A forward approach for the validation of soot sizing models using laser-induced incandescence (LII)

While validating the numerical modeling of the primary particle size distribution (PPSD) in sooting flames, a common practice is to compare the numerical results to the corresponding experimental data obtained with the Time-Resolved Laser-Induced Incandescence (TiRe-LII) technique. Since the PPSD is not directly measured by TiRe-LII, but derived with a post-processing procedure, various uncertainties and errors can potentially affect the consistency of such comparison requiring the estimation of many input parameters. On the contrary, nowadays, detailed numerical simulations provide access to a more complete set of data, which can be used to reconstruct the incandescence signal. In this work, a forward approach for the generic validation of numerical models for the PPSD is performed. It is based on the numerical reconstruction of the temporal evolution of the incandescence from the numerical results and its comparison with the measured signal. First, two indexes are proposed to quantify the agreement between the numerically synthesized and the measured signals. Then, the effectiveness of the proposed approach is demonstrated a priori by quantifying the potential errors that can be avoided with this new strategy compared to the classical approach. Finally, the feasibility of the proposed procedure is proven by comparing synthesized signals to the experimental ones available in the literature for a laminar premixed flame. It is shown that the proposed approach can be used for strengthening the analysis on numerical model performances in addition to the classical approach

A tabulated chemistry method for spray combustion

International audienceTabulated chemistry is a popular technique to account for detailed chemical effects with an affordable computational cost in gaseous combustion systems. How- ever its performances for spray combustion have not completely been identified. The present article discusses the chemical structure modeling of spray flames us- ing tabulated chemistry methods under the hypothesis that the chemical subspace accessed by a two-phase reactive flow can be mapped by a collection of gaseous flamelets. It is shown that tabulated chemistry methods based either on pure pre- mixed flamelets or on pure non-premixed flamelets fail to capture the structure of spray combustion. The reason is the complexity of the chemical structure of spray flames which exhibits both premixed-like and non-premixed-like reaction zones. To overcome this issue, a new multi-regime flamelet combustion model (called Partially-Premixed Flamelet Tabulation 2PFT) is presented in this paper. Information from premixed, partially-premixed and diffusion flames are stored in a 3-D look-up table parametrized as function of the progress variable Yc, de- scribing the progress of the reaction, the mixture fraction Yz, denoting the local equivalence ratio, and the scalar dissipation, which identifies the combustion regime. The performances of the 2PFT method are evaluated on counterflow laminar spray flames for different injection conditions of droplet diameter, liquid volume fraction and velocity. The 2PFT tabulation method better describes the chemical structure of spray flames compared to the classical techniques based on single archetypal flamelets. These results also confirm that the chemical structure of laminar spray flame can be modeled by a multi-regime flamelet combustion model based on gaseous flamelets

How to validate numerical results on primary particle diameter to experimental data from laminar sooting flames

Combustion generated soot particles have harmful effects on human health and our environment. An important aspect is to accurately determine the surface area of the particle population, which can be estimated from the particle size distribution (PSD) and morphology. Experimental investigations showed that large particles are aggregates constituted of several small primary particles [1]. Therefore, the determination of the primary particle size distribution (PPSD) is essential for the characterization of soot population. On the one side, sectional methods can be used to numerically predict the particle population of sooting flames. However, most of models assumes that large particles are spherical for all sections [2] or aggregates constituted of primary particles of identical size for all sections [3-5]. These strong assumptions can affect the results’ quality and the validity of the models themselves. On the other side, Time Resolved Laser Induced Incandescence (TiRe-LII) is a powerful, nonintrusive experimental method, which exploits the fact that the temporal decay of the LII signal is related to the primary particle diameter dp. Information on the PPSD can then be derived once the PPSD shape is presumed [6]. The general approach is to assume log-normal distribution, but Transmission Electron Microscopy measurements showed that this assumption may be not always valid [7]. In this context, the comparison of numerical results on the PPSD with experiments is extremely complex due to the strong assumptions underlying the numerical models and the fact that TiRE-LII technique does not measure directly the PPSD. In this work, we propose a new way to compare numerical to experimental data on PPSD. First, we improved our existing CFD code to obtain the mean size of the primary particles for each section, based on what proposed in [4]. Second, the TiRe-LII signal is reconstructed from the numerical PPSD and compared directly to the measured signal [8] to avoid any potential errors due to a presumed PPSD shape. This approach is applied to the investigation of an ethylene laminar-coflow diffusion flame [9], which is a target of the ISF workshop [10], and potential sources of errors are discussed

