystalline approximation (QCA) in the multiple scattering theory. The model results in a more reasonable agreement with numerical simulations. The present work is of practical importance in correctly modeling light absorptance in random media and interpreting the experimental observations in various applications for random media, such as solar energy concentration, micro/nanofluids, structural coloration, etc. It also has profound implications for the coherent scattering physics in random media with absorption. [ABSTRACT FROM AUTHOR]

A pore network (PN) model is developed to explore drying of a thin hydrophobic porous medium bounded with a gas purge channel. The PN is composed of cubic pore bodies connected by cylindrical pore throats. At the interface between a pore throat and body, a sudden geometrical expansion exists. When a meniscus advances to this interface, it will be pinned first until the pressure across the meniscus increases to a critical value. This phenomenon is called the capillary valve effect. Because of this effect, two types of invasion into pore bodies are discerned, i.e. bursting and merging invasion. The developed PN model with the capillary valve effect is validated against the experimental results. For the drying case of dominant merging invasion, a drying front of finite width is stably receded. But when bursting invasion dominates, gas invasion is a random process; the drying process can be characterized by three regimes: a surface evaporation period, a constant rate period, and a falling rate period. The total liquid saturation for transition from the constant to falling rate period is close to that at which the total area for vapor transport is maximal between the partially filled pores and their neighboring empty pores. [ABSTRACT FROM AUTHOR]

Radiation absorbers are vital components in energy conversion and detection devices such as solar cells, thermal absorbers, photodetectors and biological sensors. Recently, layered absorbers with ultrathin films have attracted increasing attentions considering the good tradeoff between high absorption ability and low cost of simple structures. Although some theories have been proposed to understand absorption enhancement by introducing ultrathin films, few of them explicitly show effects of thicknesses and materials for perfect absorption. In this manuscript, the perfect absorption in layered absorbers is attributed to admittance matching by adjusting the loss factor of artificial magnetic conductors. Realistically, unified theories based on admittance analyses are developed to achieve perfect absorption in these absorbers. Admittance matching conditions related to thicknesses and materials for multilayer layered absorbers are deduced to achieve perfect absorption. Thus optimal film materials or thicknesses of films can be obtained directly. Moreover, admittance loci diagrams are employed to clearly illustrate influences of both material properties and structural parameters on radiative behaviors. Therefore, unified analyses are not only used as explanations for absorption mechanism, but also used as practical tools for the optimization of layered absorbers with ultrathin films. [ABSTRACT FROM AUTHOR]

Cascaded latent thermal storage is an efficient technique for balancing the gap between demand and supply in renewable energy utilization. In this work, the steady cascaded thermal storage system with multiple phase-change materials (PCMs) of different phase-change temperatures is theoretically optimized from the viewpoint of thermodynamics. Analytical solutions for optimal temperatures of heat transfer fluid (HTF) and PCMs are obtained based on entropy and entransy concepts. The corresponding qualifications for optimization solutions are also discussed. With an increase in stage number or that in NTU , both thermal and exergy efficiencies increase. Compared with single-stage thermal storage, the cascaded thermal storage with multiple PCMs can not only improve the thermal performance, but also extend the application scope of thermal energy with multi-grade thermal energies provided. The temperature distribution for HTF and PCMs with entransy optimization is linear, while that with entropy optimization is geometric progression. The application situations of entropy and entransy in thermal energy utilization are identified by comparing optimal result based on entropy with that based on entransy. This optimization can not only be used to establish the steady cascaded thermal storage system, but also guide the filtration of PCMs used in such systems. [ABSTRACT FROM AUTHOR]

Cascaded latent thermal storage can find its applications in renewable thermal energies with time-dependent temperatures. This paper presents thermal performance of cascaded heat storage with unsteady inlet temperature of heat transfer fluid (HTF). The optimization of temperatures of HTF and phase-change materials (PCMs) is performed based on exergy, entropy, and entransy. Corresponding analytical/numerical solutions with these concepts are obtained. The qualifications for existence of optimization solutions are proposed. The optimization result of unsteady case is compared with that of steady case. The fluctuation of HTF temperature is transferred along the HTF flow path. The HTF temperatures in different stages exhibit similar fluctuating trend, but the fluctuating amplitude is diminished along the HTF flow path. With an increase in stage number, the difference between temperatures of outlet HTF and environment is gradually decreased with obviously improved thermal performance. As NTU increases, thermal performance is gradually increased, but with decreased increasing amplitude. The optimization result with entransy is also compared with that of entropy. The optimal exergy efficiency based on entropy is greater than that based on entransy, while the optimal thermal efficiency based on entransy is superior to that based on entropy. The optimization can be applied to select PCMs for unsteady cascaded thermal storage. [ABSTRACT FROM AUTHOR]

• Realizing tailorable bandgap-dependent selective emitters based on metasurfaces. • Selective emitter with excellent spectrally selectivity, polarization-insensitive, and angle-insensitive features. • Tunable cutoff wavelength by adjusting the geometries of nanodisks of the metasurface. • The efficiency of TPV system can be expected to be 27% at 1473 K. Selective emitters are crucial in thermophotovoltaic (TPV) systems as they can selectively tailor the incident light to match with the bandgap of photovoltaic (PV) cell, thus greatly increasing the system efficiency. However, further progress has been hampered by the limitation in designing high performance selective emitters with spectrally emissive selectivity and compatible ability to different PV cells. In this study, we propose a design strategy to realize tailorable bandgap-dependent selective emitters based on metasurfaces by combing the two-resonance response of surface plasmon polaritons (SPPs) and magnetic polaritons (MPs). The emission spectrum can be manipulated and matched to various PV cells with different bandgap by simply changing the size and period of the meta-atoms in metasurface. We design and experimentally demonstrate a selective emitter appealing to GaSb PV cells, which presents near-perfect emissivity (above 0.96) above the bandgap wavelength (from 0.9 μ m to 1.54 μ m) with polarization-insensitive and angle-insensitive features. Moreover, the emitter shows a high figure of merit (FOM, of 0.85), and the efficiency of TPV system can be expected to be 27% at 1473K. The proposed strategy and designed emitter may pave a reliable route for improving the performance of TPV systems. [ABSTRACT FROM AUTHOR]

