Double-flame Structure Research Articles

Double flame dynamics in hotspot ignition

Hotspot ignition is a key problem in fundamental flame initiation, combustion control and prevention of abnormal combustion phenomena. Depending on the characteristics of the hotspot and the thermochemical condition of the reactive mixture, subsequent reaction front initiated from a hotspot can vary substantially. The current paper focus on the dynamics and chemistry for one of the most complicated scenarios - hotspot induced double flames, which include a cool and a hot flame segment. This work adopts a recently developed compressible reacting flow simulation code COGNAC to investigate the initiation and propagation of double flame dynamics from hotspot ignition in a one-dimensional spherical coordinate. The results have shown that under atmospheric pressure, double flame initiation is not feasible for sufficiently small hotspot. Under elevated pressures, double flame can be initiated within a much wider hotspot window, exhibiting complex flame dynamics, such as variations in cool and hot flame bifurcation behaviors and their initiation locations. Analysis on thermochemical structure and dynamics of double flame has shown that the interaction of cool and hot flames in a double flame structure is inherently intermutual, in that the leading cool flame enhances trailing hot flame speed through reform of the unburnt mixture, while the trailing hot flame initiation affects the leading cool flame via thermal expansion effects.

A Numerical Study on the Ignition and Flame Propagation for Locally Stratified Methanol/n-Heptane Dual-Fuel Mixtures

ABSTRACT Flame propagation under thermal and compositional stratifications is relevant to a wide range of applications in combustion devices. In this work, numerical studies are conducted to investigate the multi-stage ignition process and flame propagation in the stratified methanol/n-heptane-air dual-fuel mixture under engine-like conditions. Firstly, the effects of the initial gas temperature of the oxidizer (TOX) and fuel stream on the ignition process and combustion modes are investigated. The results show that TOX affects the early-stage ignition, and a fuel-rich stabilization mode is observed at low temperatures. For example, at 800 K, both cool and hot flames are initiated at the regions close to the most reactive mixture fraction, . A transition from the fuel-lean to fuel-rich and then back to the fuel-lean regions in the mixture fraction space is observed at higher temperatures. Then, hot flames propagate into the methanol/air premixture, forming quasi-steady hot-flame fronts in the fuel-rich regions at TOX = 800 and 900 K owing to the lack of oxygen and low adiabatic flame temperatures. At 1000 K, another flame front propagates into the n-heptane/air mixtures owing to enhanced reaction rate at higher local temperatures. By increasing TOX, a balance owing to the gas expansion is observed at 1020 K, and a spontaneous ignition mode appears at temperatures above 1050 K owing to the high reactivity and early ignition of the methanol/air premixture. Secondly, the effect of compositional stratification on the ignition and flame structures is investigated. Compared with the double flame structure under thermal and compositional stratification conditions, only a single flame structure is observed for those cases with uniform temperatures. A high temperature in the fuel stream region accelerates the decomposition of n-heptane, resulting in the complete consumption of oxygen.

Forced ignition of premixed cool and hot DME/air flames in a laminar counterflow

Recently, low-temperature chemistry (LTC) and LTC-induced cool flames have attracted extensive interest since they can substantially influence the ignition process and accelerate the subsequent hot flame propagation. However, it is unclear how the flow affects the forced ignition of a cool flame and its subsequent transition to a hot flame. In this study, we conduct transient 2D simulations for the forced ignition processes of premixed cool and hot flames in a laminar, axisymmetric, counterflow configuration with well-defined flow field. Different ignition energies and strain rates are used to ignite and stabilize cool and/or hot flames in a counterflow. The critical conditions for the ignition of cool and hot flames are identified, and the cool flame kernel quenching and its transition to hot flame are discussed. It is found that the ignition energy determines the highest temperature and thereby controls the thermal runaway process. The strain rate influences the flame kernel propagation and the transition from cool flame to hot flame. A large strain rate may quench the cool flame, while a small strain rate may lead to the transition from cool flame to hot flame due to the sufficiently long residence time. Therefore, cool flame ignition and stabilization can only be achieved within a certain range of strain rates. Furthermore, an ignition regime diagram in terms of ignition energy and strain rate is proposed, and four regimes are identified: (I) only cool flame, (II) only hot flame, (III) ignition failure, and (IV) transition from cool flame to hot flame (with appearance of double flame structure). These results provide new insights into how the flow affects the cool flame ignition and transition.

