Thermal contact conductance (TCC) is such a common parameter that is simple to understand but difficult to determine in heat transfer process. TCC can bring a large uncertainty in the thermal analysis of the whole system in many engineering applications, such as the cooling of heat pipe reactors, heat dissipation of the chip and finned-tube heat exchangers; besides, it can also reduce the heat transfer efficiency, thus obtaining accurate TCC and controlling it can be meaningful for high-efficiency thermal management. Although enough attention was paid to TCC in the last few decades, however, there is still a gap needed to fill in TCC research. In this article, a comprehensive review on TCC of macroscale and conforming contact is summarized and commented. The review contains the trend and main progress of TCC, including theoretical analysis, experimental measurement and numerical simulation, and the limitations and advantages of these methods are mentioned and commented. As one of common and conforming contact cases, the TCC at the interface of concentric cylinder surface is mentioned and discussed. Besides, the main methods of controlling TCC is also reviewed. Finally, the prospects and challenges of TCC is addressed.

The escalating energy demands and the imperative of environmental conservation necessitate advanced sustainable energy solutions. This study introduces a novel nanofluid spectrum-splitting photovoltaic/thermal system integrated with radiative cooling (RC) technology, termed NSS-RC-PV/T. This system optimizes solar spectrum utilization, enhances thermal management, and significantly improves the efficiency and flexibility of heat, electricity, and cooling outputs. Employing a reversible PV-Ag panel, the system adapts between PV/T and RC modes based on energy demands. A comprehensive mathematical model is established to evaluate its performance under realistic environmental conditions across China. Results indicate the maximum energy output of the system is 6438 MJ/m2, which is a 33.4% increase in annual energy output compared to the conventional PV/T system. The dynamic power response model also shows an increase of 5.8% (266 MJ/m2) compared to the daylight response model. This research underscores the potential of NSS-RC-PV/T systems in advancing renewable energy technologies and meeting modern energy needs.

Silica aerogel can be applied to the concentrated solar power (CSP) plants to achieve a high operation temperature due to the high optical transparency and thermal insulation properties. However, the insulation capacity of aerogel at high-temperature will deteriorate rapidly due to the damage of the porous structure. In present study, a transparent aerogel consisting of SiO2/Al2O3 core-shell particles was proposed. The structure-dependent radiative properties were predicted through the combination of Discrete Dipoles Approximation (DDA) and Monte Carlo (MC) method. The core-shell aerogel achieves a higher solar transmittance than the alumina aerogel. The evaluation parameter of greenhouse selectivity was introduced to assess the aerogel performance by considering the solar transmission and infrared extinction simultaneously. An optimal design of SiO2/Al2O3 core-shell aerogel at different temperatures was conducted. Compared with the pure alumina aerogel, a maximum enhancement of 22.6% in greenhouse selectivity was achieved by the core-shell aerogel with phi = 95% and r(1)/r(2) = 0.4 at 700 degrees C. This work provides a promising method to achieve a high-efficient transparent thermal insulating aerogel for high-temperature CSP plants with high thermal stability.

Superhydrophobic surfaces (SHSs) have received significant attention due to their low surface energy, which facilitates droplet dewetting transition and promotes jumping condensation. However, at high subcooling temperatures, SHSs encounter challenges in maintaining their performance. They are prone to issues like nucleation-induced flooding condensation and reduced hydrophobicity. To address the limitations of SHSs at high subcooling temperatures, we propose the concept of superhydrophobic lubricant-infused surfaces (LISs). These combine the advantages of superhydrophobicity with enhanced functionality provided by a lubricant layer. Firstly, the wetting and adhesion of droplets on LISs have been explored, considering the effects of substrate characteristic and lubricant thickness. Subsequently, the surface condensation on both LISs and SHSs at high subcooling temperatures has been investigated. The failure modes of SHSs at high subcooling temperatures have been revealed. Our molecular dynamics simulations demonstrate that by infusing a superhydrophobic lubricant, the LISs can effectively overcome the limitations of SHSs. This enables the maintenance of dewetting property and ensures droplet jumping condensation, even at high subcooling temperatures. Finally, a comprehensive map is developed to compare the static wettability and dynamic wettability during condensation under different substrate characteristics and lubricant thicknesses. The present work can provide valuable insights into the potential of LISs in addressing the limitations of SHSs. It paves the way for applications in various fields, such as self-cleaning, anti-icing, moisture resistance, drag reduction, and enhanced heat transfer performance.

