Kinetic modeling of the decomposition and flames of hydrazine

Abstract

A detailed N/H reaction mechanism has been developed and validated by comparing modeling results with measurements of hydrazine pyrolysis in shock waves, and in hydrazine decomposition flames at low and atmospheric pressures. The mechanism consists of 51 reactions for 11 species. Rate constants for several decomposition reactions have been estimated employing updated thermodynamic data. Analysis of the reactions abstracting an H atom from NH 3, NH 2, NH and N 2H 4 from 1000 to 2000 K demonstrates that the Evans-Polanyi correlation holds for the radicals H, NH, and NH 2. Probably it is also valid for the radicals N, NNH and N 2H 3. Several rate constants were estimated with this assumption. No further adjustment of the mechanism was attempted. The modeling correctly reproduces the experimental rate of decomposition of hydrazine and also the product distribution. The initial decomposition of N 2H 4 into two NH 2 radicals and the subsequent reaction N 2H 4 + NH 2 → NH 3 + N 2H 3 mainly govern the decomposition of hydrazine in dilute mixtures and together with the reaction NH 2 + NH 2 → N 2H 2 + H 2 control the propagation speed of a hydrazine flame. The computed speeds of such decomposition flames agree well with low-pressure and atmospheric pressure experiments for pure hydrazine and its mixtures with Ar, N 2, H 2O and NH 3. Also the concentration profiles of major and minor species in low-pressure hydrazine flames are well reproduced. A sensitivity analysis identifies the critical reactions in particular experimental conditions. The choice of rate constants for key reactions and further development of the mechanism is discussed.