This study presents spatially-resolved temperature measurements in swirl-stabilized spray flames under elevated pressures (1-3.5 bar) using nonlinear two-line atomic fluorescence (NTLAF) thermometry. A comprehensive calibration process was established, and the calibration parameters of NTLAF were obtained under different pressures. A comparison was performed between two calibration strategies: (1) applying calibration parameters obtained at each pressure (1-3 bar) versus (2) utilizing atmospheric pressure (1 bar) calibration parameters for all pressure conditions. Comparative analysis revealed that employing 1 bar parameters introduced an overestimation (<70 K) in the mean temperature of the laminar methane flame across the tested pressure range. This finding suggests that while pressure-specific calibration parameters are preferable for optimal accuracy, atmospheric calibration parameters remain viable for applications with moderate precision requirements or challenging calibration environments. Finally, these two strategies were successfully applied to analyze temperature distributions in swirl spray combustion systems at elevated pressures. The error of utilizing atmospheric calibration parameters is <83 K, and flame characteristics were also well captured, demonstrating the methodology's capability for detailed thermal analysis in practical combustion configurations.

This study investigates the effects of coflow preheat temperature and elevated pressure on the structure of turbulent nonpremixed syngas jet flames representing conditions relevant to the integrated gasification combined cycle (IGCC) combustion systems. Experiments were conducted using the KAUST high temperature and pressure duct (HTPCD). The study begins by establishing a stability curve for different nitrogen dilution ratios versus blowoff jet velocity, revealing a nonlinear relationship between nitrogen content in the jet and blowoff velocity; the blowoff velocity decreases from approximately 76 m/s for undiluted syngas to approximately 32 m/s at 45% nitrogen dilution. Selected flame conditions, with varying jet velocities (25-45 m/s) and nitrogen content (15-30%), are analyzed under four different pressure and coflow temperature conditions: 1 bar at 295 K, 1 bar at 373 K, 5 bar at 295 K, and 5 bar at 373 K. Direct flame imaging and OH-PLIF techniques are used to examine the appearance, structure, OH corrugation, and thickness of the flame. The results demonstrate that flames with higher jet velocities are more significantly affected by elevated coflow temperatures, resulting in shorter luminous flame lengths and increased corrugation. In contrast, for lower jet velocities (25 m/s), luminous flame length and corrugation are largely unaffected by preheat temperature, owing to a competing reduction in Reynolds number caused by the lower gas density at elevated temperatures. Increasing the pressure to 5 bar enhances the corrugation of the flame in all conditions and reduces the visible length of the flame, regardless of the conditions of the jet. Furthermore, increasing either jet velocity or nitrogen dilution ratio decreases the thickness of the OH layer, although this effect diminishes in high-pressure flames. Coflow preheating consistently increases OH layer thickness (from approximately 1.4 mm to 1.8 mm at atmospheric pressure), while elevated pressure reduces it (from approximately 1.8 mm at 1 bar to approximately 1.2 mm at 5 bar). Higher coflow temperatures also increase the thickness of the OH layer regardless of pressure. One-dimensional (1-D) counterflow diffusion flame simulations provide chemical-kinetic support for the observed experimental trends, particularly the widening of the OH distribution at higher air preheat temperatures.

This paper presents an experimental investigation of how jet forming cap geometry influences hydrogen jet behavior in a prototype low-pressure direct injection (LPDI) system. The principal novelty of this work lies in the simultaneous characterization of internal inter-cap pressure dynamics and external jet morphology across a systematically varied set of cap configurations (1 to 7 orifices) under constant total discharge area, an approach that isolates geometric effects from mass flow rate influences and has not been reported previously for LPDI hydrogen injectors. High-speed schlieren imaging, microscopic backlit visualization, and direct piezoelectric cap pressure measurements were employed concurrently. Results show that increasing orifice number transitions the jet from a concentrated high-momentum plume to multiple smaller, diffusive jets, enhancing mixing at the expense of penetration. Intermediate configurations (2-4 holes) achieved more stable cap pressures and faster jet establishment. For the 1-hole cap, empirical correlations between normalized Mach disk height and diameter and the square root of the cap-to-ambient pressure ratio were derived and validated against classical compressible flow theory (R-2 > 0.94). These findings provide both fundamental understanding and practical design guidance for hydrogen injector optimization in efficient, low-emission combustion engines.

