On the existence of steady-state gaseous microgravity spherical diffusion flames in the presence of radiation heat loss

Whether steady-state gaseous microgravity spherical diffusion exist in the presence of radiation heat loss is an important fundamental question and has important implications for spacecraft fire safety. In this work, experiments aboard the International Space Station and a transient numerical model are used to investigate the existence of steady-state microgravity spherical diffusion flames. Gaseous spherical diffusion flames stabilized on a porous spherical burner are employed in normal (i.e., fuel flowing into an ambient oxidizer) and inverse (i.e., oxidizer flowing into an ambient fuel) flame configurations. The fuel is ethylene and the oxidizer oxygen, both diluted with nitrogen. The flow rate of the reactant gas from the burner is held constant. It is found that steady-state gaseous microgravity spherical diffusion flames can exist in the presence of radiation heat loss, provided that the steady-state flame size is less than the flame size for radiative extinction, and the flame develops fast enough that radiation heat loss does not drop the flame temperature below the critical temperature for radiative extinction (1130 K). A simple model is provided that allows for the identification of initial conditions that can lead to steady-state spherical diffusion flames. In the spherical, infinite domain configuration, the characteristic time for the diffusion-controlled system to effectively reach steady-state is found to be on the order of 100,000 s. Despite a narrow range of attainable conditions, flames that exhibit steady-state behavior are observed aboard the ISS for up to 870 s, even with the constraint of a finite boundary. Steady-state flames are simulated using the numerical model for over 100,000 s.     

On the global structure and asymptotic stability of low-stretch diffusion flame: Forced convection

The present work analyzes cylindrical diffusion flames (Tsuji burner) under low stretch condition, considering fuel injection also from the backward region of the burner. To highlight the fundamental aspects of this flame, some assumptions are imposed, like constant thermodynamic and transport coefficients, unitary Lewis number and no radiative heat loss. It is also considered potential flow model and incompressible Navier–Stokes model. Despite the simplicity of the former model, results from both models show good agreement. Also, an asymptotic analysis describing the problem far from the burner is able to capture the most important mechanisms controlling the flame, then the flame shape is determined and the dependence of the characteristic length scales on Peclet number (based on the burner properties), free stream velocity and stoichiometry is revealed. The results show that the flame width is proportional to the mass stoichiometric coefficient and reciprocal to the Peclet number the 1/4 power and free stream velocity the 3/4 power, and that the flame height is proportional to the square of the mass stoichiometric coefficient and to the square root of the ratio of Peclet number to free stream velocity. In addition, an asymptotic stability analysis reveals low-stretch flame extinction to be caused by reduction in fuel and oxidizer concentrations, which provides the range of the stoichiometric coefficient for stable regime, and at the same time the range of heat released.     

Structure and dynamic response of radiative diffusion flames

Nitrogen-diluted hydrogen burning in air is modeled numerically using a constant density and one-step reaction model in a plane two-dimensional counterflow configuration. An optically thin assumption is used to investigate the effects of radiation on the dynamics, structure, and extinction of diffusion flames. While there exist dual steady-state extinction limits for the 1D radiative flame response, it is found that as the 1D radiative extinction point is approached the 1D low-stretch diffusion flame exhibits oscillatory response, even with sub-unity Lewis number fuel. These radiation-induced limit cycle oscillations are found to have increasing amplitude and decreasing frequency as the stretch rate is reduced. Flame oscillation eventually leads to permanent extinction at the stretch rate which is larger than the steady-state radiative extinction value. Along the 1D radiative response curve, the transition from 1D flame to 2D structure and the differences in the resulting 2D flame patterns are also examined using a variety of initial profiles, with special emphasis on the comparison of using the initial profiles with and without a flame edge. Similar to the previous studies on the high-stretch adiabatic edge flames using the same configuration, the high-stretch radiative flames are found to resist 1D blow-off quenching through various 2D structures, including propagating front and steady cellular flames for initial profiles with and without flame edges. For all initial profiles studied, the low-stretch radiative flames are also found to exhibit different 2D flame phenomena near the 1D radiative extinction limit, such as transient cellular structures, steady cellular structures, and pulsating ignition fronts. Although the results demonstrate the presence of low-stretch and high-stretch 2D bifurcation branches close to the corresponding 1D extinction limits irrespective of the initial profile used, particular 2D flame structures in certain stretch rate range are initial profile dependent. The existence of two-dimensional flame structures beyond the 1D steady-state radiative extinction limit suggests that the flammable range is expanded as compared to that predicted by the 1D model. Hence, multi-dimensional flame patterns need to be accounted for when determining the flammability limits for a given system.     

