Spray Jet Flame Research Articles

Large Eddy Simulation of a spray jet flame using Doubly Conditional Moment Closure

A spray jet flame is modelled using Large Eddy Simulation (LES) with Doubly Conditional Moment Closure (DCMC). Since turbulent spray flames may include multiple combustion modes, the DCMC model uses both mixture fraction and reaction progress variable as conditioning variables. Conditional spray terms were included in the DCMC model to consider the coupling between evaporation and the flame structure. In the case of spatial homogeneity and in the limit of negligible mixture fraction scalar dissipation rate (SDR), the DCMC equation is shown to reproduce the flame structure of freely propagating laminar flames. For the spray jet flame, a good agreement between the simulation results and the experiments is achieved, in terms of the spray statistics, as well as the instantaneous and mean flame shape. The simulation shows important differences in the flame structure between the turbulent inner and the quasi-laminar outer flame branch. The doubly-conditional parametrisation appears to be advantageous for resolving small scale effects related to droplet evaporation. Analysis of the DCMC equation suggests that the behaviour of the flame at its anchoring point is strongly influenced by non-premixed burning modes. The solution appears to be weakly affected by terms of convective transport in the DCMC equation, but significant spatial variations and temporal fluctuations of the conditional reaction rate, around 10% of the time-based mean, persist.

The evolution of soot morphology and nanostructure along axial direction in diesel spray jet flames

This study aims at providing new insights into the soot formation and oxidation processes in diesel spray jet flames under the diesel-like conditions. Soot particles are sampled within a reacting diesel jet spray flame in a constant-volume combustion chamber under diesel-like high-temperature and pressure ambient conditions. The experiments are conducted with an injector hole of 0.28 mm and injection mass of 30 mg per shot. Soot particles at different locations along the axis of the diesel jet flame are sampled by a thermophoretic probe, and the variation of soot morphology and nanostructure of the sampled soot particles along the axial location in the flame is analyzed by using the transmission electron microscopy (TEM). At upstream of the jet flame, large “liquid-like” particles are observed in the TEM images. As these particles are delivered to the downstream, (1) there are many immature primary particles formed on the surface of large “liquid-like” particles, and (2) they are carbonized into the larger soot aggregates with the matured structure, and then at last, (3) the larger soot aggregates gradually break up into the smaller particles with relatively compact structure. Both the diameter of primary particles and radius of gyration of the soot aggregates decrease as the distance between the injector nozzle tip and the center of TEM grid is increased, while the opposite trend is found in the behavior of the fractal dimensions of soot aggregates. The high resolution TEM images show that the soot primary particles exhibit the turbostratic state at the upstream of the jet flame, while more matured structures with the graphitic crystallite in outer shell and several fine particles in the inner core are observed as the sampling distance from orifice is increased. The average fringe length increases while the average tortuosity and separation decrease with the distance from the nozzle. These results suggest that the mechanism of mature soot aggregates generated directly from the large “liquid-like” particles should be one of the ways of soot particle formation along the axis of diesel jet flames.

A DNS evaluation of mixing and evaporation models for TPDF modelling of nonpremixed spray flames

In the present work, nonpremixed temporally evolving planar spray jet flames are simulated using both direct numerical simulation (DNS) and the composition transported probability density function (TPDF) method. The objective is to assess the performance of various mixing and evaporation source term distribution models which are required to close the PDF transport equation in spray flames. Quantities which would normally be provided to the TPDF solver by spray models and turbulence models are provided from the DNS: the mean flow velocity, turbulent diffusivity, mixing frequency, and cell-mean evaporation source term. Two cases with different Damköhler numbers (Da) are considered. The low Da case (Da-) features extinction followed by reignition while extinction in the high Da case (Da+) is insignificant. The TPDF modelling considers two mixing models: interaction by exchange with the mean (IEM) and Euclidean minimum spanning trees (EMST). Three models for distribution of the evaporation source terms are considered: EQUAL which distributes them in proportion to notional particles’ mass weight, NEW which creates new particles of pure fuel, and SAT which distributes the sources preferentially to notional particles close to saturation. It is found that the IEM model overpredicts the extinction when used with any evaporation model for both Da- and Da+ cases. The EMST model captures well the trend for extinction and reignition for the Da- case when it is coupled with the EQUAL evaporation model, but it overpredicts the extinction when coupled with the NEW or SAT evaporation model. For the Da+ case, all evaporation models reasonably capture the flame dynamics when coupled with EMST. The flame temperature in the mixture fraction space was examined to further assess the model performance. In general the EMST model results in narrow PDFs with little conditional fluctuation, while the IEM model produces bimodal PDFs with burning and partial extinction branches.