Ammonia, a carbon-free compound, holds significant potential as a fuel in internal combustion engines. Recent research has revealed the limitations of gaseous ammonia use and highlighted the need to inject it in the liquid phase at higher pressures into the combustion environment to mitigate these shortcomings. This study focuses on investigating the flash-boiling effects of high-pressure liquid ammonia spray in a constant volume combustion chamber (CVCC) using a hollow-cone gasoline direct injection injector. Based on high-speed Schlieren and shadowgraph imaging data, the spray characteristics under varying ambient conditions are characterized. In particular, parameters such as the spray cone angle, penetration length, and plume ratios are studied to characterize the transformation of liquid ammonia spray into different spray regimes, characterized by flash-boiling phenomena. Furthermore, this work establishes a correlation between an increase in the ambient temperature of the CVCC and the transition into different regimes, as evidenced by the characteristics of the spray plume. Furthermore, the cooling effects of high fuel injection pressures at elevated CVCC ambient pressures are investigated to study the impact of pressure differentials on the drop in temperature caused by the injection of liquid ammonia. The findings of this study can serve as a foundation for the development of high-pressure liquid ammonia direct injection techniques, leveraging the rapid flash-boiling behavior of liquid ammonia.

A double-swirler ammonia/methane co-flame burner was developed to enhance flame stability and reduce NOx emissions in ammonia combustion. In this system, a premixed ammonia/air mixture is introduced through the inner swirler, while a lean premixed methane/air flame from the outer swirler stabilizes the ammonia/air flame. Experimental results highlight that the Reynolds number of the outer methane/air flame strongly affects flame stability and NOx emissions under fuel-lean ammonia/air conditions. Higher Reynolds numbers increase the fuel supply and combustion power of the methane/air flame but paradoxically destabilize the flame and lead to higher NOx emissions. Numerical simulations reveal the mechanisms underlying these observations. While higher Reynolds numbers increase energy and turbulence in the outer flame, they also intensify shear layer dynamics and turbulence-induced flame stretch. These effects promote localized flame quenching, disrupting the flame structure despite the additional energy in the system. Local extinction of the outer flame at high Reynolds numbers allows unburned methane/air to mix with the inner ammonia/air stream. This mixing raises the concentration of H and OH radicals in the reaction zones, promoting NO formation via the HNO pathway (e.g., HNO + H = NO + H2). To minimize NOx emissions in the double-swirler coflame burner, preventing local extinction and limiting partial mixing between the methane/air and ammonia/air streams are essential.

Ammonia (NH3) is a promising carbon-free fuel for future combustion systems, but its practical application is limited by its low reactivity and high NO emissions. To address these challenges, a stratified dual-swirler burner is developed, in which a highly reactive outer CH4/air swirl flame is used to stabilize a low-reactivity inner NH3/air flame and control NO emissions. This configuration establishes spatial reactivity stratification, allowing control over flame stabilization and fuel oxidation. As a result, ammonia conversion is enhanced while NO formation is effectively suppressed. Experiments and large-eddy simulations (LES) are conducted to investigate the coupled effects of inner and outer equivalence ratios and Reynolds number on flame structure and NO emissions. LES and OH/NO-PLIF diagnostics reveal three combustion regimes as the inner ammonia equivalence ratio increases: a methane-supported regime under ultra-lean conditions, a self-sustained NH3 flame near stoichiometric conditions, and a triple-flame regime under fuel-rich conditions. The results show that NO emissions are strongly governed by the outer methane stabilization flame. At a low outer equivalence ratio of 0.7, a weakened shear-layer flame leads to frequent local extinction, enhancing mixing between NH3 and CH4-derived hot products and radicals and promoting NO formation through HNO-related pathways. Increasing the outer equivalence ratio to 0.75 strengthens the shear-layer reaction zone, suppresses local quenching, weakens HNO-driven NO formation, and significantly reduces NO emissions. Under fuel-rich inner-stream conditions, oxygen deficiency further limits fuel-N conversion, resulting in NO emissions below 200 ppm (15% O2) while maintaining complete NH3 conversion. These results demonstrate that controlling outer-flame reactivity provides an effective pathway to regulate flame stabilization, shear-layer mixing, and NO chemistry in stratified NH3/CH4 swirl flames, offering a physically grounded strategy for low-NO ammonia combustor design.Novelty and significance statement: This work establishes a mechanistic control strategy for stratified NH3/CH4 dual-swirl combustion by regulating the reactivity of the outer methane stabilization flame. A highly reactive outer CH4/air flame stabilizes a low-reactivity inner NH3/air flame, forming a spatially stratified reaction system that enables stable ammonia oxidation and low NO emissions. The central contribution of this study is to reveal how the outer-stream equivalence ratio, Φout, modifies shear-layer extinction, flame–flame interaction, and dominant NO formation pathways. By combining OH/NO-PLIF measurements with large-eddy simulations, the study shows that increasing Φout strengthens the shear-layer reaction zone, suppresses local flame quenching, weakens HNO-mediated NO formation, and significantly reduces NO emissions while maintaining complete NH3 conversion. These findings establish a direct link between outer-flame reactivity, flame stabilization, and NO chemistry, providing physically grounded guidance for low-NO and fuel-flexible ammonia combustor design.