Large eddy simulation of spray combustion using the spray flamelet/progress variable model: Further extension and validation

The classical flamelet combustion model assumes that the combustion system consists of two homogeneous inlet streams for oxidizer and fuel. It usually requires extension and validation for configurations with different types of inlet streams. In the current work, the recently developed two-phase spray flamelet/progress variable (TSFPV) model is further extended and validated on the configurations of Delft spray flame AII and Sydney piloted ethanol spray flame EtF2, representing the systems with a single inlet and three inlets, respectively. The strategies for generating the spray flamelet library are adjusted, and an extended model is developed for modeling flame AII. Large eddy simulations of these two flames are conducted to evaluate the accuracy of the TSFPV model. The results of flame AII show that some droplets penetrate into the flame zone, and significant droplets/flame interactions occur. The flame exhibits a typical double flame structure. The predictions of gas phase temperature are in good agreement with the experimental measurements, which indicates that the current model can better consider the influence of droplet evaporation and the temperature reduction caused by interphase heat loss. The results of flame EtF2 also achieve good agreement with experimental data according to radial profiles of axial droplet velocities and gas phase temperature. Overall, the TSFPV model is well validated and exhibits sound performance on different flame configurations, indicating its widespread applicability and generalizability.

An experimental study of the sooting behavior of a partially premixed flame under moderately rich conditions

Soot and its gaseous precursors are quantified in detail (precursors up to 166 amu, volume fraction, particle size, number concentration, and light emissivity dispersion exponent) in a laminar partially premixed counterflow flame of ethylene. The investigated flame has an equivalence ratio Φ = 2.43 and a mixture fraction Zst = 0.4, resulting in a distinct double-flame structure consisting of a rich premixed flame component and a diffusion flame component, both stabilized on the fuel side of the stagnation plane. The value of the equivalence ratio makes the premixed flame the dominant contributor to soot production, with soot being oxidized completely by OH from the diffusion flame component. Particle size is measured to increase quasi-monotonically, but remains within a few nanometers throughout the soot forming region. Aromatic species are primarily formed in the post flame region of the premixed flame. Their mole fractions peak close to the premixed flame and decrease as the diffusion flame is approached. The experimentally measured gaseous species are captured well by kinetic models, with the exception of two critical species in soot chemistry: benzene and naphthalene.

Ignition and stabilization of laminar premixed n-heptane/air flames under engine-like conditions

The multi-stage ignition process of a laminar premixed n-heptane/air mixture is investigated under engine-relevant conditions. The cold fuel/air mixture is injected into a hot environment through an inflow boundary at different inlet velocities. Results show that a double-flame structure for stoichiometric mixture is observed and it transitions to a single flame at an inlet temperature (TFuel) of 700 K. The cool flame is stabilized near the inlet for the case with TFuel = 900 K before the hot flame merging with it. However, both the cool and hot flames move upstream for TFuel = 800 K due to the negative temperature coefficient (NTC) behavior. In addition, the effect of inlet velocity is also studied. By increasing the inlet velocity, the hot flame becomes lifted off. Comparisons of the ignition Damkӧhler number (Daign) defined by the ratio of the residence time to the cool- or hot-flame IDTs illustrate the different controlling mechanisms. At a low temperature of 700 K, both low-temperature combustion (LTC) and high-temperature combustion (HTC) have a Daign value of unity, indicating that the stabilization process is mainly controlled by autoignition rather than diffusion. The heating effect from the hot environment enhances the NTC characteristics of the premixed flame, thus, shortening the IDT. Whereas at 800 K, the value of Daign for the LTC is of unity, but that of HTC decreases, especially at a low inlet velocity, indicating that diffusion plays a more important role for HTC and the flame is stabilized by propagation, which can also be confirmed from the comparisons of the 0-D and 1-D ignition delay times (IDTs). Diffusion of intermediate radicals and heat suppresses the appearance of the high-temperature ignition at an inlet velocity of 1 m/s. At 900 K, high inlet velocity can increase the Daign to unity, indicating a transition in the combustion regime from propagation to autoignition.

