Julien Reveillon

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.

Spray vaporization in nonpremixed turbulent combustion modeling: a single droplet model

Abstract The injection of liquid fuel is a common procedure in turbulent combustion devices operating in the nonpremixed regime. Various numerical models may be found in the literature to calculate such turbulent flames, using either Reynolds averaged Navier-Stokes techniques (RANS) or large eddy simulation (LES). The typical inputs of nonpremixed turbulent combustion modeling are the mean and the fluctuations of the mixture fraction. In computational fluid dynamics codes, the mean source of mixture fraction may be provided by Euler-Lagrange spray modeling. However, the sources of fluctuations of mixture fraction due to vaporization require more closures. Direct numerical simulation (DNS) provides a way of estimating these sources and, using DNS of droplets evaporating in a turbulent flow, it is described how they play an important role in the time evolution of fuel/air mixing in a dilute spray. The statistical properties of the spray and of the scalar field are analyzed to propose a single droplet model (SDM) to evaluate these sources. SDM calculates mean values of the Eulerian source of fuel conditioned on the mixture fraction.

Effects of the preferential segregation of droplets on evaporation and turbulent mixing

Droplet segregation in isotropic homogeneous turbulence is analysed using a spectral direct numerical simulation solver to describe the evolution of the turbulent carrier phase, whose characteristic properties remain statistically stationary due to a semi-deterministic forcing scheme. Lagrangian dilute spray modelling is employed to describe the discrete-phase evolution. The liquid density is distributed on an Eulerian mesh to analyse the evolution of the spray and its spatial distribution. This gives results in accordance with classical methods for droplet segregation. It also allows a deeper analysis of the spray evolution. In particular, droplet segregation and vapour mass fraction may be analysed jointly. First, droplet segregation phenomena are studied through the analysis of the formation and the geometry of the droplet clusters. Then, the effects of segregation on spray evaporation are investigated from both the dispersed and carrier phase points of view. At equilibrium, droplet dynamics leads to different segregation levels that are associated with characteristic Stokes numbers. It appears that the evaporation process evolves in three different stages in time: single-droplet mode in the early stage, cluster mode in the intermediate stage and a gaseous mode in the late stage. Segregation levels strongly affect the evolution of the mean vapour mixture fraction during the second stage, while the corresponding standard deviation is affected for longer, up to the third stage in our simulations. However, from the evolution of the integral scale and the shape of the energy spectrum, it appears that turbulent mixing eliminates the segregation effects, apart from the first evaporation stage when the droplet segregation determines the vapour distribution.

Eulerian analysis of the dispersion of evaporating polydispersed sprays in a statistically stationary turbulent flow

Direct numerical simulations of a statistically stationary spatially decaying turbulence were performed to study the dispersion of evaporating droplets in a non-homogeneous flow. High-order finite-difference methods have been used to solve the compressible gas equations along with a Lagrangian solver for the polydispersed spray to produce a reliable reference solution. The key and novel point in this paper is to provide a comprehensive analysis from an Eulerian point of view of the turbulent dispersion of evaporating polydispersed sprays with various mean Stokes numbers. For this purpose, our work focuses on simplified evaporation laws and one-way coupling to isolate some of the key physical phenomena to be captured by an Eulerian description, which would be hidden by the intricacy of the numerous couplings occurring with more complex models. A special emphasis is laid on the polydispersed character of the spray in the particular chosen configuration. It is shown that the dynamics of the droplets at the g...

Direct Numerical Simulation of Sprays: Turbulent Dispersion, Evaporation and Combustion

Numerical procedures to describe the dispersion, evaporation and combustion of a polydisperse liquid fuel in a turbulent oxidizer are presented. Direct Numerical Simulation (DNS) allows one to describe accurately the evolution of the fully compressible gas-phase coupled with a Lagrangian description in order to describe two-phase flows. Standard coupling is used for the Eulerian/Lagrangian system while some practical issues related to the reactive source terms are addressed by suggesting a fast single-step Arrhenius law allowing one to capture the main fundamental properties of the flame whatever the local equivalence ratio. Then some basic procedures to describe spray preferential segregation in a turbulent reactor are described. Eventually spray combustion is addressed by first demonstrating the complex interactions caused by the presence of an evaporating liquid phase: definition of various equivalence ratios, apparition of flame instabilities for a unit Lewis number, etc. Then a history of the development of the existing spray combustion diagrams is presented to display the possible flame structures and combustion regimes encountered in spray combustion.

