Near-lean blowoff dynamics in a liquid fueled combustor

Abstract

This paper describes an analysis of the near-lean blow off(LBO) dynamics of spray flames, including the influence of fuel composition upon these dynamics. It is motivated by the fact that, while reasonable correlations exist for predicting blowoffconditions, the fundamental reasons for why flames supported by flow recirculation actually blow offare not well understood. Prior work on gaseous systems has shown that the blowoffevent is a culmination of several intermediate processes, initiating with local extinction of reactions ("stage 1"), followed by large scale changes in flame and flow dynamics ("stage 2"), finally leading to blowoff. In this study, near-LBO dynamics were characterized for ten liquid fuels with widely varying kinetic and physical properties. Results were compared at two air inlet temperatures, 450 and 300 K, as this influences the relative importance of physical and kinetic properties in controlling LBO. Extinction, re-ignition, and recovery of the flame are evident from these data, and grow in frequency as blowoffis approached. Results show that after a near-blowoffevent, the flame can move upstream at velocities much faster than the flow velocity, corresponding to re-ignition. Nonetheless, the majority of the flame recovery events appear to be associated with convection of hot products back upstream, not re-ignition. In contrast, downstream motion of the flame faster than the flow, which would correspond to bulk flame extinction, was never observed. This indicates that "extinction events"actually correspond to convection of the flame downstream by the flow when it loses its stabilization point. The dependence of the equivalence ratio when these events appear, their frequency, and event duration were quantified as a function of fuel composition and air inlet temperature. For example, the data shows a higher percentage of recovery from near-blowoffevents through re-ignition for high DCN fuels at the 450 K air temperature condition. These extinction/re-ignition results suggest that high DCN fuels are harder to blow offthan low DCN fuels through two mechanisms: (1) by delaying the onset of LBO precursor events, and (2) because they are able to recover from these precursor events through re-ignition more often.

Lean Blow-Off Scaling of Turbulent Premixed Bluff-Body Flames of Vaporized Liquid Fuels

“…Additionally, for the two kerosene flames, it was observed that the one with a higher DCN (A2 in this case) was more stable, which is consistent with the findings in Ref. [17].…”

“…Such localized increase in OH* signal may be associated with the continuous re-ignition of the flame in RZ as observed by Ref. [17].…”

“…[16]. On comparing kerosene fuels, the kerosene flames with a high derived cetane number (DCN) are more stable than those with a low DCN [15,17,18]. The spray characteristics near blow-off of alternative fuels were compared to A2 in Ref.…”

“…Rock et al [17] showed through CH* images that the flames produced with high DCN have more re-ignition events, which makes them resilient to blow-off. This enhanced re-ignition behaviour of kerosene flames prolongs the blow-off…”

“…While these prior studies have provided significant insight into the LBO limits and structure of gaseous fuels, recent studies reported that flames with heavy hydrocarbon or complex fuels show a substantial difference in terms of LBO and flame structure [13,15,17]. Tambold et al [20] showed that alcohol flames blow-off at higher equivalence ratios than methane.…”