Two-dimensional simulation of cool and double flame formation induced by the laser ignition under shock-tube conditions

The laser ignition-induced spherical cool and double flame’s initiation, propagation, and transition are studied using a two-dimensional simulation with a detailed mechanism using n-heptane/O2/Ar/He mixture. The current study’s primary goal is to reveal how the radicals and flow field produced by the laser ignition lead to cool, hot, or double flame formation and to determine the flame propagation modes. It is found that after the laser spark, the over-driven shock wave results in vortex pairs, which significantly distorts the temperature field and the spatial distribution of the radicals. A torus-like hot flame is ignited at the ignition kernel center instantaneously. The results show that the laser-ignited hot flame can transit to a cool or double flame depending on the laser pulse energy. On the one hand, the hot flame extinguishes under a small laser ignition energy condition, at which the radicals at the hot flame front are transported by the vortex pair and lead to a cool flame formation. On the other hand, when the laser spark energy is large, a transient premixed double flame structure is observed. The double flame can be localized, and the appearance location is primarily affected by the spatial distribution of laser-induced radicals. The cool flame can coexist with a trailing hot flame for milliseconds, dramatically accelerating the hot flame propagation and fundamentally impacting the flame speed interpolation. Computational singularity perturbation (CSP) analysis identifies key reactions that belong to typical low-temperature chemistry in the double flame zone. A comparative simulation case with an initial temperature above the low-temperature chemistry regime confirms that no double flame structure can be formed under high-temperature conditions. The present study provides essential insight and guidance for the flame speed measurement using laser ignition under engine-relevant conditions.

Large eddy simulation of n-dodecane spray flame: Effects of injection pressure on spray combustion characteristics at low ambient temperature

The present study uses large-eddy simulations (LES) to identify the underlying mechanism that governs the ignition and stabilization mechanisms of ECN Spray A flame for different injection pressures (Pinj) and ambient temperatures (Tam). Two Pinj of 50 MPa and 150 MPa, as well as two Tam of 750 K and 900 K are considered. The numerical model is validated against the experimental fuel penetration, radial mixture fraction distribution, ignition delay time, and lift-off length. The combustion characteristics of all four spray flames are well predicted, with a maximum relative difference of 15% to the measurements. At 900 K, high-temperature ignition (HTI) occurs in the fuel-rich mixture at the spray head of the high Pinj spray flame, but at the spray periphery of the low Pinj spray flame. This is due to the low Pinj case having fuel-richer mixture in the inner spray region. Nonetheless, the spray flames at both Pinj exhibit double-flame structure. At 750 K, HTI occurs at the fuel-rich and fuel-lean regions for spray flames with Pinj=50 MPa and 150 MPa, respectively. Reducing the Pinj leads to a lower injection velocity, less turbulent fluctuation, slower mixing, and hence the occurrence of HTI at the fuel-rich mixtures. The spray flame in the low Pinj case at 750 K exhibits a triple-flame structure at the lift-off position, while the high Pinj case exhibits a lean premixed reaction zone. This difference is attributed to the distribution of fuel-rich mixtures. Despite differences in the flame structures, auto-ignition process plays a key role to stabilize the lift-off position for all four spray flames. The auto-ignition process is also found to be dependent on the cool-flame products upstream of the lift-off position. In particular for the low Tam cases, the heat transfer effect from the main flame to the fuel-rich regions is suggested to also contribute to the flame stabilization mechanisms.

Effect of equivalence ratio and temperature on soot formation in partially premixed counterflow flames

