NOx formation in methyl ester, alcohol, and alkane droplet autoignition and combustion: PLIF measurements and detailed kinetic modeling

2014 Summer.Numerous studies have shown that diesel engines fueled by fatty-acid methyl ester biodiesel often exhibit slightly increased production of oxides of nitrogen (NOx) in comparison to petroleum diesel. A number of explanations for this increase have been proposed. One theory, which has been supported by optical engine test results, suggests that the presence of oxygen atoms in the methyl ester fuel molecule results in a leaner premixed autoignition zone, thereby increasing in-cylinder temperatures and promoting Zel'dovich NOx production. Other experiments have suggested that the unsaturated methyl esters in biodiesel cause an increase in CH radical production (and/or other potential precursors such as C2O) which in turn increases Fenimore NOx formation. In this work, these hypotheses are explored experimentally and computationally by considering autoignition and combustion of single, isolated methyl ester, alcohol and alkane droplets. Experiments were conducted in which the planar laser-induced fluorescence (PLIF) spectroscopy technique was applied to burning liquid fuel droplets in free-fall. A monodisperse stream of droplets was generated by a piezoelectric device and passed through a resistively heated ignition coil. A pulsed laser beam from a Nd:YAG-pumped dye laser (10 Hz, 10 ns width) was formed into a sheet and passed through the droplet flame. The dye laser was tuned to excite hydroxyl (OH) at 282.9 nm and nitric oxide (NO) at 226.0 nm. The resulting fluorescence was imaged by a Cooke Corporation DiCam Pro ICCD digital camera. Band pass filters were utilized to reject laser light scattering while admitting fluorescence wavelengths. Due to the small fluorescence signal, many fluorescence images were averaged together to create a useful average image; approximately 250 and 1000 images were averaged for OH and NO spectroscopy, respectively. Finally, pixel intensity of the averaged fluorescence image was integrated about the droplet center to create qualitative radial profiles of OH and NO concentration. Profiles were generated for a number of oxygenated fuels and one pure hydrocarbon: methanol, ethanol, 1-propanol, methyl butanoate, methyl decanoate, and n-heptane. To quantitatively interpret the contribution of Zel'dovich and Fenimore NOx mechanisms on NOx formation in the vicinity of igniting liquid droplets, detailed numerical droplet combustion simulations were conducted. The transient, spherically symmetric droplet combustion modeling featured detailed gas-phase kinetics, spectrally resolved radiant heat transfer, and multicomponent gas transport. New chemical kinetic mechanisms were created by appending NOx chemical kinetics to existing detailed methanol, methyl butanoate, and n-heptane mechanisms. In the computations, non-oxygenated (heptane) and oxygenated (methyl butanoate, methanol) fuel droplets are introduced into a hot (1150 K) air ambient whereupon the liquid vaporizes, thus producing a stratified fuel/air mixture that thermally autoignites after an ignition delay period. The computational results suggest that NOx formation in stratified fuel/air mixture in the vicinity of a cold liquid droplet is influenced greatly by the detailed full NOx chemistry (Fenimore, Zel'dovich and N2O) and cannot be fully explained by considering only the Zel'dovich NOx route. The computations also suggest, however, that the stoichiometry of the premixed autoignition zone in the laminar gas phase surrounding a spherical droplet differs from that observed in turbulent diesel spray ignition. In single droplets, irrespective of the fuel used, autoignition always initiates in the relatively hot lean region far from the droplet. In diesel sprays, depending on the thermodynamic conditions and fuel reactivity, ignition can occur in lean or rich regions by virtue of turbulent transport of heat and mass. In large molecular weight fuels like n-heptane or petroleum diesel fuel, this is often in mixtures which are quite rich (Φ ≈ 3). To underscore the difference between turbulent spray ignition and ignition of a single droplet, the most reactive mixture fraction and critical scalar dissipation rate were derived for the case of turbulent ignition The results show that for a turbulent non-premixed flame to ignite, two requirements must be met: (1) the fuel/air mixture fraction must be equal or similar to the most reactive mixture fraction, (2) the local scalar dissipation rate must be less than the critical scalar dissipation rate. Due to the effect of scalar dissipation rate on transport and mixing in turbulent, non-premixed flames, it is concluded that, at least as far as autoignition is concerned, autoignition of spherically-symmetric isolated fuel droplets has limitations as physical model for ignition of diesel sprays in compression ignition engines. However, the computations clearly show that transient NOx formation in presence of thermal and concentration gradients cannot be adequately described by the Zel'dovich NOx mechanism, which has consequences with regards to capability of computational engine simulations to accurately predict NOx formation

