Stoichiometric Mixture Fraction Research Articles

The design of a combustion chamber operated in MILD regime - numerical modeling of hydrogen combustion in oxygen steam mixtures

The goal of the current work is to develop a combustion chamber that can operate on recirculated steam produced by burning hydrogen in oxygen under conditions of Moderate or Intense oxygen Dilution (MILD) in atmospheric and stoichiometric conditions. The study investigates several configurations of combustor with nozzles, taking into account the overall temperature, OH radicals, and heat release rate distribution throughout the combustor’s domain. Thermal power variations of the steam generator (5 to 20 kW) were examined in conjunction with different oxygen dilutions with steam, down to 3% of O2 (by mol.). The outcomes reveal that a rise in dilution degree promotes a drop in the mean temperature across every case and reagents’ recirculation with homogeneous temperature field, suggesting the presence of MILD combustion. The highest temperature values were observed at the stoichiometric mixture fraction. Higher dilution degree revealed more efficient heat release across the domain with low fluctuations from the reference MILD combustion data. Of the two combustion models studied, the Partially Stirred Reactor model did not show flame extinction at the highest dilution degrees, unlike the Eddy Dissipation model. The selected final design of the combustion chamber was used for constructing the actual combustor dedicated for lab-scale operation.

Effect of H[formula omitted] addition on NH[formula omitted] flame structure and dynamics in a mixing layer

Hydrogen addition has been widely used to improve the combustion performance of low-reactivity fuels such as NH3. In this study, a series of two-dimensional NH3/H2 flames in mixing layers were investigated numerically. The aim is to assess and interpret the effects of H2 addition on the structure and dynamics of ammonia flames. Detailed chemistry and transport models were considered in the simulations. It was shown that for pure hydrogen or ammonia flames, a classical tribrachial structure was observed. The non-premixed branch in the tribrachial flame of NH3 combustion features high heat release rate (HRR), while the two premixed branches are weak. The mixture fraction corresponding to the maximum HRR of NH3 combustion is significantly lower than the stoichiometric mixture fraction. When blending ammonia with hydrogen, a tetrabrachial flame structure consisting of two premixed branches and two non-premixed branches was observed, where one non-premixed branch corresponds to the reaction of NH3, while the other is formed due to the reaction of H2. It was suggested that the formation of the tetrabrachial flame is related to the preferential diffusion of hydrogen. After decreasing artificially the diffusivity of hydrogen to suppress the preferential diffusion effect, the flame exhibits only three branches with the merging of the two non-premixed branches. By analyzing the flame dynamics at the leading edge, it was found that the flame speed (Sd∗) is negatively correlated with the flame stretch (κ) and scalar dissipation rate (χ), with Sd∗ decreasing almost linearly with κ or χ, consistent with previous studies of methane and hydrogen flames. As the H2 concentration is increased, the values of Sd∗ increase due to the enhanced reactivity.

Soot Measurements and a Simplified Analytical Formulation-Based Modeling in Laminar Counterflow Diffusion Flames

ABSTRACT The paper presents soot volume fraction measurements and a simplified soot formation model. Soot formation is a complex physicochemical process predominantly modeled using detailed chemistry and stochastic methods. The current work presents a simplified analytical function-based approach to calculate the net soot formation rate by coupling the local mixture-fraction and temperature fields. The validation experiments were carried out in a unique large-diameter (40 mm) counterflow burner for ethylene-air flames at various strain rates and normalized separation (L/D) distances. The soot volume fractions (fv) were determined using Laser Induced Incandescence (LII) technique calibrated by the Light Extinction (LE) method. Further model validations were carried out against the experimental measurements in the literature. The modeling work was performed on OpenFOAM®. The model predicted soot distributions for various peak flame temperatures, stoichiometric mixture-fractions, burner dimensions, fuel-oxidizer compositions, and strain rates with good accuracy. The study underscores the importance of considering two soot formation rate peaks, one at high temperature and one at low temperature, for accurate predictions, unlike a single peak employed in common analytical models. Furthermore, the effect of velocity boundary conditions on soot zone locations and the significance of providing realistic velocity conditions for the simulations are also discussed. Particle Image Velocimetry (PIV) measurements were performed to determine the actual nozzle-exit velocity profiles.

