Study On Extinction Of Near-Limit Laminar Premixed Flames At Elevated Pressures

The studies on the extinction of the near-limit and flammability limits of thepremixed laminar flames are important both for application of combustion andenrichment of combustion theory. However, so far, the studies at elevated pressureare still far from sufficient. Thus, a combined experimental and detailed numericalstudy on pressure effect on flame extinctions and thereby the flammability limits wasconducted.The numerical simulations of the1-D freely propagating and counterflowingpremixed laminar flames at elevated pressure were conducted by using the PREMIXand OPPDIF modules in CHEKMIN software with GRI3.0mechanism. The mixturesare lean CH₄/air or CH₄/N₂/O₂with He dilution, such that its Lewis number is greatthan unity. Sensitivity analysis and path analysis are adopted to assess the pressureeffect. In the meantime, modifications of the existing microgravity experimentalsystems have been conducted, including the replacement of the straight opposednozzles with smoothly converging ones, and the upgrades of the flow rate controlsystem, ignition system and image acquisition system. In addition, the commonlyused pseudo-steady state measurement method, which also was used in this study wasassessed and improved. With the upgraded experimental rig and the improvedmeasurement method, the extinction limits of the premixed CH₄/air andCH₄/N₂/O₂/He counterflowing laminar flames under two weak strain rates weremeasured at the3.6s microgravity drop tower at NMLC. Corresponding experimentswere also done at normal gravity in the laboratory. With the comparison among theexperimental data at normal and microgravity and the results of numerical simulation,the effect of natural convection, Lewis number and strain rate on the extinction limitsof the near-limits flames at elevated pressures were studied.The numerical results of near-limit1-D freely propagating premixed laminarCH₄/Air flames show that radiative heat loss, represented by the maximum flametemperature difference between the radiative flame and artificial non-radiative flame at identical conditions, first increases and then decreases as the pressure increases.The sensitivity analysis and path analysis showed that as pressure increase, the threebody reaction enhances. However, as the pressure continues to increase above5atm,the HO₂related two body reactions HO₂+CH₃=OH+CH₃O and OH+HO₂=O₂+H₂Ogradually become the dominant chain branching reaction and chain terminationreaction respectively. At the same time, the dominant chain branching reactionbecomes temperature non-sensitive and chain termination reaction becomestemperature sensitive. As the temperature decreases with fuel concentration, therecess of the main branching is less than that of main termination. Thus, the flamebecomes more resistance and flammability limit at lean side expands at high pressureThe numerical study of counterflow premixed laminar CH₄/Air flame revealedthat its extinction limits first increased and then slightly decreasing as the pressureincreases, which is different from that of the flammability limits of1-D freelypropagating flames. This is caused by the combined effect of the change in the chainreactions and the changes local strain rate of the flames at an elevated pressure.The experimental results of counterflow premixed laminar CH₄/Air flame atnormal-and microgravity both showed that the increase of Lewis number and strainrate could lead to the decrease of extinction limits. As the pressures increases, theextinction limits first increase and then slightly decrease, which match with thenumerical results. In addition, results at normal-gravity are large that those atmicrogravity, which is partly caused by the curvature and stretching effect on flameintroduced by natural convection.