Flame stabilization mechanism in reacting jets in swirling vitiated crossflow

Abstract

Flame stabilization in reacting jet in crossflow (RJICF) is often affected by interaction between different mechanisms, involving auto-ignition, partially-premixed flame behavior, and vortex-induced recirculation zones. These flame stabilization mechanisms depend on the local flow dynamics involving the interaction between the jet fluid and the crossflow. In this research a staged combustion system featuring a reacting jet in crossflow as means of secondary combustion zone is investigated. The fuel jet is injected into the crossflow through a contoured nozzle which is protruded into the crossflow to avoid flame contact with the combustor wall. with a top-hat velocity profile. The ignition dynamics of the reacting jet is discussed based on high-repetition-rate OH-PLIF measurements along the RJICF trajectory. The recirculation region in the leeward side of the jet promotes fuel–air mixing and provides conditions favorable for a sustained flame kernel which leads to complete ignition of the jet. There is a difference in the flame structure of a non-premixed H2/N2 flame as compared to a premixed natural gas flame indicating a difference in flame stabilization mechanism. The H2 flame is primarily stabilized at the stagnation plane between the crossflow and the windward shear layer of the jet. Due to the higher propensity of strain rate induced extinctions of premixed natural gas (NG)/air jet flame, a stable reaction zone is seen only in the jet recirculation region. Conditional statistics of the simultaneously computed velocity magnitude, vorticity magnitude and in-plane strain rate were extracted from the location of the flame front in the windward and leeward side of the jet. The conditional statistics show that the mean strain rate plays an important role in flame stabilization particularly for premixed NG/air jets which is found to be susceptible to strain rate induced extinction. Measurements at a plane half jet diameter below the injection location of the jet indicate that there is a continuous entrainment of PIV particles as well as reaction products and unburnt fuel-air mixture. In this study high-repetition-rate simultaneous PIV/OH-PLIF measurements along the jet cross-section are discussed.