Studies On Characteristics Of Stratification And Entrainment Of Smoke Layers In Channel Fires

A fire occurs in a confined space with relatively large ratio of length to width can be defined as a channel fire, which includes tunnel fire, corridor fire, underground passage fire and so on. Since channel fire brings about tremendous fire safety problems, it attracts increasing attentions from both fire safety engineers and fire researchers. Statistics have shown that the toxic fire smoke is the most hazardous factor in a fire. Due to the characteristic of structure and that of ventilation condition, it is difficult to exhaust fire smoke out of a channel immediately. Therefore, the characteristics of smoke transport in a channel should be concerned. Entrainment and stratification of fire smoke are closely related to fire safety design, the physics of which should be understood first.Experiments, numerical simulation and theoretical analysis were carried out to investigate the characteristics of entrainment and stratification in a channel fire. To carry out experimental study, a reduced-scale tunnel with dimensions of 66m (Length)×1.5m (Width)×1.3m (Height) were designed and constructed. Experimental techniques for stratification pattern measurement were developed. Experimental system for Particle Image Velocimetry (PIV) of fire-induced flows was also developed. Some new analytical techniques for fire-induced flow structure were proposed. This thesis focused on four issues:Effects of longitudinal air flow on the structure of smoke flow; Entrainment physics of smoke layer; the dimensionless criteria for stability of smoke layer; the differences between the vertical profiles of temperature rise and those of CO concentration. The main contents include:Vortex fields or velocity fields of the fire-induced flows were obtained from numerical simulation and PIV measurements. Proper Orthogonal Decomposition (POD) was used to extract coherent structures from the fire-induced flows. Energy spectrums of these structures were also obtained. Results indicate that, with the increase in longitudinal ventilation velocity, the energies of the large-size structures decrease but those of the small-size structures increase. Further, the ratios of energy of vertical flow pattern to that of horizontal flow pattern increase with the increase in longitudinal ventilation velocity.Experiments were conducted to investigate the physics that control air entrainment into smoke layer. Results indicate that, besides Richardson number, Reynolds number is also an important dimensionless parameter that affects the amount of air entrainment into smoke layer. Smoke flows of reduced-scale experiments have the orders of Reynolds numbers lower than 105, which are demonstrated to be lower than the critical Reynolds number that is necessary to sustain fully inertial turbulent fluctuations, and thus hardly entrain the fresh air. Whereas smoke flows of full-scale experiments have Reynolds numbers on the order of 105, which can sustain fully inertial turbulent fluctuations and the entrainment coefficients were close to the laws of Ellison and Turner.Dimensionless criteria for sustainability of stability of fire-induced smoke layer were obtained. The stratification pattern was found to fall into three regimes. Buoyancy force and inertia force, as the two dominant factors that affect the buoyant flow stratification, were correlated through the Froude number and the Richardson number. At RegionⅠ(Ri>0.9 or Fr<1.2), the buoyant flow stratification was stable, where a distinct interface existed between the upper smoke layer and the lower air layer. At RegionⅡ(0.3<2.4), the buoyant flow stratification becomes unstable, with a strong mixing between the buoyant flow and the air flow and then a thickened smoke layer.Differences between vertical profiles of CO volume concentration and those of temperature rise were found. By analyzing dimensionless transport equations and boundary conditions, the factors that are attributed to these differences can be revealed. Results indicated that, under the conditions with natural ventilation or relatively weak longitudinal ventilation, CO volume concentration decays much more slowly than temperature rise in the vertical direction, whereas, under the conditions with strong longitudinal ventilation, the vertical profiles of CO volume concentration and those of temperature rise show similarity. The intensity of heat transfer from smoke flow to wall boundaries is demonstrated to be closely related to the correlation between vertical profiles of CO volume concentration and those of temperature rise. A small amount of heat loss from smoke flow to wall boundaries lead to a higher similarity between vertical profiles of CO volume concentration and those of temperature rise.