Global warming is a major concern, and active cooling technology, which consumes substantial energy, contributes to global warming. Radiative cooling (RC) is a form of passive cooling. An optically selective porous film can provide cost-effective and efficient radiative cooling. This study aims to investigate the theoretical effects of the structural parameters on the cooling performance of porous films. An improved diffusion-limited cluster aggregation (DLCA) algorithm is established to reproduce porous structures with moisture content. A combination of discrete dipole approximation and Monte Carlo method was employed to predict the structure-related solar reflectance and “sky window” emissivity. An optimal design of a porous RC film was developed. The results demonstrate that the BaSO4 porous film can achieve high solar reflectance (∼ 97%), high “sky window” emissivity (∼ 99%), and a net cooling power of 102 W/m2 during the day. In particular, the effect of moisture content on the cooling performance of the porous RC film was analyzed. Moisture content enhances the cooling performance at night but decreases it during the daytime. The results provide guidance for designing optically selective porous films for radiative cooling.

In this paper, a 3D numerical model was established to investigate the effect of thermal pad on enhancing heat transfer of rough contact interfaces. The model is a sandwich structure and it includes chip, thermal pad and heat sink. The results indicate that the elastic modulus of the thermal pad can affect the actual contact area of rough contact interfaces, which further affects interfacial heat transfer. Besides, it is the through-plane thermal conductivity of the thermal pad that affects the heat dissipation of the chip, and increasing through-plane thermal conductivity of the thermal pad is more effective than increasing the clamping pressure for heat dissipation. Furthermore, if thermal conductivity of the thermal pad is low, the efficiency of simply increasing convective heat transfer coefficient at the side of the heat sink is limited for heat dissipation. The study also investigates thermal contact resistances among thermal pad, chip and heat sink. The results indicate that thermal contact resistance is influenced by material properties, and thermal contact resistance accounts for 2.2%–45.85% of total thermal resistance for the studied cases of this paper. The model was validated by experimental results and can be acceptable to study the heat dissipation of the sandwich structure with rough contacts.

Transparent aerogel window based on silica aerogel has been investigated for achieving energy-saving buildings. However, most of the attention was paid to the effective thermal conductivity and solar transmittance of the aerogel. The research on solar transmittance control and visual experience is still lacking. In the present study, a dynamic aerogel based on V O2/SiO2 core–shell nanospheres is proposed to serve as an energy-saving window. The effects of particle size, outer-to-inner diameter ratio, volume fraction, and thickness on the radiative properties of aerogel windows are studied theoretically by combining the Mie theory and the Monte Carlo method. In particular, the solar control ability and haze of the dynamic aerogel are computed. The results show that a 0.25 cm thick aerogel doped with core–shell nanospheres of D/d= 2 (fv= 0.01%) achieves solar control ability of 124.2 W/m2. The solar transmittance and haze are 70% (55%) and 0.067 (0.097) for the insulating (metallic) phase. The present study provides a guideline for designing highly transparent aerogel windows with switchable solar transmittance and low haze for energy-saving buildings.

Developing advanced thermal interface materials (TIMs) to bridge heat-generating chip and heat sink for constructing an efficient heat transfer interface is the key technology to solve the thermal management issue of high-power semiconductor devices. Based on the ultra-high basal-plane thermal conductivity, graphene is an ideal candidate for preparing high-performance TIMs, preferably to form a vertically aligned structure so that the basal-plane of graphene is consistent with the heat transfer direction of TIM. However, the actual interfacial heat transfer efficiency of currently reported vertically aligned graphene TIMs is far from satisfactory. In addition to the fact that the thermal conductivity of the vertically aligned TIMs can be further improved, another critical factor is the limited actual contact area leading to relatively high contact thermal resistance (20-30 K mm2 W-1) of the "solid-solid" mating interface formed by the vertical graphene and the rough chip/heat sink. To solve this common problem faced by vertically aligned graphene, in this work, we combined mechanical orientation and surface modification strategy to construct a three-tiered TIM composed of mainly vertically aligned graphene in the middle and micrometer-thick liquid metal as a cap layer on upper and lower surfaces. Based on rational graphene orientation regulation in the middle tier, the resultant graphene-based TIM exhibited an ultra-high thermal conductivity of 176 W m-1 K-1. Additionally, we demonstrated that the liquid metal cap layer in contact with the chip/heat sink forms a "liquid-solid" mating interface, significantly increasing the effective heat transfer area and giving a low contact thermal conductivity of 4-6 K mm2 W-1 under packaging conditions. This finding provides valuable guidance for the design of high-performance TIMs based on two-dimensional materials and improves the possibility of their practical application in electronic thermal management.

