Emissions of nitrogen oxides (NOx) from high-temperature combustion remain a major concern because thermal NOx formation increases sharply with temperature. This study proposes a nitrogen-dilution-based oxidizer composition control strategy inspired by membrane air separation, in which the nitrogen fraction is increased and oxygen concentration reduced without burner modification or active control. Experiments were conducted using a premixed methane-air swirl burner at atmospheric pressure under three oxidizer conditions, and NOx emissions were evaluated as EINOx. Flame structure was analyzed using CH* chemiluminescence imaging with inverse Abel transform to estimate flame volume and residence time. Under constant fuel flow, nitrogen dilution reduced NOx by up to 45% in concentration and 26% in EINOx due to reduced oxygen concentration and lower adiabatic flame temperature. Overall, the results demonstrate the effectiveness of oxidizer composition control as a practical NOx reduction strategy.
To achieve carbon neutrality in the steel industry, co-firing ammonia (NH3) with steel mill off-gases, specifically coke oven gas (COG) and blast furnace gas (BFG), has emerged as a promising decarbonization strategy. This numerical study investigates the combustion characteristics of co-firing NH(3)with COG and BFG. Using the Okafor mechanism, laminar burning velocities and NOx emissions were analyzed. Results indicate that COG, NH(3)co-firing reduces laminar burning velocity due to radical competition but increases NOx emissions. Conversely, BFG, NH(3)mixtures exhibit non-linear laminar burning velocity, recovering stability at higher fractions, and demonstrate a significant DeNOx effect with reduced emissions compared to pure NH3. These findings provide essential data for optimizing co-firing ratios to balance combustion stability and environmental compliance in steel manufacturing.
The objective of this study is to investigate the exhaust emission characteristics of surrogate e-gasoline fuels reflecting Fischer-Tropsch synthetic fuel properties under real vehicle and chassis dynamometer conditions. Three surrogate e-gasoline blends were formulated and compared with conventional gasoline using four standard driving cycles (FTP-75, HWFET, SC0(3), and US06). Regulated emissions (CO2, NOR, THC, and CO) and unregulated pollutants, including volatile organic compounds (VOCs) and carbonyl compounds, were simultaneously evaluated. The results show that surrogate e-gasoline fuels produce higher THC and CO emissions during cold-start conditions due to delayed fuel vaporization associated with lower vapor pressure. However, under stabilized and high-temperature operating conditions, improved combustion stability-particularly for fuels with a higher iso-paraffin content-leads to reduced THC and CO emissions compared to gasoline. Fuels containing MTBE exhibit a slight increase in NO(x )owing to elevated combustion temperatures, while overall CO(2 )emissions are reduced by approximately 3% as a result of higher heating value. In addition, the removal of aromatic hydrocarbons effectively suppresses hazardous VOC emissions such as benzene and toluene.
The detailed reaction mechanism for NOx formation under high-temperature conditions was generated using the Reaction Mechanism Generator (RMG). The mechanism was generated over 2000-5000 K at 1 bar and included pressure-dependent reactions. The final mechanism consists of 33 species and 292 reactions, including neutral and electronically excited species. To evaluate the generated mechanism, it was implemented in a reduced-order chemical reactor network model that accounts for the flow characteristics of a microwave plasma reactor, and the predicted NOx concentrations were compared with measurements. The simulations predicted that the NOx concentration reaches a maximum at N-2:O-2 = 6:4 and 5:5 and decreases with increasing flow rate, consistent with the experimental observations. However, the discrepancy between the simulations and experiments increased as the flow rate increased, which is attributed not to missing reaction pathways but to simplifications in the reactor model that do not fully capture the actual temperature gradient. Reaction path analysis showed that Zeldovich reactions and three-body dissociation of O-2 and NO dominate in the plasma stream. In the surrounding stream, reactions involving O atoms diffusing from the plasma stream are dominant, with O-atom recombination to form O-2 and NO-NO(2 )interconversion as the main pathways. Additionally, reactions involving electronically excited species derived from the pressure-dependent reactor were also identified. However, these excited species did not contribute to NOx formation and instead relaxed back to their ground state via collisional relaxation.
