Jet Flame Research Articles

Flame structure and broadening in turbulent premixed jet flames

High-resolution and high signal-to-noise (SNR) planar Rayleigh scattering thermometry measurements are performed to investigate preheat and reaction zone broadening, turbulent transport of high-temperature gases, and the internal flame structure in a series of highly turbulent, lean (ϕ = 0.75), premixed methane-air turbulent jet flames. These flames are characterized by turbulence intensities ranging from u′/SL of 35 to 150 and turbulent Reynolds numbers approaching 104, as determined from separate particle image velocimetry (PIV) measurements. Average flame profiles obtained using a gradient-based flame reconstruction method and probability density functions (PDFs) of the preheat, reaction, and thermal layer thicknesses show substantial broadening of the preheat and thermal layers. Instantaneous preheat zones are broadened by as much as a factor of 30 compared to laminar flames, with an average broadening approaching a factor of 14 for the most turbulent case. Moderate broadening of the reaction layer is observed with up to a factor of eight times that of a laminar premixed flame for instantaneous realizations and an average broadening of four times that of a laminar flame for the most turbulent case. Conditional statistics of temperature obtained at isodistance contours within and upstream of the preheat zone show large shifts towards significantly higher temperatures compared to laminar flame values. In addition, broad temperature distributions and reduced thermal gradients at each position provide strong evidence of persistent turbulent transport of high-temperature gases ahead of the preheat zone. Finally, a detailed analysis of scalar dissipation rate statistics and dissipation layer topology was conducted. The conditional mean scalar dissipation rates for the turbulent flames show significant reductions compared to calculated laminar flame values and a shift towards higher values of the reaction progress variable (CT) for increasing turbulence levels. PDFs show a transition from near-Gaussian to highly negatively skewed with increasing turbulence levels, where the most probable values of the scalar dissipation rate shift towards higher values of CT, indicating that the primary reaction zone and flame front are subjected to increased strain. In terms of structure, no significant broadening of the small-scale dissipation layers was observed with increasing turbulence intensity. However, as turbulence intensity increases, there is a transition from a sparse number of continuous layers to a large number of shorter, ”broken”, nearly straight segments. The reduction of individual layer lengths (i.e., ”breaking”) is favored over increases in dissipation layer wrinkling, where the sharp decreases in the local dissipation rate values are highly correlated with regions of high curvature. A final notable outcome of this work is that there were negligible differences in all statistical and topological results when moving between the two highest turbulent cases, even though the turbulence intensity, turbulent Reynolds number, and the turbulence Karlovitz number increased substantially (70 - 120%). This implies that there may be an asymptotic limit in which turbulence affects flame behavior for the current configuration.

Mechanisms of air breakdowns for aluminum conductors steel reinforced wire-wire gaps induced by propane jet flames

In recent years, events of transmission line tripping caused by wildfires have occurred frequently in many mountain areas. In order to understand the mechanism of such events, an experimental system for simulating breakdown discharge processes of model wires induced by a simulated fire source is designed. In this experiment, propane jet flame (somewhat like eruptive fires in real wild fires) is used as a simulated flame source. By setting three different gas flow rates and five different wire-wire gaps, discharge breakdown behaviors and characteristics between two ACSR (aluminum conductors steel reinforced) segments in propane flames with three flame zones are explored. The influences of these different factors on the breakdown characteristics and mechanisms of wire-wire air gaps are analyzed. Results show that breakdown voltages increase almost linearly with the increase of wire-wire gaps in continuous zone of the flames for three gas flow rates. For a short wire-wire gap like 5.0 cm, the pilot discharge channels can be formed more easily, stably and completely under a lower voltage and weaker external ionization environment. The propane jet flame conditions are complex and affected by multiple actions like initial jet force (affected by gas flow rate), temperature distribution, soot particle, ion wind. As a result, local electric field will be distorted, a streamer channel is established dependently in different cases. These factors play individual, competitive or synergistic roles at the same time in breakdowns of real ACSR wire-wire gaps.

