A three-dimensional (3-D) imaging system for studies of reactive and non-reactive flows is described. It can be used to reveal the topology of turbulent structures and to extract 3-D quantities, such as concentration gradients. Measurements are performed using a high repetition rate laser and detector system in combination with a scanning mirror. In this study, the system is used for laser-induced incandescence measurements to obtain quantitative 3-D soot volume fraction distributions in both laminar and turbulent non-premixed flames. From the acquired data, iso-concentration surfaces are visualised and concentration gradients calculated.
A new diagnostic technique is described that has the capability of making real-time, in situ measurements of the volatile fraction of diesel particulate matter (PM). LIDELS uses two laser pulses of comparable energy, separated in time by an interval sufficiently short to freeze the flow field, to measure the change in PM volume caused by laser-induced desorption of the volatile fraction. The first laser pulse produces elastic light scattering (ELS) that gives the volume of the total PM, and also deposits the energy to desorb the volatiles. ELS from the second pulse gives the volume of the remaining solid portion of the PM, and the ratio of these two measurements is the quantitative solid volume fraction. Calibration is required for the individual total PM and solid fraction to be quantitative. Applicability of the technique is demonstrated for load and EGR sweeps for a turbocharged, direct-injection diesel engine.
A multiwavelength flame emission technique is developed for high spatial resolution determination of soot temperature and soot volume fraction in axisymmetric laminar diffusion flames. Horizontal scans of line-integrated spectra are collected over a spectral range of 500-945 nm. Inversion of these data through one-dimensional tomography using a three-point Abel inversion yields radial distributions of the soot radiation from which temperature profiles are extracted. From an absolute calibration of the flame emission and by use of these temperature data, absorption coefficients are calculated, which are directly proportional to the soot volume fractions. The important optical parameters are discussed. It is shown that a uniform sampling cross section through the flame must be maintained and that variations in sampling area produce inconsistencies between measurements and theory, which cannot be interpreted as spatial averaging of the property field. The variations in cross-sectional sampling area have the largest influence on the measurements at the edges of the flame, where the highest resolution is required. Emission attenuation by soot has been shown to have minor influence on the soot temperature and soot volume fraction for the soot loading of the axisymmetric flame tested. An emission correction scheme is outlined, which could be used for more heavily sooting flames. For a refractive index absorption function E(m) = Im[(m2 - 1)(m2 + 2)] that is independent of wavelength, the soot temperatures and soot volume fractions measured with this technique are in excellent agreement with data obtained by coherent anti-Stokes Raman scattering nitrogen thermometry and two-dimensional soot extinction in the same ethylene coflow diffusion flame. The agreement of the results suggests a limit of the slope of the spectral response of E(m) to be between 0 and 20% over the spectral range examined.
The ambiguity and incorrect treatment of the evaporation term among some LII models in the literature are discussed. This study does not suggest that the correct formulation presented for the evaporation model is adequate, or that it reflects the soot evaporation process under intense evaporation. The emphasis is that the current evaporation model must be used correctly in the evaluation of the LII model against experimental data. Numerical results are presented to demonstrate the significance of the molecular weight associated with the heat of evaporation and the thermal velocity of carbon vapor on the results obtained with the evaporation model. Other errors frequently repeated in the literature are also identified.
Extinction and scattering properties at wavelengths of 250â€“5200 nm were studied for soot emitted from buoyant turbulent diffusion flames in the long residence time regime where soot properties are independent of position in the overfire region and characteristic flame residence times. Flames burning in still air and fueled with gas (acetylene, ethylene, propane, and propylene) and liquid (benzene, toluene, cyclohexane, and n-heptane) hydrocarbon fuels were considered. Measured scattering patterns and ratios of total scattering/absorption cross sections were in good agreement with predictions based on the Rayleigh-Debye-Gans (RDG) scattering approximation in the visible. Measured depolarization ratios were roughly correlated by primary particle size parameter, suggesting potential for completing RDG methodology needed to make soot scattering predictions as well as providing a nonintrusive way to measure primary soot particle diameters. Measurements of dimensionless extinction coefficients were in good agreement with earlier measurements for similar soot populations and were independent of fuel type and wavelength except for reduced values as the near ultraviolet was approached. The ratios of the scattering/absorption refractive index functions were independent of fuel type within experimental uncertainties and were in good agreement with earlier measurements. The refractive index function for absorption was similarly independent of fuel type but was larger than earlier reflectometry measurements in the infrared. Ratios of total scattering/absorption cross sections were relatively large in the visible and near infrared, with maximum values as large as 0.9 and with values as large as 0.2 at 2000 nm, suggesting greater potential for scattering from soot particles to affect flame radiation properties than previously thought.
A novel technique for two-dimensional measurements of soot volume fraction and particle size has been developed. It is based on a combined measurement of extinction and laser-induced incandescence using Nd:YAG laser wavelengths of 532 nm and 1064 nm. A low-energy laser pulse at 532 nm was used for extinction measurements and was followed by a more intense pulse at 1064 nm, delayed by 15 ns, for LII measurements. The 532-nm beam was split into a signal beam passing the flame and a reference beam, both of which were directed to a dye cell. The resulting fluorescence signals, from which the extinctionwas deduced, together with the LII signal, were registered on a single CCD detector. Thus the two-dimensional LII image could be converted to a soot volume fraction map through a calibration procedure during the same laser shot. The soot particle sizes were evaluated from the ratio of the temporal LII signals at two gate time positions. The uncertainty in the particle sizing arose mainly from the low signal for small particles at long gate times and the uncertainty in the flame temperature. The technique was applied to a well-characterized premixed flat flame, the soot properties of which had been previously thoroughly investigated.
