Spray Flame Research Articles

Large Eddy Simulation of a Turbulent Spray Burner Using Thickened Flame Model and Adaptive Mesh Refinement

Abstract The accurate simulation of two-phase flow combustion is crucial for the design of aeronautical combustion chambers. In order to gain insight into complex interactions between a flame, a flow, and a liquid phase, the present work addresses the combustion modeling for the large eddy simulation (LES) of a turbulent spray jet flame. The Eulerian–Lagrangian framework is selected to represent the gaseous and liquid phases, respectively. Chemical processes are described by a reduced mechanism, and turbulent combustion is modeled by the thickened flame model (TFM) coupled to the adaptive mesh refinement (AMR). The TFM-AMR extension on the dispersed phase is successfully validated on a laminar spray flame configuration. Then, the modeling approach is evaluated on the academic turbulent spray burner, providing a good agreement with the experimental data.

Solid-liquid equilibria in mixtures of iron(III) nitrate nonahydrate and ethanol or 1-propanol

Solutions of iron(III) nitrate nonahydrate (INN) in ethanol and 1-propanol are interesting starting materials for producing iron oxide nanoparticles by spray flame synthesis. For the design of these processes, it is important to have information on solid-liquid equilibria (SLE) in these solutions. As corresponding data were not available in the literature, we have studied the SLE of solutions of INN in ethanol and 1-propanol experimentally at temperatures between 288.15 and 308.15 K at 101.3 kPa. Unexpected phenomena occur in these solutions: one would expect an increasing amount of precipitate upon increasing the concentration of the salt INN, but the reverse behavior is observed for a wide range of states. This is caused by chemical reactions in the solutions: not INN, but iron(III) hydroxide (IH) precipitates from the solution, leading also to a strong acidity of the mixture. These chemical effects are taken into account in a physico-chemical model that was developed for describing the SLE in the studied systems. In that model, the physical non-ideality is described with the extended Pitzer model.

An investigation on early evolution of soot in n-dodecane spray combustion using large eddy simulation

Numerical simulations using large eddy simulation (LES) and Unsteady Reynolds Averaged Navier–Stokes (URANS) are carried out to identify the underlying mechanisms that govern the early soot evolution process in an n-dodecane spray flame at 21% O2 by molar concentration. A two-equation phenomenological soot model is used here to simulate soot formation and oxidation. Both ignition delay time (IDT) and lift-off length (LOL) are found to agree with experimental measurements. The transient evolution of soot mass, in particularly the soot spike phenomenon, is captured in the present LES cases, but not in the URANS cases. Hence, a comparison of numerical results from LES and URANS simulations is conducted to provide a better insight of this phenomenon. LES is able to predict the rapid increasing soot mass during the early stage of soot formation due to having a large favorable region of equivalence ratio (ϕ>1.5) and temperature (T>1800K) for soot formation. This favorable region increases and then decreases to reach a quasi-steady state in the LES case, while it continues to increase in the URANS simulation during the early time. In addition, the soot spike is a consequence of the competition between soot formation and oxidation rates. The time instance when the total soot mass reaches peak value coincides with the time instance when the total mass of soot precursor reaches a plateau. The soot spike is formed due to the continuous increase of oxidizing species in the LES case which leads to a more dominant oxidation process than the formation process.

Thermophysical Properties of Mixtures of Titanium(IV) Isopropoxide (TTIP) and 2-Propanol (iPOH)

The present work provides thermophysical data on binary mixtures of titanium(IV) isopropoxide (TTIP) and 2-propanol (iPOH), which are important precursor solutions in spray flame synthesis of nanoparticles. The specific density, viscosity, thermal conductivity, and molar isobaric heat capacity were measured at 101.3 kPa and temperatures ranging from 293.15 to 333.15 K. The vapor–liquid equilibrium (VLE) was measured at pressures ranging from 20 to 80 kPa. Furthermore, self-diffusion coefficients of pure iPOH were measured at 101.3 kPa and temperatures ranging from 293.15 to 333.15 K. A qualitative study using 1H NMR spectroscopy provides evidence for a cross-association between TTIP and iPOH in the mixture. Correlations for all studied thermophysical properties are provided. The results establish a rational basis for the design of spray flame synthesis processes.

