Characteristics of the recirculation zone in a cylindrical confined space and its suppression by an annular orifice plate

The accumulation of pollutants in the recirculation zone can worsen ventilation. It is critical to reduce recirculation zones to improve the ventilation efficiency of buildings. However, the variation rule of the recirculation zone in a cylindrical confined space (CCS) is unclear, and there are few solutions to suppress or eliminate the recirculation zone at present. In this paper, an annular deflector orifice plate for suppressing the recirculation zone was developed based on the structural characteristics of the CCS. This device is simple in structure and can be used flexibly. Through experiments and numerical simulations, the variation rule of the recirculation zone length and the influence of structural parameters of the device on the vortex suppression were explored. Firstly, empirical formulas for calculating the length of the recirculation zone in the CCS were obtained. In addition, it was proved that placing the annular orifice plate inside the CCS effectively reduced the recirculation zone and improved the ventilation efficiency. Compared to the system without the annular orifice plate, the dimensionless length of the recirculation zone was decreased by 76.3%, and the time to completely discharge the pollutants from the CCS was decreased by 16.7%. Finally, parameters of the annular orifice plate that form the best vortex suppression effect were proposed: the porosity range was 40%–50%, uniform in shape with equal ring spacing, and placed more than one inlet diameter away from the inlet. The results help guide the ventilation design of CCS.