George W. Mulholland

The existence of young soot in the exhaust of inverse diffusion flames

Knowledge of the chemical and physical structure of early soot is useful in the development of sootparticle inception models. This paper examines the hypothesis that soot exiting an inverse diffusion flame is similar in chemical and morphological structure to (1) soot precursor particles and (2) soot that exits underventilated flames. Experiments in volving soot collection from the exhaust of laminar ethylene inverse diffusion flames were performed. Soot samples were analyzed for morphology using transmission electron microscopy, for carbon-to-hydrogen ratio using elemental analysis, for organic fraction using thermaloptical analysis, and for polycyclic aromatic hydrocarbon content using laser microprobe mass spectrometry and gas chromatography/mass spectrometry. Results of these analyses support the validity of the above hypothesis. This finding is significant because exhaust collection from the inverse flame provides an opportunity to gather large samples of young soot without invading the flame with an intrusive probe (a necessary task when collecting precursors low in the center of a normal diffusion flame). Larger samples can then be subjected to more detailed analysis than previously possible. An identification of specific polycyclic aromatic hydrocarbon isomers present in young soot from diffusion flames is reported. The data are available for comparison with polycyclic aromatic hydrocarbon growth, soot inception, and soot growth models.

Structural Property Effect of Nanoparticle Agglomerates on Particle Penetration through Fibrous Filter

Most filtration studies have been conducted with spherical particles; however, many aerosol particles are agglomerates of small primary spheres. Filtration efficiency tests were conducted with silver NP agglomerates, with the agglomerate structure controlled by altering the temperature of a sintering furnace. The mobility diameter and mass of the silver NP agglomerates were measured using a differential mobility analyzer together with an aerosol particle mass analyzer. From these measurements, it was found that the fractal-like dimension, D fm, varied from 2.07 to 2.95 as the sintering temperatures was increased from ambient to 600°C. The agglomerates were essentially fully coalesced at 600°C allowing direct comparison of the filtration behavior of the agglomerate to that of a sphere with the same mobility diameter. Other agglomerate properties measured include the primary diameter, the agglomerate length and aspect ratio, and the dynamic shape factor. Agglomerate filtration modeling with no adjustable parameters has been investigated in terms of diffusion, impaction, and interception. The model results agree qualitatively with the experimental results in the particle size range of 50 to 300 nm. The results indicated that the larger interception length of agglomerates is responsible for the smaller penetration through a fibrous filter in comparison to spherical particles with the same mobility diameters.

Measurement of visible and near-IR optical properties of soot produced from laminar flames

This study describes the measurements of the dimensionless extinction constant, K e , of soot in the visible and IR spectrum using the National Institute of Standards and Technology Large Agglomerate Optics Facility. Soot was produced using a 11 mm i.d. laminar diffusion flame burner fueled with acetylene and ethene. Light extinction measurements were performed using light sources at 543.5, 632.8, 856, 1314, and 1565 nm. The mean values of present measurements of K e range from 7.95 to 10.0. These unique experiments provide accurate values of K e to be used for measurements of soot concentration and temperature in the IR spectrum. These measurements represent the first fuel-specific data available in the near-IR spectrum. The measured K e values for all wavelengths are significantly larger than values calculated using reported values of the refractive index and the Rayleigh theory. Transmission electron microscopy and optical microscopy analyses were used to analyze soot morphology and aerosol properties to estimate the influences of beam shielding and light scattering on the observed variations of K e .

The Effect of Orientation on the Mobility and Dynamic Shape Factor of Charged Axially Symmetric Particles in an Electric Field

The mobility of a nonspherical particle is a function of both particle shape and orientation. Thus, unlike spherical particles, the mobility, through its orientation, depends on the magnitude of the electric field. In this work, we develop a general theory, based on an extension of the work of Happel and Brenner (1965), for the orientation-averaged mobility applicable to any axially symmetric particle for which the friction tensor and the polarization energy are known. By using a Boltzmann probability distribution for the orientation, we employ a tensor formulation for computing the orientation-averaged mobility rather than a scalar analysis previously employed by Kim et al. (2007) for nanowires. The resulting equation for the average electrical mobility is much simpler than the expression based on the scalar approach, and can be applied to any axially symmetric structures such as rods, ellipsoids, and touching spheres. The theory is applied to the specific case of nanowires and the experimental results on the mobility of carbon nanotubes (CNT). A set of working formulas of additional mobility expressions for nanorods and prolate spheroids in the free molecular, continuum, and transition regimes are also presented. Finally, we examine the expression of dynamic shape factor common in the literature, and propose a clearer definition based on the tensor approach. Mathematica codes for the electrical mobility evaluations for five cases are provided in the Supplemental Information. Copyright 2012 American Association for Aerosol Research

Measurement of Smoke Particle Size under Low-Gravity Conditions

Smoke detection experiments were conducted in the Microgravity Science Glovebox (MSG) on the International Space Station (ISS) during Expedition 15 in an experiment entitled Smoke Aerosol Measurement Experiment (SAME). The preliminary results from these experiments are presented. In order to simulate detection of a prefire overheated-material event, samples of five different materials were heated to temperatures below the ignition point. The smoke generation conditions were controlled to provide repeatable sample surface temperatures and air flow conditions. The smoke properties were measured using particulate aerosol diagnostics that measure different moments of the size distribution. These statistics were combined to determine the count mean diameter which can be used to describe the overall smoke distribution.

