Charles W. Bruce

Trans-spectral absorption and scattering of electromagnetic radiation by diesel soot

The mass density normalized absorption and total scattering coefficients have been measured using in situ techniques at selected wavelengths from the visible to approximately 1 cm for soot generated by the open combustion of diesel fuel. Particle morphologies are complex although similar to those of soots of other hydrocarbons and methods of generation. An ellipsoidal model has been applied as an approximation to the often multiconnected, chainlike aerosol and then compared with the measured results. The experimental results show an approximate (lambda)(-1) dependence over more than five decades of wavelength data. There is only general agreement with the simplified calculations in this feature as well as in the magnitude.

Water vapor continuum absorption in the 3.5–4.0-μm region

Measurements of water vapor continuum absorption in the 3.5-4.0-microm region are presented. The measurements were made with both long-path absorption cell and spectrophone systems. A deuterium fluoride grating tunable laser was the ir source. Measurements were made at 23 degrees C and 65 degrees C with 14.3 Torr and 65 Torr of water vapor, respectively, buffered to 760-Torr total pressure by an 80/20 mixture of N(2)/O(2). Both natural water and a special sample of deuterium depleted water (one-fiftieth the normal concentration) were used. The 65 degrees C results agree with previous measurements by other workers. The 23 degrees C results indicate a continuum absorption at this temperature about a factor of 2 larger than expected based on the extrapolation scheme and high-temperature data (>/=65 degrees C) of others.

Parametric study of the absorption cross section for a moderately conducting thin cylinder.

Asystem has been developed to measure the absorption cross section for a single carbon fiber at 35 GHz as a functio of length, orientation, and diameter. Typical lengths of the fibers considered ranged from 1 to 20 mm, and diameters ranged from 3 to 8 µm. The results were compared with the modified integra equation calculations of Waterman and Pedersen that describe the scattering and absorption behavior for a wire of finite length and conductivity. Good agreement was found for all lengths, orientations, and diameters studied.

Numerical Methods for Solving the Problem of Electromagnetic Scattering by a Thin Finite Conducting Wire

Scattering, absorption and extinction by a thin finite length conducting wire are computed numerically by solving the generalized Pocklington integro-differential equation using two approaches: the method of moments (MoM) with short range pulse basis functions via the point matching scheme and the Galerkin method with long range basis functions (Legendre polynomials modified to satisfy the boundary conditions of the problem). A new development included in the computations reported here involves a more accurate rendering of wires with lower aspect (length-to-diameter) ratios. Both methods converge to the same answer and satisfy the energy balance to within one percent. A comparison is made with an existing analytical theory by Waterman and Pedersen. This theory solves a more approximate form of the Pocklington equation and is found to have anomalies for some cases. The solutions of this paper agree with the analytical theory for very thin wires, and the results yield a small but significant amplitude and resonance shift for lower aspect ratios. All three solutions are in agreement with the numerous available experimental results to within the experimental errors. The numerical approaches provide a complete direct solution to the problem and remove all the anomalies which occurred in the analytical theory by Waterman and Pedersen.

Propagation at 10 μm through smoke produced by atmospheric combustion of diesel fuel

Absorption and extinction due to the smoke produced by the atmospheric combustion of diesel fuel have been measured using a CO2 laser spectrophone at a wavelength of 10.6 microm. The absorption coefficient normalized to the aerosol mass density is 0.84 +/-0.076 m2/g, and the total scattering coefficient (the difference between the extinction and absorption values) normalized in the same way is 0.15 +/- 0.014 m2/g. The largely fibrous aerosol was found to be 80% carbon, with most of the remainder consisting of hydrocarbons which are comparatively transparent at 10.6 microm and physically resemble the unburned fuel. The normalized coefficients of this study approximate those of diesel automobile effluents. This is not surprising since the aerosol composition and morphology appear to be similar.

