Experimental investigation of methane lean blowout limit; effects of dilution, mass flow rate and inlet temperature

Lean blowout (LBO) is one of the major instability problems of premixed combustion. LBO equivalence ratio is a function of inlet temperature and pressure, mass flow or aerodynamic loading, and fuel composition. All these, except the last, vary during startup and with load. Developing gas turbine combustors capable of operating within wider range of fuel compositions requires extensive knowledge about instability limits of the combustor at different operating conditions. In this work an atmospheric variable swirl combustor was used to study the influence of inlet temperature, mass flow, swirl number and dilution on lean blowout of methane. The equivalence ratio at LBO was investigated for methane at 3 different inlet temperatures at various swirl numbers. The swirl number was varied by changing the ratio of axial and tangential flow through the combustor inlet, and was determined using Laser Doppler Anemometry. The experiments showed that increasing the swirl number reduced the lean blowout equivalence ratio for a given inlet temperature and that increasing the inlet temperature reduced the lean blowout equivalence ratio at a certain swirl number. In order to study the effect of inlet mass flow rate on lean stability limit, blowout experiments were conducted at 7 different mass flow rates. The measurements showed that the total mass flow has a nonmonotonic effect on the lean blowout limit. At total mass flow rates below 200 SLPM increasing the total mass flow extended lean stability limit whereas at mass flow rates higher than 300 SLPM the trend was reversed. The effect of fuel dilution on the LBO limit was also investigated by adding different fractions of N₂ and CO₂ to the fuel mixture. The results were compared with those for pure methane at the same swirl number. Dilution with either diluent reduced the LBO limit of methane. However at the concentrations lower than 50 % the effect of dilution on LBO equivalence ratio was relatively small and no significant difference was observed between N₂ and CO ₂ dilution.