Soot Particle Studies—Instrument Inter-Comparison—Project Overview

An inter-comparison study of instruments designed to measure the microphysical and optical properties of soot particles was completed. The following mass-based instruments were tested: Couette Centrifugal Particle Mass Analyzer (CPMA), Time-of-Flight Aerosol Mass Spectrometer—Scanning Mobility Particle Sizer (AMS-SMPS), Single Particle Soot Photometer (SP2), Soot Particle-Aerosol Mass Spectrometer (SP-AMS) and Photoelectric Aerosol Sensor (PAS2000CE). Optical instruments measured absorption (photoacoustic, interferometric, and filter-based), scattering (in situ), and extinction (light attenuation within an optical cavity). The study covered an experimental matrix consisting of 318 runs that systematically tested the performance of instruments across a range of parameters including: fuel equivalence ratio (1.8 ≤ φ ≤ 5), particle shape (mass-mobility exponent ( D _fm ), 2.0 ≤ D _fm ≤ 3.0), particle mobility size (30 ≤ d _m ≤ 300 nm), black carbon mass (0.07 ≤ m _BC ≤ 4.2 fg) and particle chemical composition. In selected runs, particles were coated with sulfuric acid or dioctyl sebacate (DOS) (0.5 ≤ Δ r _ve ≤ 201 nm) where Δ r _ve is the change in the volume equivalent radius due to the coating material. The effect of non-absorbing coatings on instrument response was determined. Changes in the morphology of fractal soot particles were monitored during coating and denuding processes and the effect of particle shape on instrument response was determined. The combination of optical and mass based measurements was used to determine the mass specific absorption coefficient for denuded soot particles. The single scattering albedo of the particles was also measured. An overview of the experiments and sample results are presented.     

Particle Morphology and Density Characterization by Combined Mobility and Aerodynamic Diameter Measurements. Part 2: Application to Combustion-Generated Soot Aerosols as a Function of Fuel Equivalence Ratio

Composition, shape factor, size, and fractal dimension of soot aerosol particles generated in a propane/O₂, flame were determined as a function of the fuel equivalence ratio (φ). Soot particles were first size-selected by a differential mobility analyzer (DMA) and then analyzed by an Aerodyne aerosol mass spectrometer (AMS). The DMA provides particles of known mobility diameter (d_m ). The AMS quantitatively measures the mass spectrum of the nonrefractory components of the particles and also provides the vacuum aerodynamic diam eter (d_va ) corresponding to the particles of known mobility diameter. The measured d_m, d_va , and nonrefractory composition are used in a system of equations based on the formulation presented in the companion article to estimate the particle dynamic shape factor, total mass, and black carbon (BC) content. Fractal dimension was estimated based on the mass-mobility relationship. Two types of soot particles were observed depending on the fuel equivalence ratio. Type 1: for φ < 4 (lower propane/O₂), d_va ; was nearly constant and independent of d_m . The value of d_va increased with increasing φ. Analysis of the governing equations showed that these particles were highly irregular (likely fractal aggregates), with a dynamic shape factor that increased with d_m and φ. The fractal dimension of these particles was approximately 1.7. These particles were composed mostly of BC, with the organic carbon content increasing as φ increased. At φ = 1.85, the particles were about 90% BC, 5% PAH, and 5% aliphatic hydrocarbon (particle density = 1.80 g/cm³). Type 2: for φ > 4 (high propane/O₂), d_va was linearly proportional to d_m . Analysis of the governing equations showed that these particles were nearly spherical (likely compact aggregates), with a dynamic shape factor of 1.1 (versus 1 for a sphere) and a fr actal dimension of 2.95 (3 for a sphere). These particles were composed of about 50% PAH, 45% BC, and 5% aliphatic hydrocarbons (particle density = 1.50 g/cm³). These results help interpret some measurement s obtained in recent field studies.     

Influence of oxide scales on heat transfer in secondary cooling zones in the continuous casting process,part 2: determination of material properties of oxide scales on steel under spray-water cooling conditions

In order to model heat transfer from the slab to the cooling agent in the continuous casting process the thermophysical properties of the surface layer must be taken into account. For this purpose thermal conductivities and thermal diffusivities of wustite as well as of two carbon steels were measured. The oxide scale growth for six steels was measured as a function of time and at temperatures between 900 and 1200°C in air and in steam. Complex heterogeneous oxide scales were examined by optical and scanning electron microscopy, by SIMS, as well as by Mössbauer spectroscopy. Segregation of alloying elements in the metal-oxide contact zone was found. Cracks due to thermal stresses result in spalling of the scale. Therefore, a straightforward use of the data for process modelling can principally not be recommended without restrictions.     

