The internal structure of igniting turbulent sprays as revealed by complex chemistry DNS

Neophytou, A.; Mastorakos, Epaminondas; Cant, RS

doi:10.1016/j.combustflame.2011.08.024

Cited by 58 publications

(53 citation statements)

References 63 publications

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“…All simulations have been carried out until t final = max(3t turb , 4t chem ), where t turb =L 11 /u is the initial turbulent eddy turnover time and t chem =D 0 /S 2 b( g =1) the chemical timescale. This simulation time is either comparable to or greater than the simulation duration used in a number of recent DNS analyses [22,24,25,27,[29][30][31][44][45][46][47], which significantly contributed to the fundamental understanding of turbulent combustion. It was shown in Ref.…”

Section: Numerical Implementationmentioning

confidence: 89%

“…Furthermore, Re d is the droplet Reynolds number, Sc is the Schmidt number, B d is the Spalding mass transfer number, Sh c is the corrected Sherwood number and Nu c is the corrected Nusselt number, which are defined as [21,33]: The droplets are coupled to the gaseous phase via additional source terms in the gaseous transport equations, which may be generically written as [18][19][20][21][22][23][24][25][26][27][28][29][30][31]:…”

Section: Mathematical Backgroundmentioning

confidence: 99%

“…In recent times, both single-step [18][19][20][21][22][23][24][25][26][27][28][29] or detailed [30,31] chemistry based DNS analyses have contributed significantly to the physical understanding and modelling of the combustion of turbulent droplet-laden mixtures. In the aforementioned DNS studies, the gaseous phase is treated in a typical Eulerian fashion and the droplets are considered as sub-grid particles which are tracked in a Lagrangian manner.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Flame Structure and Propagation in Turbulent Flame-Droplet Interaction: A Direct Numerical Simulation Analysis

Wacks

Chakraborty

2016

Flow Turbulence Combust

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Three-dimensional Direct Numerical Simulations (DNS) in canonical configuration have been employed to study the combustion of mono-disperse droplet-mist under turbulent flow conditions. A parametric study has been performed for a range of values of droplet equivalence ratio φ d , droplet diameter a d and root-mean-square value of turbulent velocity u . The fuel is supplied entirely in liquid phase such that the evaporation of the droplets gives rise to gaseous fuel which then facilitates flame propagation into the dropletmist. The combustion process in gaseous phase takes place predominantly in fuel-lean mode even for φ d > 1. The probability of finding fuel-lean mixture increases with increasing initial droplet diameter because of slower evaporation of larger droplets. The chemical reaction is found to take place under both premixed and non-premixed modes of combustion: the premixed mode ocurring mainly under fuel-lean conditions and the non-premixed mode under stoichiometric or fuel-rich conditions. The prevalence of premixed combustion was seen to decrease with increasing droplet size. Furthermore, droplet-fuelled turbulent flames have been found to be thicker than the corresponding turbulent stoichiometric premixed flames and this thickening increases with increasing droplet diameter. The flame thickening in droplet cases has been explained in terms of normal strain rate induced by fluid motion and due to flame normal propagation arising from different components of displacement speed. The statistical behaviours of the effective normal strain rate and flame stretching have been analysed in detail and detailed physical explanations have been provided for the observed behaviour. It has been found that the droplet cases show higher probability of finding positive effective normal strain rate (i.e. combined contribution of fluid motion and flame propagation), and negative values of stretch rate than in the stoichiometric premixed flame under similar flow conditions, which are responsible for higher flame thickness and smaller flame area generation in droplet cases.

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Section: Numerical Implementationmentioning

confidence: 89%

Section: Mathematical Backgroundmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Flame Structure and Propagation in Turbulent Flame-Droplet Interaction: A Direct Numerical Simulation Analysis

Wacks

Chakraborty

2016

Flow Turbulence Combust

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“…Thus, the entire body of experimental evidence suggests that flame propagation in turbulent droplet-laden mixtures is a complex physical process depending on the simultaneous interaction of evaporative heat and mass transfer, fluid dynamics and combustion thermo-chemistry and a thorough understanding of all these phenomena will be required in order to develop accurate models for the design and development of reliable, energy-efficient engines and combustors. Direct Numerical Simulations (DNS) have contributed significantly to the physical understanding and modelling of the combustion of turbulent droplet-laden mixtures in the recent past [14][15][16][17][18][19][20][21][22][23][24][25][26], where either a single-step [14][15][16][17][18][19][20][21][22][23][24] or a detailed [25,26] chemical reaction mechanism is employed. In the aforementioned DNS studies, the gaseous phase is treated in a typical Eulerian fashion and the droplets are considered as sub-grid particles which are tracked in a Lagrangian manner.…”

