Flame Structure and Propagation in Turbulent Flame-Droplet Interaction: A Direct Numerical Simulation Analysis

Three-dimensional Direct Numerical Simulations (DNS) in canonical configuration have been employed to study the combustion of mono-disperse droplet-mist under turbulent flow conditions. A parametric study has been performed for a range of values of droplet equivalence ratio ?_d, droplet diameter a_d and root-mean-square value of turbulent velocity u^′. The fuel is supplied entirely in liquid phase such that the evaporation of the droplets gives rise to gaseous fuel which then facilitates flame propagation into the droplet-mist. The combustion process in gaseous phase takes place predominantly in fuel-lean mode even for ?_d>1. The probability of finding fuel-lean mixture increases with increasing initial droplet diameter because of slower evaporation of larger droplets. The chemical reaction is found to take place under both premixed and non-premixed modes of combustion: the premixed mode ocurring mainly under fuel-lean conditions and the non-premixed mode under stoichiometric or fuel-rich conditions. The prevalence of premixed combustion was seen to decrease with increasing droplet size. Furthermore, droplet-fuelled turbulent flames have been found to be thicker than the corresponding turbulent stoichiometric premixed flames and this thickening increases with increasing droplet diameter. The flame thickening in droplet cases has been explained in terms of normal strain rate induced by fluid motion and due to flame normal propagation arising from different components of displacement speed. The statistical behaviours of the effective normal strain rate and flame stretching have been analysed in detail and detailed physical explanations have been provided for the observed behaviour. It has been found that the droplet cases show higher probability of finding positive effective normal strain rate (i.e. combined contribution of fluid motion and flame propagation), and negative values of stretch rate than in the stoichiometric premixed flame under similar flow conditions, which are responsible for higher flame thickness and smaller flame area generation in droplet cases.     

A Comparison of the Blow-Off Behaviour of Swirl-Stabilized Premixed, Non-Premixed and Spray Flames

Confined short turbulent swirling premixed and non-premixed methane and heptane spray flames stabilized on an axisymmetric bluff body in a square enclosure have been examined close to the blow-off limit and during the extinction transient with OH* chemiluminescence and OH-PLIF operated at 5 kHz. The comparison of flames of different canonical types in the same basic aerodynamic field allows insights on the relative blow-off behaviour. The flame structure has been examined for conditions increasingly closer to blow-off. The premixed flame was seen to change from a cylindrical shape at stable burning condtions, with the flame brush closing across the flow at conditions close to blow-off. The PLIF images show that for the gaseous non-premixed flame, holes appear along the flame sheet with increasing frequency as the blow-off condition is approached, while the trend is less obvious for the spray flame. Non-premixed and spray flames showed randomly-occurring lift-off, which is further evidence of localised extinction. The mean lift-off height increased with increasing fuel jet velocity and decreased with increasing air velocity and approaches zero (i.e. the flame is virtually attached) just before the blow-off condition, despite the fact that more holes were evident in the flame sheet as extinction was approached. It was found that the average duration of the blow-off event, when normalised with the characteristic flow time d/U _b (d being the bluff-body diameter and U _b the bulk velocity) was in the range 9–38 with the spray flame extinction lasting a shorter time than the gaseous flames. Finally, it was found that correlations based on a Damköhler number collapse the blow-off velocity data for all flames with reasonable accuracy. The results can help the development of advanced turbulent combustion models.     

Spray-Flame Dynamics in a Rich Droplet Array

We numerically study spray-flame dynamics. The initial state of the spray is schematized by alkane droplets located at the nodes of a face-centered 2D-lattice. The droplets are surrounded by a gaseous mixture of alkane and air. The lattice spacing s reduced by the combustion length scale is large enough to consider that the chemical reaction occurs in a heterogeneous medium. The overall spray equivalence ratio is denoted by ?_T, with ?_T = ?_L + ?_G, where ?_G corresponds to the equivalence ratio of the gaseous surrounding mixture at the initial saturated partial pressure, while ?_L is the so-called liquid loading. To model such a heterogenous combustion, the retained chemical scheme is a global irreversible one-step reaction governed by an Arrhenius law, with a modified heat of reaction depending on the local equivalence ratio. ?_T is chosen in the range 0.9 ≤ ?_T ≤ 2. Three geometries (s = 3, s = 6, s = 12) and four liquid loadings, ?_L = 0.3, ?_L = 0.5, ?_L = 0.7, ?_L = 0.85 are studied. In the rich sprays, our model qualitatively retrieves the recent experimental measurements: the rich spray-flames can propagate faster than the single-phase flames with the same overall equivalence ratio. To analyse the conditions for this enhancement, we introduce the concept of “spray Peclet number”, which compares the droplet vaporization time with the combustion propagation time of the single-phase flame spreading in the fresh surrounding mixture.     

