Enhanced near-field radiative heat transfer between borophene sheets on different substrates

Near-field radiative heat transfer(NFRHT)has the potential to exceed the blackbody limit by several orders of magnitude,offering significant opportunities for energy harvesting.In this study,we have examined the NFRHT between two borophene sheets through the calculation of heat transfer coefficient(HTC).Due to the tunneling of evanescent waves,borophene sheet allows for enhanced heat flux and adjustable NFRHT by varying its electron density and electron relaxation time.Additionally,the near field coupling is further examined when the borophene is deposited on dielectric or lossy substrates.The maximum HTC is closely related to the real part of the dielectric substrate.As a case study,the HTCs on the lossy substrate of MoO_(3),ZnSe,and SiC are calculated for comparisons.Our results indicate that MoO_(3)is the optimal substrate to get the enhanced energy transfer coefficient.It results in a remarkable value of 1737 times higher than the blackbody limit owing to the enhanced photon tunneling probability.Thus,our study reveals the effect of substrate on the HTC between borophene sheets and provides a theoretical guidance for the design of near-field thermal radiation devices.

Near-field radiative heat transfer between nanoporous GaN films

Photon tunneling effects give rise to surface waves,amplifying radiative heat transfer in the near-field regime.Recent research has highlighted that the introduction of nanopores into materials creates additional pathways for heat transfer,leading to a substantial enhancement of near-field radiative heat transfer(NFRHT).Being a direct bandgap semiconductor,GaN has high thermal conductivity and stable resistance at high temperatures,and holds significant potential for applications in optoelectronic devices.Indeed,study of NFRHT between nanoporous GaN films is currently lacking,hence the physical mechanism for adding nanopores to GaN films remains to be discussed in the field of NFRHT.In this work,we delve into the NFRHT of GaN nanoporous films in terms of gap distance,GaN film thickness and the vacuum filling ratio.The results demonstrate a 27.2%increase in heat flux for a 10 nm gap when the nanoporous filling ratio is 0.5.Moreover,the spectral heat flux exhibits redshift with increase in the vacuum filling ratio.To be more precise,the peak of spectral heat flux moves fromω=1.31×10^(14)rad·s^(-1)toω=1.23×10^(14)rad·s^(-1)when the vacuum filling ratio changes from f=0.1 to f=0.5;this can be attributed to the excitation of surface phonon polaritons.The introduction of graphene into these configurations can highly enhance the NFRHT,and the spectral heat flux exhibits a blueshift with increase in the vacuum filling ratio,which can be explained by the excitation of surface plasmon polaritons.These findings offer theoretical insights that can guide the extensive utilization of porous structures in thermal control,management and thermal modulation.

Near-field radiative heat transfer in hyperbolic materials

In the post-Moore era, as the energy consumption of micro-nano electronic devices rapidly increases, near-field radiative heat transfer(NFRHT) with super-Planckian phenomena has gradually shown great potential for applications in efficient and ultrafast thermal modulation and energy conversion. Recently, hyperbolic materials, an important class of anisotropic materials with hyperbolic isofrequency contours, have been intensively investigated. As an exotic optical platform, hyperbolic materials bring tremendous new opportunities for NFRHT from theoretical advances to experimental designs. To date, there have been considerable achievements in NFRHT for hyperbolic materials, which range from the establishment of different unprecedented heat transport phenomena to various potential applications. This review concisely introduces the basic physics of NFRHT for hyperbolic materials, lays out the theoretical methods to address NFRHT for hyperbolic materials, and highlights unique behaviors as realized in different hyperbolic materials and the resulting applications. Finally, key challenges and opportunities of the NFRHT for hyperbolic materials in terms of fundamental physics, experimental validations, and potential applications are outlined and discussed.