Three Essays in Regional Economics

This dissertation combines three studies of policies and behaviors in the context of different geographic regions to better understand their causes or effects on the economy. The first study examines the relationship between neighborhood poverty rates and both the density and revenue of Video Gambling Terminals (VGT) in Illinois. Using a combination of linear regression methods, I find evidence that VGT revenue and density are greater in neighborhoods with higher poverty rates. In the second study, I use a seemingly Unrelated Regression approach to analyze the correlation between immigrant populations in the member countries of the European Union and changes in voting for far-right parties. The results show no consistent evidence of a relationship between the number of immigrants in a country and voting for far-right parties. Finally, the third study estimates the impacts to Nevada's economy from an increase in the minimum percentage of electricity which come from renewable sources mandate by the state's Renewable Portfolio Standard (RPS). I use a Computable General Equilibrium (CGE) model to estimate the impacts of the RPS and a $10 per ton carbon tax, and find that the RPS achieves smaller reductions in CO2 emissions by the electricity generation sector than the carbon tax, but also does not reduce activity in the sector by as much

Expanding the Literature on Health and Nutrition for Individuals with Significant Disabilities

This dissertation contains three separate papers, each constituting a chapter, on health and nutrition for individuals with significant disabilities. The first paper is a narrative literature review outlining obesity prevalence, obstacles to healthy living, and promising interventions in both nutrition and physical activity for the population. The second paper is a phenomenological study detailing the lived experiences of six parents of children with significant disabilities regarding the children’s eating habits. Five distinct themes were revealed from coded transcripts and included variety of foods (including those recommended for high, moderate, and low intake); food particularity, preferences, and inflexibility; health conditions impacting eating routines; family mealtime adaptations; and variation in engagement with physical activity. The third paper is an intervention study conducted remotely with four high school students with significant disabilities. This single case research study investigated the system of least prompts to increase participants’ behavior in choosing the food representing the missing food group when shown an array of photos of foods representing the other four groups. Findings indicate that the intervention was effective for increasing all participants’ ability to choose the correct missing food. All students mastered the skill and two maintained at 6-week follow up. Key Words: nutrition education, physical activity, eating habits, significant disabilities, high school students, system of least prompts, remote instruction/learnin

Parents of Children with Significant Disabilities Describe Their Children’s Eating Habits: A Phenomenological Study

This qualitative phenomenological study, through interviews, aimed to understand the experiences of parents of children with significant multiple disabilities about their children’s eating habits. Review of literature indicated disparities in health between people with significant disabilities (SD) that include intellectual disabilities (ID), intellectual/developmental disabilities (IDD), or multiple impairments (MI) and people who are typically developing. People with significant disabilities are at a higher risk for obesity, future weight gain, underweight and/or malnutrition, adherence to a less-healthy diet, and problem behaviors during meal time. Semi-structured initial and follow-up interviews used general questions to gather data, which were subsequently coded and examined for themes across participants. Five themes emerged from the interviews and were compared to findings in the literature. This study included some unique characteristics: detailed descriptions of the children’s eating habits, family experiences around these habits, and what habits are going well for the children

Fra LBL til LGBT: En historisk undersøgelse af trans-inklusionen i Landsforeningen for bøsser og lesbiske, 1999-2008

Denne artikel undersøger trans-inklusionen i 2008 i den daværende Landsforeningen for bøsser og lesbiske, samt udviklingen fra en bøsse-lesbisk forening til en LGBT-forening. Analysen viser at udviklingen til en LGBT-forening begyndte allerede i 1999 da et medlem af foreningens egen ledelse stod frem som transkønnet. Analysen viser at delte livserfaringer (’approximations’) mellem transpersoner og homo- og biseksuelle personer var vigtige for opnåelsen af en trans-inklusion, og dermed en LGBT-udvikling. Analysen viser ydermere at den danske trans-inklusion ikke endte i en kunstig implementering, modsat mange amerikansk

Characterizing Excited State Diffusion in PCDTBT Using Transient Absorption Microscopy

Organic semiconducting polymers (OSPs) are an attractive alternative to traditional inorganic semiconductors for use in photovoltaic devices and other optoelectronic applications because they are cost effective and solution processable. Here we describe our efforts towards under-standing excited state transport in micron-scale domains of the OSP, poly [N-9’’-hepta-decanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] (PCDTBT) utilizing transient absorption microscopy (TAM). Using TAM, we directly image excited state diffusion across mi-cron scale domains of PCDTBT thin films, reducing the effects of morphological heterogeneity in these complex polymeric systems. To further understand exciton diffusion and dissociation dynamics we have begun the development of two individual experiments. This presentation will discuss the theoretical evaluation of both experiments as well as preliminary experimental development. The first experiment has been investigated by Monte Carlo simulation of exciton dissociation at microfabricated donoracceptor interfaces. The second experiment relies on the fabrication of polymer devices so that carrier diffusion can be characterized under the influence of an electric field. Investigation and implementation of these experiments was made possible by support from the Montana Academy of Sciences and will provide a more thorough understanding of the excited state transport dynamics in micron scale regions of PCDTBT