The regression rate of solid fuel is a critical performance parameter in solid-fuel ramjets, directly influencing thrust, specific impulse, and range. Therefore, accurate prediction of regression rate is essential for the design and optimization of solid-fuel ramjet propulsion systems. In this study, a coupled numerical framework combining condensed-phase pyrolysis and gas-phase combustion was developed to predict the regression rate and combustion characteristics of a fuel-rich propellant for solid-fuel ramjet applications. The regression rate was computed based on convective heat transfer from the flame and the Arrhenius-type pyrolysis relationship. Systematic analysis was conducted to examine the effects of inlet air mass flux and temperature on fuel regression and combustion performance. The predicted regression rate showed good agreement with experimental data, validating the proposed model. The results revealed that the local regression rate peaked near the reattachment point and gradually decreased downstream. Furthermore, increasing the air mass flux and temperature led to a higher regression rate. A regression rate correlation was derived, demonstrating that both air temperature and mass flux significantly influence the fuel regression rate.
This study experimentally investigates the ignition characteristics of a methane-air pre-detonator under varying equivalence ratios using ultra-high-speed visualization and dynamic pressure measurements. The ignition process was divided into initial ignition, secondary ignition induced by reflected shock waves, and exhaust stages. High-speed imaging showed that flame wrinkling increased with equivalence ratio, which is attributed to enhanced burning velocity, higher heat release rate, and intensified pressure rise under confined conditions. After passing through the DDT (deflagration-to-detonation transition) device, distinct Von-Neumann spikes were observed at downstream pressure sensors, indicating successful detonation transition. Although reflected shock waves induced secondary combustion within the chamber, their influence on transition behavior became limited once a sufficiently strong detonation wave was established. Quantitative analysis revealed that flame residence time decreased significantly with increasing equivalence ratio and approached a minimum threshold beyond unity. These results demonstrate that ignition behavior in a confined pre-detonator is governed by coupled effects of pressure rise, shock-flame interaction, and fuel-air ratio.
Ammonia is being explored as a carbon-free fuel; however, its narrow flammability limits, low laminar burning velocity, and high ignition energy make it difficult to use as a commercial fuel. To overcome these limitations, the addition of hydrogen has been widely investigated. In this study, the explosion characteristics of ammonia cracking fuel were experimentally examined in a cylindrical constant-volume combustion chamber, with the ammonia cracking ratio and equivalence ratio as key variables, and the results were compared with corresponding heat loss behavior. As the cracking ratio increased, the maximum explosion pressure (P-max) and index (K-G) increased, while the explosion duration (t(c)) and heat loss (q(tr)) decreased. With respect to the equivalence ratio, both the maximum explosion pressure and index increased under lean conditions up to Phi = 1.1 and then decreased under rich conditions. Schlieren imaging of the flame reaction zone revealed that buoyancy instability dominated at a cracking ratio of 7%, hydrodynamic instability at 16%, and thermo-diffusive instability at 62%. The thermo-diffusive instability was found to enhance the explosion characteristics.
In this experimental research, the combustion and exhaust emission characteristics of a turbo-charged spark ignition (SI) engine operating on various test fuels (bioethanol-gasoline blends) were investigated. The study analyzed the key combustion characteristics, including brake torque, exhaust gas temperature, and fuel consumption rate, to determine the influence of the ethanol blends. Furthermore, the reduction effects on exhaust emissions, specifically carbon monoxide, unburned hydrocarbons, and nitrogen oxides, were compared against those of neat gasoline. The results showed that the brake torque produced by the ethanol-blended fuels was slightly higher than that of neat gasoline. However, brake specific fuel consumption increased with higher ethanol blending ratio due to the lower heating value of ethanol. Overall, ethanol-blended fuels exhibited lower exhaust emissions than neat gasoline across all experimental conditions. The E100 reduced NOx emissions by an average of approximately 10% compared to the G100.
This study analyzed the combustion instability characteristics of retrofit and original nozzles in hydrogenmixed gas turbines using a 1D thermoacoustic model. Experiments were conducted on a laboratory-scale test rig at hydrogen mixing ratios of 0%, 30%, and 50%. Both nozzles exhibited frequency shift phenomena with increasing hydrogen content, but showed opposite trends in dynamic pressure amplitude. The original nozzle showed decreased dynamic pressure amplitude with increasing hydrogen ratio, while the retrofit nozzle exhibited increased dynamic pressure amplitude. Analysis of experimentally measured flame transfer functions revealed that the retrofit nozzle exhibited longer time delay than the original nozzle under the same conditions. Root locus analysis confirmed that this difference in time delay is the key factor determining system stability. It was identified that the center cooling air in the retrofit nozzle induces flame lift-off, increasing the time delay.
