Laminar Spray Research Articles

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.

Vaporization Damköhler Number and Enrichment Effects in Spray Diffusion Flames in an Oscillating Flow Field

The sensitivity to the vaporization Damköhler number of the behavior of a coflow laminar spray diffusion flame in an oscillating flow field is investigated. Droplet grouping induced by the host gas flow oscillations is accounted for, and its effect is described through a specially constructed model for the vaporization Damköhler number that responds to the proximity of the droplets as they cluster. A formal analytical solution is developed, and the dynamics of the spray flame front shapes and thermal fields are deduced. Computed results demonstrate how strongly the vaporization Damköhler number impacts on the type of primary homogeneous flame formed and on the possible existence of multiple flame sheets as the flow field oscillates. In addition, isolated regions of high fuel vapor concentrations are produced by fuel droplet enrichment. The presence of resulting parallel fluctuating thermal fields indicates a potential impact on undesirable pollutants production.

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.

Facets of the Propagation of Lean Laminar Premixed Polydisperse Spray Flames

A new numerical study of the multifaceted unsteady propagation of lean laminar premixed polydisperse spray flames in a two-dimensional channel is presented. The impact of the spray's initial droplet size distribution on the flame propagation is investigated for two very different initial size distributions having the same Sauter mean diameter. It is found that the initial droplet size distribution can exert a strong influence on the average flame speed, spatial instabilities, and timewise oscillations and conditions for their onset. In addition, even for the same size distribution, the actual mode of flame propagation is influenced by the initial perturbation and, here, was shown to be attracted to one of two unsteady limiting modes.

Spray flame dynamics with oscillating flow and droplet grouping

The co-flow laminar spray diffusion flame in an oscillating flow field is investigated. Mild slip is permitted between the droplets and their host surroundings and droplet grouping resulting from the host flow oscillations is accounted for. The spray is modelled using the sectional approach and a perturbation analysis using a small sectional Stokes number is utilised for solving the liquid phase governing equations. The effect of droplet grouping is described through a specially constructed model for the vaporisation Damkohler number. The large chemical Damkohler number assumption is adopted and a formal analytical solution is developed for Schwab-Zeldovitch parameters through which the dynamics of the spray flame front shapes and thermal fields are deduced. Computed results based on the solutions demonstrate how the phenomenon of droplet grouping can lead to the existence of multiple flame sheets as a result of the dynamic change in the type of the main homogeneous flame from under- to over-ventilated as the flow field oscillates. Concomitant fluctuating thermal fields are also shown to be present indicating a potential impact on undesirable pollutants production.

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.

Three-Dimensional Flamelet Simulation of Polycyclic Aromatic Hydrocarbons Formation in DI-Diesel Engines

In this study, numerical simulations for an n-heptane fueled Chaochai 6102bzl direct injection diesel engine are performed in order to predict the chemical details of the combustion process and resulting polycyclic aromatic hydrocarbons (such as benzene, naphthalene, phenanthrene and pyrene) formation. The diesel geometry and reduced kinetic mechanism of n-heptane oxidation, which includes only 86 reactions and 57 species, have been developed and incorporated into the computational fluid dynamics code, FLUENT. The diesel unsteady laminar flamelet model, turbulence model and spray model have been employed in the numerical simulations. The numerical simulation results showed that the polycyclic aromatic hydrocarbons were firstly increased with the increase of diesel crank angel and then decreased, which was mostly located at the bottom of diesel combustion chamber wall.

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.

Laminar premixed spray flame analysis with thermally sensitive intermediate kinetics

A new thermo-diffusive analysis of one-dimensional laminar lean or rich off-stoichiometric premixed spray flames has been performed using a chain branching/chain breaking chemical kinetic scheme and under the assumption that the fuel droplets evaporate in a sharp front. The sensitivity of the flame structure, speed and the location of the evaporation front to the initial droplet load have been demonstrated. A linear stability analysis reveals the way in which the spray's presence modifies the neutral stability curves.

Analysis of a spray diffusion flame and its extinction in a shear layer

A new semi-analytical solution for a laminar spray diffusion flame in the shear layer between fuel and oxidant streams is developed. The Stokes number is identified as a small spray droplet-related parameter to be used in a perturbation analysis of the liquid phase governing equations. Appropriate specification of an additional parameter ensures that similarity is achieved so that the concentration field of the liquid in the spray can be readily found. The coupled gas-phase equations are treated using the usual inverse of the large Zeldovitch number for the asymptotic analysis. Numerical results demonstrate how the distribution of the liquid phase in the developing shear layer between two unidirectional gas streams flowing over one another with (the possibility of) dissimilar velocities in their respective free-streams influences the flame shape, location, fuel vapour and temperature fields. An extinction analysis enables a parametric mapping of conditions for extinguishment of the spray diffusion flames to be drawn.

Latest technologies in plate cooling and their benefits in plate production

With extremely high cooling rates, a combined laminar jet and spray nozzle design cooling facility is the most advanced tool for the evolution of mechanical properties of high-strength steels. The main metallurgical aspects of Accelerated Cooling (ACC) and Direct Quenching (DQ) are presented. Typical plate applications are brought up. Design, engineering and operational aspects of modern cooling facilities are explained. Of particular importance for a homogenous cooling process is a powerful process control featuring advanced process models.

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.

