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Heat barrier, the unrestricted increase in airplane or rocket speeds caused by aerodynamic heating, which—without adequate provisions for cooling the exposed surfaces—can lead to the loss of a hypersonic vehicle’s reusability, maneuverability, and cost-effectiveness. To date, indirect thermal protection methods, such as regenerative cooling, film cooling, and transpiration cooling, have proven to be complex and inefficient. Here, we propose a direct liquid cooling system to mitigate the heat barrier, utilizing a blunt-sharp structured thermal armor (STA)—a recently proposed material [36] to elevate the Leidenfrost point. The fiber-metal nano-/micro-STA withstands rigorous simulated hypersonic aerodynamic heating using butane and acetylene flames, ensuring effective temperature management in scenarios where flame temperatures reach up to 3000 °C—far exceeding the melting point of the STA substrate. Systematic cycling and durability tests further confirm the STA’s exceptional tolerance and robustness under extreme conditions. This work offers an efficient thermal protection method for hypersonic vehicles. Heat barriers pose significant challenges to hypersonic flight. Here, authors demonstrate a direct liquid cooling system using a structured thermal armor that elevates the Leidenfrost point, effectively managing temperatures up to 3000 °C.

Despite trees' critical role in regulating global warming, their direct transpiration‐induced cooling (TIC) effects in response to background climate at the global scale are currently not well understood by ground observations. We used the global observation‐based SAPFLUXNET data set to quantify the trees' TIC and investigate how hydroclimatic variables affect TIC across biomes. Results show that TIC (i.e., air temperature reduction (ΔT)) was highest in tropical rainforests (3.24°C m−2 d−1) and lowest in temperate grassland deserts (0.06°C m−2 d−1). ΔT was mainly driven by air temperature and vapor pressure deficit in warm‐wet biomes, while precipitation and soil water content (SWC) in hot‐dry biomes. Globally, we found an average critical SWC threshold (SWCcrit) for ΔT (0.37 m3 m−3), with higher values in warm‐wet and lower values in hot‐dry biomes. These findings provide novel insights into the role of trees in mitigating global warming and improving the hydroclimatic constraints in models. Plain Language Summary The fact that trees play an important role in mitigating global warming, yet we still don't completely understand their transpiration cooling (TC) under changing climate using ground data. This study used the first global sap flow database to find out the trees' TC and their hydroclimatic controlling mechanisms across different biomes. We found the highest TC in tropical rainforests and the lowest in temperate grassland deserts. Among the selected site variables, air temperature, and vapor pressure deficit were important drivers of TC in warm‐wet biomes, while precipitation and soil water content in hot‐dry biomes. The amount of soil water restricted TC in different ways across biomes, with higher threshold values in warm‐wet and lower values in hot‐dry biomes. These findings are likely to develop more integrated and effective climate mitigation and adaptation strategies and improve the model's representation of hydroclimatic constraints. Key Points Transpiration‐induced cooling is highest in tropical rainforests, moderate in temperate forests, and lowest in temperate grassland deserts The transpiration‐induced cooling is mainly driven by available energy in warm‐wet biomes, while water availability in hot‐dry biomes Globally, we found an average critical soil water content threshold (SWCcrit), with higher values in warm‐wet biomes and lower values in hot‐dry biomes

Transpiration cooling is a thermal management technique that utilizes the phase change of liquid coolant to effectively dissipate heat. Porous ceramic media play a crucial role in this cooling process by facilitating liquid transport and heat exchange; however, their intermittent capillary action and inter-grain phonon scattering significantly hinder rapid cooling. Here, we propose a strategy to create AlN-based nanofiber aerogel as a transpiration thermo-cooler, featuring vertically aligned channels and monocrystalline nanofibers by combining nanoengineering and multiscale structural assembly techniques. Benefiting from the unconstrained capillarity of aerogel channels, our thermo-coolers exhibit a fast liquid transport rate of up to 8.33 ± 0.026 mm s −1 , surpassing that of state-of-the-art porous media by one to two orders of magnitude. In addition, the enhanced phonon conduction properties of single-crystal AlN nanofibers enables thermo-coolers to achieve a fast cooling rate of 156.8 °C s −1 , outperforming advanced cooling materials by a factor of five and making them ideal for various thermal management applications. Cooling failure in porous ceramics under extreme heat flux is tackled by assembling monocrystalline AlN nanofibers into vertical channels, creating fiber aerogel thermo-coolers that enable rapid liquid transfer and efficient heat exchange.