Facile collection of two-dimensional electronic spectra using femtosecond pulse-shaping technology

This letter reports a straightforward means of collecting two-dimensional electronic (2D-E) spectra using optical tools common to many research groups involved in ultrafast spectroscopy and quantum control. In our method a femtosecond pulse shaper is used to generate a pair of phase stable collinear laser pulses which are then incident on a gas or liquid sample. The pulse pair is followed by an ultrashort probe pulse that is spectrally resolved. The delay between the collinear pulses is incremented using phase and amplitude shaping and a 2D-E spectrum is generated following Fourier transformation. The partially collinear beam geometry results in perfectly phased absorptive spectra without phase twist. Our approach is much simpler to implement than standard non-collinear beam geometries, which are challenging to phase stabilize and require complicated calibrations. Using pulse shaping, many new experiments are now also possible in both 2D-E spectroscopy and coherent control.open798

All-optical retrieval of the global phase for two-dimensional Fourier-transform spectroscopy

A combination of spatial interference patterns and spectral interferometry are used to find the global phase for non-collinear two-dimensional Fourier-transform (2DFT) spectra. Results are compared with those using the spectrally resolved transient absorption (STRA) method to find the global phase when excitation is with co-linear polarization. Additionally cross-linear polarized 2DFT spectra are correctly phased using the all-optical technique, where the SRTA is not applicable.Comment: 6 pages, 7 figures, journal publicatio

Elucidation of Ultrafast Photophysics with Optical Pulse Shaping

Optical pulse shaping is an incisive tool of laser spectroscopy that allows the experimentalist extensive flexibility to manipulate the electric field of an excitation laser pulse. In this thesis, four applications of optical pulse shaping are examined. In Chapter 2, a partially non-collinear implementation of two-dimensional electronic spectroscopy is demonstrated for the first time on rubidium vapor. The use of a pulse shaper in this context is advantageous as it significantly reduces the complexity of the experimental apparatus. Unfortunately, non-ideal pulse shaping due to pixelation effects in some pulse-shaping devices leads to spurious pulse generation when the spatial masks become highly modulated. To model the effects on recovered 2D spectra, the optical Bloch model is numerically propagated with an explicit inclusion of the electric field modified by pixelation effects. Finally, steps necessary to minimize distortions are outlined. Chapter 3 describes a series of experiments designed to study the mechanism of multiple exciton generation in semiconductor quantum dots through open and closed loop coherent control schemes. The data collected with open loop control methods indicate that the initially formed exciton relaxes on an ultrafast time scale (\u3c20 fs) revealing the reason for a lack of controllability in adaptive, closed-loop optimizations. Chapter 4 of this thesis examines the process of singlet fission in tetracene thin films employing coherent control and ultrafast pulse shaping. Singlet fission is a promising avenue to achieving highly efficient third generation photovoltaic devices, and in this work, closed-loop control suggests that low-frequency nuclear motions play a mechanistic role in tetracene singlet fission. Finally, Chapter 5 examines a specific class of pulse shaping, sinusoidal spectral phase modulation, which is widely employed in the coherent control community as a route toward an intuitive probe into molecular dynamics. This work seeks to build a bridge between the fields of nonlinear spectroscopy and coherent control by describing the light-matter interactions characteristic of this type of pulse shaping using the tools of time-dependent perturbation theory. This description allows experimentalists to predict and test theories of coherent control simply and with little ambiguity, furthering the usefulness of coherent control as a spectroscopic tool

Label-Free Saturated Structured Excitation Microscopy

Micro- and nanoscale chemical and structural heterogeneities, whether they are intrinsic material properties like grain boundaries or intentionally encoded via nanoscale fabrication techniques, pose a challenge to current material characterization methods. To precisely interrogate the electronic structure of these complex materials systems, spectroscopic techniques with high spatial resolution are required. However, conventional optical microscopies are limited to probe volumes of ~200 nm due to the diffraction limit of visible light. While a variety of sub-diffraction-limited techniques have been developed, many rely on fluorescent contrast agents. Herein we describe label-free saturated structured excitation microscopy (LF-SSEM) applicable to nonlinear imaging approaches such as stimulated Raman and pump-probe microscopy. By exploiting the nonlinear sample response of saturated excitation, LF-SSEM provides theoretically limitless resolution enhancement without the need for a photoluminescent sample