Chung K. Law

This study investigates the effects of ozone addition on autoignition-assisted hydrogen-air flames with detailed kinetics and transport. For homogeneous ignition, a critical temperature ( T c ) was identified that significantly influences the reaction pathways and ignition characteristics. It is shown that below T c , the system exhibits a distinct two-stage reaction process during autoignition, characterized by initial ozone decomposition followed by high-temperature hydrogen-oxygen reactions. Above T c , the two ignition stages merge, leading to drastically reduced ignition delay time—a small temperature difference near T c can result in a hundredfold reduction. For the auto-ignitive flames, similar transition in terms of flame speeds occurs near the critical temperature, for which the proposed scaling law based on Damköhler number holds for both conditions below and above T c . Comparative analysis of zero-dimensional (0D) and one-dimensional (1D) simulations reveals pronounced differences in the evolution of key species such as H 2 , H, HO 2 and O 3 . In 1D flames, transport processes lead to more efficient radical buildup and earlier ozone consumption compared to the 0D case. The spatial coupling of the H 2 diffusion zones with O 3 consumption zones above T c was found to enhance the overall combustion process. The effects of elevated pressure have also been illustrated. These findings underscore the critical influence of transport effects and subtle temperature variations on radical accumulation, reaction pathways, and flame dynamics in ozone-assisted hydrogen combustion.

This work integrates in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments, wire microcalorimetry experiments, CFD simulations, and density functional theory (DFT) calculations to elucidate the catalytic combustion kinetics of ammonia over platinum. In situ DRIFTS measurements revealed the formation and temperature-dependent evolution of reaction intermediates during high-concentration ammonia oxidation. Distinct spectral bands corresponding to N 2 O were observed upon the onset of oxidation, a phenomenon not accounted for in the very limited number of existing NH 3 -Pt kinetic mechanisms. To address this limitation, six elementary surface steps involving N 2 O on Pt were identified via DFT calculations, and the corresponding rate constants were obtained. On this basis, a thermodynamically consistent microkinetic model with 11 surface species and 43 elementary steps was developed within the mean-field approximation. CFD simulations using this model successfully reproduced the ignition temperature, heat release profiles, and product formations observed in microcalorimetric experiments. Sensitivity and surface species analyses further clarified the dominant elementary steps and surface coverage evolution, strengthening the mechanistic understanding of the underlying chemistry. This work provides fundamental insights into the microkinetics of catalytic ammonia combustion, particularly the formation and consumption pathways of N 2 O, and supports the advancement of predictive modeling for catalytic combustion systems. Novelty and significance statement This work advances the mechanistic understanding of catalytic ammonia combustion over platinum. In situ diffuse reflectance infrared Fourier transform spectroscopy revealed substantial formation of N 2 O during the oxidation of concentrated ammonia on Pt, a pathway that has been largely overlooked in the very limited kinetic models available in the literature. To address this gap, the missing N 2 O-related elementary reactions and their rate constants were identified and quantified using density functional theory calculations. Based on these results, a thermodynamically consistent microkinetic model comprising 11 surface species and 43 elementary reactions was developed within the mean-field approximation and successfully reproduced the microcalorimetric experimental data. By explicitly incorporating the formation and consumption pathways of N 2 O, the model captures key kinetic features of ammonia oxidation on platinum. These findings bridge an important gap in the mechanistic description of catalytic ammonia combustion and provide a reliable kinetic framework for the development of predictive models and the practical utilization of carbon-free ammonia in power generation.

For collision between two droplets with sufficiently large Weber numbers (We), regimes of coalescence (III), reflexive separation (IV), and stretching separation (V) after temporary coalescence have been identified in accordance with specific ranges of the impact parameter (B) and Ohnesorge number (Oh). Recognizing that prevention of droplet breakup is critical for various applications, here we discuss the mechanism governing the transition between the breakup regimes and experimentally study the influence of surfactants on the response in the various regimes. It is shown that, compared to the surfactant-free case, adding surfactant substantially delays the occurrence of regime IV, requiring notably larger critical We (Wec) for the transition between regimes of coalescence III and IV. By increasing the surfactant concentration (Φ) over the critical micelle concentration (CMC), Wec decreases although it is still significantly larger than that of surfactant-free cases, even when Φ is 40 times the CMC. Similar suppression with surfactant addition is also observed for the transition between regimes III and V, showing at least 45% larger B required to reach regime V as compared to the surfactant-free cases. Furthermore, by comparing the collision sequences with and without surfactant, with similar values of B, Oh, and We, the nondimensional maximum length of relaxation (Lm) in the surfactant-free cases is substantially larger than that with surfactant addition. The results demonstrate significant effects of surfactant addition in droplets on the suppression of further breakup after merging.