Influence of heat and mass transfer on the ignition and NOx formation in single droplet combustion

The effect of heat and mass transfer on the ignition, and in a second step on the nitrogen oxide (NO _x ) generation, of single burning droplets is examined in a numerical study. Spherical symmetry with no gravity and no forced convection is presumed; ambient temperature is set at 500 K, below the auto-ignition point. The essentials of a forced droplet ignition by an external energy source are introduced. Two methods are applied: heat introduction at a fixed radial position r and heat introduction at a fixed local equivalence ratio ϕ _r . This study’s distinctiveness compared to previous research is its focus on and its combination of partially pre-vaporized droplets and detailed chemistry, both being technically relevant in kerosene and diesel fuel combustion. The fuel of choice is n-decane (C₁₀H₂₂), and NO _x production is studied exemplarily as a representative group of pollutant emissions. The conducted simulations show a decrease of NO _x formation with an increase of the pre-vaporization rate \Uppsi. \Uppsi. This decrease is generally valid for both methods of heat introduction. However, results on flame stabilization and NO _x production reveal a high sensitivity to parameters of the ignition model. The burning behavior during the initial stages is dominated by the ignition position. Extracting heat from the exhaust gas region of burning droplets shows no impact on the flame position nor on the relative NO _x production. As a consequence, a well-founded modeling of the investigated droplet regime needs to resort to an iterative adaptation of the heat introduction parameters based on the findings of droplet burning and exhaust gas production.     

Effects of fuel properties on the burning characteristics of collision-merged alkane/water droplets

The combustion characteristics of freely falling droplets, individually generated by the merging of colliding alkane and water droplets, were experimentally investigated. The outcome of the collision droplets was firstly studied and then the subsequent burning processes such as the flame appearance, ignition and burning behaviors were recorded, through either visual observation or microphotography with the aid of stroboscopic lightening. If the merged droplets were exhibited in an insertive manner, while the water droplet inserted into the alkane droplet, these yield the burning behaviors prior to the end of flame were very much similar to that of pure alkane. The burning was ended with droplet extinction for lower-C alkane, and with either droplet “flash vaporization” or extinction for hexadecane. And if the merged droplets were in adhesive manner, for hexadecane with large water content, they either could not be ignited for the large merged droplets, or be ignited with a much prolonged ignition delay, followed by a soot-reducing flame and an ending of droplet extinction for the small merged droplets. “Homogeneous” explosion was not observed in any of the tests, and “heterogeneous” explosion, induced by trapped air bubbles, occasionally occurred for merged droplets with C-atom in alkane is higher than dodecane. And the sudden disappearance of droplet definitely decreased the burning time and thus enhanced the burning intensity. Besides, the fuel mass consumption rates were increased, even in the cases that having droplet extinction, because of the enlargement of the surface area due to the stuffing of water droplet.     

Effects of Fuel Lewis Number on Localised Forced Ignition of Turbulent Mixing Layers

The influences of fuel Lewis number Le _F on localised forced ignition of inhomogeneous mixtures are analysed using three-dimensional compressible Direct Numerical Simulations (DNS) of turbulent mixing layers for Le _F  = 0.8, 1.0 and 1.2 and a range of different root-mean-square turbulent velocity fluctuation u′ values. For all Le _F cases a tribrachial flame has been observed in case of successful ignition. However, the lean premixed branch tends to merge with the diffusion flame on the stoichiometric mixture fraction isosurface at later stages of the flame evolution. It has been observed that the maximum values of temperature and reaction rate increase with decreasing Le _F during the period of external energy addition. Moreover, Le _F is found to have a significant effect on the behaviours of mean temperature and fuel reaction rate magnitude conditional on mixture fraction values. It is also found that reaction rate and mixture fraction gradient magnitude \(\vert \nabla \xi \vert \) are negatively correlated at the most reactive region for all values of Le _F explored. The probability of finding high values of \(\vert \nabla \xi \vert \) increases with increasing Le _F . For a given value of u′, the extent of burning decreases with increasing Le _F . A moderate increase in u′ gives rise to an increase in the extent of burning for Le _F  = 0.8 and 1.0, which starts to decrease with further increases in u′. For Le _F  = 1.2, the extent of burning decreases monotonically with increasing u′. The extent of edge flame propagation on the stoichiometric mixture fraction ξ = ξ _st isosurface is characterised by the probability of finding burned gas on this isosurface, which decreases with increasing u′ and Le _F . It has been found that it is easier to obtain self-sustained combustion following localised forced ignition in case of inhomogeneous mixtures than that in the case of homogeneous mixtures with the same energy input, energy deposition duration when the ignition centre is placed at the stoichiometric mixture. The difficultly to sustain combustion unaided by external energy addition in homogeneous mixture is particularly prevalent in the case of Le _F  = 1.2.     

