Search

We have developed a strategy for distinguishing between small-angle X-ray scattering (SAXS) from gas-phase species and newly formed nanoparticles in mixed gas- and particle-phase reacting flows. This methodology explicitly accounts for temperature-dependent scattering from gases. We measured SAXS

We present carbon-fiber and aluminum sandwich plates with millimeter thicknesses that exhibit high stiffness- and strength-to-weight ratios. These composites consist of carbon-fiber-reinforced polymer faces and waterjet-cut aluminum cores, bonded using epoxy. Relative to single-ply carbon-fiber-reinforced polymer sheets, this construction provides 22-fold increases in mass-specific flexural rigidity and 18-fold increases in mass-specific flexural strength, with areal densities of only 120–260 mg/cm2. Our work represents a simple and inexpensive platform for creating extremely lightweight structural components for microflyers and small robots.

Photophoretic levitation is a propulsion mechanism in which lightweight objects can be lifted and controlled through their interactions with light. Since photophoretic forces on macroscopic objects are usually maximized at low pressures, they may be tested in vacuum chambers in close proximity to the chamber floor and walls. We report here experimental evidence that the terrain under levitating microflyers, including the chamber floor or the launchpad from which microflyers lift off, can greatly increase the photophoretic lift forces relative to their free-space (mid-air) values. To characterize this so-called "ground effect" during vacuum chamber tests, we introduced a new miniature launchpad composed of three J-shaped (candy-cane-like) wires that minimized a microflyer's extraneous interactions with underlying surfaces. We compared our new launchpads to previously used wire-mesh launchpads for simple levitating mylar-based disks with diameters of 2, 4, and 8 cm. Importantly, wire-mesh launchpads increased the photophoretic lift force by up to sixfold. A significant ground effect was also associated with the bottom of the vacuum chamber, particularly when the distance to the bottom surface was less than the diameter of the levitating disk. We provide guidelines to minimize the ground effect in vacuum chamber experiments, which are necessary to test photophoretic microflyers intended for high-altitude exploration and surveillance on Earth or on Mars., 7 pages, 7 figures, including the Supplemental Material

Microgap thermionic energy converters and nanogap thermophotovoltaic devices can efficiently generate electricity from heat without using moving parts. However, converters that have diminutive spacings are difficult to operate and challenging to manufacture at scale. Previous efforts to establish small gaps, such as microfabricated pillars or nanobeads, have resulted in problems including excessive parasitic heat transfer and temperature limitations. To overcome these obstacles, we have developed a microfabricated spacer film that incorporates precisely positioned standoffs in order to limit the area available for conductive losses. We show that this film increases thermal resistance by more than 50% relative to previous designs and demonstrate its use in a thermionic converter to achieve a temperature difference of ~350 K over a gap of $\sim 2~\mu \text{m}$ across an area of ~1.25 cm2. These films will allow mass production of efficient, high-power-density solid-state electricity generators. [2020-0174]

The Breakthrough Starshot Initiative aims to send a gram-scale probe to Proxima Centuri B using a laser-accelerated lightsail traveling at relativistic speeds. Thermal management is a key lightsail design objective because of the intense laser powers required but has generally been considered secondary to accelerative performance. Here, we demonstrate nanophotonic photonic crystal slab reflectors composed of 2H-phase molybdenum disulfide and crystalline silicon nitride, highlight the inverse relationship between the thermal band extinction coefficient and the lightsail's maximum temperature, and examine the trade-off between the acceleration distance and setting realistic sail thermal limits, ultimately realizing a thermally endurable acceleration minimum distance of 16.3~Gm. We additionally demonstrate multi-scale photonic structures featuring thermal-wavelength-scale Mie resonant geometries, and characterize their broadband Mie resonance-driven emissivity enhancement and acceleration distance reduction. Our results highlight new possibilities in simultaneously controlling optical and thermal response over broad wavelength ranges in ultralight nanophotonic structures., Comment: 15 pages, 4 figures

Many micro-aerial vehicles can benefit from having compact and robust pre-deployment configurations that can later transform into larger and more capable forms. We demonstrate parylene-covered silicon frames that can be folded into origami-inspired aerodynamic shapes for structural applications in centimeter-scale aircraft. By changing the spacing between the frames, we can control the conformality of the parylene deposition, which determines the directionality of the fold (mountain or valley). We fabricated a foldable propeller from such polymer-coated frames and characterized its mass and thrust. [2020-0095]

We argue that light sails that are rapidly accelerated to relativistic velocities by lasers must be significantly curved in order to reduce their mechanical stresses and avoid tears. Using an integrated opto-thermo-mechanical model, we show that the diameter and radius of curvature of a circular light sail should be comparable in magnitude, both on the order of a few meters in optimal designs for gram-scale payloads. Moreover, when sufficient laser power is available, a sail's acceleration length decreases and its chip payload capacity increases as its curvature increases. Our findings provide guidance for emerging light sail design programs, which herald a new era of interstellar space exploration., 8 pages, 4 figures