The present invention relates to a method and apparatus for applying laser induced incandescence (LII) to determine a primary particle size of submicron sized particles. The present invention has found that in addition to volume fraction information, particle size can be determined using LII due to the fact that transient cooling is dependent on the diameter of the particle. The ratio of a prompt and a second time integrated measurement from the same laser pulse has been found to be a function of the particle size. A modeling process involves a solution of the differential equations describing the heat/energy transfer of the particle and surrounding gas, including parameters to describe vaporization, heat transfer to the medium, particle heating etc. The solution gives temperature and diameter values for the particles over time. These values are then converted to radiation values using Planck’s equation. Thus the technique in accordance with the invention is able to provide a more accurate particle measurement than previous LII techniques, particularly where time averaging is not possible and size measurements must be obtained from a single laser pulse. Simultaneously a particle volume fraction can be obtained in accordance with the invention. Calibration is needed to obtain a quantified volume fraction measurement. In a further embodiment of the present invention, a technique for providing absolute intensity calibration is included in the method.
Aerosols affect the Earth’s temperature and climate by altering the radiative properties of the atmosphere. A large positive component of this radiative forcing from aerosols is due to black carbonâ€”sootâ€”that is released from the burning of fossil fuel and biomass, and, to a lesser extent, natural fires, but the exact forcing is affected by how black carbon is mixed with other aerosol constituents. From studies of aerosol radiative forcing, it is known that black carbon can exist in one of several possible mixing states; distinct from other aerosol particles (externally mixed1, 2, 3, 4, 5, 6, 7) or incorporated within them (internally mixed1, 3, 7), or a black-carbon core could be surrounded by a well mixed shell7. But so far it has been assumed that aerosols exist predominantly as an external mixture. Here I simulate the evolution of the chemical composition of aerosols, finding that the mixing state and direct forcing of the black-carbon component approach those of an internal mixture, largely due to coagulation and growth of aerosol particles. This finding implies a higher positive forcing from black carbon than previously thought, suggesting that the warming effect from black carbon may nearly balance the net cooling effect of other anthropogenic aerosol constituents. The magnitude of the direct radiative forcing from black carbon itself exceeds that due to CH4, suggesting that black carbon may be the second most important component of global warming after CO2 in terms of direct forcing.
Laser-induced incandescence (LII) and laser elastic-scattering measurements have been obtained with subnanosecond time resolution from a propane diffusion flame. Results show that the peak and time integrated values of the LII signal increase with increasing laser fluence to maxima at the time of the onset of significant vaporization, beyond which they both decrease rapidly with further increases in fluence. This latter behavior for the time-integrated value is known to be characteristic for a laser beam with a rectangular spatial profile and is attributed to soot mass loss from vaporization. However, there is no apparent explanation for the corresponding large decrease in the peak value. Analysis shows that the peak value occurs at the time in the laser pulse when the time-integrated fluence reaches approximately 0.2 J/cm2 and that the magnitude of the peak value is strongly dependent on the rate of energy deposition. One possible explanation for this behavior is that, at high laser fluences, a cascade ionization phenomenon leads to the formation of an absorptive plasma that strongly perturbs the LII process.
ABSTRACT Diesel and gasoline 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 soot 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 information on the increasingly desirable size and morphology information. Quantitative LII is shown to provide a sensitive, precise, and repeatable measure of the soot concentration over a wide measurement range. The current research determined the tailpipe particulate emissions characteristics from a DISI (direct injection spark ignition) vehicle, including identifying the relative contributions of various engine modes to the total particulate emissions. The volume concentration measurements were obtained in the undilute exhaust with laser-induced incandescence (LII). Particulate measurements were also performed with ELPI instrumentation, sampling from a mini-diluter. Gravimetric filter sampling was performed to measure mass emission rate, organic/elemental carbon, and sulphates/nitrates/trace elements. The LII technique was demonstrated to be capable of real-time particulate matter measurements over all vehicle transient conditions. The wide measurement range and lower detection limit of LII make it a potentially preferred standard instrument for soot measurements.
The invention relates to a method and an apparatus for the determination of particle volume fractions with laser induced incandescence (LII) using absolute light intensity measurements. This requires a knowledge of the particle temperature either from a numerical model of particulate heating or experimental observation of the particulate temperature. Further, by using a known particle temperature a particle volume fraction is calculated. This avoids the need for a calibration in a source of particulates with a known particle volume fraction or particle concentration. The sensitivity of the detection system is determined by calibrating an extended source of known radiance and then this sensitivity is used to interpret measured LII signals. This results in a calibration independent method and apparatus for measuring particle volume fraction or particle concentrations. A modeling process involves a solution of the differential equations describing the heat/energy transfer of the particle and surrounding gas, including parameters to describe vaporization, heat transfer to the medium, particle heating etc. The solution gives temperature and diameter values for the particles over time. These values are then converted to radiation values using Planck’s equation.