A. Fuentes

Soot Volume Fraction Measurements in a Three-Dimensional Laminar Diffusion Flame established in Microgravity

ABSTRACT A methodology for the estimation of the soot volume fraction in a three-dimensional laminar diffusion flame is presented. All experiments are conducted in microgravity and have as objective producing quantitative data that can serve to estimate radiative heat transfer in flames representative of fires in spacecraft. The competitive nature of formation and oxidation of soot and its direct coupling with the streamlines (source of oxygen) require for these measurements to be conducted within the exact configuration. Thus three-dimensional measurements are needed. Ethylene is injected through a square porous burner and the oxidizer flows parallel to its surface. The methodology uses CH* chemiluminescence measurements to correct for three-dimensional effects affecting light attenuation measurements. Corrected local soot concentrations are thus obtained. All experiments are conducted during parabolic flights and the parameters varied are fuel and oxidizer flow rates.

Three-dimensional recomposition of the absorption field inside a nonbuoyant sooting flame

A remote scanning retrieval method was developed to investigate the soot layer produced by a laminar diffusion flame established over a flat plate burner in microgravity. Experiments were conducted during parabolic flights. This original application of an inverse problem leads to the three-dimensional recomposition by layers of the absorption field inside the flame. This technique provides a well-defined flame length that substitutes for other subjective definitions associated with emissions.

Modelling thermal radiation in buoyant turbulent diffusion flames

This work focuses on the numerical modelling of radiative heat transfer in laboratory-scale buoyant turbulent diffusion flames. Spectral gas and soot radiation is modelled by using the Full-Spectrum Correlated-k (FSCK) method. Turbulence-Radiation Interactions (TRI) are taken into account by considering the Optically-Thin Fluctuation Approximation (OTFA), the resulting time-averaged Radiative Transfer Equation (RTE) being solved by the Finite Volume Method (FVM). Emission TRIs and the mean absorption coefficient are then closed by using a presumed probability density function (pdf) of the mixture fraction. The mean gas flow field is modelled by the Favre-averaged Navier–Stokes (FANS) equation set closed by a buoyancy-modified k-ϵ model with algebraic stress/flux models (ASM/AFM), the Steady Laminar Flamelet (SLF) model coupled with a presumed pdf approach to account for Turbulence-Chemistry Interactions, and an acetylene-based semi-empirical two-equation soot model. Two sets of experimental pool fire data are used for validation: propane pool fires 0.3 m in diameter with Heat Release Rates (HRR) of 15, 22 and 37 kW and methane pool fires 0.38 m in diameter with HRRs of 34 and 176 kW. Predicted flame structures, radiant fractions, and radiative heat fluxes on surrounding surfaces are found in satisfactory agreement with available experimental data across all the flames. In addition further computations indicate that, for the present flames, the gray approximation can be applied for soot with a minor influence on the results, resulting in a substantial gain in Computer Processing Unit (CPU) time when the FSCK is used to treat gas radiation.

SOOTING BEHAVIOR DYNAMICS OF A NON-BUOYANT LAMINAR DIFFUSION FLAME

Local soot concentrations in non-buoyant laminar diffusion flames have been demonstrated to be the outcome of two competitive processes, soot formation and soot oxidation. It was first believed that soot formation was the controlling mechanism and thus soot volume fractions could be scaled with a global residence time. Later studies showed that this is not necessarily the case and the local ratio of the soot formation and oxidation residence times is the prime variable controlling the ultimate local soot volume fractions. This ratio is a strong function of geometry and flow field, thus a very difficult variable to properly quantify. This study presents a series of microgravity, low oxidizer flow velocity, experiments where soot volume fraction measurements have been conducted on a laminar, flat plate boundary layer type diffusion flame. The objective of the study is to determine if the above observations apply to this type of diffusion flames. The fuel is ethylene and is injected through a flat plate porous burner into an oxidizer flowing parallel to the burner surface. The oxidizer consists of different mixtures of oxygen and nitrogen, flowing at different velocities. These experiments have been complemented with numerical simulations that emphasize resolution of the flow field to simulate the trajectory of soot particles and to track their history from inception to oxidation. The results validate that local soot volume fractions are a function of the local formation and oxidation residence times and are not necessarily a function of the global residence time. For this particular geometry, an increase in oxidizer velocity leads to local acceleration that reduces the oxidation residence time, leading to higher soot concentrations. It was also observed that the flames become longer as the flow velocity is increased in contrast with the reversed trend observed in flames at higher flow velocities. This result is important because it seems to indicate the presence of a maximum in the flame length and luminosity below those encountered in natural convection. The result would have implications for fire safety in spacecrafts since the ambient gas velocities are below those observed in natural convection, and longer and more luminous flames represent a higher hazard.

