Stationary-state and oscillatory combustion of hydrogen in a well-stirred flow reactor

Abstract

The existence of stationary-state and oscillatory ignition phenomena is established in hydrogen oxidation in a jet-stirred, Pyrex glass reactor (0.55 dm 3). The experimental observations and their numerical interpretation from a detailed kinetic mechanism are reported. Compositions containing 2H 2 + O 2, H 2 + O 2, and H 2 + 2O 2 were investigated at total pressures mainly in the range 10–20 Torr, with brief excursions up to 80 Torr. The vessel temperature was varied between 670 and 790K and mean residence times were controlled in the range 0.7–8.0 s. Reactant temperatures were measured using a very fine thermocouple in the vessel and light output was monitored by photomultiplier. The experimental conditions straddle events at the second limit. Thus criticality and ignition occurs as the vessel temperature is raised under continuous flow of reactants. Ignition is an oscillatory phenomenon at low pressures, and adiabatic temperature changes occur momentarily during it. Increases of vessel temperature cause oscillatory amplitudes to fall and their frequencies to rise. Eventually the oscillations dwindle to a stable stationary state. In certain circumstances there can be an abrupt transition from low frequency to high frequency oscillations. Detailed investigations show that there can be reaction multiplicity and even birhythmicity in the vicinity of these transition points. The dependences of oscillatory reaction on pressure, temperature, and composition are investigated. The elementary kinetic scheme adopted for a numerical integration of the mass and energy conservation equations comprises 8 species in 33 reactions and is designed to embrace subcritical, “slow reaction” and high temperature ignition phenomena. The effects of third body efficiencies are taken into account where these are established quantitatively and enthalpy change is computed at each step. Heat loss rates are computed using an appropriate heat transfer coefficient at the vessel walls. Excellent agreement between the numerical and experimental results is obtained over wide ranges of conditions. Even complex phenomena such as oscillations of mixed frequency and the existence of hysteresis including birhythmicity are predicted numerically. The oscillatory phenomena that occur at the transition to ignition at the second ( p- T a) limit are due to chain branching coupled to product inhibition. Temperature change is an accompaniment and augments the effect, but it does not play an essential role. By contrast, the origins of the additional complexities associated with reaction multiplicity are thermokinetic in kind.