Double-wall transpiration cooling (DWTC) represents a sophisticated cooling technology in contemporary aircraft engines; nevertheless, its internal cooling has encountered challenges such as crossflow effects. This paper proposes a new turbine blade cooling system with a bionic base, and conducts a comparative analysis using ANSYS Fluent software for traditional base-less schemes, as well as bionic base double-wall cooling schemes with in-line arrangements. A detailed comparison is made of the overall cooling performance and internal heat transfer characteristics of the new double-wall cooling system under different crossflow mass flow ratios ( CMFR = 0, 0.1, and 0.25) and impact jet Reynolds numbers ( Re i = 12500, 18750, and 25000). The findings of this study indicate that both the crossflow effects and the presence of biomimetic bases have a significant influence on the film performance. In the range of crossflow mass flow ratios from 0 to 0.25, different designs exhibit a similar trend: as the crossflow intensity increases, the Nu increases, primarily due to the increased film coverage enhancing heat exchange on the impingement cooling walls, leading to improved film cooling efficiency. Furthermore, the new cooling structure with biomimetic bases shows a noticeable reduction in drag effects from strong crossflow at high Re i and CMFR , resulting in lower pressure loss, with film cooling efficiency improving by 8%.

Hypersonic vehicles are subjected to extreme thermal environments with heat fluxes on the order of MW/m 2 during ascent and re-entry, posing significant challenges for thermal protection systems (TPS). Transpiration cooling, a bio-inspired active cooling method analogous to sweating, represents a highly efficient approach to TPS. This study experimentally investigates phase-change transpiration cooling using porous titanium alloy plates, aiming to elucidate the cooling mechanisms of titanium alloy/phase-change fluid systems under high heat flux conditions. An oxy-acetylene heating test platform was established to simulate megawatt-level thermal loads, and cooling experiments were subsequently conducted under heat fluxes of 1MW/m 2 and 2.5MW/m 2 . The results demonstrate that phase-change transpiration cooling provides excellent thermal protection. The front-side temperature of the sample remained stable between 200–300°C, while the back-side temperature did not exceed 60°C. This work provides key experimental data and theoretical support for the engineering application of porous titanium alloy/phase-change fluid transpiration cooling systems in the thermal protection of hypersonic vehicles.

With lightweight, multifunctional, and designable characteristics, porous/lattice structures have started to be used in aerospace applications. Porous/lattice structures applied in the thermal management technology of aerospace vehicles have attracted much attention. In the past few years, many related numerical and experimental investigations on flow, heat transfer, modelling methodology, and manufacturing technology of porous/lattice structures applied in thermal management systems have been widely conducted. This paper lists the investigations and applications of porous/lattice structures applied in thermal management technology from two aspects, i.e., heat transfer enhancement by porous/lattice structures and transpiration cooling. In addition, future developments and challenges based on the previous investigations are analyzed and summarized. With the higher requirements of thermal protection for aerospace applications in the future, thermal management technology based on porous/lattice structures shows good prospects.

