Theoretical and experimental analysis of the critical velocity of interface instability in gas jet forming

Abstract

Gas jet forming can be used for creating optical reflectors, but under certain conditions, the surface shape fluctuation of the gas-liquid interface, that is, interface instability, may occur during the forming process. This study investigated the mechanism of interface instability in gas jet forming by combining the Kelvin-Helmholtz instability and the gas jet flow velocity distribution law. Theoretical analysis revealed that when the nozzle height is fixed, interface instability occurs when the gas flow rate reaches a certain value, which increases with the distance between the nozzle and the liquid surface. In order to verify the results of the theoretical analysis, experiments were carried out within the range of nozzle height from 0 to 42 mm, based on the formulas of the transition section between the potential core region and the self-similarity region in the gas jet. Within this range, the critical velocity of interface instability was found to be the minimum, 9.946 m/s, when the nozzle height was 0 mm, and the maximum, 15.562 m/s, when the nozzle height was 42 mm. The results of the theoretical analysis and experiments were used to establish a predictive model for the critical condition of interface instability, with a mean prediction deviation of 0.119 m/s. This model can provide a basis for selecting parameters for gas jet forming.