The effect of methane addition on reacting hydrogen jets in crossflow

The combustion characteristics in a jet in crossflow configuration are numerically investigated using Large Eddy Simulation (LES) with subgrid scale modelling for partially premixed combustion. The time-averaged statistics are compared with the experimental measurements. Both the temperature and velocity comparisons show good agreement with measurements. The distribution of reaction regions of nitrogen-diluted hydrogen fuel jet is then examined in both physical and mixture fraction space. The results demonstrate that the fuel consumption in potential core, near-field and far-field regions occurs at various mixture fraction ranges. In the potential core region, reactions predominantly occur between the lean and stoichiometric mixture fraction range. This range broadens to include fuel-rich conditions in the near-field region and narrows again to fuel-lean combustion in the far-field region. With these changes in the mixture fraction range, the fractional contribution from different combustion modes also changes. A closer study of these changes reveals that the fractional contribution from the non-premixed mode is highest at the end of the potential core region. The dependency of these combustion modes on the fuel composition is studied by increasing the volume fraction of methane in the jet. This resulted in altered chemical kinetics and thereby increased the fractional contribution of the non-premixed mode, particularly in the potential core region.

Dynamics of regime transition of autoignitive jet flames from conventional to MILD combustion

Turbulent combustion of jet flames in a hot diluted coflow of combustion products has been studied across a range of jet Reynolds numbers for propane, ethylene, and an ethylene-propane blend as fuels. The study revealed a transition from conventional autoignitive combustion to a regime of Moderate or Intense Low-oxygen Dilution (MILD) combustion, which is characterized by a nearly invisible flame. The flames studied are luminous at low jet velocities and become MILD at higher jet velocities. Planar Laser-Induced Fluorescence (PLIF) of formaldehyde (CH2O), a key intermediate species in hydrocarbon combustion, is combined with CH*-chemiluminescence imaging and Rayleigh scattering to investigate the phenomena. The transition to MILD combustion is found to accompany a broadening of the formaldehyde region, indicating a broader low-temperature reaction zone. As the transition is approached, the appearance of holes in the chemiluminescent front is also observed. Correspondence between the location and structure of these holes with regions of formaldehyde in the flame are illustrated. These regions are proposed to be signatures that precede the transition to a fully MILD flame. In other words, the transition to MILD combustion begins at a local level and the MILD region spreads to engulf the entire combusting mixture. Scalar gradients were imaged to further unravel the local turbulence/chemistry interactions that yield the MILD inception. The measured scalar gradients were found to be commensurate with scalar gradients obtained from chemical kinetic simulations at the stoichiometric mixture fractions, while being an order of magnitude larger at other locations; this suggests that the inception of MILD combustion occurs near the stoichiometric region.

On conserved scalars that preserve stoichiometric mixture fraction

With the increasing use of carbon-free fuels in hydrocarbon blends, multiple definitions are appearing for conserved scalars that are employed in the formulation of mixture fraction. One essential requirement for such scalars is the preservation of stoichiometric mixture fraction under conditions where the elemental ratios change locally due to preferential diffusion. This short communication clarifies that the scalar introduced by Bilger in 1989, namely β=2YCWC+YH2WH−YOWO does satisfy these conditions regardless of the gaseous fuel blend, as long as the products of complete combustion are assumed to be CO2 and H2O. An explanation of the underlying foundation of this conserved scalar is provided along with an example for the generalised cases where dilution and partial premixing may be present.

