Progress in Energy and Combustion Science最新文献

It is accepted that the thermoacoustic behavior of a given combustion system can be analyzed by investigating how its natural acoustic modes are perturbed by the flame dynamics. As a result, the resonance frequency and structure of the resulting thermoacoustic mode – understood as a perturbed acoustic mode – are slightly modified with respect to the natural acoustic mode counterpart. However, experimental evidence shows that the frequency of unstable thermoacoustic modes sometimes lies far away from the natural acoustic frequencies of the system under study. In many cases, this frequency cannot be associated with hydrodynamic or entropy-related instabilities. In recent years, the intrinsic thermoacoustic (ITA) feedback loop has been formally recognized as the responsible mechanism in some of those situations. Theory and devoted experiments have been developed that have enormously contributed to the understanding of the particular behavior of intrinsic thermoacoustic instabilities.

The present review encapsulates in a single theoretical framework the theory presented in the collection of today existing ITA papers, which spread through different cases of study regarding acoustic boundaries – anechoic, partially or fully reflecting – and geometries – duct flames, combustors composed by three coaxial ducts and annular configurations. Several examples are shown that summarize the most relevant results on ITA theory to this day. This review paper also gives special attention to the categorization of ITA modes, given the fact that there is no current agreement on the definition of an ITA mode: one example in this review paper explicitly shows that the proposed categorization methods can indeed be contradictory. Of high interest is also the review of papers illustrating the coexistence of thermoacoustic modes of acoustic and ITA nature, which in turn relate to the recently discovered exceptional points in the thermoacoustic spectrum. Additionally, this paper discusses the ‘counter-intuitive’ evidence that shows that ITA modes can be destabilized when acoustic dissipative elements are added into the system. Finally, it is shown how a single-mode Galerkin expansion may be able to model some ITA eigenfrequencies. This result is suggested in some recent works and is not obvious. The practical relevance of ITA modes in industrial combustion chambers of gas turbines is also discussed together with suggestions for future studies.

Direct air capture (DAC) is gathering momentum since it has vast potential and high flexibility to collect CO₂ from discrete sources as “synthetic tree” when compared with current CO₂ capture technologies, e.g., amine based post-combustion capture. It is considered as one of the emerging carbon capture technologies in recent decades and remains in a prototype investigation stage with many technical challenges to be overcome. The objective of this paper is to comprehensively discuss the state-of-the-art of DAC and CO₂ utilization, note unresolved technology bottlenecks, and give investigation perspectives for commercial large-scale applications. Firstly, characteristics of physical and chemical sorbents are evaluated. Then, the representative capture processes, e.g., pressure swing adsorption, temperature swing adsorption and other ongoing absorption chemical loops, are described and compared. Methods of CO₂ conversion including synthesis of fuels and chemicals as well as biological utilization are reviewed. Finally, techno-economic analysis and life cycle assessment for DAC application are summarized. Based on research achievements, future challenges of DAC and CO₂ conversion are presented, which include providing synthesis guidelines for obtaining sorbents with the desired characteristics, uncovering the mechanisms for different working processes and establishing evaluation criteria in terms of technical and economic aspects.

This paper intends to provide a comprehensive state-of-art review of recent progresses and to formulate perspectives on window-ejected fire plumes, originating from under-ventilated compartment fires (known as ‘Regime I’ fires). Various external boundary conditions are considered, as they contribute to the fire and plume dynamics, and as such affect decisions on fire prevention and firefighting. Hence this is an important fire combustion topic of both fundamental and practical significance. After discussing the general fundamentals, the paper focuses particularly on recent progresses on quantifying the ejected fire plume behavior: constrained by the presence of walls; at sub-atmospheric pressure (for fires at high altitudes) and under complex flow conditions caused by wind. Experiments, theoretical scaling analysis and basic models are reviewed. The key points cover systematically: the compartment fire evolution (and hence criteria for flame ejection through the window); flame interaction and merging behavior from two windows; air entrainment mechanisms and characteristic parameters (flame structure/dimensions, temperature profile and heat flux) of window-ejected fire plumes. Meanwhile, the limitations of present research and future challenges are also discussed.

