N-heptane Spray Flames Research Articles

Effects of liquid water injection on flame surface topology and propagation characteristics in spray flames: A direct numerical simulation analysis

The effects of water injection on flame surface topology and local flame propagation characteristics have been analyzed for statistically planar turbulent n-heptane spray flames with an overall (i.e., liquid + gaseous) equivalence ratio of unity using carrier-phase direct numerical simulations. Most fuel droplets have been found to evaporate as they approach the flame even though some droplets can survive until the burnt gas side is reached, whereas water droplets do not significantly evaporate ahead of the flame and the evaporation of water droplets starts to take place in the reaction zone and is completed within the burnt gas. However, the gaseous-phase combustion occurs predominantly in fuel–lean mode although the overall equivalence ratio remains equal to unity. The water injection has been found to suppress the fuel droplet-induced flame wrinkling of the progress variable isosurface under the laminar condition, and this effect is particularly strong for small water droplets. However, turbulence-induced flame wrinkling masks these effects, and thus, water injection does not have any significant impact on flame wrinkling for the turbulent cases considered here. The higher rate of evaporation and the associated high latent heat extraction for smaller water droplets induce stronger cooling effects, which weakens the effects of chemical reaction. This is reflected in the decrease in the mean values of density-weighted displacement speed with decreasing water droplet diameter. The weakening of flame wrinkling as a result of injection of small water droplets is explained through the curvature dependence of the density-weighted displacement speed. The combined influence of cooling induced by the latent heat extraction of water droplets and flame surface flattening leads to a decrease in volume-integrated burning rate with decreasing water droplet diameter in the laminar cases, whereas the cooling effects are primarily responsible for the drop in burning rate with decreasing water droplet diameter in the turbulent cases.

Stability and structure of lean swirling spray flames with various degrees of prevaporization

Achieving full premixing and complete evaporation in lean prevaporized premixed combustors is challenging as it depends on the spray injector characteristics, prevaporisation strategy, and flow conditions. This article experimentally explores the stability and structure of a turbulent swirling n-heptane spray flame under various degrees of prevaporization. The results show that preheating the air to 343 K and 393 K has little effect on the lean blow-off velocity, while recessing the fuel injection significantly decreases the lean stability limit. To correlate these limits, various attempts to define a Damköhler number were made, but unlike previous studies with no prevaporisation, the difficulty in defining laminar flame speed in the present case does not allow a single correlation to work for all degrees of prevaporization. Four stable cases that differ in equivalence ratio, air preheat temperature, and fuel injection recess are investigated using one-dimensional PDA, OH* chemiluminescence and CH[Formula: see text]O-planar laser-induced fluorescence (PLIF). Cases without fuel injection recess or air preheat exhibit a conical-shaped heat release zone near the shear layers. Preheating the air to 393 K reduced the Sauter mean diameter, increased prevaporization, and enabled a second heat release zone downstream of the fuel injection. Recessing the fuel injection by 25 mm reduced droplet velocities and led to a semi-spherical instead of a conical heat release zone. The CH[Formula: see text]O-PLIF signal without injection recess was high along the central axis and its distribution resembled that observed for spray jet flames. In contrast, with recessed spray injection, CH[Formula: see text]O was mainly found outside the central recirculation zone and only appeared inside during lean blow-off; similar to previous work with premixed flames. These findings show that different methods of prevaporization, which only differ by subtle changes in droplet characteristics, strongly impact flame stability. The present data can be used for turbulent flame modelling focusing on sprays and finite-rate kinetics.