Soot formation is quantified in detail (volume fraction, particle size, number concentration, and light emissivity dispersion exponent) in a series of partially premixed counterflow flames of ethylene at equivalence ratios, Φ, equal to 6.5, 5.0, and 4.0, and with maximum temperature spanning approximately 200 K. The focus is to investigate the effect of peak temperature and equivalence ratio on soot formation while maintaining constant global strain and stoichiometric mixture fraction. Oxygen is progressively displaced from the oxidizer to the fuel stream of a diffusion flame to stabilize partially premixed flames of decreasing Φ, showing a double-flame structure consisting of a rich premixed flame component stabilized on the fuel side of the stagnation plane and a diffusion flame component stabilized on the oxidizer side. Soot is detected in the region sandwiched between the two flame components, is formed in both of them, and is convected away radially at the Particle Stagnation Plane (PSP). At fixed Φ, raising the peak temperature invariably raises the soot volume fraction throughout the probed region. Vice versa, at fixed peak temperature, lowering the equivalence ratio causes the premixed flame component to shift away from the diffusion flame component, with the consequent broadening of the soot forming region and an increase in both soot volume fraction as well as soot particle sizes through an enhancement of surface growth. Detailed probing of the region in the vicinity of the PSP offers evidence of soot oxidation from molecular oxygen. Furthermore, when the maximum temperature is sufficiently low, the net soot production rate turns negative because surface oxidation overwhelms surface growth. Comparing the soot number production rate inferred from experiments to the dimerization rate of benzene (C6H6), naphthalene (C10H8), and pyrene (C16H10) reveals that only the smallest aromatics are present in flames at sufficiently large concentrations to account for soot nucleation. This observation applies to both the diffusion flame and the premixed flame components and confirms previous findings in strictly diffusion flames.

Structure of the ethanol spray flame under conventional and MILD conditions

A numerical examination is carried out on the experimental MILD turbulent spray burner of Delft Spray in Hot Co-flow, DSHC. The RANS approach is combined with the Lagrangian tracking of droplets for modeling the pressure-swirl atomizer. The EDC combustion model is employed, to take into account detailed chemical mechanisms. Predictions show that the structure of the triple-flame of liquid fuel in the atmospheric condition is different from the conventional gaseous triple-flames. Furthermore, the structure of the MILD spray flame is compared to the non-MILD spray flame. The flame index shows that the triple-flame structure in the conventional spray flame that consists of two outer and middle diffusion branches, and a premixed inner branch converts to a double-flame structure under MILD condition, including an outer diffusion and a premixed inner branch. In another step, sensitivity analysis relative to the temperature, and oxygen concentration of the co-flow stream is performed independently, which was not possible in the experiment. The parameter of ignition delay time is used for the selection of parametric study cases. Moderate preheating could lead to deviation from the MILD combustion regime, while high preheating is beneficial. NO pollutant generation shows more sensitivity to dilution rather than preheating in MILD spray flame. This study showed that increasing or decreasing each of the temperature and oxygen concentration doesn’t improve all the effective aspects toward achieving the MILD condition. So, the effective parameters should be changed according to a specific and more precise goal.

Experimental investigation of the flame structure of dilute sprays issuing into a hot and low-oxygen coflow

The combustion of liquid fuels in a hot and low-oxygen environment is commonly encountered in a range of practical situations. To enable investigation of the fundamental combustion processes relating to such applications, liquid fuels were injected into the reaction zone as dilute sprays in this study. Droplets of ethanol, n-heptane, and n-heptane/toluene blends were produced via an ultrasonic nebuliser, and were carried by air through a central jet to a hot coflow of combustion products. The resulting flames were then analysed using four simultaneous laser diagnostic techniques. Planar laser-induced fluorescence (PLIF) was implemented to perform imaging of key intermediate species, including hydroxyl (OH) and formaldehyde (CH2O), while the Mie scattering technique was used to detect the location of droplets. The sooting behaviour of these flames was also investigated, via the laser-induced incandescence (LII) technique. The existence of distinct inner and outer reaction zones is a key feature of all of the flames studied, and this “double flame structure” was found to be related to partial premixing of air and fuel, as well as penetration of droplets into the inner reaction zone. A change in the stabilisation of the inner flame front was observed with variations in fuel type, with a greater likelihood of ignition kernels in the case of the n-heptane and n-heptane/toluene flames, whereas the equivalent ethanol flame displays a bifurcation structure. Variations in the jet Reynolds number and liquid fuel loading were also found to have a notable impact on the distribution and evaporation of droplets, which was in turn found to affect the formation of the double flame structure. Due to the complex coupling between turbulence, chemistry and droplet evaporation in these flames, the accurate prediction of such results a priori is not within the limits of current modelling capabilities. These findings provide a valuable insight to enable future advancements in spray combustion modelling and the design of practical combustion devices.

