Direct numerical simulation of spray flame/acoustic interactions

Interactions between conical spray flames and sinusoidal velocity modulations due to the propagation of acoustic waves have been studied thanks to direct numerical simulations (DNS). A 2D axi-symmetric configuration has been used to capture the evolution of the pulsating laminar flames. The DNS solver has been coupled with a Lagrangian model to account for the dispersion and evaporation of the liquid fuel in the computational domain. Four main configurations, with a unitary global equivalence ratio, have been studied. Apart from a gaseous reference case, one polydispersed and two monodispersed Bunsen-type injections with various droplets density and inertia have been simulated. DNS results are in good agreement with experimental data. For significant acoustic Stokes numbers, results showed a double effect of the modulations on the flame: a direct disturbance of the flame front and a secondary impact through the local variation of the mixture fraction due to droplets preferential segregation.     

A joint experimental and numerical study of ignition in a spray burner

Partly due to stringent restrictions on pollutant emissions, aeronautical engine manufacturers target lean operating conditions which raise new difficulties such as combustion stability as well as ignition and re-ignition at high altitude. The injection of liquid fuel introduces additional complexity due to the spray-flame interaction. It is then crucial to better understand the physics behind these phenomena and to develop the capacity to predict them in an industrial context. In this work, a comprehensive joint experimental and numerical investigation of the academic swirled-confined version of the KIAI-Spray burner is carried out. Experimental diagnostics, such as Phase Doppler Anemometry (PDA), Planar Laser Induced Fluorescence (OH-PLIF), high-speed visualization and high-speed particle image velocimetry (HS-PIV), together with Large Eddy Simulations coupled to Discrete Particle Simulations are used to study spray flame structure and spray ignition. The analysis of the swirled-stabilized spray flame highlights the main effects of the presence of droplets on the turbulent combustion, and the complementarity and validity of the joint experiment and simulation approach. Ignition sequences are then studied. Both experiment and simulation show the same behaviors, and relate the flame kernel evolution and the possible success of ignition to the local non reacting flow properties at the sparking location, in terms of turbulence intensity and presence of droplets.     

Identification and analysis of instability in non-premixed swirling flames using LES

Large eddy simulations (LES) of turbulent non-premixed swirling flames based on the Sydney swirl burner experiments under different flame characteristics are used to uncover the underlying instability modes responsible for the centre jet precession and large scale recirculation zone. The selected flame series known as SMH flames have a fuel mixture of methane-hydrogen (50:50 by volume). The LES solves the governing equations on a structured Cartesian grid using a finite volume method, with turbulence and combustion modelling based on the localised dynamic Smagorinsky model and the steady laminar flamelet model respectively. The LES results are validated against experimental measurements and overall the LES yields good qualitative and quantitative agreement with the experimental observations. Analysis showed that the LES predicted two types of instability modes near fuel jet region and bluff body stabilised recirculation zone region. The mode I instability defined as cyclic precession of a centre jet is identified using the time periodicity of the centre jet in flames SMH1 and SMH2 and the mode II instability defined as cyclic expansion and collapse of the recirculation zone is identified using the time periodicity of the recirculation zone in flame SMH3. Finally frequency spectra obtained from the LES are found to be in good agreement with the experimentally observed precession frequencies.     

LES/FGM investigation of ignition and flame structure in a gasoline partially premixed combustion engine

This paper presents a joint numerical and experimental study of the ignition process and flame structures in a gasoline partially premixed combustion (PPC) engine. The numerical simulation is based on a five-dimension Flamelet-Generated Manifold (5D-FGM) tabulation approach and large eddy simulation (LES). The spray and combustion process in an optical PPC engine fueled with a primary reference fuel (70% iso-octane, 30% n-heptane by volume) are investigated using the combustion model along with laser diagnostic experiments. Different combustion modes, as well as the dominant chemical species and elementary reactions involved in the PPC engines, are identified and visualized using Chemical Explosive Mode Analysis (CEMA). The results from the LES-FGM model agree well with the experiments regarding the onset of ignition, peak heat release rate and in-cylinder pressure. The LES-FGM model performs even better than a finite-rate chemistry model that integrates the full-set of chemical kinetic mechanism in the simulation, given that the FGM model is computationally more efficient. The results show that the ignition mode plays a dominant role in the entire combustion process. The diffusion flame mode is identified in a thin layer between the ultra fuel-lean unburned mixture and the hot burned gas region that contains combustion intermediates such as CO. The diffusion flame mode contributes to a maximum of 27% of the total heat release in the later stage of combustion, and it becomes vital for the oxidation of relatively fuel-lean mixtures.     

