Effects of pressure on flame propagation in a premixture containing fine fuel droplets

Combustion experiments on fuel droplet–vapor–air mixtures have been performed with a rapid expansion apparatus which generates monodispersed droplet clouds with narrow diameter distribution using the condensation method. The effects of fine fuel droplets on flame propagation were investigated for ethanol droplet–vapor–air mixtures at various pressures from 0.2 to 1.0 MPa. A stagnant fuel droplet–vapor–air mixture, generated in a rapid expansion chamber, was ignited at the center of the chamber using an ignition wire. Spherical flame propagation under constant-pressure conditions was observed with a high-speed video camera and flame speed was measured. Total equivalence ratio, and the ratio of liquid fuel mass to total fuel mass, was varied from 0.6 to 1.4 and from zero to 56%, respectively. The mean droplet diameter of fuel droplet–vapor–air mixtures was set at 8.5 and 11 μm. It was found that the flame speed of droplet–vapor–air mixtures less than 0.9 in the total equivalence ratio exceeds that of premixed gases of the same total equivalence ratio at all pressures. The flame speed of fuel droplet–vapor–air mixtures decreases as the pressure increases in all total equivalence ratios. At large ratios of liquid fuel mass to total fuel mass, the normalized flame speed (the flame speed of droplet–vapor–air mixtures divided by the flame speed of the premixed gas with the same total equivalence ratio), increases with the increase in pressure for fuel-lean mixtures, and it decreases for fuel-rich mixtures. The outcome is reversed at small ratios of liquid fuel mass to total fuel mass; the normalized flame speed decreases with the increase in pressure for fuel-lean mixtures, and increases for fuel-rich mixtures. The results suggest that the increase in pressure promotes droplet evaporation in the preheat zone.     

Spherical turbulent flame propagation of pulverized coal particle clouds in an O2/N2 atmosphere

The present study aims to clarify the effects of turbulence intensity and coal concentration on the spherical turbulent flame propagation of a pulverized coal particle cloud. A unique experimental apparatus was developed in which coal particles can be dispersed homogeneously in a turbulent flow field generated by two fans. Experiments on spherical turbulent flame propagation of pulverized coal particle clouds in a constant volume spherical chamber in various turbulence intensities and coal concentrations were conducted. A common bituminous coal was used in the present study. The flame propagation velocity was obtained from an analysis of flame propagation images taken using a high-speed camera. It was found that the flame propagation velocity increased with increasing flame radius. The flame propagation velocity increases as the turbulence intensity increases. Similar trends were observed in spherical flames using gaseous fuel. The coal concentration has a weak effect on the flame propagation velocity, which is unique to pulverized coal combustions in a turbulent field. These are the first reports of experimental results for the spherical turbulent flame propagation behavior of pulverized coal particle clouds. The results obtained in the present study are obviously different from those of previous pulverized coal combustion studies and any other results of gaseous fuel combustion research.     

Effect of non-zero relative velocity on the flame speed of two-phase laminar flames

A numerical study of one-dimensional n-heptane/air spray flames is presented. The objective is to evaluate the flame propagation speed in the case where droplets evaporate inside the reaction zone with possibly non-zero relative velocity. A Direct Numerical Simulation approach for the gaseous phase is coupled to a discrete particle Lagrangian formalism for the dispersed phase. A global two-step n-heptane/air chemical mechanism is used. The effects of initial droplet diameter, overall equivalence ratio, liquid loading and relative velocity between gaseous and liquid phases on the laminar spray flame speed and structure are studied. For lean premixed cases, it is found that the laminar flame speed decreases with increasing initial droplet diameter and relative velocity. On the contrary, rich premixed cases show a range of diameters for which the flame speed is enhanced compared to the corresponding purely gaseous flame. Finally, spray flames controlled by evaporation always have lower flame speeds. To highlight the controlling parameters of spray flame speed, approximate analytical expressions are proposed, which give the correct trends of the spray flame propagation speed behavior for both lean and rich mixtures.     