Enhancement of Near-Field Radiative Heat Transfer based on High-Entropy Alloys

The enhancement of near-field radiative heat transfer(NFRHT)has now become one of the research hotspots in the fieldsof thermal management and imaging due to its ability to improve the performance of near-field thermoelectric devices and near-field imaging systems.In this paper,we design three structures(multilayer structure,nanoporous structure,and nanorod structure)based on high-entropy alloys to realize the enhancement of NFRHT.By combining stochastic electrodynamicsand Maxwell-Garnett's description of the effective medium,we calculate the radiative heat transfer under different parametersand find that the nanoporousstructure has the largest enhancement effect on NFRHT.The near-field heat transfer factor(q)of this structure(q=1.40×10^(9)W/(m^(2)·K))is three times higher than that of the planestructure(q=4.6×10^(8)W/(m^(2)·K)),and about two orders of magnitude higher than that of the SiO2plate.Thisresult providesa freshidea for the enhancement of NFRHT and will promote the application of high-entropy alloy materials in near-field heat radiation.

Contribution of terahertz waves to near-field radiative heat transfer between graphene-based hyperbolic metamaterials

Hyperbolic metamaterials alternately stacked by graphene and silicon（Si） are proposed and theoretically studied to investigate the contribution of terahertz（THz） waves to near-field radiative transfer. The results show that the heat transfer coefficient can be enhanced several times in a certain THz frequency range compared with that between graphene-covered Si bulks because of the presence of a continuum of hyperbolic modes. Moreover, the radiative heat transfer can also be enhanced remarkably for the proposed structure even in the whole THz range. The hyperbolic dispersion of the graphenebased hyperbolic metamaterial can be tuned by varying the chemical potential or the thickness of Si, with the tunability of optical conductivity and the chemical potential of graphene fixed. We also demonstrate that the radiative heat transfer can be actively controlled in the THz frequency range.

Enhanced near-field radiative heat transfer between core-shell nanoparticles through surface modes hybridization

Core-shell nanoparticles(CSNPs)are widely used in energy harvesting,conversion,and thermal management due to the excellent physical properties of different components.Because of the synergistic interaction between the core and the shell,the thermal radiative properties are expected to be further enhanced.In this work,we achieve near-field radiative heat transfer(NFRHT)enhancement between SiC@Drude CSNPs.Numerical results show that the total heat flux between NPs is 1.47 times and 9.98 times higher than homogeneous SiC and Drude NPs at the same radius when the core volume fraction is 0.76.Surface modes hybridization arising from the interfaces of the shell-core and shell-air contributes to the improved thermal radiation.The effect of shift frequency on the NFRHT between SiC@Drude CSNPs is studied,showing that the enhancement ratio of NFRHT between CSNPs can reach 4.34 at a shift frequency of 1×10^(14) rad/s,which is 38.34 times higher than the previous work.This study demonstrates that surface modes hybridization in CSNPs can significantly improve NFRHT and open a novel path for high-efficiency energy transport at the nanoscale.

High-quality quasi-monochromatic near-field radiative heat transfer designed by adaptive hybrid Bayesian optimization

The increasing demand for versatile and high-quality near-field radiative heat transfer(NFRHT) has created a critical need for a design approach that can handle numerous candidate structures. In this work, we employ and develop an adaptive hybrid Bayesian optimization(AHBO) algorithm to design the high-quality quasi-monochromatic NFRHT. The candidate materials include hexagonal boron nitride, silicon carbide, and doped silicon. The high-quality quasi-monochromatic NFRHT is optimized over 1.0 × 10^(8) candidate structures to maximize the evaluation factor. It is worth noting that only 2.6% of the candidate structures needed to be calculated to identify the optimal structure. The optimal structure of quasi-monochromatic NFRHT is an aperiodic multilayer metamaterial that differs from conventional periodic multilayer structures. Moreover, we investigate the robustness and mechanisms of the optimal quasi-monochromatic NFRHT with respect to the vacuum gap distance and the temperature difference between the emitter and receiver. In addition, the high-quality multi-peak NFRHT is designed using the AHBO algorithm by improving the definition of the evaluation factor. The results demonstrate that the AHBO algorithm is efficient in designing high-quality quasi-monochromatic and multi-peak NFRHT, and it can be further expanded to other structural designs in the field of energy conversion.