Repurposing existing steam methane reformer (SMR) units for ammonia (NH3) cracking has been proposed as a cost-effective and rapid route to low-carbon hydrogen (H2) production. However, the durability of reformer alloys in NH3-rich environments remains a significant concern. Consequently, protective surface coatings capable of resisting nitridation are essential for enabling long-term operation under these conditions and minimizing the need for major modifications to existing units. This study evaluates the role of aluminization and pre-oxidation in forming a protective Al2O3 coating on an HP40 alloy and identifies the key processing parameters governing oxide layer quality and stability. After aluminization, pre-oxidation was performed at 1000 degrees C with heating rates of 2, 8, 12, and 20 degrees C/min, and soaking times of 1, 2, and 24 h to assess their influence on scale growth and morphology. Results show that the interplay between heating rate and soaking time strongly affects Al2O3 formation, with a 2 degrees C/min heating rate and 1-h soaking time producing the most adherent and protective coating. This optimized aluminization, followed by a pre-oxidation procedure, significantly suppresses nitrogen ingress during high-pressure NH3 cracking, thereby improving the alloy's nitridation resistance compared with the base material. The findings provide a complementary understanding of tailoring aluminized diffusion coatings through controlled pre-oxidation to enhance material durability in NH3-rich, high-temperature, and highpressure environments.

Hydrodynamic cavitation (HC) reactors are increasingly applied in the remediation of organic pollutants in water, leveraging intense shear and hydroxyl radical (OH•) generation to accelerate degradation processes. However, scaling up HC devices remains a challenge in environmental engineering due to poorly understood effects at different scales of operation. Vortex-based HC (VDs) provide superior cavitation efficiency compared to traditional orifice and venturi devices and therefore this study examines the scale-dependent generation of OH• in VDs. Coumarin dosimetry was employed as the quantification method for OH• generation. Available published experimental datasets were analysed: (i) varying inlet pressures from 100 to 400 kPa for throat diameters of 6 and 12 mm, and (ii) throat diameters ranging from 6 to 38 mm (nominal capacities of 5 to 200 L/min) at a fixed pressure drop of 280 kPa. After normalization of the available data, seven machine learning (ML) models were trained to establish relationships between operating conditions and OH• generation performance. eXtreme Gradient Boosting (XGB) and artificial neural networks (ANN) outperformed the others, with higher R2 and lower RMSE. After using SHAP interpretation, these two models were used to elucidate scale effects on both radical yield and energy efficiency, resulting in actionable design guidelines for VDs at different scales. By combining experimental dosimetry with predictive ML, this work advances the fundamental understanding and practical implementation of cavitation-based advanced oxidation technologies, particularly for efficient and energy-optimized treatment of organic pollutants in wastewater.

