Reducing the drag of midpoint lateral orifices of exhaust air ducts by shaping them along vortex zone outlines

Abstract

This study is concerned with the development of an energy-efficient design of a duct fitting comprising a midpoint lateral exhaust orifice. This is an element often encountered in the practice of exhaust ventilation system design and engineering. A numerical study was performed to identify relationships between the drag of such an orifice and its dimensions and the ratio of the airflow entering the orifice and passing through the duct at the confluence, GO/GC. Findings are well supported by other authors’ data and by our own experimental study. For the first time, the outlines of a vortex zone occurring at the inlet of the flow have been identified and supported by experimental visualization. A relationship behind orifice dimensions and the flow rate of passing airflow has been determined. A geometric approximation has also been identified for the vortex zone, enabling outlines of any vortex zones to be traced out within the studied range of dimensions and GO/GC flow ratios. The discovered outlines have been used for an energy-efficient exhaust orifice design featuring reduced drag both for the airflow entering through and bypassing the orifice. A “one-type-fits-all” shape has been identified that minimizes drag over the entire variation range of the GO/GC ratio. Approximate formulas for all identified relationships have been derived and programmed into an online calculator to facilitate their use in the design of energy-efficient ventilation systems. By making the duct fitting energy-efficient, drag can be reduced by 20–400% compared to a standard design. Reduced pressure losses enable reduction both of power consumption and size/rating of ventilation units used in the system.