Nozzles, swirl-spray pressure

The conditions necessary for equality of particle size distribution were determined under ambient conditions. The nozzles used in the investigation can be classified as swirl-spray pressure nozzles. They accommodate a swirl insert which imparts a tangential velocity to the exiting fluid and results in a conical spray pattern. These nozzles were sufficiently different from conventional swirl nozzles (see Putnam et al., Ref. 6) to require an experimental study of particle size distribution. 

A. Datta, S. K. Som, Numerical prediction of air core diameter, coefficient of discharge and spray cone angle of a swirl spray pressure nozzle, Int. J. Heat Fluid Flow 21, 412 (2000). 

Pressure nozzles are somewhat inflexible since large ranges of flowrate require excessive variations in differential pressure. For example, for an atomiser operating satisfactorily at 275 kN/m2, a pressure differential of 17.25 MN/m2 is required to increase the flowrate to ten times its initial value. These limitations, inherent in all pressure-type nozzles, have been overcome in swirl spray nozzles by the development of spill, duplex, multi-orifice, and variable port atomisers, in which ratios of maximum to minimum outputs in excess of 50 can be easily achieved(34). 

The data indicated that droplet-size changes are primarily influenced by injection pressure and orifice size while secondary changes can be attributed to fluid properties, orifice shape, and the nozzle s internal length diameter ratio. This last point was not observed by Dombrowski and Wolfsohn (8) for more conventional swirl spray nozzles. Nevertheless, they present a useful correlation between Sauter mean diameter and operating conditions. 

N. Dombrowski, D. Hasson, The flow characteristics of swirl (centrifugal) spray pressure nozzles with low viscosity liquids, AlChE J. 15(4), 604—611 (2(X)4). 

Pressure nozzles are usually operated with a feed pressure of 30-200 bar and lead to a certain flow velocity at the orifice of the nozzle. The feed is forced into rotation in a swirl chamber within the nozzle resnlting in a cone-shaped spray at the nozzle orifice. It readily integrates into a spray as it is unstable. An increase in the feed rate leads to a less homogeneous and coarse spray with an increase in the width of the droplet size distribution. The mean size of droplets is indirectly proportional to pressnre up to 690 bar (680 atm) and directly proportional to feed rate and feed viscosity. Working with pressure nozzles results in a particle size diameter between 50 and 500 pm. °... 

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. 

The objective of the atomization process is to create drops small enough to dry in the spray drying tower. This is done with a number of high-pressure nozzles known as hollow-cone pressure swirl nozzles. These nozzles are distributed at one or more levels within the spray drying tower and have to be sufficiently distant from the wall to avoid buildup caused by wet drops sticking before they have dried sufficiently. For this reason, and for reasons of residence time, smaller towers typically run with smaller nozzles. 

Here, X is the area fraction of air core at the nozzle exit and a is spray cone angle. For a swirling conical melt-tin sheet, the spray cone angle increases nearly linearly from a = 15-55° with an increase in pressure differential (ApO on the liquid from Api = 0.4 MPa to 0.8 MPa [4, 9]. 

For composite-particle production in a spray process, a hybrid gas atomization nozzle configuration ideally fits the requirement of gas/particle/liquid dispersion and mixing in the spray flow field. A typical pressure-swirl-gas atomization (PSGA) configuration for composite-particle production is sketched in Fig. 18.51 (upper), where solid particles, continuously supplied by a particle pump and conveyed by the atomization gas, are co-injected and impacted with the liquid droplets in the secondary atomization zone. Figure 18.51 (lower) shows... 

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