A posteriori testing of algebraic flame surface density models for LES

In the application of Large Eddy Simulation (LES) to premixed combustion, the unknown filtered chemical source term can be modelled by the generalised flame surface density (FSD) using algebraic models for the wrinkling factor Ξ. The present study compares the behaviour of the various models by first examining the effect of sub-grid turbulent velocity fluctuation on Ξ through a one-dimensional analysis and by the LES of the ORACLES burner (Nguyen, Bruel, and Reichstadt, Flow, Turbulence and Combustion Vol. 82 [2009], pp. 155–183) and the Volvo Rig (Sjunnesson, Nelsson, and Max, Laser Anemometry, Vol. 3 [1991], pp. 83–90; Sjunnesson, Henrikson, and Löfström, AIAA Journal, Vol. 28 [1992], pp. AIAA–92–3650). Several sensitivity studies on parameters such as the turbulent viscosity and the grid resolution are also carried out. A statistically 1-D analysis of turbulent flame propagation reveals that counter gradient transport of the progress variable needs to be accounted for to obtain a realistic flame thickness from the simulations using algebraic FSD based closure. The two burner setups are found to operate mainly within the wrinkling/corrugated flamelet regime based on the premixed combustion diagram for LES (Pitsch and Duchamp de Lageneste, Proceedings of the Combustion Institute, Vol. 29 [2002], pp. 2001–2008) and this suggests that the models are operating within their ideal range. The performance of the algebraic models are then assessed by comparing velocity statistics, followed by a detailed error analysis for the ORACLES burner. Four of the tested models were found to perform reasonably well against experiments, and one of these four further excels in being the most grid-independent. For the Volvo Rig, more focus is placed upon the comparison of temperature data and identifying changes in flame structure amongst the different models. It is found that the few models which largely over-predict velocities in the ORACLES case and volume averaged in a previous a priori DNS analysis (Chakraborty and Klein, Physics of Fluids, Vol. 20 [2008], p. 085108), deliver satisfactory agreement with experimental observations in the Volvo Rig, whereas a few of the other models are only able to capture the experimental data of the Volvo Rig either quantitatively or qualitatively.     

Experimental and LES investigation of premixed methane/air flame propagating in a tube with a thin obstacle

In this paper, an experimental and numerical investigation of premixed methane/air flame dynamics in a closed combustion vessel with a thin obstacle is described. In the experiment, high-speed video photography and a pressure transducer are used to study the flame shape changes and pressure dynamics. In the numerical simulation, four sub-grid scale viscosity models and three sub-grid scale combustion models are evaluated for their individual prediction compared with the experimental data. High-speed photographs show that the flame propagation process can be divided into five stages: spherical flame, finger-shaped flame, jet flame, mushroom-shaped flame and bidirectional propagation flame. Compared with the other sub-grid scale viscosity models and sub-grid scale combustion models, the dynamic Smagorinsky–Lilly model and the power-law flame wrinkling model are better able to predict the flame behaviour, respectively. Thus, coupling the dynamic Smagorinsky–Lilly model and the power-law flame wrinkling model, the numerical results demonstrate that flame shape change is a purely hydrodynamic phenomenon, and the mushroom-shaped flame and bidirectional propagation flame are the result of flame–vortex interaction. In addition, the transition from “corrugated flamelets” to “thin reaction zones” is observed in the simulation.     

Evaluation of the flame propagation within an SI engine using flame imaging and LES

This work shows experiments and simulations of the fired operation of a spark ignition engine with port-fuelled injection. The test rig considered is an optically accessible single cylinder engine specifically designed at TU Darmstadt for the detailed investigation of in-cylinder processes and model validation. The engine was operated under lean conditions using iso-octane as a substitute for gasoline. Experiments have been conducted to provide a sound database of the combustion process. A planar flame imaging technique has been applied within the swirl- and tumble-planes to provide statistical information on the combustion process to complement a pressure-based comparison between simulation and experiments. This data is then analysed and used to assess the large eddy simulation performed within this work. For the simulation, the engine code KIVA has been extended by the dynamically thickened flame model combined with chemistry reduction by means of pressure dependent tabulation. Sixty cycles have been simulated to perform a statistical evaluation. Based on a detailed comparison with the experimental data, a systematic study has been conducted to obtain insight into the most crucial modelling uncertainties.     