Comparison of droplet evaporation models for a turbulent, non-swirling jet flame with a polydisperse droplet distribution

The current investigation aims to present the numerical results of a turbulent spray jet flame in the framework of Large Eddy Simulation. The injection of n-heptane liquid fuel is achieved through the use of a pressure-swirl atomiser generating a lifted spray flame. The evolution of the sub-grid probability density function of relevant scalars is accounted for by the Eulerian stochastic field method. A non-reactive spray computation is conducted in a preliminary stage. The simulation allows for a stochastic breakup formulation in combination with a stochastic dispersion model to confirm their predictive capabilities. This is achieved as the liquid properties such as droplet diameter and velocity profiles are found to be in excellent agreement between the simulated results and measurements. The main target of the work is to assess the performance of the most widely accepted evaporation models in a turbulent droplet-laden flame. In this paper, a detailed comparison of the evaporation models is conducted thanks to a recent development in measurement techniques which allow to permit the spray temperature profiles. The time-averaged droplet temperature across the spray flame is therefore investigated in order to validate several droplet vaporisation models. All the models under consideration are found to capture the formation of a double reaction zone flame and the measured lift-off height is reproduced within a satisfactory level of accuracy. This good reproduction of the flame morphology additionally confirms the performance of the pdf approach as a closure for the unknown turbulence–chemistry interaction in this type of spray flames. However, the simulated wet-bulb temperature in the hot burnt gas between the two reaction zones in addition to the profile along the spray centreline show a large discrepancy when compared to measurements. This large difference thus requires further investigation both in the modelling of evaporation and in the accuracy of measurement techniques.

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.

Multi-environment PDF modeling for n-dodecane spray combustion processes using tabulated chemistry

The multi-environment probability density function approach together with a tabulated chemistry model has been developed to numerically simulate spray combustion processes under Diesel-like combustion conditions. In this study, the joint composition PDF is approximated using the multi-environment PDF, consisting of a combination of weights and weighted scalars in both chemical composition space and physical space. To account for the auto-ignition process and subsequent flame propagation, a flamelet generated manifold is generated by utilizing steady and unsteady flamelet solutions. The spray dynamics are represented by the state-of-art physical models in context with the Eulerian–Lagrangian approach. The RANS-based computations are made for the n-dodecane non-reacting and reacting spray jets. In terms of spray and vapor penetration, the mean and rms mixture fraction, ignition delay, and lift-off height, the numerical results are in reasonably good agreement with the available experimental data of the Engine Combustion Network. Based on the numerical results, detailed discussions are made for the effects of the ambient conditions on the structures and characteristics of high-pressure Diesel-fueled spray jet flames.