A numerical analysis is performed to investigate the decomposition rate of CF4 added to the oxidizer side in non-premixed counterflow flames of methane with an initial pressure and temperature of 1 atm and 295 K, respectively, under oxygen-enhanced conditions on the oxidizer side or hydrogen added to the fuel side. Furthermore, the changes in axial velocity, maximum flame temperature, and NO concentration are analyzed in flames with a strain rate of 100 s(-1). An increase in either the oxygen enrichment rate or the hydrogen addition allows a greater amount of CF4 to be added to the oxidizer without flame quenching, while simultaneously enhancing the CF4 decomposition rate. As the extinguishing condition is approached, the maximum concentration of nitrogen oxide decreases rapidly unlike the maximum flame temperature which decreases linearly.
As waste management practices and policies have evolved, the composition of municipal waste has shifted, raising its average heating value from 1,945 kcal/kg to 4,025 kcal/kg over the past 20 years. This increase has resulted in incineration conditions that exceed the design specifications of existing facilities, with a notable rise in the combustion rate due to higher fractions of volatile synthetic resins. Consequently, instead of achieving uniform combustion as initially designed, the process now completes much faster, creating localized high-temperature zones. These zones cause issues such as facility damage, clinker formation, elevated NOx emissions, and reduced operational efficiency. To address these challenges, this study applies the MILD (moderate and intense low oxygen dilution) combustion technique using FGR (flue gas recirculation) technology in a 180 kWth stoker-type incinerator. Wall temperatures were measured with nine thermocouples, exhaust gas concentrations were analyzed with a non-dispersive infrared (ND-IR) gas analyzer. The experiments varied FGR ratios (0%, 50%, 100%) and FGR injection locations to identify optimal operating conditions. The results showed that increasing the FGR ratio significantly reduced the maximum temperature within the incinerator. The greatest temperature reduction, along with decreased CO and NO emissions, was observed when 100% FGR was injected into both the upper and lower high-temperature zones.
Ammonia-coal co-firing can reduce CO2 emissions in pulverized-coal boilers, but ammonia's low burning velocity makes flame stabilization a key design issue. This study investigates the impact of swirler geometry on flame stability in a lab-scale pulverized coal-ammonia burner. Six tangential swirlers, combining three port numbers (8, 6, 4) and two axial positions (upper: U, lower: D), were tested over 1-5 kW(th). Flame imaging and two-color pyrometry quantified apparent flame intensity and coal particle temperature, and non-reacting CFD clarified the internal recirculation and turbulence fields. The 8-port cases (U-8, D-8) secured the widest stable operating range, whereas 4-port swirlers exhibited frequent ignition failure and blow-off at low loads. D-type geometries enhanced stability by promoting recirculation of coal reaction heat into the flame root. These results provide practical design guidelines for stabilizing ammonia-coal co-firing burners.
The conversion of industrial boilers to carbon-free fuels is one of the strategies for reducing carbon emissions. Hydrogen and ammonia, as representative carbon-free fuels, emit no CO2 during combustion, and when co-fired with LNG in existing boilers, affect flame temperature, flue gas composition, and flow rate, thereby influencing overall heat transfer characteristics. To evaluate these effects, a process simulation-based model dividing the boiler into radiation and convection zones was developed. The variations in radiative and convective heat transfer characteristics according to variation in the co-firing ratio were quantitatively analyzed, and boiler efficiency was assessed. Variations in flame temperature and flue gas composition affected furnace emissivity and radiative heat transfer, which in turn influenced flue gas temperature, flow rate, and convective heat transfer. As a result of the simulation, hydrogen co-firing increased radiative heat transfer and decreased convective heat transfer, leading to improved boiler efficiency. In contrast, ammonia co-firing decreased radiative heat transfer and increased convective heat transfer, resulting in degraded boiler efficiency. These findings are expected to serve as a basis for the design and operational optimization of industrial boilers utilizing carbon-free fuels.
Hydrogen is a key fuel for reducing greenhouse gas emissions in the gas turbine sector, but its combustion characteristics differ from natural gas and may impair combustor stability. Power producers are considering its applicability, although assessing co-firing limits remains largely within OEMs. In this study, hydrogen co-firing tests were conducted under atmospheric conditions to evaluate combustion characteristics and stable operating limits. As the hydrogen ratio increased, nitrogen oxide concentrations rose while carbon monoxide decreased, and combustion dynamics exhibited pronounced low-frequency variations. The surface temperature of combustor hot-section components also showed location-dependent behavior.
