LES of atomizing spray with stochastic modeling of secondary breakup (original) (raw)
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Large eddy simulation of spray atomization with stochastic modeling of breakup
Physics of Fluids, 2010
A stochastic subgrid model for the droplet breakup in large eddy simulation is developed. An Eulerian description of the continuous phase is adopted and fully coupled with a Lagrangian definition of the dispersed phase. A stochastic model is incorporated into the equation describing the evolution of the spray probability density function in order to simulate the atomization of sprays in the framework of uncorrelated breakup events. The results of the simulations are compared with experimental data from a diesel injector and in spray in cross flows.
Large Eddy Simulation of Evaporating Spray with a Stochastic Breakup Model
Large Eddy Simulations of atomization and evaporation of liquid fuel sprays in diesel engine conditions are performed with stochastic breakup and non-equilibrium droplet heat and mass transfer models. The size and number density of the droplets generated by the breakup model are assumed to be governed by a Fokker-Planck equation, describing the evolution of the PDF of droplet radii. The fragmentation intensity spectrum is considered to be Gaussian and the scale of Lagrangian relative velocity fluctuations is included in the breakup frequency calculations. The aerodynamic interactions of droplets in the dense part of the spray are modeled by correcting the relative velocity of droplets in the wake of other droplets. The stochastic breakup model is employed together with the wake interaction model for simulations of non-evaporating and evaporating sprays in various gas temperature and pressure conditions. The predicted results for physical spray parameters, such as the spray penetration length are found to be in good agreement with the available experimental data.
International Conference on Liquid Atomization and Spray Systems (ICLASS)
At high Reynolds and Weber numbers typical of diesel-like sprays, the finest turbulent structures are highly intermittent in nature, resulting in intense fluctuations in gas phase velocities and can significantly contribute to the atomization process. In order to account for their influence on breakup of spray droplets, we introduce a new stochastic breakup model to be used in conjunction with large eddy simulation (LES) for the gaseous flow. The model is based on a stochastic parent-to-child relaxation of droplets, whose parameters are linked to the viscous dissipation rate on residual scales "seen" by a droplet along its trajectory. In order to introduce the intermittency effects on the droplet breakup, this dissipation rate is simulated stochastically, in the framework of log-normal process. The non-reacting "Spray-A" experiment from Engine Combustion Network (ECN) is used to assess the performance of the new stochastic breakup model in comparison to the standard hybrid KH-RT breakup model in terms of evolution of liquid penetration length and droplet size statistics. The results clearly show that in comparison to the hybrid KH-RT model the stochastic breakup model gives a more accurate prediction of different parameters with relatively less sensitivity to the grid resolution.
Secondary Atomisation: Simulation of Droplet Break-Up in Disturbed Flow Fields
2008
It is well known that both combustion e±ciency in diesel engines, and the the quenching of ¯res, is conditioned by the surface to volume ratio of fuel/water droplets. This requires a deeper understanding of the droplet break- up process within liquid sprays. The break-up of individual droplets follows well known behaviour although how nearby droplets in the spray in°uence this process is not well understood. By numerically simulating the break-up behaviour of two equally sized droplets in two distinct geometrical con¯gurations it is shown that the break up of each droplet is strongly in°uenced by the presence of the other.
Multi-scale spray atomization model
2019
The purpose of the present article is to present a dynamic multi-scale approach for turbulent liquid jet atomization in dense flow (primary atomization), together with the possibility to recover Interface Capturing Method (ICM) / Direct Numerical Simulation (DNS) features for well resolved liquid-gas interface. A full ICM-DNS approach should give the best comparison with experimental data, but it is not industrially affordable for the time being, therefore models are mandatory. A numerical representation based on full ICM-DNS, for the initial destabilization of the complex turbulent liquid jet, going up to the spray formation, for which well established numerical models can be used, is appealing but has not yet been applied. Indeed such an approach requires the ICM-DNS to be applied up to the formation of each individual droplet. Hence, in many situation models have to be applied to the dense, unresolved and turbulent liquid-gas flow. To achieve this goal, the most important unresol...
