Turbulent Spray Research Articles

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

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

LES Study of a Turbulent Spray Jet: Mesh Sensitivity, Mesh-Parcels Interaction and Injection Methodology

This study focuses on Large Eddy Simulations (LES) in the Eulerian-Lagrangian framework of turbulent spray jets. The choice of the numerical grid relates both to turbulence level description and parcels-grid cells interaction. The objective of the present work is to determine a case set-up capable of predicting flow fields of a turbulent n-heptane spray jet for which a set of experimental data in non-reactive conditions (isothermal) is available (Shum-Kivan Proc. Combust. Inst. 36(2), 2567–2575 (2017)). The study first focuses on the LES grid, based on simulation with only gas phase, and subsequently on the choice of the suitable number of parcels to describe the spray. The droplets behavior is analyzed using the particle Stokes number at different axial locations. Furthermore, a variation in the existing injection methodology, as available in OpenFOAMⓇ, reveals a beneficial impact on the prediction of the experimental data.

Mechanisms of flame propagation in jet fuel sprays as revealed by OH/fuel planar laser-induced fluorescence and OH* chemiluminescence

Previous work on spray flames has shown that different propagation mechanisms may occur depending on the size and number density of droplets. In this work, the structure and propagation of flames in uniformly dispersed sprays of low-volatility fuels is experimentally examined. The effect of the Sauter mean diameter (SMD) of the spray (16–33 µm) on the propagation modes, flame speed, and flame curvature is assessed in weakly turbulent sprays, with the ratio of axial velocity rms to the gaseous laminar burning velocity uz′/SL,g ranging from 0.5–2.5, and overall equivalence ratio ϕ of 0.8, 1, and 1.4. The growth of the flame is evaluated from OH*-chemiluminescence and schlieren visualisation, which combined with OH/fuel planar laser-induced fluorescence visualisation reveal details of the propagation mechanisms. The aviation fuels investigated – Jet A and a renewable alternative, ATJ-8 – exhibited similar flame speed behaviour due to changes in SMD in each of the propagation modes identified: the droplet, inter-droplet, and gaseous-like modes. Concentrated reactions around large droplets found in lean conditions (ϕ = 0.8) allowed for a slowly propagating flame front which, in turn, ignited new droplets. Stoichiometric to rich conditions (ϕ = 1, 1.4) were marked by stronger evaporation ahead of the flame and, therefore, higher and more uniform heat release across the flame. Still, droplets penetrated the flame, locally inducing regions of negative flame curvature and continuing to evaporate in the burnt products. The droplet-induced effects disappeared at low SMD (16 µm, ϕ = 1.4), giving rise to a fully gaseous layer ahead of the flame and the highest flame speeds. At rich conditions and high SMD, Jet A had a lower flame speed than ATJ-8.

Numerical simulation of secondary atomization of an emulsion fuel droplet due to puffing: Dynamics of wall interaction of a sessile droplet and comparison with a free droplet

Physical mechanisms of explosive boiling of an ethanol-in-decane emulsion droplet are investigated by numerical simulation. Vapor bubble growth due to explosive boiling leads to secondary breakup of the droplet. The ethanol mass ratio is chosen in the regime of puffing (vapor ejection and partial breakup), which is generally likely to occur for a fuel spray in a combustor. Using both free and sessile droplet configurations and varying the initial number of boiling bubbles and the depth of the bubble formation in the dispersed sub-droplets, the droplet breakup and vapor mixing processes are discussed. For both free and sessile configurations, the basic dynamics of puffing is the same. When the bubble depth is deeper in the sub-droplet, the bubble growth is eventually larger. When the number of bubbles is more, puffing occurs several times, or the bubbles coalesce leading to larger bubble size, larger breakup and enhanced vapor mixing. In the sessile droplet configuration, the vapor ejection dynamics is essentially similar, but particularly when puffing is strong, the droplet bounces from the wall, first pushed toward the wall by the ejected vapor and then pulled from the wall due to the repelling and recoiling motion. Under such a condition, the initially attached droplet finally detaches from the wall and returns into the ambient gas. This phenomenon may transiently influence the mixture formation in a combustor when a fuel spray collides with the combustor wall. The present study serves as a first step toward a goal of quantitatively evaluating the puffing/microexplosion effect on a real fuel turbulent spray in internal combustion engines as to how large it can contribute to improving combustion performance, and insights have been obtained for future modeling which is expected to fill the gap between the droplet-scale and spray-scale knowledge.

