Flame Chemistry Research Articles

Investigation of the stability, radiation, and structure of laminar coflow diffusion flames of CH4/NH3 mixtures

The stability, radiation, and structure of laminar axisymmetric CH4/NH3/air diffusion flames have been studied using photographic images, spectrally resolved measurements of flame radiation, and the spatial distribution of temperature and major species mole fractions obtained by spontaneous Raman scattering. The fit procedure for the Raman spectra of NH3 includes a hitherto unquantified overtone feature, whose inclusion in the fit significantly improves the NH3 fraction obtained. Nitrogen is used to replace NH3 to separate chemical effects of NH3 addition from those due to dilution. The results show that NH3 addition drastically reduces radiation from carbon-containing species, with progressively increasing strong chemiluminescence from excited NO2 and NH2, indicating a substantial change in flame chemistry. While the Rayleigh/Mie scattering from soot particles is still observed in the Raman spectra at 28% NH3 addition, 46% NH3 in the fuel is seen to suppresses soot formation effectively. The measured axial and radial profiles of temperature and major species indicate a substantial contribution from radial transport from the reaction zone, seriously complicating the relation between composition, mixture fraction, and the corresponding equilibrium temperature and mole fractions.

Interaction of preferential evaporation and low-temperature chemistry in multicomponent counterflow spray flames

With transportation fuels being comprised of many compounds with different physicochemical properties, it is important to consider their impact on the combustion behavior. This study investigates the effects of preferential evaporation, in which different components of a liquid fuel evaporate at different rates, in the context of laminar counterflow spray flames. To isolate these effects, simulations are performed by considering a three-component Jet-A (POSF 4658) surrogate, consisting of n-dodecane, methylcyclohexane, and m-xylene with both preferential and non-preferential evaporation models. We show that the preferential evaporation of the more volatile cycloalkane in the preheated stream changes the gas-phase composition upstream of the flame. The lower reactivity of this component results in a downstream shift in the low-temperature chemistry zone. This leads to less penetration of the spray into the flame, a lower fuel consumption speed, and a higher gaseous mixture fraction throughout the reaction zones, compared to the non-preferential model. These effects highlight the necessity of considering preferential evaporation behavior when constructing models for multicomponent spray combustion.

DNS Study of Spherically Expanding Premixed Turbulent Ammonia-Hydrogen Flame Kernels, Effect of Equivalence Ratio and Hydrogen Content

In this study, 3D premixed turbulent ammonia-hydrogen flames in air were studied using DNS. Mixtures with 75%, 50% and 25% ammonia (by mole fraction in the fuel mixture) and equivalence ratios of 0.8, 1.0 and 1.2 were studied. The studies were conducted in a decaying turbulence field with an initial Karlovitz number of 10. The flame structure and the influence of ammonia and the equivalence ratio were first studied. It was observed that the increase in equivalence ratio smoothened out the small scale wrinkles while leading to strongly curved leading edges. Increasing the amount of hydrogen in the fuel mixtures also led to increasingly distorted flames. These effects are attributed to local increases in the equivalence ratio due to the preferential diffusion effects of hydrogen. The effects of curvature on the flame chemistry were studied by looking at fuel consumption rates and key reactions. It was observed that the highly mobile H2 and H species were responsible for differential rates of fuel consumption in the positively curved and negatively curved regions of the flame. The indication of a critical amount of hydrogen in the fuel mixture was observed, after which the trends of reactions involving H radical reactions were flipped with respect to the sign of the curvature. This also has implications on NO formation. Finally, the spatial profiles of heat release and temperature for 50% hydrogen were studied, which showed that the flame brush of the lean case increases in width and that the flame propagation is slow for stoichiometric and rich cases attributed to suppression of flame chemistry due to preferential diffusion effects.