Effects of n-heptane concentration on ignition characteristics and flame propagation of dual fuel combustion under engine-like conditions

Natural gas/diesel dual fuel combustion mode is one of the most attractive strategies to achieve ultra-low emissions and efficient combustion in transportation sector. This paper numerically investigated the effects of n-heptane concentration on dual fuel combustion characteristics in terms of ignition and flame development in low-speed engine environments. The results indicate that with the decrease of fuel equivalence ratio, the ignition of dual fuel mixture is delayed and occurs in the leaner fuel mixture. The evolution of heat release rate (HRR) transfers from double peaks to single peak as the fuel equivalence ratio decreases and only rapid growth stage appears in the evolution process of peak temperature Tpeak for the case of Φfuel=2. With the development of dual fuel flame, Tpeak is maintained at the mixture fraction close to ξst, but the flame front still moves to the premixed methane/air mixtures side in physical space. Moreover, the flame propagation between the onset of ignition and the appearance of maximum Tpeak is shortened with the decreasing n-heptane concentration. It is also shown that the characteristic time τmax firstly decreases and then increases with the decrease of fuel equivalence ratio, and the minimum appears in the case with equivalence ratio of Φfuel=4. Meanwhile, the distribution of chemical explosion mode (CEM) becomes wider as the fuel equivalence ratio decreases. With the decreasing concentration gradient, the flame structure with low-intermediate-high temperature exothermic co-existence is formed in the rich fuel region, and it changes to double flame structure gradually as the combustion reaction proceeds. The low concentration gradient produced by the decreasing fuel equivalence ratio affects rection pathways of fuel molecules, which further influences the origination of ignition, flame structure and flame propagation process.

An Experimental Study on Kerosene Spray Combustion Under Conventional and Hot-Diluted Conditions

ABSTRACT The combustion of kerosene spray under hot-diluted conditions and conventional conditions was experimentally investigated. By examining flame photographs, chemiluminescence images, and in-field temperature measurements, the separate effect of different variables including oxygen concentration, temperature and velocity of the co-flowing air, fuel flow rate and injection pressure, and eventually the type of spray nozzle on multiple parameters such as flame stability, structure, luminosity, temperature field, and qualitative CH radical distribution, as well as HCO and NO2 with lower precision, in the reaction region, have been studied. It was observed that an increment in injection pressure and co-flow temperature enhances the spray flame stability, while dilution exacerbates it. Also, a solid cone spray pattern with a lower spray angle has better stability than hollow cone ones with a higher spray angle. Moreover, it was noted that liquid fuels, compared to gaseous fuels, require higher preheating temperatures, for the same dilution level, to engender a stable flame. For combustion of spray in conventional conditions, a double-flame structure was observed consisting of a bluish section at the leading edge emerging into a yellowish sooting trail. An increase in co-flow velocity, as well as injection pressure, strengthens the inner flame front, whereas raising the co-flow temperature or diluting the oxidant, deteriorates the inner flame front. In the case of highly preheated air (without dilution), the flame liftoff height is reduced to as close to the atomizer as a few millimeters, forming a single flame structure similar to gaseous flames. In this case, the peak temperature was considerably higher than the conventional combustion, yet the gain was much lower than the preheating level. Combined effects of preheating and dilution alter the spray flame structure in a way that the flame volume is reduced, the temperature field has become more homogeneous, the peak temperature is limited to less than 1500 K, and temperature fluctuations have significantly decreased, seemingly approaching MILD combustion regime conditions.

Effect of Diluent Addition on Combustion Characteristics of Methane/Nitrous Oxide Inverse Tri-Coflow Diffusion Flames

ABSTRACT The purpose of this study is to examine the dilution effect on combustion characteristics of methane/nitrous oxide inverse tri-coflow diffusion flame through the addition of various dilution gases. Two diluents, namely carbon dioxide and argon, were used. To scrutinize the overall combustion characteristics, the effects of dilution on flame appearance and pollution emissions were calculated numerically and observed experimentally. In terms of flame appearance, with increasing the central oxidizer flow rate, the flame became an inner-oxidizer-injected flame structure and then converted to a double flame structure. When 40% Ar and 20% CO2 were added to the oxidizer individually, a liftoff edge flame was formed at high central oxidizer flow rate. This theoretical prediction provided a fast and preliminary explanation of the various flame structures and concurred with the trend of IDF formation observed in the experiment. Regarding the flue gas emission, the addition of Ar in CH4/N2O diffusion flame can effectively reduce the yield of CO, whereas the addition of CO2 leads to an increase of CO concentration in the flue gas. Simulation results indicated that for Ar addition, the production of NOx is dominated by the inert effect at dilution levels lower than 40%, whereas the thermal/diffusion effect dominates at dilution levels higher than 40%. CO formation is mainly dominated by the thermal/diffusion effect. In CO2 dilution, NOx formation is dominated by the inert and chemical effects at dilution levels lower than 40%. However, at dilution levels higher than 40%, the chemical effect is more dominant. Thus, CO formation is dominated mainly by the chemical effect.

