Energy-efficient buildings require sustainable materials that combine structural performance with advanced optical and thermal functionalities to minimize energy consumption and greenhouse gas emissions. Here, we reported a new strategy to develop biodegradable transparent bamboo with a dense and ordered structure, achieved through selective delignification followed by directional pressing to align cellulose nanofibrils. This process yielded large-scale transparent bamboo with remarkable mechanical strength, 78% optical transparency in the visible spectrum, and a high haze (> 90%) that ensured uniform daylight distribution and reduced reliance on artificial lighting. To further impart dynamic solar modulation, a thin polylactic acid film containing tungsten-doped vanadium dioxide (W-VO2) nanoparticles was integrated onto the transparent bamboo substrate. The resulting thermochromic bamboo exhibited a solar modulation ability of 9.7% along with effective thermal regulation that lowered indoor heating loads in hot regions. By synergizing biodegradability, mechanical robustness, and active photothermal control, this transparent bamboo/W-VO2 composite offered a sustainable and high-performance alternative to conventional glass, holding great promise for energy-efficient building applications.

Although received considerable attention in microchannel heat sinks of thermal management systems, current topology optimizations have primarily concentrate on enhancing the thermohydraulic performance through internal channel redesign while maintaining fixed inlet and outlet configurations. In this study, the 2D topology optimization of microchannel heat sinks with different inlet and outlet widths is investigated based on the variable density method, with the aim of maximizing the total heat generation and minimizing the total power consumption. The influences of thermal objective weights and Reynolds numbers on topology optimization results are explored in detail. Meanwhile, the 3D numerical simulation is performed to validate the 2D optimized topology. It is demonstrated that the optimized Z-type flow arrangement microchannel heat sink with inlet and outlet both having half the width of the design domain can substantially enhance the thermohydraulic performance, and achieve a 46.4-62.2% reduction in pumping power with the lowest temperature of the bottom wall, compared to the conventional one.

The heterogeneous bubble growth dynamics is a fundamental aspect of nucleate boiling. Attentions have focused more on the generality in the "typical" bubble cycle in most previous studies, but less on the stochasticity of the bubble growth process. In this study, successive heterogeneous bubble nucleation-growth-departure cycles from a fixed single cavity on a horizontal surface with the static contact angle of 59.5 degrees in saturated nucleate pool boiling of deionized water are experimentally investigated, covering the wall superheat of 6.1-15.5 K and the system pressure of 18-99.2 kPa. Distinct stochasticity for both the growth time and the departure radius of the successive bubbles is manifested under the same system conditions, which is responsible for the discrepancies of the bubble growth coefficient under the classical theoretical framework. It is revealed that the instability of the surrounding fluid-thermal environment has a prominent influence on the stochasticity of the bubble growth coefficient, which is governed by the competition between the natural convection induced by the temperature difference and the rewetting convection induced by the bubble departure. The stochasticity of the bubble growth coefficient increases versus the Richardson Number, Ri, with a mutation occurring at about Ri = 10, while the stochasticity of the bubble departure radius closely depends on that of the bubble growth coefficient with a growing relevance versus the Jacob number. Furthermore, a method to predict the heterogeneous bubble growth is proposed in consideration of the effect of the stochasticity. This new insight captures the general characteristics of bubble growth through a dimensionless correlation, while the stochasticity is controlled in a separate dimensional correlation involving only the departure parameters. Within the Jacob number range of 18.9-141.4, the dimensionless correlation demonstrates good overall performance, with the deviations from experiments remaining low at a MAPE of 1.72-3.88 %.

