Turbulent Spray Jet Research Articles

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.

Development of a CPU/GPU portable software library for Lagrangian–Eulerian simulations of liquid sprays

The Lagrangian–Eulerian method is widely used for simulations of fuel sprays in turbulent combustion because of the advantage of treating the spray droplets as discrete points. One challenge of the Lagrangian–Eulerian method is the intense computational requirement when tracking the large number of Lagrangian particles needed for high fidelity. We have developed a performance-portable library, Grit, to track the Lagrangian particles in parallel on central processing unit (CPU) and graphics processing unit (GPU) accelerated high performance computing (HPC) architectures. Grit is a C++ library which employs Message Passing Interface (MPI) for distributed memory parallelism and Kokkos programming model for on-node shared memory parallelism with performance portability across different architectures of GPUs and multi-core/manycore CPUs. The parallel algorithms, key parallel kernels, and their performances on the pre-exascale supercomputer, Summit, are presented. Grit is coupled with a direct numerical simulation (DNS) solver, S3D, for multiphase simulations. A conservative formulation has been developed and implemented in Grit for phase coupling with thermodynamic consistency. The formulation separates the conservation of mass, momentum and energy from the physical models to prevent accidental violation of conservation laws due to inconsistent models. The formulation also enforces consistent definitions of enthalpies of the fuel for both phases and the latent heat of evaporation. Simulations of turbulent particle-laden flow and the evaporation of dilute turbulent spray jet are performed to verify the software implementation, and to demonstrate the scalability of Grit for large-scale multiphase simulations.

LES Study of a Turbulent Spray Jet: Mesh Sensitivity, Mesh-Parcels Interaction and Injection Methodology

This study focuses on Large Eddy Simulations (LES) in the Eulerian-Lagrangian framework of turbulent spray jets. The choice of the numerical grid relates both to turbulence level description and parcels-grid cells interaction. The objective of the present work is to determine a case set-up capable of predicting flow fields of a turbulent n-heptane spray jet for which a set of experimental data in non-reactive conditions (isothermal) is available (Shum-Kivan Proc. Combust. Inst. 36(2), 2567–2575 (2017)). The study first focuses on the LES grid, based on simulation with only gas phase, and subsequently on the choice of the suitable number of parcels to describe the spray. The droplets behavior is analyzed using the particle Stokes number at different axial locations. Furthermore, a variation in the existing injection methodology, as available in OpenFOAMⓇ, reveals a beneficial impact on the prediction of the experimental data.

Statistics of progress variable and mixture fraction gradients in an open turbulent jet spray flame

The statistical behaviours of the variance, covariance and gradients of reaction progress variable c, and mixture fraction ξ have been analysed in an ethanol turbulent spray jet flame based on the results obtained from a carrier-phase three-dimensional Direct Numerical Simulation (DNS) dataset. It has been observed that the Favre Probability Density Functions (PDFs) of c and ξ extracted from the DNS data are correctly estimated by a standard β-PDF. Furthermore, the log-normal distribution has been found to accurately represent |∇ξ|, but small discrepancies were found at the head of the distribution. By contrast, |∇c| was found to be only marginally represented by a log-normal distribution with non-negligible departures from the DNS data at both the head and the tail of the distribution. It has also been found that ∇c and ∇ξ show predominant collinear alignment throughout the flame brush. Finally, the joint-PDF of |∇c| and |∇ξ| (i.e. P|∇c|,|∇ξ|) has been compared with the product of the marginal PDFs of |∇c| and |∇ξ| (i.e. P|∇c|×P|∇ξ|) extracted from DNS (i.e. assuming statistical independence) and a good agreement has been observed. The bivariate log-normal distributions with and without correlation have also been considered and fail to capture the DNS trend, which stems from the difficulties of approximating P|∇c| and P|∇ξ| with log-normal PDFs.

Comparison of droplet evaporation models for a turbulent, non-swirling jet flame with a polydisperse droplet distribution

The current investigation aims to present the numerical results of a turbulent spray jet flame in the framework of Large Eddy Simulation. The injection of n-heptane liquid fuel is achieved through the use of a pressure-swirl atomiser generating a lifted spray flame. The evolution of the sub-grid probability density function of relevant scalars is accounted for by the Eulerian stochastic field method. A non-reactive spray computation is conducted in a preliminary stage. The simulation allows for a stochastic breakup formulation in combination with a stochastic dispersion model to confirm their predictive capabilities. This is achieved as the liquid properties such as droplet diameter and velocity profiles are found to be in excellent agreement between the simulated results and measurements. The main target of the work is to assess the performance of the most widely accepted evaporation models in a turbulent droplet-laden flame. In this paper, a detailed comparison of the evaporation models is conducted thanks to a recent development in measurement techniques which allow to permit the spray temperature profiles. The time-averaged droplet temperature across the spray flame is therefore investigated in order to validate several droplet vaporisation models. All the models under consideration are found to capture the formation of a double reaction zone flame and the measured lift-off height is reproduced within a satisfactory level of accuracy. This good reproduction of the flame morphology additionally confirms the performance of the pdf approach as a closure for the unknown turbulence–chemistry interaction in this type of spray flames. However, the simulated wet-bulb temperature in the hot burnt gas between the two reaction zones in addition to the profile along the spray centreline show a large discrepancy when compared to measurements. This large difference thus requires further investigation both in the modelling of evaporation and in the accuracy of measurement techniques.

