Premixed Stagnation Flame Research Articles

Synthesis of self-assembled spherical ZnFe2O4 nanomaterials by the premixed stagnation flame method for highly sensitive acetone sensor

The efficient and reliable detection of acetone is critical for industrial safety, environmental monitoring, and human health. Separately, ZnFe2O4 particles were synthesized by using the premixed stagnation flame method and the co-precipitation method. The premixed stagnation flame method was used to create self-assembled spherical ZnFe2O4 nanoparticles with uniform morphology, polycrystalline morphology, and excellent phase purity. The effect of the gas-sensitive performance of these sensors for two routes on acetone vapor was compared. The sensor synthesized using the premixed stagnation flame method had a response value of up to 26 at 235°C for 100 ppm acetone vapor. It revealed outstanding reproducibility, stability, and selectivity for acetone vapor, which is attributed to the 10 nm "lychee-like" particle structure uniformly dispersed on the surface of the 400–500 nm spherical particles, forming a unique self-assembled structure. The self-assembled spherical structure has a large specific surface area, abundant oxygen adsorption, and oxygen vacancies, which effectively increases the reaction sites and electron mobility inside and outside the nanoparticles. The premixed stagnation flame method of producing spherical ZnFe2O4 nanoparticles presents a viable method for developing extremely sensitive, fast-responding, and selective acetone vapor sensors.

Deposition of graphenic nanomaterials from elevated temperature premixed stagnation flames

The work examines the unique nanostructure of carbon nanoparticles deposited from sooting premixed flames with flame temperatures exceeding 2200 K. This flame temperature regime has previously been shown to transition from typical soot formation conditions to a regime whereby the flame-form carbon adopts a nanostructure considerably more ordered than soot. Graphenic carbon deposits observed by High-resolution TEM (HRTEM) are reported here corroborating previous Raman spectroscopy evidence. The use of premixed stretch-stabilized flames enables particle production in the high-temperature regime under a flow field amenable to low-dimensional flame modeling. Although the flame flow configuration is relatively simple, three sample preparation methods are used to assess the representation of true carbon properties as they exist in the flame. HRTEM imaging is carried out on carbon particle samples prepared by rapid-insertion deposition, aerosol dilution probe deposition and carbon particle film deposition. Images from rapid-insertion samples show amorphous particles in the lightly sooting flame and turbostratic particles in the heavy sooting flame. There is trace evidence of graphenic structure in rapid-insertion samples but the most striking particles on the TEM grid are graphite nanocrystals presumably formed by a new artificial crystallization process. HRTEM images of particles collected over time by diluted aerosol deposition and film deposition show clear graphenic structures. Overall, the carbon nanostructure observed by HRTEM is a mixture of amorphous, turbostratic and graphenic carbon lattices depending on the flame condition and sampling method. The current work highlights potential impacts of higher flame temperatures and higher equivalence ratio on deposited flame-formed carbon. Namely, graphenic particle structure is observed in rapid-insertion deposition samples but graphene portions are most abundant in aerosol dilution and carbon particle film deposition samples. This may indicate that graphene structures grow on the deposition surface over time.

Effects of particle collection in a premixed stagnation flame synthesis of sub-stoichiometric [formula omitted] nanoparticles

Flame synthesis is a simple method to prepare sub-stoichiometric titanium dioxide (TiO2-x) nanoparticles. A rotating stagnation plate is often used as a substrate and to provide a cooling mechanism. The collection of particles from the rotating plate could be done in two ways: the conventional interval particle collection (IPC) method and a continuous particle collection (CPC). The effects of the deposition time and the rotation speed on the properties of titanium dioxide (TiO2) particles are investigated experimentally. For IPC, it was found that the properties of the collected samples are dependent on the deposition time. This creates an undesirable correlation between properties and synthesis yield. On the other hand, CPC approach allows for a continuous synthesis in which the particle properties are invariant with respect to the synthesis yield. The tunability of the particle properties is still achievable by controlling the rotation speed in the CPC.

Thermodynamic Barrier to Nucleation for Manganese Oxide Nanoparticles Synthesized by High-Temperature Gas-to-Particle Conversion

A complementary experimental and modeling study is reported here for nucleation of manganese oxide nanoparticles in premixed stagnation flames. The current synthesis occurs at relatively high flame...

