The investigation of shock compression in highly exothermic inorganic powder mixtures leading to reaction has been a subject of interest for several decades. In particular, understanding the processes occurring within the time scale of the high-pressure shock state, resulting in the formation of new materials and phases, has garnered significant attention. Chemical reactions in shock-compressed media are generally categorized based on their time scale: i) shock-induced chemical reactions occur in the shock front or shortly behind it (in the stress pulse) during the time scale of mechanical equilibration (<1 μs), and ii) shock-assisted chemical reactions occur on the longer time scale of bulk temperature equilibration (>10 μs) after the state of stress has been released. It is worth noting that a solid-state detonation wave involves a type of combustion with a supersonic exothermic front that accelerates through a medium, ultimately supporting the leading shock front. While extensive discussions have focused on shock-induced and shock-assisted reactions, as well as the solid-state detonation, certain questions regarding the possibility of i) shock-induced reactions occurring within the time scale of high-pressure shock state, and ii) chemical reactions occurring promptly enough after the shock wave to sustain a detonation wave (ultra-fast gasless reactions), remained unanswered. In this paper, we provide a brief review of shock compression of reactive heterogeneous media, with a particular emphasis on recent experimental studies. We critically address the chemical reactions occurring within these material systems and the underlying mechanisms, supported by in-situ and ex-situ experimental evidences. Specifically, our primary focus lies on the aluminum-nickel and the metal nitride-boron systems. Based on our analysis, we conclude that the shock-induced reactions can occur in the time scale of the propagated shock wave and can be explained by the mechanically induced thermal explosion phenomena. However, the observed phenomena so far cannot be attributed to solid-state detonation, since they cannot result in a self-sustained mode of shock wave propagation.
Thermal energy storage (TES) is increasingly important due to the demand-supply challenge caused by the intermittency of renewable energy and waste heat dissipation to the environment. This paper discusses the fundamentals and novel applications of TES materials and identifies appropriate TES materials for particular applications. The selection and ranking of suitable materials are discussed through multi-criteria decision making (MCDM) techniques considering chemical, technical, economic and thermal performance. The recent advancements in TES materials, including their development, performance and applications are discussed in detail. Such materials show enhanced thermal conductivity, reduced supercooling, and the advantage of having multiple phase change temperatures (cascade PCMs). Nano-enhanced PCMs have found the thermal conductivity enhancement of up to 32% but the latent heat is also reduced by up to 32%. MXene is a recently developed 2D nanomaterial with enhanced electrochemical properties showing thermal conductivity and efficiency up to 16% and 94% respectively. Shape-stabilized PCMs are able to enhance the heat transfer rate several times (3–10 times) and are found to be best suited for solar collector and PV-based heat recovery systems. Cascade and molten slats PCMs find their best applications in the thermal management of buildings and the power sector (concentrated solar plants). Microencapsulated, nanoPCMs and shape-stabilized PCMs effectively reduce the supercooling of hydrated salts. The recent trends of TES materials in various applications, including building, industrial, power, food storage, smart textiles, thermal management, and desalination are also briefly discussed. Finally, future research in advanced energy storage materials is also addressed in this study, which is intended to help create new insights that will revolutionize the thermal management field.
Transportation electrification is a promising solution to meet the ever-rising energy demand and realize sustainable development. Lithium-ion batteries, being the most predominant energy storage devices, directly affect the safety, comfort, driving range, and reliability of many electric mobilities. Nevertheless, thermal-related issues of batteries such as potential thermal runaway, performance degradation at low temperatures, and accelerated aging still hinder the wider adoption of electric mobilities. To ensure safe, efficient, and reliable operations of lithium-ion batteries, monitoring their thermal states is critical to safety protection, performance optimization, as well as prognostics, and health management. Given insufficient onboard temperature sensors and their inability to measure battery internal temperature, accurate and timely temperature estimation is of particular importance to thermal state monitoring. Toward this end, this paper provides a comprehensive review of temperature estimation techniques in battery systems regarding their mechanism, framework, and representative studies. The potential metrics used to characterize battery thermal states are discussed in detail at first considering the spatiotemporal attributes of battery temperature, and the strengths and weaknesses of applying such metrics in battery management are also analyzed. Afterward, various temperature estimation methods, including impedance/resistance-based, thermal model-based, and data-driven estimations, are elucidated, analyzed, and compared in terms of their strengths, limitations, and potential improvements. Finally, the key challenges to battery thermal state monitoring in real applications are identified, and future opportunities for removing these barriers are presented and discussed.