Numerical Investigation on the n-Heptane Spray Flame at Hydrous Ethanol Premixed Condition

Abstract In hydrous ethanol-diesel dual-fuel combustion modes, premixed hydrous ethanol would have an impact on the diesel combustion and soot production. In this work, based on computational fluid dynamics (CFD) model coupled with kinetic mechanism, the influence of premixed hydrous ethanol on combustion and soot formation of n-heptane spray flame in the combustion vessel was investigated, and the influence of different parameters of premixed atmosphere on the n-heptane spray combustion was also explored. The simulation results indicated that the premixed atmosphere could inhibit the n-heptane spray combustion. The ignition delay (ID) time and the flame lift off length (FLoL) gradually increased, and the soot generation decreased with a rise of water content and ethanol premixed equivalence ratio. In addition, compared with the chemical inhibitory effect of oxygenated ethanol molecule on the ID time of n-heptane, the impact of the decrease in charge temperature caused by the hydrous ethanol vaporization on the ID time was more obvious. It could also be found with the reduction in the ambient O2 concentration, the ID time and FLOL of n-heptane increased, and the soot generation significantly reduced. However, if the initial ambient temperature was high as 1000 K, the combustion of hydrous ethanol would enhance the charge temperature, which counteracted some combustion delay effect caused by the premixed atmosphere.

Dilute spray flames of ethanol and [formula omitted]-heptane in the transition to mild combustion

The structure and stabilisation mechanisms of ethanol and n-heptane spray flames are investigated in this study. The burner configuration involves a dilute spray of dispersed droplets which is produced and transported via a carrier gas stream of air to the reaction zone, where the flames are stabilised by a hot coflow of combustion products. A range of coflow conditions were implemented for the different flame cases, allowing the effects of the coflow oxygen (O2) concentration and temperature to be examined independently. The resulting flames were analysed using three simultaneous laser diagnostic techniques, enabling the combined planar imaging of the hydroxyl (OH) and formaldehyde (CH2O) radicals, along with the location of droplets. For both fuel types, a noticeable shift in stabilisation behaviour was observed with a variation in the coflow O2 concentration from 11% to 3%, while the coflow temperature was not seen to have a significant impact. These flames also show an interesting departure from the typical behaviour observed for gaseous and prevaporised flames in a similar configuration, particularly for coflow conditions that are typically associated with the transition to the mild combustion regime.

Analysis of local extinction of a n-heptane spray flame using large-eddy simulation with tabulated chemistry

This work is focused on the study of flame stabilization and local extinction of a spray flame in an atmospheric non-swirled test rig referred to as the CORIA Rouen Spray Burner. This burner shows a double reaction front structure, with an outer laminar diffusion flame and an inner wrinkled partially-premixed flame undertaking local extinction. This unsteady phenomenon is investigated here using large-eddy simulations with a tabulated chemistry method based on steady and unsteady diffusion flamelets with heat loss. A validation of the numerical simulations is conducted first for the carrier and dispersed phase and good agreement with the experimental data is found for mean and fluctuating quantities. The present results were able to predict relevant parameters of the flame like the lift-off length and flame shape. Numerical results evidence some intermittency on the OH concentration with the presence of high values of formaldehyde indicating the existence of localized extinction in the leading edge of the flame. A detailed analysis showed the impact of droplets on the reacting layer and the existence of rich pockets quenching the flame front. Further downstream, it was shown that when the scalar dissipation rate reaches high values near the reaction zone, the flame front becomes thinner and wrinkled until it eventually quenches. The numerical results evidence that the applied tabulated chemistry method is capable of capturing the local extinction and re-ignition events occurring in the inner layer.

Effects of Mean Inflow Velocity and Droplet Diameter on the Propagation of Turbulent V-Shaped Flames in Droplet-Laden Mixtures