It is well documented that the lift force of hovering micro aerial vehicles can be enhanced by increasing their air-flow velocities. This is commonly accomplished using nozzles and other flow-manipulating geometries with Reynolds numbers above order 100. [1,2] However, the effects of nozzles and other geometries are not well characterized for lower Reynolds numbers within the Stokes’ regime. In general, controlled flight in low-Reynolds number conditions using conventional propulsion methods such as propellers is difficult. Instead, levitation at ultra-low Reynolds number conditions has been accomplished through other means, including photophoretically as demonstrated recently by Cortes et al. [3] and Azadi et al. [4]. These works levitated planar materials without macroscale geometric enhancements and relied strictly on the lift force created through a temperature or accommodation coefficient difference across the planar structure. In the current work, we numerically explored the feasibility of multi-scale structures operating at low-to-moderate Reynolds numbers that pair microscale photophoretic gas pumping with macroscale jet-inducing nozzles. We used ANSYS Fluent to simulate the lift forces in centimeter-scale porous membrane discs (no macroscale enhancements) and in conical nozzles created from porous membranes. Our results indicate that porous conical nozzles provide an order of magnitude lift enhancement relative to flat discs with inlet velocities as low as 10-6 m/s. In addition, we developed a semi-analytical flow model and found good agreement with the simulations. We are currently fabricating mylar structures analogous to the simulation geometries, laser machined to create porosity and adhered to lightweight frames to maintain their shape. The multi-scale structures we create will be of critical importance for exploring low-pressure environments such as Earth’s mesosphere and the Martian atmosphere. References: [1] Benedict, Moble, et al. "Experimental investigation of micro air vehicle scale helicopter rotor in hover." International Journal of Micro Air Vehicles3 (2015): 231-255. [2] Seddon, John M., and Simon Newman. Basic helicopter aerodynamics. Vol. 40. John Wiley & Sons, 2011. [3] Cortes, John, et al. "Photophoretic levitation of macroscopic nanocardboard plates." Advanced Materials16 (2020): 1906878. [4] Azadi, Mohsen, et al. "Controlled levitation of nanostructured thin films for sun-powered near-space flight." Science Advances7 (2021): eabe1127.

We report a Crookes radiometer that rotates at atmospheric pressure using architected microporous dielectric plates, known as nanocardboard, as vanes [1,2]. Compared to most light mills working at tens of Pascals [3,4], the functionality at pressures three orders-of-magnitude larger results from the metamaterial vanes’ unique features: (1) extremely low areal density (0.1 mg/cm2) that reduces the vane mass and hub friction force by almost 100 times; (2) high thermal resistivity that increases the cross-vane temperature difference; and (3) micro-channels that enable through-vane thermal transpiration gas flows. Each nanocardboard vane features a basketweave-style five-flow-channel pattern to amplify the thermal transpiration force. We manufactured these vanes using microfabrication techniques in four stages: (1) silicon mold creation using photolithography and reactive ion etching; (2) mold conformal coating using atomic layer deposition; (3) carbon nanotube drop-casting and oxygen plasma etching; and (4) mold cleaving and removing using XeF2 isotropic etching [1, 5]. We 3D-printed a 26-mm-diameter quad-arm hub and mounted the vanes to it using super glue. We measured the temperature and rotation speed of the radiometer using thermal and video cameras while illuminating it using an octagonal LED array. We found that our radiometer could operate at atmospheric pressure, and that its rotation rate increased with light intensity. To our knowledge, no other radiometers have achieved such functioning in ambient air. Lastly, we simulated the radiometer’s fluid dynamics, obtaining similar trends between its rotation speed and light intensity and achieving order-of-magnitude agreement with our experiments. Our photophoretically-propelled microstructures reveal new possibilities for light sensing and actuation, aerial microflyers, and photo-generators. References: [1] Lin, Chen, et al. “Nanocardboard as a nanoscale analog of hollow sandwich plates.” Nature Communications 9.1 (2018): 1-8. [2] Cortes, John, et al. “Photophoretic Levitation: Photophoretic Levitation of Macroscopic Nanocardboard Plates” (Adv. Mater. 16/2020). Advanced Materials 32.16 (2020): 2070127. [3] Han, Li-Hsin, et al. “Light-powered micromotor: design, fabrication, and mathematical modeling.” Journal of Microelectromechanical Systems 20.2 (2011): 487-496. [4] Wolfe, David, Andres Larraza, and Alejandro Garcia. “A horizontal vane radiometer: Experiment, theory, and simulation.” Physics of Fluids 28.3 (2016): 037103. [5] Azadi, Mohsen, et al. “Demonstration of Atmospheric-Pressure Radiometer With Nanocardboard Vanes.” Journal of Microelectromechanical Systems 29.5 (2020): 811-817.

