Soot deposits

Fig. 11. OVD process (a) soot deposition, (b) soot perform cross section, (c) preform sintering, and (d) fiber drawing.

High stack temperature can be the result of an improper air to fuel mixture. A leak of combustible material from the process side to the firetube is also a cause. It can also be the result of excessive soot deposition in the firetube. 

Unless specified, samples were from soot deposited on the chamber wall and the buffer gas was helium. Elements are those incorporated in the graphite anode, D is the nanotube diameter range, D, p is the most abundant nanotube diameter, and Crystallites refers to metal-containing particles generated by the arc process and found in the soot. 

Iron, cobalt, and nickel particles also grow in soot deposited on the chamber walls, but graphitic layers wrapping the metals are not so well-developed as those grown in the cathode soot. Figure 7 shows a TEM picture of iron particles grown in the chamber soot. They... 

The other detonability length scale is the detonation cell width, X (also called cell size) which is the transverse dimension of diamond shaped cells generated by the transverse wave stmctnre at a detonation front. It has a fish scale pattern (see Figure 4-4). Detonation cell widths are nsnally measured by the traces (soot) deposited on smoke foils inserted in test vessels or piping surfaces. The more reactive the gas-air mixture, the smaller is the cell size. The same is tme for chemical indnction length as a qualitative measure of detonability. The cell width, X, is a parameter that is of practical importance. The transition from dehagration to detonation, propagation, and transmission of a detonation, can to some extent be eval-... 

Other properties of interest are carbon residue, sediment, and acidity or neutralization number. These measure respectively the tendency of a fuel to foul combustors with soot deposits, to foul filters with dirt and rust, and to corrode metal equipment. Cetane number measures the ability of a fuel to ignite spontaneously under high temperature and pressure, and it only applies to fuel used in Diesel engines. Typical properties ol fuels in the kerosene boiling range are given in Table 1. 

Where combustion is imperfect, soot deposition occurs. This, in turn, results in ... 

Fuel treatments have been used for very many years as an aid to improving the combustion efficiency process. Old formulations often used saw dust, wood flour, common salt, zinc sludge, ground oyster shell, and similar crude ingredients, but could still provide a dramatic effect when thrown into a fire. The metallic salts present (sodium in salt, zinc in sludge, and calcium in shell) acted as catalysts that dramatically lowered the ignition temperature of soot deposits from around 1100 °F/590 °C to only 600 °C/315 °C the fire burned vigorously and the soot disappeared. 

The small particles are reported to be very harmful for human health [98]. To remove particulate emissions from diesel engines, diesel particulate filters (DPF) are used. Filter systems can be metallic and ceramic with a large number of parallel channels. In applications to passenger cars, only ceramic filters are used. The channels in the filter are alternatively open and closed. Consequently, the exhaust gas is forced to flow through the porous walls of the honeycomb structure. The solid particles are deposited in the pores. Depending on the porosity of the filter material, these filters can attain filtration efficiencies up to 97%. The soot deposits in the particulate filter induce a steady rise in flow resistance. For this reason, the particulate filter must be regenerated at certain intervals, which can be achieved in the passive or active process [46]. 

Soot deposited on the chamber wall contained mostly carbonaceous particles, where no MWNTs were contained. The deposits on the cathode consist of two portions the inside is black fragile core and the outside hard shell. The inside include MWNTs scad poljd ral graphitic nanoparticles. The outer-shell part ojnsisted of the crystd of graphite. 

By inspection windows and use of a pyrometer, visual inspection of the catalyst and temperature monitoring on-site in a contactless manner were performed. It turned out that a glowing, homogeneous texture occurs at catalyst temperatures between 900 and 1200 °C, GHSV values up to 10 h and pressures less than 1 MPa [3]. This is an indication of the absence of soot deposits. At lower temperatures or... 

Analysis of nitroaromatics found by treating diesel fuel with NO2 (column A) compared to nitroaromatics found in extracts of filters of exhaust from a diesel engine (column B) or in extracts of diesel soot deposited in a dilution tunnel of an animal exposure system (13). 

