CO concentration decay profile and ceiling jet entrainment in aircraft cargo compartment fires at reduced pressures

The present study aimed to evaluate the low pressure effects on CO (carbon monoxide) concentration decay profile in aircraft cargo compartment fires, which will affect the activation of fire detectors. A series of fire tests were conducted in a full scale simulated aircraft cargo compartment at four atmospheric pressures (100 kPa, 90 kPa, 80 kPa and 70 kPa) corresponding to the pressure within an actual aircraft cargo compartment from the sea level to the cruising altitude (about 10,000 m). CO concentration decay profiles were measured and have little dependence on the pressure, and can be predicted by a theoretical formula and a brief empirical correlation. Based on the measured CO concentration and ceiling temperature, the entrainment properties of a ceiling jet at reduced pressures were analyzed. Considering the low pressure effect on entrainment coefficient, the air entrainment ratio Cα is introduced into the correlation of the ratio of mass flow rate coefficient βr/β0 to predict the ceiling mass flow rate. Further, the entrainment coefficient of a ceiling jet at reduced ambient pressures is calculated and presented compared with the plume.