Confinement Effects on Coaxial Jet Diffusion Flame

ABSTRACT A combined experimental and numerical study has been conducted to investigate the characteristics of confined coaxial jet diffusion flames in air coflow, using methane as fuel. The main concerns were focused on the unequal fuel velocity (0.1–72 m/s)/air velocities (0.045–0.53 m/s) and confinement ratio (δ*, defined as the ratio of the confinement tube diameter and the jet diameter), and most of the flow remains laminar while the ranges of δ* and gas velocities vary widely. The flame height linearly increases with the central fuel jet velocity, until the flame tip-opening or liftoff occurs, which is similar to that of the free jet flame. The critical fuel jet velocity for flame liftoff decreases with the ratio of air velocities to δ * . The critical fuel jet velocity of flame tip-opening increases with δ*, and the critical volumetric flow rate of air increases to a maximum, then decreases, indicating a combined effect of the oxygen supplement and the mixing with the fuel. Under smaller δ* with high jet flow velocity, a circular plane flame is observed before flame extinction. The formation of a recirculation zone in a confinement with combustion reaction has a significant influence on flame behaviors. A critical nondimensional Craya-Curtet number is used to predict the onset of flow recirculation with combustion reaction. At a certain coflow air velocity, the critical number initially increases and then remains constant with δ*, while at a certain δ*, the critical number decreases with air velocity. When the Craya-Curtet number is higher than the critical point, recirculation cannot be established, resulting in the formation of a tip-closed flame. With the decrease of the Craya-Curter number, the recirculation evolves to the upstream of the flame, then, the flame may be lifted off, along with the recirculation zone moving downstream. With the effect of recirculation, the stoichiometric mixture fraction contour shrinks toward the central axis where there is a higher local velocity. Therefore, the flame will be lifted off under a lower jet velocity in a smaller δ * . With the increase of the jet velocity, the Craya-Curtet number further decreases, and the recirculation becomes much stronger than the upstream inflow, resulting in the flame being compressed into a circular plane flame at a small δ* with a relatively high jet flow velocity or blowout directly at a relatively large δ * .

The mixing field and flame structure near the reaction zone of turbulent planar flames at different levels of mixture inhomogeneity

Turbulent flames with compositionally inhomogeneous mixtures are commonly used in many combustion systems. In this work, turbulent planar jet flames issued from a concentric flow slot burner, CFSB, were used to study the impact of mixture inhomogeneity near the flame sheet. The CFSB burner can control the mixing inhomogeneity by changing the mixing length “L” between the concentric fuel and air slot nozzles. At various levels of mixture inhomogeneity, the mixing field, presented by mixture fraction, Z, distribution, and the flame structure was investigated via simultaneous Rayleigh and OH-PLIF imaging technique. Our previous study investigated the mixing field immediately downstream of the burner exit in non-reacting conditions. The PDFs of Z showed that the mixing field covered a wide range of mixture fractions, where the high flame stabilization occurred when a large portion of the PDF(Z) was located within the fuel flammability limits. This work showed that further downstream, the highly stabilized flames were also obtained when the range of fluctuations in the mixture fraction was close to the stoichiometric mixture fraction. Moreover, plotting the mixing field using the scatter plot within the mixing regime diagram for various flame conditions showed that the mixing field downstream of the burner exit consistently follows the mixing diagram classification. Moreover, a close inspection of the flame structure showed that the flame sheet varies from thick, and corrugated at a low and high levels of mixing to thin flame sheet but less corrugated and that at a particular mixing normalized length L/D = 7. The flame corrugation data showed that the flame stability occurs at a minimum corrugation factor.