Ignition of dimethyl ether/air mixtures by hot particles: Impact of low temperature chemical reactions

Understanding and characterizing ignition of flammable mixtures by hot particles is important for assessing and reducing the risk of accidental ignition and explosion in industry and aviation. Recently, many studies have been conducted for ignition of gaseous mixtures by hot particles. However, the effects of low-temperature chemistry (LTC) on ignition by hot particles received little attention. LTC plays an important role in the ignition of most hydrocarbon fuels and may induce cool flames. The present study aims to numerically assess the effects of LTC on ignition by the hot particles. We consider the transient ignition processes induced by a hot spherical particle in quiescent and flowing stoichiometric dimethyl ether/air mixtures. 1D and 2D simulations, respectively, are conducted for the ignition process by hot-particles in quiescent and flowing mixtures. A detailed kinetic model including both LTC and high-temperature chemistry (HTC) is used in simulations. The results exhibit a premixed cool flame to be first initiated by the hot particle. Then a double-flame structure with both premixed cool and hot flames is observed at certain conditions. At zero or low inlet flow velocities, the hot flame catches up and merges with the leading cool flame. At high inlet flow velocities, the hot flame cannot be initiated due to the short residence time and large convective loss of heat and radicals. Comparing the results with and without considering LTC confirms that LTC accelerates substantially ignition via HTC in a certain range of hot particle temperatures. The mechanism of ignition promotion by LTC is interpreted by analyzing the radical pool produced by the LTC and HTC surrounding the hot particle. Moreover, the influence of inlet flow velocity on ignition by hot particles is assessed. Non-monotonic change of ignition delay time with flow velocity is observed and discussed.

Numerical study of a polydisperse spray counterflow diffusion flame

A counterflow two-phase diffusion flame with polydisperse spray is numerically studied in a 2D configuration, using a Lagrangian formalism for the liquid phase and accurate combustion chemistry. Results exhibit a very complex double flame structure with diffusion and premixed flames, as well as group and individual droplet burning. Monodisperse two-phase counterflow flames are also computed to help analyse the polydisperse flame, and confirm the strong link between droplet diameter and flame regime. For small droplet diameters, the flame has the same structure than a gaseous flame at a different equivalence ratio, and the total heat release of the flame increases with the droplet size. For larger droplets, the premixed mode becomes dominant and the total heat release of the flame exceeds the maximum gaseous diffusion one.

Flammability and Propagation Dynamics of Planar Freely Propagating Dimethyl Ether Premixed Flame.

Flammability dynamics and physics play a crucial role in fire safety and combustion efficiency. This paper numerically studied the flammability dynamics of dimethyl ether/air freely propagating premixed flame over a broad range of equivalence ratios (ϕ). The results showed that the traditional flammability range should be redefined considering the impact of low-temperature chemistry. A physically stable warm-flame branch existed in the ultrarich region (ϕ = 7.58–12.59), which connected the hot and cool flame transition smoothly. However, in the lean region, the transition between hot and cool flames was completed by extinguishment or ignition. Sensitivity analysis was performed to reveal the governing chemical and diffusive processes for the flammability limits (FLs). In addition to the high-temperature reactions, low-temperature chemistry also played an important role in the lean hot-flame FL because of its double-flame structure. Heat conduction and fuel and oxygen diffusions were the most significant diffusive processes for the near-limit flame propagation. The near-limit flames had a diffusion-reaction structure, in which the flame front propagation was sustained by the heat conduction-induced ignition rather than the autoignition wave. The hot-flame extinction was induced by radiative extinguishment of the high-temperature propagating front embedded in the double-flame structure, and the cool-flame extinction was induced by excessive diffusive loss.