Influences of heat flux on extinction characteristics of steady/unsteady premixed stagnation flames

This paper investigates the extinction characteristics of premixed stagnation flames (PSFs) with controlled heat losses and flow disturbances. The low-frequency air flow pulsations that imitate the operational transients in practical combustors were specially introduced. The tunable diode-laser absorption spectroscopy (TDLAS) measurement was applied to obtain the temperature profile and wall heat flux. It is found that, for steady flame with a fixed equivalence ratio, the extinction stretch rate dramatically increases as the wall heat flux decreases. The extinction criterion is summarized as a global Karlovitz number of 0.57 by establishing a relationship between the global and local stretch rates. Numerical simulations reveal that the local extinction Karlovitz number of steady PSFs is approximately 1.0 regardless of the conditions such as heat flux and equivalence ratio. Further experiments present that the air pulsations with a repetition of ∼5 Hz significantly deteriorate the flame stability. Particularly, for unsteady perturbed flames, the extinction stretch rate exhibits a nonlinear trend, yielding two regimes with discrepant sensitivities to wall heat flux. The unsteady simulation then highlights a local stretch rate overshoot in the presence of pulsation. It is caused by the time delay between the inlet velocity and flame front movement that eventually leads to poor flame stability. Moreover, in the high heat-flux regime, a smaller local stretch rate overshoot results in the weak dependence of extinction limits on heat fluxes.

Experimental and numerical studies on electric field distribution of a premixed stagnation flame under DC power supply

In this work, we achieve a sub-breakdown electric field measurement in a premixed stagnation flame by electric-field-induced second-harmonic generation (ESHG) using a nanosecond laser. Under the application of a DC voltage, the premixed flame can transit from a flat substrate-stabilized mode to a conical nozzle-stabilized mode due to the two-way interaction between the electric and hydrodynamic responses of the flame. The average electric fields along the laser pathway for these two flame modes are measured at different heights above the burner. Combining the measurement and the numerical simulation of a charge transport model, we further elucidate that the flame affects the electric field distribution in two different ways. For the flat flame mode, the electric field strength is shielded at the flame front and then quickly increases both upstream and downstream, reaching the maximum at the electrodes. The charge transport model reveals that the electrostatic shielding effect of the flame front can be attributed to the charge redistribution in the chemi-ionized conductive layer. For the conical flame mode, the electric field strength has a large gradient near the conical tip and remains almost zero downstream. The conical flame front then serves as a conductive layer to guide the charge transport under the application of the electric field. The positive and negative charges are separated at different radial positions because of the inclined asymmetric flame front. Thus, the largest electric field gradient is generated upstream of the flame conical tip, where the net charge and the electric body force maximize and create a virtual flame stabilization mode.

Modelling soot formation in a benchmark ethylene stagnation flame with a new detailed population balance model

Numerical simulation of soot formation in a laminar premixed burner-stabilised benchmark ethylene stagnation flame was performed with a new detailed population balance model employing a two-step simulation methodology. In this model, soot particles are represented as aggregates composed of overlapping primary particles, where each primary particle is composed of a number of polycyclic aromatic hydrocarbons (PAHs). Coordinates of primary particles are tracked, which enables simulation of particle morphology and provides more quantities that are directly comparable to experimental observations. Parametric sensitivity study on the computed particle size distributions (PSDs) shows that the rate of production of pyrene and the collision efficiency have the most significant effect on the computed PSDs. Sensitivity of aggregate morphology to the sintering rate is studied by analysing the simulated primary particle size distributions (PPSDs) and transmission electron microscopy (TEM) images. The capability of the new model to predict PSDs in a premixed stagnation flame is investigated. Excellent agreement between the computed and measured PSDs is obtained for large burner-stagnation plate separation ( ≥ 0.7 cm) and for particles with mobility diameter larger than 6 nm, demonstrating the ability of this new model to describe the coagulation process of aggregate particles.

Flame temperature effect on sp2 bonds on nascent carbon nanoparticles formed in premixed flames (T > 2100 K): A Raman spectroscopy and particle mobility sizing study

Abstract The evolution in carbon particle size and carbon bonds was observed with increasing flame temperature for a fixed growth time (tp ∼ 13 ms) and equivalence ratio (Φ = 2.5) in set of sooting premixed stagnation flames. For carbon formed in each flame, detailed particle size distribution functions (PSDF) and Raman spectra (excitation energy of 2.33 eV) were measured as the maximum flame temperature increased from 1911 K

In-situ laser diagnostic of nanoparticle formation and transport behavior in flame aerosol deposition