A LES Simulation of Atomisation

Large Eddy Simulation has been used with a lot of success for single phase flows. Its extension to multiphase flows is underway. As far as liquid-gas flows are concerned, two limit cases have been addressed: In one hand, if the liquid phase corresponds to a set of droplets with diameters smaller than the LES filter size, a subgrid spray is described. In the other hand, if the characteristic sizes of the surface wrinkles are greater than the LES filters size, the surface is resolved and LES models concern the velocity field. An example of the first approach is a dilute spray and an example of the second approach is waves at ocean surface. The issue with LES simulation of atomization is that a surface resolved LES is expected close to the injector together with a subgrid sprays LES far from the injector when the spray is finally formed. If only a resolved LES is used, the drop diameter cannot be smaller than the LES filter size. It follows that smaller diameters cannot be described and the breakup process is blocked numerically at a size related to the filter size. At the contrary if a subgrid spray LES is used a model is necessarily used that is accurate only for given type of injector. The present work addresses the problem induced by this transition between resolved and unresolved spray. A first approach is proposed that is able to reach both limits. The transition is performed using a filtered surface density equation to avoid the assumption that ligaments, sheets and other surface topologies becomes spherical droplet abruptly at the subgrid level. Results will be shown to demonstrate the ability of the model to recover the essential characteristics of a spray in a Diesel like application.Copyright

Subgrid Liquid Flux and interface modelling for LES of Atomization

Traditional Discrete Particle Methods (DPM) such as the Euler-Lagrange approaches for modelling atomization, even if widely used in technical literature, are not suitable in the near injector region. Indeed, the first step of atomization process is to separate the continuous liquid phase in a set of individual liquid parcels, the so-called primary break-up. Describing two-phase flow by DPM is to define a carrier phase and a discrete phase, hence they cannot be used for primary breakup. On the other hand, full scale simulations (direct simulation of the dynamic DNS, and interface capturing method ICM) are powerful numerical tools to study atomization, however, computational costs limit their application to academic cases for understanding and complementing partial experimental data. In an industrial environment, models that are computationally cheap and still accurate enough are required to meet new challenges of fuel consumption and pollutant reduction. Application of DNS-ICM methods without fairly enough resolution to solve all length scales are currently used for industrial purpose. Nevertheless, effects of unresolved scales are generally cast aside. The Euler-Lagrange Spray Atomization model family (namely, ELSA, also call, Σ−𝑌 or Ω−𝑌) developed by Vallet and Borghi pioneering work [1], and [2], at the contrary aims to model those unresolved terms. This approach is actually complementary to DNS-ICM method since the importance of the unresolved term depends directly on mesh resolution. For full interface resolution the unclosed terms are negligible, except in the far-field spray when the unresolved terms become dominant. Depending on the complexity of the flow and the available computational resources, a Large Eddy Simulation (LES) formalism could be employed as modelling approach. This work focus on the two main terms that drive these different modelling approaches namely the subgrid turbulent liquid flux and the resolved interface. Thanks to the open source library OpenFoam® this work is an attempt to review and to release an adapted modelling strategy depending on the available mesh resolution. For validation, these solvers are tested against realistic experimental data to see the overall effect of each model proposal. DOI: http://dx.doi.org/10.4995/ILASS2017.2017.4694

DNS Study of Collision and Coalescence Over a Wide Range of Volume Fraction

Among the different processes that play a role during the atomization process, collisions are addressed in this work. Collisions can be very important in dense two-phase flows. Recently, the Eulerian Lagrangian Spray Atomization (ELSA) model has been developed. It represents the atomization by taking into account the dense zone of the spray. Thus in this context, collisions modeling are of the utmost importance. In this model results of collisions are controlled by the value of an equilibrium Weber number, We*. It is defined as the ratio between the kinetic energy to the surface energy. Such a value of We* has been studied in the past using Lagrangian collision models with various complexity. These models are based on analysis of collisions between droplets that have surface at rest. This ideal situation can be obtained only if droplet agitation created during a collision has enough time to vanish before the next collision. For a spray, this requirement is not always fulfill depending for instance on the mean liquid volume fraction. If there is not enough time, collisions will occur between agitated droplets changing the issue of the collision with respect to the ideal case. To study this effect, a DNS simulation with a stationary turbulence levels has been conducted for different liquid volume fractions in a cubic box with periodic condition in all directions. For liquid volume fraction close to zero the spray is diluted and collisions between spherical droplets can be identified. For a volume fraction close to one, collisions between bubbles are found. For a middle value of the volume fraction no discrete phase can be observed, instead a strong interaction between both liquid and gas phases is taking place. In all this case the equilibrium value of the Weber number We* can be determined. First propositions to determine We* as a function of the kinetic energy, density ratio, surface tension coefficient and the volume fraction will be proposed.Copyright

DNS and Modeling of Spray Turbulent Mixing

The injection of liquid fuel is a common procedure in turbulent combustion devices operating in the non-premixed regime. In these systems, dispersion, vaporization of the fuel droplets and turbulent combustion strongly interact. The understanding and modeling of these complex phenomena are important issues when optimizing combustion processes, to improve the economical and ecological output of the device.

Quelques aspects de la modélisation numérique appliquée à la combustion turbulente

ABSTRACT Various developments in turbulent combustion modelling are presented. First direct numerical simulations are used to study triples flames properties, and the possible role that they play in auto ignition of nonpremixed mixtures. Then, a bridge is built between probality density function and flame surface approaches. In particular, the proposed formalism, which is based on the definition of a surface density function, allows for studying the properties of isoconcentration surfaces from a purely statiscal point of view. Finally, a new technique is proposed for the modelling of turbulent combustion. It uses recent developments in the field of dynamic large eddy simulation techniques coupled with probability function approach.