Characterisation of hydrogen jet flames under different pressures with varying coflow oxygen concentrations

The elevated temperature of hydrogen combustion increases the formation of thermal NOx. Moderate or intense low oxygen dilution (MILD) combustion is known to reduce NOx emissions and increase thermal efficiency. Pressure is often also used for increasing thermal efficiency. The impact that pressure has on fluid dynamics and chemical kinetics is especially relevant in MILD combustion conditions. Hydrogen jet flames issuing into a hot and vitiated coflow were imaged using OH∗ chemiluminescence at different pressures (1–7 bar) and oxygen levels (3–9% by vol.). Laminar flame simulations complemented the experiments. The observed mean radial OH∗ width increased with increased pressure, but only at O2 content less than 9%, suggesting that pressure has greater influence on kinetics when oxygen is reduced. The integrated OH∗ signal strength remained constant at 3% coflow O2, despite an apparent increase in flame width, suggesting a spatial broadening of the flame with pressure. Numerical results indicate that at 3–6% O2, conditions for MILD combustion of H2 are met across a wide range of strains and pressures, supporting the experimental observations for 3% O2.

Effects of Adiabatic Flame Temperature and Oxygen Concentration in CH4/N2/O2 Nonswirl Jet Flames: Experimental and Numerical Study

AbstractThe combustion characteristics of oxygen-enriched air–methane (i.e., O2/N2/CH4) flames in a premixed mode are investigated using both experimentally and numerically under atmospheric conditions for emissions reduction purposes. The investigation is carried out using a gas turbine model combustor equipped with a multihole burner that mimics gas turbine micromixer burners. The resulting flame is of jet type, and the velocity of the jet is kept at 5.2 m/s for all the considered flames. Models used in the numerical study include large eddy simulation, discrete ordinate, and partially premixed combustion for turbulence, radiation, and species models, respectively. The numerical results are validated, and a suitable agreement is achieved with experimental data. The results indicated that the temperature distribution, shape, and size of O2/N2/CH4 flames are predominantly controlled by adiabatic flame temperature (Tad). However, the oxygen fraction, rather than Tad, is responsible for the reaction progress. The emission of NO, CO, and CO2 increases with an increase in oxygen fraction, and the product formation in O2/N2/CH4 flames is less compared to their oxy-fuel (i.e., O2/CO2/CH4) counterparts, because N2 is mostly inert, compared to CO2. The latter participates significantly in flame reactions, which increases the rate of product formation in O2/CO2/CH4 flames.

Experimental and simulation studies on flame characteristics and soot formation of C2H2 jet flames

This work aims to identify the relationship between the temperature and soot formation in the C2H2 laminar flames. The effects of flow rates and equivalence ratios on the flame temperatures and structures were investigated. The flame length increases with the increase of the equivalence ratio. Meanwhile, the flame color changes from pale blue to bright green from the fuel-lean to stoichiometric condition, and further to bright yellow under the ultra-rich conditions. By sampling and observing the morphology of soot at different heights of the flame, we reveal the factors affecting the size of soot. As the flame height increases, the growth rate of soot agglomerates becomes faster. The contribution of surface growth to the soot particle size decreases gradually. The collision and aggregation predominate the soot growth. To uncover the soot formation mechanism, the average particle size and particle distribution as a function of height above burner were correlated with flame temperatures. Furthermore, the underlying mechanisms behind the nucleation and growth of nascent soot particles from polycyclic aromatic hydrocarbons (PAHs) were reveal via ReaxFF molecular dynamics simulations. The results show that the aggregation ability of PAHs decreases as the temperature increases. The aggregation ability of PAHs is also affected by size, and the larger the size, the stronger the aggregation ability of PAHs. Both the experimental and simulation results provide novel insights into the development of combustion kinetic model for soot formation in sooting fuels.