Intrinsic thermoacoustic feedback loop in turbulent spray flames

This paper investigates low-frequency thermoacoustic instabilities of a turbulent spray flame, a phenomenon known as ‘rumble’. Based on experimental data, a network model analysis is performed, which suggests an intrinsic thermoacoustic (ITA) feedback loop as the root cause of instability. At first, the ITA nature of the observed instability is confirmed by a parametric analysis, which reveals the sensitivity of the instability frequency to the time delay of the flame. Then, we investigate pure acoustic modes and pure ITA modes, which couple to the full system modes such that the origin and the trajectory of each mode is trackable. Finally, we show that the unstable mode frequency scales with the inlet bulk velocity: a feature that clearly separates the observed instability from classical cavity modes. The network model results are corroborated by phasor plots, in which all relevant phase information is compiled. It reveals the phase of the Flame Transfer Function (FTF) contributing to the instability and provides an estimate of the ITA frequency, which agrees well with the dominant peak in the experimental pressure spectrum. Additionally, the obtained flame phase is used to infer the instability frequency by the experimentally measured droplet burning time τB, which reproduces similar trends as experimental and network model results. This theoretical study confirms that (1) ITA feedback loops are important for spray flames, (2) that the ITA instability of the experiment scrutinized is controlled by the droplet dynamics and (3) ITA modes appear also in acoustically closed systems.

Statistics of progress variable and mixture fraction gradients in an open turbulent jet spray flame

The statistical behaviours of the variance, covariance and gradients of reaction progress variable c, and mixture fraction ξ have been analysed in an ethanol turbulent spray jet flame based on the results obtained from a carrier-phase three-dimensional Direct Numerical Simulation (DNS) dataset. It has been observed that the Favre Probability Density Functions (PDFs) of c and ξ extracted from the DNS data are correctly estimated by a standard β-PDF. Furthermore, the log-normal distribution has been found to accurately represent |∇ξ|, but small discrepancies were found at the head of the distribution. By contrast, |∇c| was found to be only marginally represented by a log-normal distribution with non-negligible departures from the DNS data at both the head and the tail of the distribution. It has also been found that ∇c and ∇ξ show predominant collinear alignment throughout the flame brush. Finally, the joint-PDF of |∇c| and |∇ξ| (i.e. P|∇c|,|∇ξ|) has been compared with the product of the marginal PDFs of |∇c| and |∇ξ| (i.e. P|∇c|×P|∇ξ|) extracted from DNS (i.e. assuming statistical independence) and a good agreement has been observed. The bivariate log-normal distributions with and without correlation have also been considered and fail to capture the DNS trend, which stems from the difficulties of approximating P|∇c| and P|∇ξ| with log-normal PDFs.

Large Eddy Simulation-Based Turbulent Combustion Models for Reactive Sprays: Recent Advances and Future Challenges