The incident flux used in the NBS smoke chamber is only a single value, at 2.5 w/cm2, which is a relatively mild flux for a fire, and cannot, thus represent all the facets of a fire. The light source is polychromatic, which causes problems of soot deposits and optics cleaning, as compared to measurements done using a monochromatic (laser) beam. Finally, the units of the normal output of this smoke chamber are fairly arbitrary and the data is of little use in fire hazard assessment. 

The network consists of a train of molecular condensation reactions occurring in the gas phase delivering reactive intermediates that form in homogeneous reaction molecular species with low reactivity for CVD (PAH) and soot depositing loosely on the... 

Melt and autoignition temperatures for many materials are known, as are normal flame temperatures. Table 8-1 gives selected temperatures of interest to many investigators. Soot will normally not affix itself to surfaces at more than approximately 700°F (371°C). Therefore, areas of high fire intensity may have little or no soot deposits. Flame temperatures are... 

When the insert was moved downstream to L/D = 1.5, the soot-deposit problem was alleviated. When L/D = 1.5, the UHC concentration was lower than for L/D = 1.1. Longer residence time and better mixing were obtained upstream of the porous layer when the insert was moved downstream to L/D = 1.5, improving the combustion efSciency... 

When the firing rate was increased, the flames generated more soot particles, and had higher CO and UHC emissions both with and without porous inserts present. Using porous inserts would generate soot-deposit problems at very low fuel-air ratios (f> < 0.3). 

Fig. 11. Experimental filter pressure drop as a function of soot mass loading compared with the model taking into account the effect of gas compressibility (dashed line) and the effect of soot deposit compaction (continuous line). The indicative example is given for a soot aggregate size of 129 nm.

The reconstructed digital materials previously mentioned are another way to simulate soot deposition apart from the unit-cell models. An example of soot deposition in such a digital material (reconstructed with a process-based algorithm that creates and sinters grains from a pre-computed grain library ) is shown in Fig. 19(a). As time passes the transition from deep bed to cake filtration as the top regions of the filter gets filled with soot is evident in... 

Fig. 19. Simulation of soot deposition on a filter wall, (a) Evolution of soot deposits (gray) in the wall (black is solid, white is pore space) and incipient cake formation (b) pressure drop as function of challenge soot mass demonstrating the deep-bed to cake filtration transition (c) visualization of soot deposition in an extruded ceramic (granular) filter wall and (d) development of soot deposits (black) and soot mass fraction in the wall (solid material is gray) to the onset of cake formation. Soot mass fraction scale is from 0 (violet) to the inflow value (red). In (d) the velocity on a section through the filter wall is shown, with overlay of the soot deposit shapes (see Plate 9 in Color Plate Section at the end of this book).

Fig. 22. Normalized soot deposit thickness evolution for different values of the microstructural parameter a.

Fig. 23. Normalized soot deposit density profile for different times.

The evolution equation for the two layers in the thin soot deposit limit (used here for analytical convenience) are ... 

Fig. 29. Morphology of simulated soot deposit grown in a square channel for Pe = 0.1 (left) and Pe = 100 (right), at the same soot mass load in the filter (Rodriguez-Perez et al., 2004).

Fig. 31. Soot deposits grown on the different channel geometries a, b and c of Fig. 30.

The same approach is employed to describe shear-induced transport of soot particles. Based on limited amount of experimental information for such phenomena in the literature we have established a flow cell where soot entrainment from the surface of preloaded filters from the engine exhaust can be studied. Preliminary experiments at ambient conditions reveal that no soot entrainment is observed up to relevant shear rates at the entrance of DPFs. We attribute this to the moisture content in ambient conditions of the soot deposits that due to capillary condensation increases adhesive forces between the particles. In the future experiments at high temperatures are planned to evaluate experimentally the shear-entrained fluxes for soot and ash deposits. 

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