Pressure-swirl atomization

One of the limitations of plain-orifice atomizers is the narrow spray cone generated. For most practical applications, large spray cone angles are desired. To achieve a wide spray cone, a simplex, i.e., 

A hollow-cone spray can be generated via a simplex atomizer. The spray pattern varies depending on the injection pressure. At very low pressures, liquid dribbles from the nozzle orifice. With increasing pressure, the liquid emerges from the orifice as a thin, 

Various hollow-cone simplex atomizers (Fig. 2.1) have been developed for combustion applications, differing from each other mainly in the way that swirl is imparted to the issuing liquid jet. In these atomizers, swirl chambers may have conical slots, helical slots (or vanes), or tangential slots (or drilled holes). Using thin, removable swirl plates to cut or stamp the swirl chamber entry ports leads to economies of the atomization systems if spray uniformity is not a primary concern. Large simplex atomizers have found applications in utility boilers and industrial furnaces. Oil flow rates can be as high as 67 kg/min. 

Solid-cone spray atomizers usually generate relatively coarse droplets. In addition, the droplets in the center of the spray cone are larger than those in the periphery. In contrast, hollow-cone spray atomizers produce finer droplets, and the radial liquid distribution is also preferred for many industrial applications, particularly for combustion applications. However, in a simplex atomizer, the liquid flow rate varies as the square root of the injection pressure. To double the flow rate, a fourfold increase in the injection pressure is 

In a duplex atomizer (Fig. 2.2), the swirl chamber consists of two sets of tangential swirl ports primary and secondary ports. The primary ports are for low flow rates and the secondary ports are the main passage for high flow rates. During operation, the primary swirl ports are supplied first with a liquid from the primary manifold, while a spring-loaded pressurizing valve prevents the liquid from entering the secondary manifold. When a predetermined injection 

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The simplex atomizer has many other variants. 1 Designs are aimed at achieving good atomization over a wide range of flow rates by varying the effective flow area without the need for excessive hydraulic pressures. In practice, pressure-swirl atomizers used in gas... 

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 spill-return 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 j et or sheet, as recognized in the application of the shroud air in fan spray and pressure-swirl atomization. 

In pressure-swirl atomization, the complex atomization process may be conveniently subdivided into two main stages, as suggested by Lefebvre.12661 In the first stage, surface instabilities are generated as a result of the combined effects of hydrodynamic and aerodynamic forces. In the second stage, surface protuberances are... 

Table 4.4. Correlations for Mean Droplet Size Generated by Pressure-Swirl Atomizers...

The variations of the mean droplet size and the droplet size distribution with axial distance in a spray generated by pressure swirl atomizers have been shown to be a function of ambient air pressure and velocity, liquid injection pressure, and initial mean droplet size and distribution 460]... 

In fan spray atomization, the effects of process parameters on the mean droplet size are similar to those in pressure-swirl atomization. In general, the mean droplet size increases with an increase in liquid viscosity, surface tension, and/or liquid sheet thickness and length. It decreases with increasing liquid velocity, liquid density, gas density, spray angle, and/or relative velocity between liquid and surrounding air. 

Similarly to pressure-swirl atomization and air-assist atomization, the mean droplet size is proportional to liquid viscosity and surface tension, and inversely proportional to air velocity, air pressure, air density, relative velocity between air and liquid, and mass flow rate ratio of air to liquid, with different proportional power... 

Neglecting the energy for overcoming viscous force during liquid breakup, a simple equation for the theoretical energy efficiency has been derived by Yule and Dunkleyl5 for a pressure-swirl atomizer ... 

A visualization study of fuel atomization using a pulsed laser holography/photography technique indicates that basic spray formation processes are the same for both a coal-derived synthetic fuel (SRC-II) and comparable petroleum fuels (No. 2 and No. 6 grade). Measurements were made on both pressure swirl and air assisted atomizers in a cold spray facility having well controlled fuel temperature. Quality of the sprays formed with SRC-II was between that of the No. 2 and No. 6 fuel sprays and was consistent with measured fuel viscosity. Sauter mean droplet diameter (SMD) was found to correlate with fuel viscosity, atomization pressure, and fuel flow rate. For all three fuels, a smaller SMD could be obtained with the air assisted than with the pressure swirl atomizer. 

Both air assisted and pressure swirl atomizers typical of oil burning systems were used in this study. The pressure swirl atomizers utilized fuel pressure and tangential slots to create a swirl flow within an internal chamber and orifice. 

