Laminar Spray Flame Research Articles

Numerical simulation of TTIP/[formula omitted]-xylene precursor solution spray flames in the counterflow configuration

Sprays of precursor solutions are used in flame spray pyrolysis (FSP) to produce functional nanoparticles with specific properties. The present study concerns the precursor titanium(IV) isopropoxide (TTIP) in p-xylene. A dilute monodisperse spray carried by an air stream is directed against an air stream to establish a counterflowing laminar spray flame. The complex thermophysical properties of the bi-component droplets are incorporated into the code as well as the detailed chemical reactions of the system with 213 chemical reactions among 52 species; the chemical reaction mechanism includes thermal decomposition of the TTIP and the combustion of p-xylene. The droplets are modeled using the multicomponent evaporation model. Drag and gravity forces are considered in the equation of droplet motion. Monodisperse spray flame structures are presented depending on the gas strain rate, the initial droplet size, the precursor mass loading as well as the equivalence ratio. The preferential evaporation of the more volatile p-xylene provides the gaseous fuel for combustion, and the thermal decomposition of the precursor TTIP considerably reduces the flame temperature.

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.

Numerical study of multicomponent spray flame propagation

A computational study of one dimensional multicomponent laminar Jet-A/air spray flames is presented. The objective is to understand the effect of various spray parameters (diameter, droplet velocity, liquid loading) on the spray flame structure and propagation. Simulation of the Eulerian gas phase is coupled with a Lagrangian tracking of the dispersed liquid phase. Jet-A surrogate of n-dodecane, methyl-cyclohexane and xylene is considered. A discrete multicomponent model for spray vapourisation is used along with an analytically reduced chemistry for computing the gas phase reactions. Both overall lean and rich cases are examined and compared with existing literature for single component spray flames. The preferential evaporation effect, unique to multicomponent fuels causes a variation of fuel vapour composition on both sides of the flamefront and this has a direct impact on the spray flame structure and propagation speed. In the rich cases, multiple flame structures exist due to the staged release of vapours across the reactive zone. Spray flame speed correlations proposed for single component fuels are extended to the multicomponent case, for both zero and high relative velocity between the liquid and the gas. The correlations are able to accurately predict the effective equivalence ratio at which the flame burns and hence the laminar spray flame speeds of multicomponent fuels for all cases studied in this work.

AN IMPROVED MODEL FOR INVESTIGATING THE EFFECT OF SPRAY POLYDISPERSITY ON LAMINAR PREMIXED SPRAY FLAME IGNITION

AN IMPROVED MODEL FOR INVESTIGATING THE EFFECT OF SPRAY POLYDISPERSITY ON LAMINAR PREMIXED SPRAY FLAME IGNITION

Effect of non-zero relative velocity on the flame speed of two-phase laminar flames

A numerical study of one-dimensional n-heptane/air spray flames is presented. The objective is to evaluate the flame propagation speed in the case where droplets evaporate inside the reaction zone with possibly non-zero relative velocity. A Direct Numerical Simulation approach for the gaseous phase is coupled to a discrete particle Lagrangian formalism for the dispersed phase. A global two-step n-heptane/air chemical mechanism is used. The effects of initial droplet diameter, overall equivalence ratio, liquid loading and relative velocity between gaseous and liquid phases on the laminar spray flame speed and structure are studied. For lean premixed cases, it is found that the laminar flame speed decreases with increasing initial droplet diameter and relative velocity. On the contrary, rich premixed cases show a range of diameters for which the flame speed is enhanced compared to the corresponding purely gaseous flame. Finally, spray flames controlled by evaporation always have lower flame speeds. To highlight the controlling parameters of spray flame speed, approximate analytical expressions are proposed, which give the correct trends of the spray flame propagation speed behavior for both lean and rich mixtures.

