Mode transition and critical spatial aspect ratio of propane-air deflagration in long confined spaces

Propane-air deflagration in long confined spaces depends on both the initial combustible-cloud extent and the enclosure geometry. However, the combined influence of these two factors on deflagration mode transition remains unclear. In this study, a three-dimensional CFD model was developed and validated against available deflagration experiments. The model was used to examine the effects of propane concentration, premixed-region aspect ratio L 0 / d , and spatial aspect ratio L / d on flame propagation, overpressure evolution and temperature distribution. Among the concentration cases, the highest pressure, temperature, and apparent flame-front speed occur at 4.0%, close to the stoichiometric propane concentration. During flame propagation, pressure-wave reflection and vent-induced rarefaction-wave backflow lead to multi-peak overpressure oscillations and promote flame-front deformation. The premixed-region aspect ratio is the main parameter associated with the deflagration regime transition. As L 0 / d increases from 3 to 4, the maximum apparent flame-front speed and peak overpressure move from a weak-deflagration branch to a strong-deflagration branch, and the sharpest increase occurs between L 0 / d = 3.25 and 3.5. The spatial aspect ratio further affects whether strong deflagration can develop. The critical L / d required for strong deflagration decreases from 9.0 to 4.0 as the cross-sectional width increases from 0.2 m to 1.0 m, following an empirical exponential relation. The results are interpreted in terms of the fuel volume participating in the coupling between combustion-driven expansion and flame deformation, and the delay of pressure relief from the open end. These results indicate that the transition from weak to strong deflagration is governed by the combined effect of premixed-region scale and spatial confinement, providing a reference for evaluating deflagration risk in long confined structures.