PhD Candidate, University of Waterloo
Developing tools to determine the soot properties needed for climate change models and policy.
Non-CO2 emissions, such as soot, are responsible for approximately half of the harmful radiative forcing that underlies climate change. Accordingly, in order to address climate change, it is critical that we develop diagnostics capable of quantifying these emissions. Time-resolved laser-induced incandescence (TiRe-LII) is a diagnostic in which nanoparticles, like soot in flames or exhaust streams, are heated using a short laser pulse until they glow or emit incandescence. The magnitude of the incandescent signal can be used to estimate the number of nanoparticles existing in a gaseous stream. Further, as smaller nanoparticles cool more quickly, the rate of decay of the incandescence following the laser pulse can be related to the properties of the nanoparticles, including their size and thermal characteristics. The literature contains a large set of competing models that are used to interpret TiRe-LII data from soot nanoparticles. Unfortunately, the nanoscale makes it difficult to determine which of these models is most suitable and makes robust interpretation of the data challenging. The current work applies an advanced statistical technique known as Bayesian model selection to TiRe-LII data from soot. The technique acts to determine the model most likely to have produced a given set of data, thereby settling disputes about competing models. By extension, the technique can be used to verify fundamental soot properties, which have been otherwise inaccessible in the past. This will bring this diagnostic to the point that it can properly determine the soot characteristics required as inputs to environmental, human health, and combustion models and will provide the necessary scientific backing for informed policy decisions in regards to climate change.
Abstract: While time-resolved laser-induced incandescence (TiRe-LII) shows promise as a diagnostic for sizing aerosolized iron nanoparticles, the spectroscopic and heat transfer models needed to interpret TiRe-LII measurements on iron nanoparticles remain uncertain. This paper focuses on three key aspects of the models: the thermal accommodation coefficient; the spectral absorption efficiency; and the evaporation sub-model. Based on a detailed literature review, spectroscopic and heat transfer models are defined and applied to analyze TiRe-LII measurements carried out on iron nanoparticles formed in water and then aerosolized into monatomic and polyatomic carrier gases. A comparative analysis of the results shows nanoparticle sizes that are consistent between carrier gases and thermal accommodation coefficients that follow the expected trends with bath gas molecular mass and structure.
Pub.: 10 Feb '15, Pinned: 25 Aug '17
Abstract: Auto-correlated laser-induced incandescence (AC-LII) infers the soot volume fraction (SVF) of soot particles by comparing the spectral incandescence from laser-energized particles to the pyrometrically inferred peak soot temperature. This calculation requires detailed knowledge of model parameters such as the absorption function of soot, which may vary with combustion chemistry, soot age, and the internal structure of the soot. This work presents a Bayesian methodology to quantify such uncertainties. This technique treats the additional “nuisance” model parameters, including the soot absorption function, as stochastic variables and incorporates the current state of knowledge of these parameters into the inference process through maximum entropy priors. While standard AC-LII analysis provides a point estimate of the SVF, Bayesian techniques infer the posterior probability density, which will allow scientists and engineers to better assess the reliability of AC-LII inferred SVFs in the context of environmental regulations and competing diagnostics.
Pub.: 19 Jan '16, Pinned: 25 Aug '17
Abstract: We have performed a comparison of ten models that predict the temporal behavior of laser-induced incandescence (LII) of soot. In this paper we present a summary of the models and comparisons of calculated temperatures, diameters, signals, and energy-balance terms. The models were run assuming laser heating at 532 nm at fluences of 0.05 and 0.70 J/cm2 with a laser temporal profile provided. Calculations were performed for a single primary particle with a diameter of 30 nm at an ambient temperature of 1800 K and a pressure of 1 bar. Preliminary calculations were performed with a fully constrained model. The comparison of unconstrained models demonstrates a wide spread in calculated LII signals. Many of the differences can be attributed to the values of a few important parameters, such as the refractive-index function E(m) and thermal and mass accommodation coefficients. Constraining these parameters brings most of the models into much better agreement with each other, particularly for the low-fluence case. Agreement among models is not as good for the high-fluence case, even when selected parameters are constrained. The reason for greater variability in model results at high fluence appears to be related to solution approaches to mass and heat loss by sublimation.
