Stability and Liftoff of Non-premixed Large Hydrocarbon Flames in MILD Conditions

The Moderate or Intense Low-oxygen Dilution (MILD) combustion regime has received interest from the industrial furnace and gas turbine engine industries due to attractive properties of reduced NOx emissions and high thermal efficiency. MILD combustion is characterized by low oxygen concentrations (i.e. 3-9% by volume) and high reactant temperatures. The focus of the current study was to understand the mechanisms governing stability and ignition of these flames in the MILD regime using large hydrocarbon fuels. A series of experimental and numerical studies were conducted to identify the physics governing lifted large hydrocarbon flames in the MILD regime. A jet in hot coflow (JHC) burner was used to stabilize a large hydrocarbon flame in a laboratory environment. The liftoff heights of the resulting flames were measured using OH* chemiluminescence, as the flames were not always visible. Opposed flow laminar diffusion flames simulations were employed to determine how the interaction between chemistry and strain may affect flame stability. Ignition delay calculations were used to determine how ignition chemistry may affect flame liftoff without considering the effect of mixing. Oscillation of the instantaneous flame liftoff height was observed and was attributed to the cyclic advection of burned fluid downstream and the subsequent autoignition of unburned fluid. An increase in the fuel jet temperature was found to stabilize the flames closer to the jet exit, which was attributed to an increase in entrainment caused by higher fuel jet velocities. Flames in a coflow with 3% O2 at an exit temperature of 1300K were found to exhibit a decrease in liftoff height with increasing fuel jet Reynolds number. This counter-intuitive trend was not observed in flames burning in a coflow with higher temperatures or in coflows with higher O2 concentrations. The decrease in flame liftoff height with Reynolds number was attributed to the transport of formaldehyde into unburned mixture via the observed oscillations in the flame base. This conclusion was supported by both PLIF measurements performed by previous researchers on gaseous MILD flames and by numerical calculations. Monday, July 25, 2016 10:00 AM, Rogers 226 School of Mechanical, Industrial, and Manufacturing Engineering