Detonation structure and detonability of C $_2$H $_2$-O $_2$ mixtures at elevated initial temperature

Abstract. We have studied the influence of initial pressure ( $P_0=0.05$–1 bar) and temperature ( $T_0= 300$–500 K) on the detonation structure of three acetylene-oxygen based mixtures. Two were stoichiometric, C $_2$H $_2+2.5$ O $_2$ and C $_2$H $_2+2.5$ O $_2+3.5$ Ar and one was lean, C $_2$H $_2+10$ O $_2$. The gaseous mixtures were heated by the detonation tube itself which was electrically heated by Joule effect at a rate of about 100 K per minute. Experimental results show that the detonation cell size $\lambda $ of the stoichiometric mixtures increases with $T_0$ and is nearly independent of this parameter for the lean one. The evolution of the cell size with respect to the initial temperature and pressure is reasonably well predicted through the ZND model using global or detailed kinetics. Calculations show that the influence of $T_0$ on cell size depends strongly on the value of the reduced global activation energy Ea/RT $_{{\rm ZND}}$ of the given mixture.The critical diameter $d_C$ of detonation transmission from a tube into a large volume and the critical radius of curvature $R_C$ of spherical detonation initiation have been studied using the two stoichiometric mixtures at elevated initial temperature. Results indicate that the classical relationships $d_C \cong 13\lambda_{CJ}$ and $R_{C} \cong 20\lambda_{CJ}$ hold also in these conditions. As a consequence, because of increasing $\lambda $ with $T_0$, the detonability of the stoichiometric mixtures drops at elevated temperature.