An Investigation of the Behavior of Steady-State Laminar Jet Spray Flames

Abstract The theory of steady-state laminar jet spray flames is developed in the limit of small Stokes number. The spray is modeled using the sectional approach. A similarity solution is suggested, and it is shown that for typical operating conditions the leading order small Stokes number solution suffices for determining lift-off and blow-out features of the flames. The latter are demonstrated to be strongly dependent on spray properties, in contrast to the Schmidt number dependence only which was predicted by classical laminar jet gaseous flame theory. The strong influence of the ambient oxygen content and the liquid fuel’s evaporation coefficient on flame characteristics, blow-out and lift-off are established.

A consistent Artificially Thickened Flame approach for spray combustion using LES and the FGM chemistry reduction method: Validation in Lean Partially Pre-vaporized flames

A consistent modeling approach in the context of the Artificially Thickened Flame (ATF) method is proposed for the prediction of turbulent Lean Partially Pre-vaporized (LPP) spray flames. Large Eddy Simulations (LES) are carried out in an Eulerian–Lagrangian framework to represent the turbulent two-phase flow. The chemical reactions are described with the Flamelet Generated Manifold (FGM) chemistry reduction method. Various aspects of the thickening of the scalar fields in the frame of reacting two-phase flows are systematically investigated relying on simplified numerical test cases. A correction for the solution of the dispersed phase equations for droplets interacting with the flame is successfully validated for different scenarios. The influences of the presence of evaporating droplets on the reaction development are carefully analyzed along with the adopted FGM database. At this stage a new theoretical expression to predict the laminar spray flame propagation speed is derived. Enhancements on the thickening procedure could be achieved with the inclusion of mixture quality aspects. Finally, the complete designed model is applied to a turbulent LPP burner for three different operating conditions. Results of the droplets velocity, characteristic diameters, liquid volumetric fluxes and reaction progress variable are compared with experimental data. Further findings regarding the mixture stratification, the impact of the SGS wrinkling modeling and the thickening onto the flame structure as well as onto the dispersed phase behavior are also addressed.

A spray flamelet/progress variable approach combined with a transported joint PDF model for turbulent spray flames

A spray flamelet/progress variable approach is developed for use in spray combustion with partly pre-vaporised liquid fuel, where a laminar spray flamelet library accounts for evaporation within the laminar flame structures. For this purpose, the standard spray flamelet formulation for pure evaporating liquid fuel and oxidiser is extended by a chemical reaction progress variable in both the turbulent spray flame model and the laminar spray flame structures, in order to account for the effect of pre-vaporised liquid fuel for instance through use of a pilot flame. This new approach is combined with a transported joint probability density function (PDF) method for the simulation of a turbulent piloted ethanol/air spray flame, and the extension requires the formulation of a joint three-variate PDF depending on the gas phase mixture fraction, the chemical reaction progress variable, and gas enthalpy. The molecular mixing is modelled with the extended interaction-by-exchange-with-the-mean (IEM) model, where source terms account for spray evaporation and heat exchange due to evaporation as well as the chemical reaction rate for the chemical reaction progress variable. This is the first formulation using a spray flamelet model considering both evaporation and partly pre-vaporised liquid fuel within the laminar spray flamelets. Results with this new formulation show good agreement with the experimental data provided by A.R. Masri, Sydney, Australia. The analysis of the Lagrangian statistics of the gas temperature and the OH mass fraction indicates that partially premixed combustion prevails near the nozzle exit of the spray, whereas further downstream, the non-premixed flame is promoted towards the inner rich-side of the spray jet since the pilot flame heats up the premixed inner spray zone. In summary, the simulation with the new formulation considering the reaction progress variable shows good performance, greatly improving the standard formulation, and it provides new insight into the local structure of this complex spray flame.

Transported Joint Probability Density Function Simulation of Turbulent Spray Flames Combined with a Spray Flamelet Model Using a Transported Scalar Dissipation Rate

ABSTRACTNumerical simulations of a turbulent ethanol/air spray flame are performed using a combined formulation of a joint gas mixture fraction-enthalpy probability density function (PDF) and a spray flamelet model, where the scalar dissipation rate of the mixture fraction is described with a transport equation. Spray evaporation is included in the flamelet model through the pre-calculation of counterflowing laminar spray flames with a detailed reaction mechanism of 38 species and 337 elementary reactions for ethanol/air combustion. A transport equation for the mean scalar dissipation rate of the mixture fraction is derived, taking into account the spray evaporation. The computed results are compared with the experimental data provided by A. Masri, University of Sydney, Australia. All major spray flame characteristics, including the Sauter mean diameter, the droplet velocity, and its turbulent fluctuations as well as gas temperature, generally show good agreement with experiments. Different models for the scalar dissipation rate of the mixture fraction are not significantly different for the flame under consideration except for inside the chemical reaction zone, where the new model shows some improvement compared to the experiment.

