Droplet Generation 35

turbine combustors are housed within an annular passage, through which a small amount of air flows. The airflow, usually less than 1% of the total combustor airflow, is discharged at the downstream end of the passage and flows radially inward across the nozzle face. This air, termed shroud air or anticarbon air, is used primarily to protect the nozzle from overheating by the flame and to prevent the deposition of coke that could interfere with the fuel spray and block the nozzle. In addition, the shroud air also exhibits some other beneficial effects. For example, swirled shroud air may improve atomization quality by reducing the mean droplet size and assisting in holding the spray tulip open at low fuel pressure while having no effect on the cone angle of the fully developed spray. This leads to an extension of weak-extinction limits and an improvement of light-up capability. Swirled shroud air tends to widen the spray cone angle by inducing radially outward flow, while unswirled shroud air tends to narrow the spray cone angle. Thus, the shroud air also provides a means for varying and controlling the spray cone angle. However, excessive amounts of unswirled shroud air may lead to a too-narrow spray cone angle and thus may result in combustion instability.

2.1.3Fan Spray Atomization

It should be noted that the spray patterns generated with the plain-orifice and pressure-swirl atomizers are normally circular. To generate a narrow elliptical spray pattern, a fan or flat spray atomizer can be used. Flat spray atomizers may have different arrangements such as flood nozzle and flat nozzle. In a flat spray flood atomizer, a round liquid jet is arranged to impinge on a curved surface, producing a wide, flat spray pattern with a fairly uniform distribution of relatively coarse droplets. The nominal spray pattern is 120° or more. In a flat spray atomizer, the discharge orifice is the intersection of a V-shaped groove with a hemispheric cavity connecting with a cylindrical liquid inlet. The produced liquid sheet breaks up into a narrow elliptical spray pattern parallel to the major axis of the orifice. This is the most widely used type of fan spray atomizers. Using this type of

36 Science and Engineering of Droplets

atomizer, excellent atomization and spray patterns have been produced for viscous and non-Newtonian liquids. In addition, a fan spray can also be produced by cutting slots in plane or cylindrical surfaces and allowing a liquid to flow into the slots from two opposite directions. For a single fan spray injector, the liquid then issues from a shaped orifice and spreads out in the shape of a circle sector of about 75°.

The behavior of fan spray atomization has been studied by Dombrowski et al.,[95] Carr,[97] and others. The trajectory of the liquid sheet generated by a fan spray atomizer is found to be dependant on the injection pressure, sheet thickness and liquid surface tension, and less affected by liquid density.[95] The sheet thickness decreases with increasing distance from the orifice. Liquid surface tension tends to contract the edges of the sheet, leading to a curved boundary as the sheet expands. Liquid viscosity restrains the breakup of the edges of the sheet, resulting in relatively large droplet sizes. Increasing injection pressure and/or shroud air pressure can reduce mean droplet size. Ambient pressure has little effect on the spray angle produced by a fan spray atomizer. However, an increase in ambient pressure tends to increase the mean droplet size because, at a high ambient pressure, the liquid disintegration takes place closer to the nozzle exit where liquid sheet is thicker. For a given flow rate, fan spray atomizers normally generate coarser droplets than pres- sure-swirl atomizers. However, the Lucas fan spray fuel injection system[97] produces very fine droplets with a mean size less than 25 µm even for oils with viscosity as high as 8.5 × 10-6 m2/s.

Fan spray atomizers have been widely used in the spray coating industry (Fig. 2.5), in some small annular gas turbine combustors, and in other special applications that require a narrow elliptical spray pattern rather than the normal circular pattern. In particular, fan spray atomizers are ideal for small annular combustors because they can produce a good lateral spread of fuel, allowing to minimize the number of injection ports.

Droplet Generation 37

Figure 2.5. Schematic of a flat spray atomizer used for paint spray.

2.1.4Two-Fluid Atomization

As mentioned in the previous section, a major drawback of the simplex atomizer is the poor atomization quality at the lowest flow rate due to too-low pressure differential if swirl ports are sized to allow the maximum flow rate at the maximum injection pressure. This problem may be resolved by using dual-orifice, duplex, or spillreturn atomizers. Alternatively, the atomization processes at low injection pressures can be augmented via forced aerodynamic instabilities by using air or gas stream(s) or jet(s). This is based on the beneficial effect of flowing air in assisting the disintegration of a liquid jet or sheet, as recognized in the application of the shroud air in fan spray and pressure-swirl atomization.

Two-fluid atomization, sometimes also termed twin-fluid atomization, two-phase atomization or aerodynamic atomization, is one of the commonly used techniques in many areas. Two-fluid