Photophoretic or light-driven levitation has been studied extensively in the context of the motion of illuminated micron-sized particles, such as dust grains in the atmosphere under sunlight [1,2], and in relation to Crooks radiometers [3]. When heated by incident light, a micron-sized particle experiences a temperature gradient that in turn results in uneven gas-surface interactions and a net propulsive force [4]. Though thoroughly investigated for micron-sized particles, this phenomenon has rarely been studied to controllably levitate macroscopic objects. We report light-driven levitation of 0.5-um thick mylar samples that have been modified by depositing a 300-nm-thick layer of carbon nanotubes (CNTs) on a single side. The CNT layer serves three key purposes: 1) It acts as a lightweight light absorber, absorbing ~ 90% of the incident light and elevating the temperature of the sample. 2) It increases the structural rigidity of the mylar film, allowing cm-scale discs with submicron thicknesses to hold their shape. 3) It creates a structured porous surface that traps impinging gas molecules, which results in an accommodation coefficient difference between the top and bottom surfaces for gas-surface interactions. Air molecules that rebound from the CNT-coated side have on average higher velocities than those departing from the opposing uncoated mylar surface. We show that the net force thus created can be used to levitate the mylar films. Moreover, we will demonstrate our ability to manipulate a light field in order to control the flight of levitating samples for extended periods of time. References: Jovanovic, O. Photophoresis—Light induced motion of particles suspended in gas. Journal of quantitative spectroscopy & radiative transfer 110, 889–901, (2009) Horvath, Photophoresis – a forgotten force ??, KONA powder and particle journal, 31, 181–199 (2014) Ketsdever, N. Gimelshein, S. Gimelshein, and N. Selden, “Radiometric phenomena: From the 19th to the 21st century”, Vacuum 86, 1644-1662 (2012). Loesche, G. Wurm, T. Jankowski, M. Kuepper, Photophoresis on particles hotter/colder than the ambient gas in the free molecular flow. J. Aerosol Sci, 97, 22–33 (2016)

Since its invention, atomic force microscopy (AFM) has enhanced our understanding of physical and biological systems at sub-micrometer scales. As the performance of AFM depends greatly on the properties of the cantilevers, many works have been done to improving cantilevers by means of modifying their geometries via lithography [1] and ion-beam milling [2,3] that primarily involved opening areas on the cantilever’s face, resulting in high resonant frequency, low spring constant, and low hydrodynamic damping. Similar improvements were achieved using a hollow beam cantilever with nanoscale wall thickness [4]. In fact, the combination of these two approaches (in-plane opening and hollow beam) can result in unique metamaterial structures with tunable properties [5], but it has not been explored for AFM application. In this work, we explore the hollow AFM cantilevers with in-plane modifications. We accomplished this by (1) taking a commercial solid silicon cantilever, (2) making a different number of holes on the face using pulsed laser micromachining, and (3) coating them with alumina using atomic layer deposition and etching the internal silicon that results in a hollow probe with holes. We present the effects of these modifications on the cantilever’s resonant frequency, quality factor, and spring constant in air. This work provides an insight into strategies for tuning cantilever’s properties for both flexural and torsional modes. References: [1] Nilsen, M.; Port, F.; Roos, M.; Gottschalk, K.-E.; Strehle, S. Journal of Micromechanics and Microengineering 2019, 29, (2), 025014. [2] Bull, M. S.; Sullan, R. M. A.; Li, H.; Perkins, T. T. ACS Nano 2014, 8, (5), 4984-4995. [3] Hodges, A. R.; Bussmann, K. M.; Hoh, J. H. Review of Scientific Instruments 2001, 72, (10), 3880-3883. [4] Cha, W.; Nicaise, S.; Lilley, D.; Lin, C.; Bargatin, I. Solid-State Sensors, Actuators and Microsystems Workshop, Hilton Head Island, South Carolina, 2018; Transducer Research Foundation: Hilton Head Island, South Carolina, pp 232-233. [5] Lin, C.; Nicaise, S. M.; Lilley, D. E.; Cortes, J.; Jiao, P.; Singh, J.; Azadi, M.; Lopez, G. G.; Metzler, M.; Purohit, P. K.; Bargatin, I. Nature Communications 2018, 9, (1), 4442. [5] C. Lin, S. M. Nicaise, D. E. Lilley, J. Cortes, P. Jiao, J. Singh, et al., "Nanocardboard as a nanoscale analog of hollow sandwich plates," Nature Communications, vol. 9, p. 4442, 2018/10/25 2018.

