A. M. El-Leathy

Soot surface oxidation in hydrocarbon/air diffusion flames at atmospheric pressure

Abstract Soot surface oxidation was studied experimentally in laminar hydrocarbon/air diffusion flames at atmospheric pressure. Measurements were carried out along the axes of round fuel jets burning in co-flowing dry air considering acetylene-nitrogen, ethylene, propylene-nitrogen, propane and acetylene-benzene-nitrogen in the fuel stream. Measurements were limited to the initial stages of soot oxidation (carbon consumption less than 70%) where soot oxidation occurs at the surface of primary soot particles. The following properties were measured as a function of distance above the burner exit: soot concentrations by deconvoluted laser extinction, soot temperatures by deconvoluted multiline emission, soot structure by thermophoretic sampling and analysis using Transmission Electron Microscopy (TEM), concentrations of major stable gas species (N2, H2O, H2, O2, CO, CO2, CH4, C2H2, C2H4, C2H6, C3H6, C3H8, and C6H6) by sampling and gas chromatography, concentrations of some radical species (H, OH, O) by deconvoluted Li/LiOH atomic absorption and flow velocities by laser velocimetry. For present test conditions, it was found that soot surface oxidation rates were not affected by fuel type, that direct rates of soot surface oxidation by O2 estimated from Nagle and Strickland-Constable (1962) were small compared to observed soot surface oxidation rates because soot surface oxidation was completed near the flame sheet where O2 concentrations were less than 3% by volume, and that soot surface oxidation rates were described by the OH soot surface oxidation mechanism with a collision efficiency of 0.14 and an uncertainty (95% confidence) of ±0.04 when allowing for direct soot surface oxidation by O2, which is in reasonably good agreement with earlier observations of soot surface oxidation rates in both premixed and diffusion flames at atmospheric pressure.

Performance of advanced corrugated-duct solar air collector compared with five conventional designs

This paper presents the results of the experimental investigation and performance analysis of an advanced corrugated duct solar collector. The collector (air heater) is constructed of corrugated surfaces similar to those used for compact heat exchangers, with the air flowing normal to the corrugations. The collector is compared with five other conventional designs constructed and tested during the course of this work. The collectors are compared under the climatic conditions of Cairo (30°N), for the flow rate range of 0.01 to 0.1 kg/sm2 and insolation of 650 to 950 W/m2. The comparison revealed that the efficiency of the corrugated duct collector increases by a ratio of 15–43% over that of the next best conventional design (flow below flat absorber) and at double the efficiency of the base collector.

Soot Surface Growth in Laminar Hydrocarbon/Air Diffusion Flames

Thestructureandsootsurfacegrowthpropertiesofround, laminar, jetdiffusione ameswerestudied experimentally. Measurements were made along the axes of ethylene-, propylene-, propane-, and acetylene ‐benzene-fueled e ames burning in coe owing air at atmospheric pressure with the reactants at normal temperature. The measurements included soot structure, soot concentrations, soot temperatures, major gas species concentrations, some radial species (H, OH, and O) concentrations, and gas velocities. The results suggested that soot surface growth involved decomposition of theoriginal fuel to form acetyleneand H, which werethe main reactantsforsoot surface growth, and that the main effect of the parent fuel on soot surface growth involved its yield of acetylene and H for present test conditions. Measurements of soot surface growth rates (corrected for soot surface oxidation ) in laminar jet diffusion e ames were consistent with earlier measurements of soot surface growth rates in laminar premixed e ames and exhibited good agreement with existing hydrogen-abstraction/carbon-addition soot surface growth mechanisms in the literature with steric factors in these mechanisms having values on the order of unity, as anticipated.

Soot Surface Reactions in High-Temperature Laminar Diffusion Flames

The structure and the soot surface growth and surface oxidation properties of round laminar jet diffusion flames were studied experimentally. Measurements were made along the axes of acetylene/argon-fueled flames burning at atmospheric pressure in coflowing oxygen/argon mixtures to provide higher temperature soot-containing regions (2000‐2350 K) than were considered during earlier work. The measurements yielded soot surface growth and surface oxidation rates as well as the flame properties that are thought to control these rates. The present measurements of soot surface growth rates (corrected for soot surface oxidation) were consistent with earlier measurements in laminar premixed and diffusion flames having lower temperatures and exhibited good agreement with existing hydrogen-abstraction/carbon-addition soot surface growth rate mechanisms in the literature with steric factors of these mechanisms having values on the order of unity, as anticipated. Similarly, the present measurements of soot surface oxidation rates (corrected for soot surface growth) were consistent with earlier measurements in laminar premixed and diffusion flames having lower temperatures and exhibited good agreement with existing OH soot surface oxidation mechanisms in the literature, with a collision efficiency for OH of 0.13, in good agreement with past work, and supplemented to only a minor degree by direct soot surface oxidation by O2.

