Structure of incipiently sooting ethylene-nitrogen counterflow diffusion flames at high pressures

We report on the structure of C2H4/N2/O2 counterflow diffusion flames at pressures up to 2.5 MPa. The concentrations of major species, aliphatics up to decane and aromatics up to indene are measured by GC-MS analysis of samples collected along the flame centerline with a capillary probe. The data are compared with results of a computational model with detailed chemistry and transport. A first set of measurements (Series 1) is performed for a fixed stoichiometric mixture fraction Zst = 0.408, fuel mass fraction YF = 0.122, and global strain rate a = 57/s at pressures in the 0.1-0.8 MPa range. The flames are soot-free and permanently blue at all pressures but for the highest value of 0.8 MPa, when visual observation of a faint yellow luminosity reveals incipient sooting. A second set of flames (Series 2) is chosen by a similar criterion, by decreasing the fuel mass fraction to YF = 0.0975, lowering the global strain rate to 18.4/s, and covering the 0.855-2.5 MPa pressure range, which also in this case yields incipient sooting only at the highest pressure. In all cases, the flames are immune from buoyancy instabilities. Importantly, the temperature-convective time history is maintained constant in the flames in each series, which allows for a systematic exploration of the influence of only pressure, and indirectly diffusion, on the transition to incipient sooting by monitoring critical soot precursors such as the aromatics. A simple scaling based on Damköhler number suggests that just the increase in concentration of species with pressure suffices to explain the increase in sooting density. Agreement between experiments and computations is very good for major species and C2H2, even for the thinnest high-pressure flames. The profiles of these species, methane and temperature collapse for all pressures, once rescaled with a diffusive length δ varying with pressure and strain rate as (p·a)−1/2. The model properly captures the first steps in fuel consumption by hydrogen abstraction by OH and H. As the pressure rises, the preferential decrease of H mole fraction suggests an increasingly dominant role of OH in an exothermic oxidative process. The observed higher sooting tendency at high pressures correlates with the increase in mole fraction of aromatics, but the model significantly overestimates such an increase. The comprehensive experimental database provides a useful test-bed for further refinements and developments of chemistry models.