Hydrogel sweat cooling is one of the leading areas in the study of multiphase heat transfer. In this study, the principles, applications, current research status, and future trends of hydrogel sweat cooling technology are comprehensively reviewed. By combing through and analyzing the relevant literature, the research progress in hydrogel sweat cooling is presented from the application perspective, including its use in electronic devices, buildings, and clean-energy facilities. The principle of each application is illustrated, the research status is established, and pros and cons are proposed. To provide inspiration for future research, the development trend is set out. Our literature review indicates that research on advanced hydrogels is the most promising research direction, including studies on the effect of environmental and indoor factors on sweat cooling performance through numerical, experimental, and theoretical means. Challenges for future research mainly include conducting hydrogel numerical analysis which can be experimentally verified, developing advanced hydrogels in a green way, and achieving the precise regulation of hydrogel control through intelligent methods. Interdisciplinary integration might be promising as well due to the fact that it can reveal the hydrogel sweat cooling mechanism from a different perspective. This study aims to promote multiphase cooling technology in exploring the application of hydrogels in energy utilization criteria.

The study of transpiration cooling is vital for the development of high-speed aircraft. In the current work, direct numerical simulation (DNS) is performed to investigate the impacts of wall transpiration on the boundary-layer oblique breakdown over a Mach 2 flat plate. The porous injection model is used to mimic the transpiration from the equally spaced circular pores. It has been observed from the numerical results that wall transpiration leads to the amplified growth rate of the imposed oblique mode waves, steady vortex waves, and other higher-harmonic waves. As a result, the occurrence of boundary-layer transition shifts upstream. Due to the presence of transpiration, the normal gradients of both streamwise velocity and temperature are decreased at the wall, which causes reduced skin friction and heat flux in the transpiration region. In addition, when upstream transpiration is present, reductions in skin friction and heat flux can also be observed within turbulent regions. This study provides insights into the DNS investigation on compressible boundary-layer natural transitions coupled with wall transpiration, and the results indicate that more systematic investigations addressing this problem are needed.

Transpiration cooling has seen a resurgence of interest for thermal protection in hypersonic flight, rocket engine liners, and gas turbines. To date, materials for transpiration cooling have been restricted to porous ceramic composites and sintered metal foams. Advances in additive manufacturing have enabled the creation of architected lattices, which have deterministic mesostructures. One such family of lattices are triply-periodic minimal surfaces (TPMS), which are continuous, analytically-defined, repeating 3D geometries. Additively-manufactured metal TPMS structures are already being studied for biomedical applications and it is proposed that they could offer several advantages for transpiration cooling as well: high surface area-to-volume ratio, pore inter-connectivity, and mechanical strength. In this work, the fluid flow behavior through a gyroid TPMS lattice is investigated through computational fluid dynamics simulation, using the lattice Boltzmann method. A comparison is made between ideal geometry and the as-printed geometry of a sample fabricated with laser powder bed fusion and characterized using x-ray computed tomography. The as-printed part matched the design porosity to within 1%, while the as-printed permeability was found to be 14.8% lower than that of the ideal geometry. The results of this research will assist in developing a methodology for the design optimization via performance simulation of these structures to meet fluid flow requirements for transpiration cooling applications.

Taking the advantages of porous ceramic (C/C-SiC composite) and transpiration cooling (Copper alloy), C/C-SiC-Cu composite was fabricated to attain a novel anti-ablation material for high temperature structure application of aerospace systems, via chemical vapor deposition and infiltration technologies. The effects of SiC content on microstructure, thermal conductivity, mechanical and ablation properties of C/C-SiC-Cu composite were investigated. As the SiC content increases from 0 vol% to 25.12 vol%, thermal conductivity, flexural strength and ablation resistance are improved by 84.99%, 78.76% and 53.68%, respectively. Thermal conductivity and thermal diffusivity exhibit proportional linear correlation with SiC content. The high SiC content could shorten the yield stage and increase the breaking strength during the flexural fracture process. When the SiC content is 25.12 vol%, the formation continuous and compact anti-ablation (SiO2) layer is formed by oxidizing and melting fluidity processes during oxyacetylene ablation test, and the protective layer covers the ablation surface and slows down the oxidative ablation, which combines with the transpiration cooling effect of copper alloy to further improve ablation resistance of C/C-SiC-Cu composite.