A numerical study on the solid–liquid phase change in open-cell metal foams is carried out by the phase field method originated from the Ginzburg–Landau theory. The Brinkman–Forchheimer extended Darcy equation and the local thermal non-equilibrium model are adopted to take natural convection and the heat transfer between metal foams and phase change materials (PCMs) into consideration, respectively. The result proves that the phase field model is reliable and effective in modeling metal foam enhanced phase change heat transfer in thermal energy storage. The effects of key parameters, such as Rayleigh number, porosity and pore density, on the melting and solidification process are investigated and it is found that they have great influence on the solid–liquid phase change. The phase field, flow field and temperature distributions in the melting and solidification process are obtained. Furthermore, kinetic undercooling in the solidification process is studied. It is concluded that heat conduction through the ligament of metal foams plays a dominant role in the solid–liquid phase change and kinetic undercooling effect is weakened as kinetic coefficient decreases. [ABSTRACT FROM AUTHOR]

In the present paper the infrared radiative properties of electron beam physical deposition (EB-PVD) partially yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) have been studied in combination with experimental measurement and theoretical model. The method of Finite-Difference-Time-Domain (FDTD) is employed to investigate the infrared spectral reflectance and transmittance of TBCs with various microstructural parameters and validated by a following experiment. The infrared radiation transfer mechanisms inside the coatings are explored and examined in detail. The results indicate that distinctive micro-nanostructures of EB-PVD coatings have important effects on its radiative performance, and the infrared spectral reflectance and transmittance of coatings are highly affected by the primary columnar morphology and pore architectures, such as intercolumnar gaps, feathery striation and closed globular pores. Further research suggests that scattering effects caused by different microstructures can greatly affect radiative transfer across the coatings. By optimizing the microstructure of coatings properly to enhance its backscattering effects, it can effectively increase the spectral reflectance and reduce the spectral transmittance, thereby reducing the radiation heat flows and providing a better thermal protection for the metallic substrate. [ABSTRACT FROM AUTHOR]

A theoretical model on radiative properties of air plasma-sprayed (APS) 8 wt% yttria stabilized zirconia (8YSZ) thermal barrier coatings (TBCs) is proposed and validated. Starting from analysis of microstructures, pores inside the coating are regarded as scatterers. Dependent scattering effects among scatterers are thoroughly considered in the framework of quasicrystalline approximation (QCA) and Percus–Yevick (P–Y) pair distribution function for sticky hard spherical particles, in which large nonspherical pores are treated as clusters of the elemental pores. Afterwards, the Monte Carlo (MC) method is used to solve the radiative transfer problem inside TBCs based on the radiative properties. The predicted optical responses including reflectance, transmittance and absorptance of TBC slabs with different thicknesses agree well with the experimental data. The widely used independent scattering assumption in conventional studies is also investigated and examined to be inapplicable in the semitransparent spectral region of TBCs, especially between around 3.2 and 5.6 μm. This study provides a simple and physically robust direct prediction on radiative properties of TBCs solely based on their microstructures and bulk optical properties of the material. Moreover, the present method can be easily applied to the studies on radiative properties of other semitransparent porous media. [ABSTRACT FROM AUTHOR]

Pool boiling heat transfer of metal foams with gradient pore densities has been investigated at atmospheric pressure in saturated deionized water. The gradient foams are made of two foam layers. The foam materials are copper and nickel. Pore densities of foam layers are from 5 PPI to 100 PPI, while the porosity remains the fixed value of 0.98. A parametric study is performed by varying the foam pore size and thickness. The effects of alumina nanoparticles and surfactant (SDS) on pool boiling heat transfer of gradient metal foams are also investigated in the present study. Images of foam fiber deposited by nanoparticles are captured by SEM. Compared to single-layer foams, gradient metal foams significantly enhance pool boiling heat transfer and the enhancement is heavily dependent on foam thickness and pore density gradient. The effects of SDS and nanoparticles on pool boiling heat transfer of gradient foams are dependent on concentrations. [ABSTRACT FROM AUTHOR]

