Flow separation and liquid rundown in a gas-atomization process

“Rundown≓ filming mode was studied in a confined design gas-atomization nozzle of converging/ diverging construction. In this mode, liquid metal runs down to a certain distance up to ≈3 mm on the outer surface of the metal delivery tube in the form of a thin film. Atomization then takes place through the disintegration of this film by the oncoming gas. The rundown effect was simulated by water, and the supersonic gas flow outside the nozzle was visualized by Schlieren photography. It was found that rundown is caused by the liquid being drawn into a separated flow region on the wall of the delivery tube. This separation comes about as a result of the action on the boundary layer of adverse pressure gradients associated with the shock waves in the gas flow. Geometric conditions required for observing rundown are discussed, and a method is outlined for assessing whether a given nozzle design is likely to produce the effect. Evidence is also presented for the filming of the liquid on the tip of the nozzle by radial spreading.