Compressible turbulent flame speeds of highly turbulent standing flames

In this paper we present the first measurement of turbulent burning velocities of a highly turbulent compressible standing flame induced by shock-driven turbulence in a Turbulent Shock Tube. High-speed schlieren, chemiluminescence, PIV, and dynamic pressure measurements are made to quantify flame–turbulence interaction for high levels of turbulence at elevated temperatures and pressure. Distributions of turbulent velocities, vorticity and turbulent strain are provided for regions ahead and behind the standing flame. The turbulent flame speed is directly measured for the high-Mach standing turbulent flame. From measurements of the flame turbulent speed and turbulent Mach number, transition into a non-linear compressibility regime at turbulent Mach numbers above 0.4 is confirmed, and a possible mechanism for flame generated turbulence and deflagration-to-detonation transition is established.     

Estimation of laminar flame speeds using axisymmetric bunsen flames: Molecular transport effects

Intricacies associated with the estimation of laminar flame speed using the axisymmetric Bunsen flame technique were assessed, through parametric direct numerical simulations. The study involved methane-air mixtures at atmospheric pressure and temperature, and both the flame cone angle and flame surface area methods were utilized to estimate the laminar flame speeds based on conditions used in recent relevant experimental studies. The results provided insight into the details of the flame structure and allowed for the assessment of various non-idealities and the attendant uncertainties associated with the estimation of laminar flame speeds. Additionally, molecular transport effects were investigated by altering the fuel diffusivity, in order to evaluate its impact on the flame structure and propagation under the presence of negative stretch. The modification of fuel diffusivity was found to affect the burning rate as stretch varies. Under fuel rich conditions, decreasing the fuel diffusivity was found to have an opposite effect on the heat release and thus the burning rate, when compared to positively stretched flames that have been investigated recently in a similar manner. The reported results are expected to provide guidance in flame propagation experiments using the convenient Bunsen flame method at near-atmospheric or elevated pressures, as well as insight into the effects of negative stretch that has, compared to positive, attracted less attention in past studies.     

Turbulent flame propagation limits of ammonia/methane/air premixed mixture in a constant volume vessel

Ammonia is one of promising energy carriers that can be directly used as carbon-neutral fuel for combustion applications. However, because of the low-burning velocity of ammonia, it is challenging to introduce ammonia to practical combustors those are designed for general hydrocarbon fuels. One of ways to enhance the combustibility of ammonia is by mixing it with other hydrocarbon fuels, such as methane, with a burning velocity is much higher than the burning velocity of ammonia. In this study, we conducted flame propagation experiments of ammonia/methane/air using a fan-stirred constant volume vessel to clarify the effect of methane addition to ammonia on the turbulent flame propagation limit. From experimental results, we constructed the flame propagation maps and clarified the flame propagation limits. The results show that the flame propagation limits were extended with an increase in mixing a fraction of methane to ammonia. Additionally, ammonia/methane/air mixtures with the equivalence ration of 0.9 can propagate at the highest turbulent intensity, even though the peak of the laminar burning velocity is the fuel-rich side because of the diffusional-thermal instability of the flame surface. Furthermore, the Markstein number of the mixture obtained in this research successfully expressed the strength of the diffusional-thermal instability effect on the flame propagation capability. The turbulence Karlovitz number at the flame propagation limit monotonically increases with the decreasing Markstein number.     

Direct numerical simulations of turbulent flame expansion in fine sprays

Direct Numerical Simulations of expanding flame kernels following localized ignition in decaying turbulence with the fuel in the form of a fine mist have been performed to identify the effects of the spray parameters on the possibility of self-sustained combustion. Simulations show that the flame kernel may quench due to fuel starvation in the gaseous phase if the droplets are large or if their number is insufficient to result in significant heat release to allow for self-sustained flame propagation for the given turbulent environment. The reaction proceeds in a large range of equivalence ratios due to the random location of the droplets relative to the igniter location that causes a wide range of mixture fractions to develop through pre-evaporation in the unreacted gas and through evaporation in the preheat zone of the propagating flame. The resulting flame exhibits both premixed and non-premixed characteristics.     

