Meredith B. Colket

Computational and experimental study of soot formation in a coflow, laminar diffusion flame

Abstract A detailed soot growth model in which the equations for particle production have been coupled to the flow and gaseous species conservation equations has been developed for an axisymmetric, laminar, coflow diffusion flame. Results from the model have been compared to experimental data for a confined methane–air flame. The two-dimensional system couples detailed transport and finite rate chemistry in the gas phase with the aerosol equations in the sectional representation. The formulation includes detailed treatment of the transport, inception, surface growth, oxidation, and coalescence of soot particulates. Effects of thermal radiation and particle scrubbing of gas-phase growth and oxidation species are also included. Predictions and measurements of temperature, soot volume fractions, and selected species are compared over a range of heights and as a function of radius. Flame heights are somewhat overpredicted and local temperatures and volume fractions are underpredicted. We believe the inability to reproduce accurately bulk flame parameters directly inhibits the ability to predict soot volume fractions and these differences are likely a result of uncertainties in the experimental inlet conditions. Predictions of the distributions of particle sizes indicate the existence of (relatively) low-molecular-weight species along the centerline of the burner and trace amounts of the particles that escape from the flame, unoxidized. Oxidation of particulates is dominated by reactions with hydroxyl radicals which attain levels approximately 10 times higher than calculated equilibrium levels. Gas cooling effects due to radiative loss are shown to have a very significant effect on predicted soot concentrations.

Computational and experimental study of axisymmetric coflow partially premixed ethylene/air flames

Abstract Six coflowing laminar, partially premixed methane/air flames, varying in primary equivalence ratio from ∞ (nonpremixed) to 2.464, have been studied both computationally and experimentally to determine the fundamental effects of partial premixing. Computationally, the local rectangular refinement solution–adaptive gridding method incorporates a damped modified Newton’s method to solve the system of coupled nonlinear elliptic partial differential equations for each flame. The model includes a C2 chemical mechanism, multicomponent transport, and an optically thin radiation submodel. Experimentally, both probe and optical diagnostic methods are used to measure the temperature and species concentrations along each flame’s centerline. Most experimentally measured trends are well predicted by the computational model. Because partial premixing decreases the flame height when the fuel flowrate is held constant, computational and experimental centerline profiles have been plotted against nondimensional axial position to reveal additional effects of partial premixing. Heat release profiles, as well as those of several species, indicate that the majority of the partially premixed flames contain two flame fronts: an inner premixed front whose strength grows with decreasing primary equivalence ratio; and an outer nonpremixed front. As the amount of partial premixing increases, computational results predict a continual reduction in the amount of flow radially inward; the resulting decrease in radial transport is responsible for various effects observed both computationally and experimentally, including a cooling of the gases near the burner surface. At the same time, radiative losses decrease with increasing amounts of premixing, resulting in higher flame temperatures.

Computational and experimental study of soot formation in a coflow, laminar ethylene diffusion flame

A sooting, ethylene coflow diffusion flame has been studied both experimentally and computationally. The fuel is diluted with nitrogen and the flame is slightly fifted to minimize the effects of the burner. Both probe (thermocouple and gas-sampling techniques) and optical diagnostic methods (Rayleigh scattering and laser-induced incandescence) are used to measure the temperature, gas species, and soot volume fractions. A detailed soot growth model in which the equations for particle production are coupled to the flow and gaseous species conservation equations has been used to investigate soot formation in the flame. The two-dimensional system couples detailed transport and finite-rate chemistry in the gas phase with the aerosol equations in the sectional representation. The formulation includes detailed treatment of the transport, inception, surface growth, oxidation, and coalescence of soot particulates. Effects of thermal radiation and particle scrubbing of gas-phase growth and oxidation species are also included. Predictions and measurements of temperature, soot volume fractions, and selected species are compared over a range of heights and as a function of radius. The formation of benzene is primarily controlled by the recombination of propargyl radicals, and benzene production rates are found to limit the rate of inception, as well as the net rate of soot growth. The model predicted soot volume fractions well along the wings of the flame but underpredicted soot volume fractions by a factor of four along the centerline. Oxidation of particulates is dominated by reactions with hydroxyl radicals that attain levels approximately ten times higher than calculated equilibrium levels. Gas cooling effects due to radiative loss are shown to have a very significant effect on predicted temperatures.