Numerical simulations of turbulent reactive sprays are challenging owing to the presence of multiple timescales and multiphysics phenomena involving complex turbulence spray and turbulence-chemistry interactions. In turbulent spray flames, several physical phenomena such as primary and secondary atomization, droplet dispersion, interparticle collisions, evaporation, mixing, and combustion occur simultaneously, and hence it becomes a formidable task to model these complex interactions. To gain fundamental knowledge and advance current modeling capabilities, it may be appropriate to aim for progress in individual modeling of breakup, dispersion, mixing and combustion, which however cannot be viewed in complete isolation. A brief review of the development of state-of-the-art turbulent combustion models applicable to the dilute spray regime is presented. Therefore, complexities associated with the dense regime, including interparticle collisions as well as primary and secondary atomization, are not covered. Further, we restrict ourselves to a brief discussion on large eddy simulation, which has found applications in both laboratory and industrial applications of turbulent combustion without a change in phase. The gas phase-based turbulent combustion models such as flamelet, conditional moment closure and transported filtered density function methods have been developed and extensively used for combustion without a phase change. However, careful adaptation and extension of these models are necessary toward modeling of turbulent combustion with phase change. This article presents a review of recent advances and directions of future research on modeling of turbulent combustion for dilute sprays.

Fluid dynamic and autoignition characteristics of early fuel sprays using hybrid atomization LES

Early fuel spray development and subsequent autoignition are investigated using our turbulent atomization model that has enabled physically-closed primary atomization modeling. Thanks to its modeling methodology, no parameter tuning or adjustment is needed to simulate a turbulent spray. Due to turbulent atomization, a dense droplet layer is formed over the injected liquid core. With evaporation, this region has a triple layer structure, which is composed of the inner layer with many droplets, the vaporization layer and the outer layer exposed to the hot air. With combustion, hot spots are generated in the outer layer, while the inner layer is too cold and hinders the hot gas to penetrate inside. The group combustion number indicates that this is sheath evaporation/combustion mode and its structure is preserved down to the liquid core length. The liquid core thinning is determined by atomization characteristics and under evaporative/reactive conditions where the outer gas temperature is high and thus the kinematic viscosity is also high, the flow motion is larger and the core is unsteadily affected by this motion. Due to this, the Rayleigh–Taylor (RT) atomization mode is further excited and core breakup occurs relatively faster. It is also observed that the unsteady core head flapping motion leads to non-uniform distributions of droplets and vapor. Due to this non-uniformity, the initial autoignition in the outer layer does not occur uniformly. After core breakup, the droplet layer rolls up in the front region and the group combustion number becomes lower, where external group combustion behavior is observed. Multistep chemistry improves the ignition delay estimation compared with the global chemistry, but the spray dynamics is mostly determined by the above fluid dynamic effects. Therefore, the present study indicates that the accurate simulation capability of fluid dynamics, i.e. primary atomization and early spray development, is important in predicting spray combustion behavior.

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.

Experimental investigation of seawater scrubbing of SO₂ in turbulent contact absorbers and spray absorbers

Experimental investigation of seawater scrubbing of SO₂ in turbulent contact absorbers and spray absorbers

Assessment of LES-STRIP approach for modeling of droplet dispersion in diesel-like sprays

In this paper, the stochastic equations of droplet motion in turbulent flow, proposed recently by Gorokhovski and Zamansky (2018, Phys. Rev. Fluids 3, 3, 034602), are assessed for turbulent spray dispersion in diesel like conditions along with Large Eddy Simulation (LES) for the gaseous flow. For droplets above the Kolmogorov length scale, this model introduces the concept of the stochastic drag, independently of laminar viscosity. For droplets below the Kolmogorov length scale, the model equation does depend on the laminar viscosity through the Stokes drag but the particle motion is stochastically forced. Both the stochastic drag and the stochastic forcing of the Stokes drag equation are based on the simple log-normal stochastic process for the viscous dissipation (ϵ) “seen” along the droplet trajectory. In this paper, this model is applied in the framework of two-way coupling, wherein the turbulence generated by the spray inturn controls the spray dispersion. The criterion for the choice of one of the approaches, i.e., the stochastic drag or the stochastic forcing, follows the classical condition for drag coefficient based on the droplet Reynolds number (Re p). The non-vaporizing spray experiments from Engine Combustion Network (ECN) are used as test cases. In addition to the comparison of the spray penetration length, spreading angle and spray structure with the experimental data, a qualitative analysis of the statistics of the droplet acceleration and gas phase velocities is presented. It was shown that the new approach is much more effective in modeling the spray dynamics on relatively coarser mesh. Consequently, the new approach in the framework of two-way coupling may predict the preferential concentration effects better, which is important for spray combustion.