Large Eddy Simulation of CO2 diluted oxy-fuel spray flames

We report results of a computational study of oxy-fuel spray jet flames. An experimental database on flames of ethanol burning in a coflow of a O2–CO2 mixture, created at CORIA (Rouen, France), is used for model validation (Cléon et al., 2015). Depending on the coflow composition and velocity the flames in these experiments start at nozzle (type A), just above the tip of the liquid sheet (type B) or are lifted (type C) and the challenge is to predict their structure and the transitions between them. The two-phase flow field is solved with an Eulerian–Lagrangian approach, with gas phase turbulence solved by Large Eddy Simulation (LES). The turbulence-chemistry interaction is accounted for using the Flamelet Generated Manifolds (FGM) method. The primary breakup process of the liquid fuel is neglected in the current study; instead droplets are directly injected at the location of the atomizer exit at the boundary of the simulation domain. It is found that for the type C flame, which is stabilized far downstream the dense region, some major features are successfully captured, e.g. the gas phase velocity field and flame structure. The flame lift-off height of type B flame is over-predicted. The type A flame, where the flame stabilizes inside the liquid sheet, cannot be described well by the current simulation model. A detailed analysis of the droplet properties along Lagrangian tracks has been carried out in order to explain the predicted flame structure and discuss the agreement with experiment. This analysis shows that differences in predicted flame structure are well-explained by the combined effects of droplet heating, dispersion and evaporation as function of droplet size. It is concluded that a possible reason for the difficulty to predict the type A and B flames is that strong atomization-combustion interaction exists in these flames, modifying the droplet formation process. This suggests that atomization-combustion interaction should be taken into account in future study of these flame types.

Lagrangian conditional moment closure model with flame group interaction for lifted turbulent spray jet flames

A new Lagrangian conditional moment closure (CMC) model is developed for multiple Lagrangian groups of sequentially evaporating fuel in turbulent spray combustion. Flame group interaction is taken into account as premixed combustion by the eddy breakup (EBU) model in terms of the probability of finding flame groups in the burned and the unburned state. Evaporation source terms are included in the two phase conditional transport equations, although they turn out to have negligible influence on the mean temperature field during combustion. The Lagrangian CMC model is implemented in OpenFOAM [1] and validated for test cases in the Engine Combustion Network (ECN) [2,3]. Similar ignition delays and lift-off lengths are predicted by the incompletely stirred reactor (ISR) and the Eulerian CMC models due to relatively uniform conditional flame structure in the domain. The improved Lagrangian CMC model shows no abrupt reaction or oscillatory behaviour with an appropriate model constant K and gives results in better agreement with measurements lying between the predictions by ISR and Lagrangian CMC without flame group interaction.

Experimental study of local flame structures and fuel droplet properties of a spray jet flame

An n-heptane spray jet flame is characterised through quantitative measurements using laser-based techniques. The experimental set-up is composed of an annular non-swirled air co-flow that surrounds a central hollow-cone spray injector, leading to a stable flame with well- defined boundary conditions. Phase Doppler anemometry (PDA) measurements contribute to this investigation through the analysis of air and droplet aerodynamics and OH-PLIF images describe the two dimensional flame structure. The polydisperse spray distribution yields small droplets along the centreline axis while the majority of the mass is situated as big droplets along the spray borders. The flame structure presents a classical shape, with an inner wrinkled partially premixed flame front and an outer diffusion flame front. In addition Global Rainbow Refractometry Technique (GRT) has been used to measure droplet temperature in the different regions of the spray jet flame. This technique was first applied with a continuous laser (C-GRT) to get temporally averaged values of fuel droplet temperature according to the methodology developed initially by Letty et al. [1]. This technique has been extended to measure instantaneous and local fuel droplet temperature by using a pulsed laser. Therefore, conditional averaged measurements of fuel droplet temperature according to the distance of flame front are reported for the first time by coupling instantaneous GRT (I-GRT) with OH-PLIF. New insights on the thermal droplet behaviour in a spray flame are discussed.

Modeling of spray jet flame under MILD condition with non-adiabatic FGM and a new conditional droplet injection model