A moment method for low speed microflows

A moment method is proposed to study slow rarefied flows by the linearized Bhatnagar-Gross-Krook (BGK) kinetic model equation. In order to obtain a consistent treatment of boundary conditions, the velocity distribution function is expanded in orthogonal polynomials which are not continuous in the velocity space. The solution of the kinetic equation is then reduced to the solution of a system of differential equations for the expansion coefficients. For one-dimensional problems, the system of moment equations can be easily recast in an hydrodynamic-like form. The method is here applied to isothermal steady boundary driven flows, i.e. the one-dimensional Couette and Poiseuille flows and the two-dimensional cavity flow. The results show that it is possible to obtain excellent approximations of the (virtually) exact solutions of the kinetic model equation by using a small number of moments in a wide range of Knudsen numbers and suggest that it might be possible to obtain a sufficiently accurate description of slow rarefied flows by a small number of moment equations

Time-resolved spatial patterns and interactions of soot, PAH and OH in a turbulent diffusion flame

International audienceSoot control raises important fundamental issues and industrial challenges, which require a comprehensive under- standing of processes governing its formation, interactions and destruction in turbulent flames. A physical insight of the soot space-time evolution in a turbulent diffusion flame is reported in this article by combining three simultaneous high sampling rate imaging diagnostics operating at a frame rate of 10 kHz: light scattering from soot particles, planar laser induced fluorescence (PLIF) of the OH radical, a marker of the flame region, and planar laser induced fluorescence of Polycyclic Aromatic Hydrocarbons (PAHs), classically identified as soot precursors. Images issued from these diagnostics provide a spatially resolved information on the production of soot, its interaction with the turbulent flow and its link with the flame surface and with soot precursors regions. It is shown in particular that soot pockets are highly distorted by turbulent eddies forming a characteristic layered pattern. A statistical analysis is also proposed to analyze such high-speed imaging results. Information on soot-OH-PAH correlation deduced from the high speed imaging could be employed to verify the adequacy of models devised to represent soot dynamics in direct or large eddy simulations of turbulent flames

Initial interpretation of Laser-induced Incandescence (LII) signals from flame-generated TiO2 particles: Towards a quantitative characterization of the flame synthesis processes

Among various optical diagnostics for the characterization of particle formation in flames, laser-induced incandescence (LII), developed for soot particles, is attracting attention for the study of flame synthesis of metal-oxides. Among them, TiO2 nanoparticles are widely used for pigments and photocatalysts. Recent works have shown the feasibility of LII for flame-synthesized TiO2, but extensive research is still needed to quantitatively characterize TiO2 production in flames with LII measurements. In this work, the first attempt towards the characterization of TiO2 synthesis in flames is provided as a normalized volume fraction. To achieve this, TiO2 nanoparticles are generated in a laminar coflow diffusion flame of argon-diluted hydrogen and air with pre-vaporized titanium isopropoxide (TTIP). A 355 nm laser is used to irradiate the flame-generated particles. Spectral, temporal, and spatial measurements are performed at various flame heights. First, laser-induced emission (LIE) at prompt is investigated for different laser fluences to identify the operating conditions that ensure the LII-like nature of the measured signals. The LIE at high fluence presents sharp features that contain information on the atomic composition of the particles and of the vaporized species when compared to reference spectra of carbon black and high-purity TiO2 particles. Then, the LII signal at lower fluence is used to obtain an estimation of the spatial evolution of the normalized volume fraction and of the LII signal decay time. These results are finally used to discuss the major aerosol processes along the flame centerline

Large Eddy Simulation of Swirled Spray Flame Using Detailed and Tabulated Chemical Descriptions

International audienceAccurate characterization of swirled flames is a key point in the development of more efficient and safer aeronautical engines. The task is even more challenging for spray injection systems. From one side, spray interacts with both turbulence and flame, eventually affecting the flame dynamics. On the other side, the structure of turbulent spray flame is highly complex due to equivalence ratio inhomogeneities caused by evaporation and mix- ing processes. The first objective of this work is to numerically characterize the structure and dynamics of a swirled spray flame. The target configuration is the experimental bench- mark named MERCATO, representative of an actual turbojet injection system. Due to the complex nature of the flame, a detailed description of chemical kinetics is necessary and is here obtained by using a 24-species chemical scheme, which has been expressly developed for DNS of spray flames. The first LES of a swirled spray flame using such a detailed chemical description is performed here and results are analyzed to study the complex inter- actions between the spray, the turbulent flow and the flame. It is observed that this coupling has an effect on the flame structure and that flame dynamics are governed by the interactions between spray, precessing vortex core and flame front. Even if such a detailed kinetic description leads to an accurate characterization of the flame, it is still highly expensive in terms of CPU time. Tabulated techniques have been expressly developed to account for detailed chemistry at a reduced computational cost in purely gaseous configurations. The second objective is then to verify the capability of the FPI tabulated chemistry method to correctly reproduce the spray flame characteristics by performing LES. To do this, results with the FPI method are compared to the experimental database and to the results obtained with the 24-species description in terms of mean and fluctuating axial gas velocity and liquid phase characteristics (droplet diameter and liquid velocity). Moreover, the flame characterization obtained with the FPI approach is compared to the results of the 24-species scheme focusing on the flame structure, on major and minor species concentrations as well as on pollutant emissions. The potential and the limits of the tabulated approach for spray flame are finally assessed