Lewis-number-induced and buoyancy-driven self-excitations in nitrogen-diluted laminar non-premixed free-jet flames

Abstract

Experiments in horizontally and vertically injected laminar free-jet flames with applying a DC electric field of −10kV were conducted to explore distinct differences between Lewis-number-induced and buoyancy-driven self-excitations, both of which have been known to be of O(1.0) Hz and thereby indiscernible. A horizontal injection method with the DC electric field, with which one could eliminate accumulated, preheat, partially premixed mixture in front of edge flame and also attach lifted flame to nozzle, was adopted to suppress heat-loss-induced self-excitation and buoyancy effects, thereby identifying Lewis-number-induced self-excitation. Each self-excitation was also discernible in the phase diagram of displacement speed versus stand-off distance. Based on these, flame stability maps as functions of initial fuel mole fraction and nozzle exit velocity were presented in horizontally and vertically injected free-jet flame. The results showed that the Strouhal number for Lewis-number-induced self-excitation increased and then decreased in increase (decrease) of Lewis number (normalized nozzle exit velocity). The mechanism of Lewis-number-induced self-excitation in free-jet flame, formed near nozzle exit, was also discussed. The buoyancy-driven self-excitation due to the accumulation of partially premixed preheated mixture in front of edge flame was also characterized well by the function dependency of a Strouhal number upon a Richardson number, the ratio of molecular weight of corresponding fuel to air, and the ratio of flame front width to mean stand-off distance; meanwhile the buoyancy-induced self-excitation due to a flame flicker was well described by the function of Strouhal number on a Richardson number. The results also showed that the buoyancy-driven self-excitation was originated from buoyancy-induced self-excitation due to a flame flicker. Once a buoyancy-driven self-excitation was launched in a situation without having a heat-loss-induced self-excitation, it overwhelmed the other self-excitations (Lewis-number-induced and buoyancy-induced self-excitations).