Critical temperature and reactant mass flux for radiative extinction of ethylene microgravity spherical diffusion flames at 1 bar

In microgravity combustion, where buoyancy is not present to accelerate the flow field and strain the flame, radiative extinction is of fundamental importance, and has implications for spacecraft fire safety. In this work, the critical point for radiative extinction is identified for normal and inverse ethylene spherical diffusion flames via atmospheric pressure experiments conducted aboard the International Space Station, as well as with a transient numerical model. The fuel is ethylene with nitrogen diluent, and the oxidizer is an oxygen/nitrogen mixture. The burner is a porous stainless-steel sphere. All experiments are conducted at constant reactant flow rate. For normal flames, the ambient oxygen mole fraction was varied from 0.2 to 0.38, burner supply fuel mole fraction from 0.13 to 1, total mass flow rate, ṁ_total, from 0.6 to 12.2 mg/s, and adiabatic flame temperature, T_ad, from 2000 to 2800 K. For inverse flames, the ambient fuel mole fraction was varied from 0.08 to 0.12, burner supply oxygen mole fraction from 0.4 to 0.85, ṁ_total from 2.3 to 11.3 mg/s, and T_ad from 2080 to 2590 K. Despite this broad range of conditions, all flames extinguish at a critical extinction temperature of 1130 K, and a fuel-based mass flux of 0.2 g/m²-s for normal flames, and an oxygen-based mass flux of 0.68 g/m²-s for inverse flames. With this information, a simple equation is developed to estimate the flame size (i.e., location of peak temperature) at extinction for any atmospheric-pressure ethylene spherical diffusion flame given only the reactant mass flow rate. Flame growth, which ultimately leads to radiative extinction if the critical extinction point is reached, is attributed to the natural development of the diffusion-limited system as it approaches steady state and the reduction in the transport properties as the flame temperature drops due to increasing flame radiation with time (radiation-induced growth.)     

Unsteady effects on flame extinction limits during gaseous and two-phase flame/vortex interactions

In highly fluctuating flows, it happens that high values of the strain-rate do not induce extinction of the flame front. Unsteady effects minimize the flame response to rapidly varying strain fields. In the present study, the effects of time-dependent flows on non-premixed flames are investigated during flame/vortex interactions. Gaseous flames and spray flames in the external sheath combustion regime are considered. To analyse the flame/vortex interaction process, the velocity field and the flame geometry are simultaneously determined using particle imaging velocimetry and laser-induced fluorescence of the CH radical. The influence of vortex flows on the extinction limits for different vortex parameters and for different gaseous and two-phase flames is examined. If the external perturbation is applied over an extended period of time, the extinction strain-rate is that corresponding to the steady-state flame, and this critical value mainly depends on the fuel and oxidizer compositions and the injection temperature. If the external perturbation is applied during a short period of time, extinction occurs at strain-rates above the steady-state extinction strain-rate. This deviation appears for flow fluctuation timescales below steady flame diffusion timescales. This behaviour is induced by diffusive processes, limiting the ability of the flame to respond to highly fluctuating flows. With respect to unsteady effects, the spray flames investigated in this article behave essentially like gaseous flames, because evaporation takes place in a thin layer before the flame front. Extinction limits are only slightly modified by the spray, controlling process being the competition between aerodynamic and diffusive timescales.     

