Characterization of a high-pressure flame facility using high-speed chemiluminescence and OH LIF imaging

The combustor and turbine inlet pressures of modern aviation and power-generation gas turbine engines can vary between 30 and 50 bar. Innovative diagnostic methods are needed to understand the complex thermo-physical processes taking place under these conditions. Non-intrusive, spatially, and temporally resolved optical and laser diagnostic methods such as chemiluminescence and laser-induced fluorescence imaging (LIF) can be used to gain insights into flame stability, heat release, and pollutant formation processes. In this work, a laboratory-scale, optically accessible, high-pressure burner facility operating on premixed CH₄/air flames is developed and characterized using kHz-rate hydroxyl and methylidyne chemiluminescence imaging, OH-LIF imaging, and two-color OH-LIF thermometry. For the latter two measurements, the flames were stabilized up to 10 bar using a stainless-steel disk mounted above the burner surface. Approximately 10-ns duration Nd:YAG laser pulses at 283.305 nm were used to excite the Q₁(7) rotational line of the A²Ʃ⁺←X²∏ (1,0) band of the OH radical, followed by fluorescence detection from the A←X (1,1) & (0,0) bands. A linear dependence of the OH-LIF signal on the laser energy was observed. An increase in pressure from 1 to 10 bar showed a nonlinear decay of the OH-LIF signal. While quenching corrections accounted for a fraction of the signal loss, additional mechanisms, such as laser beam absorption and signal trapping, need to be considered for complete signal quantification. The measured excitation spectrum compared well with the LIFBASE model predictions. The flame equivalence ratio scans at different pressures agreed with the Cantera equilibrium flame code calculations. 2D OH-concentration distributions and two-color OH-LIF temperature maps agreed qualitatively with flame simulations performed using the ANSYS Fluent software package. This well-characterized burner facility provides a testbed for combustion and soot formation studies and investigates the role of minor species at elevated pressures at gas-turbine-relevant flame conditions.