• Near-complete violation of Kirchhoff's law has been firstly realized in the near-infrared regions using extremely high-Q resonances. • The asymmetric dispersion relations of guide modes in planar magneto-optical systems are analytically derived, which is responsible for perfect nonreciprocal thermal radiation spectra. • The explicit rules to control asymmetrically angular positions of embedded eigenstates have been established, ensuring flexibly tailor perfect nonreciprocal properties over a wide angular range. • The roles of different geometric parameters in engineering nonreciprocal thermal radiation have been revealed. Engineering nonreciprocal thermal radiation is of great importance in both fundamental thermal science and practical energy applications, but the design of perfect nonreciprocal thermal emitters in the near-infrared region has not been reported due to considerably weak magneto-optical (MO) responses of natural materials. In this work, by taking advantage of high- Q asymmetric embedded eigenstates (EEs) and quasi-EEs, near-complete violation of Kirchhoff's law has been achieved within the near-infrared waveband using epsilon-near-zero/MO dielectric/metal sandwich structures. The analytical dispersion relations of guided modes are firstly derived, perfectly predicting the asymmetric resonant peaks in absorption or emission spectra. We also establish explicit rules to build the connections between angular locations of EEs and the thickness of MO dielectric layer, which allows to flexibly tailor perfect nonreciprocal properties over a wide angular range. Besides, the influence of upper layer's thickness, material loss and different substrates in asymmetric distributions of EEs has also been discussed. The unveiled mechanisms and tunability in engineering nonreciprocal thermal radiation might pave a new way for the next-generation energy devices to break the current limit of energy transfer and conversion efficiency. [ABSTRACT FROM AUTHOR]

Abstract: In this paper the radiative properties of air plasma sprayed (APS) thermal barrier coatings are investigated by measuring the spectral normal-hemispherical reflectance and transmittance of samples with various thickness and porosities in a broad spectral band ranging from 250nm to 15μm. The spectral absorption and scattering coefficients are obtained by a four flux model based on the experimental data. The results show that the porous zirconia ceramics exhibits high scattering coefficient (>15,000m−1) and low absorption coefficient (<500m−1) in the spectral range from 0.5 to 5.5μm. Moreover, the scattering coefficient increases with increasing porosity, while the spectral absorption coefficient is not quite sensitive to the change of porosity. In order to examine the effects of microstructure of porous zirconia coatings, a theoretical model of Mie scattering is established, and a fairly good agreement with the experimental data is achieved. The results indicate that the scattering coefficient heavily depends on the microstructures of coatings, i.e., pore size, porosity and pore distribution, and this will in turn considerably affect the thermal radiation heat transfer inside the coating. [Copyright &y& Elsevier]

Abstract: Metal Foam-enhanced Cascaded Thermal Energy Storage (MF-CTES) has been proposed to solve the problem of poor heat transfer during heat exchange process, which is caused by unavoidable decrease of temperature differences. This paper conducts a theoretical study examining the overall thermal performance of Single-stage Thermal Energy Storage (STES), Cascaded Thermal Energy Storage (CTES) and MF-CTES, with both heat exchange rate and exergy efficiency being considered. The main findings are: heat exchange rate of STES is improved by CTES (up to 30%), and is further improved by MF-CTES (by 2–7 times); exergy efficiency of STES cannot be significantly improved by CTES (−15% to +30%), nor by MF-CTES; exergy transfer rate of STES is increased by CTES (up to 23%), and is further increased by MF-CTES (by 2–7 times). [Copyright &y& Elsevier]

Abstract: In the traditional inward solidification model of a sphere, the interface temperature of liquid–solid is considered as a constant freezing point, but majority of materials have a freezing temperature range and the freezing temperature descends with the change of liquid–solid two phase composition. By taking binary alloy for instance, a new liquid–solid interface model is proposed based on the solid fraction, because the solid fraction plays the bridge between the freezing temperature and the binary phase composition. The new model determines a corresponding relationship between the position of liquid–solid moving interface and the freezing temperature, and it is applied in the solidification analysis of a single encapsulated phase change material (PCM) particle. For different type of PCMs, two rules have been built. A linear rule is designed for the material with the narrow freezing temperature range; to materials with the wide freezing temperature range like Copper (Cu–Al), the solid fraction presents a complicated function relationship with the freezing temperature, so in the numerical calculation, the local linear rule is employed based on the transient interpolation of the solid fraction in the discrete points which represent the freezing temperature against solid fraction in DSC measurements or equilibrium phase diagrams. [Copyright &y& Elsevier]

Abstract: Thermal transport in metal foams has received growing attention in both academic research and industrial applications. In this paper the recent research progress of thermal transport in metal foams has been reviewed. This paper aims to provide the comprehensive state-of-the-art knowledge and research results of thermal transport in open celled cellular metal foams, which covers the effective thermal conductivity, forced convection, natural convection, thermal radiation, pool boiling and flow boiling heat transfer, solid/liquid phase change heat transfer and catalytic reactor. The forced convection and thermal conductivity have been extensively investigated, while less research were performed on two-phase (boiling and solid/liquid phase change heat transfer) and thermal radiation in metal foams. Also most research still treats the metal foam as one type of effective continuous porous media, very few researchers investigated the detailed thermal behaviours at the pore level either by numerical or experimental approaches. [Copyright &y& Elsevier]

Abstract: An analytical model is developed to characterize the radiative transport process in highly porous, open-celled metal foams having idealized cellular morphologies in terms of fundamental radiative parameters such as emissivity, reflectivity and configuration factors. In comparison with the conventional two-flux approach or the diffusion approximation utilizing the Rosseland mean coefficient, the present model is explicit and yet relatively simple. Overall, the predicted effective radiative conductivity as a function of pore size and relative density (defined as the ratio of foam density to solid density) agrees well with that measured using a guarded-hot-plate apparatus for steel alloy foams. A systematic parametric study is subsequently carried out. The contribution of reflectance to thermal radiation is found to be significant, up to 50%, but the effect of temperature gradient is relatively small. The equivalent radiative conductivity increases linearly with increasing cell size for a fixed relative density, whilst for a given cell size the variation of relative density only has a small effect on radiation due to the mixed effects of increased emission and extinction. [Copyright &y& Elsevier]