In this work, we investigate the flame aerosol deposition of TiO2 nanoparticles in a well-defined premixed stagnation flame based on two-dimensional phase-selective laser-induced breakdown spectroscopy (PS-LIBS). The deposited nanoparticles are in anatase phase with an average size of ∼10 nm. In PS-LIBS measurement, atomic emissions of titanium near the wavelength of 500 nm are extracted by spectral filter lenses and imaged for demonstrating the particle volume fraction distributions. The PS-LIBS signals can well depict the whole process of particle formation and transport, including the rapid gas-to-particle conversion near the flame front, dilution and concentrating of the nanoparticles induced by gas density variations, and dropping of particle volume fractions near the substrate. A quantitative model has been proposed to describe convection, thermophoresis, and diffusion of the nanoparticles. Combining the PS-LIBS measurement and the numerical simulation, it is found that, the nanoparticles concentrate outside the boundary layer at low substrate temperatures due to a joint effect of gas compression and thermophoresis. Further parametric analysis indicates that substrate temperatures and precursor loading rates can strongly affect the particle concentration boundary layer and the particle deposition rate.

Catalytic activity of flame-synthesized Pd/TiO2 for the methane oxidation following hydrogen pretreatments

A nanocatalyst composed of 10wt% Pd supported on TiO2 was synthesized using one-step flame spray pyrolysis with a highly-quenched premixed stagnation flame. The as-prepared nanoparticles, having a mean diameter of approximately 10nm, were then subjected to isothermal oxidation and reduction in a fixed bed reactor. The potential roles of Pd and PdO as well as possible synergistic effects between these two active species were explored during methane conversion. The lowest light-off temperature T20 was obtained at a Pd ratio of 53% and this result can be reasonably explained based on a surface Pd–PdO site pair mechanism. Additionally, differences in the hysteresis behavior during methane oxidation were noted throughout a heating–cooling cycle over the temperature range of 200–600°C, such that the degree of hysteresis was strongly correlated with the Pd ratio.

Pd-doped TiO2 film sensors prepared by premixed stagnation flames for CO and NH3 gas sensing

This paper synthesized Pd-TiO2 films by the flame stabling on a rotating surface (FSRS) technique with different doping ratios of Pd. The semiconductor metal oxide (SMO) particles together with Pd additive condensed in the post flame zone, and then deposited directly on a sensor subtract which is fixed on a rotating plant to form the sensing films. The composition and structure of sensing films were assessed with TEM, SEM, XPS, XRD and EDS techniques. It was found that TiO2 particles prepared by the FSRS technique had diameters ranging from 9 to 17 nm and are in anatase type. Pd-doped TiO2 films remained porous anatase and Pd element dispersed well in the TiO2 film. The sensing test results showed that the nano film sensors synthesized by FSRS technique responded rapidly to CO concentration change while rather slowly to NH3 concentration change. The doping Pd improved the response sensitivity of TiO2 sensor remarkably for CO sensing, and a certain extent for NH3. An optimal doping ratio existed and the TiO2 film sensor sample with 1.0 wt.% Pd-doping ratio had the best gas sensing performance.

Electrohydrodynamic instability of premixed flames under manipulations of dc electric fields.

We report an electrohydrodynamic instability in a premixed stagnation flame under manipulations of a dc electric field. This instability occurs when the electric field strength is at a certain value below the breakdown threshold, which is 0.75kV/cm in the experimental setup. Above this value the flame front suddenly transits from a substrate-stabilized near-flat shape to a nozzle-stabilized conical surface, accompanied by a jump in the electric current through the flame field. At the transition moment, the flame spontaneously propagates upstream to the nozzle while the flow velocity at the upstream of the flame front decreases to zero, as revealed by high-speed photographs and PIV measurements. These phenomena indicate a transient balance between the fluid inertia and the electric body force around the instability threshold. A quantitative model suggests that the flame instability can be explained by a positive feedback loop, where the electric field applies a nonuniform electric body force, pulling the flame front upstream, and the pulled flame front in turn enhances the local electric body force. The electrohydrodynamic instability occurs when the electric pulling is strong enough and both the growth rates and the magnitudes of the electric body force on flame exceed those of the fluid dynamic pressure.

Low-frequency AC electric field induced thermoacoustic oscillation of a premixed stagnation flame

We present a novel phenomenon of low-frequency AC electric field induced thermoacoustic oscillations by employing a wall-jet premixed stagnation flame configuration. A high speed camera and a microphone are used to record the instantaneous flame topographies and the acoustic oscillations, respectively. We find that, under manipulations of the AC electric field, the acoustic oscillation occurs after the flame transits from a ‘disc’ mode to an ‘envelope’ mode. The observed correlation between the changing rates of the flame surface area and the pressure fluctuations in the time domain indicates a significant contribution from the unsteady heat release to the acoustic oscillation; meanwhile, the other possible mechanism, i.e. the direct impact of oscillating electric body force on the generated pressure wave, can be ruled out based on a theoretical analysis of a pressure wave equation. The oscillating behaviors of the flame front show that the electric field drives the movement of the flame cusp with a large stretch rate. Assessment of the current density distribution of the weakly-ionized flame plasma is conducted by non-dimensionalizing the convective–diffusive–reactive equation, estimating the electric conductivity, and solving Ohm's Law. This simplified model divulges that the non-uniform current density induces large perturbations at the corner positions of the flame front. The unsteady flame fronts strongly influence the current response of the flame, which indicates a self-looping process among the flame current, the electric body force and the flame reaction rate.