Windows are among the weakest components of building envelopes, accounting for up to 50% of total energy loss from buildings. Conventional solar control coatings, such as silver-based low-emissivity (low-E) films, demonstrate solar spectral selectivity, effectively reducing solar heat gain in summer by blocking near-infrared radiation. However, their static optical properties also block desirable solar heat gain in winter, thereby restricting year-round energy efficiency. Herein, we propose an IHO/MgF2/VO2/MgF2/IHO/MgF2 multilayer coating that provides dynamic solar modulation, high luminous transparency, and low mid-infrared (MIR) emissivity simultaneously. This structure integrates thermochromic VO2 for dynamic near-infrared (NIR) switching with hydrogen-doped indium oxide (IHO) as a transparent low-E layer, while anti-reflective MgF2 layers maximize visible comfort. To ensure high solar heat modulation ability while balancing luminous transmission and radiative heat loss (i.e., MIR emissivity), a genetic-algorithms-coupled transfer-matrix method was employed to optimize material selection and layer thickness. The optimized design achieves a solar heat gain modulation ability of 7% and a U-value of similar to 1.78 W/(m(2)K) while maintaining a luminous transmittance of similar to 60% in double-glazed windows, representing a 250% improvement in modulation capability compared to single-layer VO2 coatings (2%) and a 33% enhancement in visible transmission compared to bare VO2 films (45%). Whole-building energy analysis indicates that applying this coating to medium office buildings reduces energy consumption by 21.8% compared to conventional double-glazed windows and 8.4% compared to low-E windows. This work provides a promising solution for next-generation smart windows that effectively balance solar heat gain with thermal insulation, offering significant potential for reducing global building energy consumption while maintaining occupant visual comfort.

The rapid advancement of high-power electronic devices has created an urgent need for more effective thermal management solutions, as inadequate heat dissipation severely limits device performance and reliability. Conventional vapor chambers could dissipate heat efficiently via liquid-vapor phase change heat transfer but are limited due to capillary pressure-permeability trade-off and the long, tortuous condensate return path, resulting in insufficient liquid supply under high thermal loads and making them unsuitable for next-generation high-power electronics. Herein, we developed a vapor chamber for high heat flux dissipation by incorporating a hierarchically biporous superhydrophilic evaporator wick for enhanced capillary-driven liquid supply, along with a superhydrophobic micro/nano-structured condenser surface that enables dropwise condensation and facilitates condensate return. This synergistic design ensures a continuous and efficient liquid supply, achieving a minimum overall thermal resistance of ∼0.17°C/W, a minimum thermal resistance of temperature uniformity of ∼0.02°C/W, and a critical heat dissipation flux exceeding 600 W/cm2-even under gravity-opposed conditions-showing significant enhancement compared to previous vapor chamber designs. This work provides a promising solution for next-generation high-power electronics requiring extreme heat flux dissipation.

The pebble-bed high temperature gas-cooled reactor (HTGR) is a type of the Generation IV reactor. For the necessary thermal and radiation protection, the walls around the pebble bed are stacked with the graphite reflector layer and the carbon bricks. The bypass flow fails to effectively cool the fuel pebbles in the pebble bed and thus influences the efficiency and safety of HTGR. In this paper, a comprehensive three-dimensional model for HN-750 self-designed by HUANENG Group of China is established. A physically realistic mathematical model is proposed to investigate the convective heat transfer and the bypass flow characteristics in HN-750. The results indicate that the maximum temperature of the pebble bed with bypass flow is about 200 K higher than that without bypass flow, while the pressure drop in pebble bed is about 13 kPa lower than that without bypass flow. The bypass flow accounts for 29.25 % of the total helium flow, 24.43 % of which is the vertical gap bypass flow. The synergistic combination of square keys and sealing strips can effectively block nearly half of the vertical gap bypass flow. The present large-scale three-dimensional numerical calculations can support the thermal-hydraulic optimization and safety analysis of pebble-bed HTGRs.

Ordered porous structures are increasingly implemented in capillary-driven systems owing to their tunable pore networks and low tortuosity, which can mitigate the inherent trade-off between capillary pressure and permeability. Predictive models for capillary rise are essential for quickly evaluating and optimizing capillary performance. However, prevailing models for random porous structures based on the homogeneous media assumption fail to accurately capture the capillary rise behavior in ordered porous structures. To address this issue, we developed a dynamic model based on energy conservation principle to predict the capillary rise in ordered porous structures represented by the inverse simple cubic structure. The dynamic model, explicitly incorporating the geometric features of the inverse simple cubic structure, accounts for the variations in surface energy, kinetic energy, gravitational potential energy, energy dissipation due to viscous and energy loss at entrance. Validated by comparing the liquid height-time (h - t) curves under identical structural and wetting conditions with experiments, this dynamic model demonstrates superior predictive accuracy for capillary height, with a deviation of only 0.5% from the experimental data. Furthermore, it facilitates detailed energy analysis during the capillary rise, revealing the impact of structural and wetting parameters on capillary performance, thereby providing guidance for the design and optimization of ordered porous structures in engineering applications such as thermal management, water transport and energy systems.

