Quantitatively OH-PLIF measurements in laminar diffusion flames of n-heptane at elevated pressures

In the hydrocarbon fuel combustion, OH radicals play an important role, participating in most of the radical reactions and marking the location of the combustion product region. In this work, a two-dimensional quantitative measurement of OH radical concentration in n-heptane laminar diffusion flames was conducted across a pressure range from 1.0 bar to 4.0 bar. The primary objective was to investigate the impact of pressure on OH radical concentration and distribution in these flames. This investigation was carried out through the utilization of a combination of planar laser-induced fluorescence (PLIF) and calibration via absorption spectroscopy. Specifically, the Q1(8) band was selected for excitation, with an excitation wavelength of 283.55 nm employed for the measurements. As the pressure increases, the area occupied by the OH radical distribution region decreases, and the positions of the maximum concentration values on both sides of the flame axis shift closer to the flame axis itself and towards the burner exit. Besides, the integrated value and max value of OH concentration in the laminar diffusion flames decreases approximately linearly as the pressure increases. These experiments data provide reference for the application of optical techniques and the development of chemical reaction kinetics of n-heptane combustion.