Measurement of OH concentrations in turbulent diffusion flames using combined LIF and Raman spectroscopy

Quantification of laser induced fluorescence (LIF) signals of a particular species requires local flame temperature to accurately predict the collisional quenching. Most of the currently available collisional quenching models of OH radical assume equilibrium temperature to evaluate the collisional removing of OH radical from the upper state. This assumption is reasonable for hydrogen flames where near equilibrium exists close to the burner region. However, an attempt to use this assumption in hydrocarbon flames incurs significant errors in the measurement of the absolute concentration of OH radical. This paper presents a technique of combined LIF and Raman spectroscopic measurements, in which local temperature is measured independently from Raman spectra. The measurement of OH radical concentration in natural gas turbulent flames is presented using a combined LIF-Raman spectroscopy technique. For quantification of absolute OH concentration, both measured collider species concentration and equilibrium composition were used. The differences are analyzed and discussed.