Cellular confinement of tunnel sections between two air curtains

Abstract

This paper compares and discusses the efficiency of various air curtains designed to ultimately operate a bilateral aeraulic confinement of a short section of a tunnel or corridor-like geometry. The tested arrangements include single- and twin-jet air curtain systems with and without air return ducts. Jet opening ratios equal to 10 and 20 are considered, with air discharge velocities ranging from 1 to 10 m/s. The facility and the experimental approach used in this work are described and justified in many details. Results are presented in terms of leakage flow rates. Information is also provided about relevant air change rates and system time constants. The global efficiency of an air curtain is greatly improved by the adjunction of an air recirculation circuit due to enhanced system stability. A critical jet exit velocity corresponding to transition from the laminar to the turbulent regime of the channel flow inside the air curtain diffuser was found. For discharge velocities larger than this critical velocity an asymptotic behaviour is observed for every air curtain tested. At the lower nozzle exit velocities, the non-recirculated twin-jet system exhibits a weird behaviour. Unexpectedly, the results show that a single jet with a given opening ratio may perform better than a twin-jet of same height with a lower opening ratio. However, the results show that the performance of an air curtain is much dependant of the conditions under which this air curtain is operated demonstrating therefore that great care should be exercised in any attempt of comparison.