An experimental investigation on spray, ignition and combustion characteristics of biodiesels

Spray, ignition and combustion characteristics of biodiesel fuels were investigated under a simulated diesel-engine condition (885 K, 4 MPa) in a constant volume combustion vessel. Two biodiesel fuels originated from palm oil and used cooking oil were used while JIS#2 used as the base fuel. Spray images were taken by a high speed video camera by using Mie-scattering method to measure liquid phase penetration and liquid length. An image intensifier combined with OH filter was used to obtain OH radical image near 313 nm. Ignition and combustion characteristics were studied by OH radical images. Biodiesel fuels give appreciably longer liquid lengths and shorter ignition delays. At low injection pressure (100 MPa), biodiesel fuels give shorter lift-off lengths than those of diesel. While at high injection pressure (200 MPa), the lift-off length of biodiesel fuel originated palm oil gives the shortest value and that of biodiesel from used cooking oil gives the longest one. Air entrainment upstream of lift-off length of three fuels was estimated and compared to soot formation distance. This study reveals that the viscosity and ignition quality of biodiesel fuel have great influences on jet flame structure and soot formation tendency.     

Dynamic behavior of a freely-propagating turbulent premixed flame under global stretch-rate oscillations

Dynamic features of a freely propagating turbulent premixed flame under global stretch rate oscillations were investigated by utilizing a jet-type low-swirl burner equipped with a high-speed valve on the swirl jet line. The bulk flow velocity, equivalence ratio and the nominal mean swirl number were 5 m/s, 0.80 and 1.23, respectively. Seven velocity forcing amplitudes, from 0.09 to 0.55, were examined with a single forcing frequency of 50 Hz. Three kinds of optical measurements, OH-PLIF, OH^* chemiluminescence and PIV, were conducted. All the data were measured or post-processed in a phase-locked manner to obtain phase-resolved information. The global transverse stretch rate showed in-phase oscillations centering around 60 (1/s). The oscillation amplitude of the stretch rate grew with the increment of the forcing amplitude. The turbulent flame structure in the core flow region varied largely in axial direction in response to the flowfield oscillations. The flame brush thickness and the flame surface area oscillated with a phase shift to the stretch rate oscillations. These two properties showed a maximum and minimum values in the increasing and decreasing stretch periods, respectively, for all the forcing amplitudes. Despite large variations in flame brush thickness at different phase angles, the normalized profiles collapse onto a consistent curve. This suggests that the self-similarity sustains in this dynamic flame. The global OH^* fluctuation response (i.e. response of global heat-release rate fluctuation) showed a linear dependency to the forcing velocity oscillation amplitudes. The flame surface area fluctuation response showed a linear tendency as well with a slope similar to that of the global OH^* fluctuation. This indicated that the flame surface area variations play a critical role in the global flame response.     

LES of the Sydney swirl flame series: A study of vortex breakdown in isothermal and reacting flows

This work examines the flow and mixing in selected non-premixed cases of the Sydney Swirl Flame series by Large Eddy Simulation. A mixture fraction approach with a steady flamelet model, based on a detailed chemical mechanism, is applied to determine the chemical state in the flame. The isothermal case N29S054 is simulated to provide insights into the flow field, the resolution requirements for the simulation, and to allow for various measures of validation and verification. For the reactive case, the high-speed hydrogen/methane flame SMH1 is chosen for its similarity to the non-swirling Sydney flames and its good stability. In experiments carried out previously at Sydney University, vortex breakdown has been observed and in the isothermal case, this is clearly predicted by the LES. However, no vortex breakdown is observed in the simulations of flame SMH1, which necessitates further studies on this and similar flames to investigate this phenomenon. Studies of the low-velocity flames SM1 and SM2 show that reactive vortex breakdown can be predicted successfully. This difficulty in the prediction of vortex breakdown is another indication that the Sydney Swirl Flame series, especially at high velocities of the central jet, is an interesting and challenging test-case for the development of combustion LES.     

A reduced-scale experimental investigation of facade flame behavior ejected from a top-hung window of fire compartment

In case of a room fire, facade flame ejected from a window could lead to catastrophic consequence. Top-hung window is a common feature in modern buildings. This paper investigates, for the first time, the ejected facade flame behavior from a top-hung window. Experiments are performed by a cubic scale model compartment with a top-hung window of four opening angles and four dimensions under various heat release rates (HRR). A total of 119 test conditions are involved to quantify generally the ejected facade flame behavior and morphological characteristics, characterized by its vertical height and horizontal depth (normal to facade). It is found that the position of neutral plane between the inflow and outflow at the top-hung window increases with opening angle, while being smaller as the window width is larger or the window height is smaller. The critical HRR for flame ejection is higher as the top-hung window opening angle is larger. The facade flame depth increases, while the flame height decreases, with increasing of opening angle of the top-hung window. A new characteristic length taking into account both the top-hung window dimensions and window opening angle is deduced by the mass balance analysis of the inflow and outflow as well as ventilation factor through the top-hung window. New dimensionless models are established to describe the critical HRR for flame ejection, the height of the facade flame and flame depth as a function of the newly derived top-hung window ventilation factor as well as characteristic length, showing good fitting of experimental results. The present study provides the basic data, understanding and model of facade flame characteristics from a top-hung window of a fire compartment, which is essential in estimation of its risk and adverse impact to urban environment as a new supplementary over previous knowledge limited for freely opened windows.     

