12. Smoke Stratification

The phenomena and formation mechanisms of smoke stratification and engineering solutions to estimate smoke stratification in tunnel fires are described. Smoke stratification is an important issue for evacuation and firefighting in tunnel fires. Smoke released from a fire contains some hazardous combustion products. If the smoke stratification in a tunnel section dissolves, tunnel users in this region could be in great danger. In no ventilation or very low ventilation conditions, smoke exists on both sides of the fire, and good stratification could exist at the early stages but generally not after the fire becomes larger. For a ventilation velocity slightly lower than the critical velocity, smoke backlayering and good stratification exist upstream of the fire; however, the smoke stratification downstream becomes worse. Under high ventilation rates, all the smoke flows towards the downstream side, and stratification is difficult to maintain even at a short distance downstream. The theory of the smoke movement along the tunnel is introduced. An empirical model of smoke stratification in tunnels with longitudinal ventilation is also presented.