The influence of design and operating variables on spray formation for pressure swirl atomizers has been well studied (e.g., Effects of fuel pressure and nominal flow rate... 

MEAN DROPLET SIZE CORRELATION FOR PRESSURE SWIRL ATOMIZER... 

Effect of Variables on Mean Droplet Size. Some of the principal variables affecting the mean droplet diameters for pressure swirl atomizers may be expressed by equation 14. 

Thep and q denote the integral exponents of D in the respective summations, and thereby explicidy define the diameter that is being used. ZV and Dt are the number and representative diameter of sampled drops in each size class i. For example, the arithmetic mean diameter, D10, is a simple average based on the diameters of all the individual droplets in the spray sample. The volume mean diameter, D3Q, is the diameter of a droplet whose volume, if multiplied by the total number of droplets, equals the total volume of the sample. The Sauter mean diameter, D32, is the diameter of a droplet whose ratio of volume-to-surface area is equal to that of the entire sample. This diameter is frequendy used because it permits quick estimation of the total liquid surface area available for a particular industrial process or combustion system. Typical values of D32 for pressure swirl atomizers range from 50 to 100 Jim. 

Hollow Sprays. Most atomizers that impart swirl to the liquid tend to produce a cone-shaped hollow spray. Although swirl atomizers can produce varying degrees of hollowness in the spray pattern, they all seem to exhibit similar spray dynamic features. For example, detailed measurements made with simplex, duplex, dual-orifice, and pure airblast atomizers show similar dynamic structures in radial distributions of mean droplet diameter, velocity, and liquid volume flux. Extensive studies have been made (30,31) on the spray dynamics associated with pressure swirl atomizers. Based on these studies, some common features were observed. Test results obtained from a pressure swirl atomizer spray could be used to illustrate typical dynamic structures in hollow sprays. The measurements were made using a phase Doppler spray analyzer. 

Pressure-driven devices (Figure 4.1-6), such as pressure swirl atomizers, use aerodynamic drag at the gas-liquid interface to amplify natural disturbance to pinch droplets off liquid sheets and columns. Finer mists can be generated by adding a centrifugal force to the process that must overcome viscous damping and surface tension. 

D.P. Schmidt, I. Nouar, P.K. Senecal, C.J. Rutland, J.K. Martin, R.D. Reitz, Pressure-swirl atomization in the near field, SAE Technical Paper Series 1999-01-0496, 1999. 

Park, H., S. D. Heister Nonlinear simulation of free surfaces and atomization in pressure swirl atomizers, Phys. Fluids 18, 052103 (2006). 

J. Oilman, A.H. Lefevbre Fuel distributions frinn pressure-swirl atomizers, AIAA Journal of Propulsion and Power, 1, 11-15 (1985). 

J. Cousin, S. J. Yotm, C. Dumouchel Coupling of Classical Linear Theory and Maximum Entropy Formalism for Prediction of Drop Size Distribution in Sprays Applicatiini to Pressure-Swirl Atomizers, Atomizatirai Sprays 6, 601-622 (1996). 

Couto, H. S., Carvalho, J. A., and Bastos-Netto, D., Theoretical Formulation for Sauter Mean Diameter of Pressure Swirl Atomizers, J. Propul. Power, Vol. 13, No. 5, 1997,... 

Radcliffe [52] studied a family of simplex/pressure swirl atomizers and demonstrated that at low Reynolds numbers Co decreases with an increase of Reynolds number, and for larger Reynolds numbers, C/j is independent of the Reynolds number. It is also weakly dependent on the injection pressure within the normal operating range [42, 43]. Discharge coefficients for swirl nozzles are provided in [1, 51, 52], among others. 

There has been some development in the numerical modeling of the sheet formation from swirl nozzles. A fully nonlinear model using an axisymmetric boundary element formulation has been developed for simulating the free surface shape and spray formed by simplex/pressure swirl atomizers [30, 32]. A linear instability analysis by Ponstein has been used to predict the number of droplets formed from each ring-shaped ligament shed from the parent surface. 

J.-H. Rhim, S.-Y. No, Breakup length of conical emulsion sheet discharged by pressure-swirl atomizer, Int. J. Automot. Technol. 2(3), 103-107 (2001). 

X. F. Wang, A. H. Lefebvre, Influence of ambient pressure on pressure swirl atomization, Atomization Spray Tech. 3, 209 (1987). 

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