The effects of the Hartman cavity on the performance of the USGA nozzle used for aluminum spray forming. [Quarterly report, July-- September 1996]

This paper addresses the effects of the Hartman cavity on performance of the USGA (Ultrasonic Gas Atomizer) used for Al spray forming. Numerical simulations of the gas flow field were done in order to establish effects of the cavity on flow development both inside and outside the air nozzles. PDPA measurements were made of gas velocity and turbulence intensity, droplet mean and fluctuating velocity, and droplet size across planes at various distances downstream. High speed imaging is used in the flow region near the orifice exit where recirculation zones are generated and there is concern about metal droplet deposition on atomizer surfaces. Shadowgraphy show presence of shock waves and cells in the emerging gas jets. It was found that the Hartman cavity has little effect on droplet sizes generated; also little effect on spray development. The rectangular slit orifices for the 2 gas jets and the liquid jet generate a spray, after impingement, which is somewhat rectangular in cross section. As the spray develops downstream, it changes shape under influence of entrainment from the gas surrounding the spray. After a distance of 254 mm from nozzle exit, width and breadth of the jet are equal but significant shape change occur further downstream. Gaussian velocity distributions result in liquid flux distributions and metal deposits with Gaussian shapes instead of deposits with uniform thickness.