2014
An experimental investigation of secondary atomization of spray in plain-jet airblast atomizers with circular array of six liquid jets, is studied. Cross flow of air with controlled velocity and pressure is applied to transverse jets. The liquid jets (Re=12000) are injected into air flow (We g of 1250-3000) analyzed experimentally for different injection diameters of 0.8-1.6 mm. Particle size distribution measurement is carried out by Malvern Mastersizer X in fully atomized region of the atomizer. The droplet size distribution in secondary or final atomization stage is also modelled by Finite stochastic Breakup model, which requires 4 parameters to be defined i.e. initial droplet diameter, maximum stable droplet size, minimum mass ratio and droplet breakup probability. The modeling approximation is in very good agreement with experimental result. This remarkable consistency between model and experiment is quite useful in terms of optimal atomization performance by considering the dy...
An Evaluation of Atomization Models for Dense Sprays
Calculations of a transient atomization process are presented, which simulates fuel injection of sprays in gasoline direct injection engines. Only non-reacting sprays are considered with the focus on the atomization process. The FIRE code, developed by AVL, is used as the platform to test three different atomization models: (i) Taylor Analogy Breakup (TAB) model; (ii) surface wave instability (WAVE) model; and the more recent (iii) FIPA (Fractionnement Induit Par Acceleration) model. Comparisons of calculations with experimental data reveal significant discrepancies regardless of the atomization model used. It is acknowledged that, in this study, only the standard model constants are adopted and that may be further optimised to improve the calculations. However, the fact remains that all the atomization models start with an initial distribution of spherical droplets at the injector tip. An assumption that is not supported by recent measurements which show that fluid elements rather than spherical droplets dominate this early zone.
A computationally efficient spray model is presented for the simulation of transient vaporizing engine sprays. It is applied to simulate high-pressure fuel injections in a constant volume chamber and in mixture preparation experiments in a light-duty internal combustion engine. The model is based on the Lagrangian-Particle/Eulerian-Fluid approach, and an improved blob injection model is used that removes numerical dependency on the injected number of computational parcels. Atomization is modeled with the hybrid Kelvin-Helmholtz/Rayleigh-Taylor scheme, in combination with a drop drag model that includes Mach number and Knudsen number effects. A computationally efficient drop collision scheme is presented, tailored for large numbers of parcels, using a deterministic collision impact definition and kd-tree data search structure to perform radius-of-influence based, grid-independent collision probability estimations. A near-nozzle sub-grid scale flow-field representation is introduced to reduce numerical grid dependency, which uses a turbulent transient gas-jet model with a Stokes-Strouhal analogy assumption. An implicit coupling method was developed for the Arbitrary Lagrangian-Eulerian (ALE) turbulent flow solver. A multi-objective genetic algorithm was used to study the interactions of the various model constants, and to provide an optimal calibration. The optimal set showed similar values of the primary breakup constants as values used in the literature. However, different values were seen for the gas-jet model constants for accurate simulations of the initial spray transient. The results show that there is a direct correlation between the predicted initial liquid-phase transient and the global gas-phase jet penetration. Model validation was also performed in engine simulations with the same set of constants. The model captured mixture preparation well in all cases, proving its suitability for simulations of transient spray injection in engines.
SAE Technical Paper Series, 2011
During the last fifteen years Computational Fluid Dynamics (CFD) has become one of the most important tools to both understand and improve the Diesel spray development in Internal Combustion Engine (ICE). Most of the approaches and models used pure Eulerian or Lagrangian descriptions to simulate the spray behavior. However, each one of them has both advantages and disadvantages in different regions of the spray, it can be the dense zone or the downstream dilute zone. One of the most promising techniques, which has been in development since ten years ago, is the Eulerian-Lagrangian Spray Atomization (ELSA) model. This is an integrated model for capturing the whole spray evolution, including primary break-up and secondary atomization. In this paper, the ELSA numerical modeling of Diesel sprays implementation in Star-CD (2010) is studied, and simulated in comparison with the Diesel spray which has been experimentally studied in our institute, CMT-Motores Térmicos. Since many of the most important characteristics of the spray development, as the penetration or the axial velocity, can be captured using 2D simulations, in this preliminary validation of ELSA model only two-dimensional simulations have been performed. Moreover, the main objective of the paper is to: firstly, obtain mesh independency for further analysis and secondly, improve the classic k-ε RANS model for ELSA model. Apart from this, several characteristics of the spray as can be the droplet formation of the liquid penetration are also showed.