The Characteristics Of Fire Smoke Trasport In Channels Under Different Ventilation Conditions

Channel is a typical kind of building style generalized from transportation tunnels, metro platform, underground commercial streets and other pedestrian subways, etc. Many channels have been built in China and other countries with the rapid development of urban construction and tunnel engineering. In recent years, several catastrophic fire accidents have occurred in channels, which have greatly draw people's attention to the channel fire safety aspect. It is widely acknowledged that the incomplete combustion in channel fire always produces large amount of toxic smoke together with high temperature, which is the major fire hazard to occupants in the field. Therefore, the study on smoke transport characteristics in channels has significant meanings for occupant safety.First, this thesis studied the temporal growth characteristics of CO concentration in channels under natural ventilation by means of theoretical and experimental analysis. The distribution of CO and smoke temperature along the channel was comparatively analyzed. Then, the smoke transport characteristics on both horizontal and vertical direction under mechanical extraction were investigated through experiments and numerical simulations. Detailed research work is as follows:Previous researchers worked on the spatial distribution of CO concentration mainly by using its steady value or peak value, but conducted few quantitative study on its temporal growth characteristics. With the notice of some special phenomena during its transport process in channels, the smoke layer was arbitrarily divided into several zones on the longitudinal direction. An exponential model was derived for the temporal CO growth in each zone, which was then verified correct by experiments conducted in a model channel.Experimental results also indicated that the distribution characteristics of CO concentration and smoke layer temperature along the longitudinal direction displayed large difference. The smoke temperature decreased along the channel at all elevations in the layer, this trend also applied to CO concentration at higher elevations. However, the CO concentration increased largely on the longitudinal direction at lower elevations, which resulted in higher concentration at far-fire field than near-fire field at these heights.Experiments were conducted to investigate effect of mechanical extraction rates on the horizontal smoke transport characteristics. The real time smoke stratification configurations were captured with the application of laser sheet visualization technique. Results showed that the smoke layer maintained good stratification under small extraction rate, whereas mixing occurred at the layer interface while increasing the extraction rate. Stronger mixing was observed at locations near the channel portal. The Froude number could be used to well explain these phenomena. The normalized maximum temperature rise and smoke flow velocity both decayed exponentially with extraction rate.The plug-holing of smoke layer occurs during the vertical smoke transport process, which could reduce the performance of mechanical extraction system. CFD (computational fluid dynamics) simulations were adopted to study the development of layer plug-holing. Variations of the layer depth and temperature with mechanical extraction rates under the vent were quantitatively revealed. A transition zone and a complete plug-holing zone of the extraction rates were observed from the curve. It was noticed that the extraction efficiency decreased with the increase of plug-holing extent.By introducing a parameter of Ventilation System Output (VSO), the effect of plug-holing on the performance of extraction system was quantified based on experimental data, then some recommendations on practical smoke extraction optimization was also proposed. Results indicated that all the VSO values were comparatively low due to the occurrence of plug-holing. The maximum VSO for two fire sizes were 36.0% (10.4kW) and 17.5% (7.8kW) respectively, which then decreased to a stable level of 12.5% and 7.5% with the increase of extraction rates.