Measurement and scaling of turbulent burning velocity of ammonia/methane/air propagating spherical flames at elevated pressure

This study reports the accurate laminar burning velocity, turbulent burning velocity and its correlations of ammonia/methane/air propagating spherical flames. The experiments were carried out on a medium-scale, fan-stirred cylindrical combustion chamber with ammonia molar content varying from 20% to 60% and the initial pressure up to 3 bar. The turbulent burning velocity decreases with the ammonia content due to the weakening effect of the laminar burning velocity under all turbulence intensities and pressures studied. Since the weakening of flame chemistry is dominated by the enhancement of turbulence eddies, the normalized turbulent burning velocity increases with the ammonia content. The turbulent expanding flame of ammonia/methane/air is self-similar under different ammonia content. This self-similar propagation follows the one-half power-law correlation between the normalized turbulent burning velocity, ST/SL, and the turbulent flame Reynolds number, which is quantitatively consistent with that of unity Lewis number methane/air flames (Chaudhuri 2012). The pressure dependence of turbulent burning velocity can be represented roughly as a 0.4 power law of ST/SL and (u′/SL)(P/P0). However, there is a quantitative gap between the pre-exponential factor of the present experimental data and the literature data based on this correlation, which could attribute to the difference in the turbulence eddy scales of different experimental apparatus. The integral length scale characterized the largest turbulence eddies is introduced to consider the turbulent length scale effect. A modified general correlation ST/SL∼(LI/L0)0.5[(u′/SL)(P/P0)]0.41 with the consideration of the integral length scale effect is obtained, which is able to predict a variety of spherical flame data regardless of temperatures, pressures, and fuel types. In addition, it is verified that turbulent burning velocity of ammonia flame could be expressed by the correlation of Karlovitz and Damköhler numbers: ST/SL∼Ka·Da=ReT,flow0.5. It can be seen that ammonia has similar turbulent combustion characteristics as hydrocarbon fuel. These findings indicate that it is feasible to simulate and optimize ammonia combustors utilizing previous turbulent burning velocity correlations based on hydrocarbon fuel.

Carbon fiber oxidation in combustion environments—Effect of flame chemistry and load on bundle failure

Carbon fiber (CF)-based composites are susceptible to fire attack. In this work, we assess the effect of flame chemistry and load on the oxidation-induced failure of three polyacrylonitrile-based CFs with different tensile modulus. Traditional thermal analyses initially showed the influence of microstructure and heating rate on the oxidative behavior. Two different flame-based tests were implemented to address the CF behavior under true fire conditions. Filaments were inserted into a premixed methane/air flame. Subsequent scanning electron microscopy revealed prompt and heterogeneous fiber pitting even after short exposures (t≈ 0.5s). The flame stoichiometry, characterized by the fuel/oxidizer ratio (ϕ), was varied to assess its effect on the CF bundle failure considering time-to-failure (TTF) as an indicator of fire resistance. The trends are similar for the three CF types. The stoichiometric flame (ϕ=1.0) yielded the least aggressive conditions despite being the hottest. Slightly faster bundle failures were caused by the fuel-rich configuration (ϕ=1.2), suggesting a contribution from OH radicals. Higher tensile loads tended to reduce this difference. The enhanced aggressiveness of the fuel-lean condition (ϕ=0.7), i.e., oxygen-rich flame, was confirmed by a TTF at least 50% shorter than its stoichiometric counterpart. Moreover, we found a striking one order-of-magnitude difference in TTF between the high and standard/intermediate modulus CF bundles, regardless of ϕ. The results highlight the critical importance of flame chemistry and fiber microstructure in the fire resistance of CF-based structures.