SOOT SURFACE GROWTH IN LAMINAR HYDROCARBON/AIR DIFFUSION FLAMES AT ATMOSPHERIC PRESSURE

The soot surface growth and flame structure properties of round laminar coflowing jet diffusion flames were studied experimentally. Test conditions involved ethylene-, propyleneand propane-fueled diffusion flames burning in coflowing air at atmospheric pressure. Measurements were limited to the axes of the flames and included soot concentrations, soot temperatures, soot structure, major gas species concentrations, radical (H,OH,O) species concentrations, and gas velocities. The results show that fuel decomposition yields significant acetylene concentrations at fuel-rich conditions, that significant soot formation begins when significant H-radical concentrations are present, and soot formation ends when acetylene concentrations become small. Hydrogen, OH and O radicals are present throughout the soot formation region so that soot formation and oxidation proceed at the same time. Strong rates of soot growth compared to rates of soot nucleation early in the soot formation process, combined with increased rates of soot nucleation and oxidation as soot formation proceeds, causes primary soot particle diameters to reach a maximum relatively early in the soot formation process. Present measurements in diffusion flames exhibit encouraging agreement with existing Hydrogen-Abstraction/Carbon-Addition (HACA) growth mechanisms, with earlier soot growth measurements in acetylene-fueled diffusion flames, and with earlier soot growth measurements in premixed flames fueled with a variety of hydrocarbons.

Soot Formation and Oxidation in Laminar Flames

The flame and soot structure, including soot primary particle nucleation, surface growth and oxidation properties, of 6 premixed and 20 diffusion flames were studied experimentally for various fuels, at temperatures of 1400-2350 K and at pressures of 13-811 kPa (0.1-8.0 atm). Measurements were made along the axes of flames with the reactants at normal temperature (300 K). The following properties were measured as a function of distance from the burner exit: soot concentrations by deconvoluted laser extinction, soot temperatures by deconvoluted multiline emission, soot structure by thermophoretic sampling and analysis using transmission electron microscopy (TEM), concentrations of major stable gas species by isokinetic sampling and gas chromatography, concentrations of radial species (H, OH, O) by deconvoluted Li/LiOH atomic absorption, and flow velocities by laser velocimetry. The measurements were analyzed to determine local soot surface growth, oxidation and nucleation rates, as well as local flame properties that are thought to affect these rates. The measurements of soot surface growth rates (corrected for soot surface oxidation) were consistent with the measurements in laminar premixed and diffusion flames involving a variety of hydrocarbons at variable pressure. In addition, the growth rates from all the available flames were in good agreement with each other and with existing hydrogen- abstraction/carbon-addition (HACA) soot surface growth mechanisms available in the literature yielding values of steric factors on the order of unity, as expected. Measurements of early soot surface oxidation rates (corrected for soot surface growth and prior to consumption of 70 % of the maximum mass of the primary soot particles) in laminar diffusion flames were consistent with all flames regardless of flame types, fuels, temperatures and pressures. The soot surface oxidation rates from all available flames could be explained by reaction with OH, having a collision efficiency of 0.12, and supplemented to only a minor degree by direct soot surface oxidation by O2.

Effects of Flow Properties on Soot Formation and Oxidation Rates in Flame Environments

Soot and flame structure were measured in a variety of round laminar premixed and nonpremixed (diffusion) jet flames fueled with hydrocarbons to study the properties of soot formation (primary soot particle nucleation and surface growth) and surface oxidation. The following properties were measured along the axes of the flames: soot concentrations by laser extinction, soot temperatures by multiline emission, soot structure by thermophoretic sampling and electron microscopy, concentrations of major gas species by sampling and gas chromatography, concentrations of some radical species (H, OH and O) by atomic absorption and flow velocities by laser velocimetry. These measurements were analyzed to yield local primary soot particle surface growth and surface oxidation rates, as well as primary soot particle nucleation rates. It was found that local soot surface growth rates could be correlated effectively using Hydrogen-Abstraction/CarbonAddition (HACA) soot surface growth rate mechanisms in the literature; that local soot surface oxidation rates could be correlated effectively using OH/O2 mechanisms in the literature; and that local soot particle nucleation rates (observable using electron microscopy techniques) could be correlated by assuming that the rate of development of large PAH molecules by the HACA mechanism controlled nucleation rates. These results were found considering a broad range of conditions in laminar flame environments: premixed and diffusion flames, various hydrocarbon fuel types (methane, acetylene, ethylene, propylene, propane and benzene), gas temperatures of 1500-2350 K and pressures of 10-100 kPa. NOMENCLATURE