Investigation of the transition from lightly sooting towards heavily sooting co-flow ethylene diffusion flames

Laminar, sooting, ethylene-fuelled, co-flow diffusion flames at atmospheric pressure have been studied experimentally and theoretically as a function of fuel dilution by inert nitrogen. The flames have been investigated experimentally using a combination of laser diagnostics and thermocouple-gas sampling probe measurements. Numerical simulations have been based on a fully coupled solution of the flow conservation equations, gas-phase species conservation equations with complex chemistry and the dynamical equations for soot spheroid growth. Predicted flame heights, temperatures and the important soot growth species, acetylene, are in good agreement with experiment. Benzene simulations are less satisfactory and are significantly under-predicted at low dilution levels of ethylene. As ethylene dilution is decreased and soot levels increase, the experimental maximum in soot moves from the flame centreline toward the wings of the flame. Simulations of the soot field show similar trends with decreasing dilution of the fuel and predicted peak soot levels are in reasonable agreement with the data. Computations are also presented for modifications to the model that include: (i) use of a more comprehensive chemical kinetics model; (ii) a revised inception model; (iii) a maximum size limit to the primary particle size; and (iv) estimates of radiative optical thickness corrections to computed flame temperatures.

Successes and Uncertainties in Modeling Soot Formation in Laminar, Premixed Flames

A model for soot formation in laminar, premixed flames is presented. The analysis is based on a simplified inception model, detailed kinetic calculations of soot surface growth, and coalescing particle collisions. A sectional aerosol dynamics algorithm which involves solving a master equation set for the densities of different particle size classes provides an efficient solution scheme. The calculation of surface growth and coalescence sectional coefficients has been simplified and extended to the entire temperature range of interest in flame simulations. In order to test convergence properties, the former geometric limitation on the number of size classes has been relaxed. Convergence of the soot volume fraction typically requires only a few size classes and balance equations. Several possible soot surface growth models have been compared. The inception and surface growth models require profiles of temperature and important species like benzene, acetylene, and hydrogen atoms, and oxidizing species. Extensive comparisons have been made with well-characterized flame data by using experimental temperature profiles and calculating the concentrations of the important species with a burner code. The calculated species concentrations and surface growth/oxidation rates are input to the aerosol dynamics program, which calculates the evolution of various soot size and density parameters. While aspects of the model are highly simplified, on balance it appears to give agreement with experiment that is comparable to that obtained from more elaborate models. The calculated sensitivity of soot growth to temperature and the important inception and coalescence parameters is discussed.

Distributed-memory parallel computation of a forced, time-dependent, sooting, ethylene/air coflow diffusion flame

Forced, time-varying laminar flames help bridge the gap between laminar and turbulent combustion as they reside in an ever-changing flow environment. A distributed-memory parallel computation of a time-dependent sooting ethylene/air coflow diffusion flame, in which a periodic fluctuation (20 Hz) is imposed on the fuel velocity for four different amplitudes of modulation, is presented. The chemical mechanism involves 66 species, and a soot sectional model is employed with 20 soot sections. The governing equations are discretised using finite differences and solved implicitly using a damped modified Newtons method. The solution proceeds in parallel using strip domain decomposition over 40 central processing units (CPUs) until full periodicity is attained. For forcing amplitudes of 30%, 50%, 70% and 90%, a complete cycle of numerical predictions of the time-resolved soot volume fraction is presented. The 50%, 70% and 90% forcing cases display stretching and pinching off of the sooting region into an isolated oval shape. In the 90% forcing case, a well-defined hollow shell-like structure of the soot volume fraction contours occurs, in which the interior of the isolated sooty region has significantly lower soot concentrations than the shell. Preliminary comparisons are made with experimental measurements of the soot volume fraction for the 50% forcing case. The experimental results are qualitatively consistent with the model predictions.