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.

Combustion noise analysis of a turbulent spray flame using a hybrid DNS/APE-RF approach

Combustion generated noise of a turbulent spray flame has been investigated using a hybrid Computational Fluid Dynamics/Computational Aero-Acoustics (CFD/CAA) framework. In this two-step framework, the reacting flow-field of a flame is simulated using Direct Numerical Simulations (DNS) in the first step, while the acoustic wave propagation in a non-uniform mean flow is captured by solving the Acoustic Perturbation Equations extended to Reacting Flows (APE-RF) in the second step. The numerical approach used in this study is therefore, termed as the hybrid DNS/APE-RF approach. First, this hybrid DNS/APE-RF approach is used to simulate a benchmark experimental open turbulent non-premixed flame, and the results obtained for the non-premixed flame’s flow-field statistical quantities, as well as radiated sound intensities are extensively validated against experimental data. Next, the hybrid DNS/APE-RF approach is applied to an experimental open turbulent spray flame, for simulating its two-phase reacting flow-field along with its combustion generated acoustic field, to predict and analyze the noise radiation behavior. The DNS results of the spray flame are compared with measurements, in terms of droplet velocity statistics and gas-phase temperatures, and favorable agreement is observed. The computed acoustic power spectra of the spray flame exhibit a power law dependence of the form f−2.4 (where f is the frequency) in the frequency range 300 Hz < f < 1000 Hz. And, the computed noise spectra of the spray flame contain a nearly constant sound pressure level plateau, for frequencies greater than 1000 Hz and up to 3000 Hz. Acoustic refraction effects induced by sound speed variations in the flame, resulting in the attenuation of high-frequency noise radiated in the flame downstream direction, are also captured in the simulation. Noise radiation characteristics of the turbulent spray flame are found to resemble those of turbulent premixed and non-premixed flames.

Five kHz thermometry in turbulent spray flames using chirped-probe pulse femtosecond CARS, part I: Processing and interference analysis

We have applied chirped-probe-pulse (CPP) femtosecond (fs) coherent anti-Stokes Raman scattering for 5 kHz temperature measurements in turbulent spray flames. The CPP fs CARS technique has previously been used to perform spectroscopic temperature measurements in highly turbulent laboratory burners with excellent accuracy, precision, temporal resolution, and spatial resolution. In this paper, ultrafast CARS measurements in spray flames are presented as part of a larger effort to provide spatially and temporally resolved temperature fields in harsh spray environments. The Sydney Needle Spray Burner (SYNSBURNTM) was used to stabilize turbulent spray flames of acetone and ethanol. The burner features a retractable fuel injector so that the droplet density at the nozzle exit could be systematically varied. Results from selected regions of the turbulent spray flames are discussed in detail to highlight the challenges of CPP fs CARS temperature measurements. Sources of spectral distortion due to interaction with droplets are discussed along with an uncertainty analysis. The passage of fuel through the probe volume caused varying levels of signal degradation and resulted in complete signal loss on approximately 10% of the laser shots for dense spray conditions. The interferences are attributed to two separate phenomena and are categorized based on the probable phase of the fuel – liquid or gas. Interference caused by liquid fuel was unavoidable in certain regions at certain operating conditions, but easily identified and removed. Interference from vapor fuel was more problematic as the nitrogen signal was only moderately corrupted in the high-frequency portion of the spectrum, and the temperature was generally biased to higher values. Rejecting individual signal spectra, based on a fitting error threshold, was shown to be effective in excluding shots with significant interference from fuel droplets, but shots with only minor interference require a more-advanced rejection criterion. Analysis of the temperature fields for a few selected conditions is presented showing trends with the atomization quality of the liquid fuel. Fourier analysis revealed hydrodynamic instabilities in the shear layer and relatively weak thermoacoustic instabilities in the reaction zone.

