Experimental determination of the static equivalent pressures of detonative explosions of cyclohexane/O2/N2-mixtures in long and short pipes

Abstract

In the past 5 years the Safety Engineering Group of BASF has determined the static equivalent pressures (“pstat”) of the eight detonative pressure scenarios which explosive gas mixtures can exhibit in long and in short pipes. More precisely, for different combustibles the pstat-values of the corresponding ternary mixtures combustible/O2/N2 were determined on the stoichiometric line and on the O2-line of the explosion triangle. By doing so, the pstat-values of all other compositions inside the explosion triangle could be predicted by extrapolation with an accuracy sufficient for practical applications. Furthermore, a proposal of how to transfer these results to the huge number of other combustibles not investigated so far was provided. A key-parameter in this context was the ratio R between the static equivalent pressure at the point where the deflagration-to-detonation transition occurs in the long pipe and the static equivalent pressure in the region of the stable detonation.In the present work the pstat-values of the new ternary mixture cyclohexane/O2/N2 are reported. Cyclohexane is of special interest because its autoignition temperature (AIT) in air is substantially lower than the AIT-values of all combustibles that have been tested before. According to our hitherto understanding the low AIT should noticeably reduce the ratio R. The experiments, however, did not confirm this hypothesis. After presenting the experimental results, which actually confirm the findings for the combustibles investigated so far, an explanation for the unexpected behaviour regarding R will be presented in terms of the differences between the low-temperature and the high temperature oxidation mechanism. As kind of spin-off, this explanation also allows to better understand quantitatively the degree of precompression in the yet unreacted mixture required for the occurrence of the deflagration-to-detonation Transition (DDT).