Time-resolved Laser-induced Incandescence Signals Research Articles

Simultaneous Inversion of Particle Size Distribution, Thermal Accommodation Coefficient, and Temperature of In-Flame Soot Aggregates Using Laser-Induced Incandescence.

Measuring the size distribution and temperature of high-temperature dispersed particles, particularly in-flame soot, holds paramount importance across various industries. Laser-induced incandescence (LII) stands out as a potent non-contact diagnostic technology for in-flame soot, although its effectiveness is hindered by uncertainties associated with pre-determined thermal properties. To tackle this challenge, our study proposes a multi-parameter inversion strategy-simultaneous inversion of particle size distribution, thermal accommodation coefficient, and initial temperature of in-flame soot aggregates using time-resolved LII signals. Analyzing the responses of different heat transfer sub-models to temperature rise demonstrates the necessity of incorporating sublimation and thermionic emission for accurately reproducing LII signals of high-temperature dispersed particles. Consequently, we selected a particular LII model for the multi-parameter inversion strategy. Our research reveals that LII-based particle sizing is sensitive to biases in the initial temperature of particles (equivalent to the flame temperature), underscoring the need for the proposed multi-parameter inversion strategy. Numerical results obtained at two typical flame temperatures, 1100 K and 1700 K, illustrate that selecting an appropriate laser fluence enables the simultaneous inversion of particle size distribution, thermal accommodation coefficient, and initial particle temperatures of soot aggregates with high accuracy and confidence using the LII technique.

Near-threshold soot formation in premixed flames at elevated pressure

Soot formation at lean-threshold conditions referred to as “near-threshold sooting conditions” (i.e., with stoichiometry, φ, around 1.90 for ethene as a fuel) are studied in laminar premixed ethylene/air flames at pressure from 1 to 10 bar. Laser extinction is used to measure the soot volume fraction. Time-resolved laser-induced incandescence (TiRe-LII) is used to determine particle diameters from the LII signal temporal decay after pulsed laser heating. Thermophoretic sampling is applied to extract particle samples from the flame and ex situ transmission electron microscopy (TEM) is used to measure particle sizes and morphology. The soot volume fraction scales with pressure in a power-law function with the parameter n as 1.4 to 1.9 for flames at the equivalence ratio (φ = 2.1) even at the onset of soot formation. The elevated dependence of soot volume fraction on height above burner is detected with increasing pressure in the near-threshold sooting conditions. The measured soot diameter increases with pressure and equivalence ratio and its sensitivity to the equivalence ratio increases with increasing pressure. The TiRe-LII signal decay varies only little with height above burner and laser fluence in the near-threshold sooting flame (φ = 1.90–1.95), which indicates that the soot particle surface growth and oxidation are balanced. For a slightly sooting flame, TEM measurements from thermophoretically-sampled soot agree well with the LIIsim-evaluated particle size, indicating the reliability of TiRe-LII particle diameter determination under near-threshold conditions.

Particle Size Distribution of Soot from a Laminar/Diffusion Flame

ABSTRACTA practical method is proposed to determine the inflame soot particle size distribution via an explicit solution derived from time-resolved laser-induced incandescence (TiRe-LII). Three appropriate time intervals are selected from the TiRe-LII decay signal. The equivalent mean sizes as well as relative ratios of number densities for three classes of monodisperse particles are determined with the mono-exponential fit to each interval, which allows an explicit solution for the particle size distribution. Simulations show that inversed log-normal distributions from the explicit solution are coincident with the input parameters in terms of trend, and there exist critical time intervals where inversed results are most insensitive to the variation of interval length. The error of inversion is critically dependent on the geometric standard deviation but weakly dependent on the count median particle diameter. Influences of experimental conditions on the inversed error are additionally evaluated. The results show that flame temperature has a significant impact on the error of inversion. Thus, a database of the error as a function of flame temperature at a fixed aggregate size is established. The error database allowed the inversed results from experimental TiRe-LII signals to be readily corrected at various flame locations by interpolation. The corrected inversed log-normal distributions were consistent in trend with those determined from the established non-linear regression method, and the modeling LII signals reproduced agree with the experimental data. The small deviation of the results potentially stemmed from the statistical noise contained in recorded LII signals.

