Ignition of ethane, propane, and butane in counterflow jets of cold fuel versus hot air under variable pressures

This study investigates experimentally the nonpremixed ignition of ethane, propane, n-butane, and isobutane in a configuration of opposed fuel versus heated air jets. For each of these fuels we explore the effects of inert dilution, system pressure, and flow strain rate, for fuel concentrations ranging between 3-100% by volume, pressures between 0.2 and 8 atm, and strain rates of 100-600 s^-1. Qualitatively, these fuels share a number of characteristics. First, flame ignition typically occurs after an interval of mild oxidation, characterized by minimal heat release, fuel conversion, and weak light emission. The temperature extent of this regime decreases with increasing the fuel concentration, the ambient pressure, or the flow residence time. Second, the response to strain rate, pressure, and fuel concentration is similar for all investigated fuels, in that the ignition temperatures monotonically decrease with increasing fuel content, decreasing flow strain, and increasing ambient pressure. The C₄ alkanes, however, exhibit three distinct p-T ignition regimes, similar to the homogeneous explosion limits. Finally, at 1 atm, 100% fuel, and a fixed flow strain rate the ignition temperature increases in the order of ethane < propane < n-butane < i-butane. Numerical simulation was conducted for ethane ignition using detailed reaction kinetics and transport descriptions. The modeling results suggest that ignition for all fuels studied at pressures below 5 atm is initiated by fuel oxidation following the high-temperature mechanism of radical chain branching and with little contribution by low-to- intermediate temperature chemistry.