This paper presents a numerical study on turbulent mixed convection heat transfer in a horizontal tube under uniform heat flux and non-uniform heat flux by Large Eddy Simulation (LES). The effects of solar elevation angle (W) and Grashof number on instantaneous turbulent mixed convection behavior, turbulence statistics, friction factor, and Nusselt number are systematically analyzed. The results show that the natural convection can suppress turbulent fluctuation in the upper fluid zone under uniform heat flux, which causes heat transfer deterioration and localized high temperature. While the natural convection can improve the turbulent intensity at the top of the tube under non-uniform heat flux with W = 90 degrees, which helps to avoid the local overheating. Additionally, the solar elevation angle can significantly influence the turbulent mixed convection characteristics. The phenomenon of turbulent flow laminarization under non-uniform heat flux with W = 30 degrees reduces the Nusselt number compared to that under non-uniform heat flux with W = 45 degrees and W = 60 degrees. At last, under non-uniform heat flux with W = 90 degrees, with the increase of Grashof number, the velocity fluctuation intensity near the wall decreases first and then increases. While the flow resistance and heat transfer performance gradually increase. It is recommended that the heat transfer enhancement measures should avoid obstructing secondary flow in the cross section. And it is beneficial to use longitudinal vortex generators for laminarized regions and spherical dimples for turbulent regions.

This study aims to design a new multi-scale coupling method and develop a corresponding code for the core of pebble-bed high-temperature gas-cooled reactor (HTGR). This research independently designed a multiscale coupling calculation framework that integrates macro-mesoscopic-microscopic models, specifically for pebble-bed high-temperature gas-cooled reactors, based on the C/C++ language. Macroscopically, a two-dimensional axisymmetric modeling method is adopted, and thermal conduction, fluid dynamics and neutron transport processes are coupled to establish the corresponding physical models. Mesoscopically and microscopically, the initial results from macroscopic calculations are used to calculate the temperature distribution at the fuel pebble scale and the TRISO particle scale using the heat source decomposition method, enhancing computational efficiency. The results of the study demonstrate that the independently developed multi-scale coupling code can effectively simulate various operating conditions of HTR-PM and HTR-10 reactor types, providing significant theoretical support and technical assurance for their design and safety assessment.

Glazed windows contribute up to 60% of energy losses from the building envelope. Designing energy-efficient retrofitting materials for windows that can block near-infrared radiation and retard environmental heat is crucial for reducing building energy loss and greenhouse gas emissions. Here, we proposed a visible transparent, near-infrared opaque, and thermal insulating silica aerogel film embedded with indium tin oxide (ITO) nano-particles to reduce the energy consumption of buildings in hot regions. A coupled heat conduction, convection, and radiation transfer model was developed to assess how aerogel thickness, diameter, and concentration of ITO nanoparticle affect the optical and thermal performance of silica aerogel film on windows. The results show that a 5-mm-thick aerogel doped with 0.099 vol% ITO nanoparticles measuring 4 nm in diameter can achieve a Uvalue of 2.30 W/(m2 center dot K), a solar heat gain coefficient of 0.60, a luminous transmittance of 0.60 and a near-infrared transmittance of 0.27. The proposed silica aerogel composite film can minimize energy loss without blocking visible solar energy, which has great potential as a retrofit to increase the energy efficiency of windows, particularly in hot regions.

The growing demand for decarbonization and the rapid advancement of high-power electronic devices have created an urgent need for more efficient thermal management solutions. Boiling heat transfer dissipates heat efficiently through liquid vaporization, offering a promising solution for effective thermal management. However, simultaneously improving both boiling heat transfer coefficient (HTC) and critical heat flux (CHF) remains challenging due to the conflicting requirements on nucleation-site density. Herein, we address the trade-off by designing superbiphilic surfaces fabricated by simply spraying superhydrophobic dots onto solution-processed superhydrophilic substrate. The superhydrophilic substrate enhances liquid supply, the superhydrophobic dots facilitate bubble nucleation, and their spatial separation suppresses bubble coalescence. These synergistic effects enable simultaneous enhancements in HTC and CHF, reaching 101.39 kW/m2 & sdot;K and 203.80 W/cm2, respectively-representing improvements of 206% and 184% compared to a neutral smooth surface with a contact angle of 90 degrees . Furthermore, the superbiphilic surfaces offer excellent thermal durability during 72-h long-term boiling and 5-repeated thermal cycling tests, as well as reliable liquid-vapor separation across large areas. This work highlights the potential of surface wettability engineering in overcoming the traditional trade-off between HTC and CHF and provides a potential thermal management solution for next-generation high-power electronic systems and broader decarbonization efforts.