Flame synthesis of novel ternary nanocatalysts Pd/CeO2[sbnd]TiO2 with promotional low-temperature catalytic oxidation properties

One-step flame-assisted spray pyrolysis with highly-quenched premixed stagnation flame is developed to synthesize the supported palladium catalysts on the mixed ceria-titania oxides. Promotional oxygen reducibility and inherent high activity of the catalysts are discovered in contrast to the same loading palladium catalyts on either single support, and the complete methane oxidation is achieved at as low as 450 °C. Palladium valence electronic structure analysis from X-ray photoemission spectroscopy (XPS) indicates that the metal palladium is highly positively charged and electrons transfer from palladium clusters to support oxides during the flame synthesis process. Similar electron transfer can even occur during the cyclic catalytic process for the initially non-charged clusters. From the cyclic experiments, theelectrostatic interaction built up at the metal–support interface plays a major role in the superior sintering resistance for palladium-based catalysts during the combustion process. Moreover, the introduction of water vapor to the feedstock induces a negative effect on the methane oxidation at low temperatures. XPS spectra reveals that the palladium 3d binding energy varies and the interfacial electron transfer flow changes over heating–cooling cycles when exposed to water vapor. More importantly, the excellent thermal stability performances remain even under the water vapor environment.

NO formation in rich premixed flames of C1–C4 alkanes and alcohols

A comprehensive study of prompt nitric oxide (NO) formation is presented for rich (φ=1.3) premixed flames of alkanes and alcohols. Measurements of the flame burning rate, the temperature, and the CH radical and NO species are performed in atmospheric-pressure laminar premixed stagnation flames. New data, obtained for all isomers of the C4 fuels, butane and butanol, are merged with existing data for C1–C3 fuels obtained by the same methodology. The combined dataset consists of all isomeric combinations of the fuel molecules, and provides a complete description of pollutant trends related to fuel type and branching structure for C1–C4 alkanes and alcohols. These data show alcohols produce less NO than alkanes of equivalent carbon chain length. This behavior is linked to the tendency of alcohols to produce lower concentrations of the CH radical, due to inhibition of the formation of methyl radicals. The results show a linear scaling of prompt-NO formation with the CH radical concentrations, when these are scaled by the characteristic residence time in the flame. This residence time depends on the burning velocity of the fuels, which decreases with increasing fuel branching. These data are useful for thermochemical model development using detailed simulations of the experiments and a comparative diagnostics approach. Further adjustments to all of the thermochemical models are needed to capture the observed trends. This dataset is made available and provides validation and optimization targets for future model revisions.

NO formation in premixed flames of C1–C3 alkanes and alcohols

The influence of alkane and alcohol molecular structures on nitric oxide (NO) pollutant formation trends is investigated through the study of C1–C3 alkanes and alcohols in premixed stagnation flames at atmospheric pressure. The flame diagnostics consist of one-dimensional (1-D) Planar Laser-Induced Fluorescence (PLIF) for measurements of NO and CH concentrations, 1-D NO-LIF thermometry, and 1-D Particle Tracking Velocimetry (PTV) for the determination of flame reactivity and burning rates. The results show that alcohols produce less NO than their alkane equivalents. This behaviour is linked to the lower flame temperatures of alcohol flames and the tendency of alcohols to produce lower concentrations of the CH radical, due to inhibition of the formation of methyl groups. These trends are captured in the available thermochemical models; however, there are discrepancies regarding the formation pathways of NO and its intermediates which are linked to variability in model rate coefficients. Further adjustments to all of the thermochemical models are needed to accurately capture the nature of NOx pollutant formation in premixed alkane and alcohol flames, and this experimental study provides target data for this effort.

