Confined deflagration of propanal/air in the presence of intrinsic instability at elevated pressures

Abstract

The partial oxidation process is a chemical technique with a high risk of deflagration, which involves directly mixing of fuels with oxidizers at elevated conditions. Under non-standard conditions, the deflagration behaviors of fuels are often unavailable to the general public, which hampers progress in further basic research. In this study, the deflagration behaviors of propanal-air mixtures at 75 °C and 0.1–1.0 MPa were investigated, and the effect of initial pressure (P0) and equivalence ratio (φ) on the deflagration characteristics of propanal was discussed. To explain the similarities and differences in the deflagration characteristics under various conditions, the buoyancy instability, hydrodynamic instability and thermal-diffusion instability suffered by the flames were also analyzed. Results show that the pressure generated by the burned mixture is a cubic function of time, and the state of flame propagation can be determined by the monotonicity of the second-order derivative of pressure. Moreover, the peak deflagration pressure (Pm) is more sensitive to changes in φ on the fuel-lean side, but Pm/P0 at the fuel-rich condition is markedly higher than Pm/P0 at the fuel-lean condition. At all φ, an increase in P0 effectively shortens the ignition time and reduces the minimum ignition energy. Nevertheless, there are discernible differences in the variation patterns of the deflagration time (θm) with P0 at various φ. Specifically, θm gradually prolongs with increasing P0 at fuel-lean conditions, whereas it tends to be the same at different P0 and φ = 1. Furthermore, under fuel-rich conditions, θm progressively shortens as P0 increases. The decrease in θm is attributed to the increasing flame speeds in the presence of intrinsic instability, and the self-accelerating flame yields pressure waves, which travel faster than the flame. Then a series of reflected pressure waves are formed after the pressure waves strike the vessel wall, interacting with the flame front and resulting in the pressure oscillation. Moreover, increasing P0 advances the pressure oscillation and augments the intensity of pressure oscillation.