微尺度扩散火焰特性的数值解析

本文以均匀空气流中圆管形成的甲烷射流扩散火焰为对象,用数值解析的方法研究了微尺度扩散火焰的火焰结构和燃烧特性。燃烧反应采用甲烷/空气一步总包反应,喷管壁面采用绝热条件。在Re一定情况下,改变喷口尺寸和喷口流速考察了微扩散火焰的结构和火焰熄灭的尺度效应。数值结果表明,随着喷口直径的增大,微火焰的上方出现回流; Re=12条件下,在喷口直径=0.07 mm时存在熄灭极限;稳定燃烧区的最小发热率约为0.5 W;微尺度条件下,Da数对火焰结构和火焰的熄灭有一定的影响。     

Computational and experimental study of JP-8, a surrogate, and its components in counterflow diffusion flames

Non-sooting counterflow diffusion flames have been studied both computationally and experimentally, using either JP-8, or a six-component JP-8 surrogate mixture, or its individual components. The computational study employs a counterflow diffusion flame model, the solution of which is coupled with arc length continuation to examine a wide variety of inlet conditions and to calculate extinction limits. The surrogate model includes a semi-detailed kinetic mechanism composed of 221 gaseous species participating in 5032 reactions. Experimentally, counterflow diffusion flames are established, in which multicomponent fuel vaporization is achieved through the use of an ultrasonic nebulizer that introduces small fuel droplets into a heated nitrogen stream, fostering complete vaporization without fractional distillation. Temperature profiles and extinction limits are measured in all flames and compared with predictions using the semi-detailed mechanism. These measurements show good agreement with predictions in single-component n-dodecane, methylcyclohexane, and iso-octane flames. Good agreement also exists between predicted and measured variables in flames of the surrogate, and the agreement is even better between the experimental JP-8 flames and the surrogate predictions.     

The two-dimensional structure of low strain rate counterflow nonpremixed-methane flames in normal and microgravity

The structure and extinction of low strain rate nonpremixed methane–air flames was studied numerically and experimentally. A time-dependent axisymmetric two-dimensional (2D) model considering buoyancy effects and radiative heat transfer was developed to capture the structure and extinction limits of normal gravity (1-g) and zero gravity (0-g) flames. For comparison with the 2D modelling results, a one-dimensional (1D) flamelet computation using a previously developed numerical code was exercised to provide information on the 0-g flames. A 3-step global reaction mechanism was used in both the 1D and 2D computations to predict the measured extinction limit and flame temperature. Photographic images of flames undergoing the process of extinction were compared with model calculations. The axisymmetric numerical model was validated by comparing flame shapes, temperature profiles, and extinction limits with experiments and with the 1D computational results. The 2D computations yielded insight into the extinction mode and flame structure. A specific maximum heat release rate was introduced to quantify the local flame strength and to elucidate the extinction mechanism. The contribution by each term in the energy equation to the heat release rate was evaluated to investigate the multi-dimensional structure and radiative extinction of the 1-g flames. Two combustion regimes depending on the extinction mode were identified. Lateral heat loss effects and multi-dimensional flame and flow structure were also found. At low strain rates in 1-g flames (‘regime A’), the flame is extinguished from the weak outer edge of the flame, which is attributed to a multi-dimensional flame structure and flow field. At high strain rates, (‘regime B’), the flame extinction initiates near the flame centreline owing to an increased diluent concentration in the reaction zone, similar to the extinction mode of 1D flames. These two extinction modes can be clearly explained by consideration of the specific maximum heat release rate.     

Use of laser-induced spark for studying ignition stability and unburned hydrogen escaping from laminar diluted hydrogen diffusion jet flames

Ignition and unburned hydrogen escaping from hydrogen jet diffusion flames diluted with nitrogen up to 70% were experimentally studied. The successful ignition locations were about 2/3 of the flame length above the jet exit for undiluted flames and moved much closer to the exit for diluted flames. For higher levels of dilution or higher flow rates, there existed a region within which a diluted hydrogen diffusion flame can be ignited and burns with a stable liftoff height. This is contrary to previous findings that pure and diluted hydrogen jet diffusion cannot achieve a stable lifted flame configuration. With liftoff, the flame is noisy and short with significant amount of unburned hydrogen escaping into the product gases. If ignition is initiated below this region, the flame propagates upstream quickly and attaches to the burner rim. Results from measurements of unburned hydrogen in the combustion products showed that the amount of unburned hydrogen increased as the nitrogen dilution level was increased. Thus, hydrogen diffusion flame diluted with nitrogen cannot burn completely.     