Leading-Edge Reaction Zones in Lifted-Jet Gas and Spray Flames

An investigation of the leading edge characteristics in lifted turbulent methane-air (gaseous) and ethanol-air (spray) diffusion flames is presented. Both combustion systems consist of a central nonpremixed fuel jet surrounded by low-speed air co-flow. Non-intrusive laser-based diagnostic techniques have been applied to each system to provide information regarding the behavior of the combustion structures and turbulent flow field in the regions of flame stabilization. Simultaneous sequential CH-PLIF/particle image velocimetry and CH-PLIF/Rayleigh scattering measurements are presented for the lifted gaseous flame. The CH-PLIF data for the lifted gas flame reveals the role that ``leading-edge' combustion plays as the stabilization mechanism in gaseous diffusion flames. This phenomenon, characterized by a fuel-lean premixed flame branch protruding radially outward at the flame base, permits partially premixed flame propagation against the incoming flow field. In contrast, the leading edge of the ethanol spray flame, examined using single-shot OH-PLIF imaging and smoke-based flow visualization, does not exhibit the same variety of leading-edge combustion structure, but instead develops a dual reaction zone structure as the liftoff height increases. This dual structure is a result of the partial evaporation (hence partial premixing) of the polydisperse spray and the enhanced rate of air entrainment with increased liftoff height (due to co-flow). The flame stabilizes in a region of the spray, near the edge, occupied by small fuel droplets and characterized by intense mixing due to the presence of turbulent structures. This revised version was published online in July 2006 with corrections to the Cover Date.     

Large Eddy Simulation of a Polydisperse Ethanol Spray Flame

Ethanol is identified as an interesting alternative fuel. In this regards, the predictive capability of combustion Large Eddy Simulation approach coupled to Lagrangian droplet dynamic model to retrieve the turbulent droplet dispersion, droplet size distribution, spray evolution and combustion properties is investigated in this paper for an ethanol spray flame. Following the Eulerian-Lagrangian approach with a fully two way coupling, the Favre-filtered low Mach number Navier-Stokes equations are solved on structured grids with dynamic sub-grid scale models to describe the turbulent carrier gas phase. Droplets are injected in polydisperse manner and generated in time dependent boundary conditions. They evaporate to form an air-fuel mixture that yields spray flame. Part of the ethanol droplets evaporates within the prevaporization area before reaching the combustion zone, making the flame to burn in a partially premixed regime. The chemistry is described by a tabulated detailed chemistry based on the flamelet generated manifold approach. The fuel, ethanol, is modeled by a detailed reaction mechanism consisting of 56 species and 351 reversible reactions. The simulation results including excess gas temperature, droplet velocities and corresponding fluctuations, droplet mean diameters and spray volume flux at different distances from the exit plane show good agreement with experimental data. Analysis of combustion spray features allows gaining a deep insight into the two-phase flow process ongoing.     

Effects of Spray and Turbulence Modelling on the Mixing and Combustion Characteristics of an n-heptane Spray Flame Simulated with Dynamic Adaptive Chemistry