Three-dimensional carrier phase Direct Numerical Simulations of V-shaped n-heptane spray flames have been performed for different initially mono-sized droplet diameters to investigate the influence of mean flow velocity on the burning rate and flame structure at different axial locations from the flame holder. The fuel is supplied as liquid droplets through the inlet and an overall (i.e., liquid + gaseous) equivalence ratio of unity is retained in the unburned gas. Additionally, turbulent premixed stoichiometric V-shaped n-heptane flames under the same turbulent flow conditions have been simulated to distinguish the differences in combustion behaviour of the pure gaseous phase premixed combustion in comparison to the corresponding behaviour in the presence of liquid n-heptane droplets. It has been found that reacting gaseous mixture burns predominantly under fuel-lean mode and the availability of having fuel-lean mixture increases with increasing mean flow velocity. The extent of flame wrinkling for droplet cases has been found to be greater than the corresponding gaseous premixed flames due to flame-droplet-interaction, which is manifested by dimples on the flame surface, and this trend strengthens with increasing droplet diameter. As the residence time of the droplets within the flame decreases with increasing mean inflow velocity, the droplets can survive for larger axial distances before the completion of their evaporation for the cases with higher mean inflow velocity and this leads to greater extents of flame-droplet interaction and droplet-induced flame wrinkling. Mean inflow velocity, droplet diameter and the axial distance affect the flame brush thickness. The flame brush thickens with increasing droplet diameter for the cases with higher mean inflow velocity due to the predominance of fuel-lean gaseous mixture within the flame. However, an opposite behaviour has been observed for the cases with lower mean inflow velocity where the smaller extent of flame wrinkling due to smaller values of integral length scale to flame thickness ratio arising from higher likelihood of fuel-lean combustion for larger droplets dominates over the thickening of the flame front. It has been found that the major part of the heat release arises due to premixed mode of combustion for all cases but the contribution of non-premixed mode of combustion to the total heat release has been found to increase with increasing mean inflow velocity and droplet diameter. The increase in the mean inflow velocity yields an increase in the mean values of consumption and density-weighted displacement speed for the droplet cases but leads to a decrease in turbulent burning velocity. By contrast, an increase in droplet diameter gives rise to decreases in turbulent burning velocity, and the mean values of consumption and density-weighted displacement speeds. Detailed physical explanations have been provided to explain the observed mean inflow velocity and droplet diameter dependences of the flame propagation behaviour.

Effects of ambient methanol on pollutants formation in dual-fuel spray combustion at varying ambient temperatures: A large-eddy simulation

Large-eddy simulation with a finite-rate chemistry model is carried out to investigate the formation of soot and nitrogen oxides (NOx) in the dual-fuel spray combustion. Liquid n-heptane is injected into a constant volume chamber, filled with a premixed methanol–air mixture with an equivalence ratio (ϕm) of 0.3. Three dual-fuel cases are simulated under initial temperatures of 900, 950 and 1000 K. Three single-fuel cases, with the same configurations, but with pure air being used as ambient gas composition are also simulated and used as baselines for comparison purposes. The paper aims to identify the main mechanisms of soot and NOx reduction in dual-fuel spray combustion under conditions relevant to internal combustion engines. The numerical model is validated using the Engine Combustion Network n-heptane fuel experimental data. It is found that soot emission has a strong non-linear dependence on ambient temperature in dual-fuel combustion. At high temperatures, soot emission is enhanced whereas at lower temperatures, it is suppressed. For the presently studied cases, the results show that the enhanced mixing is the primary reason for soot reduction in the 900 and 950 K cases, whereas the onset auto-ignition in ambient methanol/air mixture, which leads to a shortened lift-off length of the n-heptane spray flame and reduced ambient oxygen concentration, is the mechanism behind the enhanced soot emission in the 1000 K case. The methanol dilution effects of oxygen concentration and heat capacity on ignition delay time and the maximum flame temperature are minor in the current dual-fuel configurations. Regarding NOx emission in dual-fuel combustion, it is found that the effect of methanol on NOx formation also depends on the ambient temperatures. The NOx formation rate in the dual-fuel case is lower than that of the single-fuel case at 900 K. However, an opposite trend of NOx formation rate is observed in the 1000 K cases. The main reason for the increased NOx emission is the larger high temperature region resulted from the interaction of the spray flame and the ambient mixture ignition.

