Ian G. Shepherd

Modeling light scattering from Diesel soot particles

The Mie model is widely used to analyze light scattering from particulate aerosols. The Diesel particle scatterometer, for example, determines the size and optical properties of Diesel exhaust particles that are characterized by the measurement of three angle-dependent elements of the Mueller scattering matrix. These elements are then fitted by Mie calculations with a Levenburg-Marquardt optimization program. This approach has achieved good fits for most experimental data. However, in many cases, the predicted complex index of refraction was smaller than that for solid carbon. To understand this result and explain the experimental data, we present an assessment of the Mie model by use of a light-scattering model based on the coupled-dipole approximation. The results indicate that the Mie calculation can be used to determine the largest dimension of irregularly shaped particles at sizes characteristic of Diesel soot and, for particles of known refractive index, tables can be constructed to determine the average porosity of the particles from the predicted index of refraction.

Experimental criteria for the determination of fractal parameters of premixed turbulent flames

The influence of spatial resolution, digitization noise, the number of records used for averaging, and the method of analysis on the determination of the fractal parameters of a high Damköhler number, methane/air, premixed, turbulent stagnation-point flame are investigated in this paper. The flow exit velocity was 5 m/s and the turbulent Reynolds number was 70 based on a integral scale of 3 mm and a turbulent intensity of 7%. The light source was a copper vapor laser which delivered 20 nsecs, 5 mJ pulses at 4 kHz and the tomographic cross-sections of the flame were recorded by a high speed movie camera. The spatial resolution of the images is 155 × 121 μm/pixel with a field of view of 50 × 65 mm. The stepping caliper technique for obtaining the fractal parameters is found to give the clearest indication of the cutoffs and the effects of noise. It is necessary to ensemble average the results from more than 25 statistically independent images to reduce sufficiently the scatter in the fractal parameters. The effects of reduced spatial resolution on fractal plots are estimated by artificial degradation of the resolution of the digitized flame boundaries. The effect of pixel resolution, an apparent increase in flame length below the inner scale rolloff, appears in the fractal plots when the measurent scale is less than approximately twice the pixel resolution. Although a clearer determination of fractal parameters is obtained by local averaging of the flame boundaries which removes digitization noise, at low spatial resolution this technique can reduce the fractal dimension. The degree of fractal isotropy of the flame surface can have a significant effect on the estimation of the flame surface area and hence burning rate from two-dimensional images. To estimate this isotropy a determination of the outer cutoff is required and three-dimensional measurements are probably also necessary.

Reaction rates in premixed turbulent flames and their relevance to the turbulent burning speed

An experimental procedure to measure the local reaction rate, in premixed turbulent flames is presented. It utilizes the flame crossing frequency sub-model for the reaction rate formulated by Bray-Champion-Libby (BCL) for turbulent flames with unstrained or equally strained flamelets. The experiments involve measuring the flame crossing frequencies and the conditioned mean velocity of two components using respectively Mie scattering and two color LDA techniques. A method of analysis has also been developed to deduce the turbulent burning speed from. The turbulent/laminar burning speed ratio is obtained by integrating the reaction rates measured along 2D mean Lagrangian flowlines through the flame brush. The flowlines are traced automatically using feedback control for positioning the LDA probe. The method has been applied to study five v-flames and four large bunsen type conical flames. The distribution of are well predicted by the BCL model. The turbulent flame speed results based on are in excellent agreement with those obtained by the conventional flame orientation method. The main advantage of using this method to determine the turbulent burning speed is that the uncertainties are lowered. Furthermore, it can be applied successfully to flames with complex geometries such as the flame tip region of a conical flame.

An experimental estimation of flame surface density and mean reaction rate in turbulent premixed flames

An experimental method based on laser induced Rayleigh scattering for premixed turbulent flames has been developed to obtain a more complete description of the scalar field in the time domain. The Rayleigh scattering data are analyzed to infer the flamelet crossing frequency and the flamelet transit time. An important consequence of these measurements is the development of a means to determine the mean reaction rate at a point within the flame brush without making drastic assumptions about the instantaneous flamelet structure and geometry. The method has been applied to methane-air open conical turbulent premixed flames in the flamelet regime, where the scalar field temporal statistics are determined on the centerline. The variation of the flamelet transit times with the flame and flow parameters is discussed. It is shown that the cold flow integral length scale is the regulating parameter of the flamelet surface to volume ratio for the investigated turbulent combustion regime. The local mean reaction rate is estimated directly from the Rayleigh scattering data.

Simulation of lean premixed turbulent combustion

There is considerable technological interest in developingnew fuel-flexible combustion systems that can burn fuels such ashydrogenor syngas. Lean premixed systems have the potential to burn thesetypes of fuels with high efficiency and low NOx emissions due to reducedburnt gas temperatures. Although traditional scientific approaches basedon theory and laboratory experiment have played essential roles indeveloping our current understanding of premixed combustion, they areunable to meet the challenges of designing fuel-flexible lean premixedcombustion devices. Computation, with itsability to deal with complexityand its unlimited access to data, hasthe potential for addressing thesechallenges. Realizing this potential requires the ability to perform highfidelity simulations of turbulent lean premixed flames under realisticconditions. In this paper, we examine the specialized mathematicalstructure of these combustion problems and discuss simulation approachesthat exploit this structure. Using these ideas we can dramatically reducecomputational cost, making it possible to perform high-fidelitysimulations of realistic flames. We illustrate this methodology byconsidering ultra-lean hydrogen flames and discuss how this type ofsimulation is changing the way researchers study combustion.

Micro scalar timescales in premixed turbulent combustion

Time series measurements of gas density have been obtained in lean (=0.65–0.8) premixed turbulent flames stabilized on a Bunsen-type burner (U0=2.3, 3.5 m/s). Two Eulerian scalar timescales were obtained from spectral analyses of these time series. The first, T^, characterizes the flame front wrinkling process, describable by random telegraph signal statistics, and the second is a small scalar timescale, τD. This was shown to be due to the transitions of the flame fronts as they pass through the probe volume. The ratio between these two Eulerian scales τD/T^=0.4 in the present flames, is a measure of the scale range of scalar fluctuations and is potentially an interesting factor at high turbulent Reynolds numbers when broadening of the preheat zone by turbulent mixing affects this ratio directly by broadening the “dissipation range” in the spectrum. A further timescale, a direct measure of the flame front transit time, was also determined from these data. Estimates of the local mean scalar dissipation were obtained from this data using flamelet assumptions and an interior distribution of the scalar, tested by fitting to numerical simulations of laminar flames, for the local scalar gradients. A simple expression was derived which related the mean scalar disspation to the burning fraction of the scalar probability density function, γ, and a chemical time characteristic of the flame front. It was found that the dissipation rate is affected more by the local scalar gradients than by γ. A dissipation time scale was deduced from this rate and was found to be proportional to the ratio of the scalar integral timescale and the flame transit time.

Structure of Turbulent Premixed Flames as Revealed by Spectral Analysis

Although spectral analysis has proved a powerful tool for studying non-reacting turbulent flows, its application in turbulent reacting flow investigations has been limited. With the development of recent theoretical models which place more emphasis on the temporal characteristics of the scalar and veloCity fluctuations/1/, experimental investigation of their spectral behaviour would be useful to further the development of turbulent combustion models and to infer the physics of the turbulence-combustion interaction mechanisms which control the overall reaction rate.