Detonation in ammonia-oxygen and ammonia-nitrous oxide mixtures

Abstract

The sensitivity to detonation of ammonia-oxygen (NH3-O2) and ammonia-nitrous oxide (NH3-N2O) mixtures 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 numerical 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-O2 mixtures and between 7 and 23 mm for NH3-N2O mixtures in the equivalence ratio and pressure ranges ϕ=0.6–1.5, and P1=43–100 kPa, and ϕ=0.3–1.25, and P1=41–80 kPa, respectively. These cell widths are larger than for CH4-O2 mixtures under similar conditions. The mixture with N2O is more sensitive to detonation than the mixture with O2 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 detonation in ammonia-based mixtures show pathological detonation behavior at low pressure. The major exothermic reaction is NH3 + OH = NH2 + H2O in NH3-O2 but it is outweighed by H + N2O = N2 + OH in NH3-N2O mixture. One of the dominant radicals is OH, which is supplied by H + O2 = O + OH at low pressure and by H2O2 (+M) = 2OH (+M) at elevated pressure in NH3-O2 mixture; and by H + N2O = N2 + 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.