Five kHz thermometry in turbulent spray flames using chirped-probe-pulse femtosecond CARS, part II: Structure of reaction zones

Temperature was measured in turbulent spray flames of ethanol and acetone stabilized on the piloted Sydney Needle Spray Burner (SYNSBURNTM) using single-laser-shot, chirped-probe-pulse femtosecond coherent anti-Stokes Raman spectroscopy (CPP-fs-CARS) with a repetition rate of 5 kHz. The burner features air-blast atomization of liquid injected from a needle that can be translated by a length Lr within a co-flowing air stream so that piloted spray flames ranging from dilute to dense can be studied. Part I of these investigations has reported on the CPP-fs-CARS technique and extensive details of data processing methodology. Part II is concerned with the structure of the reaction zones at different spray loadings and for different departures from blow-off. While not performed simultaneously, measurements of the size distribution of liquid fragments are also reported and discussed in conjunction with the measured temperature. Measured probability density functions of temperature show that for flames with the same liquid loading but different recess lengths, Lr, the near-field spray structure that forms upstream of x/D = 10 affects flame structure and stability further downstream. As the spray exiting the burner becomes denser, with a higher proportion of ligaments and ‘irregular’ shaped objects, the entrainment of hot pilot gases into the spray envelope is affected, hence changing the rates of vaporization and subsequent combustion. The reported results will also form a useful platform for validating sub-models of atomization and combustion in turbulent, dilute to dense spray flames.

Modelling of sprays: recent results and future challenges

A brief overview of some recent developments and future challenges in spray modelling is presented. The focus is on the comparative analysis of Lagrangian and Fully Lagrangian (Osiptsov) approaches to spray modelling and the applications of the latter approach and its generalisations to the solution of engineering problems, recent developments in modelling the heating and evaporation of spherical and non-spherical droplets and films, and application of the method of integral manifolds in the analysis of spray heating, evaporation and ignition in Internal Combustion (IC) engine-like conditions. Future challenges described in the paper include the generalisation of the Fully Lagrangian approach to enable it to model realistic turbulent sprays, further development of the models of heating and evaporation of deformed droplets, modelling of heating and evaporation of droplets in trance- and super-critical conditions and further development of the integral manifold methods to enable their application to the solution of realistic engineering problems, with particular focus on IC engines. All models are expected to be developed in formats that enable their relatively simple implementation into commercial Computational Fluid Dynamics (CFD) codes.

Turbulent Spray Combustion Modeling Using Various Kinetic Solvers and Turbulence Models

Turbulent spray combustion of n-dodecane was modeled at relevant engine conditions using two combustion models (direct integration of chemistry (DIC) and flamelet generated manifolds (FGM)) and multifidelity turbulence models (dynamic structure large eddy simulation (LES) and renormalization group (RNG) Reynolds-averaged Naiver–Stokes (RANS)). The main objective of this work is to study the effect of various combustion and turbulence models on spray behavior and quantify these effects. To reach these objectives, a recently developed kinetic mechanism and well-established spray models were utilized for the three-dimensional turbulent spray simulation at various combustion chamber initial gas temperature and pressure conditions. Fine mesh with a size of 31 μm was utilized to resolve small eddies in the periphery of the spray. In addition, a new methodology for mesh generation was proposed and investigated to simulate the measured data fluctuation in the CFD domain. The pressure-based ignition delay, flame lift-off length (LOL), species concentrations, spray, and jet penetrations were modeled and compared with measured data. Differences were observed between various combustion and turbulence models in predicting the spray characteristics. However, these differences are within the uncertainties, error, and variations of the measured data.

