Gaseous Detonations. VII. A Study of Thermodynamic Equilibration in Acetylene-Oxygen Waves

Abstract

The observed detonation velocities in acetylene-oxygen mixtures containing from 5 to 50% acetylene and thermo-hydro-dynamic calculations which assume complete equilibrium to be attained in the Chapman-Jouguet plane agree within 0.5%. This is of special interest because in successive parts of this composition range each of the several reactions, viz. dissociation of excess oxygen, carbon dioxide, water, and hydrogen becomes the main endothermic process. Temperatures may be calculated from the detonation velocities which agree with the theoretical equilibrium values to better than 1% over the range from 3000 to 4500°K. In acetylene-oxygen mixtures containing 53 and 55% of the former, the observed detonation velocities agree with equilibrium calculations only when it is assumed that undersaturated carbon vapor, taken to be in equilibrium with unburned acetylene, has 170 Kcal as its heat of sublimation. The velocities in mixtures containing from 60 to 70% acetylene agree quite well with calculations assuming homogeneous equilibrium. However, supersaturation of carbon vapor with respect to the solid must be assumed. In mixtures containing 71% acetylene a sudden rise in detonation velocity is observed partway down the tube. The final velocity, as well as the constant velocities in still richer mixtures, are consistent with calculations assuming complete equilibrium, including solid carbon. The discontinuity observed with 71% mixtures is interpreted as the overtaking of the Chapman-Jouguet plane of the primary detonation wave by a secondary shock wave developed by the delayed precipitation of carbon. It is suggested that because of the dependence of the delays of nucleation on the degree of supersaturation of a vapor, high velocities corresponding to complete equilibrium could be observed with leaner mixtures also, if detonation velocities were measured far enough from the point of initiation.