Near-field radiative heat transfer enhancement by multilayers and gratings in the thermophotovoltaic system

The near-field effect can be used to improve the output power of the near-field thermophotovoltaic device(NTPV).The nearfield radiative heat transfer in the near-field thermophotovoltaic device can be enhanced by the excitation of hyperbolic modes and the coupling of surface plasmon polaritons.In this study,we design a two-body near-field thermophotovoltaic system based on hyperbolic metamaterial.The multilayer structure on the emitter is composed of Ga-doped ZnO(GZO)and hafnium dioxide(HfO2).The gratings are on the InAs photovoltaic cell.Fluctuational electrodynamics and rigorous coupled-wave method are employed to calculate radiative heat transfer.It is found that the NTPV system with multiple microstructures performs better than the NTPV system just with single micro-structures.This NTPV system performs better in a wider vacuum gap.The output power and efficiency are enhanced by the GZO-HfO2surface plasmon polaritons in multilayer structure.The gratings can monitor the spectral heat flux to match the cell band gap to enhance the performance of the near-field thermophotovoltaic system.This investigation provides a novel approach for improving the output power of a two-body near-field thermophotovoltaic system.

Near-field radiative heat transfer between general materials and metamaterials

We investigated the near-field radiative heat transfer between general materials and metamaterials.We studied the effects of metamaterial parameters on the radiative heat exchange and used three kinds of natural or artificially-constructed materials such as Al,boron-doped Si and metamaterials as examples.We calculated and analyzed the near-field radiative heat transfer processes between two semi-infinite bodies.The numerical results indicate that the radiative heat exchange between the two different materials may be less or more than the radiative heat exchange between the corresponding identical materials.It was found out to depend on the radiative properties of the materials.The work would provide a valuable reference for the selection of practical mate-rials.

Zonal method solution of radiative heat transfer in a one-dimensional long roller-hearth furnace in CSP

A radiative heat transfer mathematical model for a one-dimensional long furnace was set up in a through-type roller-hearth furnace （TTRHF） in compact strip production （CSP）. To accurately predict the heat exchange in the furnace, modeling of the complex gas energy-balance equation in volume zones was considered, and the heat transfer model of heating slabs and wall lines was coupled with the radiative heat transfer model to identify the surface zonal temperature. With numerical simulation, the temperature fields of gas, slabs, and wall lines in the furnace under one typical working condition were carefully accounted and analyzed. The fundamental theory for analyzing the thermal process in TI＇RI-IF was provided.

Effect of Particle Size Distribution on Radiative Heat Transfer in High-Temperature Homogeneous Gas-Particle Mixtures

The weighted-sum-of-gray-gas(WSGG)model and Mie theory are applied to study the influents of particle size on the radiative transfer in high temperature homogeneous gas-particle mixtures,such as the flame in aero-engine combustor.The radiative transfer equation is solved by the finite volume method.The particle size is assumed to obey uniform distribution and logarithmic normal(L-N)distribution,respectively.Results reveal that when particle size obeys uniform distribution,increasing particle size with total particle volume fraction fvunchanged will result in the decreasing of the absolute value of radiative heat transfer properties,and the effect of ignoring particle scattering will also be weakened.Opposite conclusions can be obtained when total particle number concentration N0 is unchanged.Moreover,if particle size obeys L-N distribution,increasing the narrowness indexσor decreasing the characteristic diameter Dˉwith the total particle volume fraction fvunchanged will increase the absolute value of radiative heat transfer properties.With total particle number concentration N0 unchanged,opposite conclusions for radiative heat source and incident radiation terms can be obtained except for radiative heat flux term.As a whole,the effects of particle size on the radiative heat transfer in the high-temperature homogeneous gas-particle mixtures are complicated,and the particle scattering cannot be ignoring just according to the particle size.

Modeling of unsteady MHD free convection flow with radiative heat transfer in a rotating fluid

In this paper, a numerical simulation has been carried out on unsteady hydromagnetic free convection near a moving infinite flat plate in a rotating medium. The temperatures involved are assumed to be very high so that the radiative heat transfer is significant, which renders the problem highly non-linear even with the assumption of a differential approximation for the radiative heat flux. A numerical method based on the Nakamura scheme has been employed to obtain the temperature and velocity distributions which are depicted graphically. The effects of the different parameters entering into the problem have been discussed extensively.