Radiative heat transfer plays a crucial role in heat transfer within silica aerogel. Due to the complex optical properties of silica aerogel, approximate models often introduce non-negligible deviations in radiative thermal conductivity estimation. Although partitioning radiation into spectral bands is adequate for achieving accurate numerical modeling of radiative heat transfer, this approach incurs high computational costs. In this study, a Beer-Lambert law-based general acceleration approach is proposed to simplify the radiative heat transfer calculations in silica aerogel. The proposed approach exploits the significant variation in spectral optical thickness across bands. Specifically, it truncates the source function integration in the incident radiation calculation and neglects contributions from distant regions in bands with large optical thickness. Results show that the proposed approach reduces computational time by up to 33% while keeping the relative deviation in radiative heat flux below 0.01%. Moreover, the computational efficiency further improves as the optical thickness increases.

In this work, an extension of the discrete unified gas-kinetic scheme (DUGKS) to gas–liquid phase change problems is proposed based on the conservative Allen–Cahn phase-field model. The kinetic framework of the DUGKS for gas–liquid phase change problems comprises three components: the conservative Allen–Cahn phase-field equation to describe the interface evolution, the Navier–Stokes equations for fluid flow, and the energy equation for the temperature field. To model the phase change, a source term is introduced based on the heat conduction balance across the interface. By applying Chapman–Enskog analysis, these kinetic equations can be correctly recovered to the corresponding macroscopic equations. The DUGKS approach, which integrates the strengths of the lattice Boltzmann method and the finite volume method, can capture the non-equilibrium phenomenon of phase change more precisely and simulate under the non-uniform mesh with high accuracy. The main contribution of this work lies in formulating a gas–liquid phase change phase-field model by embedding a physically consistent source term into the conservative Allen–Cahn equation under the framework of the DUGKS, enabling an accurate description of the interfacial dynamics during phase change. Including one-dimensional Stefan problem, the evaporation of the stationary droplet, the falling evaporating droplet under gravity in a container, and the saturated film boiling, four benchmark cases are conducted to validate the proposed models and methods. It is demonstrated that the present method can accurately capture the interface dynamics and simulate the gas–liquid phase change problems.

Flow boiling instability in large length-to-diameter ratio microchannels poses significant challenges for large power and high heat flux dissipation in spacecraft. This study presents a two-dimensional numerical model based on the Volume of Fluid (VOF) approach to investigate flow boiling instability in such microchannels with integrated reservoirs. Key factors, including the compressible volume fraction in the reservoir, saturation pressure, and the diverging angle of the microchannel, are analyzed to understand their impact on pressure drop, velocity, and temperature instability. The results reveal that flow boiling instability arises from the complex interplay of bubble generation, growth, and coalescence, rather than being solely induced by compressible volume. Increasing the compressible volume fraction in the reservoir mitigates pressure drop fluctuations but amplifies oscillations in streamwise velocity and inner wall temperature. Lower saturation pressures intensify fluctuations in pressure drop, velocity, and wall temperature, resulting in greater instability. Conversely, diverging microchannel geometries reduce instability by facilitating smoother flow, delaying flow pattern transitions, and preventing local dryout. These findings provide critical insights for the design and optimization of microchannel systems to enhance flow boiling heat transfer performance. A balanced consideration of compressible volume, saturation pressure, and diverging angle is essential for achieving stable and efficient thermal management in practical applications.

In this paper, an analytical hydrodynamic model for the microlayer under heterogeneous bubbles in pool boiling is developed accounting for the intrinsic association between the peripheral viscous boundary layer outside the microlayer and the residual flow inside the microlayer. The development of the viscous boundary layer is evaluated using the self-similarity transformation method. Then the microlayer structure is derived based on the mass and momentum conservation between the residual flow and the loss of the viscous boundary layer, characterizing as a time-dependent quadratic logarithmic function. Ulteriorly, the concept of the initial microlayer thickness is redefined by distinguishing the narrowly termed microlayer from the macrolayer. The dependence of the microlayer structure as well as the initial microlayer thickness on the bubble growth exponent is also revealed. The proposed theoretical model agrees well with available experimental data in the literatures. This model will be conducive to improving the fundamental understanding about the mechanism of microlayer formation.