On Laminar Rich Premixed Polydisperse Spray Flame Propagation with Heat Loss

A mathematical analysis of laminar premixed spray flame propagation with heat loss is presented. The analysis makes use of a distributed approximation of the Arrhenius exponential term in the reaction rate expression and leads to an implicit expression for the laminar burning velocity dependent on the spray-related parameters for the fuel, gas-related parameters and the intensity of the heat losses. It is shown that the initial droplet load, the value of the evaporation coefficient, and the initial size distribution are the spray-related parameters which exert an influence on the onset of extinction. The combination of these parameters governs the manner in which the spray heat loss is distributed spatially and it is this feature that is the main factor, when taken together with volumetric heat loss, which determines the spray’s impact on flame propagation and extinction.

Analysis of segregation and bifurcation in turbulent spray flames: A 3D counterflow configuration

The understanding of spray combustion processes is of primary importance, as it is encountered in a wide range of industrial applications. In the present work, mesoscale-resolved simulations of a 3D turbulent counterflow spray configuration are conducted. Primary focus is on examining the effect of the coupling between turbulence, evaporation, mixing, and combustion. By considering different initial droplet diameters and through comparisons with turbulent and laminar configurations at the same operating condition, it is shown that preferential concentration can lead to conditions of locally high mixture-fraction composition. In addition, local variability in strain rate and droplet diameter introduces a bifurcation of the spray flame. This bifurcation consists of spray flame structures exhibiting single-reaction or double-reaction structures. It is shown that this bimodal behavior is linked to the existence of a hysteresis in the laminar spray flame structure for droplet diameter variations, as well as the occurrence of a bifurcation for strain rate variations. These results have direct implications for flamelet-based tabulation methods, since identifying the appropriate flamelet structure in turbulent spray flames would require informations about boundary conditions and the flamelet history.

A tabulated chemistry method for spray combustion

Tabulated chemistry is a popular technique to account for detailed chemical effects with an affordable computational cost in gaseous combustion systems. However its performances for spray combustion have not completely been identified. The present article discusses the chemical structure modeling of spray flames using tabulated chemistry methods under the hypothesis that the chemical subspace accessed by a two-phase reactive flow can be mapped by a collection of gaseous flamelets. It is shown that tabulated chemistry methods based either on pure premixed flamelets or on pure non-premixed flamelets fail to capture the structure of spray combustion. The reason is the complexity of the chemical structure of spray flames which exhibits both premixed-like and non-premixed-like reaction zones. To overcome this issue, a new multi-regime flamelet combustion model (called Partially-Premixed Flamelet Tabulation 2PFT) is presented in this paper. Information from premixed, partially-premixed and diffusion flames are stored in a 3-D look-up table parametrized as a function of the progress variable Yc, describing the progress of the reaction, the mixture fraction Yz, denoting the local equivalence ratio, and the scalar dissipation χ∗, which identifies the combustion regime. The performances of the 2PFT method are evaluated on counterflow laminar spray flames for different injection conditions of droplet diameter, liquid volume fraction and velocity. The 2PFT tabulation method better describes the chemical structure of spray flames compared to the classical techniques based on single archetypal flamelets. These results also confirm that the chemical structure of laminar spray flame can be modeled by a multi-regime flamelet combustion model based on gaseous flamelets.

