Detonation in ammonia-oxygen and ammonia-nitrous oxide mixtures

摘要

The sensitivity to detonation of ammonia-oxygen (NH3-O 2 ) and ammonia-nitrous oxide (NH3-N2O) mix-tures has been investigated experimentally and numerically. Detonation were studied in a stainless steel tube with a length of 4.6 m and an inner diameter of 78 mm. The initiation of the detonation wave was achieved using a weak electric spark and a Shchelkin spiral to trigger flame acceleration and transition to detonation. The soot foil technique was employed to determine the detonation sensitivity. For the numer-ical simulations, the Shock and Detonation Toolbox in Cantera was employed. The pressure in experiment was below 100 kPa and was extended to 4.5 MPa in the numerical study using a real gas model based on the Peng-Robinson equation of state. Overall, detonation in ammonia-based mixtures have an irregular structure and do not demonstrate a high sensitivity. At ambient temperature, the experimental cell width ranges between 14 and 54 mm for NH3-O 2 mixtures and between 7 and 23 mm for NH3-N2O mixtures in the equivalence ratio and pressure ranges phi = 0 . 6 -1.5, and P 1 = 43 -100 kPa, and phi = 0 . 3 -1.25, and P 1 = 41 -80 kPa, respectively. These cell widths are larger than for CH4-O 2 mixtures under similar condi-tions. The mixture with N2O is more sensitive to detonation than the mixture with O 2 at low pressure, but becomes less sensitive at elevated pressure. Increasing pressure also tends to stabilize the detonation by raising the isentropic coefficient. Through detailed thermo-chemical analysis, it was shown that det-onation in ammonia-based mixtures show pathological detonation behavior at low pressure. The major exothermic reaction is NH3 + OH = NH2 + H2O in NH3-O 2 but it is outweighed by H + N2O = N 2 + OH in NH3-N2O mixture. One of the dominant radicals is OH, which is supplied by H + O 2 = O + OH at low pres-sure and by H2O2 (+M) = 2OH (+M) at elevated pressure in NH3-O 2 mixture; and by H + N2O = N 2 + OH in NH3-N2O mixture. The characteristic length scale, i.e., the induction distance, is sensitive to reactions responsible for supplying OH radical in both mixtures. In NH3-N2O mixture, the induction distance is also sensitive to reactions involving the oxidizer, N2O. (c) 2023 The Combustion Institute. Published by Elsevier Inc. All rights reserved.