Accurate treatment of size distribution effects in polydisperse spray diffusion flames: multi-fluid modelling, computations and experiments

An Eulerian multi-fluid model is validated by comparison with experimental measurements in the test case of laminar spray counterflow diffusion flames. Special attention is devoted, both from the modelling and experimental point of view, to the treatment of the droplet distribution tail, characterized by the rare occurrence of relatively large droplets carrying a non-negligible amount of mass. The Eulerian multi-fluid approach is shown to capture the dynamics, evaporation and heating of the droplets with a limited number of sections and, thus, at a modest cost. This simplification will be essential for the use of multi-fluid methods in multi-dimensional problems.

Curvature Effects in Burke-Schumann Spray Flame Extinction

The effect of curvature on the extinction at the tip of laminar spray diffusion flames in coflow is analyzed. In particular, we look at the way spray-related parameters alter the tip curvature and what impact this influence has on flame-tip extinction. The spray is described using the sectional approach and is assumed to be quasi-monodisperse. The analysis reveals that the flame curvature is strongly changed by the droplet loading and the vaporization Damkohler parameter. It is found that the tip curvature decreases rather sharply for a certain range of values of the vaporization Damkohler number. The extent of the decrease increases with the initial droplet load. Under these circumstances flame extinction is inhibited. However, beyond this range flame extinction can actually be enhanced because of the presence of the droplets. This behavior, with its linkage to the flame-tip radius of curvature, is related to the manner in which the spray parameters dictate the shape and temperature of these spray flames.

Soret effects in laminar counterflow spray diffusion flames

The impact of vapour-phase Ludwig–Soret effects on the structure of steady laminar counterflow spray diffusion flames is investigated numerically using complex chemistry and multicomponent molecular transport laws of Chapman-Enskog kinetic theory. These Ludwig–Soret effects, which tend to drive light molecular species towards hot regions and heavy molecular species towards cold regions of the flow, not only influence streamwise diffusion within the planar diffusion flame structure, but also the structure of the fuel vapour diffusion boundary layer near each suspended fuel droplet.Our numerical results provide a quantitative assessment of the impact of Ludwig–Soret effects on the flame structures and on droplet boundary layer structures for typical experimental flames. Computationally, Ludwig–Soret effects are found to vary from being negligible for the experiments considered with nitrogen as the diluent to decreasing the peak temperature by nearly 125 K for the experiment considered with helium as the diluent. In addition, Ludwig–Soret effects do not significantly influence vaporization rates in the zones where the vaporization process is fully developed. Only the droplet boundary layer structure is modified with higher surface fuel vapour concentrations approximately compensating reduced fuel diffusion around the droplet.

A general superscalar for the combustion of liquid fuels

A new integral solution is found to apply to liquid fuel combustion for a wide range of configurations; isolated droplet burning, droplet array burning, liquid film burning or liquid stream burning; steady, quasisteady, or (in some cases) unsteady gas fields: transient or steady liquid thermal conditions; and stagnant, forced-convective, or natural-convective gas fields. The major constraints are one-step chemical kinetics and unitary Lewis number. In particular, the superscalar S is contrived and shown after integration of the governing equations to be uniform (but possibly time varying) over the gas field. S = h + v ( Q − L e f f 1 − Y F s ) Y O + L e f f 1 − Y F s Y F where h, YO, and YF are the specific enthalpy, oxidizer mass fraction, and fuel vapor mass fraction, respectively, with the subscript s indicating a value at the liquid surface; Q, Leff, and v are the fuel heating value, energy flux per unit mass flux transferred to the liquid surface, and the fuel-to-air mass stoichiometric ratio, respectively. The value of S can be determined without a need to engage the continuity and momentum equations. Similarly, the limiting flame temperature (for infinite kinetic rate) Tf and the liquid surface temperature T, will depend only on ambient conditions, fuel properties, and Leff. That is, S, Tf and Ts are shown to depend on the hydrodynamics, transport phenomena, and geometrical configurations only through Leff and are readily calculable. Variations in the form of the superscalar are discussed for the cases of multicomponent liquid fuels and vaporization without burning. Comparisons of these results with a few previous results in certain classical configurations validate the new findings. Extensions to other configurations are discussed. The use of the superscalar to validate subgrid modeling for laminar and turbulent spray combustion is examined.

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 of laminar coflow spray flames at different pressures

Experiments were conducted on laminar spray diffusion flames of ethanol/argon burning in oxygen at pressures of 1 and 3 atm. The flames were physically characterized by measuring droplet velocities and sizes by phase-Doppler anemometry, and gas temperature by thin-filament pyrometry and thermocouples. The flames exhibited a cold core, where little vaporization occurred, surrounded by a primary diffusion flame enveloping most of the droplet cloud. Their structure was strongly influenced by the Peclet number for heat transfer that, by regulating the heat penetrating into the core, and, consequently., the growth of the thermal boundary layer, affected the droplet vaporization history. At pressures above the atmospheric, because of density effects on thermal diffusivity, the boundary layer growth above the burner was reduced and so was the vaporization region. Complete evaporation of the droplets before they reached the primary diffusion flame was ensured if a suitably defined Damkohler number of evaporation, Da v , was smaller than 1. Conversely, if Da v >1, droplets penetrated the flame, ignited by a flame transfer process that was captured photographically, and burned isolated on the oxidizer side. These conditions of internal or partial group combustion prevailed in the lower part of the flame. Farther up in the flame, this combustion regime progressively shifted toward that of external group combustion, with fewer and fewer direct droplet/flame interactions.