Failure of Cylindrical Brittle Deposits Impacted by a Supersonic Air Jet

Abstract

The failure of a cylindrical brittle material impacted by a supersonic air jet is investigated. Gypsum was cast around steel tubes to simulate the deposit formed on tube surfaces in industrial boilers. The breakup behavior of two deposit sizes, positioned at several distances from the nozzle exit, was visualized and documented using a high-speed video camera. Three deposit failure behaviors were observed: (i) crack formation and propagation along the longitudinal axis of the cylinder, (ii) surface pitting followed by axial crack formation, and (iii) surface pitting followed by spalling. These types of failure depend on the ratio of jet diameter to deposit diameter, which affects the magnitudes of compressive, tensile, and shear forces induced within the material. By analyzing the breakup movies, characteristics of the broken deposits, such as the breakup duration and the amount of deposit removed, were measured. Also, the effects of deposit thickness and distance from the nozzle exit on these characteristics were investigated.