Combustion of hydrogen and oxygen with ammonia and nitrous oxide—Laminar flame speeds and flammability limits at low pressure for ternary mixtures

Abstract

Flame speeds relative to the burnt gas ( S B ) and composition limits of flammability have been measured at subatmospheric pressure for the ternary systems H 2 + O 2 + NH 3 and H 2 + O 2 + N 2O, and flame speeds for some mixtures of H 2 and O 2 with N 2. In the H 2 + O 2 + NH 3 system, the flammability limits were close to those predicted by the Le Chatelier rule. Flame speed contours on a triangular diagram reflected the form of the limits. In the H 2 + O 2 + N 2O system, lean mixtures effectively obeyed the Le Chatelier rule, whereas the region of flammability for rich mixtures was conspicuously wider than predicted. The composition dependence of the flame speed showed anomalouos behaviour, nitrous oxide and oxygen asisting one another in the combustion of hydrogen. The fastest flame speed occurred in a mixture with a composition per cent H 2 + 12 per cent O 2 + 30 per cent N 2O. For ternary mixtures containing ammonia or nitrous oxide excess of either reactant did not survive the flame, but was decomposed into its constituent elements. Adiabatic flame temperatures and equilibrium compositions of burnt gases were calculated for stoichiometric mixtures and values of the burning velocity relative to unburnt gas ( S B ) were derived, allowing comparisons with other work at atmospheric pressure, and hence estimates for the overall kinetic order n to be made. The burning velocities of binary H 2 + O 2 mixtures decreased as the pressure diminished, leading to a value for n of ca. 2–5. For the binary systems NH 3 + O 2 and H 2 + N 2O and ternary mixtures with a composition 30 per cent H 2 + 30 per cent O 2 + 40 per cent NH 3 burning velocities showed little dependence on pressure, indicating an overall kinetic expression of second order.