Finite chemical kinetic effects in a subsonic turbulent hydrogen flame

Departures from chemical equilibrium appear in nonpremixed turbulent flames at very high mixing rates, as shown by dimensional analysis based on Damköhler number (characteristic time of mixing over characteristic time of chemical reaction). This paper presents an experimental study that shows departures from chemical equilibrium in a hydrogenair flame, which is often erroneously considered to have an infinitely fast chemical rate and therefore to be at chemical equilibrium. These departures from chemical equilibrium are measured with nonintrusive laser diagnostics. Instantaneous and spatially resolved measurements of major combustion species (H₂, O₂, H₂O, and N₂), density, and temperature are performed by means of Raman and Rayleigh scattering in a turbulent jet flame with a fuel of 22 mole percent argon in hydrogen. From these measurements we infer the local fuel mixture fraction f. Departures from chemical equilibrium are manifested by the comparison between the measured temperature and the equilibrium temperature deduced from the value of f. We vary the Damköhler number by adjusting either the aerodynamic conditions or the chemical rate. In the first case, a range of Reynolds numbers is explored-Re = 8500, Re = 17,000, and RRe = 20,000-using the same fuel. The experimental results show a dramatic effect of the Reynolds number on the extent of departure from the limit of chemical equilibrium. Differences between the measured temperature and the inferred equilibrium temperature are as large as 450K as the flame approaches blowoff conditions. In the second case, we hold the aerodynamic conditions constant, and alter the chemical reaction rate by diluting the fuel with increasing amounts of nitrogen. These last experiments also show a difference between measured temperature and the inferred equilibrium temperature. Consequently, departures from the limit of chemical equilibrium are achieved through increasing the rate of mixing or by decreasing the rate of chemical reaction.