David R. Snelling

A calibration-independent laser-induced incandescence technique for soot measurement by detecting absolute light intensity

Laser-induced incandescence (LII) has proved to be a useful diagnostic tool for spatially and temporally resolved measurement of particulate (soot) volume fraction and primary particle size in a wide range of applications, such as steady flames, flickering flames, and Diesel engine exhausts. We present a novel LII technique for the determination of soot volume fraction by measuring the absolute incandescence intensity, avoiding the need for ex situ calibration that typically uses a source of particles with known soot volume fraction. The technique developed in this study further extends the capabilities of existing LII for making practical quantitative measurements of soot. The spectral sensitivity of the detection system is determined by calibrating with an extended source of known radiance, and this sensitivity is then used to interpret the measured LII signals. Although it requires knowledge of the soot temperature, either from a numerical model of soot particle heating or experimentally determined by detecting LII signals at two different wavelengths, this technique offers a calibration-independent procedure for measuring soot volume fraction. Application of this technique to soot concentration measurements is demonstrated in a laminar diffusion flame.

Two-dimensional imaging of soot volume fraction in laminar diffusion flames

A technique for acquiring two-dimensional soot-volume-fraction measurements in laminar flames has been demonstrated. The technique provides a map of very low noise concentration over a range of wavelengths (250-1100 nm). A noise level of 0.0007 in extinction and a spatial resolution of 30-40 microm for soot concentration were achieved with an arc lamp source that was filtered to provide greater spatial coherence and a CCD detector. The broadband arc lamp source also allowed us to avoid the added noise resulting from speckle with coherent laser sources. Beam steering, due to refractive-index gradients in the flame, was measured and compared with theoretical predictions. The optical arrangement to minimize the effect of beam steering is described. As a result the beam steering had no effect on the soot measurements in the flames examined. Flame-transmission maps obtained with this system in an ethylene/air laminar diffusion flame are presented. Tomographic analysis from use of an Abel inversion of the line-of-sight data to obtain radial profiles of soot concentration is described.

Influence of nitrogen dilution and flame temperature on soot formation in diffusion flames

Abstract The line-of-sight soot surface temperatures and soot volume fractions were measured as a function of axial position in overventilated coflow laminar diffusion flames. A comparison of the influence of nitrogen dilution and the flame temperature on soot formation in diffusion flames of ethylene was made, and the relative importance of the two effects was quantified. To isolate the influence of the dilution and flame temperature, a reference condition was specified, such that the temperature of the reactants was 623 K without nitrogen dilution of the fuel gas. In dilution experiments, the temperature of reactants was at the reference temperature and the nitrogen dilution of the ethylene was varied from 0 to 0.78 mole fraction. In a second set of experiments, where dilution was zero, the flame temperature was varied by reducing the temperature of the reactants from 623 to 298 K in six steps. The reduction in soot formation is due to both lowered temperature and fuel concentration in dilution experiments, and due to only lowered temperature in undiluted flames since the temperature was varied by controlling the temperature of the reactants. The difference between the two sets of results, for the same flame temperature, gives the influence of dilution on soot formation. The observed soot concentrations in both diluted and undiluted flames, after allowance is made for differences in flame heights and diameters, are fitted to a simple Arrhenius rate expression where the preexponential factor is linear in fuel mole fraction and the measured activation energy is 200 kJ/mol. This rate expression correctly predicts the observation that the effect of dilution on the maximum soot volume fraction is greater than that of temperature up to a diluent fraction of about 0.7. At this point the contributions of both factors are equal; further increase in the diluent amount makes the contribution of the temperature larger.

Diffuse-light two-dimensional line-of-sight attenuation for soot concentration measurements

A technique of diffuse-light two-dimensional line-of-sight attenuation (diffuse 2D-LOSA) is described and demonstrated that achieves very high levels of sensitivity in transmissivity measurements (optical thicknesses down to 0.001) while effectively mitigating interferences due to beam steering. An optical system is described in which an arc lamp coupled with an integrating sphere is used as a source of diffuse light that is imaged to the center of the particulate laden medium. The center of the medium is then imaged onto a CCD detector with 1:1 magnification. Comparative measurements with collimated 2D-LOSA in nonpremixed flames demonstrate the accuracy and improved optical noise rejection of the technique. Tests in weakly sooting, nonpremixed methane-air flames, and in high pressure methane-air flames, reveal the excellent sensitivity of diffuse 2D-LOSA, which is primarily limited by the shot noise of the lamp and CCD detector.