Experimental and kinetic studies of laminar burning velocities of ammonia with high Lewis number at elevated pressures

The present study aims to expand the available database of laminar burning velocities to a higher initial pressure for ammonia kinetic model validation and to investigate the ammonia flame chemistry especially for the effect of N2 addition on N2 and NOx production. In the present study, a high-pressure constant-volume combustion vessel with the pressure release tank was developed and validated. The variations of Lewis number with diluent (He/N2, He/Ar) ratio and the relationships of Lewis number and adiabatic flame temperature with initial pressures were investigated to prevent the flame instability effects and create safe experiments at high pressures. The laminar burning velocities of NH3/O2/He/Ar mixtures with Lewis number around 1.5 and the fixed adiabatic flame temperature of 2250 K at elevated pressures (up to 1.5 MPa) were measured and the overall reaction order of about 1.6 was determined, besides, the effects of pressure on laminar burning velocities and NOx formations were investigated. Moreover, to provide more useful validation data at high pressure, the laminar burning velocities of NH3/O2/He/Ar mixtures at different adiabatic flame temperatures and initial pressure of 1.0 MPa were presented. Additionally, the predictions of four kinetic models on the experimental data were compared and the discrepancies between the calculated and measured results were found. Since ammonia contains nitrogen element, the effects of N2 addition on the laminar burning velocities and the production of N2 and NOx in ammonia combustion were investigated and the physical and chemical effects of N2 addition were separated. Results showed that the laminar burning velocities and NOx concentration for NH3/O2/N2 flames decreased with the increase of N2 dilution and a slight non-linear tendency was observed. Both physical and chemical effects inhibit the N2 production in the NH3/O2/N2 mixtures. The chemical effects play a dominant role in the inhibition of N2 production at the N2 dilution rates below 0.3. The main kinetic effect of N2 addition on NO production is that the addition of N2 shifts the reaction N + NO <=> N2 + O to the left-hand side.

MID-IR laser absorption spectroscopy of 1- and 2-butanol in a shock tube facility

E-fuels are CO2-neutral fuels generated using renewable fuels such as wind and solar energy, where the carbon is taken out of the atmosphere or from biomass. One of the suitable e-fuel classes are alcohols with a low number of carbon atoms. Obtaining the low pollutant emissions on combusting liquid fuels such as fuel oil and kerosene, e-fuels with lean, premixed, pre-vaporized (LPP) concept plays a pivotal role. So, in this framework, the present work focuses on the combustion properties in the low pressure and high-temperature regime of 1- and 2-butanol isomers to evaluate their potential as future sustainable aviation fuels. The study investigates the fuel-lean conditions of 1- and 2-butanol in the fuel/oxygen equivalence ratios of 0.25, 0.5, and 0.9 relevant to the LPP (Yan et al. 2015) combustion concept for its applications in aviation. The flame chemistry of butanol isomers has been widely investigated; however, more investigations on ignition chemistry are needed. For this, tunable diode laser absorption spectroscopy is utilized to obtain the time-resolved CO formation history, ignition delay time (IDT), and progress in the butanol consumption by applying the central wavelength of 2059.91 cm−1 and 2948.27 cm−1, respectively. The measurements were performed in the temperature range of 1150–1500 K near 1.5 and 3 bar. The experimentally obtained IDT were validated with several recent reaction mechanisms available in the literature. The correlation between measured IDT, CO formation, and butanol consumption data was captured by the simulations satisfactorily. However, the CO formation from the simulation shows disagreement with the experimentally obtained signal. In addition, discrepancies in the absolute CO were observed between simulation and experiment. Moreover, a sharp drop was observed for the butanol consumption histories predominantly at ϕ= 0.9 in both 1-and 2-butanol, which could not be predicted from the available mechanisms. Furthermore, by updating the rate constants of several key reactions near the sharp fall region, a better prediction from the mechanism was seen. The time-resolved CO profile and butanol consumption history, along with the absorption cross-section data for 1- and 2-butanol isomers, are reported for the first time in this paper.

