Effects of jet-breaker dimensional characteristics on drop manhole hydraulic performance

Abstract

Drop manholes prevent excessive flow velocity and provide energy dissipation in steep urban sewer and drainage systems. Their poor hydraulic performance under Regime R2, in which the inlet jet collides with the manhole outlet, was improved by the jet-breaker. However, this device should still be properly sized to efficiently enhance manhole operation. In this paper, effects of jet-breaker length, width, sagitta, angle, and inlet pipe filling ratio on drop manhole performance, were investigated experimentally. Two-level factorial design and dimensional analysis were both utilized to design the experiments to study drop manhole energy dissipation and air demand as responses. Statistical analysis of the results revealed both significant design factors and regression models for each response. Neither jet-breaker length nor width had significant effects. However, the responses were improved by increasing both jet-breaker angle and inlet pipe filling ratio. Jet-breaker sagitta had a different effect on each response; a flat plate was more appropriate for energy dissipation while a curved plate decreased air demand. Simultaneous analysis of regression models determined the proper levels of significant design factors as 80% filling ratio of the inlet pipe, jet-breaker angle at 70°, and its sagitta equal to 0.