Instantaneous planar measurements of nitric oxide concentration in a turbulent n-heptane spray flame

Abstract

Instantaneous and mean nitric oxide (NO) concentrations are measured in a turbulent, lifted, n-heptane jet spray flame at an atmospheric pressure using planar laser-induced fluorescence (PLIF). The flame-front is simultaneously located using OH-PLIF. The NO excitation line with the least temperature-quenching dependence is selected, and the associated NO-LIF dependence is corrected. LIF optimization and data reduction are based on previous work [Mulla et al. Combust. Flame 203 (2019) 217–229] where the NO-LIF signal was simulated at the spray flame condition (temperature and gas composition from LES) using a spectroscopic model. In the present work, to reduce interferences from polycyclic aromatic hydrocarbons (PAH) and soot, the detection bandwidth is progressively reduced from broadband (F6), intermediate (F3), to narrowband (F1). F6 results in a significant interference, while F1 contains the least interference. Instantaneous NO mole fraction (χNO) is deduced from F1 dataset using a novel instant-wise mapped interference subtraction approach (F1-Inst). The conditional mean NO concentrations derived from F6, F3, F1, and F1-Inst are in agreement (within uncertainty limits); however, F1-Inst appears to be the most reliable. Instantaneous flame-structure (OH) and NO concentration fields are analyzed. The spray flame structure consists of the inner (B1) and outer (B2) flame branches. Local extinctions are observed along B1. Small-scale (<1 mm) extinctions do not alter the NO concentration, whereas large-scale extinctions result in a sharp (50–100%) χNO drop, possibly due to a reduced temperature. Instantaneous χNO along B2 shows a sharp growth until 55 mm height above burner (HAB), beyond which the growth saturates. The deceleration of NO growth and χNO reduction near the sooting region is most likely due to the soot-radiation induced temperature drop. Mean χNO along B1 measures ∼ 25 ppm with a small axial variation (20–30 ppm). Expected lower temperature and nearly constant χNO with residence time suggest the nitrous oxide and prompt NO formations routes. Mean χNO in the outer diffusion branch (B2) increases from 35 ppm at the flame-base to 75 ppm at 60 mm HAB. The NO growth and potentially high temperature in B2 indicate the thermal route of NO formation.