Combustion of C1 and C2 PFAS: Kinetic modeling and experiments

ABSTRACT A combustion model, originally developed to simulate the destruction of chemical warfare agents, was modified to include C1-C3 fluorinated organic reactions and kinetics compiled by the National Institute of Standards and Technology (NIST). A simplified plug flow reactor version of this model was used to predict the destruction efficiency (DE) and formation of products of incomplete combustion (PICs) for three C1 and C2 per- and poly-fluorinated alkyl substances (PFAS) (CF4, CHF3, and C2F6) and compare predicted values to Fourier Transform Infrared spectroscopy (FTIR)-based measurements made from a pilot-scale EPA research combustor (40–64 kW, natural gas-fired, 20% excess air). PFAS were introduced through the flame, and at post-flame locations along a time-temperature profile allowing for simulation of direct flame and non-flame injection, and examination of the sensitivity of PFAS destruction on temperature and free radical flame chemistry. Results indicate that CF4 is particularly difficult to destroy with DEs ranging from ~60 to 95% when introduced through the flame at increasing furnace loads. Due to the presence of lower energy C-H and C-C bonds to initiate molecular dissociation reactions, CHF3 and C2F6 were easier to destroy, exhibiting DEs >99% even when introduced post-flame. However, these lower bond energies may also lead to the formation of CF2 and CF3 radicals at thermal conditions unable to fully de-fluorinate these species and formation of fluorinated PICs. DEs determined by the model agreed well with the measurements for CHF3 and C2F6 but overpredicted DEs at high temperatures and underpredicted DEs at low temperatures for CF4. However, high DEs do not necessarily mean absence of PICs, with both model predictions and limited FTIR measurements indicating the presence of similar fluorinated PICs in the combustion emissions. The FTIR was able to provide real-time emission measurements and additional model development may improve prediction of PFAS destruction and PIC formation. Implications: The widespread use of PFAS for over 70 years has led to their presence in multiple environmental matrixes including human tissues. While the chemical and thermal stability of PFAS are related to their desirable properties, this stability means that PFAS are very slow to degrade naturally and potentially difficult to destroy completely through thermal treatment processes often used for organic waste destruction. In this applied combustion study, model PFAS compounds were introduced to a pilot-scale EPA research furnace. Real-time FTIR measurements were performed of the injected compound and trace products of incomplete combustion (PICs) at operationally relevant conditions, and the results were successfully compared to kinetic model predictions of those same PFAS destruction efficiencies and trace gas-phase PIC constituents. This study represents a significant potential enhancement in available tools to support effective management of PFAS-containing wastes.

On the controlling parameters of the thermal decomposition of inhibiting particles: A theoretical and numerical study

Alkali metal compounds, such as (NaHCO3)s and (Na2CO3)s, are widely used fire suppressants that can be spread as powders or aqueous solutions. Their efficiency depends strongly on the capacity of the alkali metal particles to decompose inside the flame front. The paper aims at providing a theoretical basis for: (1) manufacturing efficient powders; (2) selecting inhibitor candidates that could be efficient against a given reactive mixture. To do so, analytical expressions for the decomposition position of these inhibiting particles are sought to unveil the influence of flame and powder properties on the thermal decomposition problem. The latter is first solved analytically using a simple model for the thermal decomposition of alkali metals combined with a 2-step chain-branching mechanism for the flame chemistry. The analytical solutions are then confronted to numerical simulations using detailed chemistry for both CH4/air and H2/air flames. The validity of the analytical derivations is discussed in terms of combustion regime, characterized in this paper by the ratio of chain-termination to chain-branching time scales. Based on these analytical derivations, a model is proposed for the critical particle size above which mild to no inhibition may be observed. This model accounts for the thermo-chemical properties of the flame/particle system, while being perfectly suited for fine-tuning using either numerical or experimental data. Finally, the paper opens a discussion on the capacity of alkali metal powders to efficiently mitigate highly reactive mixtures, e.g.,C2H2/air and H2/air flames, given the predicted particule sizes needed for fast decomposition in such cases.