Accurate modelling of spray combustion process is essential for efficiency improvement and emissions reduction in practical combustion engines. In this work, both unsteady Reynolds-averaged Navier-Stokes (URANS) simulations and large eddy simulations (LES) are performed to investigate the effects of spray and turbulence modelling on the mixing and combustion characteristics of an n-heptane spray flame in a constant volume chamber at realistic conditions. The non-reacting spray process is first simulated with URANS to investigate the effects of entrainment gas-jet model on the penetration characteristics and fuel vapor distributions. It is found that the droplet motion near the nozzle has significant influence on the fuel vapor distribution, while the liquid penetration length is controlled by the evaporation process and insensitive to gas-jet model. For the case considered, both URANS with the gas-jet model and large eddy simulations can properly predict the vapor penetration. For the combustion characteristics, it is found that LES yields better predictions in the global combustion characteristics. The URANS with gas jet model yields a comparable flame length and lift-off-length (LOL) to LES, but results in a larger ignition delay time compared to the experimental data. Another focus of this work is to qualify the convergence characteristics of the dynamic adaptive chemistry (DAC) method in these transient combustion simulations, where DAC is applied to reduce the mechanism locally and on-the-fly to accelerate chemistry calculations. The instantaneous flame structures and global combustion characteristics such as ignition delay time, flame lift-off length and emissions are compared between simulations with and without DAC. For URANS, good agreements are observed both on instantaneous flame structures and global characteristics. For LES, it is shown that the errors incurred by DAC are small for scatter distributions in composition space and global combustion characteristics, while they may significantly affect instantaneous flame structures in physical space. The study reveals that for DAC application in transient simulations, global or statistic information should be used to assess the accuracy, such as manifolds in composition space, conditional quantities and global combustion characteristics. For the cases investigated, a speed-up factor of more than two is achieved by DAC with a 92-species skeletal mechanism with less than 0.2 % and 3.0 % discrepancy in ignition delay and LOL, respectively.     

Large-Eddy Simulation of Interactions Between a Reacting Jet and Evaporating Droplets

Large-eddy simulation of a turbulent reactive jet with and without evaporating droplets is performed to investigate the interactions among turbulence, combustion, heat transfer and evaporation. A hybrid Eulerian–Lagrangian approach is used for the gas–liquid flow system. Arrhenius-type finite-rate chemistry is employed for the chemical reaction. To capture the highly local interactions, dynamic procedures are used for all the subgrid-scale models, except that the filtered reaction rate is modelled by a scale similarity model. Various representative cases with different initial droplet sizes (St ₀) and mass loading ratios (MLR) have been simulated, along with a case without droplets. It is found that compared with the bigger, slow responding droplets (St ₀ = 16), smaller droplets (St ₀ = 1) are more efficient in suppressing combustion due to their preferential concentration in the reaction zones. The peak temperature and intensity of temperature fluctuations are found to be reduced in all the droplet cases, to a varying extent depending on the droplet properties. Detailed analysis on the contributions of respective terms in a transport equation for grid-scale kinetic energy (GSKE) shows that the droplet evaporation effect on GSKE is small, while the droplet momentum effect depends on St ₀. When the MLR is sufficiently high, the bigger (St ₀ = 16) droplets can have profound influence on GSKE, and consequently on the formation and evolution of large-scale flow structures. On the other hand, the turbulence level is found to be lower in the droplet cases than in the pure flame case, due to the dissipative droplet dynamic effect.     

Microgravity experiments and numerical studies on ethanol/air spray flames

Spray flames are known to exhibit amazing features in comparison with single-phase flames. The weightless situation offers the conditions in which the spray characteristics can be well controlled before and during combustion. The article reports on a joint experimental/numerical work that concerns ethanol/air spray flames observed in a spherical chamber using the condensation technique of expansion cooling (based on the Wilson cloud chamber principle), under microgravity.We describe the experimental set-up and give details on the creation of a homogeneous and nearly monosized aerosol. Different optical diagnostics are employed successfully to measure the relevant parameters of two-phase combustion. A classical shadowgraphy system is used to track the flame speed propagation and allow us to observe the flame front instability. The complete characterization of the aerosol is performed with a laser diffraction particle size analyser by measuring the droplet diameter and the droplet density number, just before ignition. A laser tomography device allows us to measure the temporal evolution of the droplet displacement during flame propagation, as well as to identify the presence of droplets in the burnt gases. The numerical modelling is briefly recalled. In particular, spray-flame propagation is schematized by the combustion spread in a 2-D lattice of fuel droplets surrounded by an initial gaseous mixture of fuel vapour and air.In its spherical expansion, the spray flame presents a corrugated front pattern, while the equivalent single-phase flame does not. From a numerical point of view, the same phenomena of wrinkles are also observed in the simulations. The front pattern pointed out by the numerical approach is identified as of Darrieus–Landau (DL) type. The droplets are found to trigger the instability. Then, we quantitatively compare experimental data with numerical predictions on spray-flame speed. The experimental results show that the spray-flame speed is of the same order of magnitude as that of the single-phase premixed flame. On the other hand, the numerical results exhibit the role played by the droplet radius in spray-flame propagation, and retrieve the experiments only when the droplets are small enough and when the Darrieus–Landau instability is triggered. A final discussion is developed to interpret the various patterns experimentally observed for the spray-flame front.     

