Drop manholes are commonly employed in sewer and drainage systems to reduce pipes slope. The operation of these structures is dominated by their flow regime. Poor hydraulic performance of them under Regime R2 was improved with the jet-breaker, which intersects the inlet jet; yet its proper dimensions were needed to be precisely determined. In this paper, effects of jet-breaker length, width, sagitta, and angle on drop manhole pool free-surface height were experimentally studied under 80% filling ratio of the inlet pipe. The modern statistical Design of Experiment (DoE) methodology and dimensional analysis were utilized to design the experiments in accordance with the 24-1IV fractional factorial design. Consequently, nine specific jet-breakers were built and examined at two different angles, and under various flow rates. The statistical analysis of the results shown that manhole pool height was significantly decreased when jet-breaker length, width, and sagitta were 1, 1.4, and 0.7 times the inlet pipe diameter, respectively, and its angle was at 70°. The use of DoE resulted in 21% reduction in experimental runs, cost, and time, while it provided comprehensive data analysis and objective conclusion.

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Drop structures and especially drop manholes are extensively employed in supercritical routes of sewer and drainage systems. Drop manholes remarkably affect hydraulic features of their downstream system, while their operation is dominated by the flow regime inside them. Poor hydraulic performance of these structures under Regime R2 was improved with the jet-breaker, yet its proper dimensions were needed to be precisely determined. In this paper, effects of the jet-breaker length, width, sagitta, and angle on drop manhole energy dissipation and air demand (as responses), under the inlet pipe 80% filling ration, were experimentally studied. The modern statistical design of experiment (DoE) methodology and dimensional analysis were utilized to design the experiments in accordance with the 24-1IV fractional factorial design. Ten specific jet-breakers were examined and more than 135 tests were performed. The statistical analysis of the results revealed that both responses were significantly improved when the jet-breaker length and width were 2 and 1.4 times the inlet pipe diameter, respectively; its sagitta was equal to 0, and its angle was at 70°. The use of DoE resulted in 21% reduction in experimental runs, straightforward data analysis, and unbiased concluding.

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.

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