Stationary combustion regimes and extinction limits of one-dimensional stretched premixed flames in a gap between two heat conducting plates

Stationary combustion regimes, their linear stability and extinction limits of stretched premixed flames in a narrow gap between two heat conducting plates are studied by means of numerical simulations in the framework of one-dimensional thermal-diffusion model with overall one-step reaction. Various stationary combustion modes including normal flame (NF), near-stagnation plane flame (NSF), weak flame (WF) and distant flame (DF) are detected and found to be analogous to the same-named regimes of conventional counterflow flames. For the flames stabilized in the vicinity of stagnation plane at moderate and large stretch rates (which are NF, NSF and WF) the effect of channel walls is basically reduced to additional heat loss. For distant flame characterized by large flame separation distance and small stretch rates intensive interphase heat transfer and heat recirculation are typical. It is shown that in mixture content / stretch rate plane the extinction limit curve has ε-shape, while for conventional counterflow flames it is known to be C-shaped. This result is quite in line with recent experimental findings and is explained by extension of extinction limits at small stretch rates at the expense of heat recirculation. Analysis of the numerical results makes possible to reveal prime mechanisms of flame quenching on different branches of ε-shaped extinction limit curve. Namely, two upper limits are caused by stretch and heat loss. These limits are direct analogs of the upper and lower limits on conventional C-shaped curve. Two other limits are related with weakening of heat recirculation and heat dissipation to the burner. Thus, the present study provides a satisfactory explanation for the recent experimental observations of stretched flames in narrow channel.     

辐射传热对微重力环境下预混火焰传播特性的影响

1引言在火焰中，辐射过程是一种重要的传热方式。对该过程尽可能精确的计算，对于改进燃烧设备的设计、改善设备的运行性能十分有益。在正常重力环境下，与其它的释热现象相比，预混火焰中的辐射热损失十分微弱，因而，过去对预混火焰的分析中，往往忽略了辐射热损失的影响。近年来，对微重力（ug）环境下的预混火焰的研究结果表明，可燃极限与＃s最小点火能无关，自媳灭火焰（SEFs）发生时；其释放的能量比通常观察到的点火极限时的能量大几个数量级山，因此火焰伸张并不能解释“g环境下观察到的实验结果，辐射热损失可能是影响＃g火焰可…     

Transport mechanisms controlling soot production inside a non-buoyant laminar diffusion flame

This study integrates new and existing numerical modeling and experimental observations to provide a consistent explanation to observations pertaining flame length and soot volume fractions for laminar diffusion flames. Integration has been attempted by means of scaling analysis. Emphasis has been given to boundary layer flames. For the experiments, ethylene is injected through a flat porous burner into an oxidizer flowing parallel to the burner surface. The oxidizer is a mixture of oxygen and nitrogen, flowing at various velocities. All experiments were conducted in microgravity to minimize the role of buoyancy in distorting the aerodynamics of the flames. A previous numerical study emphasizing fuel transport was extended to include the oxidizer flow. Fictitious tracer particles were used to establish the conditions in which fuel and oxidizer interact. This allowed establishing regions of soot formation and oxidation as well as relevant characteristic length and time scales. Adequate scaling parameters then allow to establish explanations that are consistent for different burner configurations as well as “open-tip” and “closed-tip” flames.     

