Simultaneous two-phase flame velocity measurement using laser-induced incandescence particle image velocimetry (LII-PIV)

In a previous study we demonstrated a novel two-phase PIV technique based on the laser-induced incandescence (LII) signal from black submicron tungsten carbide particles (WC), which achieved velocity measurements for both dispersed-form (large water droplet) and continuous-form (gas). Submicron WC particles are intentionally seeded into a two-phase flow, and heated by a light sheet generated by a double-pulsed PIV laser running at high energy. The 200 nm diameter, light absorbing WC particles are heated to several thousand degrees to emit strong incandescence signals, whilst the temperature rise in liquid droplets or large particles remains negligible. The small particles follow the gas phase flow, unlike the droplets which may have a different velocity. Droplets are detected via the Mie scatter signal at the same incident wavelength, whereas the LII signal from small WC particles is detected at a suitably different wavelength within the LII emission spectrum, thus allowing discrimination of velocities between phases. The LII-PIV technique had been implemented with a low-speed CCD PIV camera in non-reacting flows. In flames, the strong flame luminosity saturated the second frame due to the long exposure time as the characteristics of the device. To solve this problem, in the present study, we synchronized two high-speed CMOS cameras to a low speed laser. One records the LII signal and the other records the Mie scatter signal from 36.6 µm water droplets. The scattering from WC particles appears only as a weak background signal in the Mie image, which can be easily removed by applying a high-pass filter. Simultaneous velocity measurements for both gas and liquid phase are demonstrated in an air jet, a cold impinging flow, and finally in a Bunsen flame. The last two cases are repeated using the traditional two-phase PIV technique based on image segmentation so as to conduct a fair comparison of both techniques. We show that LII-PIV can achieve the same level of accuracy as the segmentation method in non-reacting flows, and can be applied to measure in flames with two-phase flows with less stringent requirements regarding seeding quality.