Measurement of unstable burning velocities of iso-octane–air mixtures at high pressure and the derivation of laminar burning velocities

A new technique is reported for measuring burning velocities at high pressures in the final stages of two inwardly propagating flame kernels in an explosion bomb. The flames were initiated at diametrically opposite spark electrodes, close to the wall, in quiescent mixtures. Measurements of pressure and flame kernel propagation speeds by high-speed photography showed the burning velocities to be elevated above the corresponding laminar burning velocities as a result of the developing flame instabilities. The enhancement increased with increase in pressure and decreased with increase in Markstein number. When the Markstein number was negative, instabilities could be appreciable, as could the enhancement. For the iso-octane–air mixtures investigated, where the mixtures had well-characterised Markstein numbers or critical Peclet numbers at the relevant pressures and temperatures, it was possible to explain the enhancement quantitatively by the spherical explosion flame instability theory of Bechtold and Matalon, provided the critical Peclet number was that observed experimentally, and allowance was made for the changing pressure. With this theoretical procedure, it was possible to derive values of laminar burning velocity from the measured values of burning velocity over a wide range of equivalence ratios, pressures, and temperatures. The values became less reliable at the higher temperatures and pressures as the data on Markstein and critical Peclet numbers became less certain. It was found that with iso-octane as the fuel the laminar burning velocity decreased during isentropic compression.     

Flame propagation speed and Markstein length of spherically expanding flames: Assessment of extrapolation and measurement techniques

Laminar burning velocities are of great importance in many combustion models as well as for validation and improvement of chemical kinetic schemes. Determining laminar burning velocities with high accuracy is quite challenging and different approaches exist. Hence, a comparison of existing methods measuring and evaluating laminar burning velocities is of interest. Here, two optical diagnostics, high speed tomography and Schlieren cinematography, are simultaneously set up to investigate methods for evaluating laminar flame speed in a spherical flame configuration. The hypothesis to obtain the same flame propagation radii over time with the two different techniques is addressed. Another important aspect is the estimation of flame properties, such as the unstretched flame propagation speed and Markstein length in the burnt gas phase and if these are estimated satisfactorily by common experimental approaches. Thorough evaluation of the data with several extrapolation techniques is undertaken. A systematic extrapolation approach is presented to give more confidence into results generated experimentally. The significance of the linear extrapolation routine is highlighted in this context. Measurements of spherically expanding flames are carried out in two high-pressure, high-temperature, constant-volume vessels at RWTH in Aachen, Germany and at ICARE in Orleans, France. For the discussion of the systematic extrapolation approach, flame speed measurements of methane / air mixtures with mixture Lewis numbers moderately away from unity are used. Conditions were varied from lean to rich mixtures, at temperatures of 298–373 K, and pressures of 1 atm and 5 bar.     

Role of large scale flow features on cycle-to-cycle variations of spark-ignited flame-initiation and its transition to turbulent combustion

This study investigates the influence of large-scale flow features, including flow structure and velocity magnitude, on the early-burn period variability in a homogenous-charge spark-ignited engine fueled with premixed propane-air mixture. Particle image velocimetry and in-cylinder pressure measurement data from a previous study - were processed to enable simultaneous flow characterization and flame-front tracking as well as apparent heat-release analysis. By combining probability analysis of flame development with conditional sampling of fast and slow early-burn cycles using 10% fuel mass fraction burned, it is shown that an undesirable flow structure produces an asymmetric flame development at the initial flame growth period. This asymmetric flame structure persists through the whole initial-to-turbulent transition period until the flame becomes fully turbulent. The undesirable flow condition is characterized by large-scale convective flows near spark plug rather than flows that lead to increased flame spread in multiple directions. The simultaneous flow and flame characterization enables the quantifications of flame-front propagation speed, unburned fuel-air mixture velocity ahead of flame front and local burning velocity at flame surface. Here the local burning velocity is referred to as laminar or turbulent flame speed. A simplified approach is introduced to derive integrated values for these quantities per crank-angle-degree, enabling the quantitative comparison of the trend-wise difference in these integrated metrics between fast and slow early-burn cycles. It is revealed that for the transition period, the CCV in the velocity magnitude of unburned fuel-air mixture ahead of the flame front accounts for nearly 50% to the variability of flame propagation speed. The burning velocity provides the remaining source of the flame propagation variability in this period. The flame propagation variations in the initial flame growth and fully turbulent periods are smaller than those in the transition period and are primarily dependent on the variability of large-scale flow features.     

