Quantitative OH concentration measurement using OH(2,0) band bi-directional LIF method for high-pressure and high-temperature symmetrical flames

Combustion measurements using laser diagnostics are vital for experimentally revealing both qualitative and quantitative flame characteristics. This paper focuses on the application of the quantitative OH laser-induced fluorescence (OH-LIF) method for high-pressure and high-temperature flames. Under high-pressure conditions, the intense OH chemiluminescence drastically decreases the signal to noise ratio (S/N). However, OH(2,0) band excitation is an alternative method to the typical OH(0,0) or OH(1,0) band excitation LIF under high-pressures. OH(2,0) band LIF sufficiently maintains the S/N under high-pressure because the OH fluorescence can be detected away from the peak OH chemiluminescence wavelength. In addition, the bi-directional LIF method is known as a quantitative OH concentration measurement that can ignore the quenching factor. Therefore, this study proposed a quantitative OH concentration measurement by combining the OH(2,0) band LIF and bidirectional LIF methods for high-pressure and high-temperature flames. First, an H-2/air premixed flame created by a McKenna burner under atmospheric pressure was used to validate the quantitative OH(2,0) band bidirectional LIF method. Furthermore, a CH4/O-2/N-2 oxygen-enriched premixed flame created by a multi-hole calibration burner was used to verify the method under high-pressure (similar to 0.5 MPa) and high temperature (similar to 2900 K) conditions. A comparison of the experimental and numerical results revealed that the calculated OH concentrations were within the experimental uncertainty for the McKenna burner configuration. The experimental OH concentration results under high-pressure conditions deviated approximately 10%-20%, but they were also in good correlation with the calculation. In addition, the qualitative correlations with equivalence ratio and pressure variation were also acceptable. Although further consideration of the method is desirable for high-pressure turbulent flames, the results in this study showed that the OH(2,0) band bi-directional LIF method is feasible for measuring OH concentration in high-pressure and high-temperature flames.

Automated summary

This paper focuses on the application of quantitative OH laser-induced fluorescence (OH-LIF) method for high-pressure and high-temperature flames. The OH(2,0) band LIF combined with bidirectional LIF methods is proposed for quantitative measurement of OH concentration. The results show that the OH(2,0) band bidirectional LIF method is feasible for measuring OH concentration in high-pressure and high-temperature flames.