Temperature-dependent ion chemistry in nanosecond discharge plasma-assisted CH4 oxidation

Ion chemistry with temperature evolution in weakly ionized plasma is important in plasma-assisted combustion and plasma-assisted catalysis, fuel reforming, and material synthesis due to its contribution to plasma generation and state transition. In this study, the kinetic roles of ionic reactions in nanosecond discharge (NSD) plasma-assisted temperature-dependent decomposition and oxidation of methane are investigated by integrated studies of experimental measurements and mathematical simulations. A detailed plasma chemistry mechanism governing the decomposition and oxidation processes in a He/CH4/O2 combustible mixture is proposed and studied by including a set of electron impact reactions, reactions involving excited species, and ionic reactions. A zero-dimensional model incorporating the plasma kinetics solver ZDPlasKin and the combustion chemical kinetics solver CHEMKIN is used to calculate the time and temperature evolution of the ion density. Uncertainty analysis of ionic reactions on key species generation is conducted by using different referenced data, and insignificant sensitivity is found. The numerical model is consistent with experimental data for methane consumption and generation of major species including CO, CO2, and H2. By modeling the temporal evolution of key ions, it is observed that O2 + presents the largest concentration in the discharge stage, followed by CH4 +, CH3 +, and CH2 +, which is in accordance with the traditional ion chemistry in hydrocarbon flames and agrees well with molecular-beam mass spectrometer investigations. The path flux shows that the concentrations of key species, including electrons, O, OH, H, O(1D), O2(a1Δg), O2 +, CH3 +, and CH4 +, change within 1–2 orders of magnitude and that the transition from a homogeneous state to a contracted/constricted state does not occur. The path flux and sensitivity analysis reveal the significant roles of cations in the stimulation of active radical generation, including CH, O, OH, and O(1D), thus accelerating methane oxidation. This work provides a deep insight into the ion chemistry of temperature-dependent plasma-assisted CH4 oxidation.

Chemical effects of carbon dioxide in ethylene, ethanol and DME counter-flow diffusion flames: An experimental reference for the fictitious CO2 flame

The chemical effects of carbon dioxide on flame chemistry were investigated numerically and experimentally in ethylene, ethanol, and dimethyl-ether (DME) counter-flow diffusion flames. In this work, a method that could add an experimental reference flame for the fictitious CO2 flame (FCO2) was proposed. The fuel/oxidant carrier gas (FCO2+argon+helium) in the FCO2 flames was replaced by an inert gas mixture (nitrogen+argon+helium) whose composition was optimized to maintain approximately the same thermal and dilution effects as FCO2 addition. The differences in the flame temperatures and the measured species concentration between CO2 flame and the inert gas mixture flame could then be attributed to the pure chemical effects. The new experimental strategy was applied to ethylene, ethanol, and dimethyl-ether (DME) counter-flow diffusion flames. Good agreement was found between the experimental and the computational results. The chemical effects of CO2 addition, indicated by both the numerical FCO2 flame and the experimental inert gas mixture flame, both decreased the peak flame temperatures and the concentration of CH4, H2, and C2H2 except CO. The effects of CO2 addition on the production pathways of CO and C2H2 were very similar in ethylene, ethanol, and DME flames. Decreased CO oxidation rate through CO+OH = CO2+H was a significant feature of the chemical effects identified by both the FCO2 flame and the inert gas mixture flame. Thermal effects of CO2 addition decreased the formation rate of C2H2 through C2H3(+M) = H+C2H2(+M) while the chemical effects on this pathway were weaker.

Conditional moment closure method with tabulated chemistry in adiabatic turbulent nonpremixed jet flames

The conditional moment closure (CMC) method with tabulated chemistry is proposed to reduce the computational load for integration of stiff reaction steps in the conventional CMC methods. It is based on the assumption that temperature and all species except CO and CO2 adjust fast enough so that they can be uniquely determined by the single reaction progress variable given as the sum of CO and CO2 mass fractions. The CMC equations are solved for the conditional reaction progress variable, Qpv, whereas the other species mass fractions and the reaction rates including that for Qpv are retrieved from lookup tables. A steady homogeneous form of the conditional variance and covariance equations are solved to construct the second-order closure table to be consistent with Steady Laminar Flamelet Model (SLFM)-based first-order closure. Good agreement is shown between the results by integrated and tabulated chemistry in both the first-order and the second-order CMC for Sandia Flames D, E and F. The overall computational time is reduced to about 0.5% of that by the conventional CMC method involving integration of the detailed reaction mechanism for all species and temperature.

