On the dependence of the laser-induced incandescence (LII) signal on soot volume fraction for variations in particle size

“The laser-induced incandescence (LII) signal is proportional to soot volume fraction” is an often used statement in scientific papers, and it has – within experimental uncertainties – been validated in comparisons with other diagnostic techniques in several investigations. In 1984 it was shown theoretically in a paper by Melton that there is a deviation from this statement in that the presence of larger particles leads to some overestimation of soot volume fractions. In the present paper we present a detailed theoretical investigation of how the soot particle size influences the relationship between LII signal and soot volume fraction for different experimental conditions. Several parameters have been varied; detection wavelength, time and delay of detection gate, ambient gas temperature and pressure, laser fluence, level of aggregation and spatial profile. Based on these results we are able, firstly, to understand how experimental conditions should be chosen in order to minimize the errors introduced when assuming a linear dependence between the signal and volume fraction and secondly, to obtain knowledge on how to use this information to obtain more accurate soot volume fraction data if the particle size is known

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

Experimental and theoretical comparison of spatially resolved laser-induced incandescence signals in a sooting flame

A detailed experimental and theoretical investigation has been made on the use of Laser-Induced Incandescence (LII) in two configurations; right-angle LII and backward LII. Both right-angle and backward LII imaging measurements were conducted in simultaneous experiments at various pulse energies. The theoretically calculated LII signals were based on a heat transfer model for soot particles exposed to laser radiation, and were compared with the experimental LII images. Both the experimental and theoretical results from this initial comparison showed similar general behaviour, for example the broadening of the spatial LII distribution and the hole-burning effect at centre for increasing laser pulse energies

On the Use of Laser-Induced Incandescence for Soot Diagnostics: From Theoretical Aspects to Applications in Engines

The laser-induced incandescence technique (LII) is a laser-based diagnostic technique for measurements of soot volume fraction and particle size. The technique relies on detection of incandescent light from soot particles heated to around 4000 K using nanosecond laser pulses. A theoretical model for LII has been implemented and improved in order to provide a tool for predicting the signal response from soot particles when exposed to the laser pulse for various experimental conditions. Specifically, the model is capable of predicting the signal response from arbitrarily shaped measurement volumes defined by non-uniform spatial distribution of laser energy, and also from primary particle size distributions. The model has been applied in order to investigate the influence of various physical and experimental parameters on evaluated primary particle size, the uncertainties introduced when measuring quantitative soot volume fractions in high-pressure environments using atmospheric calibration flames and on the relationship between the LII signal and the soot volume fraction. The model has also been applied in order to predict the appearance of experimentally obtained spatially resolved LII signals in a methane diffusion flame. The spatial distribution of laser energy, which has a profound influence on the LII signal behaviour, was measured using a beam profile CCD camera, the data being input to the model. A generally good agreement between theoretical and experimental data was found, but the results indicated that the theoretical model overpredicted the signal response at high fluence. Two experimental investigations using laser diagnostics for in-cylinder measurements of internal combustion engines have been undertaken. In one of the studies the laser-induced fluorescence (LIF) technique was applied to measure the flame propagation inside a spark-ignition engine by detection of fluorescence from intermediate species in the end gas. Two Nd:YAG lasers operating at 355 nm and two ICCD detectors were used in order to provide two independent images of the unburnt gas region within single engine cycles. An image evaluation scheme was developed in order to evaluate the velocity field of the flame propagating inside the engine. In a second study the laser-induced incandescence technique was applied to measure quantitative soot volume fractions inside a high-speed direct-injection passenger car Diesel engine. The quantitative information was attained by relating the signals obtained in the engine to those obtained in a calibration flame

Lines of Reasoning When Designing Education for Municipal Councillors in Sweden

Although elected representatives play an imperative role for the functioning of a formal democracy, educational research has so far not given much attention to the education and training offered to this group of people. A democratic dilemma may arise in the design and organisation of this education that relate to local governance and policy processes. This paper investigates introductory education that Swedish municipalities offer to municipal councillors and explore the reasons behind its design. The study draws on a comprehensive set of empirical material, consisting of educational programmes from 261 Swedish municipalities and interviews with municipal representatives. The results suggest three different lines of reasoning, denoted system-oriented, relationship-oriented, and market-oriented lines, behind the design of this education. The importance of these results can be considered in relation to previous findings that a strained relation exist between elected representatives and local administrations in Sweden.Funding agencies: Vetenskapsrådet [grant number 2016-05330].</p

Characteristics of laser-induced incandescence from soot in studies of a time-dependent heat- and mass-transfer model

The temporal behavior of the laser-induced incandescence (LII) signal is often used for soot-particle sizing, which is possible because the cooling behavior of a laser-heated particle is dependent on the particle size. The heat- and mass-transfer model describing the temporal LII-signal behavior has in this work been extended to include the influence of the primary particle-size distribution and the spatial distribution of laser energy. When evaluating primary particle size, a monodisperse size distribution is often assumed, although it is well known that a polydisperse distribution is a better description of the real situation. In this work the impact of this assumption is investigated for Gaussian and lognormal size distributions of different widths, and the result is a significant bias towards larger particle sizes because of the higher influence of larger particles on the LII signal. Moreover, the dependence of the LII signal on the laser fluence is studied for different spatial distributions of the laser energy. The top-hat, Gaussian sheet and Gaussian beam distributions were tested and it is established that the LII signal is strongly dependent on the choice of distribution. However, in this case the influence of particle size is minor