Heat transfer characteristics of premixed methane-air flame jet impinging on a hemispherical surface

A combined experimental and numerical investigation was conducted to study the heat transfer characteristics of premixed methane-air flame jet impinging on a hemispherical surface. The effects of the mixture equivalence ratio (ϕ = 0.9–1.5), burner to plate distance (H/d = 1–6), the mixture Reynolds number (Re = 300–800) and curvature ratio (d/D = 0.1–0.133) on the heat transfer characteristics were investigated. CFD software was used to simulate the laminar methane-air flame impinging on a hemispherical surface for different operation conditions, and the heat flux distribution and average heat flux on the impingement surface were obtained. The heat transfer efficiency is presented to quantify the thermal performance of impinging flame jet. It was found that the equivalence ratio has an important effect on the flame height, which in turn affects the heat transfer efficiency, and the stoichiometric mixture has the lowest heat transfer efficiency because its lowest flame height. The burner-to-plate distance plays a decisive role in the heat transfer characteristics of impinging flame jet, and when the flame premixed cone just touches the impingement surface, it has the best thermal performance. Higher mixture Reynolds number has higher flame height, temperature and heat flux distribution, but lower heat transfer efficiency. The configuration of impinging flame jet with smaller curvature ratio has better thermal performance.

A direct numerical simulation study on the structures and turbulence–flame interactions of a laboratory-scale lean premixed jet flame in cross-flow

In the present work, direct numerical simulation of a laboratory-scale lean premixed reacting jet in cross-flow was performed to explore the flow–flame structures and turbulence–flame interactions. A jet of lean premixed ethylene–air mixtures (equivalence ratio $\phi = 0.6$ ) was injected into a hot vitiated cross-flow. Both non-reacting and reacting cases were simulated. It was found that the reacting jet penetrates deeper in the cross-flow with a weaker shear layer compared with the non-reacting one. The wake of the non-reacting and reacting jet is characterized by vertical vortices and recirculation zones, respectively. As for the flame structure of the reacting case, the reaction intensity varies considerably in different flame zones. The heat release rate on the leeward side is higher than that on the windward side, but lower than that of the corresponding laminar flame. The analysis of the turbulence–flame interactions of the reacting case showed that the large local Damköhler number ( $Da$ ) related to reaction-induced dilatations results in an increased tendency of the scalar gradient to align with the most extensive strain rate, which is more evident in the regions with high heat release rate on the leeward side. Negative dilatation regions with positive tangential strain rate and negative normal strain rate are observed on the windward side. High positive dilatations appear on the flame front of the leeward side. The tangential strain rate is negatively correlated with the normal strain rate and curvature. Regions with a high local $Da$ on the windward side correspond with high positive curvature regions.

A species-weighted flamelet/progress variable model with differential diffusion effects for oxy-fuel jet flames

A flamelet/progress variable (FPV) model accounting for the differential diffusion (DD) effects is proposed and applied to large eddy simulation (LES) of turbulent oxy-fuel flames (Sevault et al. 2012). Based on tabulations with the detailed molecular diffusion and the equal diffusion (ED) assumption, a species-weighted flamelet (SWF) model is developed to represent the DD effects. The influences of combustion progress, mixture fraction, and turbulence on variable Lewis numbers are incorporated into the model. The model assessments are first conducted on a laminar coflow flame, showing that the effect of DD on temperature and species is accurately captured, in that the importance of DD is seen in the laminar flame. Subsequently the model is implemented in a fully coupled LES of oxy-fuel jet flames. The LES results show that DD plays an important role in the reaction zone and regions near the fuel nozzle, and its effect decreases farther downstream, consistent with the experimental observations. The LES with the SWF model yields good predictions on the mean temperature and major species at the fuel-rich side while slight deviation (less than 6%) at the fuel-lean side. In comparison, LES with the original unity Lewis numbers flamelet model (Pierce et al. 2004) predicts a full extinction because of neglecting the DD characteristics, while the variable Lewis number flamelet model provides the maximum deviation of 26% because of ignoring the ED characteristics produced by turbulent disturbance. The trend and location of localized extinction of oxy-fuel flame are also well predicted using the SWF model.