Spherical Diffusion Flames of Ethylene in Microgravity: Multidimensional Effects

The joint American–Russian Space Experiment Flame Design (Adamant) was implemented on the International Space Station (ISS) in the period from 2019 to 2022. The objectives of the experiment were to study the radiative extinction of spherical diffusion flames (SDF) around a porous burner (PB) under microgravity conditions, as well as the mechanisms of control of soot formation in the SDF. The objects of the study were the normal and inverse SDFs of gaseous ethylene in an oxygen atmosphere with nitrogen dilution at room temperature and pressures ranging from 0.5 to 2 atm. The paper presents the results of transient 1D and 2D calculations of 24 normal and 13 inverse SDFs with and without radiative extinction. The 1D calculations revealed some generalities in the evolution of SDFs with different values of the stoichiometric mixture fraction. The unambiguous dependences of the ratio of flame radius to fluid mass flow rate through the PB on the stoichiometric mixture fraction were shown to exist for normal and inverse SDFs. These dependences allowed important conclusions to be made on the comparative flame growth rates, flame lifetime, and flame radius at extinction for normal and inverse SDFs. The 2D calculations were performed for a better understanding of the various observed non-1D effects like flame asymmetry with respect to the center of the PB, flame quenching near the gas supply tube, asymmetrical flame luminosity, etc. The local mass flow rate of fluid through the PB was shown to be nonuniform with the maximum flow rate attained in the PB hemisphere with the attached fluid supply tube, which could be a reason for the flame asymmetry observed in the space experiment. The evolution of 2D ethylene SDFs at zero gravity was shown to be oscillatory with slow alterations in flame shape and temperature caused by the incepience of torroidal vortices in the surrounding gas. Introduction of the directional microgravity, on the level of 0.01g , led to the complete suppression of flame oscillations.

An investigation on flame structure and NOx formation in a gas turbine model combustor using large eddy simulation

In this work, large eddy simulations (LES) of a Gas Turbine Model Combustor (GTMC) are done using a five-step global mechanism that includes separate thermal and non-thermal NOx formation parts. To verify the accuracy of the solution, time-averaged profiles of the flow variables and fluctuations are compared to the available experimental and numerical data. The LES results show that the vortical structures inside the chamber are highly connected to the temperature field and chemical reactions, and despite having a major role in fast premixing and consequent NOx reductions, they contribute to NOx generation by forming high temperature spots inclusive of chemical radicals. Also, the importance of the baroclinic torque in vorticity creation is demonstrated by comparing the corresponding values to vortex stretching in upstream parts of the chamber. It is shown that the baroclinic torque mostly takes action between high vorticity and high strain regions and can possibly intensify the strong vortices, while the vortex stretching is mostly active near the strong vortices. Furthermore, observation of detailed statistics shows that most of the heat release occurs in samples with mixture fractions near the global value, while NO generation is highly biased toward the strong vortices and the stoichiometric mixture fraction. To investigate the role of the radicals in more details, a chemical reactor network (CRN) is created by clustering the LES solution. Also, the integration of Partially Stirred Reactors (PaSRs) with Perfectly Stirred Reactor (PSR) networks is used to improve the accuracy of predicting the reactant jet penetration and ignition radicals.

Investigation of structure and soot formation in laminar inverse diffusion flames by atomic ratio measurements using laser-induced breakdown spectroscopy

This experimental study reports the two-dimensional C/O and H/O atomic ratio distributions of oxy-ethylene laminar inverse diffusion flames (IDFs) with elevated stoichiometric mixture fractions (Zst) using laser-induced breakdown spectroscopy (LIBS), and investigates the effects of Zst on the structure and soot formation of IDFs. The LIBS diagnostic system consisting of a pulsed Nd: YAG laser and a fiber optic spectrometer measures the atomic ratios of IDFs with Zst of 0.2–0.5, and the influence of laser energy on atomic ratio calibrations and measurements is corrected to improve accuracy. IDFs are divided into five zones, namely the oxidant zone, the chemiluminescence zone, the gap, the soot zone, and the fuel zone. The C/O atomic ratio of the chemiluminescence zone and the gap remains 0.35–0.5, and the soot zone is 0.4–0.7. Except for the gap, where the H/O atomic ratio remains at 0.55–0.60, the H/O atomic ratio of all other zones is greatly affected by Zst. These atomic ratio results demonstrate that the C/O atomic ratio has the advantage of differentiating the structure of IDFs. Experimental results show a C/O atomic ratio of 0.5–0.6 is required for soot inception in IDFs, so the elevated Zst delays soot inception by lowering the C/O atomic ratio near the flame sheet, and it also compresses the residence time and C/O atomic ratio space favorable for soot formation. Additionally, the decreased local equivalence ratio determined from the H/O atomic ratio further confirms that the increase of Zst makes IDFs unsuitable for soot formation. This study proves that LIBS is an effective flame diagnostic tool and the atomic ratios hold promise for studying a broader range of flame configurations.

