Reaction propagation, leading to developing detonation, in a rapid compression machine

Optical imaging and pressure measurements are employed to indicate the nature of the varied changes occurring subsequent to compression in a Rapid Compression Machine, RCM, prior to their ultimate termination, usually in a detonation. A stoichiometric mixture of i-octane with variable inert diluents was investigated, enabling different compression temperatures to be attained and the varied nature of the subsequent changes to be studied. This was done with eleven i-octane/oxygen stoichiometric mixtures, all with different concentrations. The mixtures included some with an autoignitive Negative Temperature Coefficient, NTC. The mixtures were optically observed after compression to 2.0 MPa and pressures continually recorded until the completion of reaction. The compression temperature, Tc, was progressively increased by changing the compositions of the inert diluents.

At the lowest compression temperature laminar flame propagation was observed, with some wrinkling of the flame. As the temperature increased, calculation and observation confirmed a transition to autoignitive propagation, with a progressively increasing propagation speed of the predominantly autoignitively propagating front. At Tc = 740K, in the NTC regime, the autoignitive speed reached a maximum value of about 323 m/s. The NTC had little direct influence upon the ultimate detonation. Further increases in mixture temperature generated increasingly strong acoustic waves. As the acoustic velocity was approached, whether these changes would culminate in a detonation was dependent upon whether the approaching excitation temperature for the necessary accompanying heat release would be sufficient. Ultimately, it depended on whether the value of ξ = a/u_a was low enough, and that of ε = (r_0 )/(aτ_e ) was high enough to reside within the Detonation Peninsula. Ultimately quite strong detonations were obtained, verging on super-knock. As the different changes developed, the pressure records showed fluctuations of increasing amplitude. The normalised pressure fluctuations, ΔP/P, were found to increase with a Detonation Parameter, β=(ε/ξ).