Nanostructured thin films open the door to sustained sunlight-powered flight in the near space., We report light-driven levitation of macroscopic polymer films with nanostructured surface as candidates for long-duration near-space flight. We levitated centimeter-scale disks made of commercial 0.5-micron-thick mylar film coated with carbon nanotubes on one side. When illuminated with light intensity comparable to natural sunlight, the polymer disk heats up and interacts with incident gas molecules differently on the top and bottom sides, producing a net recoil force. We observed the levitation of 6-mm-diameter disks in a vacuum chamber at pressures between 10 and 30 Pa. Moreover, we controlled the flight of the disks using a shaped light field that optically trapped the levitating disks. Our experimentally validated theoretical model predicts that the lift forces can be many times the weight of the films, allowing payloads of up to 10 milligrams for sunlight-powered low-cost microflyers at altitudes of 50 to 100 km.

Shock tube ignition delay times and species time history measurements for Primary Reference Fuels (PRFs) such as normal heptane provide targets for the validation of combustion models, which in turn are used to develop more fuel-efficient engines that have smaller environmental footprints. However, a review of the literature has revealed that most of the shock tube ignition delay time and species measurement data for normal heptane have been obtained at elevated pressures, rather than at relatively low pressures where many other important experimental techniques such as jet-stirred reactors and flow reactors can provide corroborating results. One central problem preventing previous shock tube studies from examining first-stage ignition at these lower pressures was that ignition times were too long under these conditions to be measured within the available shock tube test times. To address this issue, recent advances in shock tube techniques for achieving long uniform test times have been applied in order to measure low-pressure first-stage ignition times of normal heptane together with normal heptane fuel time-history records at times up to about 30 ms. These measurements were performed in the Negative Temperature Coefficient (NTC) region ( T = 664 − 792 K) in lean mixtures (21%O2/Ar, equivalence ratio ϕ = 0.5 ) at pressures of roughly P = 3.5 atm using both the conventional and Constrained Reaction Volume (CRV) shock tube filling strategies. The data have been used to evaluate the performance of several combustion models, have been compared with other higher-pressure shock tube first-stage ignition times in n-heptane/20–21%O2 mixtures found in the literature, and have been fit using a two-zone Arrhenius model for first-stage ignition delay times. The results showed that some combustion models yield ignition delay time predictions that differ from the experimental results by as much as an order of magnitude, that under these conditions n-heptane first-stage ignition times scale by roughly P − 0.71 but are insensitive to ϕ, and that the fraction of fuel remaining after first-stage ignition increases with increasing initial experimental temperature.

In atomic force microscopy, the cantilever probe is a critical component whose properties determine the resolution and speed at which images with nanoscale resolution can be obtained. Traditional cantilevers, which have moderate resonant frequencies and high quality factors, have relatively long response times and low bandwidths. In addition, cantilevers can be easily damaged by excessive deformation, and tips can be damaged by wear, requiring them to be replaced frequently. To address these issues, new cantilever probes that have hollow cross-sections and walls of nanoscale thicknesses made of alumina deposited by atomic layer deposition are introduced. It is demonstrated that the probes exhibit spring constants up to ≈100 times lower and bandwidths up to ≈50 times higher in air than their typical solid counterparts, allowing them to react to topography changes more quickly. Moreover, it is shown that the enhanced robustness of the hollow cantilevers enables them to withstand large bending displacements more readily and to be more resistant to tip wear.

We performed a study of the evolution of soot composition and fine structure, i.e., maturity level, in an atmospheric ethylene-air diffusion flame. We used laser-induced incandescence (LII) to provide information about maturity level of the bulk primary particle and X-ray photoelectron spectroscopy (XPS) to provide complementary information about particle-surface-maturity level. The results demonstrate that the bulk material and the particle surface evolve separately in the flame. Increased soot-maturity level is associated with increased long-range order of the particle fine structure. This increased order leads to an increase in the absorption cross-section in the visible and near-infrared and a shift of the absorption to longer wavelengths with increasing maturity level of the bulk particle. These trends result in a decrease in the dispersion exponent (ξ) and increase in the absorption cross-section scaling factor (β), as inferred from LII measurements. LII measurements demonstrate that bulk-ma...

The purpose of this article is to explore the dependence of calculated postshock thermodynamic properties in shock tube experiments upon the vibrational state of the test gas and upon the uncertain...