Study on the combustion, entrainment, and plume flow behaviors of annular pool fires.

Annular fire source is a common combustion form in fire accidents. Effects of Din/Dout (the ratio of inner to outer diameters of the floating-roof tank) on the flame morphology and plume entrainment mechanisms of annular pool fires were studied by numerical simulation. Results show, as Din/Dout increases, the area with low combustion intensity near the central axis of the pool surface gradually increases. Combined with the time-series HRR and the stoichiometric mixture fraction line of the fire plume, it reveals that the combustion of annular pool fire is dominated by non-premixed diffusion flame. The pressure near the pool outlet decreases with Din/Dout, while the plume turbulence presents an opposite trend. Based on the time-sequential plume flow and the gas-phase material distribution data, the flame merging mechanism of the annular pool fires is revealed. Furthermore, based on the similarity criterion, it verifies that the applicability of the above scaled simulation conclusions could also be extended to guide full-scale fires.

Soot structure and flow characteristics in turbulent non-premixed methane flames stabilised on a bluff-body

The soot properties of methane in turbulent regimes are not well characterised but are highly desirable. Methane is the main constituent of natural gas that is broadly used in many industrial combustors. Investigation of turbulent methane flames under well-defined boundary conditions is therefore useful for interpreting soot formation in practical burners and can be used for further model development. This study presents a joint experimental and numerical study of a series of turbulent non-premixed bluff-body flames fuelled with pure methane for three values of the momentum flux ratio of fuel jet to co-flowing air. Soot volume fraction (SVF) and flowfield are measured simultaneously using planar laser-induced incandescence (P-LII) and 2D-polarised particle image velocimetry (P-PIV). Additionally, time-averaged temperature, mixture fraction, OH and C2H2 concentrations are estimated numerically using RANS models. The global flame structure for all three flames features a recirculation zone with a double-vortex structure, a jet-propagating zone, and a neck zone connecting the two regions. The soot distribution within the recirculation zone shows clear distinct features, which is attributed to the mean mixture fraction distribution in this zone. Increasing the momentum flux ratio shifts the location of the mean stoichiometric mixture fraction to the rich inner vortex core, leading to a distinct peak of the total integrated soot in the inner vortex of the recirculation zone that is not observed in other cases. Also, it is deduced that the soot inception starts earlier in the recirculation zone for the flame with the highest momentum flux ratio and in the jet zone for the other two flames. Much higher soot concentration and lower intermittency are found with ethylene-based flames stabilised on the same burner and with the same operating conditions. In addition, the study has generated a database of soot and flowfield results, which can be helpful for future model validations.

Detailed Study of the Formation of Soot Precursors and Soot in Highly Controlled Ethylene(/Toluene) Counterflow Diffusion Flames