A model of laminar flames in droplet suspensions

A mathematical model of steady laminar flame propagation through a suspension of liquid droplets was proposed, and numerical calculations within the framework of this model were performed. The model is constructed based on one-dimensional differential equations of the theory of laminar flames in homogeneous gaseous mixtures in conjunction with the theory of droplet burning in uniform monodisperse suspensions. The chemical process was described using a multistage kinetic scheme. A comparison of model predictions with the available experimental data demonstrated satisfactory agreement.     

High-speed visualization of flame propagation in explosions

Flow visualization data is presented to describe the structure of flames propagating in methane-air explosions in semi-confined enclosures. The role of turbulence is well established as a mechanism for increasing burning velocity by fragmenting the flame front and increasing the surface area of flames propagating in explosions. This area increase enhances the burning rate and increases the resultant explosion overpressure. In real situations, such as those found in complex process plant areas offshore, the acceleration of a flame front results from a complex interaction between the moving flame front and the local blockage caused by presence of equipment. It is clear that any localised increase in flame burn rate and overpressure would have important implications for any adjacent plant and equipment and may lead to an escalation process internal to the overall event. To obtain the information required to quantify the role of obstacles, it is necessary to apply a range of sophisticated laser-based, optical diagnostic techniques. This paper describes the application of high-speed, laser-sheet flow visualization and digital imaging to record the temporal development of the flame structure in explosions. Data is presented to describe the interaction of the propagating flame with a range of obstacles for both homogeneous and stratified mixtures. The presented image sequences show the importance of turbulent flow structures in the wake of obstacles for controlling the mixing of a stratified concentration field and the subsequent flame propagation through the wake. The data quantifies the flame speed, shape and area for a range of obstacle shapes.     

稀甲烷/氢气预混湍流传播火焰实验研究

本文采用定容湍流燃烧弹获取了稀甲烷/氢气/空气在强湍流条件下的火焰发展历程,研究了湍流火焰在负马克斯坦数条件下的传播特性.结果表明,湍流火焰呈现自相似传播特性,即使在强湍流条件下,湍流传播火焰仍然会受到不稳定性的影响.并且随着马克斯坦数的减小,不稳定性对湍流传播火焰的影响增强。同时,本文获得一种新的湍流燃烧速度拟合公式,包含了负马克斯坦数条件下不稳定性对湍流燃烧速度的影响。     

Flame enhancement of ethylene/methane mixtures by ozone addition

As one of the longest lasting species in plasma-assisted combustion, ozone has a pronounced effect on ignition and flame propagation. Many previous studies, however, have only investigated the combustion enhancement by ozone for single-component fuels. In the present study, the impact of ozone addition on multi-component fuel mixtures is examined through one-dimensional laminar flame simulations across a range of temperatures, pressures, residence times, and mixture compositions. Due to the presence of an alkene (ethylene), ozone is consumed through pre-flame ozonolysis reactions even at room temperature. The flame speed is shown to be dependent on the domain length (residence time), and a new reference flame speed is defined for ozonolysis-assisted flame propagation. It is also found that the flame speed enhancement by ozone is highly nonlinear, as a small amount of ethylene produces a disproportionate boost in the laminar flame speed. Finally, the competition between ozonolysis, ozone decomposition, and other ozone reactions in a mixture of alkenes and alkanes is examined in detail. Increases in the pressure, temperature, and equivalence ratio (for rich mixtures) favor ozonolysis reactions over other ozone reactions. The results of this study provide important insights into the timescales, length scales, and reaction pathways that govern ozone-assisted combustion of multi-component fuels in real combustors.     