Turbulent spray flames of intermediate density: Stability and near-field structure

This paper presents an experimental study of turbulent spray jets and flames where the liquid loading is intermediate between the two extremes of dense and dilute; a region of significant importance in practical applications. A new piloted burner is introduced for this purpose featuring air-blast atomization, with liquid injection from a needle that can be translated within a co-flowing air stream. The stability characteristics are presented for three recess lengths using acetone as fuel. High-speed shadowgraph imaging is performed in the atomization region of jets and flames having different liquid loading and recess distances. Three types of fluid fragments are used to map the evolution of these sprays: droplets, ligaments and ‘irregular’ shapes. Statistics for each class of these fluid shapes are presented to map the boundary conditions at the jet exit plane and to track their evolution with downstream distance along the jet. These show clearly how ligaments and irregular shapes break down to supply more droplets further downstream. Measurements of liquid surface area show a peak in the mean area and its rms of fluctuations around two jet diameters from the point of injection marking the region of maximum atomization. It is also evident that the process of primary and secondary atomization is completed by about seven jet diameters from the liquid injection point. This burner provides a versatile platform for studying these flows and the resulting information is already proving to be extremely useful in the development and validation of related models for turbulent spray jets and flames.

Lagrangian conditional moment closure model with flame group interaction for lifted turbulent spray jet flames

A new Lagrangian conditional moment closure (CMC) model is developed for multiple Lagrangian groups of sequentially evaporating fuel in turbulent spray combustion. Flame group interaction is taken into account as premixed combustion by the eddy breakup (EBU) model in terms of the probability of finding flame groups in the burned and the unburned state. Evaporation source terms are included in the two phase conditional transport equations, although they turn out to have negligible influence on the mean temperature field during combustion. The Lagrangian CMC model is implemented in OpenFOAM [1] and validated for test cases in the Engine Combustion Network (ECN) [2,3]. Similar ignition delays and lift-off lengths are predicted by the incompletely stirred reactor (ISR) and the Eulerian CMC models due to relatively uniform conditional flame structure in the domain. The improved Lagrangian CMC model shows no abrupt reaction or oscillatory behaviour with an appropriate model constant K and gives results in better agreement with measurements lying between the predictions by ISR and Lagrangian CMC without flame group interaction.

Large eddy simulation of a turbulent spray jet generated by high-pressure injection: impact of the in-nozzle flow

ABSTRACTThe turbulence properties of a gas spray jet generated through the injection of a high-pressure atomising spray were studied in the context of the Euler–Lagrange formulation using a large eddy simulation (LES) approach. The study's main aim was to investigate nozzle flow effects on flow and turbulence statistics for a two-phase jet flow in the near and far fields of the spray. The study investigated the injection of an existing in-nozzle flow at the spray inlet. Simulation results were compared with measurements obtained for non-evaporating sprays under quasi-steady conditions, which, in practice, correspond to a long-duration injection. Both simulated high-injection pressure cases showed a good agreement with experimental data. A two-stage evolution of the spray was observed. The flow of the gas jet first developed along the spray cone, followed by the appearance of highly vortical structures around and within the spray. Cavitation in the nozzle flow resulted in an observed asymmetry of the generated gas spray jet: on the gas spray jet side of the nozzle (relative to the cavitation side), stronger jet spreading occurred, and droplets were smaller and slower. The spray jet typically exhibited an ellipsoidal cross-sectional shape. The paper provides a characterisation of the asymmetry that persisted for the duration of the spray evolution (at least for the distances considered here).