F211 よどみ流予混合火炎のOH分布に与える表面反応の影響に関する実験的検討(OS-03:革新的技術のための燃焼研究(3))

F211 よどみ流予混合火炎のOH分布に与える表面反応の影響に関する実験的検討(OS-03:革新的技術のための燃焼研究(3))

Diagnostics and Modeling of Stagnation Flames for the Validation of Thermochemical Combustion Models for NOx Predictions

This article presents a combined experimental and modeling approach for the study of NO formation in premixed stagnation flames. In the experiments, one-dimensional (1-D) Particle Image Velocimetry (PIV), 1-D spectrally resolved Planar Laser-Induced Fluorescence (PLIF), and 1-D NO-LIF thermometry are used to measure high resolution profiles, which are directly compared to numerical simulations. The simulations are performed with Cantera using 1-D hydrodynamics, transport, and detailed thermochemical models. Accurate measurements of premixed gas composition, gas velocity, temperature, and spread rate yield all necessary inlet boundary conditions. Use of a temperature-controlled stagnation plate allows for first-order heat loss effects to be imposed on the numerical simulation, rather than relying on external temperature corrections. The experiments provide a sensitive test of NO formation, result in multiple validation targets which do not rely on extrapolations, and allow for accurate specification of measurement uncertainties when comparing experiments to simulations. This article provides a discussion of the diagnostic techniques and compares experimental results for methane–air flames to numerical predictions using a number of natural gas thermochemical models (GRI-Mech 3.0, GDF-Kin 3.0, CRECK 1212, and Konnov 0.6) and their associated submodels for NOx formation. The reported results indicate that further adjustments to thermochemical combustion models are needed to accurately predict NOx pollutant formation in small hydrocarbons under atmospheric-pressure conditions.

Effects of heat conduction and radical quenching on premixed stagnation flame stabilised by a wall

The premixed stagnation flame stabilised by a wall is analysed theoretically considering thermally sensitive intermediate kinetics. We consider the limit case of infinitely large activation energy of the chain-branching reaction, in which the radical is produced infinitely fast once the cross-over temperature is reached. Under the assumptions of potential flow field and constant density, the correlation for flame position and stretch rate of the premixed stagnation flame is derived. Based on this correlation, the effects of heat conduction and radical quenching on the wall surface are examined. The wall temperature is shown to have great impact on flame bifurcation and extinction, especially when the flame is close to the wall. Different flame structures are observed for near-wall normal flame, weak flame, and critically quenched flame. The fuel and radical Lewis numbers are found to have opposite effects on the extinction stretch rate. Moreover, it is also demonstrated that only when the flame is close to the wall does the radical quenching strongly influence the flame bifurcation and extinction. The extinction stretch rate is shown to decrease with the amount of radical quenching for different fuel and radical Lewis numbers. Besides, the coupling between the wall heat conduction and radical quenching is found to greatly influence the bifurcation and extinction of the premixed stagnation flame.

An experimental and reduced modeling study of the laminar flame speed of jet fuel surrogate components

The laminar flame speed is an essential combustion parameter used in the validation of chemical kinetic mechanisms. In recent years, mechanisms tailored for jet fuel surrogate components have been partially validated using the laminar flame speeds of pure components, which were derived using both linear and non-linear extrapolation techniques. However, there remain significant deviations between the results from different studies that motivate further investigation. In this study, laminar, atmospheric pressure, premixed stagnation flames are investigated for the surrogate fuels n-decane, methylcyclohexane and toluene, which are representative of the alkane, cycloalkane and aromatic components of conventional aviation fuel, respectively. Numerical simulations are directly compared to velocity profile measurements to assess the predictive capabilities of the recently proposed JetSurF 2.0 chemical kinetic mechanism. Simulations of each experiment are carried out using the CHEMKIN-PRO software package together with the detailed mechanism, with accurate specification of the necessary boundary conditions from experimental measurements. Furthermore, a skeletal version of the detailed mechanism is deduced for improved computational speed using a species sensitivity reduction method, here referred to as Alternate Species Elimination (ASE). Toluene experimental data are further compared to a detailed toluene mechanism, termed the Stanford mechanism. The experimental and numerical reference flame speeds are used to infer the true laminar flame speed of the compounds following a recently proposed direct comparison technique that is similar to a non-linear extrapolation to zero flame stretch. JetSurF 2.0 and the skeletal ASE mechanisms demonstrate excellent overall agreement with experiment for n-decane and methylcyclohexane flames, for which the original model was optimized, but poor agreement for toluene, which was not an optimization target. Improved agreement for toluene is observed between the Stanford mechanism and experiment. Results confirm that the direct comparison method yields consistent laminar flame speed data irrespective of the reactivity accuracy of the chemical kinetic model employed. The laminar flame speed results from this study are essential for the further development of chemical kinetic mechanisms and contribute to the surrogate modeling of jet fuel combustion.