We present a combined experimental and probabilistic simulation study of soot-precursor. The experiments were conducted using aerosol mass spectrometry coupled with tunable vacuum ultraviolet radiation from the Advanced Light Source at Lawrence Berkeley National Laboratory. Mass spectra and photoionization efficiency (PIE) curves of soot precursor species were measured at different heights in a premixed flat flame and in a counter-flow diffusion flame fueled by ethylene and oxygen. The PIE curves at the pyrene mass from these flames were compared with reference PIE scans recorded for pyrene. The results demonstrate that other C16H10 isomers than pyrene are major components among species condensed onto incipient soot in this study, which is in agreement with the simulations. Species with mass 202 u only have a high prevalence in incipient soot particles drawn from the premixed flame, but hydrocarbon species with sizes in the range 200–400 u are important to incipient-soot formation in both flames. The simulations predict that some species form through combination reactions involving relatively large radicals and bypass traditional molecular-growth pathways through addition of small hydrocarbon species. The experimental results support this prediction; they demonstrate that these species have higher relative abundances in particles formed close to the fuel outlet than smaller, lighter molecular species and indicate that these species are important to early formation of incipient-soot precursors. The results also imply that a leading role in incipient-soot precursor formation is played by species with lower thermal stability than the even-carbon numbered, unsubstituted polycyclic aromatic hydrocarbons known as “stabilomers”.

High-temperature Arrhenius ignition delay time correlations are useful for revealing the underlying parameter dependencies of combustion models, for simplifying and optimizing combustion mechanisms for use in engine simulations, for scaling experimental data to new conditions for comparison purposes, and for guiding in experimental design. We have developed a scaling relationship for Fatty Acid Methyl Ester (FAME) ignition time data taken at high temperatures in 4%O2/Ar mixtures behind reflected shocks using an aerosol shock tube: τign[ms]=2.24×10-6[ms](P[atm])-0.41(ϕ)0.30(Cn)-0.61exp37.1[kcal/mol]Ru[kcal/mol K]T[K] Additionally, we have combined our ignition delay time data for methyl decanoate, methyl palmitate, methyl oleate, and methyl linoleate with other experimental results in the literature in order to derive fuel-specific oxygen-mole-fraction scaling parameters for these surrogates. In this article, we discuss the significance of the parameter values, compare our correlation to others found in the literature for different classes of fuels, and contrast the above expression’s performance with correlations obtained using leading FAME kinetic models in 4%O2/Ar mixtures.

Several techniques have been developed for obtaining long, constant-pressure test times in reflected shock wave experiments in a shock tube, including the use of driver inserts, driver gas tailoring, helium gas diaphragm interfaces, driver extensions, and staged driver gas filling. These techniques are detailed here, including discussion on the most recent strategy, staged driver gas filling. Experiments indicate that this staged filling strategy increases available test time by roughly 20 % relative to single-stage filling of tailored driver gas mixtures, while simultaneously reducing the helium required per shock by up to 85 %. This filling scheme involves firstly mixing a tailored helium–nitrogen mixture in the driver section as in conventional driver filling and, secondly, backfilling a low-speed-of-sound gas such as nitrogen or carbon dioxide from a port close to the end cap of the driver section. Using this staged driver gas filling, in addition to the other techniques listed above, post-reflected shock test times of up to 0.102 s (102 ms) at 524 K and 1.6 atm have been obtained. Spectroscopically based temperature measurements in non-reactive mixtures have confirmed that temperature and pressure conditions remain constant throughout the length of these long test duration trials. Finally, these strategies have been used to measure low-temperature n-heptane ignition delay times.

This article introduces an algorithm that determines the thermodynamic conditions behind incident and reflected shocks in aerosol-laden flows. Importantly, the algorithm accounts for the effects of droplet evaporation on post-shock properties. Additionally, this article describes an algorithm for resolving the effects of multiple-component-fuel droplets. This article presents the solution methodology and compares the results to those of another similar shock calculator. It also provides examples to show the impact of droplets on post-shock properties and the impact that multi-component fuel droplets have on shock experimental parameters. Finally, this paper presents a detailed uncertainty analysis of this algorithm’s calculations given typical experimental uncertainties.

Ignition delay times of normal heptane have been measured at temperatures ranging from 651 to 823 K and at pressures between 6.1 and 7.4 atm at an equivalence ratio of 0.75 in 15%O 2 /5%CO 2 /Ar and in 15%O 2 /Ar mixtures behind reflected shock waves in a shock tube. Time-history measurements of fuel, OH, aldehydes (mostly CH 2 O), CO 2 , H 2 O, and temperature were also measured under these conditions. These time-histories provide critically needed kinetic targets to test and refine large reaction mechanisms. Measurements were acquired using a novel constrained reaction volume approach, wherein a sliding gate valve confined the reactant mixture to a region near the endwall of the shock tube. A staged-driver gas filling strategy, combined with driver section extensions, driver inserts, and driver gas tailoring, was used to obtain constant-pressure test times of up to 55 ms, allowing observations of the chemistry in the Negative Temperature Coefficient (NTC) region. Experiments with conventional shock tube filling were also performed, showing similar overall ignition behavior. Comparisons between current data and simulations using the Mehl et al. n -Heptane mechanism (2011) are provided, revealing that the mechanism generally under-predicts first-stage ignition delay times in the NTC region, and that at low temperatures it over-predicts the extent of fuel decomposition during first stage ignition.