An innovative coaxial dual-swirl combustor was developed to address the challenges of combustion instability and NOx emissions inherent to ammonia combustion. The combustor employs an inner swirler supplying a premixed ammonia/air stream and an outer swirler generating a premixed methane/air flame that stabilizes the inner ammonia flame. Combined large-eddy simulations (LES) and planar laser-induced fluorescence (PLIF) measurements were conducted to systematically examine the effects of the inner-stream equivalence ratio and outer-stream Reynolds number on NO emissions. The results demonstrate a marked reduction in NO emissions for stoichiometric to fuel-rich ammonia/air flames, while lean flames near the blowout limit exhibit strong sensitivity of NO emissions to the outer-stream Reynolds number. LES and PLIF analyses reveal that flame-flame interactions in the shear layer between the two flames govern this behavior. Depending on the inner equivalence ratio, merged single reaction zone, distinct twin reaction zones or triple-flame structures form, altering radical concentrations and NO formation pathways. The central recirculation zone (CRZ), originating from vortex breakdown, also plays a key role in stabilizing the flames and oxidizing residual fuel. Hot gases from the outer flame recirculate into the inner ammonia stream, promoting complete combustion similar to the Rich-Quench-Lean (RQL) concept. Overall, NO emissions are primarily governed by the intensified flame-flame interactions. At higher outer-stream Reynolds numbers, lean flames (Phi NH3 = 0.4) exhibit enhanced NO formation via the HNO pathway, driven by local flame extinction, partial mixing of methane and ammonia through extinction holes, and subsequent downstream oxidation. Near-stoichiometric flames show reduced NO emissions due to dilution and radical pool modification, without evidence of local extinction. In contrast, fuel-rich flames (Phi NH3 = 1.4) exhibit effective de-NOx reduction with only moderate sensitivity to the Reynolds number, owing to the robust triple-flame structure. This study provides critical insights into flame-flame interactions and NOx formation in ammonia/methane dual-swirl flames, offering a pathway to more stable, low-emission ammonia combustion technologies and advancing the practical deployment of ammonia as a carbon-free fuel.

This paper presents an experimental methodology for quantifying internal jet-forming cap pressure losses in hydrogen direct-injection systems through non-intrusive analysis of external near-nozzle under-expanded jet structures. Based on a customized single-hole gasoline direct injection injector with synchronized high-speed schlieren imaging, microscopic shadowgraphy, and needle lift measurements, we establish empirical correlations between observable Mach disk dimensions and upstream flow conditions across pressure ratios from 10 to 96. Key contributions include: (1) needle lift stabilizes at similar to 60 mu m, independent of injector driving parameters; (2) a validated momentum-based rate of the injection method achieving nozzle discharge coefficient characterization of similar to 0.9, demonstrating excellent agreement with needle-lift-derived theoretical predictions; (3) the finding that Mach disk stabilization time scales universally with needle actuation time, independent of pressure ratio, indicating that shock structure development is governed by the establishment of boundary conditions rather than local acoustic phenomena; (4) non-intrusive quantification of inner cap pressure losses yielded a value range of xi = 0.729-0.756 using empirical Mach disk scaling laws; (5) identification of self-similarity establishment within 0.2 ms providing critical time scales for mixture formation modeling. These findings resolve a key disconnect between classical jet theory and practical injector performance, providing a systematic framework for diagnosing internal flow restrictions from external optical measurements, a capability essential for hydrogen injector optimization where direct internal instrumentation is impractical.

Multiphase flows play a central role in many natural and engineered systems, with applications ranging from chemical processes to biological environments. If the two phases involved are gas and liquid, modelling is often based on computational fluid dynamics, using methods such as the Volume-of-Fluid (VoF) or the Level-Set. While the latter are particularly effective at capturing the evolution of complex interfaces, the computational time required is often prohibitive for practical applications such as the optimisation and control of large-scale systems. Neural Operators (NOs) have emerged as a powerful tool to predict the evolution of partial differential equations. Among NOs, Fourier Neural Operators (FNOs) are particularly effective with fluid dynamic problems. This work describes the application of FNOs to predict two-dimensional multiphase interfaces from VoF simulation data. Two benchmark cases are studied: an autoregressive breaking-of-a-dam simulation and a time-conditioned signed-distance surrogate for 1000 randomly generated falling-liquid configurations used to test geometric generalisation. On the first test, the mean square error on the volume fraction ranges from approximately 1e-6 to 1e-5, while for the falling liquid blobs validation set, the surrogate attains a coefficient of determination of 0.95, a mean absolute error of 1.69 grid cells, and a liquid-volume conservation error of less than 2%. The two benchmarks demonstrate FNOs’ ability to accurately simulate complex systems with computational time 3 to 4 orders of magnitude lower. These results support the use of operator learning as a fast predictive component for future digital-twin, monitoring, or control-support workflows.

