High-speed volumetric imaging of formaldehyde in a lifted turbulent jet flame using an acousto-optic deflector

Abstract

The development of high-speed volumetric laser-induced fluorescence measurements of formaldehyde ($\hbox {CH}_2\hbox {O}$-LIF) using a pulse-burst laser operated at a repetition rate of ${100} \hbox { kHz}$ is presented. A novel laser scanning system employing an acousto-optic deflector (AOD) enables quasi-4D $\hbox {CH}_2\hbox {O}$-LIF imaging at a scan frequency of ${10}\hbox { kHz}$. The diagnostic capability of time-resolved volumetric imaging is demonstrated in a partially premixed DME/air lifted turbulent jet flame near the flame base. Simultaneous imaging of laser beam profiles is performed to account for the laser pulse energy fluctuation and laser sheet inhomogeneity. With the accurate registration of laser sheet positions, the volumetric reconstruction of $\hbox {CH}_2\hbox {O}$-LIF signals is performed within a detection volume of $17.3 \times 11.9 \times {2.3}\, \hbox { mm}^3$ with an average out-of-plane spatial resolution of ${250}\,\upmu \hbox {m}$. A surface detection algorithm with adaptive thresholding is used to determine the global maximum intensity gradient by calculating gradient percentiles. The flame topology characteristics are investigated by evaluating the 3D curvatures of $\hbox {CH}_2\hbox {O}$ surfaces. Curvatures calculated using 2D data systematically underestimate the full 3D curvature due to the lack of out-of-plane information. The inner surfaces near the turbulent fuel jet exhibit higher probabilities of large mean curvature than the outer surfaces. The saddle and cylindrical structures are dominant on both the inner and outer surfaces and the elliptic structures occur with lower probability. The results suggest that the damping of turbulent fluctuations by the temperature increase through the $\hbox {CH}_2\hbox {O}$ region reduces the curvature, but the local structure topology remains self-similar.

Graphic abstract