Characteristics of Premixed Flames of Hot and Diluted Mixtures

Internal recirculation of product gases and mixing with fresh reactants, to form a hot and diluted combustible mixture, has been demonstrated to result in lower pollutant emissions. With sufficient amount of product gas recirculation, the mixture can reach above its auto-ignition temperature with low oxygen mass fractions (<4 wt%), resulting in distributed reactions instead of a flame. Some of the technologies with this feature are Moderate or Intense Low oxygen Dilution (MILD) combustion, FLameless OXidation (FLOX) and Colorless Distributed Combustion (CDC). However, if the amount of product gas recirculation is not enough and the auto ignition condition is not reached, premixed reactants of the hot and diluted mixture can be established. In this study, characteristics of such hot and diluted mixtures is studied which include the ignition delay time tau(ign) and the flame attributes such as laminar flame speed, S-L and thermal flame thickness, delta(th). A perfectly stirred reactor (PSR) using the hot and diluted mixture as the inlet was further examined to simulate the main combustor and combustion efficiency eta, and CO and NOx emissions were obtained. The investigations are performed at different product gas recirculation amounts xi, equivalence ratios phi and operating pressures P, using chemical kinetic simulations. The thermo-chemical state of the recirculated product gases was obtained using a PSR with varying residence times (r) to simulate different "burntness" levels (smaller tau simulates lower "burntness" level). The hot and diluted mixture temperature is higher, while oxygen and fuel mole fraction are lower for higher xi, phi, P, and tau. Radical species' mole fraction is higher for higher xi, phi, and lower P, tau. Lower tau(ign) is observed for higher xi, phi, P and smaller tau. Higher S-L is observed for higher xi, phi and lower P. Lower delta(th) is observed for higher xi, phi and P. S-L and delta(th) are found to be higher or lower with variation in r, depending on xi, phi and P, owing to variation in temperature and radicals' concentration. Higher eta is observed for higher xi, P, lower phi, tau, and higher residence time of gases (tau(reac)) inside the main combustor. Lower CO is observed with increase in xi, P and decrease in tau, while exhaust NOx increases with phi and mostly remains unchanged with increase in xi, while inconsistent trends are observed with change in P and tau.