Modeling of nonreacting and reacting turbulent spray jets using a fully stochastic separated flow approach

Numerical modeling of several turbulent nonreacting and reacting spray jets is carried out using a fully stochastic separated flow (FSSF) approach. As is widely used, the carrier-phase is considered in an Eulerian framework, while the dispersed phase is tracked in a Lagrangian framework following the stochastic separated flow (SSF) model. Various interactions between the two phases are taken into account by means of two-way coupling. Spray evaporation is described using a thermal model with an infinite conductivity in the liquid phase. The gas-phase turbulence terms are closed using the k– ε model. A novel mixture fraction based approach is used to stochastically model the fluctuating temperature and composition in the gas phase and these are then used to refine the estimates of the heat and mass transfer rates between the droplets and the surrounding gas-phase. In classical SSF (CSSF) methods, stochastic fluctuations of only the gas-phase velocity are modeled. Successful implementation of the FSSF approach to turbulent nonreacting and reacting spray jets is demonstrated. Results are compared against experimental measurements as well as with predictions using the CSSF approach for both nonreacting and reacting spray jets. The FSSF approach shows little difference from the CSSF predictions for nonreacting spray jets but differences are significant for reacting spray jets. In general, the FSSF approach gives good predictions of the flame length and structure but further improvements in modeling may be needed to improve the accuracy of some details of the predictions.

A mixture fraction framework for the theory and modeling of droplets and sprays

A mixture fraction is carefully defined for evaporation and combustion of droplets and sprays. The definition is valid at points in either the liquid or gas phases and care is taken to distinguish between definitions based on conserved scalars appropriate for heat transfer and those for mass transfer. Results are presented for Spalding B numbers and values of the mixture fraction at the droplet surface for the fast chemistry case and for the case where the droplet cannot sustain an envelope flame. The classical theory for an isolated droplet with spherical symmetry yields simple formulae when expressed in mixture fraction terms. New results are then readily obtained for several quantities of interest in spray modeling. The formulation provides a seamless unification of droplet evaporation processes with gas-phase mixing and reaction. Mixing in a turbulent spray jet is identified as a model problem that clarifies the role of large scale structures in the overall mixing process. Important constraints on the parameter space for sprays are shown to be greatly clarified when expressed in the mixture fraction framework. It is shown how the classical approach for segregated flow with Eulerian/Lagrangian modeling of dispersion and transfer processes in turbulent sprays can be upgraded to include fluctuations in the temperature and composition surrounding the droplets on top of those coming from the turbulent velocity fluctuations. Such preliminary calculations that assume a simple chemically reacting system can readily be upgraded using flamelet functions derived from counterflow experiments or computations: these can then form the starting point for full chemistry calculations using such approaches as conditional moment closure.

Effects of turbulence and carrier fluid on simple, turbulent spray jet flames

This paper presents simultaneous LIF images of OH and the two-phase acetone fuel concentration as well as detailed single-point phase-Doppler measurements of velocity and droplet flux in three turbulent spray flames of acetone. This work forms part of a larger program to study spray jets and flames in a simple, well-defined geometry, aimed at providing a platform for developing and validating predictive tools for such flows. Spray flames that use nitrogen or air as droplet carrier are investigated and issues of flow field, droplet dispersion, size distribution, and evaporation are addressed. The joint OH/acetone concentration images reveal a substantial similarity to premixed flame behavior when the carrier stream is air. When the carrier is nitrogen, the reaction zone has a diffusion flame structure. There is no indication of individual droplet burning. The results show that evaporation occurs close to the jet centerline rather than in the outer shear layer. Turbulence does not have a significant impact on the evaporation rates. A small fraction of the droplets escapes the reaction zone unburned along the centerline and persists far downstream of the flame tip. The proportion of this droplet residue increases with shorter residence times as observed for the higher velocity flame.

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.

SIMPLE DESCRIPTION OF THE COMBUSTION STRUCTURES IN THE STABILIZATION STAGE OF A SPRAY JET FLAME

The structure of a two-phase flame has been investigated within its stabilization region in the nearfield of a spray jet. The spray is produced by a coaxial air-blast injector fed with liquid methanol. We focus on a specific structure of the two-phase flame, observed experimentally where the flame presents a double structure with a predominant diffusion character for each reaction zone. The analysis is justified from experimental results of phase Doppler anemometry and planar laser-induced fluorescence of OH. The dynamics of the spray obtained from phase Doppler velocimetry are studied in terms of size classes defined from the Stokes number. The size classification shows that, where the flame stabilizes, the spray is composed of two fluids, one with high inertia (high Stokes number), the other characterized by a low Stokes number. The structure of the two-phase flame is analyzed in the low-inertia part of the spray. The emphasis is put on a regime where tau(ch) < tau(vap) < tau(mix); a double structure may develop in what we called the vaporization regime, since droplets carl cross reaction zones. Such a double structure has been predicted by Continillo and Sirignano [25] and by Greenberg and Sarig [26, 27] through numerical modeling of a hue-phase counterflow flame. The present article gives an experimental confirmation of a real occurrence of such a flame structure in turbulent spray jets and proposes a simplified description in the low-inertia part of the spray and for the flame sheet approximation.