Influence of water mist on propagation and suppression of laminar premixed flame

The combustion of premixed gas mixtures containing micro droplets of water was studied using one-dimensional approximation. The dependencies of the burning velocity and flammability limits on the initial conditions and on the properties of liquid droplets were analyzed. Effects of droplet size and concentration of added liquid were studied. It was demonstrated that the droplets with smaller diameters are more effective in reducing the flame velocity. For droplets vaporizing in the reaction zone, the burning velocity is independent of droplet size, and it depends only on the concentration of added liquid. With further increase of the droplet diameter the droplets are passing through the reaction zone with completion of vaporization in the combustion products. It was demonstrated that for droplets above a certain size there are two stable stationary modes of flame propagation with transition of hysteresis type. The critical conditions of the transition are due to the appearance of the temperature maximum at the flame front and the temperature gradient with heat losses from the reaction zone to the products, as a result of droplet vaporization passing through the reaction zone. The critical conditions are similar to the critical conditions of the classical flammability limits of flame with the thermal mechanism of flame propagation. The maximum decrease in the burning velocity and decrease in the combustion temperature at the critical turning point corresponds to predictions of the classical theories of flammability limits of Zel'dovich and Spalding. The stability analysis of stationary modes of flame propagation in the presence of water mist showed the lack of oscillatory processes in the frames of the assumed model.     

Complex chemistry simulations of spark ignition in turbulent sprays

Three-dimensional DNS of two-phase flows with the point-source approximation and with complex chemistry for n-heptane has been used to extract physical information on the structure of igniting kernels following localised heat deposition in turbulent monodisperse sprays. Consistent with experiment, small sparks fail to ignite and sprays ignite later than premixed gaseous mixtures. Reaction rates are intense in spherical zones near droplets and much lower in the interdroplet spacing, resulting in a highly wrinkled flame surface. The propagation of these reaction zones was observed. The flame shows a locally non-premixed character, with reactions proceeding at a wide range of mixture fractions, which increases as evaporation progresses. The distribution of various chemical species is presented. The results constitute a database for model validation and physical analysis.     

Experimental observation of turbulent jet ignition of pre-chamber with scavenging system for carbon dioxide diluted mixtures

The exhaust gas recirculation (EGR) method can suppress knock and improve the thermal efficiency of engines. But it will also deteriorate the combustion stability and engine power. Turbulent jet ignition (TJI) is a reliable ignition resource for improving ignition stability and burning rate. However, the residual productions in the pre-chamber will worsen the performance of the TJI. To this end, a self-designed pre-chamber with a scavenging system has been proposed. In this study, the ignition process and flame propagation phenomena under different EGR dilution ratios for H₂/N₂/O₂ and CH₄/N₂/O₂ mixtures were conducted in a constant-volume combustion chamber. The results suggested that the increase in EGR dilution weakens the influence of cellular instability and causes buoyancy instability, the latter of which could be mitigated by the passive TJI method. For the passive TJI mode, the exit time of the hot jet was delayed, and the turbulent flame speed decreased with the increase of EGR dilution ratio. Four ignition phenomena, namely jet re-ignition, flame buoyancy, re-ignition failure, and misfire, were distinctly identified. However, EGR tolerance cannot be extended by passive pre-chambers. Therefore, the pre-chamber with a scavenging system that can effectively extend the lean combustion tolerance with EGR dilution compared to SI and passive TJI was proposed. The effects of air and fuel injection quantities on ignition and flame propagation were investigated. The flame propagation velocity was positively related to the air injection mass, whereas an optimum fuel mass was required to achieve fast flame propagation. The EGR limit based on dual injections in the pre-chamber was obviously extended. Moreover, under near EGR tolerance conditions, a leaner fuel injection in the pre-chamber was required to realize successful ignition in the main chamber, as strong turbulence could cause high heat transfer loss with the cool unburnt mixture and suppress the occurrence of re-ignition.     

Turbulent flame speed and morphology of pure ammonia flames and blends with methane or hydrogen

Ammonia appears a promising hydrogen-energy carrier as well as a carbon-free fuel. However, there remain limited studies for ammonia combustion especially under turbulent conditions. To that end, using the spherically expanding flame configuration, the turbulent flame speeds of stoichiometric ammonia/air, ammonia/methane and ammonia/hydrogen were examined. The composition of blends studied are currently being investigated for gas turbine application and are evaluated at various turbulent intensities, covering different kinds of turbulent combustion regimes. Mie-scattering tomography was employed facilitating flame structure analysis. Results show that the flame propagation speed of ammonia/air increases exponentially with increasing hydrogen amount. It is less pronounced with increasing methane addition, analogous to the behavior displayed in the laminar regime. The turbulent to laminar flame speed ratio increases with turbulence intensity. However, smallest gains were observed at highest hydrogen content, presumably due to differences in the combustion regime, with the mixture located within the corrugated flamelet zone, with all other mixtures positioned within the thin reaction zone. A good correlation of the turbulent velocity based on the Karlovitz and Damköhler numbers is observable with the present dataset, as well as previous experimental measurements available in literature, suggesting that ammonia-based fuels may potentially be described following the usual turbulent combustion models. Flame morphology and stretch sensitivity analysis were conducted, revealing that flame curvature remains relatively similar for pure ammonia and ammonia-based mixtures. The wrinkling ratio is found to increase with both increasing ammonia fraction and turbulent intensity, in good agreement with measured increases in turbulent flame speed. On the other hand, in most cases, the flame stretch effect does not change significantly with increasing turbulence, whilst following a similar trend to that of the laminar Markstein length.     