Passive daytime radiative cooling (PDRC), a cooling method that needs no additional energy, has become increasingly popular in recent years. The combination of disordered media and polymeric photonics will hopefully lead to the large-scale fabrication of high-performance PDRC devices. This work aims to study two typical PDRC structures, the randomly distributed silica particle (RDSP) structure and the porous structure, and systematically investigates the effects of structural parameters (diameter D, volume fraction fv, and thickness t) on the radiative properties of the common plastic materials. Through the assistance of the metal-reflective layer, the daytime cooling power P-net of the RDSP structures is slightly higher than that of the porous structures. Without the metal-reflective layer, the porous PC films can still achieve good PDRC performance with P-net of 86 W/m(2). Furthermore, the effective thermal conductivity of different structures was evaluated. The single-layer porous structure with optimally designed architecture can achieve both good optical and insulating performance, and it is the structure with the most potential in PDRC applications. The results can provide guidelines for designing high-performance radiative cooling films.

Realizing accurate measurement for thermal contact conductance (TCC) is difficult and needs many additional measurements. In this paper, we design a three-layer back propagation artificial neural network (ANN) model to retrieve TCC of two contact solid specimens. The model can retrieve TCC only according to the measured temperatures and loading pressures, and the effects of loading pressure, temperature and surface roughness on TCC are considered. Besides, thermal conductivity, heat flux and the parameter of the TCC empirical fitting formula can be obtained simultaneously. The retrieved results are validated by experimental values. The results show that for two contact pairs with different surface roughness, the retrieved parameters of TCC empirical fitting formula are clear reasonable and can be explained conceptually compared with the published studies.

In this paper, thermal contact resistance (TCR) of three different contacts of a composite are experimentally investigated when the contact interfaces are filled with air. The surface roughnesses of the three contacts are 0.95/16.91 mu m (Contact A, titanium alloy/composite), 9.54/9.73 mu m (Contact B, composite/composite), and 11.53/6.85 mu m (Contact C, composite/titanium alloy). The measured TCRs of the three contacts decrease with an increase in loading pressure and interface temperature. For Contact B, the effect of temperature on thermal contact resistance gradually decreases with an increasing loading pressure. For Contact C when loading pressure increases to 3.6 MPa, the decreasing trend of the thermal contact resistance with increasing loading pressure becomes mild. Additionally, experimental uncertainties are analyzed and calculated.

This study investigates the effect of thermal expansion on thermal contact resistance prediction, proposing a dual-iterative coupling method (DICM). The contact surfaces in the simulation model are reconstructed based on either the actual measured topography or the hypothetical topography, and a mathematical formulation for numerically predicting the thermal contact resistance (TCR) is established. The DICM includes four steps: first, mechanical analysis is conducted based on the ideal single point contact condition, according to the elastic–plastic constitutive equations. Second, heat transfer analysis is carried out based on the deformed geometry originating from the prior mechanical analysis. Third, another step of mechanical analysis is implemented to consider the effect of thermal expansion with the temperature distribution determined in the second step. Fourth, another step of heat transfer analysis is carried out based on the deformed geometry originating from the second-step mechanical analysis. The conventional prediction method only contains the first and second steps, and is known as the single sequential coupling method (SSCM). The TCRs of two engineering examples are predicted using both DICM and SSIM. The results show that the mechanical–thermal-mechanical–thermal dual-iterative coupling method, i.e., DICM, should be recommended for simulating contact pairs with axisymmetric geometries, while SSCM is suggested for contact pairs with non-axisymmetric geometry.

In this paper, thermal contact resistance between a pair of 8-harness satin woven pierced composites is numerically investigated when the interface gaps are filled with air. Numerical model of the rough surfaces is based on measured results of actual specimens by a microscope. The results show that thermal contact resistance decreases with an increase in both loading pressure and temperature. The effects of interfacial thermal radiation on the predicted thermal contact resistance increase with an increase in temperature, but less than 5%. The percentage of heat transfer rate through solid contact regions occupies less than 17% of overall heat transfer rate for the cases studied with solid thermal conductivity around 10 W.m(-1).K-1 and loading pressure up to 2.37 MPa. For the studied composite pair (averaged surface roughness of 12.30 10.54 mu m) thermal contact resistance is in a range of 9.8 x 10(-4) - 5.7 x 10(-4) K.m(2).W-1.

The precise prediction or test of thermal contact resistance is a key issue on increasing or decreasing thermal energy transmission efficiency between two solids. This paper raises a thermal contact resistance prediction model based on measuring actual surface topography under different loading pressures and different heating temperatures. The actual topography of contact surfaces is measured by a 3-D optical microscope named Bruker Contour GT-K. The contact surfaces are reconstructed with language Python according to the data of surface topography from the microscope and the numerical contact model is generated. Then the thermal contact resistance simulation is implemented with software ABAQUS. Based on the elastic-plastic constitutive equations and steady state heat conduction theory, finite element analysis of mechanical and heat transfer performance of the contact model is performed with ABAQUS in the light of sequential coupling method. The studied material pairs are Ti-6Al-4V-Ti-6Al-4V with three kinds of different interstitial material e.g., vacuum, air and conductive silicone grease. The effect of radiation on thermal contact resistance under air and vacuum atmosphere is further studied and analyzed. Besides, the solid thermal conductivity on thermal contact resistance is investigated. To verify the accuracy of the method, the simulated results from ABAQUS are compared with the experimental results of air gap with the same boundary conditions. The maximum deviation between simulation results and experimental results is 9.57% while 75% of the deviations are within 5%. A correlation of thermal contact conductance with the average contact surface temperature and loading pressure is proposed. The results show that this method has high precision to predict thermal contact resistance in the engineering application.