Biofuels are promising alternatives to fossil fuels due to diminishing reserves and increasing environmental concerns. This review focuses on recent progress in understanding the combustion kinetics of oxygenated biofuels derived from biomass. The review begins with fundamental concepts and research methodologies in reaction kinetics, intended as a primer for engineering researchers. Subsequently, kinetic studies from the past decade on typical oxygenated biofuels are summarized, including alcohols, fatty acid methyl esters (FAMEs), ketones, ethers, and carbonates. Emphasis is placed on the influence of different oxygenated functionalities and their positions within the molecule on combustion characteristics and reaction pathways. Distinct reaction patterns for each class are highlighted. Alcohols exhibit a characteristic unimolecular dehydration reaction. FAME kinetics are similar to long-chain hydrocarbons, with unsaturation significantly impacting low-temperature oxidation. Ketone oxidation is influenced by the formation of resonance-stabilized radicals, while straight-chain ethers demonstrate a unique double negative temperature coefficient (NTC) behavior. Carbonates, relevant to lithium-ion battery safety, have gained research attention and can undergo a distinctive reaction pathway identified as CO 2 elimination reaction. To advance predictive kinetic models for biomass-derived oxygenated fuels, several targeted research directions are essential. First, there is a critical need to expand experimental datasets that capture the combustion behavior of diverse oxygenated compounds, particularly under low-temperature conditions. This must be coupled with enhanced combustion diagnostics capable of resolving key reaction intermediates characteristic of oxygenated fuel oxidation. Second, detailed quantum chemical calculations and theoretical explorations of potential energy surfaces are required to accurately determine reaction rate parameters for oxygen-involved pathways, which are often determinant in fuel decomposition and pollutant formation. Finally, progress in model predictability will depend on the adoption of advanced computational methods, including automated mechanism generation for complex oxygenated structures, systematic optimization frameworks leveraging experimental data, and the incorporation of physics-informed artificial intelligence approaches tailored to oxygenated fuel chemistries.

BACKGROUND: The 21-item Fall Risk Index questionnaire (FRI-21) was developed to screen for fall risk in older adults. It showed great potential in assessing the fall risk in stroke population. However, no previous study investigated its reliability and validity in people with stroke in Hong Kong. AIM: This study aimed to translate FRI-21 to Chinese and investigate: 1) the FRI-21 scores between people with stroke and healthy older adults; 2) the test-retest reliability of the FRI-21 in people with stroke; 3) the convergent validity by correlated of the FRI-21 with Berg Balance Scale (BBS); 4) the predictive ability of FRI-21 on the fall occurrence in the 2 years follow-up; 5) the optimal FRI-21 cut-off score that distinguishes faller and non-faller among people with stroke in the 2 years follow-up; and 6) the ceiling and floor effects of the Chinese version of the FRI-21. DESIGN: Cross-sectional study. SETTING: University-based rehabilitation laboratory. POPULATION: In total, 57 people with stroke and 31 healthy older adults. METHODS: The FRI-21 test was assessed in people with stroke on Day1 and Day 2 (7 days after Day 1), and assessed in healthy older adults on Day 1 only. The BBS was also assessed in Day 1. RESULTS: The mean FRI-21 scores in subjects with stroke was 7.37. The FRI-21 demonstrated good inter-rater reliability (intraclass correlation [ICC] 0.74) and good test-retest reliability (ICC=0.798) in people with stroke. The FRI-21 scores demonstrated significant negative correlations with the BBS (r=-0.308). The FRI-21 score was found to be a significant predictor (OR 1.40 [95% CI 1.06-1.85], P=0.018) of fall in the 2 years of follow-up. The receiver operating characteristic curve analysis identified an optimal FRI-21 cutoff score of 7.5, showing an acceptable diagnostic power in distinguishing faller and non-faller among people with stroke (area under curve = 0.723, P=0.002), with moderate sensitivity (80.0%) and specificity (60.5%). Ceiling and floor effects are negligible. CONCLUSIONS: This study reflects the reliability and validity of the FRI-21 as self-administered tool for assessing fall risk in individuals aged 50 and over with stroke, and without cognitive impairments. A cut-off score of 7.5 was identified to distinguish faller and non-faller in people with stroke. The FRI-21 score was a significant predictor of fall in people with stroke. It effectively differentiates fall risk between people with stroke and healthy older adults. Future research should increase the sample size to enhance the generalizability of the findings. CLINICAL REHABILITATION IMPACT: Clinicians can use this tool to efficiently identify high-risk individuals among stroke survivors and implement targeted early interventions. This early fall risk screening tool allows healthcare providers to initiate preventive measures and intensive rehabilitation protocols for those at greatest risk, potentially reducing secondary complications, improving functional outcomes, and decreasing hospital readmission rates.