We perform spatially resolved measurements of temperature, gaseous species up to three-ring Polycyclic Aromatic Hydrocarbons (PAHs), and soot in atmospheric pressure counterflow diffusion flames. First, we characterize fully a baseline ethylene flame and then a toluene-seeded flame in which an aliquot of ethylene in the feed stream is replaced with 3500 ppm of prevaporized toluene. The goal is twofold: to investigate the impact of a common reference fuel component of surrogates of transportation fuels and bypass the main bottleneck to soot formation from aliphatic fuels, that is, the formation of the first aromatic ring. The composition of the fuel and oxidizer streams are adjusted to maintain a constant stoichiometric mixture fraction and global strain rate, thereby ensuring invariance of the temperature-time history in the comparison between the two flames and decoupling the chemical effects of the fuel substitution from other factors. Major combustion products and critical radicals are fixed by the baseline flame, and profiles of critical C2-C5 species precursors to aromatic formation are invariant in both flames. On the other hand, doping with toluene boosts the aromatic content and soot volume fraction, increasing the mole fraction of benzenoid structures and soot volume fraction by a factor of 2 or 3, relative to the baseline ethylene flame. This finding is consistent with the expectation that the formation of the first aromatic ring is no longer a bottleneck to soot formation in the doped flame. In addition, toluene bypasses completely benzene formation, opening a radical recombination pathway to soot precursors through the production of C14H14 (via dimerization of benzyl radical) and pyrene (through dimerization of indenyl radical).

Experimental analysis and theoretical lift-off criterion for H2/air flames stabilized on a dual swirl injector

Stabilization mechanisms of partially premixed H2/air flames on a coaxial dual swirl injector are investigated at atmospheric conditions. Hydrogen is injected through a central duct, and the air by the outer annular channel. Both channels are swirled and two stabilization modes are observed depending on the geometrical configuration of the injector and on the operating conditions. In certain regimes, the H2/air flame stabilizes on the injector lips as a diffusion flame. For other operating conditions, the flame is lifted from the injector and burns mainly in partially premixed regime leading to limited NOx emissions. PIV measurements in cold flow conditions and direct observations of the flame indicate that the flame stabilization mode is mainly controlled by the inner hydrogen swirl level, the injector recess and the hydrogen velocity. For a given air flowrate, a minimum hydrogen velocity to lift the flame is determined for each combination of inner swirl level and injector recess. Assuming the flame close to the injector lips behaves like an edge flame, a model for flame stabilization based on the triple flame speed and the location of the stoichiometric mixture fraction line is built. According to this model, the flame is anchored to the injector if the triple flame can propagate to the inner injector lips, i.e., if the velocity along the stoichiometric line is lower than the triple flame speed. The model is tested using hydrogen diluted with argon and air diluted with nitrogen. Two cases producing predicted opposite trends are verified. First, the stoichiometric line is moved in the direction of lower velocity zone keeping the triple flame speed constant in order to anchor a lifted flame. Next, the stoichiometric line is kept constant and the triple flame speed is reduced in order to lift an anchored flame. The mechanisms driving flame stabilization are discussed.

Characterisation of ethylene flames under a range of low-oxygen concentrations

New understanding of turbulent ethylene (C2H4) jet flames, issuing into a range of preheated coflowing oxidisers with reduced oxygen (O2) concentrations are reported. These conditions emulate moderate or intense low-oxygen dilution (MILD) combustion. To resolve previously reported non-monotonic trends, comparisons are made for coflow O2 concentrations of 3%, 4%, 5%, 6%, 9% and 11% and coflow temperatures of 1250K, 1315K and 1385K. Instantaneous and simultaneously planar imaging measurements of temperature, hydroxyl radicals (OH) and formaldehyde (CH2O) were taken at eight downstream locations ranging from 9mm to 75mm. The new data reveal non-monotonic lift-off trends in the OH and CH2O formation heights at all three temperatures. Compared with extant measurements, the higher resolution and greater control of coflow composition provide a detailed exploration of the transitional behaviour that occurs when MILD combustion conditions are achieved. The results experimentally demonstrate non-monotonic variation in lift-off height, and show that the same behaviour is present in both the OH and CH2O formation heights, and occurs at all three temperatures, which is due to the movement of the location of the stoichiometric and most reactive mixture fractions to the coflow side of the jet shear layer. The results also show that increasing the oxidiser temperature does not have a significant effect on the OH number density for a given temperature but does result in a significant decrease in lift-off height and CH2O concentration.

