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

Graduation date: 2017The 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 NO[subscript x] emissions and high thermal efficiency. MILD combustion is characterized by low oxygen concentrations (i.e. 3%-9% by volume) and high reactant temperatures. A fundamental understanding of the physics governing MILD combustion is required to design effective practical combustion devices. While the physics relevant to MILD combustion of small hydrocarbon fuels such as methane and ethylene have been well-characterized, the behavior of large hydrocarbon fuels, such as Jet-A, have not. This is significant because many practical devices such as internal combustion engines and gas turbine engines are designed to operate using large hydrocarbon fuels. With this background and motivation, the focus of the current study was to understand the mechanisms governing stability and ignition of these flames in the MILD regime.\ud \ud To this end, 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 coflow used a premixed CH₄/H₂ secondary burner to provide an oxidizer stream at high temperature and with low oxygen concentration, which emulates MILD conditions. The coflow temperature was varied between 1300K and 1500K and the oxygen concentration was varied between 3% and 9% by volume. Three different large hydrocarbon fuels (i.e. Jet-A and two experimental fuels) were vaporized and issued into the hot coflow, with Reynolds numbers based on the inner jet diameter ranging from 3,750 to 10,000. The fuel jet exit temperature was varied from 525K to 625K. 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. \ud \ud Several conclusions were made from the measurements and simulations. 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. Opposed flame simulations indicated that formaldehyde production was increased with strain rate, which is analogous to an increase in the fuel jet velocity. Ignition delay calculations indicated that formaldehyde addition decreased ignition delay times, which results in lower flame liftoff heights. Opposed flow flame simulations indicated that the effect of changes in CH₂O production was diminished at increased coflow oxygen levels (i.e. 6% and 9%) and elevated coflow temperatures (i.e. 1400K and 1500K) due to lower formaldehyde production