This paper reports a numerical study on the Delft Spray-in-Hot-Coflow (DSHC) flame with a new OpenFOAM solver developed by the authors, in which the Flamelet Generated Manifolds (FGM) model has been implemented, and used to account for the Turbulence-Chemistry Interaction (TCI). The enthalpy loss effect due to droplet vaporization is considered by employing an additional controlling parameter in the FGM library. Different configurations for generating three dimensional non-adiabatic FGM library are tested. Analysis of the DSHC experimental data suggested that flash boiling influences the atomization of liquid fuel in the DSHC burner. This introduces new challenges for specifying precise spray boundary conditions. A conditional injection model is proposed to provide precise spray information at the injector exit plane (Z=0mm). In this conditional injection model, the droplets have an asymmetric distribution around the spray half angle, in agreement with experimental observations. Also, the possible range of droplet injection angle is conditioned upon the droplet size (mass). Droplet initial velocity magnitude is scaled such that it peaks at the spray half angle direction and reaches a minimum at the edge of the spray cloud. The results obtained show that this model significantly improves the prediction of all the properties examined compared to standard injection model. Comparison between Unsteady Reynolds Averaged Navier Stokes (URANS) and Large Eddy Simulation (LES) is made, and it is found that the LES predicted similar gas phase velocity and better temperature profiles compared to experimental data than URANS, mainly due to the better performance of the dynamic model for mixture fraction variance used in LES. The improvement of the temperature prediction by the non-adiabatic FGM was clearly demonstrated by comparing the results obtained with an adiabatic FGM table. Strong droplet–flame interaction was shown to exist in the DSHC flame. Large droplets have ballistic trajectories at the early stage of their lifetime, and can penetrate the flame and survive until far downstream outside the main reaction region. Movement of these large droplets strongly deform the shape of the flame. Rapid evaporation of droplets in the reaction region may also cause local quenching. Small droplets are quickly convected towards the central region and vanish soon, forming a fuel rich and high temperature condition in the center. These phenomena have been captured with the LES/non-adiabatic FGM modeling approach developed in this paper.

Effects of ambient pressure and precursors on soot formation in spray flames

The effects of ambient pressure on soot formation in spray jet flames are investigated by means of a two-dimensional direct numerical simulation (DNS). In addition, the effects of precursors on soot formation are also discussed. The inception models considering acetylene and polycyclic aromatic hydrocarbon (PAH) precursors are employed and compared in this study. The extended flamelet/progress-variable approach (EFPV) in which heat transfer between a droplet and the surrounding fluid can be considered is employed as a combustion model. Jet A is used as a liquid fuel with considering the detailed chemistry including 274 chemical species and 1537 elemental reactions by the flamelet library. The evaporating droplets’ motions are tracked by the Lagrangian method and the non-equilibrium Langmuir–Knudsen model is used as an evaporation model. Results show that the spray jet flame structure and the soot formation characteristics are considerably affected by the ambient pressure condition. The soot volume fraction increases with increasing the ambient pressure. It is also revealed that the dominant process to promote the soot formation is different between the acetylene and PAH precursor models. It is essential to model a soot inception process with appropriate precursors in order to understand a detail mechanism of soot formation, since the precursor which plays the important role depends on the type of fuel and the combustion conditions.

Effects of Operating Conditions of a Spray Flame on Characteristics of Carbon Fine Particle Formation

A two-dimensional direct numerical simulation is applied to spray jet flames, and effects of background pressure and droplet size on soot formation behavior is investigated in detail in the mono-dispersed droplet size conditions. N-decane (C10H22) is used as a liquid fuel, and droplet motion is calculated by the Lagrangian method. The extended flamelet/progress-variable approach is emplyed for the turbulent combustion chemistry. A kinetically based soot formation model with the flamelet model is employed to predict soot formation. The results show that the soot is mainly formed in the large vortex structure in the downstream region and the soot formation behavior is significantly affected by the pressure. It is found that the soot volume fraction and soot particle diameter increases with increasing the pressure due to that the concentration of soot precursor increases and the soot particle's residence time is enlongated in the structure.

Two-dimensional direct numerical simulation of spray flames - Part 1: Effects of equivalence ratio, fuel droplet size and radiation, and validity of flamelet model

The effects of equivalence ratio, fuel droplet size, and radiation on jet spray flame are investigated by means of two-dimensional direct numerical simulation (DNS). In addition, the validity of an extended flamelet/progress-variable approach (EFPV), in which heat transfer between droplets and ambient fluid including radiation is exactly taken into account, is examined. n-decane (C10H22) is used as liquid spray fuel, and the evaporating droplets’ motions are tracked by the Lagrangian method. The radiative heat transfer is calculated using the discrete ordinate method with S8 quadrature approximation. The results show that the behavior of jet spray flame is strongly affected by equivalence ratio and fuel droplet size. The general behavior of the jet spray flames including the heat transfer between droplets and ambient fluid with radiation effect can be captured by EFPV.