The impact of ship emission reductions can be maximised by considering climate, health and environmental effects simultaneously and using solutions fitting into existing marine engines and infrastructure. Several options available enable selecting optimum solutions for different ships, routes and regions. Carbon-neutral fuels, including low-carbon and carbon-negative fuels, from biogenic or non-biogenic origin (biomass, waste, renewable hydrogen) could resemble current marine fuels (diesel-type, methane and methanol). The carbon-neutrality of fuels depends on their Well-to-Wake (WtW) emissions of greenhouse gases (GHG) including carbon dioxide (CO₂), methane (CH₄), and nitrous oxide emissions (N₂O). Additionally, non-gaseous black carbon (BC) emissions have high global warming potential (GWP). Exhaust emissions which are harmful to health or the environment need to be equally removed using emission control achieved by fuel, engine or exhaust aftertreatment technologies. Harmful emission species include nitrogen oxides (NO_x), sulphur oxides (SO_x), ammonia (NH₃), formaldehyde, particle mass (PM) and number emissions (PN). Particles may carry polyaromatic hydrocarbons (PAHs) and heavy metals, which cause serious adverse health issues. Carbon-neutral fuels are typically sulphur-free enabling negligible SO_x emissions and efficient exhaust aftertreatment technologies, such as particle filtration. The combinations of carbon-neutral drop-in fuels and efficient emission control technologies would enable (near-)zero-emission shipping and these could be adaptable in the short- to mid-term. Substantial savings in external costs on society caused by ship emissions give arguments for regulations, policies and investments needed to support this development.

The shift in the energy sector toward green resources makes fuel cells increasingly relevant as a supplier of green and sustainable energy. However, factors such as expensive catalysts, anodic poisoning, and fuel crossover reduce the lifetime and performance of the fuel cells, necessitating catalysis improvement. This review article presents the unique capabilities of metal chalcogenides (MC) as tailored catalysts, elucidating their synthesis, testing techniques, and performance evaluations. MC catalysts are matured via various physical and chemical methods to control their morphology, quantity, dimension, and size. Upon synthesis, the catalyst performance is quantified using three-electrode cells, followed by tests in fuel-cell prototypes. As anodic catalysts, MCs oxidize various fuels such as methanol, ethanol, urea, and impure H₂ at high current densities and low onset potentials, while hindering the poisoning species. As cathodic catalysts, MCs exhibit current values similar to that exhibited by their noble metal counterparts while reducing oxygen selectively in the vicinity of the fuels via four electron transfers at a wide range of potentials.

This is a comprehensive, critical, and pedagogical review of volumetric emission tomography for combustion processes. Many flames that are of interest to scientists and engineers are turbulent and thus inherently three-dimensional, especially in practical combustors, which often contain multiple interacting flames. Fortunately, combustion leads to the emission of light, both spontaneously and in response to laser-based stimulation. Therefore, images of a flame convey path-integrated information about the source of light, and a tomography algorithm can be used to reconstruct the spatial distribution of the light source, called emission tomography. In a carefully designed experiment, reconstructions can be post-processed using chemical kinetic, spectroscopic, and/or transport models to extract quantitative information. This information can be invaluable for benchmarking numerical solutions, and volumetric emission tomography is increasingly relied upon to paint a more complete picture of combustion than point, linear, or planar tools. Steady reductions in the cost of optical equipment and computing power, improvements in imaging technology, and advances in reconstruction algorithms have enabled a suite of three-dimensional sensors that are regularly used to characterize combustion. Four emission modalities are considered in this review: chemiluminescence, laser-induced fluorescence, passive incandescence, and laser-induced incandescence. The review covers the reconstruction algorithms, imaging models, camera calibration techniques, signal physics, instrumentation, and post-processing methods needed to conduct volumetric emission tomography and interpret the results. Limitations of each method are discussed and a survey of key applications is presented. The future of volumetric combustion diagnostics is considered, with special attention paid to the advent and promise of machine learning as well as spectrally-resolved volumetric measurement techniques.