Fuel burning, atomized droplets

Liquid fuel was seldom utilized in the previous ACC studies because it was not only difficult to actuate liquid-fuel injection at high frequencies, but the combustion delays associated with liquid-fuel atomization, droplet heating, vaporization, and burning processes made such a control extremely slow for fast-response in situ type controllers. As a result, the use of liquid fuel was confined to either steady injection process ]13] or upstream addition of prevaporized fuel ]4, 7] which limited the ACC flexibility associated with temporal responsiveness. The goal of this project is to make ACC more practical for propulsion systems by studying direct liquid-fueled ACC in a closed-loop controller setting. 

A characteristic of old diesel engines was black soot in their exhausts caused by the combustion process itself in which very small atomized droplets of fuel burning in hot compressed air left an unbumt core of fine carbon particles onto which other species in the exhaust gas adsorbed [26-28]. The total particulate matter emissions of diesel exhaust are comprised of three main components. One component is the solid carbonaceous fraction, which is the visible soot emissions commonly associated with diesel exhaust A second component is the soluble organic fraction (SOF). The SOF can exist either as a vapor or as an aerosol depending of the temperature of the diesel exhaust These liquids arise from imbumed or partially burned diesel fuel or lubricating oil swept from the cylinder walls of the engine [29]. 

Liquid fuel is injected through a pressure-atomizing or an air-blast nozzle. This spray is sheared by air streams into laminae and droplets that vaporize and bum. Because the atomization process is so important for subsequent mixing and burning, fuel-injector design is as critical as fuel properties. Figure 5 is a schematic of the processes occurring in a typical combustor. 

The sensitivity of atomic absorption can often be enhanced by aspirating solutions in organic solvents. The increased sensitivity is due to a number of factors, but can be attributed in large part to the lower viscosity and surface tension as compared to aqueous solutions. The flow rate is increased and smaller droplets are formed which are more efficiently vaporized. When organic solvents are aspirated, a fuel lean flame must be used in order to burn the solvent. 

The observed flame features indicated that changing the atomization gas (normal or preheated air) to steam has a dramatic effect on the entire spray characteristics, including the near-nozzle exit region. Results were obtained for the droplet Sauter mean diameter (D32), number density, and velocity as a function of the radial position (from the burner centerline) with steam as the atomization fluid, under burning conditions, and are shown in Figs. 16.3 and 16.4, respectively, at axial positions of z = 10 mm, 20, 30, 40, 50, and 60 mm downstream of the nozzle exit. Results are also included for preheated and normal air at z = 10 and 50 mm to determine the effect of enthalpy associated with the preheated air on fuel atomization in near and far regions of the nozzle exit. Smaller droplet sizes were obtained with steam than with both air cases, near to the nozzle exit at all radial positions see Fig. 16.3. Droplet mean size with steam at z = 10 mm on the central axis of the spray was found to be about 58 /xm as compared to 81 pm with preheated air and 96 pm with normal unheated air. Near the spray boundary the mean droplet sizes were 42, 53, and 73 pm for steam, preheated air, and normal air, respectively. The enthalpy associated with preheated air, therefore, provides smaller droplet sizes as compared to the normal (unheated) air case near the nozzle exit. Smallest droplet mean size (with steam) is attributed to decreased viscosity of the fuel and increased viscosity of the gas. 

In order for fuel to combust and bum efficiently, it must be atomized into extremely small droplets. Fuel injectors aid tremendously in performing this role. However, if fuel viscosity is high, atomization into small droplets becomes difficult. Highly viscous fuel will not disperse freely after being sprayed from the fuel injector. The fuel instead sprays as a stream or large drops rather than as a fine mist. The result is a decrease in fuel efficiency and power due to incomplete burning of larger fuel droplets. 

Fuel Oil 1. Fuel oil 1 is a petroleum distillate that is one of the most widely used of the fuel oil types. It is used in atomizing burners that spray fuel into a combustion chamber where the tiny droplets burn while in suspension. It is also used as a carrier for pesticides, as a weed killer, as a mold release agent in the ceramic and pottery industry, and in the cleaning industry. It is found in asphalt coatings, enamels, paints, thinners, and varnishes. 

The burners used in flame spectroscopy are most often premixed, laminar flow burners. Figure 28-11 is a diagram of a typical commercial laminar-flow burner for atomic absorption spectroscopy that employs a concentric tube nebulizer. The aerosol flows into a spray chamber, where it encounters a series of baffles that remove all but the finest droplets. As a result, most of the sample collects in the bottom of the spray chamber, where it is drained to a waste container. Typical solution flow rates are 2 to 5 mL/min. The sample spray is also mixed with fuel and oxidant gas in the spray chamber. The aerosol, oxidant, and fuel are then burned in a slotted burner, which provides a flame that is usually 5 or 10 cm in length. 

The sulfur is preheated and delivered to a burner as a liquid at a temperature of about 145 °C. The degree of atomization and mixing are key factors for an efficient combustion. Atomization is accomplished by spray nozzles or by a mechanically driven spinning cup (Figure 6.3.2). Some burners also contain secondary air inlets to promote mixing. The atomizer breaks the liquid sulfur into microscopic droplets that burn in suspension in a refractory-lined furnace. Sulfur combustion in air is self-supporting and no supplementary fuel is required. 

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