Radiative Properties Of Soot Particles Research Articles

Radiative properties of soot fractal superaggregates including backscattering and depolarization

The modeling of radiative properties of soot particles can be addressed by a number of methods. Among them, the canonical Rayleigh-Debye-Gans (RDG) theory and the Superposition T-Matrix Method (STMM) are particularly well adapted to the task. Here, both RDG and STMM are used to model the radiative properties of two types of fractal aggregates indicative of soot relevant to lidar sensing. One type, a canonical aggregate with a chain-like morphology, corresponds to low-sooting flames and is well characterized by a single fractal dimension. The other type, a superaggregate, exhibits multiple fractal dimensions and is seen in heavily sooting flames. Radiative properties such as the differential scattering cross-section, total scattering cross-sections, backscattering cross-sections, and linear depolarization-ratio (LDR) are calculated for a range of wavelengths from 300−1100 nm while including the wavelength dependence of the soot refractive index. Of particular interest is that depolarization by a superaggregate is found to be approximately ten times larger than the depolarization by a canonical chain-like aggregate and both exhibit a power-law with wavelength. Results for specific wavelengths of interest to lidar (355, 532, and 1064 nm) are tabulated for lidar applications. Additionally, RDG theory yields fairly accurate results for modeling the radiative properties, including lidar relevant parameters such as backscattering cross-sections, only for canonical soot fractal aggregates.

Sensitivity analysis of morphology on radiative properties of soot aerosols.

Absorption cross section (Cabs), scattering cross section (Csca) and asymmetry parameter (ASY) of soot particles in different atmospheric aging status were investigated under fixed equivalent volume radius (RV) using the numerically exact multiple-sphere T-matrix method. The radiative properties of soot particles would be largely diverse in different aging status even RV is fixed. However, there are many insensitive parameters under different aging status. The Cabs and ASY is insensitive to monomers number (Ns) when Ns is larger than a threshold value. For bare and thinly coated soot aggregates, Cabs is insensitive to fractal dimension (Df) when the RV is small, where the relative errors of Cabs for different Df are within 2.5%. However, the effects of Df is obvious for large soot due to the shielding effects of large monomers, and the relative errors for different Df can reach to 18% for bare soot. For thinly coated soot, the changes of ASY with soot volume fraction (fsoot) is small due to the little changes of the fractal structure when the RV is fixed. In addition, for thickly coated soot, ASY is insensitive to Ns due to the unchanged overall spherical structure. Our results give a further understanding of the influences of morphology on radiative properties. It may be helpful for model selection and model simplification.

Morphology and mixing state of aged soot particles at a remote marine free troposphere site: Implications for optical properties

AbstractThe radiative properties of soot particles depend on their morphology and mixing state, but their evolution during transport is still elusive. Here we report observations from an electron microscopy analysis of individual particles transported in the free troposphere over long distances to the remote Pico Mountain Observatory in the Azores in the North Atlantic. Approximately 70% of the soot particles were highly compact and of those 26% were thinly coated. Discrete dipole approximation simulations indicate that this compaction results in an increase in soot single scattering albedo by a factor of ≤2.17. The top of the atmosphere direct radiative forcing is typically smaller for highly compact than mass‐equivalent lacy soot. The forcing estimated using Mie theory is within 12% of the forcing estimated using the discrete dipole approximation for a high surface albedo, implying that Mie calculations may provide a reasonable approximation for compact soot above remote marine clouds.

Flame radiation characteristics in pulsed combustion of liquid motor fuels in a constant-volume combustion chamber

Flame combustion of liquid hydrocarbon fuels occurs in augmented engines of transport vehicles and stationary power installations. Calculation of the radiative heat transfer from the flame to the combustion chamber walls represents a complicated problem whose solution requires reliable data on the in-chamber temperature distribution and flame radiation characteristics. Physically, the flame can be considered a complex disperse system consisting of gases and solid particles of soot carbon. The radiative properties of soot particles depend on a number of factors: their geometry, concentration, and optical properties. The burning in augmented small-size combustion chambers of diesel and gas turbine engines and other similar power units proceeds in either pulsed or strongly nonstationary regimes that differ considerably from stationary combustion. The difference consists in the short time of visible burning, the high rate of change in the main parameters of the combustion process, and the soot particle geometry, as well as the cyclicity of the working process. Radiation from the soot particles predominates over radiation of polyatomic gases in the process of heat transfer between the flame and the chamber walls. For this reason, the radiative properties of soot particles and the peculiarities of the heat transfer inside the chamber are given the most attention in the studying of pulsed combustion of liquid fuels. Experimental evidence and calculations are presented for the radiative characteristics of the flame in pulsed combustion of gasolines, kerosenes, and diesel fuels in a constant-volume chamber. The experimental results, along with literature data on the flame emission characteristics under stationary combustion of liquid fuels, are generalized in the form of empirical formulas that permit estimation of the flame radiation properties depending on combustion conditions (parameters). A comparison of the flame emission properties and radiative heat transfer rate calculated by the method developed by the authors shows satisfactory agreement with the results of measurements of these characteristics in diesel combustion chambers. Therefore the obtained formulas are recommended for an engineering analysis of radiative heat transfer in combustion chambers of internal combustion engines and other similar power installations.

Extinction efficiencies of elongated soot particles

Knowledge of the radiative properties of soot particles is needed in many areas of research and practical applications. Soot particles play a significant role in radiative heat-transfer modeling, efficiency of mobile and stationary combustion systems and air pollution studies. The effects of soot shape on flame radiation have often been addressed with much attention given to the limiting shapes of agglomeration: spheres and long chains modeled as infinite cylinders. In the present study, we investigate the existing approximate solutions for cylinder-like agglomerated soot. A new approximate expression for this type of agglomerate is presented that accurately describes the extinction efficiency of combustion-generated soot as a function of the particle radius and its refractive indices.