A novel hybrid hypervolume indicator and reference vector adaptation strategies based evolutionary algorithm for many-objective optimization

Engineering with Computers - A novel hybrid many-objective evolutionary algorithm called Reference Vector Guided Evolutionary Algorithm based on hypervolume indicator (H-RVEA) is proposed in this...     

Simulation of capillary shrinkage cracking in cement-like materials

In drying suspensions, water loss leads to a capillary pressure build-up in the liquid phase. This effect may also be observed in fresh cement-based materials subjected to evaporation at an open surface. If under decreasing water content the near-surface solid particles are no longer covered by a plane water film, menisci develop along with an associated build-up of negative capillary pressure, resulting in shrinkage and possibly in cracking. A 2D model for simulating the described physical process is presented. For arranging the particles in the 2D specimen a stochastic–heuristic algorithm is used. Subsequently, the course of the water front between the particles is calculated by assuming a constant curvature of the water surface. Particle mobility is taken into account by adopting interparticle forces and performing equilibrium iterations. The model allows one to study the influences of the particle size distribution as well as of the properties of the liquid phase on the capillary pressure build-up and on the cracking risk.     

Computational aspects of inverse analyses for determining softening curves of concrete

Inverse analyses of laboratory experiments for determining strain softening curves of cementitious materials are an important tool of applied fracture mechanics and provide the basis for computational simulations of failure processes in civil structures. In these analyses, experimental results are approximated by repeated numerical simulations of the tests. A new optimisation method for this fitting process is proposed. It is based on an evolutionary algorithm including local neighbourhood attraction for convergence improvement. Several aspects of applicability and performance of the method as well as the objectivity of the results are discussed and applications are presented.     

Bestimmung von Materialeigenschaften zementgebundener Werkstoffe mittels inverser Analyse

Determining properties of cement‐based materials by inverse analysis. The direct experimental determination of certain material properties of cement‐based materials is technically difficult or even impossible. In such cases, a combined application of experimental and numerical methods might be required. The material properties are thereby determined by inverse analyses of experiments. In the field of material testing, two examples for properties which may be determined by inverse analysis are the strain softening curve and the moisture dependent water vapour diffusion coefficient, respectively. These material properties are of special importance for the characterisation of structural durability. Sufficient accuracy and efficiency of an inverse analysis may only be achieved if an adequate physical model of the experiment and a suitable optimisation method are used. For the problems to be solved here, evolutionary algorithms have proved to be appropriate means for the optimisation. The principle of the inverse analysis is explained and two example applications are presented. The latter are the determination of softening curves on the basis of fracture tests and the determination of diffusion coefficients on the basis of drying experiments.     

Auf Kapillardruckmessung basierende Nachbehandlung von Betonflächen im plastischen Materialzustand

Early Age Concrete Curing Based on Capillary Pressure Measurement Due to evaporation of water at fresh concrete surfaces, a capillary pressure is built up in the pore system of the material leading to shrinkage deformations. At a certain material dependent pressure, air penetrates into the pore system. This air entry appears to be a local event and results in an inhomogeneous strain field and possibly in cracks. By controlling the capillary pressure build‐up the risk of cracking in the early age, i.e. within the first about 6 hours after casting, may be reduced significantly. A method of controlled concrete curing is proposed. It is based on in situ measurements of the capillary pressure. If the latter reaches a previously defined threshold value the concrete surface is rewetted. Thereby, the capillary pressure decreases temporarily, but is not completely relieved. Consequently, the rewetting does not create a continuous water film on the concrete surface which might degrade the near‐surface material properties due to an increased water‐cement ratio.     

Influence of oxide scales on heat transfer in secondary cooling zones in the continuous casting process,part 1: heat transfer through hot-oxidized steel surfaces cooled by spray-water

For the cooling of steels in the continuous casting process it is necessary to know the heat transfer from the solidifying strand to the cooling water to enable calculation of the secondary cooling zone. Previous investigations have only determined this variable for non-oxidizing metallic surfaces. For many steels cast in practice, however, the formation of oxide layers prevents a direct transfer of the previous results. In the present research the influence of the oxide layers on the heat transfer has been investigated for spay-water cooling. Results have shown that heat transfer in the range of stable film boiling is determined for a constant spray-water temperature in the same way as for non-oxidizing metals, i.e. using the water mass flux density ·_s only. The changed surface qualities resulting from the oxide formation cause the Leidenfrost temperature, however, to shift considerably to higher values.