The correlation of species concentration with heat release rate in an auto-igniting turbulent n-heptane spray flame

The quantification of the heat release process is important for engine combustion, for which species concentrations are often employed as the heat release rate indicators (HRRIs) considering the difficulty in the direct measurement of the local heat release rate (HRR). In this study, the correlation between HRR and species concentrations in auto-ignition and auto-ignition assisted flame propagation of n-heptane/air mixtures is first quantified to validate the commonly used formaldehyde-based HRRIs (i.e. [HCO] and [OH][CH2O]). Potential candidates for HRRIs are then proposed and further investigated in the auto-igniting n-heptane spray flame obtained with direct numerical simulations using a 44-species mechanism. It is found that [HCO] and [OH][CH2O], as well as the potential candidates [C2H3], [CH2], [OH][HO2] and [H][HO2], in general all have good linear correlations with the HRR in the spray flame, and [H][HO2] has the best performance. Chemical explosive mode analysis is then performed to further analyze the performance of the HRRIs in the three combustion regimes: post-flame, auto-ignition and flame propagation. It is shown that the primary combustion mode in this spray flame is auto-ignition. Regime-dependent correlation has been observed and the above HRRIs yield the best linear correlation for the region with the Damköhler number being far more than unity in this turbulent auto-igniting n-heptane flame.

Simultaneous imaging of soot volume fraction, PAH, and OH in a turbulent n-heptane spray flame

Soot formation and oxidation in a turbulent n-heptane jet spray flame are investigated through simultaneous imaging of quantitative soot volume fraction (fv), flame-front (OH), and soot-precursor marked by smaller (2–4 ring) polycyclic aromatic hydrocarbons (PAH). Laser-induced incandescence (LII) and laser-induced fluorescence (LIF) techniques are used. The spray flame is composed of a dual branch axisymmetric structure. The inner composite branch (B1) consists of fuel-lean and non-premixed reaction zones, whereas combustion in the outer branch (B2) occurs in a non-premixed mode. Frequent local flame extinctions occur along B1, while no extinctions are observed in B2. Consequently, the soot inception occurs near B2. In the onset of soot detection (fv ≥ 0.03 ppm) region, soot layers are thinner than those from downstream regions where soot occupies nearly the same area as of PAH. Soot generally oxidizes across the flame-front as evidenced from the absence of soot-OH overlap. The soot structures are intermittent with peak soot probability of 60%. The most probable peak fv measures 1 ppm, while the conditional mean is 0.42 ppm which is 2.5 times that of time-averaged fv. Conditional mean fv shows monotonically increasing trend with height. The flame lift-off height and local extinctions do not instantaneously influence the soot concentration, possibly due to larger convective and soot formation timescales. Soot-PAH correlations are explored. However, no strong instantaneous correlations are noted, most likely due to different chemical timescales of soot and the imaged smaller-PAH. Nevertheless, a few distinct trends are noted in soot-PAH correlations. fv weakly increases with the PAH-LIF intensity. The PAH intensity from soot onset to peak region decreases, while both soot and PAH are nearly consumed at a local flame-tip (oxidation) region. The soot-PAH overlap decreases consistently from soot onset to oxidation region. The reported database and deduced insights can aid in the development and validation of soot models in two-phase reacting flows.

Numerical Study on the Combustion Process of n-heptane Spray Flame in Methane Environment Using Large Eddy Simulation

ABSTRACT Large eddy simulation is applied for the combustion process of n-heptane spray flame in methane environment. The influence of methane (CH4) concentration on the auto-ignition process and flame structures are discussed in detail. By increasing the CH4 concentration in the ambient gas, auto-ignition is delayed obviously at a low initial temperature because of the delayed first-stage ignition. While at a high temperature, the auto-ignition time is less influenced by the addition of CH4. However, after the addition of CH4, the concentration of the mixture where the high-temperature ignition is initialed becomes much richer and it is independent of the initial temperatures. In addition, the chemical explosive mode analysis (CEMA) method shows that the mixture in high-temperature regions is non-explosive without CH4 addition, and the abrupt transition between the explosive and non-explosive mixture at the tip of the reacting spray is observed by adding CH4 in the ambient gas, indicating that a flame front is developing downstream. This flame front exists for a long time at low CH4 concentration. The eigenvalue of the Jacobian matrix for the CH4/air mixture in the ambient gas is increased significantly with the addition of CH4, indicating that the combustion process is dominated by auto-ignition. Finally, results show that the development of the flame is accelerated by the addition of CH4 in the ambient gas.