Effect of Different Fuels on Combustion Instabilities in an Annular Combustor

Abstract Combustion instability in annular combustors of jet engines is a recurring issue. In this study, the characteristics of instabilities for different fuels are investigated by combining the instability maps obtained in a laboratory-scale annular combustor equipped with multiple swirling spray injectors (MICCA-spray) and flame describing functions (FDFs) from a single sector configuration (SICCA-spray). Two types of liquid fuels are injected as hollow cone sprays: heptane, which is fairly volatile, and dodecane, which is less volatile. Experiments are also conducted with gaseous propane, perfectly premixed with air, which serves as a reference. An instability map is systematically drawn by varying the global equivalence ratio and thermal power. The data indicate that the amplitude and frequency of instabilities depend, for the same operating point, on the fuel injection conditions (premixed or spray) and fuel type. Overall trends show that premixed propane is unstable in a broad operating domain. Injection of liquid fuels induces changes in flame time lag that modifies the unstable regions. For heptane, the instability map is closer to the propane reference map, whereas dodecane exhibits wider stable regions. An attempt is made to understand these features by examining the FDF, which gives the ratio of relative fluctuations in heat release rate to the relative fluctuations in velocity. The FDFs measured in a single sector configuration give access to gain and phase information that can be used to determine unstable bands and calculate an instability index guiding the interpretation of the differences in instabilities of the three fuels.

Synthesis of anticorrosive and flame-retardant coating based on turmeric (Curcuma longa) and magnesium hydroxide

The green corrosion inhibitor is used in epoxy coating system by extraction of turmeric in dimethyl formamide (DMF). The major component extracted is curcumin. Three concentrations have been studied for anticorrosive performance i.e., X, 2X and 4X. Among this X based concentration showed the best performance. Magnesium hydroxide Mg(OH)2 has been treated with 3-glycidoxypropyltrimethoxysilane (GPTMS). It was added into coating formulation at three concentrations namely 5%, 7% and 10% (w/w). The best anticorrosive performance has been observed for 5% concentration. As the concentration increases the anticorrosive performance decreases but flame retardancy increases. The 7% Mg(OH)2 concentration has been combine studied with turmeric X concentration. It is observed that combination of extract with Mg(OH)2 gives good anticorrosive performance as well as flame retardant properties. The anticorrosive performance is evaluated by salt spray and electrochemical impedance spectroscopy (EIS) and flame retardancy is evaluated by Limiting oxygen index (LOI) and UL-94 study.

Compression ignition and spark assisted ignition of direct injected PRF65 spray

In this study the spark assisted compression ignition combustion (SACI) developments were investigated using PRF65 (low octane fuel), a mixture of 65% isooctane (by volume) and 35% n-heptane (by volume) with a RON of 65. Characteristics like the cumulative heat release (CHR) and the peak heat release rates (HRR) were studied pressure data from experiments conducted in a constant volume combustion chamber (CVCC) for more precise control of the tested conditions. Spray flame images were also studied using high speed imaging systems to understand the effect of the conditions tested in the luminosity of the flame. Experiments were performed to understand the effects of oxygen concentration and ambient temperatures. Results show that the heat release rate increases initially and then decreases with the increase in the ambient temperature and the peak heat release rate appears around 650 K to 700 K. The peak heat release rate timing is advanced with the increase of the ambient temperature or oxygen level. Flame luminosity was also found to increases with the increase in ambient temperature. Under a low ambient temperature, the oxygen level plays a major role in affecting the peak heat release rate. Under lower oxygen levels, the flame becomes darker, the ignition delay becomes longer, and the combustion process takes more time to complete. A well timed spark timing was found to advance the peak HRR and shorten ignition delay, but this effect becomes minor when the temperature increases.