Viscoelastic Effects on Unsteady Two-Dimensional and Mass Transfer of a Viscoelastic Fluid in a Porous Channel with Radiative Heat Transfer

An analysis of oscillatory flow of a viscoelastic fluid and mass transfer along a porous oscillating channel with radiative heat transfer in presence of first-order chemical reaction is considered. The problem is concerned with the flow through a channel in which the viscoelastic fluid is injected on one boundary of the channel with a constant velocity, while it is sucked off at the other boundary with the same velocity. The two boundaries are considered to be in close contact with the two plates placed parallel to each other. The effect of temperature oscillations at the plate (upper wall) where the suction takes place is taken into consideration. The plates are supposed to be oscillating with a given velocity in their own planes. Analytical expressions for velocity profile, the temperature, concentration profile, wall shear stress on the upper wall are obtained. The profiles of the velocity and skin friction have been presented graphically for different values of the viscoelastic parameters with the combination of the other flow parameters encountered in the problem under investigation. It is observed that velocity decrease with the increasing values of the viscoelastic parameter in comparison with Newtonian fluid. Also, the wall shear stress increase with the increasing values of the viscoelastic parameter.

Radiative Heat Transfer and Thermocapillary Effects on the Structure of the Flow during Czochralski Growth of Oxide Crystals

A numerical study was carried out to describe the flow field structure of an oxide melt under 1) the effect of internal radiation through the melt (and the crystal), and 2) the impact of surface tension-driven forces during Czochralski growth process. Throughout the present Finite Volume Method calculations, the melt is a Boussinnesq fluid of Prandtl number 4.69 and the flow is assumed to be in a steady, axisymmetric state. Particular attention is paid to an undulating structure of buoyancy-driven flow that appears in optically thick oxide melts and persists over against forced convection flow caused by the externally imposed rotation of the crystal. In a such wavy pattern of the flow, particularly for a relatively higher Rayleigh number , a small secondary vortex appears nearby the crucible bottom. The structure of the vortex which has been observed experimentally is studied in some details. The present model analysis discloses that, though both of the mechanisms 1) and 2) end up in smearing out the undulating structure of the flow, the effect of thermocapillary forces on the flow pattern is distinguishably different. It is shown that for a given dynamic Bond number, the behavior of the melt is largely modified. The transition corresponds to a jump discontinuity in the magnitude of the flow stream function.

3D Radiative Transfer Equation Coupled with Heat Conduction Equation with Realistic Boundary Conditions Applied on Complex Geometries

This paper presents the solution of coupled radiative transfer equation with heat conduction equation in complex three-dimensional geometries. Due to very different time scales for both physics, the radiative problem is considered steady-state but solved at each time iteration of the transient conduction problem. The discrete ordinate method along with the decentered streamline-upwind Petrov-Galerkin method is developed. Since specular reflection is considered on borders, a very accurate algorithm has been developed for calculation of partition ratio coefficients of incident solid angles to the several reflected solid angles. The developed algorithms are tested on a paraboloid-shaped geometry used for example on concentrated solar power technologies.

Combining the Radiative, Conductive and Convective Heat Flows in and around a Skylight

Normal skylights bring light into the spaces located below them. By the use of IR （infrared radiation） transmissive polymer films and IR-emitting and absorbing gases, an advanced version of the skylight may supply passive cooling and thermal insulation to the room located below it. This novel radiative skylight can, in its cooling mode, lead heat from the room below, to the cool skies located above the skylight. When cooling is no longer needed or attainable, the skylight will in its cooling mode provide the room with an optimal amount of thermal resistance. This article is a progress reporting on the modeling of the skylight. The main work is done to combine the different heat transfer methods into one single model by the use of the commercial program Comsol 4.1. The results show that a cooling effect of 100 W/ma is achievable when the skylight is compared with a similar skylight containing only air.

Surface plasmon-coupled radiative heat transfer between graphene-covered magnetic Weyl semimetals