Instantaneous planar measurements of nitric oxide concentration in a turbulent n-heptane spray flame

Instantaneous and mean nitric oxide (NO) concentrations are measured in a turbulent, lifted, n-heptane jet spray flame at an atmospheric pressure using planar laser-induced fluorescence (PLIF). The flame-front is simultaneously located using OH-PLIF. The NO excitation line with the least temperature-quenching dependence is selected, and the associated NO-LIF dependence is corrected. LIF optimization and data reduction are based on previous work [Mulla et al. Combust. Flame 203 (2019) 217–229] where the NO-LIF signal was simulated at the spray flame condition (temperature and gas composition from LES) using a spectroscopic model. In the present work, to reduce interferences from polycyclic aromatic hydrocarbons (PAH) and soot, the detection bandwidth is progressively reduced from broadband (F6), intermediate (F3), to narrowband (F1). F6 results in a significant interference, while F1 contains the least interference. Instantaneous NO mole fraction (χNO) is deduced from F1 dataset using a novel instant-wise mapped interference subtraction approach (F1-Inst). The conditional mean NO concentrations derived from F6, F3, F1, and F1-Inst are in agreement (within uncertainty limits); however, F1-Inst appears to be the most reliable. Instantaneous flame-structure (OH) and NO concentration fields are analyzed. The spray flame structure consists of the inner (B1) and outer (B2) flame branches. Local extinctions are observed along B1. Small-scale (<1 mm) extinctions do not alter the NO concentration, whereas large-scale extinctions result in a sharp (50–100%) χNO drop, possibly due to a reduced temperature. Instantaneous χNO along B2 shows a sharp growth until 55 mm height above burner (HAB), beyond which the growth saturates. The deceleration of NO growth and χNO reduction near the sooting region is most likely due to the soot-radiation induced temperature drop. Mean χNO along B1 measures ∼ 25 ppm with a small axial variation (20–30 ppm). Expected lower temperature and nearly constant χNO with residence time suggest the nitrous oxide and prompt NO formations routes. Mean χNO in the outer diffusion branch (B2) increases from 35 ppm at the flame-base to 75 ppm at 60 mm HAB. The NO growth and potentially high temperature in B2 indicate the thermal route of NO formation.

Propagation of Spherically Expanding Turbulent Flames into Fuel Droplet-Mists

The effects of droplet diameter and the overall (liquid+gas) equivalence ratio on flame topology and propagation statistics in spherically expanding turbulent n-heptane spray flames have been analysed based on three-dimensional Direct Numerical Simulations (DNS) data. It has been found that the range of both mean and Gauss curvatures of the flame surface, and the probability of finding saddle topologies increase with increasing droplet diameter and overall equivalence ratio. The presence of droplets affects the displacement speed and consumption speed statistics principally through the reaction rate of the mixture composition in the reaction zone. The magnitudes of the components of density-weighted displacement speed arising from mixture inhomogeneity and droplet evaporation remain small in comparison to the magnitudes of the reaction rate and molecular diffusion rate components. The presence of large droplets decreases the mean density-weighted displacement speed $$ {S}_d^{\ast } $$ and increases the probability of finding negative $$ {S}_d^{\ast } $$ values, except for overall fuel-lean equivalence ratios. The mean consumption speed shows an increasing trend with increasing droplet diameter for fuel-lean overall equivalence ratios, whereas the mean consumption speed decreases with increasing droplet diameter for overall stoichiometric and fuel-rich mixtures. The mean consumption speed remains greater than the mean density-weighted displacement speed for all cases considered here. An alternative flame speed, which represents the growth rate of the flame surface area, has been found to provide an approximate measure of mean consumption flame speed. By contrast, an alternative flame speed, which represents the growth rate of burned gas volume, has been found to approximate the mean density-weighted displacement speed for large droplets in the case of stoichiometric and fuel-rich overall equivalence ratios.