Synergistic effect on PAH and soot formation in ethylene counterflow diffusion flames by the addition of 1,3-dioxolane - a bio-hybrid fuel

Bio-hybrid fuels combine the utilization of bio-based feedstocks and carbon dioxide with renewable electricity to achieve a carbon-neutral and low-emissions future for the transportation sector. A potential candidate is the heterocyclic acetal 1,3-dioxolane (C3H6O2). However, little is known about the mechanism behind the impact of 1,3-dioxolane on soot formation. In this study, we investigated ethylene counterflow diffusion flames with 1,3-dioxolane addition in different amounts. The equilibrium temperature, the stoichiometric mixture fraction, and the strain rate were kept nearly constant to highlight the chemical effect of 1,3-dioxolane addition. Measurements of soot volume fraction with laser-induced incandescence showed that the addition of 1,3-dioxolane up to 30 % leads to a synergistic effect on soot formation with a maximum at 10 %. To reveal the chemistry behind the synergistic effect and its attenuation, speciation measurements were performed on three representative flames (0 %, 10 %, and 30 % fuel mole fraction of 1,3-dioxolane) with gas chromatography-mass spectrometry. The experimental data were compared to kinetic model simulations for interpretation. It was found that the addition of 1,3-dioxolane enhances methyl radical formation, which promotes C3 species formation via C1+C2 pathways. As a consequence, C3 species increase monotonically, whereas some C4 species show a non-monotonic behavior. Pathway analyses revealed that benzene dominantly forms via the C3 route. The main soot precursor naphthalene primarily forms via C4+2C3 pathways through C7 species, and benzene-based pathways, i.e., C6+C4 and H-abstraction-C2H2-addition (HACA), play a minor role. The observed synergistic effect on naphthalene formation is promoted by C3 species for 1,3-dioxolane addition in small amounts. At higher fuel mole fractions of 1,3-dioxolane, C4 species become the bottleneck in naphthalene formation and the synergistic effect subsides. Naphthalene passes this behavior mainly via the HACA pathway to larger polycyclic aromatic hydrocarbons and subsequently to soot.

Upward Flame Spread over Two Parallel Paper Sheets under Reduced Ambient Pressure and Elevated Oxygen Concentration

ABSTRACT The effects of separation distance and environmental parameters (such as ambient pressure and oxygen concentration) on the upward flame spread over two parallel paper sheets were investigated in this work, which would be conducive to enriching the fire dynamic theories and providing a reference for the risk evaluation in building and space fires. It was found that, with the increase of separation distance, the flame spread regimes can be roughly divided into four classes: the flame quenching regime (without inner flame), where the interaction of two parallel papers is mainly dominated by the solid surface radiation; the complete fusion regime, limited by the oxygen diffusion; the partial fusion regime, dominated by the convection; the separated flames regime, dominated by the gas-phase radiation. The flames inside the narrow space are fused under the flame-induced horizontal flow, and extinguished for the oxygen starvation, which reflects the interaction of two parallel samples. Interestingly, although the flames inside and outside the parallel sheets often have different flame lengths and burning intensities, they spread at almost the same rate, which is ascribed to the overlapping of the pyrolysis region on both sides of the thermal thin paper. A critical separation distance was found, where the flame spread rate is largest due to the stoichiometric mixture fraction. The orange-blue layered flames were analyzed, which is related to the boundary layer of the flow field and the mixture of fuel and fresh air. The oxygen concentration has nearly equivalent impacts on the flame-induced buoyancy and horizontal flows, and thus a constant flame fusion angle is maintained. Nevertheless, the enhanced ambient pressure mainly promotes the buoyancy and reduces the flame fusion angle. The flames inside the narrow space are apt to be extinguished under lower ambient pressure with weak buoyancy similar to the reduced gravity and microgravity environments. The transformation of the dominating regimes were observed at a moderate separation distance: the flame quenching regime at pressures below 18 kPa, the complete fusion regime at pressures between 18 ~ 29 kPa, and the partial fusion regime at pressures over 29 kPa.