Drag coefficients of spherical liquid droplets Part 2: Turbulent gaseous fields

The drag of non-evaporating, spherical, liquid droplets was measured in turbulent flow fields at parametric ranges relevant to spray combustion, characterized by the droplet Reynolds number, and the intensity and spatial scales of turbulence. The experimental apparatus comprised a wind-tunnel and a piezo-electric droplet generator. The procedure was to inject water droplets of uniform size co-currently and continuously with vertical turbulent air flows while droplet velocity was measured at different elevations using laser-Doppler velocimetry. Turbulence was characterized using hot-wire anemometry prior to droplet injection. Drag coefficients were calculated using these main measurements and the law of conservation of mechanical energy. Reynolds numbers were investigated in the range 10–100, in terms of the equivalent spherical diameter of a droplet, and the mean relative speed between the ambient gaseous field and the droplets. Weber numbers were much less than unity so droplets were effectively spherical. Relative intensities of turbulence were investigated in the range 20–65 percent, in terms of the mean relative speed. Spatial scales of turbulence were large in comparison to the droplets; the ratio between the spatial integral scale and the droplet diameter was in the range 11–38, and the Kolmogorov scale was comparable in size or smaller than the droplet diameter. Experimental data showed that the drag in turbulent fields under these conditions is not significantly different than that of solid spheres in a quiescent field at the same Reynolds number.The financial support of the Natural Sciences and Engineering Research Council of Canada and the Manufacturing Research Corporation of Ontario is gratefully acknowledged.     

Effects of Fuel Lewis Number on Localised Forced Ignition of Globally Stoichiometric Stratified Mixtures: a Numerical Investigation

The influences of fuel Lewis number Le_F on localised forced ignition of globally stoichiometric stratified mixtures have been analysed using three-dimensional compressible Direct Numerical Simulations (DNS) for cases with Le_F ranging from 0.8 to 1.2. The globally stoichiometric stratified mixtures with different values of root-mean-square (rms) equivalence ratio fluctuation (i.e. ?^′= 0.2, 0.4 and 0.6) and the Taylor micro-scale l_? of equivalence ratio ? variation (i.e. l_?/l_f= 2.1, 5.5 and 8.3 with l_f being the Zel’dovich flame thickness of the stoichiometric laminar premixed flame) have been considered for different initial rms values of turbulent velocity u^′. A pseudo-spectral method is used to initialise the equivalence ratio variation following a presumed bi-modal distribution for prescribed values of ?^′ and l_?/l_f for global mean equivalence ratio 〈?〉=1.0. The localised ignition is accounted for by a source term in the energy transport equation that deposits energy for a stipulated time interval. It has been observed that the maximum values of temperature and the fuel reaction rate magnitude increase with decreasing Le_F during the period of external energy deposition. The initial values of Le_F, u^′/S_b(?=1), ?^′ and l_?/l_f have been found to have significant effects on the extent of burning of the stratified mixtures following localised ignition. For a given value of u^′/S_b(?=1), the extent of burning decreases with increasing Le_F. An increase in u^′ leads to a monotonic reduction in the burned gas mass for all values of Le_F in all stratified mixture cases but an opposite trend is observed for the Le_F=0.8 homogeneous mixture. It has been found that an increase in ?^′ has adverse effects on the burned gas mass, whereas the effects of l_?/l_f on the extent of burning are non-monotonic and dependent on ?^′ and Le_F. Detailed physical explanations have been provided for the observed Le_F, u^′/S_b(?=1), ?^′ and l_?/l_f dependences.     

Transient combustion of a heating droplet in a gravitational environment

The transient combustion characteristics of a droplet suddenly exposed to the envelope flames in an atmospheric environment are studied numerically. Combustion can be divided into a droplet heating-up and a constant-droplet-temperature burning. The naturally-convective flow is not knowna priori, but provided as part of the solution. During the heating-up stage, the temperature and evaporation rate of droplet increase sharply, and the square of diameter decreases slightly as time proceeds. In the following stage, the droplet temperature remains constant, the evaporation rate and droplet diameter decrease with time. The flowfield of natural convection is also presented to demonstrate its interaction with the flame and the transient process. Finally, the fuel accumulation phenomenon is identified and it results in an reduction of evaporation constant.     