Prediction of electron and ion concentrations in low-pressure premixed acetylene and ethylene flames

Flame stabilisation and extinction in a number of different flows can be affected by application of electric fields. Electrons and ions are present in flames, and because of charge separation, weak electric fields can also be generated even when there is no externally applied electric field. In this work, a numerical model incorporating ambipolar diffusion and plasma kinetics has been developed to predict gas temperature, species, and ion and electron concentrations in laminar premixed flames without applied electric fields. This goal has been achieved by combining the existing CHEMKIN-based PREMIX code with a recently developed methodology for the solution of electron temperature and transport properties that uses a plasma kinetics model and a Boltzmann equation solver. A chemical reaction set has been compiled from seven sources and includes chemiionisation, ion-molecule, and dissociative–recombination reactions. The numerical results from the modified PREMIX code (such as peak number densities of positive ions) display good agreement with previously published experimental data for fuel-rich, non-sooting, low-pressure acetylene and ethylene flames without applied electric fields.

Fuel Additive Effects on Soot across a Suite of Laboratory Devices, Part 1: Ethanol

The impact of a variety of non-metallic fuel additives on soot was investigated in a collaborative university, industry and government effort. The main objective of this program was to obtain fundamental understanding of the mechanisms through which blending compounds into a fuel affects soot emissions. The research team used a suite of laboratory devices that included a shock tube, a well-stirred reactor, a premixed flat flame, an opposed-jet diffusion flame, and a high pressure turbulent reactor. The work reported here focuses on the effects of ethanol addition to ethylene on soot. The addition of ethanol led to substantial reductions in soot in all of the devices except for the opposed-jet diffusion flame. Modeling of the premixed flame and opposed-jet diffusion flame was used to obtain insights into the mechanism behind the opposing effects of ethanol addition in these two flames.

FUEL ADDITIVE EFFECTS ON SOOT ACROSS A SUITE OF LABORATORY DEVICES, PART 2: NITROALKANES

This is the second in a series of papers to summarize results of the impact of nonmetallic fuel additives on soot. The research was conducted by a university, industry, and government team with the primary objective of obtaining fundamental understanding of the mechanisms through which additive compounds blended into a fuel affect soot emissions. The work involved coordinated testing across a suite of laboratory devices: a shock tube, a well-stirred reactor, a premixed flat flame, an opposed-jet diffusion flame, and a high-pressure turbulent reactor. This article summarizes results on the addition of nitroalkanes to a base fuel consisting of n-heptane and toluene as a simple surrogate for jet fuels. In these experiments, the nitroalkanes serve as chemical probes of key reactions leading to soot. The effects of nitroalkane addition on soot were found to be device and condition dependent with no simple trends across the suite of devices.

Modeling Soot Formation in a Stirred Reactor

The formation of carbonaceous particulate matter and polycyclic aromatic hydrocarbons has recently been studied (Stouffer, et al, 2002 and Reich, et al, 2003) in a toroidal well-stirred reactor using ethylene as the fuel, with and without the additive ethanol. In the later work, modeling of the gas-phase species was performed and compared to the experimental trends. In the present study, a modified version of the CHEMKIN-based code for ‘perfectly stirred reactors’ has been used to model soot particle formation, including computations of particle mass and smoke number. Detailed soot formation routines have been extracted from Hall and coworkers (1997), who modeled soot formation in flames. Experimental trends are accurately modeled by the code with quantitative accuracies generally within 50%. The importance of accurate knowledge and control of reactor temperature is discussed. In fact, scatter in the original experimental study can be largely attributed to inadequate temperature control. Speculation for differences between the model and experiment are offered while additive effects and the well known ‘soot bell’ are discussed. For the initial experiments examined by Stouffer et al, the effect of the additive is largely due to temperature differences.© 2004 ASME