Use of simulated fractals in the correlation of aerodynamic properties with structure for agglomerates

Abstract An algorithm for generating clusters with constant H-B (Hausdorff-Besicovitch) dimension was developed using a suggestion by Sullivan. This algortithm is used to examine the relation between the fractal dimensions determined from the projections of the cluster and the actual three dimensional cluster. The structure and size distribution of soot particles which were classified according to their electrical mobility is examined.

Analytical expression for the friction coefficient of DLCA aggregates based on extended Kirkwood–Riseman theory

ABSTRACT We use a self-consistent field method, which we have previously validated, to calculate the translational friction coefficient of fractal aerosol particles formed by diffusion-limited cluster aggregation (DLCA). Our method involves solving the Bhatnagar–Gross–Krook model for the velocity around a sphere in the transition flow regime. The velocity and drag results are then used in an extension of Kirkwood–Riseman theory to obtain the drag on the aggregate. Our results span a range of primary sphere Knudsen numbers from 0.01 to 100 for clusters with up to N = 2000 primary spheres. Calculated friction coefficients are in good agreement with experimental data and approach the correct continuum and free molecule limits for small and large Knudsen numbers, respectively. Results show that particles exhibit more continuum-like behavior as the number of primary spheres increase, even when the primary particle is in the free molecule regime; as an illustrative example, the friction coefficient for aggregates with primary sphere Kn = 1 is approximately equal to the continuum friction coefficient for N > 500. We estimate that our calculations are within 10% of the true values of the friction coefficients for the range of Kn and N presented here. Finally, we use our results to develop an analytical expression (Equation (38)) for the friction coefficient over a wide range of aggregate and primary particle sizes. Copyright

Pyrolysis Smoke Generated Under Low-Gravity Conditions

A series of smoke experiments were carried out in the Microgravity Science Glovebox on the International Space Station (ISS) Facility to assess the impact of low-gravity conditions on the properties of the smoke aerosol. The smokes were generated by heating five different materials commonly used in space vehicles. This study focuses on the effects of flow and heating temperature for low-gravity conditions on the pyrolysis rate, the smoke plume structure, the smoke yield, the average particle size, and particle structure. Low-gravity conditions allowed a unique opportunity to study the smoke plume for zero external flow without the complication of buoyancy. The diameter of average mass increased on average by a factor of 1.9 and the morphology of the smoke changed from agglomerate with flow to spherical at no flow for one material. The no flow case is an important scenario in spacecraft where smoke could be generated by the overheating of electronic components in confined spaces. From electron microcopy of samples returned to earth, it was found that the smoke can form an agglomerate shape as well as a spherical shape, which had previously been the assumed shape. A possible explanation for the shape of the smoke generated by each material is presented. Copyright 2015 American Association for Aerosol Research

Condensation Particle Counter Proportionality Calibration from 1 Particle·cm−3 to 104 Particles·cm−3

This article presents a process to calibrate a condensation particle counter (CPC) over the concentration range from 1 particle·cm−3 to 104 particles·cm−3 with traceability to SI units as realized by the National Institute of Standards and Technology (NIST). The process combines two independent steps: a proportionality assessment over the range of the CPC using a consistent diluter and an absolute calibration of the CPC with an aerosol electrometer (AE) at high concentrations. The proportionality assessment, which is the focus of the article, is analogous to the attenuation method for testing the proportionality of high-power laser detectors. This procedure tests the proportionality of one CPC by itself and does not require the use of a calibrated reference CPC. The primary calibration of the CPC with the AE at high concentrations allows for an absolute calibration with NIST traceability, resulting in a simple one-parameter correction to the measured CPC data. The proportionality test enables uncertainties to be assigned to the CPC over an extended range of concentration beyond the lower detection limit of the AE, thereby establishing measurement traceability for low concentrations. The relative expanded uncertainty of the CPC with a coverage factor of k = 2 is 2.8% over the range of about 1 particle·cm−3 to 104 particles·cm−3.

Particle Morphology and Size Results from the Smoke Aerosol Measurement Experiment-2

Results are presented from the Reflight of the Smoke Aerosol Measurement Experiment (SAME-2) which was conducted during Expedition 24 (July-September 2010). The reflight experiment built upon the results of the original flight during Expedition 15 by adding diagnostic measurements and expanding the test matrix. Five different materials representative of those found in spacecraft (Teflon, Kapton, cotton, silicone rubber and Pyrell) were heated to temperatures below the ignition point with conditions controlled to provide repeatable sample surface temperatures and air flow. The air flow past the sample during the heating period ranged from quiescent to 8 cm/s. The smoke was initially collected in an aging chamber to simulate the transport time from the smoke source to the detector. This effective transport time was varied by holding the smoke in the aging chamber for times ranging from 11 to 1800 s. Smoke particle samples were collected on Transmission Electron Microscope (TEM) grids for post-flight analysis. The TEM grids were analyzed to observe the particle morphology and size parameters. The diagnostics included a prototype two-moment smoke detector and three different measures of moments of the particle size distribution. These moment diagnostics were used to determine the particle number concentration (zeroth moment), the diameter concentration (first moment), and the mass concentration (third moment). These statistics were combined to determine the diameter of average mass and the count mean diameter and, by assuming a log-normal distribution, the geometric mean diameter and the geometric standard deviations can also be calculated. Overall the majority of the average smoke particle sizes were found to be in the 200 nm to 400 nm range with the quiescent cases producing some cases with substantially larger particles.