Numerical Modeling for N-Heptane Spray Combustion Processes with Various Ambient Oxygen Concentrations

This work has numerically analyzed the structure of spray flames with various ambient oxygen concentrations. To realistically predict the spray evaporation characteristics in the high-pressure diesel-like environment, the present study has employed the high-pressure effective conductivity vaporization model. The detailed n-heptane/air chemistry is represented by 114 elementary steps and 44 chemical species. The Representative Interactive Flamelet (RIF) model is adopted to consider the interactions between turbulence and chemistry in the fast transient reacting flows. In order to account for the spatial inhomogeneity of the scalar dissipation rate, the multiple RIFs are introduced. Moreover, the present approach realistically models the vaporization effects on the mixture fraction fluctuations. For the various conditions of ambient oxygen concentration, numerical results are compared with experimental data including the ignition delay and the lift-off length. These results indicate that the present MRIF approach yields the reasonably good agreement with experimental data in terms of ignition delay time and lift-off length. To gain the physical insight into the two-stage ignition behavior of diesel fuels, the detailed discussions are also made for the temporal evolution of several key species (CH2O, OH, CO) mass fraction and maximum temperature of the transient flamelet as well as the flame stabilization mechanism of the n-heptane spray flames with the much lower ambient oxygen concentration.

Large Eddy Simulation of Multiple-Stage Ignition Process of n-Heptane Spray Flame

Large eddy simulation of n-heptane spray flames is conducted to investigate the multiple-stage ignition process under extreme (low-temperature, low oxygen, and high-temperature, high-density) conditions. At low oxygen concentrations, the first-stage ignition initiates in the fuel-rich region and then moves to stoichiometric equivalence ratio regions by decreasing the initial temperature. It is also clear that at high temperatures, high oxygen concentrations, or high densities, the reactivity of the mixture is enhanced, where high values of progress variable are observed. Analysis of key intermediate species, including acetylene (C2H2), formaldehyde (CH2O), and hydroxyl (OH) in the mixture fraction and temperature space provides valuable insights into the complex combustion process of the n-heptane spray flames under different initial conditions. The results also suggest that C2H2 appears over a wider range in the mixture fraction space at higher temperature or oxygen concentration condition, implying that it mainly forms at the fuel-rich regions. The initial oxygen concentration of the ambient gas has great influence on the formation and oxidization of C2H2, and the maximum temperature depends on the initial oxygen concentration. OH is mainly formed at the stoichiometric equivalence ratio region, which moves to high-temperature regions very quickly especially at higher oxygen concentrations. Finally, analysis of the premixed and nonpremixed combustion regimes in n-heptane spray flames is also conducted, and both premixed and nonpremixed combustion coexist in spray flames.

Quantitative imaging of nitric oxide concentration in a turbulent n-heptane spray flame

The present work reports quantitative planar imaging of Nitric Oxide (NO) concentration in a dilute spray flame at atmospheric pressure, contributing to CORIA Rouen Spray Burner (CRSB) database. Mean NO mole fraction (χNO) is measured using planar laser-induced fluorescence (PLIF) technique, while the flame front is located simultaneously using OH-PLIF. An optimum NO excitation scheme is selected to minimize temperature-quenching dependence based on LIF simulations. Furthermore, the temperature and collisional quenching effects on NO fluorescence are corrected by using the temperature and gas composition obtained from large eddy simulation performed in previous work [Proc. Combust. Inst., 36 (2017), 2567-2575]. Additionally, interference in NO-PLIF from polycyclic aromatic or unburned hydrocarbon fluorescence is corrected by detuned (NO off-transition) signal subtraction. The spatial distribution of χNO is discussed in the context of a spray flame topology, which exhibits two distinct branches. The results suggest the formation of prompt NO on the inner composite branch (fuel-lean and non-premixed) and thermal NO on the outer pure non-premixed branch. χNO at 60 mm height above burner measures 33 ppm near the inner branch, and 75 ppm around the outer branch.