Experimental investigation on a diffusion jet flame performance using a developed flame holder supplied with air from a concentric pipe

In this study, a diffusion jet flame consisting of a newly developed flame holder with three coaxial pipes to transfer air and fuel to the combustion zone, is investigated experimentally to simulate the ramjet effect. The flame holder is a double cone with three fuel ports on the top cone to improve the mixing of fuel and air. The inner pipe transfer fuel and is connected to the flame holder (which serves as a fuel injection point) and the middle pipe transfer primary air and the outer pipe transfer secondary air to the combustion zone (to investigate the impact of removing still air from around the flame as in ramjet applications). The flame holder and the primary air exit were at fixed positions (Datum Z4= 0) while the secondary air exit position changed many times below and above the datum line. The results showed that the secondary air exit location had an effect on the flame temperature distribution, flame shape, flame length, and flame width. The optimum position of the secondary air pipe was found above the datum line at Z5 = 7.5 mm, where the flame temperature and length are considered relatively high compared to other secondary air pipe positions.

The influence of the evaporation model on ethanol turbulent jet flame simulation

High-precision numerical simulation of aero-engine combustors requires a suitable evaporation model. At present, quantitative research on the effects of the evaporation model on two-phase turbulence combustion simulation is insufficient. Ethanol has a higher latent heat of evaporation than other fuels like acetone and aero kerosene, thus it is especially sensitive to the evaporation model. This paper uses LES to simulate turbulence in an aero-engine combustor, and TPDF to describe the interaction between turbulence and chemical reactions. Numerical simulation results of the Abramzon-Sirignano (A-S), Spalding, and thick exchange layer (NC-TEL) evaporation models for high-Reynolds number ethanol combustion in two-phase turbulent jet flames are analyzed and compared with experimental data. Results suggest that per unit time, the A-S evaporation model has the largest evaporation mass, followed by the NC-TEL evaporation model, while the Spalding evaporation model has the smallest evaporation mass. Through quantitative analysis of the different evaporation models’ influence on temperature, mixture fraction, and liquid phase distribution, it is found that the A-S evaporation model fits the experimental data better in the central region of jet flame but has worse performance in the other flame regions. The simulation result of the Spalding model is good in the X/D = 10 section, and that of the NC-TEL model shows the best result at X/D > 10 sections and in the outer regions of the jet flame. The maximum difference between the NC-TEL model results and the experimental data was 21.856 % in liquid phase velocity. Hence, the NC-TEL model has the best simulation performance.

Closure modeling for the conditional pressure gradient in turbulent premixed combustion

In turbulent premixed combustion, turbulence is affected by both the influence of combustion heat release and turbulent shear effects. At low Karlovitz number, heat release effects are more dominant than turbulent shear effects, but, at high Karlovitz number, turbulent shear effects dominate the turbulence dynamics. Closure models for the velocity and scalar fields that only account for one of those effects are incapable of generally describing combustion-turbulence interactions at any finite Karlovitz number. A manifold-based turbulence model that solves the conditional mean momentum transport equation conditioned on a progress variable has the potential to provide such a general description, but closure models for the equation are required. Among the unclosed terms, the conditional mean pressure gradient is significant at both low and high Karlovitz numbers but has fundamentally different behaviors in each regime. In this work, a general closure model is developed to capture the conditional pressure gradient in both Karlovitz number regimes by considering heat release and turbulent shear effects. The deviation of the conditional pressure gradient from the unconditional pressure gradient is modeled with two components. One represents the pressure decrease across the flame due to combustion heat release, and the other represents the hydrodynamic pressure fluctuations due to turbulent shear. The heat release component depends on the heat release rate and the flame-normal vector, and the turbulent shear component depends on the conditional Reynolds stresses and the gradient of the progress variable probability density function. The model is validated against Direct Numerical Simulations of spatially-evolving turbulent premixed hydrogen/air planar jet flames at low and high Karlovitz numbers. The proposed model well captures the conditional pressure gradient at both Karlovitz numbers and shows a natural transition of its dominant term: the heat release component at low Karlovitz number and the turbulent shear component at high Karlovitz number.

