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22.8.3 The Air-Blast/Air-Assist Atomizer Model

In order to accelerate the breakup of liquid sheets from an atomizer, an additional air stream is often directed through the atomizer. The liquid is formed into a sheet by a nozzle, and air is then directed against the sheet to promote atomization. This technique is called air-assisted atomization or air-blast atomization, depending on the quantity of air and its velocity. The addition of the external air stream past the sheet produces smaller droplets than without the air. Though the exact mechanism for this enhanced performance is not completely understood, it is thought that the assisting air may accelerate the sheet instability. The air may also help disperse the droplets, preventing collisions between them. Air-assisted atomization is used in many of the same applications as pressure-swirl atomization, where especially fine atomization is required.

FLUENT's air-blast atomization model is a variation of the pressure-swirl model. One important difference between them is that the sheet thickness is set directly in the air-blast atomizer model. This input is necessary because of the variety of sheet formation mechanisms used in air-blast atomizers. Hence the air-blast atomizer model does not contain the sheet formation equations that were included in the pressure-swirl atomizer model (Equations  22.8-23- 22.8-26). You will also specify the maximum relative velocity that is produced by the sheet and air. Though this quantity could be calculated, specifying a value relieves you from the necessity of finely resolving the atomizer internal flow. This feature is convenient for simulations in large domains, where the atomizer is very small by comparison.

An additional difference is that the air-blast atomizer model assumes that the sheet breakup is always due to short waves. This assumption is a consequence of the greater sheet thickness commonly found in air-blast atomizers. Hence the ligament diameter is assumed to be linearly proportional to the wavelength of the fastest-growing wave on the sheet, and is calculated from Equation  22.8-34.

Other inputs are similar to the pressure-swirl model - the user must provide the mass flow rate and spray angle. The angle in the case of the air-blast atomizer is the initial trajectory of the film as it leaves the end of the orifice. The value of the angle is negative if the initial film trajectory is inward, towards the centerline. Specification of the inner and outer diameters of the film at the atomizer exit are also required, in addition to the dispersion angle whose default value is 6 $^{\circ}$ (which can be modified in the GUI).

Since the air-blast atomizer model does not include internal gas flows, the user must create the air streams surrounding the injector as boundary conditions within the FLUENT simulation. These streams are ordinary continuous-phase flows and require no special treatment.


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