Abstract: The forced convection heat transfer characteristics in high porosity open-cell metal-foam filled tube heat exchangers are analysed in this paper. The Brinkman-extended Darcy momentum model and two-equation heat transfer model for porous media are employed for the analysis of the heat transfer performance. The morphological effects of metal foams on overall heat transfer are examined. The optimal foam-area ratio for a metal-foam filled counter-flow tube-in-tube heat exchanger is predicted. The study shows that the thermal performance of a metal-foam heat exchanger can be superior to that of conventional finned tube heat exchangers. [Copyright &y& Elsevier]

Abstract: This paper presents an analytical study of the forced convection heat transfer characteristics in high porosity open-cell metal-foam filled pipes. The Brinkman-extended Darcy momentum model and two-equation heat transfer model for porous media were employed. Based on the analytical solutions, the velocity and temperature distributions in metal-foam filled pipes were obtained. The effects of the microstructure of metal foams on overall heat transfer were examined. The results show that the pore size and porosity of metal-foams play important roles on overall heat transfer performance. The use of metal-foam can dramatically enhance the heat transfer but at the expense of higher pressure drop. [Copyright &y& Elsevier]

Abstract: This paper presents a combined experimental and numerical study on natural convection in open-celled metal foams. The effective thermal conductivities of steel alloy (FeCrAlY) samples with different relative densities and cell sizes are measured with the guarded-hot-plate method. To examine the natural convection effect, the measurements are conducted under both vacuum and ambient conditions for a range of temperatures. The experimental results show that natural convection is very significant, accounting for up to 50% of the effective foam conductivity obtained at ambient pressure. This has been attributed to the high porosity (ε >0.9) and inter-connected open cells of the metal foams studied. Morphological parameters characterizing open-celled FeCrAlY foams are subsequently identified and their cross-relationships are built. The non-equilibrium two-equation energy transfer model is employed, and selected calculations show that the non-equilibrium effect between the solid foam skeleton and air is significant. The study indicates that the combined parameter, i.e., the porous medium Rayleigh number, is no longer appropriate to correlate natural convection by itself when the Darcy number is sufficiently large as in the case of natural convection in open-celled metal foams. Good agreement between model predictions and experimental measurements is obtained. [Copyright &y& Elsevier]

This paper presents results from experimental measurements on radiative transfer in FeCrAlY (a steel based high temperature alloy) foams having high porosity (95%) and different cell sizes, manufactured at low cost from the sintering route. The spectral transmittance and reflectance are measured at different infrared wavelengths ranging from 2.5 to 50 μm, which are subsequently used to determine the extinction coefficient and foam emissivity. The results show that the spectral quantities are strongly dependent on the wavelength, particularly in the short wavelength regime (<25 μm). Whilst the extinction coefficient decreases with increasing cell size, the effect of cell size on foam reflectance is not significant. When the temperature is increased, the total extinction coefficient increases but the total reflectance decreases. An analytical model based on geometric optics laws, diffraction theory and metal foam morphology is developed to predict the radiative transfer, with cell size (or cell ligament diameter) and porosity identified as the two key parameters that dictate the foam radiative properties. Close agreement between the predicted effective foam conductivity due to radiation alone and that measured is observed. At fixed porosity, the radiative conductivity of the metal foam increases with increasing cell size and temperature. [Copyright &y& Elsevier]

• Near-field radiative heat transfer is theoretically investigated in asymmetric three-grating systems. • The forward and reverse scenarios in asymmetric three-grating systems provide a high flexibility of manipulation the heat transfer rate. • The difference in the coupling of evanescent modes on two sides of the asymmetric modulator is responsible for significantly changing the performance. • The temperature of modulator can be viewed as a signal of the coupling of modes in different channels. The investigations of near field radiative heat transfer have been extended to many-body systems for further improving the heat transfer rate. Here, we develop a general formulation for characterizing near field radiative heat transfer in three-grating systems, which can be applied in asymmetric scenarios. By introducing asymmetric modulators between thermal emitters and absorbers, we show a remarkable amplification of heat flux in three-grating systems than two-grating counterparts. Moreover, the forward and reverse scenarios in asymmetric three-grating systems provide a high flexibility of manipulation the heat transfer rate, ascribed to the difference in the coupling of evanescent modes on two sides of asymmetric modulator. The proposed formulation and the designed near field radiative heat transfer systems may pave a new avenue for further enhance and modulate heat transfer rate in many penitential applications. [ABSTRACT FROM AUTHOR]