Measurements and modeling of laser-induced incandescence of soot at different heights in a flat premixed flame

Laser-induced incandescence (LII) measurements were conducted in a laminar flat rich premixed CH4/O2/N2 flame at atmospheric pressure using a Nd:YAG laser operated at 1,064 nm. Time-resolved LII signals were measured at 610 nm (20 nm FWHM) at different heights above the burner exit surface and at different laser fluences. The averaged soot temperature over 10 ns around the laser peak was determined from spectrally resolved LII signals over the spectral range of 500–630 nm. Flame temperature and soot primary particle diameter and aggregate size at several flame heights were obtained using the NO-LIF thermometry and from image analyses of sampled soot. The time-resolved LII signal and the averaged soot temperature were modeled using two LII models. To achieve reasonable agreement between the modeled and experimental LII signal and soot temperature at low laser fluences, it is necessary to use flame height-dependent soot absorption functions. Such practice can be attributed to the continuous variation of soot thermal and/or radiative properties during the surface growth process after inception. At high laser fluences, both LII models are unable to reproduce the overall shape of the normalized time-resolved LII signal profiles and the averaged soot temperatures.

Influence of long pulse duration on time-resolved laser-induced incandescence

Time-resolved laser-induced incandescence (LII) signal of soot in an ethylene laminar diffusion flame was measured with varying laser pulse durations in the range 50–600 ns. This study presents original results since the majority of LII studies reported are based on 7–10-ns pulse duration. The LII signal from soot is a combination of heating and cooling processes of different time scales, and the influence of the pulse duration is therefore particularly relevant. The most striking finding is that when the pulse durations is longer than approximately 100 ns, the time-resolved LII signal reveals a rebound of the LII signal during its slow decaying part. This feature occurs preferably at high fluence and is unexpected as none of the physical and chemical processes known to control LII signal behaviour, and their models suggest such an effect. The phenomenon occurs with both top hat and near Gaussian temporal laser shapes. Inspection of the time-resolved emission spectra shows no indication of a laser-induced fluorescence effect, although gas-phase PAH generated during the laser heating of soot particles cannot be rejected. Other hypotheses are that the mechanism responsible for that behaviour is linked to a slow rate change of the soot morphological characteristics or to the generation of new particles during the long-duration laser excitation. Finally, experiments show that soot volume fraction measured by integrating the temporal LII signal is not affected by the pulse duration in any regions of the flame, implying that the LII method is applicable with long pulse duration lasers.

Measurement of in-cylinder soot particles and their distribution in an optical HSDI diesel engine using time resolved laser induced incandescence (TR-LII)

Soot particles generated during combustion process in a single cylinder optical diesel engine were measured using time-resolved laser induced incandescence technique (TR-LII). Experimentally measured LII data were fitted with theoretically simulated LII data to determine the count median particle diameter and the geometric width of the particle size distribution. TR-LII signals were acquired simultaneously at two different wavelengths for crank angles in the range from 48.4° to 111.4° after top dead centre (aTDC) and for various engine operating conditions. Particle size was found to be dependent on the engine load but no significant change in particle sizes were observed for different fuel injection timings. The average size of the primary particles measured over different crank angles were larger at higher engine loads compared to lower engine loads. A general trend of the soot particle diameter decreasing with crank angle was observed due to oxidation, but towards the end of the expansion stroke a marginal increase in size was noted. For all operating conditions the width of the particle size distribution was found to decrease with crank angle until 61.4°aTDC, and thereafter it increases to reach a well defined size distribution for a given fuel injection quantity despite the differences in the injection timing. The soot particles are randomly transported by the complex fluid motion present within the combustion chamber leading to strong cycle to cycle fluctuations of the measured time resolved LII data. The in-cylinder soot volume fraction derived from TR-LII data revealed that for different fuel injection timings, a relatively higher amount of soot was produced for shorter ignition delay compared to larger ignition delays.

Influence of soot particle aggregation on time-resolved laser-induced incandescence signals

Laser-induced incandescence (LII) is a versatile technique for quantitative soot measurements in flames and exhausts. When used for particle sizing, the time-resolved signals are analysed as these will show a decay rate dependent on the soot particle size. Such an analysis has traditionally been based on the assumption of isolated primary particles. However, soot particles in flames and exhausts are usually aggregated, which implies loss of surface area, less heat conduction and hence errors in estimated particle sizes. In this work we present an experimental investigation aiming to quantify this effect. A soot generator, based on a propane diffusion flame, was used to produce a stable soot stream and the soot was characterised by transmission electron microscopy (TEM), a scanning mobility particle sizer (SMPS) and an aerosol particle mass analyzer coupled in series after a differential mobility analyzer (DMA-APM). Despite nearly identical primary particle size distributions for three selected operating conditions, LII measurements resulted in signal decays with significant differences in decay rate. However, the three cases were found to have quite different levels of aggregation as shown both in TEM images and mobility size distributions, and the results agree qualitatively with the expected effect of diminished heat conduction from aggregated particles resulting in longer LII signal decays. In an attempt to explain the differences quantitatively, the LII signal dependence on aggregation was modelled using a heat and mass transfer model for LII given the primary particle and aggregate size distribution data as input. Quantitative agreement was not reached and reasons for this discrepancy are discussed.