Investigations in the TECFLAM swirling diffusion flame: Laser Raman measurements and CFD calculations

A standard burner for confined swirling natural gas flames is presented which was developed within the German TECFLAM cooperation. The aims of the TECFLAM research program are the establishment of an extensive experimental database from selective flames and the validation and improvement of mathematical combustion models. In this paper, results from joint PDF measurements of temperature, mixture fraction, and major species concentrations in a turbulent diffusion flame with 150 kW thermal load, equivalence ratio 0.833, and swirl number 0.9 are presented. Major aspects of the investigation are the general quantitative characterization of the flame and the study of the thermochemical state, e.g. effects of turbulence–chemistry interactions. Scatterplots of temperature, CH₄, and CO mole fractions as well as mean mixture fraction and temperature fields are presented and discussed. Furthermore, CFD calculations have been performed using the code Fluent 5 as an example of a commercially available code that is frequently used for technical applications. The comparison between the calculated and measured results reveals some significant deviations which are discussed with respect to the applicability of this code to swirling turbulent flames. Received: 19 April 2000 / Revised version: 15 June 2000 / Published online: 5 October 2000     

Investigation of lengthscales, scalar dissipation, and flame orientation in a piloted diffusion flame by LES

This work investigates the structure of a diffusion flame in terms of lengthscales, scalar dissipation, and flame orientation by using large eddy simulation. This has been performed for a turbulent, non-premixed, piloted methane/air jet flame (Flame D) at a Reynolds-number of 22,400. A steady flamelet model, which was represented by artificial neural networks, yields species mass fractions, density, and viscosity as a function of the mixture fraction. This will be shown to suffice to simulate such flames. To allow to examine scalar dissipation, a grid of 1.97 × 10⁶ nodes was applied that resolves more than 75% of the turbulent kinetic energy. The accuracy of the results is assessed by varying the grid-resolution and by comparison to experimental data by Barlow, Frank, Karpetis, Schneider (Sandia, Darmstadt), and others. The numerical procedure solves the filtered, incompressible transport equations for mass, momentum, and mixture fraction. For subgrid closure, an eddy viscosity/diffusivity approach is applied, relying on the dynamic Germano model. Artificial turbulent inflow velocities were generated to feature proper one- and two-point statistics. The results obtained for both the one- and two-point statistics were found in good agreement to the experimental data. The PDF of the flame orientation shows the tilting of the flame fronts towards the centerline. Finally, the steady flamelet approach was found to be sufficient for this type of flame unless slowly reacting species are of interest.     

Simulation of diesel spray combustion using LES and a multicomponent vapourisation model

Diesel spray and combustion in a constant-volume engine cylinder was simulated by a large eddy simulation (LES) approach coupling with a multicomponent vapourisation (MCV) modelling. The simulation focused on the inclusion of the interaction between fuel spray and gas-phase turbulence flow at the sub-grid scale. The LES was based on the dynamic structure sub-grid model, and an additional source term was added to the filtered momentum equation to account for the effect of drop motion on the gas-phase turbulence. The multicomponent drop vapourisation modelling was based on the continuous thermodynamics approach using a gamma distribution to describe the complex diesel fuel composition and was capable of predicting a more complex drop vapourisation process. The effect of gas-phase turbulence flow on the fuel drop vapourisation process was evaluated through the solution of the gas-phase moments of the distribution in the present LES framework. A non-evaporative spray in a constant-volume engine cylinder was first simulated to examine the behaviours of LES, in comparison with a Reynolds-averaged Navier–Stokes (RANS) simulation based on the RNG k–? model. More realistic diesel spray structures and improved agreement on liquid penetration length with the corresponding experimental data were predicted by the LES, using a grid resolution close to that of RANS. A more comprehensive simulation of diesel spray and combustion in cylindrical combustor was also performed. Predicted distributions of soot particles were compared to the experimental image, and improved agreement with the experimental data was also observed by using the present LES and MCV models. Consequently, results of the present models proved that improved overall performance of the fuel spray simulation can be achieved by the LES without a significant increase in the computational load compared to the RANS.     

Theoretical and experimental investigation of holographic reconstruction and prediction of multi-source hybrid acoustic field

Recently, NAH has been developed as an efficient tool for noise identification, noise localization and acoustic field visualization. Compared with the traditional acoustic ra-diation calculation problem, the solving problem in the NAH is an inverse acoustic problem. By measuring partial acoustic field information, such as complex pressures or particle velocities on the hologram surface, but not the surface normal velocities of the vibrating body, the surface information can be reconstructed,…