Pub.: 25 Apr '07, Pinned: 25 Aug '17
Abstract: Publication date: Available online 9 September 2015 Source:Progress in Energy and Combustion Science Author(s): H.A. Michelsen, C. Schulz, G.J. Smallwood, S. Will The understanding of soot formation in combustion processes and the optimization of practical combustion systems require in situ measurement techniques that can provide important characteristics, such as particle concentrations and sizes, under a variety of conditions. Of equal importance are techniques suitable for characterizing soot particles produced from incomplete combustion and emitted into the environment. Additionally, the production of engineered nanoparticles, such as carbon blacks, may benefit from techniques that allow for online monitoring of these processes. In this paper, we review the fundamentals and applications of laser-induced incandescence (LII) for particulate diagnostics in a variety of fields. The review takes into account two variants of LII, one that is based on pulsed-laser excitation and has been mainly used in combustion diagnostics and emissions measurements, and an alternate approach that relies on continuous-wave lasers and has become increasingly popular for measuring black carbon in environmental applications. We also review the state of the art in the determination of physical parameters central to the processes that contribute to the non-equilibrium nanoscale heat and mass balances of laser-heated particles; these parameters are important for LII-signal analysis and simulation. Awareness of the significance of particle aggregation and coatings has increased recently, and the effects of these characteristics on the LII technique are discussed. Because of the range of experimental constraints in the variety of applications for which laser-induced incandescence is suited, many implementation approaches have been developed. This review discusses considerations for selection of laser and detection characteristics to address application-specific needs. The benefits of using LII for measurements of a range of nanoparticles in the fields mentioned above are demonstrated with some typical examples, covering simple flames, internal-combustion engines, exhaust emissions, the ambient atmosphere, and nanoparticle production. We also remark on less well-known studies employing LII for particles suspended in liquids. An important aspect of the paper is to critically assess the improvement in the understanding of the fundamental physical mechanisms at the nanoscale and the determination of underlying parameters; we also identify further research needs in these contexts. Building on this enhanced capability in describing the underlying complex processes, LII has become a workhorse of particulate measurement in a variety of fields, and its utility continues to be expanding. When coupled with complementary methods, such as light scattering, probe-sampling, molecular-beam techniques, and other nanoparticle instrumentation, new directions for research and applications with LII continue to materialize.
Pub.: 10 Sep '15, Pinned: 25 Aug '17
Abstract: This paper describes the application of time-resolved laser-induced incandescence (TiRe-LII), a combustion diagnostic used mainly for measuring soot primary particles, to size silicon nanoparticles formed within a plasma reactor. Inferring nanoparticle sizes from TiRe-LII data requires knowledge of the heat transfer through which the laser-heated nanoparticles equilibrate with their surroundings. Models of the free molecular conduction and evaporation are derived, including a thermal accommodation coefficient found through molecular dynamics. The model is used to analyze TiRe-LII measurements made on silicon nanoparticles synthesized in a low-pressure plasma reactor containing argon and hydrogen. Nanoparticle sizes inferred from the TiRe-LII data agree with the results of a Brunauer–Emmett–Teller analysis.
Pub.: 19 Dec '13, Pinned: 25 Aug '17
Abstract: Abstract This paper presents a comparative analysis of time-resolved laser-induced incandescence measurements of iron, silver, and molybdenum aerosols. Both the variation of peak temperature with fluence and the temperature decay curves strongly depend on the melting point and latent heat of vaporization of the nanoparticles. Recovered nanoparticle sizes are consistent with ex situ analysis, while thermal accommodation coefficients follow expected trends with gas molecular mass and structure. Nevertheless, there remain several unanswered questions and unexplained behaviors: the radiative properties of laser-energized iron nanoparticles do not match those of bulk molten iron; the absorption cross sections of molten iron and silver at the excitation laser wavelength exceed theoretical predictions; and there is an unexplained feature in the temperature decay of laser-energized molybdenum nanoparticles immediately following the laser pulse.AbstractThis paper presents a comparative analysis of time-resolved laser-induced incandescence measurements of iron, silver, and molybdenum aerosols. Both the variation of peak temperature with fluence and the temperature decay curves strongly depend on the melting point and latent heat of vaporization of the nanoparticles. Recovered nanoparticle sizes are consistent with ex situ analysis, while thermal accommodation coefficients follow expected trends with gas molecular mass and structure. Nevertheless, there remain several unanswered questions and unexplained behaviors: the radiative properties of laser-energized iron nanoparticles do not match those of bulk molten iron; the absorption cross sections of molten iron and silver at the excitation laser wavelength exceed theoretical predictions; and there is an unexplained feature in the temperature decay of laser-energized molybdenum nanoparticles immediately following the laser pulse.
Pub.: 19 Dec '16, Pinned: 25 Aug '17
Abstract: The capabilities of time-resolved laser-induced incandescence (TiRe-LII), a combustion diagnostic used almost exclusively to measure soot primary particles, could potentially be extended to size aerosolized metal nanoparticles. In order to do this, however, it is necessary to characterize the thermal accommodation coefficient, α, which specifies the heat conduction rate between the laser-energized nanoparticles and the surrounding gas. This paper extends a molecular dynamics (MD) methodology to calculate α for Fe/He, Fe/Ar, Mo/He, and Mo/Ar systems. A comparative analysis of the results shows that α is most strongly influenced by the potential well between the gas molecule and nanoparticle surface. Finally, the MD-derived value for α is used to recover the nanoparticle size distribution for TiRe-LII measurements made on molybdenum nanoparticles in argon.
Pub.: 22 May '13, Pinned: 25 Aug '17