Detonation structure and detonability of C2H4-O2 mixtures at elevated initial temperature

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, C2H2 + 2.5 O-2 and C2H2 + 2.5 O-2 + 3.5 Ar and one was lean, C2H2 + 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 To 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 To on cell size depends strongly on the value of the reduced global activation energy Ea/RTZND of the given mixture. The critical diameter de of detonation transmission from a tube into a large volume and the critical radius of curvature Re of spherical detonation initiation have been studied using the two stoichiometric mixtures at elevated initial temperature. Results indicate that the classical relationships d(C) congruent to 13 lambda (CJ) and R-C congruent to 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.