Quantitative measurement of chemiluminescence is a challenging work that limits the development of combustion diagnostics based on chemiluminescence. Here, we present a feasible method to obtain effective quantitative chemiluminescence data with an integrating sphere uniform light source. Spatial distribution images of OH* and CH* radiation from methane laminar diffusion flames were acquired using intensified charge-coupled device (CCD) cameras coupled with multiple lenses and narrow-band-pass filters. After the process of eliminating background emissions by three filters and the Abel inverse transformation, the chemiluminescence intensity was converted to a radiating rate based on the uniform light source. The simulated distributions of OH* and CH* agree well with the experimental results. It has also been found that the distribution of OH* is more extensive and closer to the flame front than that of CH*, demonstrating that OH* is more representative of the flame structure. Based on the change in the reaction rate of different formation reactions, OH* distributions can be divided into three regions: intense section near the nozzle, transition section in the middle of the flame, and secondary section downstream the flame, whereas CH* only exists in the first two regions. In addition, as the velocity ratio of methane and co-flowing air increases, the main reactions become more intense, while the secondary reaction of OH* becomes weaker.

Combustion stability is the major concern for engines operated in highly diluted conditions, particularly during the mode transition between controlled autoignition and spark ignition. In this research, studies were performed to investigate the influence of the dilution of internal exhaust gas recirculation and the dilution of external exhaust gas recirculation on early flame development and combustion stability in a single-cylinder optical engine. It is found that a higher external exhaust gas recirculation rate slows do‘wn the early flame development, which is responsible for the higher cyclic fluctuation of combustion. The cyclic variation in the normalized flame area matches well the coefficient of variation of the early flame development period, which decreases with development of the flame. The dilution of the internal exhaust gas recirculation shows a more complicated influence on the combustion than the dilution of the external exhaust gas recirculation does. Although more hot residual gas is trapped in the cylinder, the increase in the internal exhaust gas recirculation rate does not contribute to the promotion of the in-cylinder thermal state in the studied cases. However, with a moderate increase in the internal exhaust gas recirculation rate from 9.8% to 13%, faster initial flame kernel formation is observed, which benefits the combustion by accelerating the early flame development, advancing the combustion timing and stabilizing the combustion. With the internal exhaust gas recirculation rate further increased to 19.6%, a sharp decrease in the mean expansion speed of the flame front results in a retarded early flame development and a slower heat release, which leads to severe deterioration in the combustion stability. In addition, by substituting part of the external exhaust gas recirculation with internal exhaust gas recirculation, an advanced combustion timing, a shorter combustion duration and an improved combustion stability can be achieved; this implies that a higher total exhaust gas recirculation tolerance can be achieved with the same limitation of the coefficient of variation in the indicated mean effective pressure. This result can be instructive in optimizing the control strategy of the valve timing and the external exhaust gas recirculation rate during the mode transition between controlled autoignition and spark ignition.

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Chemiluminescence information is of great significance for characterization of flame structure and combustion characteristics. An atmospheric low swirl burner was developed to investigate the chemiluminescence characteristics of OH* and CH* in low swirl flames, with the equivalence ratio varying from 0.8 to 1.2 and the swirl number from 0.2 to 0.6. The chemiluminescence images were captured via ICCD cameras coupled with narrow-bandpass filters, and an Abel inversion method was introduced to transform the line-of-sight-integrated image into two-dimensional radial distributions. The results show that the distribution of CH* is smaller than that of OH* and concentrated more upstream of the flame near the burner. The equivalence ratio has a relatively more direct influence on chemical reactions, while the swirl number has a more evident effect on the flame structure. As the equivalence ratio increases, the peak value of OH* and CH* increases and the peak position moves downstream of the flame, suggesting that the chemical reactions become more intense. In contrast, the height and width of chemiluminescence distribution increase linearly with increasing swirl number. Moreover, it is found that the equivalence ratio and swirl number can be feasibly estimated based on chemiluminescence measurement results, using the correlation between them derived from this study.

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