Effect of injection timing on the ignition process of n-heptane spray flame in a methane/air environment

There is significant interest in using pilot fuel as a source of ignition to enhance the performance of a natural gas engine. The pilot fuel injection timing is a critical factor that influences the combustion phasing and emissions characteristics. In this study, the effect of injection timing on the development of n-heptane spray flame in a methane/air environment is performed by using large eddy simulation (LES) in a constant-volume combustion vessel under engine-like conditions. The results indicate that injection retarding accelerates the ignition of n-heptane spray and reduces the interval between the ignition of n-heptane and methane/air mixture. The contributions of premixed and non-premixed combustion on heat release are distinguished by introducing the flame index (FI). Furthermore, the reason why retarding injection leads to earlier ignition is investigated by sensitivity analysis in a zero-dimension simulation. It is concluded that although methane addition delays the ignition of n-heptane, the pre-ignition oxidative reactions of methane/air mixture occurs prior to the n-heptane injection, thereby increasing the temperature and pressure and producing significantly intermediate radicals and consequently promoting the n-heptane ignition. Additionally, the n-heptane spray flame, in turn, further ignites the methane/air mixture and induces the high-temperature combustion. The results also indicate that the influence of temperature on the ignition is maximum, which is followed by the influence of important intermediate species, formaldehyde (CH2O), while the influence of pressure is minor. The present study provides a theoretical basis for an in-depth understanding of the effect of injection timing on the combustion of dual-fuel engines.

Effects of oxygen concentrations on the ignition and quasi-steady processes of n-heptane spray flames using large eddy simulation

Exhaust Gas Recirculation (EGR) is a frequently used technique to reduce the production of NOx. The effect of EGR on the early flame evolution, two-stage ignition process and spray flame structures for n-heptane spray flames are investigated using large eddy simulation. The two-stage ignition process is identified based on the formation of key species and early heat release process. Results demonstrate that a longer ignition delay (ID) and flame lift-off length (LOL) under lower oxygen concentration conditions could increase the mixing time for fuel and air. However, the first-stage ignition still initiates in fuel-richer regions for the cases with higher EGR rates due to the lack of oxygen. In contrast, compared to the case with the same initial oxygen content but at a higher gas temperature of 1000 K, the first-stage ignition moves to stoichiometric mixture fraction regions at 900 K. The combustion mode analysis based on hydroxyl and formaldehyde is conducted to distinguish between the low- and high-temperature combustion regions. Most importantly, to study the stabilization mechanism, the chemical explosive mode analysis (CEMA) is conducted based on analysis on the local flow time scale and the chemical time scale. During the early stage of ignition, a balance between reaction and mixing implies that cool flame propagates from the ignition spots through the entire flow field. And during the quasi-steady state, autoignition plays a dominant role.

Temperature and reaction zone imaging in turbulent swirling dual-fuel flames

Gaseous and liquid dual-fuel flames present both a practical approach to emissions reduction and a challenge to current state-of-the-art combustion modelling. This paper uses simultaneously imaged temperature and normalised OH signal fields to investigate flame structure and provide experimental data for model validation across a range of swirl-stabilised n-heptane spray flames. These data are obtained by non-linear excitation regime two-line atomic fluorescence (NTLAF) of indium, and planar laser-induced fluorescence (OH-PLIF), respectively. Swirling gas streams are varied by flowrate (63–88% of blow-off), premixed equivalence ratio (including air-only), and by type of gaseous fuel (natural gas and hydrogen). Results are used to describe how hot combustion products interact with the fuel spray: heating and diluting the region above the apex of the spray cone at low air flowrates but drawing fuel into outer branches of the flame with increasing air flowrates. Adding natural gas to the swirling air stream, at a concentration below the lean flammability limit, gives rise to a temperature increase in the outer branches with little effect on the hot region above the apex of the spray, along the burner centreline. The size of this region is significantly reduced; however, using hydrogen. As the concentration of gaseous fuel increases towards the lean flammability limit, peak temperatures shift towards the outer branch of the flame. Exceeding the lean flammability limit, an additional reaction zone begins to form in the premixed swirling stream, adjacent to the outer branch of the swirl flame. Stable outer branches of the swirl flame, however, become less prevalent and the peak temperatures of the spray flame return to burner centreline. This study provides insight into the complex behaviour of dual-fuel flames, a complementary dataset to related, PLIF-only studies and validation data for the development of numerical modelling tools.

