Investigation of electric field control on soot production in a small-scale biodiesel jet diffusion flame

Combining information on particle size and optical properties for in situ detection of largely oxidized soot particles: A demonstration of feasibility in laminar coflow diffusion flames

ABSTRACT Gas sampling methods have been crucial for the advancement of combustion science, enabling analysis of reaction kinetics and pollutant formation. However, the measured composition can deviate from the true ones because of the potential residual reactions in the sampling probes. This study formulates the composition estimation in stiff chemically reactive systems as a Bayesian inference problem, solved using the No-U-Turn Sampler (NUTS), and systematically investigates the causes of information loss for the initial value inference. Theoretical analysis shows that information loss arises from the restriction of system dynamics by low-dimensional attracting manifold, where constrained evolution causes initial perturbations to decay in fast eigen-directions in composition space. The methodological framework is demonstrated in the Robertson system and autoignition of hydrogen/air mixtures. Furthermore, a gas sample collected from a one-dimensional hydrogen diffusion flame is analyzed to investigate the effectiveness of the frozen temperature for alleviating information loss. The research highlights the importance of species covariance information from observations in improving estimation accuracy and identifies the intrinsic connection between inference failures and the rank reduction in the sensitivity matrix representing the system dynamics. Critical failure times for species inference are defined and analyzed in the Robertson and hydrogen autoignition systems are analyzed, providing insights into the limits of inference reliability and its physical significance.

ABSTRACT This study introduces the effect of strain rate on the NO formation characteristics using simple one‐dimensional counterflow diffusion flame model. This study found that NO formation strongly depends on the global strain rate in the counterflow diffusion flame. Depending on the global strain rate, the NO formation either monotonically or nonmonotonically increases with increasing oxygen content in the oxidizer. This is primarily attributed to the increase in the net NO formation rate with increasing strain rate, leading to a narrower flame zone. The overestimation in the GRI‐Mech 3.0 mechanism is mainly associated with the NO formation within the HNO pathway, especially with H + HNO ↔ H2 + NO, HNO + M ↔ H + NO + M, and HNO + OH ↔ H2O + NO reactions. The Mendiara–Glarborg and Glarborg 2018 mechanisms conform well with the measured trend regardless of the range of global strain rate, while GRI‐Mech 3.0 and Okafor's mechanisms demonstrate reasonably good agreement, except at low strain rate corresponding to laminar flame.

This study establishes an experimental platform consisting of an adjustable inclined surface and a cross-slope wind system. Turbulent diffusion flames are investigated by examining the variation characteristics of flame morphology under slope angles of 10–40°, cross-slope wind velocities of 0.8–2.0 m/s, and heat release rates of 15.38–61.50 kW. The results show that variations in slope angle change the components of buoyancy in the normal and tangential directions. The normal component influences the lifting of the flame perpendicularly to the slope, while the tangential component, together with differences in air entrainment on both sides of the flame, promotes flame inclination and spreading along the slope surface. The cross-slope wind enhances the horizontal stretching and attachment tendency of the flame through inertial shear, while simultaneously suppressing flame height and its development along the slope. The coupled effects of these factors cause the flame morphology to gradually transition from a nearly vertical state to an attached state. Based on dimensionless analysis, empirical correlations of flame morphology parameters are established by introducing the cross-slope wind Froude number, dimensionless heat release rate, the density ratio of propane to air, and a slope function. Within the experimental range of this study, the data under various conditions show good collapse and correlation under the selected dimensionless parameters.

Investigation of soot formation characteristics in C2H4 / NH3 laminar diffusion flames: Effects of different gradient blending ratios of NH3

Extension of jet diffusion flames impinging on metal foam barriers

Effects of oxygen-fuel velocity ratio on soot formation and flame structure in CH4-CO2-O2 inverse diffusion flames

ABSTRACT Numerical simulations were performed to examine NOₓ formation in counterflow diffusion flames fueled by ammonia and syngas. The study utilized the Li, Han, and Mei reaction mechanisms to predict flame structure and emissions under varying pressures and compositions. A primary focus was the comparison of three fueling strategies: equimolar replacement of syngas, fixed hydrogen content, and fixed carbon monoxide content. Results show that although increasing the ammonia fraction generally raises NOₓ levels, the specific ratio of H2 to CO significantly alters the emission magnitude. Among the tested methods, maintaining a fixed H2 concentration proved to be the most effective strategy for emission control. Detailed pathway analysis attributes this reduction to the suppression of the thermal NO mechanism, which was identified as the governing route for NO production under all conditions. The study also confirms a linear relationship between oxygen concentration and NO formation. These findings highlight the importance of fuel composition control in the design of cleaner ammonia – syngas combustors.