A Simple Model of Off-Stoichiometric Premixed Fuel Rich Spray Flames with Droplet Evaporation and Pyrolysis

Fuel droplets that manage to traverse the main reaction zone of a one-dimensional off-stoichiometeric fuel-rich laminar spray flame enter a hot, oxygen-free environment under which conditions their pyrolysis occurs. Under such circumstances the droplets release pyrolysis products which may diffuse back towards the flame front thereby effectively supplying a second (pool of) fuel different to the (first) fuel that was released due to pre-flame droplet evaporation. A new simple mathematical model of the propagation of a spray flame that includes this phenomenon is presented in this work for the first time and a formula is given for the burning velocity. It is shown that under certain operating conditions flame velocity enhancement may occur as a result of the additional fuel supply. Although the effect is limited to a few percent it is pointed out that it may be significant in cases of borderline flame stability and/or ignition/extinction.

Examples of the Potential of DNS for the Understanding of Reactive Multiphase Flows

The objective of this article is to point out the ability of the multiphase flow DNS (Direct Numerical Simulation) to help to understand basic physics and to interpret some experimental observations. To illustrate the DNS' potential to give access to key phenomena involved in reactive multiphase flows, several recent results obtained by the authors are summed up with a bridge to experimental results. It includes droplet dispersion, laminar spray flame instability, spray combustion regimes or acoustic modulation effect on a two-phase flow Bunsen burner. As a perspective, two-phase flow DNS auto-ignition is considered thanks to a skeletal mechanism for the n-heptane chemistry involving 29 species and 52 reactions. Results highlight evaporating droplet effects on the auto-ignition process that is generally dramatically modified by spray distribution resulting from the turbulent fluid motion. This paper shows that DNS is a powerful tool to understand the intricate coupling between the evaporating spray, the turbulent fluid motion and the detailed chemistry, inseparable in the experimental context.

Propagation of Rich Premixed Polydisperse Spray Flames

A new numerical study of rich laminar polydisperse spray flame propagation in a two-dimensional channel is presented, within the framework of a thermal-diffusional model. Two different polydisperse sprays having the same Sauter mean diameter were considered. Attention is focused on the development of spray induced instabilities and the way they are sensitive to the initial droplet size distribution. Previous work in this field was based on linear stability analyses. The current work allows nonlinear stability aspects to be captured. It was found that both cellular and oscillating instabilities can develop, depending on the evaporation coefficient of the liquid fuel. However, the transitions from planar propagation to cellular propagation and from the latter to oscillating propagation occur at different critical values of the evaporation coefficient depending on what the initial droplet size distribution actually is. Therefore, use of the Sauter mean diameter to represent a polydisperse spray can be misleading in the current context and the real size distribution must be considered.

Physical characterization of laminar spray flames in the pressure range 0.1–0.9 MPa

An experimental study is reported on the physical characterization of the structure of ethanol/argon/oxygen coflow laminar spray diffusion flames in the pressure range 0.1–0.9 MPa. Diagnostic techniques include phase Doppler anemometry to measure the droplet size distribution and the axial and radial velocity components of the droplets. The gas-phase velocity is determined using measurements from the smallest (low Stokes number) droplets and is corrected for thermophoretic effects. Temperature information is obtained using thin-film pyrometry combined with an infrared camera. All flames present a cold inner core, in which little or no vaporization takes place, surrounded by an envelope flame buried in a thermal boundary layer, where most of the droplet evaporation occurs. The thickness of this thermal boundary layer scales with the inverse of the Peclet number. Especially near the base of the flame, photographic evidence of streaks, which in some case even reveal the presence of soot, suggests that some droplets survive the common envelope flame and burn isolated on the oxidizer side in a mixed regime of internal/external group combustion. The reconstruction of the entire droplet vaporization history confirms this evidence quantitatively. A criterion for droplet survival beyond the envelope flame based on the critical value of a suitably defined vaporization Damköhler number is proposed. The scaling and self-similar behavior of the investigated flames suggest that a mixed regime is established, with a momentum-controlled cold core and a buoyancy-controlled high-temperature boundary layer, the thickness of which varies significantly with pressure, as expected from Peclet number scaling. The growth of this layer and the thickness of the vaporization region are reduced at pressures above atmospheric because of density effects on thermal diffusivity. Some aspects of the design of the combustion chamber and of the atomizer system are discussed in detail since they are critical to the suppression of instabilities and to the establishment of a well-defined high-pressure quasi-steady laminar environment.