A thick reaction zone model for premixed flames in two-dimensional channels

Direct interactions between the flow field and the chemical reaction in premixed flames occur when the reaction zone thickness is comparable to, or greater than flow length scales. To study such interactions, a laminar model is considered that has direct bearings to steadily propagating deflagrations in a Hele-Shaw channel with a background plane Poiseuille flow. The study employs asymptotic analyses, pertaining to large activation energy and lubrication theories and considers a distinguished limit where the channel width is comparable to the reaction zone thickness, with account being taken of thermal-expansion and heat-loss effects. The reaction zone structure and burning rates depend on three parameters, namely, the Peclet number, , the Lewis number, and the ratio of channel half-width to reaction zone thickness, . In particular, when the parameter is small wherein the reaction zone is thick, transport processes are found to be controlled by Taylor's dispersion mechanism and an explicit formula for the effective burning speed is obtained. The formula indicates that for , which interestingly coincides with a recent experimental prediction of the turbulent flame speed in a highly turbulent jet flame. The results suggest that the role played by differential diffusion effects is significant both in the laminar and turbulent cases. The reason for the peculiar dependence can be attributed, at least in our laminar model, to Taylor dispersion. Presumably, this dependence may be attributed to a similar but more general mechanism in the turbulent distributed reaction zone regime, rather than to diffusive-thermal curvature effects. The latter effects play however an important role in determining the effective propagation speed for thinner reaction zones, in particular, when is large in our model. It is found that the magnitude of heat losses at extinction, which directly affects the mixture flammability limits, is multiplied by a factor in comparison with those corresponding to the no-flow case in narrow channels.

10-kHz-rate two-color dual-plane stereo-PIV using an optical parametric oscillator

10 kHz-rate dual-plane stereoscopic particle-image velocimetry (DSPIV) was applied in a swirling jet flame measurement simultaneously with hydroxyl planar laser-induced fluorescence (OH-PLIF). A high-speed burst-mode laser system with an optical parametric oscillator were used in double-pulse laser generation of 532 nm for one plane and 607 nm for the second plane. Temporally-resolved particle image velocimetry (PIV) image sequences, nine-component velocity gradient tensor, and simultaneous OH-PLIF images are presented. Flow vorticities and strain rates are also obtained with the measured velocity gradients. Compared with previous polarization-based dual-plane PIV technique, two-color based DSPIV measurement has distinctive advantages in elimination of signal interference from different PIV plane due to polarization leakage and hence improves measurement accuracy.

Experimental Study on Geometrical Characteristics of a Square Turbulent Buoyant Jet Flame

This paper gives the experimental results for the buoyant jet diffusion flame in square burners. The flame height and lift-off were measured and discussed. The results show that the normalized flame height and lift-off height of square flames are lower than that of round flames with the same experimental conditions, which indicates enhanced air entrainment in square flames. The model of flame height based on the flame Froude-number, which integrates a correction factor to account for the enhancement of air entrainment, reasonably collapses the square flame data under different experimental cases. The critical Froude number for the transition from buoyancy to momentum controlled regime of a square jet flame is highly dependent on the fuel type and burner size. The lift-off height of the square jet flames increases linearly when the initial velocity increases, but also depends on the fuel type and burner size. A unified correlation using a dimensionless flow number derived from the mixedness-reactedness flamelet model is established, which reasonably predicts the lift-off distances of square jet flames.

