Ignition in laminar and turbulent nonpremixed counterflow

Investigations into nonpremixed ignition were conducted to examine the influence of complex chemistry and flow turbulence as found in practical combustion systems. The counterflow configuration, where a hot air jet ignited a cold (298K) fuel jet, was adopted in experiments and calculations.; The study of the ignition of large alkane hydrocarbons focused on the effects of fuel structure by investigating the reference fuels n-heptane and iso-octane. The ignition response of these fuels was similar to smaller fuels with similar molecular structures. This conclusion was reinforced by showing that the ignition temperature became nearly insensitive to fuel molecule size above C₄, but continued to depend on whether the structure was linear or branched.; The effects of turbulence were studied by adding perforated plates to the burner to generate controlled levels of turbulence. This configuration was examined in detail experimentally and computationally without reaction, and subsequently the effects of turbulence on ignition were studied with hydrogen as the fuel. The results indicated that at low turbulence intensities, ignition is enhanced relative to laminar ignition, but as the turbulence intensity increases the ignition temperature also increases, demonstrating that optimal conditions for ignition exist at low turbulence intensities. At high pressures, where HO₂ chemistry is important, all turbulent ignition temperatures were higher than laminar ones, and the increasing temperature trend with turbulence intensity was still observed. At low fuel concentrations, a different ignition mode was observed where the transition from a weakly reacting state to a flame occurred over a range of temperatures where the flame was repeatedly ignited and extinguished.; Turbulent ignition was modeled by solving a joint scalar PDF equation using a Monte Carlo technique. The absence of significant heat release prior to ignition enabled the use of a frozen flow solution, solved separately, in the scalar calculation. The results did not reproduce the qualitative trends noted in the experiments and the influence of turbulence intensity was not apparent in the calculated results. These discrepancies were attributed to shortcomings in the molecular mixing models in low turbulent Reynolds number flows and where reaction rates are much lower than in a flame.