• We systematically investigate the thermal radiation properties of nanofluids with nanoparticle aggregation from experiment and theory. • On the experimental side, TiO2@Ag nanoparticles are synthesized and the plasmonic nanofluids are prepared. The TEM images show that the size of clusters in nanofluid prepared in distilled water only is much larger than that in nanofluid with adding SDS surfactant. The measured thermal radiation properties demonstrate that the cluster size has a significant effect on the absorption of nanofluids. • On the theoretical side, a theoretical model for calculating the thermal radiation properties of nanofluids with nanoparticle aggregation is developed. The spectral transmissivity of nanofluids with nanoparticle aggregation is calculated and the effect of nanoparticle aggregation, fractal dimensions and cluster sizes are investigated. • By comparing the experimental results with modeling results, it demonstrates that our theoretical model can predict the thermal radiation properties of nanofluids with nanoparticle aggregation more reasonably. This is of great significance to understand the mechanism that affects the absorption of nanofluids with nanoparticle aggregation. Nanofluids show significant enhancement in the absorption of solar energy. However, little work has been done on the effect of nanoparticle aggregation on the thermal radiation properties of nanofluids up to now. In this work, we systematically investigate the thermal radiation properties of nanofluids with nanoparticle aggregation from experiment and theory. On the experimental side, titanium dioxide/silver (TiO 2 @Ag) plasmonic nanofluids are prepared in distilled water and the spectral transmittance of nanofluids with different degrees of aggregation is measured. The result shows that the size of aggregated nanoparticle cluster has a significant effect on the absorption of nanofluids. On the theoretical side, a theoretical model is developed to calculate the thermal radiation properties of nanofluids with nanoparticle aggregation. By applying the multiple sphere T-matrix (MSTM) method and the Monte Carlo (MC) method, we investigate the effects of nanoparticle aggregation, cluster fractal dimensions and cluster sizes on the thermal radiation properties of nanofluids. The results indicate that our theoretical model can predict the thermal radiation properties of nanofluids with nanoparticle aggregation more reasonably. This is of great significance to understand the mechanism that affects the thermal radiation properties of nanofluids and to actively design nanofluids. [ABSTRACT FROM AUTHOR]

• Significant melting enthalpy reduction was found in NPCMs. • Interaction between nanoparticles and PCM contributes to the reduction. • A parameter was defined to evaluate the effect of the interaction. • Dense phase is the internal reason for NPCM melting enthalpy reduction. In order to enhance the thermal conductivity of phase change material (PCM), many studies on preparation of composite PCM with nanoparticles (NPCM) have been performed. However, most of the studies only focused on PCM thermal conductivity enhancement, and ignored the melting enthalpy reduction caused by nanoparticles. In present paper, molecular dynamics (MD) method was adopted to study the effect of nanoparticles (CuO) on PCM (paraffin) melting enthalpy and to reveal the mechanism of PCM melting enthalpy reduction. The results showed that the melting enthalpy quickly decreased with the increase of nanoparticle mass fractions. For the NPCM with 19.72 wt.% nanoparticles, the melting enthalpy decreased by about 51.5%. The interactions between nanoparticles and PCM also contribute to melting enthalpy reduction except the nanoparticle mass fraction. Then, a parameter (F c) was defined to reveal the effect of the interactions on NPCM melting enthalpy. With the nanoparticle mass fraction increasing, the interaction was strengthened and the value of F c was quickly decreased, especially when the nanoparticle mass fraction was lower than 10 wt.%. The dense phase of paraffin was found around nanoparticles. The existence of dense phase caused by the interaction between nanoparticles and PCM is the internal reason for NPCM melting enthalpy reduction. [ABSTRACT FROM AUTHOR]

• Synthesis processes heavily affect the thermal conductivity of heterostructures. • Lattice mismatch enhances the thermal conductivity due to the strain effects. • Thermal conductivity of bilayers is not sensitive to the interlayer rotation. Van der Waals heterostructures formed by stacking different two-dimensional transition metal dichalcogenide sheets exhibit unusual properties compared with single-layer counterparts and are finding promising applications in optics and electronics. Different synthesis processes may lead to various defects, such as lattice mismatch, interlayer rotation and species intermixing. We adopt non-equilibrium molecular dynamics simulations to explore the influence of these factors on the thermal conductivity of bilayer heterostructures that are formed by single-layer MoS 2 and MoSe 2 sheets. The thermal conductivity of the bilayer made up of unstrained MoS 2 and MoSe 2 layers is 121% larger than the AB stacked one. Detailed phonon scattering analysis reveals that the tensile strain-induced enhancement of anharmonicity is the main reason. The study of the twisted bilayer structure shows that the interlayer rotation considerably alters the phonon band structure but has little impact on the thermal conductivity. We also find that thermal conductivity varies inversely with the species intermixing ratio due to the increase of localized phonon modes. Our findings in this work could provide some guidelines for the synthesis process of van der Waals heterostructures with exceptional thermal properties. [ABSTRACT FROM AUTHOR]

• Monolayer graphene can completely compensate for the mismatch betweentwo dissimilar materials. • The considerable Rabi frequencies of different strongly coupled polaritonic modes can be confirmed. • The maximum relative amplification factor and absolute amplification factor are 134 and 140, respectively. • Adjustable NFRHT between dissimilar materials can be achieved by manipulation of the strong coupling effects. Near-field radiative heat transfer (NFRHT) between dissimilar materials is important for near-field applications, such as near-field photonic thermal diode and nanoscale thermal imaging. It is demonstrated that monolayer graphene lead to a significant enhancement in the NFRHT between dissimilar materials. The presence of graphene is not only able to permit a large variation and amplification of NFRHT through its chemical potential, but also to fully compensate the mismatch between the resonance frequencies of the two dissimilar materials. And then we investigate the role of the strong coupling effects among different surface polaritonic modes on NFRHT. Especially, we identify strongly coupled polaritonic modes with considerable Rabi frequencies. Therefore, we analyze the graphene-assisted configurations by calculating both the near-field dispersion relations and Rabi frequencies. To quantify the role played by the monolayer graphene, we have defined two amplification coefficients of NFRHT. Finally, the contributions of thickness of the two different polar dielectrics to the strong coupling effects have been investigated. As a result, we demonstrate that the strong coupling effect we highlight is robust with respect to the mismatch between resonance frequencies, laying the foundation for an active control and enhancement of NFRHT between dissimilar materials. [ABSTRACT FROM AUTHOR]