While Spray and combustion characteristics have been extensively characterized at low ambient densities (14.8-22.8 kg/m(3)), limited understanding exists under ultra-high ambient density (similar to 50 kg/m(3)) representative of high-pressure or supercritical combustion conditions, particularly with multiple injection strategies. This knowledge gap arises from experimental challenges in controlling extreme thermodynamic conditions. The present study investigates the spray and combustion characteristics of real fuels and their surrogates operated with single, double, triple, and quadruple injection strategies under ultra-high ambient density conditions. This work is motivated by recent advances in isobaric combustion, where maintaining constant pressure combustion through multiple injections has shown improved efficiency over conventional diesel combustion. A high-pressure constant-volume combustion chamber (CVCC) capable of achieving pressures up to 300 bar was employed to reproduce engine-relevant isobaric combustion conditions. The study focused on achieving vessel pressure of 150 bar with ambient temperature of 1000 K, corresponding to ultra-high ambient bulk density of 50 kg/m(3). A high-speed chemiluminescence imaging is employed to analyze the combustion characteristics, examining parameters such as ignition delay time (IDT), mixing period, flame natural luminosity (NL), rate of heat release (ROHR), and flame lift-off lengths (FLOL). Conventional gasoline and diesel were used alongside fuel surrogates, iso-octane and n-heptane, to emulate the behavior of real fuels. The findings indicate that the variations in IDTs among the tested fuels are negligible, whereas increasing the number of injections significantly prolongs the mixing period, nearly doubling from single to quadruple injections. The maximum FLOL decreases by over 20%, indicating earlier flame stabilization under multiple-injection operation. Diesel exhibited the longest effective combustion duration, exceeding other fuels by approximately 0.25 ms across all injection strategies, indicating diffusion-controlled combustion. Among the surrogates, n-heptane showed strong ROHR sensitivity to injection phasing under multiple injections, reflecting its pronounced low-temperature chemistry. Comparisons between real and surrogate fuels revealed that simplified surrogates cannot fully reproduce the ROHR and soot-formation behavior of real fuels under multiple injection strategies. Overall, this work provides insight into the fundamental understanding of multiple-injection combustion at high pressures and underscores the need for complementary numerical simulations to further elucidate the complex mechanisms governing high-pressure combustion.

This study presents an experimental analysis of a high-flow direct injection hydrogen injector for hydrogenfueled internal combustion engines. High-speed schlieren imaging and shadowgraphy techniques were employed to investigate the transient dynamics and near-nozzle under-expanded jet structures across a range of injection parameters and ambient pressure conditions. Rate of injection measurements demonstrated that increasing injection pressure and duration directly enhance injection rates. The characteristic injection profiles exhibited transient behavior, characterized by rapid initial peaks, subsequent declines, and secondary rises. Jet visualization revealed that elevated injection pressures significantly increased jet penetration and projected area, accompanied by pronounced plume interactions and expansion waves, particularly under reduced ambient pressure conditions. Time-resolved jet penetration correlated with jet momentum and density, adhering to a single proportionality constant across multiple operating conditions. Near-nozzle visualization identified a barrel-shaped under-expanded jet structure with a distinct Mach disk, the transient height of which varied proportionally with injection pressure. Pressure measurements within the jet-forming cap, derived via empirical correlation, uncovered significant pressure losses across the inner needle valve. Notably, increasing upstream injection pressure reduced the pressure loss coefficient across the inner-cap needle while inducing localized temperature drops in the inner-cap region. These findings underscore the critical role of injector jet-forming cap design and operating parameters in optimizing hydrogen distribution for combustion efficiency. Future research will focus on the direct experimental quantification of inner-cap pressure and temperature to further validate the theoretical models underpinning these findings.

Rapid industrialization worldwide has led to the accumulation of diverse environmental pollutants in water, air, and soil, posing significant threats to the earth’s ecological balance and making environmental remediation an urgent priority. Among the various remediation techniques, ultrasound-assisted advanced oxidation processes (AOPs) have emerged as a promising approach, leveraging the unique effects of acoustic cavitation to enhance pollutant degradation in water. By generating reactive oxygen species (e.g., hydroxyl radicals) through bubble collapse and improving mass transfer, ultrasound amplifies the efficiency of AOPs using catalysts, photo-mediated processes, and chemical reagents. This review synthesizes advancements in ultrasound-assisted AOPs over the past decades, including mechanism of the reactions and novel hybrid systems, offering a global perspective on their potential for large-scale environmental remediation. We systematically explore the mechanisms of ultrasound enhancement, the types of AOPs integrated with ultrasound, and their applications in degrading persistent pollutants. Additionally, we analyze the underlying synergistic effects and discuss current challenges, and future directions for hybrid technologies based on ultrasound for efficient water treatment