Investigation of soot inception limits and chemiluminescence characteristics of laminar coflow diffusion flames in C/O ratio space

This study investigates soot inception limits of laminar coflow diffusion flames in the carbon-to-oxygen atom (C/O) ratio space, as well as the mechanism of flame structure affecting soot inception. And chemiluminescence characteristics in flames are evaluated. Oxy-ethylene flames with the same adiabatic flame temperature (2500 K) and different stoichiometric mixture fractions (Zst, 0.3–0.6) are investigated by relocating CO2 from oxidant to fuel. The position of soot inception and the chemiluminescence of CH*, C2*, and CO2* are measured by a hyperspectral imager. A numerical model with a detailed kinetic mechanism is used to obtain the temperature and gas-phase species concentration. The results prove that the critical C/O ratio favorable for soot inception is 0.53, and the critical temperatures on the flame centerline and sheet are 1322 K and 1632 K, respectively. The comparatively low temperature on the flame centerline can cause soot inception, as the polycyclic aromatic hydrocarbons (PAHs) concentration and residence time can guarantee the collision and cluster formation of PAHs. Although fuel pyrolysis is promoted on the flame sheet, oxygen-containing species dilutes the concentrations of fuel pyrolysis products and PAHs to limit soot inception, necessitating high temperatures for chemical bond formation. Furthermore, the mixture fraction, local C/O ratio, and net reaction and production rate are calculated, and the key reaction paths are also identified. The results demonstrate that the increase of Zst achieves the suppression of soot inception by lowering the formation of benzene and PAHs in the high C/O ratio (>0.5) zone, improving the oxygenation in the low C/O ratio (<0.5) zone, and compressing the C/O ratio space favorable for soot formation. Meanwhile, the (CH* and C2*) chemiluminescence zone with a C/O ratio of roughly 0.5 exists in the flame. With Zst increases, the CH* and C2* chemiluminescence diminishes at the flame root and enhances downstream, whereas CO2* chemiluminescence remains unaltered.

Effect of equivalence ratio and temperature on soot formation in partially premixed counterflow flames

Soot formation is quantified in detail (volume fraction, particle size, number concentration, and light emissivity dispersion exponent) in a series of partially premixed counterflow flames of ethylene at equivalence ratios, Φ, equal to 6.5, 5.0, and 4.0, and with maximum temperature spanning approximately 200 K. The focus is to investigate the effect of peak temperature and equivalence ratio on soot formation while maintaining constant global strain and stoichiometric mixture fraction. Oxygen is progressively displaced from the oxidizer to the fuel stream of a diffusion flame to stabilize partially premixed flames of decreasing Φ, showing a double-flame structure consisting of a rich premixed flame component stabilized on the fuel side of the stagnation plane and a diffusion flame component stabilized on the oxidizer side. Soot is detected in the region sandwiched between the two flame components, is formed in both of them, and is convected away radially at the Particle Stagnation Plane (PSP). At fixed Φ, raising the peak temperature invariably raises the soot volume fraction throughout the probed region. Vice versa, at fixed peak temperature, lowering the equivalence ratio causes the premixed flame component to shift away from the diffusion flame component, with the consequent broadening of the soot forming region and an increase in both soot volume fraction as well as soot particle sizes through an enhancement of surface growth. Detailed probing of the region in the vicinity of the PSP offers evidence of soot oxidation from molecular oxygen. Furthermore, when the maximum temperature is sufficiently low, the net soot production rate turns negative because surface oxidation overwhelms surface growth. Comparing the soot number production rate inferred from experiments to the dimerization rate of benzene (C6H6), naphthalene (C10H8), and pyrene (C16H10) reveals that only the smallest aromatics are present in flames at sufficiently large concentrations to account for soot nucleation. This observation applies to both the diffusion flame and the premixed flame components and confirms previous findings in strictly diffusion flames.