Soot particle sizing during high-pressure Diesel spray combustion via time-resolved laser-induced incandescence

Single-pulse time-resolved laser-induced incandescence (TiRe-LII) signal transients from soot particulates were acquired during unsteady high pressure Diesel combustion in a constant volume cell for typical top dead center conditions during a Diesel engine cycle. Measurements were performed for initial gas pressures between 1 and 3 MPa, injection pressures between 50 and 130 MPa and laser probe timings between 5 and 16 ms after start of fuel injection. In separate experiments and for the same cell operating conditions gas temperatures were deduced from spectrally resolved soot pyrometry measurements. Implementing the LII model of Kock et al. [Combust. Flame 147 (2006) 79–92] ensemble mean soot particle diameters were evaluated from least-squares fitting of theoretical cooling curves to experimental TiRe-LII signal transients. Since in the experiments the environmental gas temperature and the width of an assumed particle size distribution were not known, the effects of the initial choice of these parameters on retrieved particle diameters were investigated. It is shown that evaluated mean particle diameters are only slightly biased by the choice of typical size distribution widths and gas temperatures. For a fixed combustion phase mean particle diameters are not much affected by gas pressure, however they become smaller at high fuel injection pressure. At a mean chamber pressure of 1.39 MPa evaluated mean particle diameters increased by a factor of two for probe delays between 5 and 16 ms after start of injection irrespective of the choices of first-guess fitting variables, indicating a certain robustness of data analysis procedure.

Laser-induced incandescence for soot-particle sizing at elevated pressure

This paper describes the applicability of laser-induced incandescence (LII) as a measurement technique for primary soot particle sizes at elevated pressure. A high-pressure burner was constructed that provides stable, laminar, sooting, premixed ethylene/air flames at 1–10 bar. An LII model was set up that includes different heat-conduction sub-models and used an accommodation coefficient of 0.25 for all pressures studied. Based on this model experimental time-resolved LII signals recorded at different positions in the flame were evaluated with respect to the mean particle diameter of a log-normal particle-size distribution. The resulting primary particle sizes were compared to results from TEM images of soot samples that were collected thermophoretically from the high-pressure flame. The LII results are in good agreement with the mean primary particle sizes of a log-normal particle-size distribution obtained from the TEM-data for all pressures, if the LII signals are evaluated with the heat-conduction model of Fuchs combined with an aggregate sub-model that describes the reduced heat conduction of aggregated primary soot particles. The model, called LIISim, is available online via a web interface.

Influence of the temporal response of the detection system on time-resolved laser-induced incandescence signal evolutions

Time-resolved LII (TIRE-LII) measurements are performed simultaneously at two different wavelengths in a sooting, premixed, flat acetylene flame under atmospheric pressure conditions. The influence of temporal response of the detection system on the measured evolution of the LII signal is discussed. The effect of the temporal response on the determination of particle size distributions is quantified for data evaluation starting some nanoseconds after the maximum particle ensemble temperature. Furthermore, it is investigated how the temporal response of a slow detection system affects the determination of accommodation parameters, e.g. thermal accommodation coefficients, and evaporation coefficients, if TIRE-LII signals are modelled including particle heating as well as particle cooling, and if deconvolution techniques are not applied to the measured LII signal.

Experimental and theoretical comparison of spatially resolved laser-induced incandescence (LII) signals of soot in backward and right-angle configuration

In-situ measurements of soot volume fraction in the exhausts of jet engines can be carried out using the laser-induced incandescence (LII) technique in backward configuration, in which the signal is detected in the opposite direction of the laser beam propagation. In order to improve backward LII for quantitative measurements, we have in this work made a detailed experimental and theoretical investigation in which backward LII has been compared with the more commonly used right-angle LII technique. Both configurations were used in simultaneous visualization experiments at various pulse energies and gate timings in a stabilized methane diffusion flame. The spatial near-Gaussian laser energy distribution was monitored on-line as well as the time-resolved LII signal. A heat and mass transfer model for soot particles exposed to laser radiation was used to theoretically predict both the temporal and spatial LII signals. Comparison between experimental and theoretical LII signals indicates similar general behaviour, for example the broadening of the spatial LII distribution and the hole-burning effect at centre of the beam due to sublimation for increasing laser pulse energies. However, our comparison also indicates that the current heat and mass transfer model overpredicts signal intensities at higher fluence, and possible reasons for this behaviour are discussed.

