Measurements of pressure and flame speed during explosions of CH4/O2/N2/CO2 mixtures

The explosion behavior of CH4/O2/N2/CO2 mixtures was investigated by coupling spatial and temporal measurements of pressure with the temporal signals of photodiodes placed along the vessel length. The O2/CH4 ratio was always stoichiometric, the oxygen air enrichment factor, E = O2/(O2 + N2), was set to 0.8, and the dilution (CO2%v/v) was changed from 30 to 40%v/v. The signals of the photodiodes reveal that the flame speed varies from 7 to 100 m/s and, thus, the burning velocity is within the range of 0.7-10 m/s. The pressure signals exhibit oscillations and, at the lowest CO2 content (30%v/v), a pressure peak exceeding the adiabatic value is found. The pressure signal and the flame speed values definitely demonstrate that the over-adiabatic peak cannot be attributed to detonation or DDT, but it can be related to the coupling between deflagration and rapid phase transition of water (combustion-induced Rapid Phase Transition, c-RPT).