Experimental and numerical investigation of flame structures for stretched lean premixed planar/tubular flames and oxygen-enhanced planar/coflow flames.

摘要

In this work, advanced non-intrusive laser diagnostics techniques and numerical simulation are applied to investigate two different types of flame structures: counterflow and coflow flames.; A wide range of equivalence ratio flames, generated by premixed methane-air or propane-air impinging upon hot products with a counterflow burner, are studied to simulate inhomogeneous combustion processes occurring in combustion chambers such as in a direct-injection spark ignition engine. The effects of stretch, equivalence ratio and thermal-diffusion are studied separately. Hot products generated by the lean hydrogen-air flame can extend the lean flammability limit to a very low level and enhance the combustion process, which indicates the potential of ultra lean combustion. A diffusion controlled "negative flame speed flame" exists when the pre-mixed lean hydrocarbon-air mixture burns with the support of hot products from the lean hydrogen/air flame. Seven kinetic mechanisms (C1, C2, GRI-3.0, Williams, M5, Optimized, Mueller) are evaluated by data-model comparison. For this wide range of equivalence ratio flames, no universal mechanism applies and each mechanism has a specific range for a specific fuel.; Oxygen-enhanced flames have unique characteristics and potential economic benefits. Detailed flame structures measured by Raman scattering are compared to detailed simulations in an oxy/fuel coflow burner and an opposed jet burner. For planar opposed jet oxygen-enhanced flames, increasing oxygen concentration of reactants changes the flame temperature dramatically and produces more thermal NOx with nitrogen present. For diluted methane vs. air opposed jet flames, the calculated extinction limit of the minimum methane in the diluted fuel (19% CH4 in N 2) is much lower than the measured value (28% CH4 in N 2).; Three axisymmetric co-flow flames with different oxygen enhanced levels are studied. Flame A is formed from 65% CH4/35% N2 fuel stream co-flowed by air; flame B is formed from 65% CH4/35% N 2 fuel stream co-flowed by 100% O2 stream and flame C is formed from 20% CH4/80% N2 fuel stream co-flowed by 100% O2 stream. Pure oxygen as the oxidizer causes intensive chemical reaction and makes flame B (∼2400 K) and flame C (∼2900 K) much shorter, stronger, and brighter than flame A. Model-data comparisons for major species concentrations and temperature give very good agreement for flames A and B. The general trend is predicted for flame C.