On the impact of fuel injection angle in Euler–Lagrange large eddy simulations of swirling spray flames exhibiting thermoacoustic instabilities

This study deals with the fundamental problem of combustion dynamics in gas turbine combustors operating with liquid fuel. In this framework the present work proposes the study of an academic liquid fueled combustor sensitive to thermoacoustic instabilities, simulated via high-fidelity Large Eddy Simulations. The experimental setup addressed is SICCA-spray from EM2C laboratory featuring both stable and unstable flames depending on the combustion chamber length. The proposed analysis, based on the Euler-Lagrange modeling approach, studies the impact of the spray injection angle θ on both the stable flame and the triggering of the longitudinal combustor acoustic mode when using a longer quartz tube. For the liquid injection modeling, the FIM-UR semi-empirical model is adopted with three different θ values: θ=35∘,45° and 60°. In stable conditions, the spray angle is proven to have a negligible impact on the flame anchoring point, however, the mean flame length and fuel distribution are found to be slightly modified by the velocity at which droplets enter the combustion chamber. For the thermoacoustically unstable conditions, two well-established stable limit cycles with the same frequency and similar amplitudes are obtained when fuel is injected at θ=45∘ and 60°. Contrarily, the system stabilizes when θ=35∘ pointing to the importance of the dynamics of the liquid film layer formed inside the injector for this setup. Likewise, this liquid film layer dynamics and its modeling appear critical as already suggested by previous studies on the same configuration. The detailed analysis of the thermoacoustically unstable different predictions is then performed through the investigation of the spatial fields of the local Rayleigh index obtained following the novel extension in the frequency domain of the Rayleigh criterion complemented by the application of Dynamic Mode Decomposition. It confirms that the injection angle of the liquid spray has a significant effect on the thermoacoustic response of the system. Indeed the influence of θ on the dynamics of the liquid fuel when entering the combustion chamber is proven to have an impact on the synchronization mechanism governing the liquid phase with respect to acoustics sustaining the observed limit cycles. More specifically, couplings at the liquid phase level are evidenced by introducing two novel indices correlating the fluctuations of liquid fuel volume fraction and evaporation rate with pressure.

Numerical Prediction of Turbulent Spray Flame Characteristics Using the Filtered Eulerian Stochastic Field Approach Coupled to Tabulated Chemistry

The Eulerian stochastic fields (ESF) method, which is based on the transport equation of the joint subgrid scalar probability density function, is applied to Large Eddy Simulation of a turbulent dilute spray flame. The approach is coupled with a tabulated chemistry approach to represent the subgrid turbulence–chemistry interaction. Following a two-way coupled Eulerian–Lagrangian procedure, the spray is treated as a multitude of computational parcels described in a Lagrangian manner, each representing a heap of real spray droplets. The present contribution has two objectives: First, the predictive capabilities of the modeling framework are evaluated by comparing simulation results using 8, 16, and 32 stochastic fields with available experimental data. At the same time, the results are compared to previous studies, where the artificially thickened flame (ATF) model was applied to the investigated configuration. The results suggest that the ESF method can reproduce the experimental measurements reasonably well. Comparisons with the ATF approach indicate that the ESF results better describe the flame entrainment into the cold spray core of the flame. Secondly, the dynamics of the subgrid scalar contributions are investigated and the reconstructed probability density distributions are compared to common presumed shapes qualitatively and quantitatively in the context of spray combustion. It is demonstrated that the ESF method can be a valuable tool to evaluate approaches relying on a pre-integration of the thermochemical lookup-table.

Multiple optical diagnostics on effects of fuel properties on spray flames under oxygen-enriched conditions

Spray flames development in diesel engines is typically diffusion combustion process controlled by mixing rate, which may cause considerable NOx and soot emissions. Breakthroughs in the field of oxygen preparation technology make it possible to apply oxygen-enriched combustion of spray flames, which has the potential to improve the thermal efficiency of engines and reduce pollutant emissions. In this paper, multiple optical diagnostics were applied to investigate spray flames at oxygen-enriched conditions. The experiments were conducted in an optical constant volume vessel, simulating the in-cylinder conditions of diesel engines. High-speed imaging, two-color pyrometry, and OH* chemiluminescence were used to investigate flame structure, soot flame temperature and KL factor, and flame lift-off length, respectively. The main fuel properties were separated by contrasting different fuels, including diesel (D100), 70% diesel + 30% n-heptane (D70H30), 70% diesel + 30% iso-octane (D70O30), 70% diesel + 30% n-butanol (D70B30), all of which were mixed by volume. The oxygen concentration of the ambient gas was in the range of 21%-70%. Experimental results show that the oxygen concentration has significant influence on the shape, structure, temperature, and species distributions of spray flame, while the fuel properties including cetane number, latent heat of vaporization and oxygen content, have more effects on flame ignition and soot formation under relatively lower oxygen concentration. With the increase of oxygen concentration, the flame lift-off length (FLoL) experiences a fast decrease firstly and then goes down slightly until it remains basically unchanged. The overall trend of FLoL under high oxygen concentrations shows that the FLoL of D100 is the shortest while the FLoL of D70B30 is the longest, and FLoL values of D70H30 and D70O30 are almost under the same level. With the increase of oxygen concentration, the effects of fuel properties on the ignition and development of the spray flame are weakened, the natural luminosity of spray flame increases, the length and width of spray flame reduce in size. At higher oxygen concentrations, the differences in KL factors and soot flame area among all test fuels become smaller. Under the same concentration, pixel averaged flame temperature of D100 is slightly higher than that of the other three fuels, while D70B30 is the lowest. The temperature differences among four fuels are no more than 100 K.