TDLAS-based in situ diagnostics for the combustion of preheated ultra–lean dimethyl ether/air mixtures

A combined experimental and modelling study of the structure of a laminar premixed ultra–lean (ϕ=0.33) dimethyl ether/air flame at atmospheric pressure and an elevated temperature was carried out. The work aimed to apply tunable diode laser absorption spectroscopy to the identification of various flame regimes that are relevant to the oxidation of dimethyl ether. One-dimensional calculations employing burner-stabilized flame assumptions confirmed the significance of low-temperature combustion chemistry. A stable double-flame structure was predicted using state-of-the-art chemical kinetic schemes and was revealed by the experimental observations. The feasibility of the novel experimental strategy based on the preheated flat-flame burner and scanned wavelength modulation spectroscopy was investigated in this context. The absorption features of hot water and the hydroxyl radical near 1572 nm were selected as appropriate targets for distinguishing the transition from the cool flame regime to the hot flame regime. Discrepancies in the water line position and intensities were found within the 1509 nm region.

A regularized deconvolution model for sub-grid dispersion in large eddy simulation of turbulent spray flames

In this study, a sub-grid scale (SGS) dispersion model is developed for simulating turbulent spray flames. This model employs a regularized deconvolution method (RDM) to determine the sub-grid fluctuations from the filtered velocity field. The model is examined in simulations of a turbulent counterflow n-dodecane spray flame, and benchmarked against a simplified Langevin model (SLM) and simulations without dispersion closure (NOM). To identify conditions that characterize the importance of the spray-flame coupling, a regime diagram is constructed from physical arguments and theoretical analysis by considering idealized conditions. Based on the proposed criteria, a range of operating conditions is considered, involving different Stokes- and Reynolds numbers as well as inert and reacting conditions. A priori DNS analyses are performed to compare the probability density functions (PDFs) of slip velocities. Significant discrepancies between models are observed for smaller Stokes numbers. Subsequently, large eddy simulations (LES) are performed and effects of mesh resolution on statistical flow-field quantities are investigated. Qualitative and quantitative differences are observed for coarse-grid simulations with larger droplets, where a double-flame structure is predicted by DNS and LES-RDM, whereas a single flame is predicted by LES-NOM and LES-SLM. For the finer grid, all three LES models predict the double-flame structure. Turbulence modulation by the spray is expected to play a role in creating different flame topologies, and the RDM-based SGS-closure model is shown to provide improved predictions for the dispersed and carrier phases for a wider range of grid resolution.

Modelling of diesel spray flames under engine-like conditions using an accelerated Eulerian Stochastic Field method

This paper aims to simulate diesel spray flames across a wide range of engine-like conditions using the Eulerian Stochastic Field probability density function (ESF-PDF) model. The ESF model is coupled with the Chemistry Coordinate Mapping approach to expedite the calculation. A convergence study is carried out for a number of stochastic fields at five different conditions, covering both conventional diesel combustion and low-temperature combustion regimes. Ignition delay time, flame lift-off length as well as distributions of temperature and various combustion products are used to evaluate the performance of the model. The peak values of these properties generated using thirty-two stochastic fields are found to converge, with a maximum relative difference of 27% as compared to those from a greater number of stochastic fields. The ESF-PDF model with thirty-two stochastic fields performs reasonably well in reproducing the experimental flame development, ignition delay times and lift-off lengths. The ESF-PDF model also predicts a broader hydroxyl radical distribution which resembles the experimental observation, indicating that the turbulence–chemistry interaction is captured by the ESF-PDF model. The validated model is subsequently used to investigate the flame structures under different conditions. Analyses based on flame index and formaldehyde distribution suggest that a triple flame, which consists of a rich premixed flame, a diffusion flame and a lean premixed flame, is established in the earlier stage of the combustion. As the combustion progresses, the lean premixed flame weakens and diminishes with time. Eventually, only a double-flame structure, made up of the diffusion flame and the rich premixed flame, is observed. The analyses for various ambient temperatures show that the triple-flame structure remains for a longer period of time in cases with lower ambient temperatures. The present study shows that the ESF-PDF method is a valuable alternative to Lagrangian particle PDF methods.