Machine learning modelling and evaluation of jet fires from natural gas processing, storage, and transport

AbstractJet fires and their repercussions play a significant role in catastrophic incidents that typically have a cascading impact in process industries. Several hydrocarbon experiments from 19 papers were incorporated into the current endeavour to develop simulations of jet flames using machine learning (ML) models. Dimensionless characteristics have been used as output and input variables, including mass flow rates, fuel density, jet flame length, and heat release fluxes. When training three layers of the multi‐layer feedforward neural network (MLFFNN) method, a Bayesian regularization backpropagation approach was adopted and evaluated with the radial based functions (RBF) algorithm. Through an optimization procedure, the first and second hidden layers of the MLFFNN have been optimized to include 10 and five neurons, respectively. The RBF algorithm with 40 neurons in a single layer has been computed using the same method. The best mean square error (MSE) validation results for RBF and MLFFNN were 0.006 and 0.0002, respectively, for 40 and 100 epochs. The MLFFNN and RBF models' respective regression statistical analysis outputs were 0.9949 and 0.9645. The ML method has been identified as a potentially useful technique for precisely predicting the geometrical and radiative characteristics of jet flames.

The Effects of Differential Diffusion on Turbulent Non-Premixed Flames LO2/CH4 under Transcritical Conditions Using Large-Eddy Simulation

In this paper, a large-eddy simulation (LES) of turbulent non-premixed LO2/CH4 combustion under transcritical conditions is performed based on the Mascotte test rig from the Office National d’Etudes et de Recherches Ae´rospatiales (ONERA), and the aim is to understand the effects of differential diffusion on the flame behaviors. In the LES, oxygen was injected into the environment above the critical pressure while the temperature was below the critical temperature. The flamelet/progress variable (FPV) approach was used as the combustion model. Two LES cases with different species diffusion coefficient schemes—i.e., non-unity and unity Lewis numbers—for generating the flamelet tables were carried out to explore the effects of differential diffusion on the flame and flow structures. The results of the LES case with non-unity Lewis numbers were in good agreement with the experimental data. It was shown that differential diffusion had evident impacts on the flame structure and flow dynamics. In particular, when unity Lewis numbers were used to evaluate the species diffusion coefficient, the flame length was underestimated and the flame expansion was more significant. Compared to laminar counterflow flames, turbulence in jet flames allows chemical reactions to take place in a wider range of mixture fractions. The density distributions of the two LES cases in the mixture fraction space were very similar, indicating that differential diffusion had no significant effects on the phase transition under transcritical conditions.

Electric field assisted reduction of NOx emission: A numerical study

The paper reports simulation results on the influence of a direct-current driven radial electric field on the emission characteristics; especially NOx and CO of a premixed methane/air laminar jet flame. A multi-physics computational model is developed in the OpenFOAM framework to simulate electric-field-coupled premixed combustion process. The computational framework consists of coupled species, momentum and energy conservation together with a Poisson’s equation solver to resolve the electric field distribution. Electron and ion conservation equations are resolved to consider the ionic wind body force in the momentum conservation equation and the associated possible electric field distortion due to the space charge distribution. The simulations are conducted for a stochiometric and fuel rich condition and over a range of jet flow rates for a configuration representative of a test-scale experimental setup. The model predictions show that for an applied voltage of 50 kV, the flame structure changes significantly for both the stoichiometric and fuel rich conditions. The flame is stretched significantly by the electric field due to ionic wind. For the fuel rich condition, the ionic wind allows additional mixing of the fuel rich stream with the surrounding air and drastically altering the flame structure. The electric field was found to reduce the NOx emission significantly for both stoichiometric and rich conditions. Over the entire range of flowrate conditions, the stochiometric fuel-oxidizer mixture showed a decrease in maximum NOx by a factor of 1.6 in presence of electric field. For the fuel rich case, however as the flow rate is increased, the NOx reduction factor decreased from 12.0 to 1.6. For CO emissions, the presence of electric field reduces the concentration under fuel rich conditions and vice versa for the stoichiometric flame. The role of kinetics is analyzed and discussed.

