Chemiluminescence and CO pollutant formation in premixed counterflow flames

Abstract

In order to reduce pollutant emissions and increase efficiency, modern combustion systems, for example gas turbines or automotive engines, are operated in lean premixed mode. Combustion under premixed fuel lean conditions reduces the combustion temperature and, therefore, reduces NOx formation. However, low combustion temperature and shorter residence time of fuel mixture in the reacting zone lead to higher level of CO emissions and increased tendency for unstable combustion, which can lead to high amplitude combustion oscillations. The above effects become more pronounced during increased variability of the supplied fuel composition. The availability of optical sensors that measure local heat release rate, equivalence ratio, fuel composition and CO emissions can be useful for flame monitoring and, possibly, control of combustion in industrial burners. This thesis evaluates and applies two optical sensors to counterflow premixed flames. The first is a chemiluminescence sensor that measures the emitted light from flames to quantify local heat release rate, equivalence ratio and fuel composition. The second is a laser based sensor, namely Planar Two Photon Absorption Laser Induced Fluorescence of CO (Planar CO TALIF), that measures the instantaneous CO spatial distribution in flames. Chemiluminescence is the natural light emission from electronically excited molecules, mainly OH*, CH*, C2* and CO2*, formed in a flame. Due to its natural occurrence, chemiluminescence offers an economic non-intrusive diagnostic tool for combustion processes. In the late 1950s, chemiluminescence was identified as a heat release rate, reaction zone and equivalence ratio marker. However, only few quantitative correlations between chemiluminescent emission and heat release rate, equivalence ratio and fuel composition over a limited range of conditions are available in the literature and the available physical understanding of the underlying mechanisms is limited. Therefore, the present study conducts chemiluminescent intensity measurements in premixed opposed jet flames with two complementary optical systems with different spatial resolution, over wide ranges of equivalence ratio, flame strain rate and fuel composition. The results demonstrated that the CO2*, OH* and CH(A) chemiluminescent intensities can indicate well the heat release rate. The OH*/CH(A) chemiluminescent intensity ratio can be used as good indicator for equivalence ratio, when the contribution of the CO2* emission is removed from the corresponding intensities to eliminate non-monotonic behaviour. Intensity ratios between CO2* and other chemiluminescent species are good indicators of the reacting fuel blend composition for hydrogen/methane, propane/methane and carbon dioxide/methane binary blends. The self-absorption of chemiluminescent intensity and the thermal state of the excited molecules were evaluated for different chemiluminescent species and different fuels for the first time. A comprehensive detailed reaction mechanism for all the main chemiluminescent species, including CO2*, is proposed and evaluated at various conditions for the first time. This mechanism provided chemical insight and interpretation of the observed behaviour of the chemiluminescent intensity measurements. The effect of different fuel-oxidant reaction mechanisms and fuel compositions on the performance of the chemiluminescence detailed reaction mechanism was investigated. The modelled OH*, CH(A), C2* chemiluminescent intensities and their ratios were in a good agreement with the experimental results. Detailed chemical mechanism analysis provided new chemical insight on the origin of the correlation between chemiluminescence and heat release rate. The understanding of the origin of CO2* chemiluminescence led to the proposition of a new CO2* chemiluminescence chemical mechanism, which includes new thermodynamic data, intersystem exchange reaction and new formation path for hydrocarbon combustion. The new mechanism provided improved prediction of CO2* chemiluminescence and CO and hydrocarbon combustion. Since there is a lack of available information on the instantaneous CO spatial distribution in flames, planar two photon absorption of CO laser induced fluorescence was developed. Planar CO TALIF in combusting flows was performed for the first time in counterflow flames and provided two-dimensional spatial distribution of CO relative to the reaction zone. The strain rate and equivalence ratio effects on the level and spatial distribution of CO emission were quantified for the first time. A new correlation between CO pollutant formation and chemiluminescent emissions from flames was established.