Statistical Analysis of Scalar Dissipation Rate Transport in Turbulent Partially Premixed Flames: A Direct Numerical Simulation Study

Statistically planar turbulent premixed and partially premixed flames for different initial turbulence intensities are simulated for global equivalence ratios ??>?=?0.7 and ??>?=?1.0 using three-dimensional Direct Numerical Simulations (DNS) with simplified chemistry. For the simulations of partially premixed flames, a random distribution of equivalence ratio following a bimodal distribution of equivalence ratio is introduced in the unburned reactants ahead of the flame. The simulation parameters in all of the cases were chosen such that the combustion situation belongs to the thin reaction zones regime. The DNS data has been used to analyse the behaviour of the dissipation rate transports of both active and passive scalars (i.e. the fuel mass fraction Y _F and the mixture fraction ξ) in the context of Reynolds Averaged Navier–Stokes (RANS) simulations. The behaviours of the unclosed terms of the Favre averaged scalar dissipation rates of fuel mass fraction and mixture fraction (i.e. \(\widetilde {\varepsilon }_Y =\overline {\rho D\nabla Y_F^{\prime \prime } \cdot \nabla Y_F^{\prime \prime } } /\overline{\rho }\) and \(\widetilde {\varepsilon }_\xi =\overline {\rho D\nabla \xi ^{\prime \prime }\cdot \nabla \xi ^{\prime \prime }} /\overline {\rho })\) transport equations have been analysed in detail. In the case of the \(\widetilde {\varepsilon }_Y \) transport, it has been observed that the turbulent transport term of scalar dissipation rate remains small throughout the flame brush whereas the terms due to density variation, scalar–turbulence interaction, reaction rate and molecular dissipation remain the leading order contributors. The term arising due to density variation remains positive throughout the flame brush and the combined contribution of the reaction and molecular dissipation to the \(\widetilde {\varepsilon }_Y \) transport remains negative throughout the flame brush in all cases. However, the behaviour of scalar–turbulence interaction term of the \(\widetilde {\varepsilon }_Y \) transport equation is significantly affected by the relative strengths of turbulent straining and the straining due to chemical heat release. In the case of the \(\widetilde {\varepsilon }_\xi \) transport, the turbulent transport term remains small throughout the flame brush and the density variation term is found to be negligible in all cases, whilst the reaction rate term is exactly zero. The scalar–turbulence interaction term and molecular dissipation term remain the leading order contributors to the \(\widetilde {\varepsilon }_\xi \) transport throughout the flame brush in all cases that have been analysed in the present study. Performances of existing models for the unclosed terms of the transport equations of \(\widetilde {\varepsilon }_Y \) and \(\widetilde {\varepsilon }_\xi \) are assessed with respect to the corresponding quantities obtained from DNS data. Based on this exercise either suitable models have been identified or new models have been proposed for the accurate closure of the unclosed terms of both \(\widetilde {\varepsilon }_Y \) and \(\widetilde {\varepsilon }_\xi \) transport equations in the context of Reynolds Averaged Navier–Stokes (RANS) simulations.     

A RANS simulation toward the effect of turbulence and cavitation on spray propagation and combustion characteristics

A multidimensional computational fluid dynamic code was developed and integrated with probability density function combustion model to give the detailed account of multiphase fluid flow. The vapor phase within injector domain is treated with Reynolds-averaged Navier–Stokes technique. A new parameter is proposed which is an index of plane-cut spray propagation and takes into account two parameters of spray penetration length and cone angle at the same time. It was found that spray propagation factor (SPI) tends to increase at lower r/d ratios, although the spray penetration tends to decrease. The results of SPI obtained by empirical correlation of Hay and Jones were compared with the simulation computation as a function of respective r/d ratio. Based on the results of this study, the spray distribution on plane area has proportional correlation with heat release amount, NO _x emission mass fraction, and soot concentration reduction. Higher cavitation is attributed to the sharp edge of nozzle entrance, yielding better liquid jet disintegration and smaller spray droplet that reduces soot mass fraction of late combustion process. In order to have better insight of cavitation phenomenon, turbulence magnitude in nozzle and combustion chamber was acquired and depicted along with spray velocity.     

