Experimental and theoretical analysis of spill fire dynamics in confined spaces: Heat release rate estimation based on gas concentration variations

Spill fires in confined spaces, such as those in underground substations and ship engine rooms, pose significant threats to fuel safe utilization. In this study, a 3 m × 3 m × 4 m experimental platform was developed to investigate n-heptane spill fires under fuel discharge rates ranging from 5.8 to 12.6 g/s. By characterizing spread process and gas concentration evolution (O₂, CO₂, CO), a novel method for estimating the heat release rate (HRR) was proposed. The combustion process was categorized into four distinct stages: burning layer spread stage, transition stage, ghosting fire stage, and extinction stage. A parameter τ was proposed to represent the relationship between fuel discharge rate and confined space volume, and the extinction time t_e was found to be positively correlated with τ. Oxygen concentration followed a three-phase decline pattern (slow-rapid-slow decline), with extinction thresholds stabilizing at 15.0 % O₂, 3.8 % CO₂, and 1200 ppm CO, regardless of the discharge rate. Furthermore, an HRR calculation model tailored for confined space spill fires was established and validated for accuracy. The research results can be used for fire risk assessment in confined space to ensure the fuel safe utilization.