We have developed and built a small porous-plug burner based on the original McKenna burner design. The new burner generates a laminar premixed flat flame for use in studies of combustion chemistry and soot formation. The size is particularly relevant for space-constrained, synchrotron-based X-ray diagnostics. In this paper, we present details of the design, construction, operation, and supporting infrastructure for this burner, including engineering attributes that enable its small size. We also present data for charactering the flames produced by this burner. These data include temperature profiles for three premixed sooting ethylene/air flames (equivalence ratios of 1.5, 1.8, and 2.1); temperatures were recorded using direct one-dimensional coherent Raman imaging. We include calculated temperature profiles, and, for one of these ethylene/air flames, we show the carbon and hydrogen content of heavy hydrocarbon species measured using an aerosol mass spectrometer coupled with vacuum ultraviolet photoionization (VUV-AMS) and soot-volume-fraction measurements obtained using laser-induced incandescence. In addition, we provide calculated mole-fraction profiles of selected gas-phase species and characteristic profiles for seven mass peaks from AMS measurements. Using these experimental and calculated results, we discuss the differences between standard McKenna burners and the new miniature porous-plug burner introduced here.

© 2017 American Chemical Society. We present a critical evaluation of photoionization efficiency (PIE) measurements coupled with aerosol mass spectrometry for the identification of condensed soot-precursor species extracted from a premixed atmospheric-pressure ethylene/oxygen/nitrogen flame. Definitive identification of isomers by any means is complicated by the large number of potential isomers at masses likely to comprise particles at flame temperatures. This problem is compounded using PIE measurements by the similarity in ionization energies and PIE-curve shapes among many of these isomers. Nevertheless, PIE analysis can provide important chemical information. For example, our PIE curves show that neither pyrene nor fluoranthene alone can describe the signal from C16H10isomers and that coronene alone cannot describe the PIE signal from C24H12species. A linear combination of the reference PIE curves for pyrene and fluoranthene yields good agreement with flame-PIE curves measured at 202 u, which is consistent with pyrene and fluoranthene being the two major C16H10isomers in the flame samples, but does not provide definite proof. The suggested ratio between fluoranthene and pyrene depends on the sampling conditions. We calculated the values of the adiabatic-ionization energy (AIE) of 24 C16H10isomers. Despite the small number of isomers considered, the calculations show that the differences in AIEs between several of the isomers can be smaller than the average thermal energy at room temperature. The calculations also show that PIE analysis can sometimes be used to separate hydrocarbon species into those that contain mainly aromatic rings and those that contain significant aliphatic content for species sizes investigated in this study. Our calculations suggest an inverse relationship between AIE and the number of aromatic rings. We have demonstrated that further characterization of precursors can be facilitated by measurements that test species volatility. (Graph Presented).

© 2017 American Chemical Society. We have measured photoionization-efficiency curves for pyrene, fluoranthene, chrysene, perylene, and coronene in the photon energy range of 7.5-10.2 eV and derived their photoionization cross-section curves in this energy range. All measurements were performed using tunable vacuum ultraviolet (VUV) radiation generated at the Advanced Light Source synchrotron at Lawrence Berkeley National Laboratory. The VUV radiation was used for photoionization, and detection was performed using a time-of-flight mass spectrometer. We measured the photoionization efficiency of 2,5-dimethylfuran simultaneously with those of pyrene, fluoranthene, chrysene, perylene, and coronene to obtain references of the photon flux during each measurement from the known photoionization cross-section curve of 2,5-dimethylfuran.

Gas-phase mid-infrared (IR) absorption spectra (2500–3400 cm−1) for eleven fatty acid methyl esters (FAMEs) have been quantitatively measured at temperatures between 25 and 500 °C using an FTIR spectrometer with a resolution of 1 cm−1. Using these spectra, the absorption cross section at 3.39 μm, corresponding to the monochromatic output of a helium–neon laser, is reported for each of these fuels as a function of temperature. The data indicate that the 3.39 μm cross section values of saturated FAMEs vary linearly with the logarithm of the number of C H bonds in the molecule.

Ignition delay times for a variety of low-vapor-pressure biodiesel surrogates were measured behind reflected shock waves, using an aerosol shock tube. These fuels included methyl decanoate (C 11 H 22 O 2 ), methyl laurate (C 13 H 26 O 2 ), methyl myristate (C 15 H 30 O 2 ), methyl palmitate (C 17 H 34 O 2 ), and a methyl oleate (C 19 H 36 O 2 )/Fatty Acid Methyl Ester (FAME) blend. Experiments were conducted in 4% oxygen/argon mixtures with the exception of methyl decanoate which was studied in 1% and 21% oxygen/argon blends. Reflected shock conditions covered initial temperatures from 1026 to 1388 K, pressures of 3.5 and 7.0 atm, and equivalence ratios from 0.3 to 1.4. Arrhenius expressions describing the experimental ignition delay time data are given and compared to those derived from applicable mechanisms available in the literature. Graphical comparisons between experimental data and mechanism predictions are also provided. Experiments of methyl laurate, methyl myristate, and methyl palmitate represent the first shock tube ignition delay time measurements for these fuels. Finally, experiments with methyl palmitate represent, to the authors’ knowledge, the first neat fuel/oxidizer/diluent gas-phase shock tube experiments involving a fuel which is a waxy solid at room temperature.