Analysis of weakly turbulent dilute-spray flames and spray combustion regimes

Spray combustion is analysed using a full simulation of the continuous gaseous carrier phase, while dilute-spray modelling is adopted for the discrete phase. The direct numerical simulation of the flow is performed in an Eulerian context and a Lagrangian description is used for the spray. The numerous physical parameters controlling spray flames are first studied to construct two synthetic model problems of spray combustion: a laminar spray flame that propagates freely over a train of droplets and a weakly turbulent spray-jet with coflowing preheated air. It is observed that the flame structures can be classified with respect to three dimensionless quantities, which characterize the fuel/air equivalence ratio within the core of the spray-jet, the ratio between the mean distance between the droplets and the flame thickness, and the ratio between an evaporation time and a flame time. A large variety of reaction zone topologies is found when varying those parameters, and they are scrutinized by distinguishing between premixed and diffusion combustion regimes. Partially premixed combustion is observed in most of the spray-jet flames and the spray parameters that make the flame transition from non-premixed to premixed combustion are determined. A combustion diagram for dilute-spray combustion is then proposed from the identification of those various regimes.

Time-Averaged Observation of Laminar Counter-Flow Premixed Spray Flames and Some Discussions.

A series of our counter-flow premixed spary flame experiments were reviewed, ranging from flames dominated by a rather intense turbulence to sufficiently laminar ones. It was found that highly turbulent flames were dominated by a number of large-scaled droplet clusters burning in the droplet group combustion mode, whereas sufficiently laminar flames hardly showed the group combustion characteristics. Then the detailed structure of a laminar spray flame was observed in two-dimensional counter-flow configuration, where a spray flame having a flame front stable enough except of dynamic irregularities of 1 mm or so peculiar to two-phase flames. The velocity was determined for various sizes of droplets across the flame front by phase-Doppler anemometry (PDA), and the laminar burning velocity was estimated, from the minimum value in velocity of minute droplets around the flame front, to be ca. 0.34 m/s. The Lagrangian histories of evaporating or burning droplets were tracked by the particle-tracking velocimetry (PTV) across the flame front.

Structure and extinction of laminar ethanol-air spray flames

A numerical study of structures of laminar ethanol-air spray flames in the counterflow configuration is presented. The model describes a thin spray where the gas phase is formulated in Eulerian coordinates and the spray is described through Lagrangian equations. The study presents structures and extinction conditions for atmospheric spray flames, and the focus is the fuel ethanol that has not been studied so far in the literature. Moreover, both mono- and bidisperse sprays are studied for strain rates that range from 55 s−1 up to extinction. The different droplet size groups that are considered in the discrete droplet model are treated completely separately from each other so that their individual history is represented by the model. The emphasis is the possible representation of bidisperse sprays through a Sauter mean radius for the above specified conditions which has a specific significance to the consideration of laminar spray flame structures in turbulent spray flame computations where a flamelet model may be employed. The results show that ethanol-air spray flames are more stable than methanol-air spray flames and their extinction flame temperature is above that of methanol-air spray flames all the way up to extinction. The extinction conditions of the bidisperse and the monodisperse ethanol-air spray flames with a Sauter mean radius differ considerably. This is attributable to the occurrence of droplet reversal and oscillation. The study also shows the effect of the spray dispersity on the species profiles.

Heat Transfer in Fuel Flow as a Source of Laminar Flame Oscillation

The oscillatory motion encountered in a laminar spray flame as described in some experimental works is different from any reported previously in the literature. In the published works this phenomenon is associated with heat loss, droplet loading or gravity. In this paper a simplistic physical model for oscillations in the position of flame zone relative to the fuel nozzle is described. It is shown that this model is sufficient to demonstrate that the flame position instability may occurs due to some characteristics of the fuel stream warming at the entrance to combustion zone. An additional important advantage of this model is the easy possibility to graphical visualizes the effect of all of the flow properties and thermodynamic characteristic incorporated in the model.