Local extinction mechanisms analysis of spray jet flame using high speed diagnostics

This paper reports an experimental study where flame structure, flow topology and local extinction mechanisms of n-heptane spray flames are investigated. The burner consists of an annular non-swirling co-flow of air that surrounds a central hollow-cone spray injector, leading to a lifted spray flame. The experiments include measurements of droplet size and velocity by Phase Doppler Anemometry (PDA), flame structure by High-Speed Planar Laser Induced Fluorescence of OH radical (HS-OH-PLIF) simultaneously recorded with the velocity fields of the reactive flow obtained by High-Speed Particle Image Velocimetry (HS-PIV). The poly-disperse spray distribution yields small droplets along the centerline axis while the majority of the mass is located as large droplets along the spray borders. These large droplets associated with high velocities have ballistic trajectories and strongly interact with the inner wrinkled partially premixed flame front and the outer diffusion flame front. Simultaneous HS-OH-PLIF and HS-PIV images characterize the dynamics of extinction events in the spray jet flame. In the inner reaction zone, local flame extinctions are mainly controlled by the shear layer induced by the co-flow and the fuel–air heterogeneities due to the evaporation of small droplets in the vicinity of the flame front. The large scales of turbulence in the shear layer play a significant role in the dynamics of these extinctions. It is also found that the large inertial droplets penetrate the lower part of the inner front reaching the burned gases, where they evaporate rapidly. They also disturb the outer reaction zone due to the low droplets temperature and the rich mixture in the wake of droplet. These new results on local extinction of spray flames and droplet–flame interactions will also strengthen the CORIA Rouen Spray Burner (CRSB) database for the improvement of evaporation and combustion models for reacting sprays.

Large eddy simulation of the low temperature ignition and combustion processes on spray flame with the linear eddy model

Large eddy simulation coupled with the linear eddy model (LEM) is employed for the simulation of n-heptane spray flames to investigate the low temperature ignition and combustion process in a constant-volume combustion vessel under diesel-engine relevant conditions. Parametric studies are performed to give a comprehensive understanding of the ignition processes. The non-reacting case is firstly carried out to validate the present model by comparing the predicted results with the experimental data from the Engine Combustion Network (ECN). Good agreements are observed in terms of liquid and vapour penetration length, as well as the mixture fraction distributions at different times and different axial locations. For the reacting cases, the flame index was introduced to distinguish between the premixed and non-premixed combustion. A reaction region (RR) parameter is used to investigate the ignition and combustion characteristics, and to distinguish the different combustion stages. Results show that the two-stage combustion process can be identified in spray flames, and different ignition positions in the mixture fraction versus RR space are well described at low and high initial ambient temperatures. At an initial condition of 850 K, the first-stage ignition is initiated at the fuel-lean region, followed by the reactions in fuel-rich regions. Then high-temperature reaction occurs mainly at the places with mixture concentration around stoichiometric mixture fraction. While at an initial temperature of 1000 K, the first-stage ignition occurs at the fuel-rich region first, then it moves towards fuel-richer region. Afterwards, the high-temperature reactions move back to the stoichiometric mixture fraction region. For all of the initial temperatures considered, high-temperature ignition kernels are initiated at the regions richer than stoichiometric mixture fraction. By increasing the initial ambient temperature, the high-temperature ignition kernels move towards richer mixture regions. And after the spray flames gets quasi-steady, most heat is released at the stoichiometric mixture fraction regions. In addition, combustion mode analysis based on key intermediate species illustrates three-mode combustion processes in diesel spray flames.