Evaporating droplets in turbulence studied with statistically stationary homogeneous direct numerical simulation

The present paper investigates droplet laden turbulent gas flows with point droplet direct numerical simulations. A novel flow configuration is presented, which allows us to simulate statistically stationary homogeneous isotropic regions of turbulent sprays. This configuration enables a physical analysis with small statistical errors under controllable conditions. The length scales of clusters are analyzed with spectra of the droplet number density and vapor mass fraction fluctuations. The local conditions in clusters and voids are analyzed with probability density functions of the local droplet number density, local vapor mass fraction, and evaporation rate. Effects of the mass loading and Stokes number on the clustering of droplets and the mixing of vapor are characterized, and the compositions of clusters and voids are examined. As the mass loading increases from 2.5% to 12.5%, the fluctuations in the vapor mass fraction and droplet number density increase almost equally at all length scales. Clusters contain more droplets that together release more vapor, but their characteristic size remains unchanged. As the mean Stokes number increases from 1 to 4, the fluctuations in the vapor mass fraction and droplet number density decrease predominantly at small length scales. Clusters contain fewer droplets that together release less vapor, and their characteristic size increases. At a mass loading of 7.5% and a mean Stokes number of 2, clusters contain considerably more droplets and vapor than voids. Droplets located in clusters therefore evaporate more slowly than droplets located in voids.

Chirped-probe-pulse femtosecond CARS thermometry in turbulent spray flames

This paper presents temperature measurements in turbulent dilute and dense spray flames using single-laser-shot chirped-probe-pulse femtosecond coherent anti-Stokes Raman spectroscopy (CPP-fs-CARS). This ultrafast technique, with a repetition rate of 5 kHz, is applied to the piloted Sydney Needle Spray Burner (SYNSBURNTM). The burner system features air-blast atomization of liquid injected from a needle that can be translated within a co-flowing air stream. The pilot-stabilized spray flames can range between the two extremes of dense and dilute by physically translating the needle tip relative to the burner's exit plane. The CPP-fs-CARS set-up has achieved integration times of 3 picoseconds (ps) as well as spatial resolution of approximately 800 µm along beam propagation and 60 µm in the transverse dimension. Brief details of the technique, calibration, correction of interferences, and spectral fitting processes are presented along with estimates of the associated error. The measurements are compared against well-established, line Raman–Rayleigh data for temperature collected in a turbulent CH4/air jet diffusion flame, which is largely non-sooting. At peak gaseous flame temperatures of up to 2512 K, the relative accuracy and precision were 2.8% and ±3.4%, respectively. Measurements in turbulent spray flames are shown after applying the relevant corrections based on non-resonant background (NRB) behavior and camera saturation effects on the shape of the CARS signal spectrum. Preliminary mapping of the temperature fields demonstrates the wealth of information available in this dataset which will provide insights into the spatio-temporal structure of spray flames once relevant statistical analysis is applied.

Coupling of turbulence on the ignition of multicomponent sprays

This study examines the effect of turbulence on the ignition of multicomponent surrogate fuels and its role in modifying preferential evaporation in multiphase turbulent spray environments. To this end, two zero-dimensional droplet models are considered that are representative of asymptotic conditions of diffusion limit and the distillation limit are considered. The coupling between diffusion, evaporation and combustion is first identified using a scale analysis of 0D homogeneous batch reactor simulations. Subsequently, direct numerical simulations of homogeneously dispersed multicomponent droplets are performed for both droplet models, in decaying isotropic turbulence and at quiescent conditions to examine competing time scale effects arising from evaporation, ignition and turbulence. Results related to intra-droplet transport and effects of turbulence on autoignition and overall combustion are studied using an aviation fuel surrogate. Depending on the characteristic scale, it is shown that turbulence can couple through modulation of evaporation time or defer the ignition phase as a result of droplet cooling or gas-phase homogenization. Both preferential evaporation and turbulence are found to modify the ignition delay time, up to a factor of two. More importantly, identical droplet ignition behavior in homogeneous gas phase can imply fundamentally different combustion modes in heterogeneous environments.