In this study, a theoretical analysis of the sensitivity coefficients of ignition delay times is performed by using eigenvalue analysis. Using H 2 /O 2 and CH 4 /H 2 /O 2 kinetic systems as examples, it is demonstrated that the analytical solutions achieve accurate predictions of the sensitivity coefficients of the ignition delay time as compared with the computational solutions. Furthermore, the temperature dependence of the sensitivity coefficients can also be predicted by the theory. For the H 2 /O 2 system, the sensitivity coefficients of the key elementary reactions can be predicted with high accuracy at wide ranges of temperature, pressure, and equivalence ratio. For the CH 4 /H 2 /O 2 system, due to the complexity of the kinetic interactions, while the sensitivity of some reactions with relatively low values can be less accurate, the key reactions with high sensitivity can all be accurately predicted. The comparison of theory with various sizes shows that high-dimensional analysis can achieve closer prediction with the same computational cost. The potential to calculate sensitivity coefficients based on the eigenvalue is noteworthy and encouraging.

Effects of bio-fuel diethyl ether (DEE) addition on the near-field spray tip characteristics of iso-octane have been investigated at different ambient temperatures experimentally by using high-speed imaging with a long-distance microscope. The near-field spray screening is focused on the early stage of ∼0.2 ms after starting the injection and the measuring length scale is less than 3.0 mm below the nozzle exit. Results show that, first, the DEE blending ratio and ambient temperature significantly affect the state of the previous injection residual fuel, which leads to four typical near-field spray tip patterns: mushroom, umbrella, helmet, and vapor hemisphere, as the liquid phase residual fuel is vaporized. Second, quantitative description of the near-field spray tip has been conducted. High ambient temperature and DEE blending ratio favor faster tip penetration because of the enhanced residual fuel vaporization and reduced axial obstruction. Finally, different near-field spray tip patterns resulting from the residual fuel state are shown as function of the ambient temperature and DEE blending ratio, followed by a conceptual scheme regarding the state of the residual fuel and its evolution after the start of injection, at different ambient temperatures and DEE blending ratios.

Light pollution has become an increasingly knotty environmental management problem, but little has been done to review and compare light pollution controls across the world. To address this research gap, a comparative review study has been undertaken. Among the light pollution laws of the most light-polluted regions, those pertaining to Shanghai, New York, Hong Kong, Seoul, London and Valletta were examined. We systematically evaluate the impact of legal systems, regulatory approaches and control parameters on light pollution regulation. The findings reveal that civil law jurisdictions, such as Shanghai and Seoul, typically adopt dedicated legislation while common law jurisdictions, like New York and London, often rely on bolt-on regulations to broader environmental laws. The study also finds that jurisdictions employing dedicated legislation and a metrics-based system offer a more comprehensive and preemptive solution to light pollution challenges. However, certain exceptions are noted, and the balance between regulatory certainty and flexibility is highlighted. The nuanced relationship between environmental protection and legal instruments is discussed, and the potential for unintended consequences of stringent regulation is acknowledged. The paper closes with a call for ongoing research and iterative regulatory reviews, emphasizing the need to incorporate scientific advancements and stakeholder interests into regulatory updates.

In this work, effects of ozone (O 3 ) addition on ethylene–oxygen (C 2 H 4 –O 2 ) mixtures are computationally studied through the explosion limit profiles. The results show that the addition of minute quantities of ozone (with a mole fraction of 0.06% in the oxidizer) shifts the explosion limit of the C 2 H 4 –O 3 –O 2 mixtures to the low-temperature regime. Further increases in the ozone concentration gradually strengthen the negative temperature coefficient (NTC) behavior at the second limit. That is because the explosion limit is primarily controlled by the ethylene ozonolysis reaction, and both the sensitivity analysis and chemical reaction rate perturbation method reveal specific kinetic reasons. Furthermore, it is shown that with the increasing equivalence ratio, the explosion limit curve with minute ozone addition rotates counterclockwise around a crossover point, while the explosion limit curve becomes complicated and the NTC behavior appears on the second limit with larger quantities of ozone addition. Furthermore, the effects of dilutions of nitrogen (N 2 ), argon (Ar), carbon dioxide (CO 2 ), and water (H 2 O) on the explosion limits are also studied. To elucidate the different wall elimination effects of different explosion limit regimes, the impacts of surface reactions of six radicals (H, O, OH, HO 2, H 2 O 2, and HCO) have been examined and the dominant radicals are found to be H and HO 2 . The H radicals significantly influence the first explosion limit, while the HO 2 radicals impact the entire explosion limit.