The Oxygen Index on Soot Production in Propane Diffusion Flames

An experimental study of the effect of oxygen index (OI) on soot formation in laminar coflow propane diffusion flames is presented. The OI was defined as the oxygen volumetric concentration in the oxidizer flow, O2/(O2+N2), which was varied from 21% to 37%. The influence of the OI was quantified by means of three variables: the flame height, the soot volume fraction, and the vertical distribution of radiative heat flux. The flame height was based on CH* spontaneous emission and found to vary inversely with the OI, following the classical theory of Roper. As the OI increases, the rates of soot growth and soot oxidation are enhanced, and the maximum soot volume fraction and the peak of integrated soot volume fraction also increase. Moreover, the evolution of the peak of radiative heat flux and the maximum soot volume fraction are found to follow the same evolution with the OI.

Videosensor for the Detection of Unsafe Driving Behavior in the Proximity of Black Spots

This paper discusses the overall design and implementation of a video sensor for the detection of risky behaviors of car drivers near previously identified and georeferenced black spots. The main goal is to provide the driver with a visual audio alert that informs of the proximity of an area of high incidence of highway accidents only if their driving behavior could result in a risky situation. It proposes a video sensor for detecting and supervising driver behavior, its main objective being manual distractions, so hand driver supervision is performed. A GPS signal is also considered, the GPS information is compared with a database of global positioning Black Spots to determine the relative proximity of a risky area. The outputs of the video sensor and GPS sensor are combined to evaluate a possible risky behavior. The results are promising in terms of risk analysis in order to be validated for use in the context of the automotive industry as future work.

Evaluation of the Extinction Factor in a Laminar Flame Established over a PMMA Plate in Microgravity

A methodology for estimating the extinction factor at λ=530 nm in diffusion flames is presented. All experiments have been in microgravity and have as their objective the production of quantitative data that can serve to evaluate the soot volume fraction. A better understanding of soot formation and radiative heat transfer is of extreme importance to many practical combustion related processes such as spacecraft fire safety. The experimental methodology implements non-axisymmetric configurations that provide a laminar diffusion flame at atmospheric pressure. PMMA is used as fuel. The oxidizer flows parallel to its surface. Optical measurements are performed at the 4.74 s ZARM drop tower.

Influence of radiative property models on soot production in laminar coflow ethylene diffusion flames

Two axisymmetric laminar coflow non-smoking and smoking ethylene diffusion flames are studied numerically in order to assess the influence of different radiative property models on the soot formation and oxidation processes. Simulations are carried out by considering the Steady Laminar Flamelet (SLF) concept and a modified two-equation acetylene-based model to describe the soot nucleation, surface growth and oxidation processes. Several radiative property models are considered: the simple Optically Thin Approximation (OTA), the Weighted-Sum of Grey-Gases (WSGG), the Grey-Wide-Band model (GWB), the Statistical Narrow Band Correlated-k model (SNBCK) and the Full Spectrum Correlated-k model (FSCK). Comparisons between calculations carried out with the SNBCK model and experimental data show a reasonable agreement. Model results show that the choice of the radiative property models influence largely the soot prediction, especially in the upper part of the flame where oxidation occurs. Simulations show that the reabsorption of spectral gas and soot is an important feature and thus the commonly used OTA or grey models introduce large discrepancies. The GWB model leads to improved solutions, but it should be avoided if accurate predictions are desired. The FSCK provides equivalent results as compared to the SNBCK model with a substantial gain in CPU time.

Analysis of Soot Propensity in Combustion Processes Using Optical Sensors and Video Magnification

Industrial combustion processes are an important source of particulate matter, causing significant pollution problems that affect human health, and are a major contributor to global warming. The most common method for analyzing the soot emission propensity in flames is the Smoke Point Height (SPH) analysis, which relates the fuel flow rate to a critical flame height at which soot particles begin to leave the reactive zone through the tip of the flame. The SPH and is marked by morphological changes on the flame tip. SPH analysis is normally done through flame observations with the naked eye, leading to high bias. Other techniques are more accurate, but are not practical to implement in industrial settings, such as the Line Of Sight Attenuation (LOSA), which obtains soot volume fractions within the flame from the attenuation of a laser beam. We propose the use of Video Magnification techniques to detect the flame morphological changes and thus determine the SPH minimizing observation bias. We have applied for the first time Eulerian Video Magnification (EVM) and Phase-based Video Magnification (PVM) on an ethylene laminar diffusion flame. The results were compared with LOSA measurements, and indicate that EVM is the most accurate method for SPH determination.

Soot propensity detection by Eulerian video magnification

This work presents the theoretical fundamentals of a novel approach to determine the threshold over which flames release soot into the environment, characterized by the Smoke Point Height concept. The recognition of this critical point requires the detection of particular morphological changes in the flame, identified in this approach through their amplification using Eulerian Video Magnification. The application of this methodology over an ethylene laminar diffusion laboratory-scale flame is presented. The results show an effective amplification of the flames morphology, making possible the recognition of subtle changes in the flame tip. Furthermore, a validation process based on Line of Sight Attenuation measurements is presented to confirm that the results obtained via EVM are correct. The results obtained along with the successful implementation of the proposed validation show that this novel approach could be applied to detect Soot propensity in practical combustion devices.