Effect of flame retardants on side-wall quenching of partially premixed laminar flames

A combined experimental and numerical investigation of partially premixed laminar methane-air flames undergoing side-wall quenching (SWQ) is performed. A well-established SWQ burner is adapted to allow the seeding of the main flow with additional gaseous products issued from a (secondary) wall inlet close to the flame’s quenching point. First, the characteristics of the partially premixed flame that quenches at the wall are assessed using planar laser-induced fluorescence measurements of the OH radical, and a corresponding numerical simulation with fully-resolved transport and chemistry is conducted. A boundary layer of enriched mixture is formed at the wall, leading to a reaction zone parallel to the wall for high injection rates from the wall inlet. Subsequently, in a numerical study, the wall inflow is mixed with dimethylmethylphosphonat (DMMP), a phosphor-based flame retardant. The DMMP addition allows the assessment of the combined effects of heat loss and flame retardants on the flame structure during flame-wall interaction. With an increasing amount of DMMP in the injected mixture, the flame stabilizes further away from the wall and shows a decrease in the local heat-release rate. Thereby, the maximum wall heat flux is significantly reduced. That results in a lower thermal load on the quenching wall. The flame structure analysis shows an accumulation of the intermediate species HOPO at the wall similar to the CO accumulation during the quenching of premixed flames without flame retardant addition. The study shows how the structure of a partially premixed flame is influenced by a wall that releases either additional fuel or a mixture of fuel and flame retardant. The insights gained from the canonical configuration can lead to a better understanding of the combined effects of flame retardants and heat losses in near-wall flames.     

On the chemical characteristics and dynamics of n-alkane low-temperature multistage diffusion flames

We demonstrate experimentally, perhaps for the first time, the existence of low-temperature multistage diffusion flames of n-alkanes. Multistage diffusion flames of n-heptane, n-decane, and n-dodecane are established in an atmospheric counterflow burner. Planar laser-induced fluorescence, chemiluminescence, and thermometry are used to probe the structures of such flames. In the first flame zone, the majority of the fuel is partially oxidized via low-temperature peroxy chemistry. In the second flame zone, the intermediate species produced are further oxidized via intermediate-temperature chemistry. The two stages of the flame are coupled such that significant fuel and oxidizer leakage occur, respectively, from the first and second reaction zones. The fuel is then further consumed, in the second stage, after the radical pool is replenished by the oxidation of the intermediates. The structure of the n-alkane multistage flame is found to be consistent with that previously observed for acyclic ethers. Owing to the different classes of temperature-dependent chemistries dominating the first and second stages, the reaction zone structure of multistage diffusion flames is dramatically influenced by the reactant concentrations and flame temperatures. The first stage is relatively favored at lower temperatures whereas the second stage is favored at elevated temperatures. Moreover, near extinction where the flame temperature is low, the multistage flame dynamics are controlled by the first oxidation stage, governed by peroxy chemistry, whereas the second oxidation stage, governed by intermediate chemistry, is dominant near high-temperature ignition conditions. Finally, by doping the oxidizer with ozone, we demonstrate the role of ozone doping on the multistage flame structure and the existence of a separate low-temperature ozone-assisted burning mode.     

CO Imaging in piloted liquid-spray flames using femtosecond two-photon LIF

Liquid-spray flames are encountered in many practical combustion devices such as gasoline direct injection and diesel engines, gas turbine combustors as well as industrial furnaces. As opposed to gaseous fuels, additional phase-change steps present in liquid sprays not only complicate the overall combustion process, but also make in-situ, laser-based combustion diagnostics challenging. In particular, the formation of carbon monoxide (CO) due to incomplete fuel-air mixing and partial oxidation becomes a major challenge. In this study, we apply femtosecond, two-photon laser-induced fluorescence (fs-TPLIF) to measure CO concentration in piloted liquid-spray flames, taking into account possible signal interferences in the 230.1-nm, B¹Σ⁺←X¹Σ⁺ excitation scheme. A modified, flat-flame McKenna burner fitted with a direct-injection high-efficiency nebulizer (DIHEN) was used to produce piloted liquid-methanol spray flames. Although single-laser-shot OH-PLIF images show the presence of strong turbulent interactions in the core region, shot-averaged OH-PLIF images indicate that near the nozzle-exit region, the primary reaction takes place in an annular region around the droplet cloud, in general. A detailed spectroscopic study reveals that the signal interference at 460?nm originating from the second-order scattering of the excitation laser, which becomes approximately an order of magnitude stronger than CO fluorescence spectral lines near the nozzle exit region. The specific spectral filtering scheme introduced in our recent work is proved to be capable of suppressing interferences primarily originating from C₂ Swan-band emissions. Two-dimensional CO maps along with OH-PLIF flame structure data provide key insights into the CO formation in piloted liquid-spray flames, while providing critical validation datasets for advanced computational models.     