Abstract: Thermal barrier coatings (TBCs) are not the pure bulk materials, and they contain many different micro/nano structures caused by the plasma spraying process. The microstructures in the coatings play a very important role on mechanical and thermal properties, therefore, the coating properties differ hugely from those of the corresponding bulk materials. The influence of microstructures on thermal conductivity has been investigated by many researchers, however their effect on radiative heat transfer is seldom studied. The radiative heat transfer will become increasingly important as the temperature rises, and this warrants the study on the influence of microstructures on radiative properties. In the present paper, Finite-Difference-Time-Domain (FDTD) method was employed to simulate the radiative heat transfer behaviors of TBCs with different types of microstructures. The relationship between microstructures and radiative properties of the coating is investigated. Furthermore, the parameters of microstructures such as defect size, shape coefficient, porosity and orientation angle are examined in detail. The results will help us to further understand the radiative heat transfer process across the TBCs and will provide us a theoretical guide to design TBCs with a high thermal insulation property. [Copyright &y& Elsevier]

Abstract: A phase field model is for the first time employed to solve the phase change problem in a metal foam-embedded Latent Thermal Energy Storage (LTES) system. A phase field model deals with free boundary problems without tracing their positions, and therefore provides potentials of being extended to consider more complicated mechanisms: multi-dimension and volume change. The coupled heat transfer between PCMs and metal foams is solved based on the non-equilibrium heat transfer theory. Two phase fields are introduced to deal with phase change and volume change. Comparisons are made between the model predictions and the experimental data available in existing literatures. Two dimensionless groups of the material and structure parameters are identified to control the effectiveness of the system. An Effectiveness Map is also produced to distinguish the conditions under which incorporating metal foam into the PCMs is sensitive, lowly sensitive or irrelevant. The map provides a useful tool to guide the metal selection and structure design of the metal foams when enhancing the heat transfer of PCMs. Finally case studies are also carried out, in which the minimum thermal conductivity of copper/aluminium foams and the maximum k int can be determined by referring to the obtained Effectiveness Map. [Copyright &y& Elsevier]

Abstract: The current study investigated the saturated pool boiling heat transfer of deionized water with added surfactant on a horizontal metallic foam surface with V-shape grooves under atmospheric pressure. The influences of the groove configurations, sodium dodecyl sulfate (SDS) concentration, foam thickness, and thermal conductivity of foam material on heat transfer performance and bubble growth patterns were studied. SDS with concentrations of 100, 400, and 800ppm was used as the surfactant. The foam porosity was fixed at 0.95. V-shape grooves with various widths and groove number were manufactured in the foam samples, with three pore densities of 20, 100, and 130PPI. Results showed that the effects of the groove configuration, SDS concentration, and thickness on boiling heat transfer are heavily dependent on foam pore density. The counter-flow between the released bubbles and sucked liquid plays a significant role in heat transfer performance. The existence of sufficient grooves delays the critical heat flux (CHF), whereas SDS can achieve CHF earlier. [Copyright &y& Elsevier]

Abstract: The two equation numerical model has been applied for parallel flow double-pipe heat exchanger filled with open cell metal foams. The model fully considered solid–fluid conjugated heat transfer process coupling heat conduction and convection in open cell metal foam solid matrix, interface wall and fluid in both inner and annular space in heat exchanger. The non-Darcy effect and the wall thickness are also taken into account. The interface wall heat flux distribution along the axial direction is predicted. The numerical model is firstly verified and then the influences of solid heat conductivity, metal foam porosity, pore density, relative heat conductivity and inner tube radius of the heat exchanger on dimensionless temperature distribution and heat transfer performance of heat exchanger are numerically studied. It is revealed that the proposed numerical model can effectively display the real physical heat transfer process in the double pipe heat exchanger. It is expected to provide useful information for the design of metal foam filled heat exchanger. [ABSTRACT FROM AUTHOR]

• A selective emitter for thermophotovoltaic systems is theoretically and experimentally demonstrated. • The wavelength-selective emission is realized by multipole resonances excited in dielectric nanodisks. • The coupling between multipole resonances and the tungsten layer induces magnetic dipolar resonances. • The anapole-induced emission peak can couple with the lattice resonance and achieve higher emissivity. • The proposed selective emitter is successfully fabricated and experimentally verified. [Display omitted] Thermophotovoltaic systems can harvest electric energy from heat sources with a potential efficiency exceeding the Shockley–Queisser limit due to the selective emission of an elaborate thermal emitter. In this work, a two-dimensional nanodisks/thin-film metamaterial is proposed as a wavelength-selective emitter, which can coordinate well with the photovoltaic cell in a thermophotovoltaic system. Compared to conventional emitters based on surface plasmon polaritons, the emittance peaks of the proposed emitter are realized by the excitations of both electric dipole and anapole modes in silicon nanodisks, which can be easily tailored due to the non-dispersive optical constants of dielectric materials. Meanwhile, the effect of polarization and polar angle on the emittance spectra is also investigated, suggesting that the proposed emitter has high emittance and efficiency not only in the normal direction but also at large oblique angles. Electromagnetic field and current density distributions reveal that the coupling between multipole resonances and the bottom tungsten layer can induce a magnetic dipolar resonance. Therefore, the wavelengths of both emittance peaks are sensitive to the period paralleled to the incident electric field. Besides, the anapole-induced mode can couple with the lattice resonance, resulting in higher emittance. Moreover, the proposed emitter is successfully fabricated, and the measured spectra agree well with the theoretical results. The fundamental understanding and insights obtained here will facilitate the active design and application of novel multipole-based emitters in enhancing energy conversion. [ABSTRACT FROM AUTHOR]

