Flow Structures and Their Relevance to Passive Scalar Transport in Fume Cupboards

Abstract

This paper describes an experimental investigation of air flow patterns and related passive scalar transport inside an open-fronted containment facility, with emphasis on the interaction with the laboratory environment. Time-averaged and instantaneous structures of the turbulent flow were examined by visualization and local measurements inside the enclosure and at the front aperture of 1200 mm × 500 mm cross-section. The cabinet was scavenged at an extract flowrate of 0.310 m³/s with 95 per cent of the air passing through the front aperture at a spatially averaged air velocity of 0.5 m/s and 5 per cent supplied through an auxiliary aperture of 1200 mm × 15 mm cross-section located at the front edge of the ceiling of the enclosure. Sulphur hexafluoride was used to mark the flow and the distributions of its time-averaged concentration over the front aperture and inside the cabinet were used to deduce the degree of mixing and passive scalar transport.

The geometrical features of the cabinet provided a flow field with a spanwise vortex at the upper frontal part of the enclosure, whose cross-sectional dimension was larger close to the side walls where the velocity of the inflowing air was lower than in the central region. The spanwise vortex implemented to improve capture efficiency at low inflowing air velocities can be detrimental to the stability of the inflowing air stream along the sash handle, leading to severe leakage of the passive scalar encapsulated within the vortex itself. Without external disturbances, the leakage from the vortex occurs with the periodic bursts generated by the oscillations at the periphery of the spanwise vortex, but these oscillations become more random with the cross-draught. The structure and the orientation of the vortex were vulnerable to external disturbances in that the vortex was tilted and pushed below the sash level on one side, resulting in asymmetrical leakage.