ABSTRACT Transition toward sustainable energy systems requires stable and efficient utilization of weakly reactive renewable gaseous fuels such as biogas. This work experimentally investigates the coupled effects of burner geometry and oxygen enrichment on the stability of non-premixed biogas flames in a low-swirl burner (S ≈ 0.39). Two configurations, bluff-body (BB) and non-bluff-body (NBB), were examined using a surrogate biogas mixture (60% CH4/40% CO2) under air (21% O2) and oxygen-enriched (24% O2) co-flow conditions. The results showed that flame stabilization depends on both the burner geometry and co-flow enrichment. The results also revealed that there is a clear interplay between these two factors on flame stability. More importantly, even a mild oxygen enrichment of the co-flow is found to greatly expand the operation of biogas flame especially under lean fuel conditions. PIV and OH*-chemiluminescence data were exploited to explain flame blowout mechanisms. The findings clearly demonstrated the relative roles of swirl, geometry, and enrichment in stabilizing biogas diffusion flame.

The performance of diffusion flame (DF) burners strongly depends on how effectively combustion gases mix and retain heat, yet the influence of exhaust nozzle geometry on these processes remains insufficiently characterized. This study examines how varying exhaust nozzle angle affects the thermal behavior and emissions of a methane (CH4) diffusion flame under atmospheric conditions. A laboratory-scale burner with interchangeable exhaust nozzles (0°, 25°, and 50°) was operated at 1.8 kW using a fixed methane flow of 3 L/min and co-swirled air and fuel at 30°, across equivalence ratios (Φ) of 1.0, 0.7, and 0.5. Axial temperature measurements and exhaust gas analyses (Carbon dioxide (CO2) and Carbon monoxide (CO)) were conducted to assess mixing, heat retention, and post-flame oxidation. Results show that exhaust nozzle geometry notably influences flame position and heat distribution, producing non-monotonic temperature trends with equivalence ratio. The 25° nozzle angle yielded the highest near-stoichiometric downstream and flue temperatures, reaching about 204 °C at x = 45 cm and 277 °C in the flue, compared with 72 °C and 177 °C for the 0° nozzle. In contrast, the 50° nozzle produced more uniform downstream temperatures (about 150–160 °C) and the lowest CO emissions, approaching zero near Φ ≈ 1.0. These findings demonstrate that coordinated control of swirl and exhaust nozzle angle can enhance thermal response and CO reduction in diffusion flame burners without significantly changing CO2 levels.

ABSTRACT In this study, an ad hoc analytical model is developed to describe the axial profile of the time-averaged heat release rate in the diffusion flame of a Liquid Rocket Engine coaxial injector operating under transcritical conditions (resulting in a simple density stratification, without vaporization). The model is derived from the Burke-Schumann diffusion flame model with the infinitely fast combustion assumption, augmented by adding turbulent diffusivity and velocity dispersion. It is therefore built upon a series of physically motivated assumptions which are systematically validated against experimental and numerical data. The predictive capability of the model is then assessed through detailed comparisons with Large Eddy Simulation results from the literature, demonstrating its ability to capture key features of the flame structure as it gives an accurate representation of the flame profile and effectively captures the total time-averaged heat release rate. Furthermore, a preliminary extension explores the ability of the model to predict the effects of forcing amplitude and frequency on the flame response. The model exhibits some key features of the flame response to acoustic forcing. Designed to be integrated into flame transfer function models, this work represents a milestone toward fully analytical flame transfer functions.

To investigate the effects of oxygenated fuel blending on soot formation and physicochemical properties during kerosene combustion, pure kerosene, kerosene/butanol, and kerosene/2,5-dimethylfuran (DMF) blends were studied. A liquid-fuel diffusion combustion platform was established to examine flame morphology and soot characteristics under stable conditions. Flame images were captured using a CCD camera, and soot from the flame tip was collected via capillary sampling. The soot was characterized by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR). Results showed that oxygenated fuel blending shortened and darkened the flame, with DMF exhibiting a stronger soot suppression effect. XPS analysis revealed increased surface oxygen content and O/C ratio at 10% blending, while a decrease was observed at 20%, indicating nonlinear behavior. Raman results indicated that butanol increased structural disorder, whereas DMF promoted more ordered aromatic structures. FTIR analysis showed enhanced O–H and C–O groups for butanol blends, while DMF favored C = O and C–O formation, especially at higher blending ratios. Overall, fuel molecular structure and blending ratio significantly influence both combustion behavior and soot evolution, providing insights for cleaner kerosene combustion.