An approach is proposed to predict the thermal contact resistance (TCR) of rough surfaces. The practical rough topography of surfaces is measured by a contour profiler and is reconstructed to numerically analyze the mechanical and thermal contact performance. The studied material pairs are Ti-6Al-4V—Ti-6Al-4V and C/C-SiC—high temperature ceramic (HTC). The TCR with air gap and vacuum gap conditions are calculated. The approach is validated by the comparison with experimental results of surfaces with the same topography at the same temperatures and loading pressures. The influence of thermal contact conductance between real contact asperities on the TCR of rough surfaces is studied. The approach can be used to predict TCR of different materials with different gap medium under different temperatures and loading pressures. The results show that the real contact area increases approximate linearly, while TCR decreases with the increasing pressure.

The 3D C/C-SiC composite is a kind of widely used material in thermal protection system or other functional parts of the hypersonic vehicles. This paper conducts numerical simulation to predict the thermal contact resistance of the 3D C/C-SiC needled composite pairs. The practical surface topography is measured by a 3D optical microscope named Bruker Contour GT-K, and the rough surfaces for simulation are reconstructed in ANSA software with the help of Python code. The measured arithmetical mean roughness (Ra) of the two specimens are 12.04 mu m and 11.75 mu m respectively. The prediction is divided into two steps, static analysis for revealing the contact spot distribution of the contact interfaces, and thermal analysis for the temperature distribution of the contact interfaces to calculate the thermal contact resistance. Both two steps are finished with Abaqus. The prediction results show that actual contact area only occupies a small part of nominal contact area (when pressure is 5.52 MPa, the actual contact area just accounts for 7% of the nominal contact area), and the dependency curve approximately shows linearity between the proportion of actual contact area to nominal contact area and loading ressure. Besides, thermal contact resistance decreases with an increase in loading pressure and temperature. Thermal contact resistances of the studied composite materials are in the order of 6 x 10(-4)-1.2 x 10(-3) K.m(2.)w(-1). (C) 2018 Elsevier Ltd. All rights reserved.

Optimization design of thermal protection system of aerospace vehicles with progressively increasing flying speeds needs a deepened study on the thermal conduction characteristics of corresponding materials. As a potential thermal protection material, C/C-SiC 8-harness (8HS) satin woven composites with piercing fiber bundles are numerically studied in this work. The numerical model is established based on the investigation of 8HS satin weave textile. A full and a half unit cell models are formulated according to three non-orthogonal translational and one further 180° rotational symmetries, respectively. The two unit cells have much smaller sizes and also less computational costs than the rectangle unit cell available in previous literatures. The thermal boundary conditions are derived rigorously and are verified by the identical numerical results obtained by the two unit cells. The developed unit cell method and the related boundary conditions derivation process can be references for other multi-harness satin woven composites. The numerical model is validated by the experimental result obtained in this work and that available in the literature. The influences of constituents (carbon fiber, matrix C, matrix SiC and porosities) volume fractions, piercing fiber bundles and thermal contact resistance between matrix and fiber bundles on the effective thermal conductivities are studied.

The effective properties of composites can be calculated based on a representative volume element which can be called unit cell for a textile reinforced composite. A general approach of unit cell formulation for thermal conduction study of textile reinforced composites is proposed in this paper. The utilization of translational, reflectional and 180 degrees rotational structure symmetries in the geometry formulation boundary conditions derivation of unit cells are clarified. Several typical two-dimensional textile reinforced composites including plain, 5-harness satin and 8-harness satin woven composites are studied. Unit cells of different sizes are established and corresponding thermal analysis are conducted.

In the present paper,a patched-type multi-block lattice Boltzmann method is adopted to study the influences of contact pressure,roughness,gas thermal conductivity of interstice media on the thermal contact resistance.The results show that thermal contact resistance of aluminum pairs decreases with the increasing contact pressure,while increases with roughness.Besides,the thermal contact resistance of bulk aerogel pairs changes slightly with the increase of contact pressure when the interstices are filled with air,while significantly decreases if the interstice is approaching vacuum.In addition,the gas conduction of interstice media plays a key role on the thermal contact resistance.At low contact pressure,the thermal contact resistance of the vacuum gap is about 50 times of that of the air gap.