A Study on Time-Resolved Laser Induced Incandescence Analysis Method for the Measurement of Primary Particle Size in Diesel Exhaust

A simple correlation between particle size and the time-resolved laser induced incandescence signal is introduced and applied to a diesel engine for the measurement of primary particle size in the raw exhaust gas. The measurements from time-resolved LII signal are calibrated for all experimental conditions using Transmission Electron Microscope photographs to investigate the differences between particle size from TEM photographs and that estimated by LII. An evaluation of uncertainty in the LII measurements is also presented. From the comparison between the two measurements and uncertainty analysis, it can be concluded that the time-resolved LII theory employed in this study can be used as a useful tool for measuring primary particle size in engine exhaust since the measurements from LII are reliable and in a good agreement with those from TEM imaging. From a practical point of view, the strategy for particle size measurements presented here are simple and straightforward in its application.

The effect of oxygen and carbon dioxide concentration on soot formation in non-premixed flames

The influence of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation was studied by Time Resolved Laser Induced Incandescence (TIRE-LII) and TEM photography in non-premixed co-flowing flames. TIRE-LII method was used to measure the distribution of two-dimensional soot volume fraction and primary particle size. The soot was directly sampled by the thermophoretic method, and its diameter was examined by TEM photography. Two suitable delay times of the TIRE-LII method affecting measurable range and sensitivity were determined by comparing TEM photographs with the TIRE-LII signal. The effects of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation were investigated with these calibrated techniques. An O 2+(CO 2, N 2, and [Ar+CO 2]) mixtures in co-flow were used to isolate carbon dioxide effects systematically. The primary particle number concentration and soot volume fraction were abruptly decreased by the addition of carbon dioxide to co-flow. This suppression was resulted from the short residence time in inception region because of the late nucleation and the decrease of surface growth distance by the low flame temperature due to the higher thermal capacity and the chemical change of carbon dioxide. The increase of oxygen concentration in the co-flow caused an enhancement of soot nucleation and thus the residence time increase, but the specific growth rate showed almost the same value regardless of the co-flow mixture in the growth region. This result suggests that the specific growth rate has a weak dependence on the relative change of co-flow conditions in non-premixed co-flowing flames.

98/02301 A study on final volatile yield of pulverized coal pyrolysis with thermobalance

The influence of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation was studied by Time Resolved Laser Induced Incandescence (TIRE-LII) and TEM photography in non-premixed co-flowing flames. TIRE-LII method was used to measure the distribution of two-dimensional soot volume fraction and primary particle size. The soot was directly sampled by the thermophoretic method, and its diameter was examined by TEM photography. Two suitable delay times of the TIRE-LII method affecting measurable range and sensitivity were determined by comparing TEM photographs with the TIRE-LII signal. The effects of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation were investigated with these calibrated techniques. An O2+(CO2, N2, and [Ar+CO2]) mixtures in co-flow were used to isolate carbon dioxide effects systematically. The primary particle number concentration and soot volume fraction were abruptly decreased by the addition of carbon dioxide to co-flow. This suppression was resulted from the short residence time in inception region because of the late nucleation and the decrease of surface growth distance by the low flame temperature due to the higher thermal capacity and the chemical change of carbon dioxide. The increase of oxygen concentration in the co-flow caused an enhancement of soot nucleation and thus the residence time increase, but the specific growth rate showed almost the same value regardless of the co-flow mixture in the growth region. This result suggests that the specific growth rate has a weak dependence on the relative change of co-flow conditions in non-premixed co-flowing flames.

98/02295 Soot microstructure in steady and flickering laminar methane/air diffusion flames

The influence of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation was studied by Time Resolved Laser Induced Incandescence (TIRE-LII) and TEM photography in non-premixed co-flowing flames. TIRE-LII method was used to measure the distribution of two-dimensional soot volume fraction and primary particle size. The soot was directly sampled by the thermophoretic method, and its diameter was examined by TEM photography. Two suitable delay times of the TIRE-LII method affecting measurable range and sensitivity were determined by comparing TEM photographs with the TIRE-LII signal. The effects of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation were investigated with these calibrated techniques. An O2+(CO2, N2, and [Ar+CO2]) mixtures in co-flow were used to isolate carbon dioxide effects systematically. The primary particle number concentration and soot volume fraction were abruptly decreased by the addition of carbon dioxide to co-flow. This suppression was resulted from the short residence time in inception region because of the late nucleation and the decrease of surface growth distance by the low flame temperature due to the higher thermal capacity and the chemical change of carbon dioxide. The increase of oxygen concentration in the co-flow caused an enhancement of soot nucleation and thus the residence time increase, but the specific growth rate showed almost the same value regardless of the co-flow mixture in the growth region. This result suggests that the specific growth rate has a weak dependence on the relative change of co-flow conditions in non-premixed co-flowing flames.