A generalised spray-flamelet formulation by means of a monotonic variable

The external structure of the spray-flamelet can be described using the Schvab–Zel'dovich–Liñan formulation. The gaseous mixture-fraction variable as function of the physical space, , typically employed for the description of gaseous diffusion flames leads to non-monotonicity behaviour for spray flames due to the extra fuel supplied by vaporisation of droplets distributed into the flow. As a result, the overall properties of spray flames depend not only on Z and the scalar dissipation rate, χ, but also on the spray source term, . We propose a new general coordinate variable which takes into account the spatial information about the entire mixture fraction due to the gaseous phase and droplet vaporisation. This coordinate variable is based on the cumulative value of the gaseous mixture fraction and is shown to be monotonic. For pure gaseous flow, the new cumulative function, , yields the well-established flamelet structure in Z-space. In the present manuscript, the spray-flamelet structure and the new equations for temperature and mass fractions in terms of are derived and then applied to the canonical counterflow configuration with potential flow. Numerical results are obtained for ethanol and methanol sprays, and the effect of Lewis and Stokes numbers on the spray-flamelet structure is analysed. The proposed formulation agrees well when mapping the structure back to physical space thereby confirming our integration methodology.

On the impact of fuel injection angle in Euler-Lagrange Large Eddy Simulations of swirling spray flames exhibiting thermoacoustic instabilities

On the impact of fuel injection angle in Euler-Lagrange Large Eddy Simulations of swirling spray flames exhibiting thermoacoustic instabilities

On the impact of fuel injection angle in Euler-Lagrange Large Eddy Simulations of swirling spray flames exhibiting thermoacoustic instabilities

On the impact of fuel injection angle in Euler-Lagrange Large Eddy Simulations of swirling spray flames exhibiting thermoacoustic instabilities

On the impact of fuel injection angle in Euler-Lagrange Large Eddy Simulations of swirling spray flames exhibiting thermoacoustic instabilities

On the impact of fuel injection angle in Euler-Lagrange Large Eddy Simulations of swirling spray flames exhibiting thermoacoustic instabilities

Visualization of flow and high temperature area in a diesel spray flame using an RCEM

Visualization of flow and high temperature area in a diesel spray flame using an RCEM

Characteristics of wall heat transfer from impinging diesel spray flame in low oxygen concentration ambient

The objective of this study is to obtain an enhanced understanding of the effect of diesel spray/flame impingement on transient heat flux to the wall. By using a constant volume vessel under engine-like condition, surface heat flux of the wall at spray/flame impingement was measured with three thin film thermocouple heat flux sensors. Fuel was injected using a single-hole injector with a 0.133 mm diameter nozzle. In order to investigate the relation between diesel flame and wall heat loss, two color-method was applied to observe flame temperature distribution. 21 % and 16 % as the various oxygen concentration were investigated at each injection pressure of pressure 80, 120, and 180 MPa. The results showed that the lower oxygen concentration was generated heat loss regardless of injection pressures. The lower flame temperature and flame contact area and period were influence factor on heat loss on the wall. Furthermore, the relation between Nusselt number (Nu) and Reynolds number (Re) were also investigated to understand the heat transfer phenomena in diesel engines.