Weyl semimetals(WSMs)have recently attracted considerable research attention because of their remarkable optical and electrical properties.In this study,we investigate the near-field radiative heat transfer(NFRHT)between graphene-covered Weyl slabs,particularly focusing on the supported coupled surface plasmon polaritons(SPPs).Unlike bare Weyl slabs where the epsilon-near-zero(ENZ)effect contributes the most to the NFRHT,adding a monolayer graphene sheet yields coupled SPPs,i.e.,the coupling of graphene SPPs(GSPPs)and Weyl SPPs(WSPPs),which dominates the NFRHT.The graphene sheet greatly suppresses the ENZ effect by compressing the parallel wavevector,thereby enabling the heat transfer coefficient(HTC)to be significantly changed.Further,for the graphene-covered magnetic Weyl slab configuration,an increase in the number of Weyl nodes suppresses the SPP coupling and ENZ effect,thereby weakening the NFRHT with a regulation ratio of 4.4 whereas an increase in the Fermi level slightly influences the NFRHT.Several typical heterostructures are also proposed for comparison,and results show that a mono-cell structure has the largest total HTC.Our findings will facilitate the understanding of surface plasmon-coupled radiative heat transfer and enable opportunities in energy harvesting and thermal management at the nanoscale based on WSM-based systems.

Heat Transfer Characteristics of Boiler Convective Heating Surface Under Pressurized Oxygen-fuel Combustion Conditions

增压富氧燃烧是一项极具前景的减排CO2新技术。对增压富氧燃烧条件下,对流受热面换热特性进行研究具有重要的意义。该文以一台实际300MW等级机组煤粉锅炉为计算对象,采用维里方程及Chun等的计算方法计算确定增压富氧燃烧烟气物性,采用宽带关联k模型计算富氧燃烧烟气辐射特性。进行了常规空气燃烧以及φ（O2）：φ（CO2）=21：79、φ（O2）：φ（CO2）=30：70两种比例的0.1、0.5、1.0、1.5、6 MPa五种压力下增压富氧燃烧各对流受热面的热力计算,分析了增压富氧燃烧条件烟气压力变化对各受热面换热特性的影响。研究结果表明：随烟气压力的升高,烟气流速下降,但烟气的Re却基本保持不变,对流换热系数有所增加。增压富氧燃烧烟气的辐射换热系数比空气燃烧烟气辐射换热系数大。实现同样的换热量,增压富氧燃烧条件下（φ（O2）：φ（CO2）=21：79、φ（O2）：φ（CO2）=30：70）对流受热面所需换热面积比常规空气燃烧条件下少。

Global heat transfer model and dynamic ray tracing algorithm for complex multiple turbine blades of Nibased superalloys in directional solidification process

High-quality solidification microstructure during directional solidification relies on precise temperature gradient control, so accurate calculation of the temperature field is critical. In this study, a 3D transient global heat transfer model of directional solidification by the Bridgman method based on the finite difference method is developed. The radiation heat in this model is calculated by the discrete transfer method, and a modified method of external surface area for irregular geometric models is proposed to reduce the zigzag shape caused by finite difference grids. Considering the radiative heat transfer between any surface elements of all materials in the directional solidification furnace, a dynamic ray tracing algorithm is developed to simulate the entire process of directional solidification. Then, the simulated results are compared with the theoretical results and experimental results, respectively. Finally, based on the present model and method, the simulation program developed is applied to the directional solidification of actual castings. The simulated results are in good agreement with the experimental results, which indicate that the model and method developed in this study is effective and practical.

Near-Field Heat Transfer Enhancement of SiC-hBN-InSb Thermophotovoltaic System by Graphene Strong Coupling Effects

Near-field thermophotovoltaic(NTPV)devices comprising a SiC-hBN-graphene emitter and a graphene-InSb cell with gratings are designed to enhance the performance of the NTPV systems.Fluctuational electrodynamics and rigorous coupled-wave analysis are employed to calculate radiative heat transfer fluxes.It is found that the NTPV systems with two graphene ribbons perform better due to the graphene strong coupling effects.The effects of graphene chemical potential are discussed.It is demonstrated that near-field radiative heat transfer of thermophotovoltaic devices is enhanced by the coupling of surface plasmon polaritons,surface phonon polaritons,hyperbolic phonon polaritons,and magnetic polaritons caused by the graphene strong coupling effects.Rabi splitting frequency of different polaritons is calculated to quantify the mutual interaction of graphene strong coupling effects.Finally,the effects of cell grating filling ratio are investigated.The excitation of magnetic polaritons is affected by the graphene ribbon and the cell filling ratio.This investigation provides a new explanation of the enhancement mechanism of graphene-assisted thermophotovoltaic systems and a novel approach for improving the output power of the near-field thermophotovoltaic system.