38 Science and Engineering of Droplets

atomization may be divided into two categories: air-assist atomization and air-blast atomization.[98]-[102] In principle, the two atomization processes are exactly the same. In both processes, the bulk liquid to be atomized is first transformed into a stream/jet or sheet. The liquid stream/jet or sheet at a relatively low velocity is then exposed to high-velocity air (gas) stream(s). Air is used to improve and maintain the quality of atomization over a wide range of liquid flow rates. The kinetic energy of the high-velocity air stream(s) is used to shatter the liquid jet into ligaments that subsequently disintegrate into droplets. Large scale eddy structures in the air flow impact upon the liquid jet, causing destabilization, stretching, and flapping of the liquid jet. Due to the complex liquid-air interactions, the droplet sizes in aerodynamic atomization are very widely dispersed. The significant difference between the two processes is in the quantity and velocity of the air used. In air-assist atomization, the air flow rate is to be kept to a minimum, but there is no special restriction on air pressure. The air is supplied from a compressor or a high-pressure cylinder, passing through annular orifices surrounding the liquid jet, making a very high air velocity. In air-blast atomization, the air velocity corresponds to the pressure differential across the combustor liner and hence is limited to a certain value, usually up to 150 m/ s. Thus, a large quantity of air (mass flow ratio of gas to liquid >1.0) is necessary to produce good atomization. In addition to atomization, the air also acts as a medium for the transportation and complete combustion of the droplets in the combustion zone. Therefore, airassist atomization is characterized by a relatively small quantity of air with very high velocity, whereas air-blast atomization distinguishes itself by a large quantity of air with limited velocity.

In air-assist atomization, air is needed usually to augment the atomization process only at low liquid flow rates when the pressure differential is too low to produce satisfactory pressure atomization. In some designs, however, air assistance may be required over the entire range of operating conditions if the atomization quality achieved with a pressure atomizer alone is always poor. In an air-assist atomization process, the impingement of a low-velocity liquid stream by a high-velocity air stream may occur either within or outside the

Droplet Generation 39

atomizer, referred to as internal-mixing (Fig. 2.6) and externalmixing (Fig. 2.7), respectively. In an internal-mixing atomizer, intense mixing of gas and liquid occurs within the mixing chamber, and the gas flow can be a single convergent jet or multiple Y-jets. An internal-mixing atomizer is well suitable for highly viscous liquids. Good atomization quality can be achieved for very low liquid flow rates. The spray cone angle can be controlled by air/gas flow. However, the aerodynamic and fluid dynamic flow patterns are highly complex. For example, in a Y-jet atomizer, a cylindrical liquid sheet may form within the atomizer due to shearing by highvelocity gas.[103] The sheet, filled with a cloud of stray droplets, is accelerated and thinned out by the gas as it travels at a velocity at least an order of magnitude lower than the gas, and disintegrates (outside the atomizer) into a spray more even than from a swirl atomizer. In an external-mixing atomizer, a high-velocity stream of gas is arranged to impinge at some angle onto a liquid jet or conical sheet at the center of the atomizer. An external-mixing atomizer can be designed to keep the spray cone angle constant for all liquid flow rates. The air utilization and energy efficiency of an external-mixing atomizer are less than an internal-mixing atomizer, and hence more energy input is required for atomization. However, an external-mixing atomization system is more robust in the sense that there is no danger of liquid flowing in the air line to interrupt the atomization operation.

A variety of designs of air-assist atomization systems have been developed and widely used in industrial gas turbines and oilburning furnaces, as described by Gretzinger and Marshall,[102] Mullinger and Chigier,[103] Bryce et al.,[104] Sargeant,[105] and Hurley and Doyle.[106] Air-assist atomization systems are well suitable for large industrial engines particularly as the high-pressure air is needed only during engine light-up and acceleration. It is the need for an external supply of high-pressure air that prevents these systems from being applied to aircraft engines. For the atomization of coal-water slurries and other liquids that are difficult to atomize with a conventional pressure atomizer, the Parker Hannifin atomizer (Fig. 2.8) may be used in which air assistance is employed over the entire

40 Science and Engineering of Droplets

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42 Science and Engineering of Droplets

range of operating conditions. In this atomizer, two clockwise swirled air streams, one inside and the other outside an annular, anticlockwise swirled liquid sheet, are used to shear the liquid sheet at the nozzle tip. The widely used types of air-assist atomizers include, for example, the National Gas Turbine Establishment atomizer,[101] the Y- jet atomizer[104][105] used in large oil-fired boiler plants, the combined pressure and twin-fluid atomizer,[107] and those designs of Delavan company used in industrial furnaces.

In air-blast atomization, a large amount of air is used as primary driving force of atomization. Fine droplets can be produced at low liquid pressures. Due to the thorough mixing of air and droplets, very low soot formation and a blue flame with low luminosity can be achieved in the ensuing combustion process, with concomitant decrease in flame radiation and exhaust smoke. In addition, air-blast atomizers are inherently simple in design. These advantages over pressure atomizers make air-blast atomizers particularly suitable for application to combustion systems operating at high pressures. Therefore, air-blast atomizers have been used in a broad range of aircraft, marine, and industrial gas turbines.

In air-blast atomization, a liquid can be injected into a highvelocity air stream in the form of one or more discrete jets. The jets disintegrate into droplets during flight in the air stream. An example of this type of design is the plain-jet air-blast atomizer used in gas turbines. Alternatively, a liquid is first arranged to form a continuous thin sheet before being exposed to the impact and/or shear by a highvelocity air. This is the commonly used type of air-blast atomizers, i.e., prefilming type. In this design, as illustrated in Fig. 2.9,[84] the liquid is introduced from equispaced tangential ports into a weir; through that, it flows over the prefilming surface as a thin film and then is discharged as a thin sheet at the atomizing lip with the help of two air streams. One air stream flows through a central circular passage, deflected radially outward by a pintle, and impacts the inner surface of the liquid sheet. The other air stream flows through an annular passage surrounding the main body of the atomizer and contacts the outer surface of the liquid sheet at a high velocity after