Ignition delay times for methyl oleate (C 19 H 36 O 2 , CAS: 112-62-9) and methyl linoleate (C 19 H 34 O 2 , CAS: 112-63-0) were measured for the first time behind reflected shock waves, using an aerosol shock tube. The aerosol shock tube enabled study of these very-low-vapor-pressure fuels by introducing a spatially-uniform fuel aerosol/4% oxygen/argon mixture into the shock tube and employing the incident shock wave to produce complete fuel evaporation, diffusion, and mixing. Reflected shock conditions covered temperatures from 1100 to 1400 K, pressures of 3.5 and 7.0 atm, and equivalence ratios from 0.6 to 2.4. Ignition delay times for both fuels were found to be similar over a wide range of conditions. The most notable trend in the observed ignition delay times was that the pressure and equivalence ratio scaling were a strong function of temperature, and exhibited cross-over temperatures at which there was no sensitivity to either parameter. Data were also compared to the biodiesel kinetic mechanism of Westbrook et al. (2011) [10] , which underpredicts ignition delay times by about 50%. Differences between experimental and computed ignition delay times were strongly related to existing errors and uncertainties in the thermochemistry of the large methyl ester species, and when these were corrected, the kinetic simulations agreed significantly better with the experimental measurements.

Many oxygenated hydrocarbon species formed during combustion, such as furans, are highly toxic and detrimental to human health and the environment. These species may also increase the hygroscopicity of soot and strongly influence the effects of soot on regional and global climate. However, large furans and associated oxygenated species have not previously been observed in flames, and their formation mechanism and interplay with polycyclic aromatic hydrocarbons (PAHs) are poorly understood. We report on a synergistic computational and experimental effort that elucidates the formation of oxygen-embedded compounds, such as furans and other oxygenated hydrocarbons, during the combustion of hydrocarbon fuels. We used ab initio and probabilistic computational techniques to identify low-barrier reaction mechanisms for the formation of large furans and other oxygenated hydrocarbons. We used vacuum-UV photoionization aerosol mass spectrometry and X-ray photoelectron spectroscopy to confirm these predictions. We show that furans are produced in the high-temperature regions of hydrocarbon flames, where they remarkably survive and become the main functional group of oxygenates that incorporate into incipient soot. In controlled flame studies, we discovered ∼100 oxygenated species previously unaccounted for. We found that large alcohols and enols act as precursors to furans, leading to incorporation of oxygen into the carbon skeletons of PAHs. Our results depart dramatically from the crude chemistry of carbon- and oxygen-containing molecules previously considered in hydrocarbon formation and oxidation models and spearhead the emerging understanding of the oxidation chemistry that is critical, for example, to control emissions of toxic and carcinogenic combustion by-products, which also greatly affect global warming.

The purpose of this study was to quantify the ultraviolet B (UVB) output and in vitro previtamin D3 synthesis over time from various artificial light sources. Three incandescent lamps, T-Rex Active UVHeat 160 watt spot, T-Rex Active UVHeat 160 watt flood, and ZooMed PowerSun 160 watt flood, and two 1.2 m fluorescent lamps, Sylvania Blacklight 350 BL and ZooMed Reptisun 5.0, were studied. Total UVB irradiance and concentration of previtamin D synthesized using an in vitro ampoule model were quantified initially and at monthly intervals for 1 year. Incandescent lamps were measured at distances of 0.9 and 1.5 m while fluorescent lamps were measured at distances of 30.5 and 45.7 cm at the lamp's center, using both the radiometer and ampoules. Fluorescent lamp irradiance was also measured at the lamp's ends. Data were analyzed as a repeated measures split-plot in time using SAS with all mean differences determined using Least Squares Means. Incandescent lamp irradiance differences were seen at various distances. The UVHeat lamps had consistently higher previtamin D3 production and irradiance readings compared with the PowerSun lamp. Reptisun 5.0 was consistently higher in UVB irradiance over Sylvania BL 350 at both 30.5 and 45.7 cm. However, there were no differences when comparing conversion of 7-dehydrocholesterol to previtamin D3. Irradiance differences were detected between the centers and ends of the fluorescent lamps. Until UVB requirements for vitamin D3 synthesis in animals are determined, it is impossible to state that one light is superior to another. Zoo Biol 29:741–752, 2010. © 2010 Wiley-Liss, Inc.