In-Situ Real-Time Characterization of Particulate Emissions from a Diesel Engine Exhaust by Laser-Induced Incandescence

Diesel engines face tightening particulate matter emissions regulations due to the environmental and health effects attributed to these emissions. There is increasing demand for measuring not only the concentration, but also the size distribution of the particulates. Laser-induced incandescence has emerged as a promising technique for measuring spatially and temporally resolved particulate volume fraction and size. Laser-induced incandescence has orders of magnitude more sensitivity than the gravimetric technique, and thus offers the promise of real-time measurements and adds the increasingly desirable size and morphology information. The usefulness of LII as a diagnostic instrument for the precise measurement of particulate concentration and primary particle size has been demonstrated. Measurements have been performed in the exhaust of a single cylinder DI research diesel engine. Simultaneous gravimetric filter measurements were made for direct comparison with the LII technique. Quantitative LII is shown to provide a sensitive, precise, and repeatable measure of the particulate concentration over a wide dynamic range. LII and gravimetric measurements are shown to correlate well over a wide range of operating conditions. A novel method for determining the primary particle size is shown to be precise enough to distinguish particle sizes for different engine operating conditions, and subsequently the number density of primary particles was determined. LII has also been shown to be sensitive in differentiating the PM performance between four different fuels. The LII technique is capable of real-time particulate matter measurements over any engine transient operation. The wide dynamic range and lower detection limit of LII make it a potentially preferred standard instrument for particulate matter measurements. INTRODUCTION From an environmental perspective, there is an urgent need to decrease the total emissions from transportation engines. The undesirable exhaust emissions include CO2, NOx, and particulate matter (PM). CO2 is a recognized greenhouse gas, and as a result of the Kyoto Protocol, industrialized countries have committed to reducing emissions of CO2. This can be primarily achieved by reductions in fuel consumption, and diesel engines offer the highest efficiency for road-going vehicles. The concession is that the emissions reduction systems for other pollutants are not as well developed for diesel engines as they are for spark-ignited engines. Demand for improved environmental performance has led to increasingly restrictive emission regulations for diesel-powered vehicles throughout Europe, North America, and Japan. Proposed regulations indicate that this trend to lower emissions levels will continue for the foreseeable future. Although PM is regulated for environmental reasons, from an operational point of view, particulate formation is not desirable. A significant portion of atmospheric particulates arises from combustion of fuels in various engines and furnaces. In urban areas, mobile sources are major contributors to ambient PM concentrations. The particulate emissions from diesel engines are in the form of complex aerosols consisting primarily of soot and volatile organics. For regulatory purposes, particulate matter emissions are defined as the mass of the matter that can be collected from a diluted exhaust stream on a filter kept at 52°C. This includes the organic compounds that condense at lower temperatures, but excludes the condensed water. This measurement provides the timeaveraged PM emissions over the period during which the particulates are collected on the filter, making measurements of the transient behavior of PM emissions impractical. Since the collected PM and other

Investigation of Absorption and Scattering Properties of Soot Aggregates of Different Fractal Dimension at 532 nm Using RDG and GMM

Radiative properties of numerically generated fractal soot aggregates of different fractal dimensions were studied using the numerically accurate generalized Mie-solution method (GMM) and the Rayleigh-Debye-Gans (RDG) approximate theory. Fractal aggregates of identical prefactor but different fractal dimensions, namely, 1.4, 1.78, and 2.1, were generated numerically using a tunable algorithm of cluster–cluster aggregation for aggregates containing up to 800 primary particles. Radiative properties of these aggregates were calculated at a wavelength of 532 nm assuming a soot refractive index of 1.6 + 0.6i. Four commonly used structure factors in the RDG approximation were used to investigate the effect of structure factor on the differential and total scattering cross-sections and the asymmetry factor. The differential and total scattering properties calculated using the RDG approximation become increasingly sensitive to the structure factor with increasing the fractal dimension. Primary particle interactions are the fundamental mechanism for the aggregate absorption enhancement for small aggregates and the shielding effect for larger aggregates. The extent of these two competing factors is dependent on the fractal dimension and aggregate size. RDG reasonably predicts the effect of fractal dimension on the scattering properties, but fails to account for the effect of aggregation or fractal morphology on the absorption property of fractal soot aggregates, though the error is in general less than 15%. Copyright 2013 American Association for Aerosol Research

Concurrent Quantitative Laser-Induced Incandescence and SMPS Measurements of EGR Effects on Particulate Emissions from a TDI Diesel Engine

