Deflagration characteristics of N 2 diluted propanal/air mixtures at elevated pressures

Hydrocarbon fuels, in addition to being direct sources of energy, also serve as crucial raw materials for numerous industrial processes. In this study, the oxidation of propanal to propionic acid under elevated pressures has garnered our attention due to the substantial heat release during the mixing of propanal and oxidants, posing considerable hazards to production. Fortunately, the introduction of N 2 effectively mitigates these risks by reducing the laminar burning speed ( S L ) and peak deflagration pressure ( P m ), and even preventing the ignition of mixtures. On this basis, the deflagration characteristics of propanal/N 2 /air mixtures were investigated in a 20 L spherical vessel at 348 K under various equivalent ratios ( phi , 0.5 - 2.0), dilution rates ( D , 0 - 0.5) and initial pressures ( P 0 , 0.1 - 1.0 MPa). The combustion characteristics of propanal/N 2 /air were also analyzed through the 1-D laminar flame speed model, and the sensitivity analysis was performed to elucidate the effect of phi , D and P 0 on the combustion of propanal. Results reveal that the addition of N 2 stabilizes the flame propagation, and the P m of propanal/N 2 /air mixtures decreases with the increasing D , with a sudden drop observed at a certain D level. However, the reduction of S L does not undergo an abrupt decrease across the entire dilution rate range. Furthermore, P m exhibits a linear relationship with P 0 at lower D , but this linear trend disappears once D approaches the critical suppression concentration. Interestingly, the critical suppression concentration of N 2 increases with P 0 at fuel-rich conditions, yet remains unaffected by P 0 at fuel-lean conditions. Near the critical suppression concentration, a limiting burning speed ( S lim ) emerges, below which the flame is incapable of spontaneous propagation. Notably, S lim is not a fixed value, but varies according to the composition and initial state of the mixture. For a given gas component, an increase in P 0 leads to a decline in S lim , while for gases with varying components, S lim is observed to be lower under fuel-rich conditions compared to fuel-lean scenarios. Also, both the addition of N 2 and the augmentation of P 0 significantly reduce the S L . When S L is comparable to the gas rise speed due to natural convection, the flame propagation becomes vulnerable to buoyancy instability. Meanwhile, the flame propagation may also be influenced by thermal diffusion instability and hydrodynamic instability. Under the combined effect of these instabilities, the flame is prone to be unstable. More importantly, when the flame contacting the upper wall of the is in a critical state where it can or cannot propagate downward, the irregularity of the disturbed flame may introduces uncertainty into the subsequent propagation, resulting in markedly different P m even among mixtures with identical compositions.