The effect of jet mixing on the combustion efficiency of a hot, fuel-rich cross-flow

Many combustion systems inject air into a hot fuel-rich cross-flow to minimize carbon monoxide and unburned hydrocarbon emissions. Examples include staged air injection in fluidized bed combustors, gas turbine engines, and electric arc furnace exhaust afterburners. Low CO and unburnt hydrocarbon emissions are important design objectives. Efforts in modeling combustion have failed in accurately predicting the levels of these emissions. The main aim of this study is to improve our understanding of the relationship between mixing, chemical kinetics, and combustion efficiency for air jets in a hot reacting cross-flow.; Carbon monoxide and hydrogen concentration measurements have been made for six different round jet configurations issuing air into a hot reacting cross-flow. The study of the performance of four different 9 round jets modules having different jet diameters indicated that the overall equivalence ratio has a significant effect on the maximum combustion efficiency. An equivalence ratio of approximately 0.8 leads to the best combustion efficiency for all of the 9 round jet modules. This result is similar to what is observed in premixed combustion. The predominance of the effect of the equivalence ratio suggests that some premixing between the jets and the cross-flow takes place prior to combustion. The rest of the experiments were run such that the optimum mixing (according to cold flow theory) occurs at Φ = 0.8. The experiments also showed that, although the equivalence ratio does not control the location of the optimum point, it does affect the level of emissions. In fact, at constant equivalence ratio, low momentum flux ratios yield higher emissions. The number of jets affects the level of emissions. Larger number of jets provided better combustion efficiency. The unmixedness was measured for the 18 round jets module. This measurement showed the momentum flux ratio is a driving factor affecting mixing. The comparison of the combustion efficiency of H₂ and CO suggest that the reaction rate affect the level of combustion efficiency without affecting the location of the optimum.