• A scattering-type multi-probe scanning thermal microscope (s-m-SThM) which is configured with multi-probe to improve the signal is proposed. • A theory to determine the far-field signal of s-m-SThM is presented. This theoretical model is based on diploe approximation and consider the multibody mutual interactions between the probes and the sample. • The far-field signal of two-probe (Au) configuration and three-probe (Au) configuration is investugated, and the effects of the multibody mutual interactions on the signal of s-m-SThM is analyzed. • For a three-probe (Au) configuration, the results show that the far field signal has been increased more than 200 times compared with a single probe (Au) configuration. As one of the most potential applications of near-field thermal radiation, near-field scanning thermal microscope has its own advantage compared with commercial near-field scanning optical microscope, which takes the thermally excited near-field radiation as light source and does not need external illumination. However, the signal of near-field scanning thermal microscope is very weak. Ones have to heat the tip or sample to improve the signal, or apply a sensitive detector. In this work, we describe a scattering-type multi-probe scanning thermal microscope (s-m-SThM) based on near-field thermal radiation which is configured with multi-probe to improve the signal, and present a theory to determine its far-field signal. The theoretical model is based on dipole approximation and consider the multibody mutual interactions between the probes and the sample. The results demonstrate that multibody mutual interactions between the probes and the sample can effectively improve the far-filed signal of s-m-SThM. For a three-probe (Au) configuration, the results show that the far field signal has been increased more than 200 times compared with a single probe (Au) configuration. This work will help to improve the applicability of scanning thermal microscope based on near-field thermal radiation. [ABSTRACT FROM AUTHOR]

• Effects of surface thermal diffusivity and effusivity on the Leidenfrost temperature are clarified. • A theoretical model is developed for the Leidenfrost temperature. • The dependency of the Leidenfrost temperature on the thermal effusivity and the impact velocity is disclosed. Droplet impact on hot surfaces results in either droplet-surface contact or droplet-surface non-contact, i.e., the Leidenfrost state. The Leidenfrost droplet is levitated upon its vapor, deteriorating the heat transfer. The Leidenfrost temperature depends on the thermal properties of the surface, which are usually characterized by two parameters: the thermal diffusivity and the thermal effusivity. In this paper, the effects of these two parameters on the Leidenfrost temperature are clarified experimentally by varying the one of interest while keeping the other one constant. The experimental results indicate that the Leidenfrost temperature is affected by the thermal effusivity rather than by the thermal diffusivity; the higher the thermal effusivity is, the lower the Leidenfrost temperature; and the increase of the Leidenfrost temperature with the droplet impact velocity is greater for the surface with the lower thermal effusivity. To further understand the experimental findings, a theoretical model is proposed, which considers the transient heat transfer in the surface. The theoretical analysis shows that the Leidenfrost temperature scales with the inverse of the thermal effusivity and the root square of the droplet impact velocity, in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

• A new fractal theoretical model is established to predict the transport in complex porous media. • Solutions to the transport parameters of single-phase fluid in porous media are derived. • Solutions to the transport parameters of two-phase fluid in porous media are derived. • Analytical results of the proposed model are compared with the previous model. • The predicted results of the theoretical model agree well with experimental data. A new fractal theoretical model with periodic pore morphology, which idealizes the pore channels of the porous media as gourd-shaped structure, is established to model the transport in complex porous media. Based on the fractal model, the flow/thermal/mass transport properties of single-phase fluid and two-phase fluid flowing through porous structures are predicted, and analytical solutions to the permeability, thermal conductivity, diffusion coefficient of single-phase fluid in porous media, and the permeability, and the thermal conductivity of two-phase fluid in unsaturated porous media are derived. The analytical results of the proposed model for predicting the single-phase gaseous flow, heat and mass transfer in porous media are compared with the previous model data based on the idealized tubule-shaped structure of the real pore channels in porous media, which shows that the previous tubule-shaped structure is a limiting case of the present gourd-shaped structure. Analytical expressions of absolute permeability, relative permeability, and effective thermal conductivity of two-phase flow and heat transport in unsaturated porous media are derived for the present model and the previous tubule model. A parametric analysis is performed for presenting the effects of the structural parameters and fluid properties on the effective thermal conductivity, permeability and effective diffusion coefficient predicted. Meanwhile, the results show that the predicted data of the theoretical model are in good agreement with experimental data. The present study is significant for enhancing the flow, heat and mass transfer in accurately predicting transport properties of real porous media applications, and can provide an important parameter basis and guidance for the design of the transport system incorporating porous media. [ABSTRACT FROM AUTHOR]

• A thermochemical heat storage reactor with topology optimized fins is developed. • Superior heat transfer capability of topology optimized fins is confirmed. • The enhanced heat transfer mechanisms of topology optimized reactor are studied. • The heat transfer rate can be further accelerated through a proper heat transfer fluid pipe distribution. Thermochemical heat storage has a promising application prospect due to its high energy density and ability of seasonal energy storage with little heat loss. However, most of thermochemical heat storage systems suffer from very poor thermal conductivity that limit the heat transfer rate and hinder the implementation of this technology. This study presents a novel solution to the poor heat transfer problem in a multi-tube thermochemical heat storage unit through topology optimization of highly conductive fins. For the formulation of the topology optimization problem, the dehydration process of Ca(OH) 2 was considered, and a density-based approach was employed. Results indicated that the topology optimized geometry had a superior heat transfer performance and a 43% improvement was achieved with respect to the normally used longitudinal design in terms of reaction time. It was found that during the dehydration process of Ca(OH) 2 , heat conduction was the dominant factor while convection heat transfer was only observed in a small region near the porous gas channel. Furthermore, the effects of the number of heat transfer fluid (HTF) pipes were explored. It was shown that bifurcations of intermediate branches tend to vanish with the increase of the number of HTF pipes and the charging process can be further enhanced through a more uniform HTF pipe distribution. Finally, the reconstructed model was validated, and the optimized fins were tested under the hydration process of CaO, which further confirmed the superiority of the topology design. [ABSTRACT FROM AUTHOR]