Simulation of an Industrial Carbon Black Reactor Using Collision Kinetics

Purpose: Objective of the work was to test efficacy of the proposed flame chemistry and collision kinetics for prediction of process parametres through determination of the effect of basic process parameters on yield (which includes but not limited to grade of carbon black produced).&#x0D; Methodology: The research methodology in this work was simulation of an industrial Carbon Black Reactor based on reaction kinetics from flame chemistry which assumes that primary particle formation and particle growth is strictly by collision of molecular nuclei with gas molecule as proposed by the collision theory. Decant oil from the Fluid Catalytic Cracking Unit (FCCU) of the Warri Refinery and Petrochemical Company Limited of Nigeria represented by naphthalene was used as feedstock in the simulation while methane gas is the fuel for combustion needed to attain the reaction temperature.&#x0D; Findings: Results showed an excellent quantitative prediction of trends by models. Qualitative predictions gave far higher parameter values, something easily attributable to the excessively high values of kinetic data used for model testing. &#x0D; Recommendation: The simplifying assumptions of these models completely ignored microscopic phenomena such as interface mass and heat transfer and other similar processes. Consequently, the model can be improved upon by introducing some of these processes as identified.

Flame aerosol synthesis of hollow alumina nanoshells for application in thermal insulation

Commercial implementation of hollow nanoshell materials is often limited by difficult scale-up and high cost. Here we present a scalable, continuous, and potentially low-cost route to hollow nanostructured materials using a High Temperature Reducing Jet flame aerosol reactor. The key advantage of this process over traditional flame aerosol synthesis methods is separation of flame chemistry and particle formation, which allows production of hollow nanoshells from an aqueous salt solution without any template or surfactant. We produced amorphous hollow alumina with an average diameter of 181 nm and shell thickness below 10 nm, and demonstrated the generality of the method by also producing hollow SiO2, ZrO2, Ga2O3, and Co2O3. We characterized the morphology, phase transition, thermal stability, and thermal conductivity of the flame-made hollow alumina by electron microscopy, x-ray diffraction, and thermal analysis methods. As a prototypical application, we incorporated hollow alumina into thermal insulation, combining it with common fiberglass insulation to reduce the thermal conductivity by 30% relative to pure fiberglass, to 0.028 W m−1 K−1 while maintaining robust fire-resistance and elasticity.

Experimental and numerical study on sooting transition process in iso-octane counterflow diffusion flames: Diagnostics and combustion chemistry

The present work focused on the combustion chemistry in sooting transition process in iso-octane counterflow diffusion flames. A series of iso-octane flames ranging from sooting-free to heavy-sooting were investigated by stepwise increasing the oxygen concentration (XO) in the oxidizer stream. In particular an optical approach with two different algorithms based on the three wavelengths distribution of flame images captured by digital single lens reflex (DSLR) camera was proposed to identify the critical condition for the sooting transition process. The results showed that the sharp increase in signal ratio of red luminosity with simultaneously obvious decrease in signal rations of blue luminosity could be used as an effective indicator of sooting onset in flames. On-line gas chromatography (GC) measurements were used to investigate the flame chemistry and determine quantitative mole fraction profiles of stable and reactive species formed in the combustion process. Particular attention was given to the combustion chemistry changes caused by the occurrence of sooting transition. Numerical kinetic simulations were performed along with the comparisons with experimental results to obtain deep insights into the sooting transition process, with focus on benzene chemistry considering the critical role of benzene formation in the subsequent large aromatic species/soot formation process. In most cases, the trend observed in the experiments could be satisfactorily reproduced and explained by the models. For major species and C1–C4 species, it can be seen that the concentration increased with the increase of XO in sooting transition process. For the decomposition of fuel and the formation of benzene, the influence in sooting transition process was mainly reflected in the contributions of different pathways due to temperature effects. The impact of sooting transition on flame chemical structure was mainly reflected in the growth of PAHs. The increasing trend for the PAHs growth caused by the increase of oxygen concentration in the transition process was more and more obvious.