Section: Introductionmentioning

confidence: 99%

Statistical Analysis of Turbulent Flame-Droplet Interaction: A Direct Numerical Simulation Study

Wacks

Chakraborty

Mastorakos

2015

Flow Turbulence Combust

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Turbulent combustion of mono-disperse droplet-mist has been analysed based on three-dimensional Direct Numerical Simulations (DNS) in canonical configuration under decaying turbulence for a range of different values of droplet equivalence ratio (φ d ), droplet diameter (a d ) and root-mean-square value of turbulent velocity (u ). The fuel is supplied in liquid phase and the evaporation of droplets gives rise to gaseous fuel for the flame propagation into the droplet-mist. It has been found that initial droplet diameter, turbulence intensity and droplet equivalence ratio can have significant influences on the volume-integrated burning rate, flame surface area and burning rate per unit area. The droplets are found to evaporate predominantly in the preheat zone, but some droplets penetrate the flame front, reaching the burned gas side where they evaporate and some of the resulting fuel vapour diffuses back towards the flame front. The combustion process in gaseous phase takes place predominantly in fuel-lean mode even for φ d > 1. The probability of finding fuel-lean mixture increases with increasing initial droplet diameter because of slower evaporation of larger droplets and this predominantly fuel-lean mode of combustion exhibits the attributes of low Damköhler number combustion and gives rise to thickening of flame with increasing droplet diameter. The chemical reaction is found to take place under both premixed and non-premixed modes of combustion and the relative contribution of non-premixed combustion to overall heat release increases with increasing droplet size. Combust (2016) 96:573-607 of the flame propagation and mode of combustion have been analysed in detail and detailed physical explanations have been provided for the observed behaviour.

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“…Because of their key role in numerous technological applications, combustion and vaprization of fuel sprays have been the subject of many modelling efforts (see Faeth 1983;Sirignano 1983;Williams 1985;Annamalai & Ryan 1992;Li 1997;Aggarwal 1998;Crowe, Sommerfeld & Tsuji 1998 Knudsen & Pitsch 2010;Luo et al 2011;Shashank 2011;Neophytou, Mastorakos & Cant 2012), including disparate length and time scales associated with the chemistry and with the multiphase nature of the flow, which is highly turbulent in most applications.…”

Section: Introductionmentioning

confidence: 99%

Dynamics of thermal ignition of spray flames in mixing layers

et al. 2013

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Conditions are identified under which analyses of laminar mixing layers can shed light on aspects of turbulent spray combustion. With this in mind, laminar spray-combustion models are formulated for both non-premixed and partially premixed systems. The laminar mixing layer separating a hot-air stream from a monodisperse spray carried by either an inert gas or air is investigated numerically and analytically in an effort to increase understanding of the ignition process leading to stabilization of high-speed spray combustion. The problem is formulated in an Eulerian framework, with the conservation equations written in the boundary-layer approximation and with a one-step Arrhenius model adopted for the chemistry description. The numerical integrations unveil two different types of ignition behaviour depending on the fuel availability in the reaction kernel, which in turn depends on the rates of droplet vaporization and fuel-vapour diffusion. When sufficient fuel is available near the hot boundary, as occurs when the thermochemical properties of heptane are employed for the fuel in the integrations, combustion is established through a precipitous temperature increase at a well-defined thermal-runaway location, a phenomenon that is amenable to a theoretical analysis based on activation-energy asymptotics, presented here, following earlier ideas developed in describing unsteady gaseous ignition in mixing layers. By way of contrast, when the amount of fuel vapour reaching the hot boundary is small, as is observed in the computations employing the thermochemical properties of methanol, the incipient chemical reaction gives rise to a slowly developing lean deflagration that consumes the available fuel as it propagates across the mixing layer towards the spray. The flame structure that develops downstream from the ignition point depends on the fuel considered and also on the spray carrier gas, with fuel sprays carried by air displaying either a lean deflagration bounding a region of distributed reaction or a distinct double-flame structure with a rich premixed flame on the spray side and a diffusion flame on the air side. Results are calculated for the distributions of mixture fraction and scalar dissipation rate across the mixing layer that reveal complexities that serve to identify differences between spray-flamelet and gaseous-flamelet problems.

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The internal structure of igniting turbulent sprays as revealed by complex chemistry DNS

Cited by 58 publications

References 63 publications

Flame Structure and Propagation in Turbulent Flame-Droplet Interaction: A Direct Numerical Simulation Analysis

Flame Structure and Propagation in Turbulent Flame-Droplet Interaction: A Direct Numerical Simulation Analysis

Statistical Analysis of Turbulent Flame-Droplet Interaction: A Direct Numerical Simulation Study

Dynamics of thermal ignition of spray flames in mixing layers

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