A comparison of scanning mobility particle sizer (SMPS) and laser-induced incandescence (LII) measurements of diesel particulate matter (PM) was performed. The results reveal the significance of the aggregate nature of diesel PM on interpretation of size and volume fraction measurements obtained with an SMPS, and the accuracy of primary particle size measurements by LII. Volume fraction calculations based on the mobility diameter measured by the SMPS substantially over-predict the space-filling volume fraction of the PM. Correction algorithms for the SMPS measurements, to account for the fractal nature of the aggregate morphology, result in a substantial reduction in the reported volume. The behavior of the particulate volume fraction, mean and standard deviation of the mobility diameter, and primary particle size are studied as a function of the EGR for a range of steady-state engine speeds and loads for a turbocharged direct-injection diesel engine. Both the SMPS and LII techniques demonstrate good repeatability and consistency with each other. Increasing the EGR results in a sharp rise in the volume fraction of particulates for all engine speeds and loads. At all speed and load conditions the primary particle size decreases with increasing EGR. (Less)

The structure of the dense core region in transient diesel sprays

Intermittent and highly transient dense diesel sprays were investigated using two-dimensional laserlight scattering and transmission techniques to decipher the internal spray structure. The experiments were conducted at maximum injection pressures ranging from 19 to 32 MPa using an electronic diesel injector with a single-hole nozzle. The experiments involved the imaging of the unperturbed structure of the dense core region of a full-cone intermittent diesel spray on photographic film at high resolution and the simultaneous measurement of laser sheet transmission along the centerline of the spray by an array consisting of 2048 diodes. At all injection pressures, line-of-sight laser sheet transmission measurements showed that the dense core region is fragmented very near to the nozzle exit, about 25–30 nozzle diameters downstream, and perhaps much closer. Further downstream from this location, the transmission measurements and simultaneous 90° scattering images revealed that the structure has an intermittent appearance with pockets of dense spray separated by relatively void regions. Two-dimensional images displayed a highly atomized spray structure beyond 50 nozzle diameters downstream, with no indication of an intact liquid core for the range of injection pressures studied. After eliminating the secondary scattering present by applying a low-pass filter and subtracting the filtered image from the original image, it was demonstrated for enlarged views of the spray that the scattering is from randomly spaced point sources, which were interpreted as droplets, rather than liquid columns, ligaments, or large blobs of fuel. It seems that pressure-atomized and fully pulsed round liquid sprays have a significantly different near-field structure than their steady counterparts. These findings, along with the recent work of others, are inconsistent with the existence of a long (on the order of 100 nozzle diameters or more) intact liquid core in unsteady diesel sprays.

Effects of the Fractal Prefactor on the Optical Properties of Fractal Soot Aggregates

The effects of prefactor on the optical properties of numerically generated fractal soot aggregates were investigated using the numerically exact generalized multi-sphere Mie-solution method (GMM) and the approximate Rayleigh-Debye-Gans (RDG) theory. The numerically generated fractal aggregates consist of 50 to 400 primary particles of 30 nm in diameter. The considered incident laser wavelength is 266 nm. Attention is paid to the effect of prefactor on the vertical-vertical differential scattering cross section, since such quantity has often been used to infer the fractal dimension and prefactor based on the RDG formulation. The fractal prefactor affects the optical properties of the numerically generated soot aggregates through its influence on the compactness of the structure. Using GMM to calculate the optical properties of the numerically generated aggregates results in a lower aggregate absorption cross section, but a higher total scattering cross section with increasing the prefactor. The difference between the RDG results and those of GMM is primarily caused by multiple scattering and such effect is found significant, especially for the higher value of prefactor considered. The fractal dimension derived from the GMM non-dimensional differential scattering cross section agrees well with the morphological value in the case of the lower prefactor of 1.3 considered; however, the derived fractal dimension is much higher than the morphological value for fractal soot aggregates with a prefactor of 2.3. The light scattering derived prefactor is in general lower than the morphological value, especially when the morphological prefactor is higher.Copyright

Effect of EGR on Heavy-Duty Diesel Engine Emissions Characterized With Laser-Induced Incandescence

The regulations governing diesel engine particulate matter (PM) and oxides of nitrogen (NOx ) emissions are becoming increasingly stringent. New instrumentation is urgently needed to make accurate and precise measurements of PM emissions from low-emitting engines and emission control systems in a reasonable amount of time. Laser-induced incandescence (LII) is a technique for making temporally resolved measurements of soot volume fraction. LII offers real-time particulate concentration measurements over several orders of magnitude, and adds desirable information about particulate size and surface area. In this study, the exhaust gas recirculation (EGR) system of a heavy-duty diesel engine was tuned at eight speed/load conditions using quantitative LII. Soot concentrations measured by LII correlated strongly with measurements taken using the standard gravimetric technique and an AVL smoke meter.Copyright