Soot formation in methane reforming reactors can deactivate catalysts and obstruct flow. This study applies an inverse diffusion flame (IDF), representative of such reactors, to decouple chemical and physical effects on soot size evolution. Combined light extinction and angular light scattering with numerical simulations were applied to quantify the soot volume fraction (fv), primary particle diameter (dp), number density (np), and surface area per unit volume (S). Physical effects were examined by varying pressure (3–5 bar) in nitrogen (N2) diluted flames, while chemical effects were assessed by comparing N2 and carbon dioxide (CO2) dilution at 5 bar. fv increases with pressure, and the soot region moves toward the flame centerline with both height and pressure. CO2 dilution suppresses fv by ∼ 62% and reduces S by ∼ 40% relative to N2 at 5 bar, indicating restricted surface growth. dp increases with both pressure and height, reaching ∼30 nm. A pronounced rise in dp for the N2-diluted flame at 4 bar indicates a possible transition in pressure-dependent growth. Simultaneous dp increase and np decrease suggests particle coalescence, whereas larger dp values at higher pressures confirm enhanced surface growth. Numerical simulations reproduce the experimental trends despite overpredicting dp. While predicted temperature differences at the peak fv locations remain small, CO2 dilution decreases calculated acetylene (C2H2) concentration, and reduces peak H radical concentration by >50%, identifying chemical effects as the dominant soot-suppression mechanism. At elevated pressure, the dominance of the Hydrogen-Abstraction-Carbon-Addition (HACA) mechanism relative to the adsorption of polycyclic aromatic hydrocarbons (PAHs) further enhances the impact of H-radical concentration. These results offer insights into soot size evolution under different conditions in IDFs and enable validation of soot models based on a hybrid method of moments in methane-reforming applications.Novelty and significance statement This study presents the first in situ characterization of soot size evolution in methane inverse diffusion flames at elevated pressures under CO2 and N2 dilution, conditions relevant to methane reforming. By combining light extinction and angular scattering with numerical predictions of temperature, chemical species, and soot growth rates, this study decouples chemical (diluent-driven) and physical (pressure-driven) effects on soot volume fraction and primary particle size. A transition in particle growth is identified at 4 bar. The competing effects of CO2 dilution and pressure provide insights into their roles in soot formation and growth. Pressure enhances surface growth, whereas increasing height promotes soot coalescence. CO2 dilution reduces H and C2H2 concentrations, suppressing HACA-driven surface growth and decreasing soot volume fraction (by 62%) and particle size. These fundamental insights provide guidance for soot mitigation strategies in industrial methane reforming reactors.

Ammonia presents itself as a high-hydrogen dense and carbon-free alternative for industrial heating, power generation, and transportation. Nevertheless, the challenges of its low flame speed and elevated NOx (nitrogen oxides) emissions pose significant challenges in combustor applications. This study investigates a novel two-stage burner employing a radial injection staging technique and explores various NOx reduction strategies for hydrocarbon-assisted ammonia flames. These strategies include premixing, fuel staging, balanced fuel staging, air staging, and sequential premixing. The focus is on LPG (liquid petroleum gas)-stabilized ammonia flames. The experiments are conducted at a constant thermal input of 20 kW (10 kW LPG + 10 kW NH3), with global equivalence ratios ranging from 0.7 to 1.4. This approach aims to provide valuable insights into the effectiveness of different staging strategies for NOx reduction in ammonia combustion. Experimental analysis is undertaken to ascertain the flame stabilization, flame temperature and its reaction zone, intermediate species and major emissions like NOx and NH3 of the burner. Staging configuration strongly influenced flame stabilization, heat release distribution, and thermal field, with downstream-shifted combustion zones lowering peak temperatures and NOx formation. Among the tested strategies, fuel staging and sequential premixing consistently achieved the greatest NOx reduction across the entire operating range compared to the premixed baseline, without compromising flame stability. Chemical kinetics analysis further reveals the dominant NO formation pathways, highlighting the key roles of HNO and NHi radicals. Additionally, this analysis helps identify dominant reaction routes and the role of intermediate species in NO formation and reduction processes. The combined experimental and kinetic insights provide a mechanistic basis for optimizing staged combustion of ammonia-hydrocarbon blends for lower NOx emissions.