High-power compact electronic devices often generate concentrated heat in small areas, leading to serious cooling challenges. A hybrid heat sink that combines microchannels with pin-fins is a practical solution to handle these hotspots more effectively. However, most existing studies focus on regular fin geometries or uniform heating conditions and rarely consider hotspot-oriented shape co-design. To identify fin shapes that better address local hotspot issues, 50 pin-fin shape designs using CFD simulations are tested to evaluate their thermal resistance and pumping power. Then, a correlation model is built to show how geometry affects performance and a multi-objective genetic algorithm (NSGA-II) is used to find the best design. The fin shape of the TOPSIS solution is considered optimal, as it effectively enhances local boundary layer disruption near the hotspot region, thereby enhancing localized convective heat transfer. At the same time, the fin design maintains smooth flow channels, minimizing additional pressure losses. Under the same pumping power, the optimized hybrid heat sink achieves a 16.5 % reduction in total thermal resistance. Conversely, when thermal resistance is kept constant, the required pumping power is reduced by 62.9 % compared to a traditional microchannel heat sink. These results show that optimizing the shape of pin-fins can greatly improve the thermal-hydraulic performance of a microchannel pin-fin hybrid heat sink.

In aerospace applications, the thermal protection system (TPS) plays a pivotal role in safeguarding high-speed vehicles and spacecraft from extreme heat. Porous ablative composites, commonly used in TPS, mitigate overheating and structural failure through heat absorption and dissipation. However, current approaches to optimizing the thermal protection performance of these composites often rely on random or heuristic porosity distributions, which lack systematic design frameworks. This study introduces a method for optimizing the thermal protection performance of porous ablative composites by integrating neural network models with optimization algorithms. The results demonstrate that an optimized two-section porosity scheme with higher porosity near the heated surface significantly improves thermal protection, reduces bondline temperature by 21.88 K compared to traditional designs, which accounts for 11.49 % of the temperature rise. Additionally, the optimized composite exhibits 6.5 % reduced equivalent density, offering an efficient solution with superior thermal protection. The study also finds that smoothing the porosity transition zone does not improve performance and adds unnecessary manufacturing complexity. These findings provide a robust framework for future design of porous ablative composites, enabling more efficient and cost-effective solutions for high-speed aerospace missions.

Silica aerogel, renowned for its exceptional insulation properties, exhibits extremely low thermal conductivity at room temperature, with radiative heat transfer contributing significantly to its overall thermal performance at higher temperature. Its radiative thermal conductivity is predicted by the extensively-employed Rosseland diffusion approximation model developed under the optically thick hypothesis. It is imperative to ascertain its applicability, as failing to determine the appropriate conditions for the Rosseland model can result in significant prediction discrepancy under varying optical thickness. A coupled radiative and conductive heat transfer model is developed in this study, where radiative heat transfer is solved by a spectral band method applicable at any optical thickness. The effects of temperature, Rosseland optical thickness, and boundary surface emissivity on the radiative thermal conductivity are systematically analyzed while comparing with predictions from the Rosseland model. Finally, the applicable scope of the Rosseland diffusion approximation model in silica aerogel is obtained.

As the main region where charged particles are accelerated by the electric field and accumulated near the cathode, the near-cathode region has significant non-local equilibrium characteristics and plays a crucial role in exploring the energy and mass transport properties of the thermal plasma from the arc column region to the hot cathode surface. However, there is a lack of universally adopted models and accepted theories for the study of the near-cathode region due to the complexity of the physical mechanisms involved in the sheath. According to the physical characteristics of the arc discharge, an external circuit is usually used to regulate the total current applied to the electrodes to maintain stable discharge. Therefore, the energy and mass transport properties of atmospheric thermal plasma driven by a current source coupled to an external circuit are investigated in this work to explore the transport mechanism of charged particles in the near-cathode region based on an implicit particle-in-cell Monte Carlo collision method. Firstly, the current-driven model in this work is compared with the present voltage-driven model and fluid model to verify the correctness of this model. Then, the collisions between charged particles and gas atoms inside the sheath are analyzed, and the spatial distributions of particle current density and particle heating rate inside the sheath are also studied. Finally, the variations of typical parameters of thermal particles under different current densities (10 6 –10 7 A m −2 ) are analyzed, including maximum particle number density, maximum particle spatially-averaged temperature, sheath thickness, charge density and electric field strength in the cathode.