Two-dimensional direct numerical simulation of spray flames – Part 2: Effects of ambient pressure and lift, and validity of flamelet model

The effect of ambient pressure on spray flames is investigated by means of two-dimensional direct numerical simulation (DNS), and the validity of an extended flamelet/progress-variable approach (EFPV) is examined under the high-pressure condition. The DNS is performed not only for a simple jet spray flame with a pilot burner but also for a lifted recirculation spray flame without any pilot burner at ambient pressures of 0.1 and 0.5MPa. n-decane (C10H22) is used as liquid spray fuel, and the evaporating droplets’ motions are tracked by the Lagrangian method. The results show that the behaviors of jet and lifted recirculation spray flames are strongly affected by ambient pressure. The effects of the change of the ambient pressure on these spray flame behaviors can be well captured by EFPV and EFPV coupled with G-equation model (EFPV-G), respectively.

LES-CMC of a dilute acetone spray flame

A combined Large-Eddy-Simulation (LES) – Conditional Moment Closure (CMC) method has been applied to model spray combustion. Additional source terms in the CMC transport equation that originate from the evaporation of the fuel have been modelled and their influence on flame structure and global quantities such as temporally averaged temperature profiles has been analysed. The predictions of a laboratory spray jet flame are generally good, the temperature profiles are in good agreement with measurements, and predictions of droplet velocities are satisfactory. The evaporation source term leads to a flux in mixture fraction space and peak temperatures are shifted towards higher mixture fraction regions. The results, however, also highlight some limitations associated with mixture fraction as a single conditioning variable for mixture fraction based approaches if applied to liquid fuel combustion.

Numerical Simulation of Soot Formation in Spray Jet Flames

Numerical Simulation of Soot Formation in Spray Jet Flames

高圧下における同軸噴流噴霧火炎構造とすす生成領域の実験的観察(熱工学,内燃機関,動力など)

高圧下における同軸噴流噴霧火炎構造とすす生成領域の実験的観察(熱工学,内燃機関,動力など)

1203 高圧下における同軸噴流噴霧火炎の数値シミュレーション(GS-6 燃焼(1))

1203 高圧下における同軸噴流噴霧火炎の数値シミュレーション(GS-6 燃焼(1))

G0601-6-5 高圧下における同軸噴流噴霧火炎構造の実験的観察 : 噴霧油滴と燃焼領域の観察(熱工学(6)燃焼(1))

G0601-6-5 高圧下における同軸噴流噴霧火炎構造の実験的観察 : 噴霧油滴と燃焼領域の観察(熱工学(6)燃焼(1))

Analysis and flamelet modelling for spray combustion

The validity of a steady-flamelet model and a flamelet/progress-variable approach for gaseous and spray combustion is investigated by a two-dimensional direct numerical simulation (DNS) of gaseous and spray jet flames, and the combustion characteristics are analysed. A modified flamelet/progress-variable approach, in which total enthalpy rather than product mass fraction is chosen as a progress variable, is also examined. DNS with an Arrhenius formation, in which the chemical reaction is directly solved in the physical flow field, is performed as a reference to validate the combustion models. The results show that the diffusion flame is dominant in the gaseous diffusion jet flame, whereas diffusion and premixed flames coexist in the spray jet flame. The characteristics of the spray flame change from premixed–diffusion coexistent to diffusion-dominant downstream. Comparisons among the results from DNS with various combustion models show the modified flamelet/progress-variable approach to be superior to the other combustion models, particularly for the spray flame. Where the behaviour of the gaseous total enthalpy is strongly affected by the energy transfer (i.e. heat transfer and mass transfer) from the dispersed droplet, and this effect can be accounted for only by solving the conservation equation of the total enthalpy. However, even the DNS with the modified flamelet/progress-variable approach tends to underestimate the gaseous temperature in the central region of the spray jet flame. To increase the prediction accuracy, a combustion model for the partially premixed flame for the spray flame is necessary.