Effects of stretch-chemistry interaction on chemical pathways for strained and curved hydrogen/air premixed flames

In most combustion scenarios, stretch-chemistry interaction can directly alter the flame structure and combustion properties of a flammable mixture. In this study, the effects of stretch-chemistry interaction on chemical pathways of hydrogen oxidation are numerically investigated by considering laminar Inwardly Propagating Flames (IPF) and Outwardly Propagating Flames (OPF). One-dimensional transient simulations considering detailed chemistry and transport are conducted for different hydrogen/air mixtures with Lewis numbers well below and above unity. It is observed that the chemical pathways are affected by both flame stretch and flame propagation process. The relative roles of dominant elementary reactions vary with the Lewis number. In IPF and OPF, the negative and positive stretch rate are found to have opposite effects on the chemical pathway. When the relative importance of a reaction is enhanced by the positive stretch in OPF, it is weakened by the negative stretch in IPF. Furthermore, the chemical pathways obtained in this study are compared with other canonical flame configurations in previous work. It is observed that the relative importance of individual reaction is similar in different flame configurations, while disagreement is noticed in the quantitative contributions to the heat release and radical production. The disagreement is due to the different combinations of strain and curvature embodied in the stretch rate. To illustrate the individual role of the two stretch components respectively originating from hydrodynamic strain and flame curvature, a recently developed composition space model is used to decouple the influence of strain and curvature. It is found that these two stretch components have different effects upon chemical pathways. Overall, the hydrogen chemical pathway is more sensitive to the stretch originating from the curved flame propagation than to the flow field strain rate. Though the stretch rate is sufficient to characterize the flame chemistry for fuel-rich hydrogen/air flames with a negative stretch rate, more specific information on strain and curvature is required for fuel-lean and stoichiometric hydrogen/air flames.

Fuel molecular structure effect on soot mobility size in premixed C6 hydrocarbon flames

Soot formation in premixed laminar flames is examined for a canonical set of flames burning C6 hydrocarbon fuels. Particle mobility size and flame temperature measurements are complemented by flame structure calculations using detailed flame chemistry. Specifically, the evolution of the detailed soot particle size distribution (PSDF) is compared for n-hexane, n-hexene, 2-methylpentane, cyclohexane and benzene at a carbon-to-oxygen ratio of 0.69 and maximum flame temperature of 1800 K. Under this constraint, the overall sooting process is comparable as evidenced by similar time resolved bimodal PSDF. However, the first inception of particles and the persistence of nucleation-sized particles with time are depend upon the structure of the parent fuel. For the given conditions, the fastest onset of soot is observed in cyclohexane and benzene flames and the observed evolution of the PSDF also shows that nucleation-sized particles disappear sooner in cyclohexane and benzene flames. Flame structure computations incorporating detailed chemistry show a clear connection between the early onset of soot particles as fuel specific routes to PAH formation are predicted in the pre-flame region of the cyclohexane and benzene flames. These observations illustrate the impact of alkane, alkene, cycloalkane and aromatic fuel structure on soot formation in premixed flames. Analysis of soot particle morphology by atomic force microscopy indicates that most of size distribution is composed of aggregates. Simple aggregate mobility diameter analysis shows the spherical assumption taken to interpret the mobility diameter does not impact the PSDF number density result but the inferred volume fraction for aggregates deviates by up to an order of magnitude depending on the morphology assumptions adopted.