This study computationally investigates the auto-ignition delay times and explosion limits of H2/O2 and C1-C7 n-alkanes/O2 mixtures based on detailed chemical kinetics. Comprehensive ignition responses are consolidated through three-dimensional response surfaces of the ignition delay time as function of the system temperature and pressure, yielding the essential kinetic characteristics governing ignition. The intersection curves of the ignition surface with the cross-section perpendicular to the three coordinate axes are the characteristic response curves of the ignition delay time and explosion limit. Therefore, the ignition delay time and explosion limit are the changing characteristics of the three-dimensional ignition surface observed from different perspectives. Sensitivity analysis demonstrates that while the significant variations of the reaction pathways among different fuels are the primary reason of the wide spreading of the explosion limits curves for the various fuels in the low-temperature regime, their chain branching reactions tend to be similar in the intermediate temperature regime such that the explosion limit curves of different fuels are closely coalesced. Furthermore, in the high-temperature regime, the chain branching reactions of alkanes are primarily governed by small radicals, such as CH3, while it is still dominated by the H+O2=OH+O for H2.

-dodecane) as the representative fuel and the Redlich-Kwong equation of state (EoS) as the real gas description. It is demonstrated that the real gas description yields a shorter ignition delay time (IDT) compared with the ideal gas description, especially in low-temperature regimes which could encompass the negative temperature coefficient (NTC) phenomena and has a stronger dependence on the molecular volume than the attractive potential. The study further shows that high pressure facilitates low-temperature reaction pathways, where the compressibility factors of key reactants contribute to real gas effects. Moreover, the results suggest that accounting for real gas behavior leads to an increase in the formation of polycyclic aromatic hydrocarbons (PAHs), which, in turn, promotes soot generation.

Abstract Reduction of large combustion mechanisms is usually conducted based on the detection and elimination of redundant species and reactions. Reaction elimination methods are mostly based on sensitivity analysis, which can provide insight into the kinetic system, while species elimination methods are more efficient. In this work, the species‐targeted local sensitivity analysis (STLSA) method is proposed to evaluate the importance of species and eliminate non‐crucial species and their related reactions to simplify kinetic models. This paper comprehensively evaluates the effectiveness of STLSA across various combustion scenarios, including high and low‐temperature ignition and laminar flame speed, using diverse mechanisms like USC Mech II, JetSurf 1.0, POLIMI_TOT_1412, NUIGMech1.1 and so on. Comparisons with graph‐based methods, such as DRG and DRGEP, highlight STLSA's superior efficiency and accuracy. Moreover, STLSA is compared to species‐targeted global sensitivity analysis (STGSA), demonstrating significant computation cost savings and comparable model reduction capabilities. The study concludes that STLSA is a robust and versatile tool for mechanism reduction, offering substantial improvements in computational efficiency while maintaining high accuracy in predicting key combustion properties.

The catalytic total oxidation of propane over platinum was investigated experimentally and numerically at pressures of 1–7 bar and catalyst temperatures up to 700 K. A wire microcalorimeter was employed to determine the global rate parameters of the catalytic reaction within the kinetically controlled regime. For 1 bar pressure, the dissociative adsorption of C 3 H 8 on Pt and its subsequent decomposition were modeled as two lumped steps based on global reaction parameters. A detailed and thermodynamically consistent catalytic mechanism was constructed by incorporating these lumped steps with an existing atmospheric-pressure H-C 2 elementary reaction model. Two-dimensional CFD simulations using the developed global and detailed reaction mechanisms closely reproduced the measured heat release rates . The intricate dependence of catalytic ignition and reactivity on pressure was further elucidated. Ignition temperatures were found to be linearly correlated to pressures, due to the weaker net adsorption of oxygen compared to that of propane, which progressively aggravated at higher pressures and in turn hindered ignition. More importantly, a non-monotonic pressure dependence of the C 3 H 8 catalytic reactivity on Pt, which gradually diminishes with increasing temperatures, is reported for the first time. The temperature range of this non-monotonic behavior (< 650 K) is of special importance for part-load and idling operations of gas turbines using hybrid hetero-/homogeneous combustion approaches and for normal operations of recuperative microreactors . Thus, this work provides key information for the design and optimization of such devices utilizing Pt as catalyst.