A high-speed OH* chemiluminescence imaging diagnostic was developed to image the structure and homogeneity of combustion events behind reflected shock waves in the Stanford Constrained Reaction Volume Shock Tube. An intensified high-repetition-rate imaging system was used to acquire images of OH* chemiluminescence (near 308 nm) through a fused quartz shock tube end-wall window at 10–33 kHz during the combustion of n-heptane (21 % O $$_{2}$$ /Ar, $$\phi = 0.5$$ ). In general, the imaging technique enabled observation of the main ignition event in the core of the shock tube that corresponded to typical markers of ignition (e.g., pressure rise), as well as localized ignition near the wall that preceded the main core ignition event for some conditions. Case studies were performed to illustrate the utility of this novel imaging diagnostic. First, by comparing localized wall ignition events to the core ignition event, the temperature homogeneity of the post-reflected shock gas near the end-wall was estimated to be within 0.5 % for the test condition presented ( $$T=1159\,\hbox {K}, P=0.25\,\hbox {MPa}$$ ). Second, the effect of a recession in the shock tube wall, created by an observation window, on the combustion event was visualized. Localized ignition was observed near the window, but this disturbance did not propagate to the core of the shock tube before the main ignition event. Third, the effect of shock tube cleanliness was investigated by conducting tests in which the shock tube was not cleaned for multiple consecutive runs. For tests after no cleaning was performed, ignition events were concentrated in the lower half of the shock tube. In contrast, when the shock tube was cleaned, the ignition event was distributed around the entire circumference of the shock tube; validating the cleaning procedure.

We have designed and constructed a Hencken-type burner that produces a 38-mm-long linear laminar partially premixed co-flow diffusion flame. This burner was designed to produce a linear flame for studies of soot chemistry, combining the benefit of the conventional Hencken burner's laminar flames with the advantage of the slot burner's geometry for optical measurements requiring a long interaction distance. It is suitable for measurements using optical imaging diagnostics, line-of-sight optical techniques, or off-axis optical-scattering methods requiring either a long or short path length through the flame. This paper presents details of the design and operation of this new burner. We also provide characterization information for flames produced by this burner, including relative flow-field velocities obtained using hot-wire anemometry, temperatures along the centerline extracted using direct one-dimensional coherent Raman imaging, soot volume fractions along the centerline obtained using laser-induced incandescence and laser extinction, and transmission electron microscopy images of soot thermophoretically sampled from the flame.

Biodiesel is an oxygenated fuel composed of large fatty acid methyl esters (FAME) and is derived from plant oils and animal fats. Its combustion is associated with significant decreases in pollutant emissions such as hydrocarbons, particulate matter, and carbon monoxide relative to fossil diesel [1]. However, combustion of methyl esters in diesel engines has yielded several troublesome problems, including changes in ignition timing and significant increases in nitrogen oxide emissions [2]. These problems can be overcome by developing and applying detailed chemical kinetic models to the design of diesel engines and fuel blends [3, 4]. Chemical kinetic models are based on experimental combustion data, such as those gathered in jet-stirred reactors, rapid compression machines, flow reactors, laminar flame burners, and shock tube experiments.

We have developed a shock tube that features a sliding gate valve in order to mechanically constrain the reactive test gas mixture to an area close to the shock tube endwall, separating it from a specially formulated non-reactive buffer gas mixture. This second-generation Constrained Reaction Volume (CRV) strategy enables near-constant-pressure shock tube test conditions for reactive experiments behind reflected shocks, thereby enabling improved modeling of the reactive flow field. Here we provide details of the design and operation of the new shock tube. In addition, we detail special buffer gas tailoring procedures, analyze the buffer/test gas interactions that occur on gate valve opening, and outline the size range of fuels that can be studied using the CRV technique in this facility. Finally, we present example low-temperature ignition delay time data to illustrate the CRV shock tube's performance.

Stanford Linear Accelerator Center (SLAC) was faced with the need to secure its PeopleSoft/Oracle business system in an academic environment which only has a minimal firewall. To provide protected access to the database servers for NT-based users all over the site while not hindering the lab's open connectivity with the Internet, we implemented a pseudo three-tier architecture for PeopleSoft with Windows Terminal Server and Citrix MetaFrame technology. The client application and Oracle database were placed behind a firewall, and access was granted via an encrypted link to a thin client. Authentication in the future will be through two-factor token cards. NT workstations in the business system unit were further secured through switched network ports and an automated installation process that included SMB signing and disabling LM Authentication in favor of NTLMv2. The hardened workstations then accessed the business system through the Citrix Secure ICA client. How these security measures affected our mixed environment (Windows9x, Samba, Transarc AFS clients, Pathworks, developers, researchers) is discussed.