ABSTRACT Argan nut shell biomass is abundant in southern Morocco and represents a promising feedstock for thermochemical energy conversion through gasification. In this study, a throated downdraft gasifier was designed and constructed to produce syngas from ANS. The physicochemical properties of ANS were first characterized. The gasification process was evaluated in terms of gas yield and syngas heating values. The effects of operating conditions, including the ANS feedstock load and the air flow rate, which determine the equivalence ratio, were analyzed. The gasification characteristics were investigated based on temperature measurements within the gasifier, the instantaneous composition of the produced syngas, the lower heating value (LHV), and the gasification efficiency. A portion of the produced syngas was flared to generate a laminar jet diffusion flame in ambient air. Four flames characterized by relatively low H2/CO ratios were studied. Each flame corresponded to different H2/CO and N2/CO2 ratios, as well as varying instantaneous CH4 concentrations and residual O2 levels. This analysis aimed to evaluate the influence of syngas composition on laminar diffusion flame characteristics.

ABSTRACT An experimental study of combustion in arrays of inverse diffusion flames (IDF) was conducted. Different circular arrays under conditions of pilot IDF absence and presence, with its co- and counter-rotation with respect to peripheral IDF, were tested. PIV studies of array flow structures in the longitudinal and transverse cross-sections were performed, and high-speed recordings of the ignition process were conducted. Investigations of array flow structures revealed that the presence of a counter-rotating pilot IDF leads to the decay of the array structure at the z/D out = 2 section, where a strong axial flow helicity is observed, which is not present in other array configurations. Additionally, the pilot IDF improves combustion stability, which is particularly important at lean mixtures with 0.7 ≤ ϕ ≤ 1.0, where axial flame oscillations are observed. Due to these factors, combustion in the 7-IDF array with counter-current rotation of the pilot flame occurs more intensely and produces the lowest flame lengths. However, this type of array takes longer to ignite (1100 ms in average) compared to the 7-jet array with the same direction of rotation (700 ms). This occurs the flow structure specifics, while large-scale vortex structures are more evenly distributed in all sections of the array with co-rotating IDF, whereas the array with counter-rotating central IDF is characterized by displacement of vortices from their symmetrical positions, as well as their asymmetrical merging. These factors contribute to uneven flame propagation across the circumference of the array, leading to a longer ignition time.

Soot formation in a laminar co-flow C2H4-NH3 diffusion flame at different oxygen indices

Influence of pressure on the structure and chemistry of counterflow diffusion flames from 1 to 6 bar

Effects of oxygen-enrichment and CO2 dilution on the physicochemical properties of soot in a propane laminar diffusion flame

Biogas is a sustainable energy source that contributes to the closed carbon cycle by reducing greenhouse gas emissions. Nevertheless, concerns exist over its emissions, NO, and CO. This study investigates the potential of blending biogas with dimethyl ether (DME), an eco-friendly fuel, to enhance combustion characteristics and reduce emissions. Numerical analysis was conducted on DME/biogas blends (30%–60%) in a laminar diffusion flame under atmospheric pressure, considering the chemical effects of CO 2 in biogas. The addition of DME expands the reaction zone and flame thickness, increasing fuel consumption. Adding DME slightly increases the NO emission index, at a rate of 0.02 (g-NO / kg-fuel) per 10% DME increment. It significantly reduces the CO emission index, which decreases by 16–20 (g-CO / kg-fuel). 50:50 DME/biogas blend provides the optimal trade-off between NO and CO emissions. The analysis reveals that the NNH and thermal routes are the dominant contributors to NO production, and their influence increases upon DME addition. The N 2 O and prompt routes have minimal impact. Adding DME improves NO consumption through the Ch i -reburning sub-pathway, involving HCCO and CH i=1–3 radicals, partially compensating for the increase in thermal and NNH routes / NOx. More DME reduces CO production by inhibiting the forward reaction HCO + M => H + CO + M.

Experimental and numerical investigations on diffusion flame structures of cracked fuels