Highlights • Thermochemical heat storage materials have great fractal characteristic. • Fractal porous media resembling actual calcium oxide materials are reconstructed. • Materials with larger fractal dimension have more diffusion resistance. • Maxwell model overestimates transport process of gas within the complicated pores. Abstract Mass transfer is the key characteristic that affects the charging and discharging performance of the porous materials used in thermochemical heat storage systems. Therefore, an algorithm is required to simulate the transport process within heat storage materials and to accurately evaluate the effective diffusion coefficient. This study is about porous calcium oxide, which is widely used in CaO-Ca(OH) 2 or CaO-CaCO 3 heat storage systems. In this paper, the fractal porous media that resemble real heat storage materials are reconstructed by a random walk method. Furthermore, the gas diffusion process through complicated porous microstructures within fractal porous media is simulated by an efficient lattice Boltzmann method. Results indicate that thermochemical heat storage materials have great fractal characteristic and materials with larger fractal dimension have more diffusion resistance. The effective gas diffusion coefficients of CaO and Ca(OH) 2 are obtained from the ratios of effective gas diffusion coefficient and bulk gas diffusion coefficient that are 0.228 and 0.188 for CaO and Ca(OH) 2 , respectively. Additionally, it is found that calcium oxide samples prepared at higher temperatures have the poorer exothermic performance because of the poorer gas diffusion. Moreover, the correlation between the effective diffusion coefficients and the porosity of porous materials is numerically predicted and evaluated. The fractal model-LBM method developed in this paper may serve as a great tool for easy estimation of effective diffusion coefficients and mass transfer. [ABSTRACT FROM AUTHOR]

• The thermoelectric properties of isotope graphene nanoribbons via machine learning guided manipulation of disordered antidots and interfaces is enhanced up to 5.69 times that of the pristine one. • The heteroatoms (isotope) tend to form a continuous interface barrier perpendicular to the carrier transport direction to suppress the propagation of phonons through pure isotope-substituted armchair graphene nanoribbons. • The antidot structure significantly improves armchair graphene nanoribbon thermoelectric properties by reducing thermal conductivity and increasing thermoelectric power factor compared to the pure isotope structure. Structural manipulation at the nanoscale breaks the intrinsic correlations among different energy carrier transport properties, achieving high thermoelectric performance. However, the coupled multifunctional (phonon and electron) transport in the design of nanomaterials makes the optimization of thermoelectric properties challenging. Machine learning brings convenience to the design of nanostructures with large degree of freedom. Herein, we conducted comprehensive thermoelectric optimization of isotopic armchair graphene nanoribbons (AGNRs) with antidots and interfaces by combining Green's function approach with machine learning algorithms. The optimal AGNR with ZT of 0.894 by manipulating antidots was obtained at the interfaces of the aperiodic isotope superlattices, which is 5.69 times larger than that of the pristine structure. The proposed optimal structure via machine learning provides physical insights that the carbon-13 atoms tend to form a continuous interface barrier perpendicular to the carrier transport direction to suppress the propagation of phonons through isotope AGNRs. The antidot effect is more effective than isotope substitution in improving the thermoelectric properties of AGNRs. The proposed approach coupling energy carrier transport property analysis with machine learning algorithms offers highly efficient guidance on enhancing the thermoelectric properties of low-dimensional nanomaterials, as well as to explore and gain non-intuitive physical insights. Graphical Abstract [Display omitted]. [ABSTRACT FROM AUTHOR]

• A theoretical model for pool boiling on nano-porous structure is developed. • MD simulations on boiling on nano-porous structures are performed. • The relationship between the pore sizes and CHF is disclosed. • Effects of void space and area of solid surfaces on boiling inception time is revealed. • CHF occurs when vapor film is formed at the surface of the nano-porous structure. The nano-porous structures have been employed to enhance the boiling heat transfer. To optimize the nano-porous structures so as to improve the boiling heat transfer, it is necessary to disclose the impacts of the nano-porous structures on the characteristics of boiling. Experimentally, it is difficult to observe the phenomena of boiling inside the nano-porous structure. An alternative way is to resort the theoretical and modeling analysis. In the present study, we fist perform a theoretical analysis on boiling on the nano-porous structures from the macroscopic perspective. We find that the boiling inception time is proportional to the ratio of the void volume to the solid surface area; and that the critical heat flux is a power law function of the pore size, with the exponent of 1.5. To demonstrate these theoretical analysis, we then perform the molecular dynamics simulations on boiling of argon on the copper based nano-porous structures. The molecular dynamics simulations demonstrate the theoretical analysis. It is revealed that the boiling inception time is a power law function with respect to the ratio of the void volume to the solid surface area. The exponent for the power law relation between the critical heat flux and the pore size is 1.232. The critical heat flux occurs at the moment when the surface of the nano-porous structures is completely covered by the vapor film. [ABSTRACT FROM AUTHOR]