Dilution effects on laminar jet diffusion flame lengths

Many studies have examined the stoichiometric lengths of laminar gas jet diffusion flames. However, these have emphasized normal flames of undiluted fuel burning in air. Many questions remain about the effects of fuel dilution, oxygen-enhanced combustion, and inverse flames. Thus, the stoichiometric lengths of 287 normal and inverse gas jet flames are measured for a broad range of nitrogen dilution. The fuels are methane and propane and the ambient pressure is atmospheric. Nitrogen addition to the fuel and/or oxidizer is found to increase the stoichiometric lengths of both normal and inverse diffusion flames, but this effect is small at high reactant mole fraction. This counters previous assertions that inert addition to the fuel stream has a negligible effect on the lengths of normal diffusion flames. The analytical model of Roper is extended to these conditions by specifying the characteristic diffusivity to be the mean diffusivity of the fuel and oxidizer into stoichiometric products and a characteristic temperature that scales with the adiabatic flame temperature and the ambient temperature. The extended model correlates the measured lengths of normal and inverse flames with coefficients of determination of 0.87 for methane and 0.97 for propane.     

Critical conditions at liftoff limit of a laminar n-butane-air jet diffusion flame

The stability mechanism of laminar coflow jet diffusion flames in normal gravity has been studied computationally and experimentally. N-butane, the heaviest alkane in a gaseous state at ambient temperature and pressure, is used as the fuel since the reaction mechanism is similar to that of higher (liquid) hydrocarbons. The critical mean n-butane jet and coflowing air velocities at flame stability limits are measured using a small fuel tube burner (0.8 mm inner diameter). The time-dependent, axisymmetric numerical code with a detailed reaction mechanism (58 species and 540 reactions), molecular diffusive transport, and a radiation model, reveals a flame structure. A fuel-lean peak reactivity spot (i.e., reaction kernel), possessing the hybrid nature of diffusion-premixed flame structure at a constant temperature of ≈1560 K, is formed at the flame base and controls the flame stability. In a near-quiescent environment, the flame base resides below the fuel tube exit plane and thereby premixing is limited. As the coflowing air velocity is increased incrementally under a fixed fuel jet velocity, the flame base moves slightly above (≈1 mm) the burner exit and vigorous premixed combustion becomes prevailing. The local heat-release rate at the reaction kernel nearly doubles due to the increased convective oxygen flux (i.e., a blowing effect). The local Damköhler number, newly defined as a ratio of the square root of the local heat-release rate and the local velocity, decreases gradually first and drops abruptly at a critical threshold value and the flame base lifts off from the burner rim. The calculated coflow air velocity at liftoff is ≈0.38 m/s at the fuel jet velocity of 2 m/s, which is consistent with an extrapolated measured value of 0.41 m/s. This work has determined the critical Damköhler number at the stability limit quantitatively, for the first time, for laminar jet diffusion flames.     