Simulations of turbulent acetone spray flames using the conditional source term estimation (CSE) approach

Turbulent dilute acetone spray flames are simulated in an Eulerian–Lagrangian framework using Conditional Source-term Estimation (CSE) for closure of the mean reaction rate. The objective of this work is to assess the performance of the coupled CSE-spray approach, investigated for the first time for the selected configuration. One non-reacting case and four non-premixed flames, AcF1–AcF3 and AcF5 are chosen. Detailed tabulated chemistry is included. Predictions of mean axial velocity, gas temperature and spray statistics are compared with available experimental data. Good agreement between the predicted gas temperatures and experimental values is found. The peak temperature tends to be overpredicted and shifted radially outwards in the simulations, in particular farther downstream. Same trends are observed in all flames, but the predictions deteriorate with flame AcF5. Possible sources of discrepancy are inaccurate turbulent mixing field, increased level of premixing for flame AcF5, the neglect of spray effect in the chemistry tabulation and a larger experimental uncertainty in the temperatures in the high-temperature regions. The mean droplet velocity profiles are in good agreement with the experiments for the four flames. However, the velocity was underpredicted, in particular near the centreline, for flames AcF1 and AcF2.

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

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

Spontaneous Raman–LIF–CO–OH measurements of species concentration in turbulent spray flames

This paper presents new measurements of species concentrations, temperature and mixture fraction in selected regions of a turbulent ethanol spray flame. The line-Raman–LIF–COOH setup developed at the Sandia's Combustion Research Facility is utilised to probe regions of a spray flame where laser breakdown of liquid droplets is avoided and the remaining interferences can be corrected. The spray flame is stabilised on the piloted Sydney needle spray burner, where axial translation of the liquid injecting needle in the air-blast stream can transition the spray from dilute to dense. The solution to obtaining successful measurements is found to be multifaceted and includes: the appropriate selection of flame conditions; high sensitivity of the Raman detection system permitting reduced laser energies; development of a pre-processing algorithm to reject strong droplet interferences; and application of the hybrid matrix inversion method combined with wavelet denoising to account for interference corrections and noise at the very low signal levels obtained. Unique and necessary for the successful measurements reported in this paper, a pre-processing algorithm is outlined that removes data points corrupted with strong interferences from droplets. These interferences arise from a range of sources, but the most intense are due to the laser interaction with surrounding mist or liquid fragments, such that measurements near the jet centreline are corrupted and hence discarded. Reliable measurements of mixture fraction, temperature obtained from the sum of the species number densities, and species mole fractions are reported for regions in the flames sufficiently far from the centreline. The paper demonstrates the feasibility of the judicious use of Raman scattering in turbulent spray flames, the results of which will be extremely useful for validating numerical simulations.

Examination of the evolution of iron oxide nanoparticles in flame spray pyrolysis by tailored in situ particle sampling techniques

In this report, a flame spray pyrolysis setup has been examined with various in situ extraction methods of particle samples along the flame axis. First, two precursor formulations leading to the formation of iron oxide nanoparticles were used in a standardized SpraySyn burner system, and the final particle outcome was characterized by a broad range of established powder characterization techniques (TEM/HRTEM, SAXS, XRD, BET). The characterization of the powder products evidenced that mostly homogeneous gas-to-particle conversion takes place when applying an acidic precursor solution, whereas the absence of the acid leads to a dominant droplet-to-particle pathway. Our study indicates that a droplet-to-particle-pathway could be present even when processing the acidic formulation. However, even if a secondary pathway might take place in this case as well, it is not dominant and nearly negligible. Subsequently, the in situ particle structure evolution was investigated for the dominant gas-to-particle pathway, and particles were extracted along the flame axis for online SMPS and offline TEM/HRTEM analysis. Due to the highly reactive conditions within the flame (high temperatures, turbulent flow field, high particle number concentrations), the extraction of representative samples from spray flames is challenging. In order to handle the reactive conditions, two extraction techniques were tailored in this report. To extract an aerosol sample within the flame for SMPS measurement, a Hole in a Tube probe was adjusted. Thus, the mobility particle diameter as well as the corresponding distribution widths were obtained at different heights above the burner along the flame axis. For TEM/HRTEM image analysis, particle samples were collected thermophoretically by means of a tailored shutter system. Since all sampling grids were protected until reaching the flame axis and due to the low sampling time, momentary captures of local particle structures could be extracted precisely. The particle morphologies have clearly shown an evolution from spherical and paired particles in the flame center to fractal and compact agglomerates at later synthesis stages.