Plug-holing height and complete plug-holing phenomenon in naturally ventilated tunnel fires with vertical shaft

Abstract

The plug-holing phenomenon in naturally ventilated tunnel fires with vertical shaft has been receiving considerable attention, however, there is still no simple physical quantity to represent the degree of plug-holing. A series of numerical simulations were conducted to analyze the ventilation performance of vertical shaft in tunnel fires by considering different shaft geometries. Plug-holing height and complete plug-holing were proposed to describe the degree of plug-holing. The influences of shaft geometry on smoke extraction efficiency and plug-holing height were likewise analyzed. Results show that the total mass flow rate in the vertical shaft increases with the increase of shaft length and height, and is more sensitive to shaft length. By contrast, the increase in the pure smoke in the shaft is relatively limited. Moreover, the dimensionless plug-holing height is considerably sensitive to and significantly increases with the shaft length. However, the dimensionless plug-holing height has lower sensitivity to the shaft height and shows no evident monotony with a change in shaft height. Accordingly, a prediction model of the dimensionless plug-holing height is eventually established on the bases of simulated data. Critical criteria for determining the occurrence of complete plug-holing in car fire scenarios have been identified and can be described as Ri∗0.09α1.22=5.16.