Concept and combustion characteristics of ultra-micro combustors with premixed flame

Under micro-scale combustion influenced by quenching distance, high heat loss, shortened diffusion characteristic time, and flow laminarization, we clarified the most important issues for the combustor of ultra-micro gas turbines (UMGT), such as high space heating rate, low pressure loss, and premixed combustion. The stability behavior of single flames stabilized on top of micro tubes was examined using premixtures of air with hydrogen, methane, and propane to understand the basic combustion behavior of micro premixed flames. When micro tube inner diameters were smaller than 0.4 mm, all of the fuels exhibited critical equivalence ratios in fuel-rich regions, below which no flame formed, and above which the two stability limits of blow-off and extinction appeared at a certain equivalence ratio. The extinction limit for very fuel-rich premixtures was due to heat loss to the surrounding air and the tube. The extinction limit for more diluted fuel-rich premixtures was due to leakage of unburned fuel under the flame base. This clarification and the results of micro flame analysis led to a flat-flame burning method. For hydrogen, a prototype of a flat-flame ultra-micro combustor with a volume of 0.067 cm³ was made and tested. The flame stability region satisfied the optimum operation region of the UMGT with a 16 W output. The temperatures in the combustion chamber were sufficiently high, and the combustion efficiency achieved was more than 99.2%. For methane, the effects on flame stability of an upper wall in the combustion chamber were examined. The results can be explained by the heat loss and flame stretch.     

Kinetics and extinction of non-premixed cool and warm flames of dimethyl ether at elevated pressure

The growing demand of clean and efficient propulsion and energy systems has sparked an interest in understanding low-temperature combustion at high pressure. Cool flame transition and extinction limits as well as oxygen concentration dependence at elevated pressures provide insights of the low-temperature and high-pressure fuel reactivity. A new experimental high-pressure counterflow burner platform was designed and developed to achieve the studies of high-pressure cool flames. Dimethyl ether (DME) was chosen to study its non-premixed cool flame in high-pressure counterflow burner at pressure up to 5 atm, perhaps for the first time. This paper investigates the effects of pressure on cool flame structure, extinction and transition limits, and oxygen concentration dependence as well as ozone assisted warm flames of DME in experiments and numerical simulations. The results show that the reignition transition from cool flame to hot flame occurs either with the decrease of the strain rate at a given fuel concentration and pressure or with the increase of fuel mole fraction or pressure at a given strain rate. Furthermore, it is shown that the higher pressure shifts the cool flame to higher strain rates and results in higher cool flame extinction strain rates. However, the existing kinetic model of DME fails in predicting the cool flame extinction limit at elevated pressures. Besides, the cool flame extinction limits are proportional to nth power of the oxygen concentration, [O₂]ⁿ, and the increase of pressure leads to stronger extinction limit dependence (larger n) on oxygen concentration. The present experiment and detailed kinetic analysis show clearly that increasing pressure promotes the low-temperature chemistry including the oxygen addition reactions. In addition, stable warm flame was first experimentally observed by using DME at elevated pressure with ozone sensitization.     

Soot particle size distribution measurements in a turbulent ethylene swirl flame

There is a need to better understand particle size distributions (PSDs) from turbulent flames from a theoretical, practical and even regulatory perspective. Experiments were conducted on a sooting turbulent non-premixed swirled ethylene flame with secondary (dilution) air injection to investigate exhaust and in-burner PSDs measured with a Scanning Mobility Particle Sizer (SMPS) and soot volume fractions (f_v) using extinction measurements. The focus was to understand the effect of systematically changing the amount and location of dilution air injection on the PSDs and f_v inside the burner and at the exhaust. The PSDs were also compared with planar Laser Induced Incandescence (LII) calibrated against the average f_v. LII provides some supplemental information on the relative soot amounts and spatial distribution among the various flow conditions that helps interpret the results. For the flame with no air dilution, f_v drops gradually along the centreline of the burner towards the exhaust and the PSD shows a shift from larger particles to smaller. However, with dilution air f_v reduces sharply where the dilution jets meet the burner axis. Downstream of the dilution jets f_v reduces gradually and the PSDs remain unchanged until the exhaust. At the exhaust, the flame with no air dilution shows significantly more particles with an f_v one to two orders of magnitude greater compared to the Cases with dilution. This dataset provides insights into soot spatial and particle size distributions within turbulent flames of relevance to gas turbine combustion with differing dilution parameters and the effect dilution has on the particle size. Additionally, this work measures f_v using both ex situ and in situ techniques, and highlights the difficulties associated with comparing results across the two. The results are useful for validating advanced models for turbulent combustion.