Millimeter-wavelength investigation of fibrous aerosol absorption and scattering properties

The measurements here are used to examine agreement with a recently developed theory for long-wavelength fibrous aerosol attenuative properties (extinction and components absorption, scattering). This is intended to be the final phase of a long and systematic examination of the theorys key features. In this case the parameters are high conductivities coupled with a broad range of fiber diameters. It is clear that there is a limit on the extinction efficiency or effective extinction cross section per unit fiber volume. This limit is represented by the fiber diameter of translucency, that is, the diameter at which the fiber is not completely opaque to the electromagnetic energy. The transition is approximated by the classical skin depth of the fiber. Above this diameter the peak extinction efficiency decreases with an increase in diameter at approximately the same rate for all conductors. The scattering resonance producing this peak becomes stronger as the diameter increases. Our data confirm that for fiber diameters below the skin depth the character of the attenuation is that of absorption.

Extinction efficiencies for metallic fibers in the infrared

Mass density normalized extinction has been both measured and computed throughout the infrared for distribution of well-separated synthesized silver fibers. The computational basis is a code originally generated for use with Drudian thin fibers at millimeter wavelengths and modified for application at wavelengths that include molecular and structural (crystalline) resonances as well as thicker fibers. The computation involved convolution of fiber responses over distributions for both fiber lengths and diameters. Agreement between the measured and the computed results was found to be close.

Development of Cleaner-Burning Brick Kilns in Ciudad Juarez, Chihuahua, Mexico

Abstract The following results provide a comparison between net airborne contamination produced by the traditional form of kiln used in Northern Mexico and by those modified according to a design by Dr. Robert Marquez. What has become known as the MK style kiln was intended to significantly reduce contaminant emissions. The concept involves covering the kiln with a dome and channeling the output of an active kiln through a second, identical loaded kiln for its additional filtration of the effluents. Kilns of a pair are connected via clay brick channels. The roles are reversed after the initial kiln is refilled. Significant reductions in the particulate and gaseous emissions were achieved in the prototype system, but a connectional problem with recent kiln pairs has also limited the degree of operational success. The problem did not mask the potential of the MK kiln, as will be shown. Additional anticipated benefits to the owners of MK kilns, such as reduced operating cycles and decreased quantities of fuel, also have been verified. Key measurements made during all of the burns were of aerosol densities and buoyancies in the flues, kiln temperatures, and, on a number of occasions, chemical analyses of both aerosol and gaseous effluents. Continuous time histories of aerosol densities for most burns (of a total of ∼40) provide a basis for examining features and the effects of differing styles of operation with respect to burn efficiency and net contaminant masses. Covering the active kiln with a dome produces a net reduction in dry aerosol effluent mass of a factor between 5 and 10, whereas the addition of a filter kiln produces a net reduction of about a factor of 2. The use of used motor oil as a fuel further reduced aerosol contamination by ∼1 order of magnitude.

Experimentally determined relationship between extinction coefficients and liquid water content.

Simultaneous environmental chamber measurements have been made of 10.27-microm extinction coefficients and liquid water content of droplet distributions with sizes spanning those of light to heavy fogs. The measurements yield a linear relation that is nearly independent of droplet size distribution, in agreement with recent calculated results and predictions. Liquid water content varied from 0.01 to 3.3 g/m(3), and droplet size distributions with both single and bimodal differential extinction coefficient curves were included. The spectral variation of the extinction ceofficient between 9.2 and 10.8 microm was also measured. The results are in good agreement with the variation calculated for typical droplet size distributions.

Electromagnetic Scattering by Finite Conducting Fiber: Limitation of a Previous Published Code

In this paper, the authors examine the low aspect ratio limits of a computational code whose development and application they reported several years ago. A detailed comparison has been made between this code, based on the Moment Method with point matching scheme, and the T-matrix method for the solution of the problem of electromagnetic scattering and absorption by cylinders of finite conductivity. The Alyones et al. code (A-code) has previously been shown to be applicable to relatively thin multilayered fibers, and the T-matrix method has been shown to be valid for low aspect ratios. . . both composed of bulk materials. Here the A-code is tested for low aspect ratio solid fibers and larger radial size parameters (ka) for both low and high conductivity fibrous materials. For very low values of the radial size parameter, the two codes enter agreement at lower aspect ratios, generally at a value of about 10, and as the radial size parameter increases, the agreement occurs at smaller aspect ratios until there is no convergence between the two methods. The criterion is stricter for higher conductivities. A combined code has been formed to handle the calculation of electromagnetic scattering by finite conducting fibers.