Longitudinal distributions of CO concentration and temperature in buoyant tunnel fire smoke flow in a reduced pressure atmosphere with lower air entrainment at high altitude

Abstract

Smoke temperature and CO (carbon monoxide) concentration are two most important parameters concerning human safety in case of a tunnel fire. Their longitudinal distributions in the smoke flow along the tunnel are both closely related to fresh air entrainment from the surroundings; meanwhile heat loss process also contributes to the temperature decay but not to the CO concentration dilution at the same time. However, previous researches are all considering in default the condition with normal pressure, which is needed to be extended for condition at reduced pressure atmosphere such as at high altitude (for example, Tibet). This paper reports new findings for the distributions of these two parameters in a reduced pressure atmosphere with lower air density and thus lower air entrainment. The longitudinal distributions of smoke flow temperature and CO concentration for a tunnel fire near sea level (1atm) and at high altitude (0.64atm) have been correspondingly computed and compared by Fire Dynamics Simulator (FDS). It is found that the longitudinal decay profiles of CO concentration are similar in these two pressures, as both the air entrainment mass flow rate during the smoke flow traveling (contributing to the dilution) and the air entrainment of the fire plume (dominating the initial mass flow rate of the smoke flow) are proportional to ambient pressure thus their ratio is independent of pressure. However, the longitudinal decay of the smoke flow temperature is faster with distance along the tunnel in the reduced pressure atmosphere, as the air entrainment of the fire plume (dominating the initial mass flow rate of the smoke flow) is lower in the reduced pressure atmosphere, meanwhile the heat loss term is independent of pressure giving their ratio (heat loss to initial mass flow rate) is larger in the reduce pressure. Therefore, the difference between normalized longitudinal profiles of CO concentration and smoke temperature in a tunnel fire is larger, as indicated by a higher λ coefficient value, in the reduced pressure atmosphere at higher altitude than that in the normal pressure atmosphere, although their values of λ for both these two atmospheric pressure can be well correlated by a reciprocal function with longitudinal air flow speed.