Preliminary Study on Exhaust Efficiency of Smoke Management System in Tunnel Fires

Abstract

Exhaust efficiency in tunnel fires with mechanical means was discussed. Heat exhaust efficiency and smoke exhaust efficiency were defined and studied under two different exhaust modes (longitudinal exhaust and transverse exhaust) respectively. Exhaust efficiency depends on smoke generation rate, exhaust rate, heat loss through walls of tunnel, configuration of extraction installations and so on. Under smoke extraction, smoke exhaust efficiency equals 1 if a quasi-steady vented stage without upstream outflow forms, otherwise smoke exhaust efficiency is less than 1. Heat exhaust efficiency is always less than 1 since part of the heat released by the fire is lost through the solid boundaries of tunnel by radiative and convective heat transfer between the smoke and ceiling lining. Based on experimental data obtained from a series of fire tests in a model tunnel, heat exhaust efficiency was estimated and influencing factors were discussed. In tunnel fires with transverse mechanical exhaust system, heat exhaust efficiency varies from 20% to 50%. Heat exhaust efficiency increases along with the reduction of the number of exhaust inlets turned on while extracting and decreases along with the increase of the area of exhaust inlet. However, quantitative influence of inlet position on exhaust efficiency still need further study and will be reported later.