The influence of a plasma actuator embedded in a bluff-body on the stabilization of nonpremixed jet flames

This study is to experimentally investigate the influence of an annular plasma actuator embedded in a bluff body burner on flame stabilization, extending the previous work on non-reacting jets to propane jet diffusion flames. The objective of the actuator is to produce a vortex ring in the vicinity of the jet exit, subsequently to alter the local flow velocity, and thus to achieve flame stabilization. Results show that repetitive flame reattachment may occur with the presence of the induced vortex ring. Flame reattachment frequency and attachment time are analyzed with fast Fourier transform (FFT) and a probability density function (PDF), respectively, and both results suggest that the degree of interaction between the flame and the vortex ring has strong dependence on the imposed flow momentum, characterized by the momentum ratio of the vortex ring to the main jet. A flame describing function (FDF), characterizing the dynamic flame response to the induced flow, exhibits a trough where the vortex ring momentum is comparable to the jet momentum. Given the constant vorticity independent of the jet momentum, a greater degree of interaction between the vortex ring and the fuel jet leads to more repetitive flame reattachment, resulting in more steady heat release.

Large Eddy Simulation of turbulent nonpremixed sooting flames: Presumed subfilter PDF model for finite-rate oxidation of soot

Modeling soot evolution in turbulent reacting flows using Large Eddy Simulation (LES) is challenging due to the complex subfilter soot-turbulence-chemistry interactions. Soot particles form at fuel-rich mixtures and are subsequently oxidized as they are transported toward fuel-lean mixtures. In previous work, this phenomenology was explicitly encoded into a presumed subfilter PDF model for soot by confining soot strictly to mixtures where growth rates exceed oxidation rates. However, this model implicitly assumed that oxidation is infinitely fast. In this work, a new presumed subfilter PDF model for soot is proposed to account for finite-rate soot oxidation. The distribution of soot with respect to the flame structure (mixture fraction) is modeled by comparing the local relative motion of diffusionless soot particles with respect to mixture fraction iso-contours with the local oxidation rate. When the oxidation rate is suppressed or the transport rate is very fast, soot is allowed to penetrate further into fuel-lean mixtures. This model can allow for soot leakage across the flame, a critical phenomenon in smoking flames. The new model is validated a priori against Direct Numerical Simulation (DNS) databases of turbulent nonpremixed jet flames and then a posteriori against experimental measurements in a laboratory-scale turbulent jet flame. A priori results show remarkably good agreement with filtered DNS data and are shown to correctly allow for soot leakage at low Damköhler number. Finally, LES results of the turbulent sooting jet flame show an improvement of soot prediction using the new soot subfilter model compared to previous works. In this flame, as in experiments, evidence of soot leakage is found near the beginning of the sooting region, and this soot leakage cannot be predicted with previous subfilter models that presume infinitely fast oxidation.

Influence of Dust Layers in Connecting Pipes on Explosion Propagation Characteristics of Flake Aluminum Powder in Cylindrical Interconnected Vessels.

Aluminum dust explosion has become an important type of dust explosion accident. In the present work, an interconnected system is established to study the influence of accumulated dust layers in connecting pipes on explosion propagation characteristics. A high-precision computational fluid dynamics (CFD) method is applied to study the overpressure and flame development of aluminum powder explosion under the central ignition condition of two cylindrical vessels with a volume ratio of 1:5. The results show that pressure build-up in the secondary container is mainly due to the oscillating pressure wave. Moreover, compared with the ignition container, the thickness of the dust layer has a more obvious enhancement effect on the peak overpressure for the secondary container. When the ignition occurs in a large container, the dust layer is lifted in the connecting pipe under the action of the precursor pressure wave. After the jet flame enters the connecting pipe, the lifted aluminum dust participates in the explosion reaction, which significantly increases the explosion intensity. With the participation of the accumulated dust layer, the maximum overpressure of the explosion of flake aluminum dust in the interconnected system is higher than 30 bar, and the jet flame velocity is increased by 3 times. The present work can provide reference for the safety design of process equipment involving dust.