Exploring fuel isomeric effects on laminar flame propagation of butylbenzenes at various pressures

This work reports an experimental and kinetic modeling investigation on the laminar flame propagation of three butylbenzene isomers (n-butylbenzene, iso-butylbenzene and tert-butylbenzene)/air mixtures. The experiments were performed in a high-pressure constant-volume cylindrical combustion vessel at the initial temperature of 423 K, initial pressures of 1–10 atm, and equivalence ratios (?) of 0.7–1.5. The laminar burning velocities of butylbenzene/O₂/He mixtures were also measured at 423 K, 10 atm and ? = 1.5 to provide additional experimental data under conditions that the butylbenzene/air experiments are susceptible of cellular instability. Comparison among the laminar burning velocities of butylbenzenes including both the three isomers investigated in this work and sec-butylbenzene investigated in our recent work [Combust. Flame 211 (2020) 18–31] shows remarkable fuel isomeric effects, that is, iso-butylbenzene has the slowest laminar burning velocities, followed by n-butylbenzene and tert-butylbenzene, while sec-butylbenzene has the fastest laminar burning velocities. A kinetic model for butylbenzene combustion was developed to simulate the laminar flame propagation of butylbenzenes. Sensitivity analysis was performed to reveal important reactions in laminar flame propagation of butylbenzenes, including both small species reactions and fuel-specific reactions. Kinetic effects are concluded to result in the different laminar burning velocities of four butylbenzene isomers. Small species reactions control the laminar flame propagation under lean conditions, which results in small differences of laminar burning velocities. Chain termination reactions, especially fuel-specific reactions, have important contributions to inhibit the laminar flame propagation under rich conditions. The structural features of butylbenzene isomers can significantly affect the formation of some crucial radicals such as methyl, cyclopentadienyl and benzyl radicals under rich conditions, which leads to remarkable fuel isomeric effects on their laminar burning velocities, especially at high pressures.     

Effects of fuel stratification on ignition kernel development and minimum ignition energy of n-decane/air mixtures

Fuel-stratified combustion has broad application due to its promising advantages in extension of lean flammability limit, improvement of flame stabilization, enhancement of lean combustion, etc. In the literature, there are many studies on flame propagation in fuel-stratified mixtures. However, there is little attention on ignition in fuel-stratified mixtures. In this study, one-dimensional numerical simulation is conducted to investigate the ignition and spherical flame kernel propagation in fuel-stratified n-decane/air mixtures. The emphasis is placed on assessing the effects of fuel stratification on the ignition kernel propagation and critical ignition condition. First, ignition and flame kernel propagation in homogeneous n-decane/air mixture are studied and different flame regimes are identified. The minimum ignition energy (MIE) of the homogeneous n-decane/air mixture is obtained and it is found to be very sensitive to the equivalence ratio under fuel-lean conditions. Then, ignition and flame kernel propagation in fuel-stratified n-decane/air mixture are investigated. The inner equivalence ratio and stratification radius are found to have great impact on ignition kernel propagation. The MIEs at different fuel-stratification conditions are calculated. The results indicate that for fuel-lean n-decane/air mixture, fuel stratification can greatly promote ignition and reduce the MIE. Six distinct flame regimes are observed for successful ignition in fuel-stratified mixture. It is shown that the ignition kernel propagation can be induced by not only the ignition energy deposition but also the fuel-stratification. Moreover, it is found that to achieve effective ignition enhancement though fuel stratification, one needs properly choose the values of stratification radius and inner equivalence ratio.