Visualization Study of Helium-Air Exchange Flow using Optical Systems

Buoyancy-driven exchange flows of helium-air through horizontal and inclined small openings were investigated. Exchange flows may occur following the opening of a window for ventilation, when fire breaks out in a room, as well as when a pipe ruptures in a high temperature gas-cooled nuclear reactor. The experiment in this paper was carried out in a test chamber filled with helium and the flow was visualized using the smoke wire method. The flow behaviour was recorded by a high-speed camera combined with a computer system. The image of the flow was transferred to digital data, and the flow velocity was measured by PTV and PIV software. The mass fraction in the test chamber was measured using electronic balance. The detected data was arranged by the densimetric Floude number of the exchange flow rate derived from the dimensional analysis. A method of mass increment was developed and applied to measure the exchange flow rate. By removing the cover plate placed on the top of the opening, the exchange flow was initiated. Air entered the test chamber and the mass of the gas mixture in the test chamber increased. The volumetric exchange flow rate was evaluated from the mass increment data. In the case of inclined openings, the results of both methods were compared. The inclination angle for the maximum densimetric Floude number decreased with the increase of the length-to-diameter ratio of the opening. For a horizontal opening, the results from the method of mass increment agreed with those obtained by other authors for a water-brine system.