Solid propulsion systems are widely used in aerospace and defense due to their high energy density and storability. However, irreversible combustion and long-term exposure to harsh environments require accurate assessment of material degradation. Conventional health monitoring relies on destructive testing, which is time-consuming and costly. Recently, sensor-based structural health monitoring(SHM) and digital twin technologies have gained attention as alternatives, though their application to solid propulsion remains limited due to nonlinear degradation and sensor integration challenges. This paper reviews degradation mechanisms in solid motors and explores SHM and digital twin strategies applied in other propulsion systems. A framework combining embedded sensors, multiphysics simulation, and AI-driven prediction is proposed to enable real-time diagnostics and condition-based maintenance for solid propulsion systems.
This study compared the combustion characteristics of a commercial ignition coil and two multi-stage ignition coils in a single-cylinder DISI engine. The multi-stage ignition coils each comprise two coils and operate in simultaneous (S-mode) and continuous (C-mode) modes; simulated EGR was applied to create a combustion-unstable environment. At 30% EGR, the S-mode showed lower combustion pressure, longer combustion duration, and reduced combustion stability than the commercial ignition coil, resulting in higher indicated specific fuel consumption. The C-mode exhibited performance comparable to the commercial ignition coil at 30% EGR. At 35% EGR, the C-mode maintained higher combustion pressure and shorter ignition delay, yielding lower IMEP COV and an ISFC reduction exceeding 10% relative the commercial ignition coil.
This study aims to establish design parameters for an oxy-fuel burner to efficiently combust LDG, a low-calorific fuel from steelmaking processes. Using a 100,000 kcal/h experimental burner, combustion characteristics were analyzed with design parameters including Re, J and S/D number. The results indicated that the momentum flux of the oxygen jet significantly impacts combustion. Variations in the S/D number affected the flow of fuel and external air. CO emissions increased when the S-F/D-A number exceeded 1.5 due to insufficient fuel entrainment. To reduce NO emissions, minimizing external air entrainment by adjusting the number of oxygen nozzles and maximizing the S/D number was effective. This research provides essential design parameters for stable and efficient combustion of low-calorific fuels such as LDG.
The thermoacoustic network model is a low-order modeling approach used to analyze and predict thermoacoustic instabilities arising from the positive interaction between unsteady heat release and a combustor's acoustic field. In this study, we question the conventional assumption of a compact flame in thermoacoustic network models by systematically analyzing the effect of flame length on this low-order framework. We first compare the growth rates and mode frequencies of thin and long flames using a three-dimensional analysis tool, revealing a clear discrepancy between the two cases. We then evaluate a one-dimensional network modeling strategy that segments the flame. The results indicate that both the thin-flame assumption and the segmented-flame approach fail to reproduce the full thermoacoustic dynamics within the combustor. We also investigate the influence of varying gain and time delay parameters on growth rates and mode frequencies. The findings of this study demonstrate that flame length has a significant impact on thermoacoustic behavior, and this influence should be appropriately incorporated into low-order thermoacoustic models. Based on these observations, we suggest future research directions for improving existing thermoacoustic network models.
This study investigates the regression rate and combustion characteristics of solid fuel using a simple one-dimensional counterflow diffusion flame analysis. A coupled computational framework was developed by integrating gas-phase and condensed-phase reaction models. Simulation results were validated using regression rates from counterflow combustion experiments. Furthermore, the effects of oxygen concentration, ambient pressure, and oxidizer momentum flux on the combustion behaviors of solid fuel were systematically examined. The predicted regression rates showed good agreement with experimental data across the range of conditions tested in this study, although discrepancies were observed at lower pressures. An increase in oxygen concentration, ambient pressure, and oxidizer momentum flux was found to enhance the regression rate. In particular, the increase in momentum flux and pressure shifts the peak temperature location closer to the fuel surface. This proximity intensifies heat feedback to the solid surface, thereby accelerating the pyrolysis process and increasing the regression rate. Furthermore, the correlation analysis identified the oxygen mass fraction as the most influential of the three examined parameters affecting the regression rate. These findings offer fundamental insight into the pyrolysis and combustion of solid fuel relevant to air-breathing solid fuel propulsion systems.
In this study, the effect of an electric field on thermo-acoustic instability in a laminar premixed flame was investigated. Flame surface fluctuations and sound pressure were measured using a high-speed camera and a microphone. The results showed that the flame surface oscillated under the no-voltage condition, with these oscillations intensifying under -3 kV and decreasing under +3 kV. This behavior remained consistent even as the inlet velocity increased. In the frequency domain, applying -3 kV shifted the dominant frequency to a higher range, whereas no distinct peak was observed under the +3 kV condition. Regarding the sound pressure level, -3 kV had little impact, whereas +3 kV reduced it by up to 23.8% under stoichiometric conditions. Furthermore, this effect remained effective even under lean premixed conditions.