Effects of oxygen on soot formation in laminar co-flow flames of binary mixtures of ethane, DME, and oxygen

Soot volume fractions of binary mixtures of ethane, DME and oxygen were evaluated experimentally in a co-flow burner. Soot volume fractions were measured by diffuse-light line-of-sight attenuation technique. The carbon mass flow rate and the C/H ratio were kept constant for all mixtures, while oxygen was introduced into the fuel stream either as an additive or as intramolecular oxygen in DME or both. Visible flame heights were decreased with increasing oxygen in the fuel stream. Binary mixtures of ethane and DME did not show any synergistic effects on soot formation. The maximum soot volume fraction was decreased from 1.41 ppm in the neat ethane flame to 0.08 ppm in the neat DME flame. On the other hand, oxygen addition to either ethane or DME resulted in an increase in soot concentrations. The maximum soot volume fraction reached to 2.1 ppm and 0.29 ppm in ethane/oxygen and DME/oxygen mixtures, respectively. The peak soot yield initially decreased for all mixtures studied as oxygen was introduced into the fuel tube. A unique transition was observed in DME/oxygen mixtures at an equivalence ratio of 6.8. When more oxygen was added beyond this ratio, the effect of oxygen on soot formation was suddenly reversed, and the peak soot volume fraction and yield were significantly decreased. This sudden reversal was explained as a result of a transition into a more premixed mode of combustion, supported by the flame images as well as chemiluminescence measurements. A distinct two-zone flame structure, that is, an inner rich zone and an outer stoichiometric zone separated by a dark region, was observed for the neat DME flame and DME/oxygen mixture flames. It is argued that as oxygen becomes available from DME decomposition, it produces a premixed region, and the flame chemistry is significantly altered beyond a critical oxygen concentration. Current radially resolved soot volume fraction measurements suggest that the effects of oxygen on soot formation can be quite complex with nonmonotonic behaviour, sudden effect reversals and shifts in soot formation and oxidation zones.

Numerical Modeling of Flame Shedding and Extinction behind a Falling Thermoplastic Drip

The dripping of molten thermoplastics is a widely observed phenomenon in cable and façade fire, where the large drips can often carry a blue chain flame during the free fall to ignite other flammable materials and escalate the fire hazard. This work simulated the flame evolution behind a falling thermoplastic drip with the DNS model and finite-rate flame chemistry. The accelerated free-fall of drip was modeled by fixing the position of drip, increasing the upward airflow, and setting a fuel jet on the top of the drip. Modeling reproduces the dripping flame and reveals the flame shedding to be a combination of a lifted flame and a vortex street, where the lifted flame caused by the gravity acceleration of drip is identified as the critical factor that governs the shedding formation. As the diameter of drip decreases, the falling drip becomes difficult in forming a stable shedding structure in the wake region, so that the dripping extinction occurs due to the dilution and cooling of airflow, agreeing well with the experimental observation. This work reveals the underlying mechanism of stabilizing the dripping flame and helps evaluate the fire risk and hazard of dripping phenomena.

Potential of hydrogen addition to natural gas or ammonia as a solution towards low- or zero-carbon fuel for the supply of a small turbocharged SI engine

Nowadays there is an increasing interest in carbon-free fuels such as ammonia and hydrogen. Those fuels, on one hand, allow to drastically reduce CO2 emissions, helping to comply with the increasingly stringent emission regulations, and, on the other hand, could lead to possible advantages in performances if blended with conventional fuels. In this regard, this work focuses on the 1D numerical study of an internal combustion engine supplied with different fuels: pure gasoline, and blends of methane-hydrogen and ammonia-hydrogen. The analyses are carried out with reference to a downsized turbocharged two-cylinder engine working in an operating point representative of engine operations along WLTC, namely 1800 rpm and 9.4 bar of BMEP. To evaluate the potential of methane-hydrogen and ammonia-hydrogen blends, a parametric study is performed. The varied parameters are air/fuel proportions (from 1 up to 2) and the hydrogen fraction over the total fuel. Hydrogen volume percentages up to 60% are considered both in the case of methane-hydrogen and ammonia-hydrogen blends. Model predictive capabilities are enhanced through a refined treatment of the laminar flame speed and chemistry of the end gas to improve the description of the combustion process and knock phenomenon, respectively. After the model validation under pure gasoline supply, numerical analyses allowed to estimate the benefits and drawbacks of considered alternative fuels in terms of efficiency, carbon monoxide, and pollutant emissions.