Transport Time Lag Effect on Smoke Flow Characteristics in Long-Narrow Spaces

Abstract

This paper focuses on the smoke transport lag time at the early stage of fires in long-narrow spaces, which is defined as the time from fire onset to the time when smoke reaches a given position on the ceiling. For a heat detector at a specific location on the ceiling, the smoke transport lag time is a part of the response time of the heat detector. Especially when the heat release rate is relatively small at the early stage of fires, the smoke transport lag time will be very long, which will hence lead to the increase of heat detector response time. It is clear that the prediction of smoke transport lag time is critical to the activation time of the heat detector. However, previous studies have much focused on fire characteristics in long-narrow spaces, leaving very few on the transport time lag. Therefore, in this study, a theoretical model regarding smoke transport time lag was developed for both steady and time-dependent fires based on the weak-plume theory. This model was validated by a series of reduced-scale experiments. It can be concluded from comparison that the predictions of this model agree reasonably well with the corresponding experimental results. Using the proposed method, the dimensionless equations of smoke transport time lag, velocity and temperature considering the smoke lag effect in a long-narrow space for time-squared fires were also theoretically deduced. Additionally, to further determine the applicability of ‘Quasi-steady’ state assumption for time-squared fires, a calculation method regarding the critical time was also developed. The outcomes from this study will be beneficial to the development of fire detection model in long-narrow spaces.