This study investigates the effect of pressure on soot formation in turbulent oxy-fuel methane jet flames using Planar Laser-Induced Incandescence (PLII) calibrated using Light Extinction Measurements (LEM). The bulk jet velocity was held constant while the pressure was increased from 1 bar (Re = 3500) to 3 bar (Re = 10,600) and 5 bar (Re = 17,700) inside our new high-pressure vessel. DSLR imaging revealed increased flame luminosity and reduced flame length with rising pressure, consistent with previous literature. Planar soot volume fraction (SVF) measurements showed a significant increase in soot levels with pressure, with the maximum mean SVF increasing by factors of 7 and 14 at 3 bar and 5 bar, respectively. The intermittency analysis indicated a more continuous soot field at elevated pressures, while the Probability Density Function (PDF) analysis demonstrated a shift toward higher soot concentrations with reduced variability. Radially and volumetrically integrated SVF further confirmed a pressure dependence, with total soot loading (volume-integrated SVF) showing a P2.1 pressure dependence. However, when normalized by the fuel mass flow rate, this dependence weakens to show almost linear behavior, suggesting that both pressure and additional carbon input contribute to the observed trends. These findings provide new insights into the pressure-driven mechanisms governing soot formation in high-pressure turbulent oxy-fuel flames and provide dataset for validating numerical models.

Oxidative desulfurization (ODS) technology has attracted increasing attention as a practical approach for producing cleaner fuel oil. This study synthesized a novel phosphomolybdic acid (PMoA) based mesoporous catalyst with a high density of oxygen vacancies using a facile one-pot method. The role of oxygen vacancies in the ODS mechanism was detailed and proposed through density functional theory (DFT) simulation. The ODS catalytic performance of different model oil compounds over the series of catalysts was systematically evaluated. Remarkably, dibenzothiophene (DBT) achieved a desulfurization efficiency of 99.5% within 10 min at 60 degrees C and an O/S ratio of 3 in a batch reactor using hydrogen peroxide (H2O2) as the oxidant. The ODS reactivity order followed DBT > BT (benzothiophene) > 4,6-DMDBT (4,6-Dimethyldibenzothiophene). Furthermore, the ODS of Arabian Extra Light oil (AXL) was investigated under optimized conditions using an extraction-adsorption process. Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) and Nuclear Magnetic Resonance (NMR) results revealed that O2S and O2S2 species with lower carbon numbers (<30) can be easily separated due to their higher polarity property for the AXL ODS experiment. DFT results confirmed that oxygen vacancies significantly enhance H2O2 adsorption, thereby improving the efficiency of the ODS process. This study provides theoretical insights into the rational design of ODS catalysts and demonstrates that the synthesized catalyst is a promising candidate for industrial ODS applications.

The development of sustainable aviation fuels (SAFs) is essential for reducing carbon emissions in the aviation industry. Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK) has emerged as a promising SAF candidate, and hydrogen enrichment offers a viable strategy to enhance its combustion performance. This study systematically investigates the laminar flame characteristics of FT-SPK blended with hydrogen under varying conditions, including hydrogen blending ratios from 0 % to 95 % (mole fraction), pressures of 1 to 2 bar, and equivalence ratios ranging from 0.7 to 1.2. Experimental measurements and chemical kinetic modeling were performed to assess the effects of hydrogen addition on combustion stability, flame propagation, and the underlying mechanisms governing flame speed enhancement. Results show that hydrogen enrichment significantly increases the laminar flame speed of FT-SPK/air mixtures, with the stronger effect observed under fuel-rich conditions. Furthermore, the global activation energy of FT-SPK/H2/air flames shows distinct trends concerning equivalence ratio: hydrogen increases the activation energy under lean conditions by enhancing chain-branching reactions, while it decreases the activation energy under rich conditions by facilitating low-activation-energy reaction pathways. The developed chemical kinetic model accurately predicts experimental flame speeds across all tested conditions, providing a valuable tool for future studies on hydrogen-enriched SAFs. These findings provide critical insights into the role of hydrogen in enhancing the combustion performance of FT-SPK and support the development of low-carbon aviation fuel technologies. Novelty and significance statement This study presents a comprehensive and first-of-its-kind investigation into the laminar flame speed of hydrogen-enriched FT-SPK/air mixtures, which is a novel sustainable aviation fuel. By integrating detailed experimental measurements with validated chemical kinetic modelling, it reveals the dominant role of hydrogen in enhancing flame speed through chemical and thermal effects. The work also quantifies the impact of hydrogen on global activation energy across lean and rich conditions, offering novel insights into reactivity trends and radical chemistry. These findings provide a critical foundation for advancing hydrogen-enriched SAF technologies in low-carbon aviation.