Pre-Chamber Ignition Mechanism: Simulations of Transient Autoignition in a Mixing Layer Between Reactants and Partially-Burnt Products

The structure of autoignition in a mixing layer between fully-burnt or partially-burnt combustion products from a methane-air flame at ? = 0.85 and a methane-air mixture of a leaner equivalence ratio has been studied with transient diffusion flamelet calculations. This configuration is relevant to scavenged pre-chamber natural-gas engines, where the turbulent jet ejected from the pre-chamber may be quenched or may be composed of fully-burnt products. The degree of reaction in the jet fluid is described by a progress variable c (c = taking values 0.5, 0.8, and 1.0) and the mixing by a mixture fraction ξ (ξ = 1 in the jet fluid and 0 in the CH₄-air mixture to be ignited). At high scalar dissipation rates, N₀, ignition does not occur and a chemically-frozen steady-state condition emerges at long times. At scalar dissipation rates below a critical value, ignition occurs at a time that increases with N₀. The flame reaches the ξ = 0 boundary at a finite time that decreases with N₀. The results help identify overall timescales of the jet-ignition problem and suggest a methodology by which estimates of ignition times in real engines may be made.     

Direct Numerical Simulations of Localised Forced Ignition in Turbulent Mixing Layers: The Effects of Mixture Fraction and Its Gradient

The effects of mixture fraction value ξ and the magnitude of its gradient |∇ξ| at the ignitor location on the localised forced ignition of turbulent mixing layers under decaying turbulence is studied based on three-dimensional compressible Direct Numerical Simulations (DNS) with simplified chemistry. The localised ignition is accounted for by a spatial Gaussian power distribution in the energy transport equation, which deposits energy over a prescribed period of time. In successful ignitions, it is observed that the flame shows a tribrachial structure. The reaction rate is found to be greater in the fuel rich side than in stoichiometric and fuel-lean mixtures. Placing the ignitor at a fuel-lean region may initiate ignition, but extinction may eventually occur if the diffusion of heat from the hot gas kernel overcomes the heat release due to combustion. It is demonstrated that ignition in the fuel lean region may fail for an energy input for which self-sustained combustion has been achieved in the cases of igniting at stoichiometric and fuel-rich locations. It is also found that the fuel reaction rate magnitude is negatively correlated with density-weighted scalar dissipation rate in the most reactive region. An increase in the initial mixture fraction gradient at the ignition centre for the ignitor placed at stoichiometric mixture decreases the spreading of the burned region along the stoichiometric mixture fraction isosurface. By contrast, the mass of the burned region increases with an increase in the initial mixture fraction gradient at the ignition location, as for a given ignition kernel size the thinner mixing layer includes more fuel-rich mixture, which eventually makes the overall burning rate greater than that compared to a thicker mixing layer where relatively a smaller amount of fuel-rich mixture is engulfed within the hot gas kernel. Submitted as a full-length article to Flow Turbulence and Combustion.     

Droplet combustion experiments in varying forced convection using microgravity environment

A new microscopic model of the interaction between droplet flames and fine vortex tubes which compose a coherent structure of turbulence was developed. Three non-dimensional numbers were introduced to extend the length scale and time scale so as to be suitable for microgravity experiments using droplets of combustion of about 1 mm in diameter. An experimental apparatus for combustion of a single droplet and that of an array of two droplets in varying airflow was developed, and experiments were performed in microgravity and normal gravity at pressures up to 2.0 MPa for n-nonane and ethanol as fuels. Variations of the instantaneous burning rate constant, K_i, in response to the varying flow velocity was successfully observed. At high pressure, the effects of droplet Reynolds number Re on K_i was clearly seen, while the effects of natural convection, which increases K_i with Re, was seen in normal gravity even in the forced airflows. As for the experiments on combustion of an array of two droplets, K_i reduction of the downstream droplet became weak when the flow direction was varied. However, the K_i reduction of the downstream droplet for flow direction variations was clearly seen for n-nonane droplets but almost not for ethanol droplets. The interaction mechanism between upstream and downstream droplets is considered to result from the elimination of oxidizer supply to the downstream droplet, indicating strong interaction effects of n-nonane droplets for a stoichiometric oxygen-fuel ratio of n-nonane (i.e., 14.0) greater than that of ethanol (i.e., 3.0).