Flash boiling atomization is a promising approach to enhance spray atomization with internal energy as the driving force as well. Past investigations primarily focused on the morphologies and macroscopic characteristics of flash boiling sprays. Recently, with the advances in experimental techniques and the need in developing cleaner and more efficient combustion systems, thorough and detailed analyses were carried out on flash boiling atomization. This review article will introduce and discuss recent flash boiling advances using experimental approaches. This work will first discuss the gas-liquid, two-phase features in the nozzle and the impacts on the primary breakup of flash boiling sprays. Then, the characteristics of the external flash boiling spray plumes will be discussed with a dense vapor and sparse droplet feature. Furthermore, practical issues in adopting flash boiling atomization such as injector tip-wetting and spray wall impingement effects are covered in the flash boiling regime. Finally, practical applications of flash boiling atomization in combustors such as reciprocating internal combustion engines are presented. It is aimed that this review can provide an up-to-date summary of the current state-of-the-art of flash boiling atomizations and shed light on the future development of active flashing atomization techniques.
Recent discoveries and developments on the dynamic process of premixed turbulent spark ignition are reviewed. The focus here is on the variation of turbulent minimum ignition energies (MIET) against laminar MIE (MIEL) over a wide range of r.m.s. turbulence fluctuation velocity (uʹ) alongside effects of the spark gap between electrodes, Lewis number, and some other parameters on MIE. Two distinguishable spark ignition transitions are discussed. (1) A monotonic MIE transition, where MIEL sets the lower bound, marks a critical uʹc between linear and exponential increase in MIET with uʹ increased. (2) A non-monotonic MIE transition, where the lower bound is to be set by a MIET at some uʹc, stems from a great influence of Lewis number and spark gap despite turbulence. At sufficiently large Lewis number >> 1 and small spark gap (typically less than 1 mm), turbulence facilitated ignition (TFI), where MIET < MIEL, occurs; then MIET increases rapidly at larger uʹ > uʹc because turbulence re-asserts its dominating role. Both phenomena are explained by the coupling effects of differential diffusion, heat losses to electrodes, and turbulence on the spark kernel. In particular, the ratio of small-scale turbulence diffusivity to reaction zone thermal diffusivity, a reaction zone Péclet number, captures the similarity of monotonic MIE transition, regardless of different ignition sources (conventional electrodes versus laser), turbulent flows, pressure, and fuel types. Furthermore, TFI does and/or does not occur when conventional spark is replaced by nanosecond-repetitively-pulsed-discharge and/or laser spark. The latter is attributed to the third lobe formation of laser kernel with some negative curvature segments that enhance reaction rate through differential diffusion, where MIEL < MIET (no TFI). Finally, the implications of MIE transitions relevant to lean-burn spark ignition engines are briefly mentioned, and future studies are suggested.
Polymer electrolyte fuel cells, including acidic proton exchange membrane fuel cells (PEMFCs) and alkaline anion exchange membrane fuel cells (AEMFCs), are the types of the most promising high-efficiency techniques for conversion hydrogen energy to electricity energy. However, the catalysts’ insufficient activity and stability toward oxygen reduction reaction (ORR) at the cathodes of these devices are still the important constraints to their performance. So far, carbon black supported platinum (Pt/C) and its alloys are still the most practical and best-performing type of catalysts. However, the scarcity of Pt is highly challenging and the high price of commercial catalyst will continue to drive up the cost of both PEMFCs and AEMFCs. Moreover, the traditional carbon black support is susceptible to corrosion especially under electrochemical operation, itself inactive for ORR and weakly binding with Pt-based nanoparticles. In this review, the advanced carbons synthesized by various template methods, including hard-template, soft-template, self-template and combined-template, are systematically evaluated as low-Pt catalyst supports and non-noble catalysts. For the templates-induced carbon-based catalysts, this review presents a comprehensive overview on the carbon supported low-Pt catalysts from aspect of composition, size and shape control as well as the non-noble carbon catalysts such as transition metal-nitrogen-carbons, metal-free carbons and defective carbons. Furthermore, this review also summarizes the applications of low/non-Pt carbon-based catalysts base on the template-induce advanced carbons at the cathodes of PEMFCs and AEMFCs. Overall, the templates-induced carbons can show some perfect attributes including ordered morphology, reasonable pore structure, high conductivity and surface area, good corrosion resistance and mechanical property, as well as strong metal–support interaction. All of these features are of particular importance for the construction of high-performance carbon-based ORR catalysts. However, some drawbacks mainly involve the removal of templates, maintenance of morphological structure, and demetalation. To address these issues, this review also summarizes some effective strategies, such as employing the easily removed hard/soft-templates, developing the advantageous self-templates, enhancing the metal–support interaction by formation of chemical binds, etc. In conclusion, this review provides an effective guide for the construction of template-induced advanced carbons and carbon-based low/non-Pt catalysts with analysis of technical challenges in the development of ORR electrocatalysts for both PEMFCs and AEMFCs, and also proposes several future research directions for overcoming the challenges towards practical applications.
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